late devonian carbonate margins and foreslopes ... - GeoScienceWorld

Journal of Sedimentary Research, 2015, v. 85, 1334–1361 Research Article DOI: http://dx.doi.org/10.2110/jsr.2015.84

LATE DEVONIAN CARBONATE MARGINS AND FORESLOPES OF THE LENNARD SHELF, CANNING BASIN, WESTERN AUSTRALIA, PART A: DEVELOPMENT DURING BACKSTEPPING AND THE AGGRADATION-TOPROGRADATION TRANSITION TED E. PLAYTON*1

AND

CHARLES KERANS2

1

The University of Texas at Austin, Department of Geological Sciences/Bureau of Economic Geology, Austin, Texas 78712, U.S.A. 2 The University of Texas at Austin, Department of Geological Sciences, 1 University Station C1100, Austin, Texas 78712, U.S.A. e-mail: [email protected]

ABSTRACT: Carbonate reefal margin and foreslope settings are characteristically heterogeneous and difficult to predict due to a spectrum of sediment source factories, resedimentation processes, resultant deposit types, and controlling parameters. In particular, the effects of changes in long-term accommodation on the composition, architecture, and sediment distribution patterns of carbonate margin–foreslope–basin systems are poorly understood. Upper Devonian (Frasnian) outcrop exposures along the Lennard Shelf, northeast Canning Basin, Western Australia, were investigated to assess the development of reefal margin and foreslope settings during long-term 1) platform backstepping with aggradational pulses, and 2) across the transition from platform aggradation to progradation. Measured sections tied to interpreted photomosaics and detailed mapping using aerial photographs were collected from the South Lawford Range and Windjana Gorge areas. The exposures reveal distinctive differences in foreslope grain composition, deposit characteristics and proportions, margin morphology, and stratigraphic expression 1) during platform evolution between backstepping events, and 2) depending on position within the long-term accommodation setting. Between backstepping events, aggradational margins can be classified as “growth escarpments” with associated grain-dominated, onlapping foreslope deposits. Margins across the long-term transition from aggradation to progradation evolved from erosional escarpments with onlapping debris deposits to accretionary, interfingering configurations. Development of growth escarpments between backstepping events was a function of vertical reefal growth from sustained high accommodation conditions during the long-term transgressive systems tract, coupled with a Frasnian reefal assemblage that responded to light and tracked relative sea level. This net vertical reefal growth also resulted in relative margin stability and the deposition of grain-dominated foreslopes. Conversely, margins were highly unstable and underwent repeated failure across the longterm aggradation-to-progradation transition, reflecting a lack of underlying substrate to support basinward advance, and resulting in debris-dominated foreslopes. These observations provide relationships that predict margin and foreslope associations of faciesscale heterogeneity and seismic-scale geometry within a low-frequency sequence stratigraphic framework.

INTRODUCTION

Carbonate Margins, Foreslopes, and Significance of the Lennard Shelf Dataset Numerous case studies and compilations of modern, outcrop, and subsurface carbonate margin, slope, and basinal depositional systems undertaken over the past half-century have elevated our understanding of the range of deposits, architectures, variability, organization, and controls that result in the complex heterogeneity we observe in these settings (e.g., Cook et al. 1972; Mountjoy et al. 1972; McIlreath and James 1978; Schlager and Ginsburg 1981; Droxler and Schlager 1985; Mullins and Cook 1986; Eberli and Ginsburg 1989; Kenter 1990; Grammer and Ginsburg 1992; Adams and Schlager 2000; Della Porta et al. 2003; Janson et al. 2011; Collins et al. 2013).

* Present Address: Chevron Energy Technology Company, Carbonate Stratigraphy Research & Development, 1500 Louisiana Street, Houston, Texas 77002, U.S.A. Published Online: November 2015 Copyright E 2015, SEPM (Society for Sedimentary Geology)

Despite this excellent foundation, challenges remain in developing conceptual predictive models of carbonate margin and foreslope settings, in part associated with the high degree of lateral and stratigraphic depositional heterogeneity inherent to these settings, limited datasets of acceptable outcrop exposure or seismic imaging quality, and overall lesser research efforts to date (when compared to deep-water siliciclastic or carbonate platform-top systems) stemming from historically restricted hydrocarbon production. Increasing importance of carbonate slope and basin systems, both economically (e.g., Devonian–Carboniferous reservoirs of the Pricaspian Basin, Kazakhstan, e.g., Collins et al. 2013; Permo-Carboniferous reservoirs of the Midland Basin, west Texas, e.g., Clayton and Kerans 2013) and as robust recorders of carbonate platform evolution, warrants the development of a uniform characterization approach and synthesis of these complex settings. Playton (2008) and Playton et al. (2010) propose classifications and end-member models for carbonate slope and basin deposit types, styles of platform-to-slope (margin) transitions, spatial (strike) variability, and stratal architecture, and discuss the interplay of intrinsic and extrinsic controls behind such

1527-1404/15/085-1334/$03.00

JSR

CANNING BASIN CARBONATE SLOPES AND MARGINS, PART A

1335

TABLE 1.— Terminological schemes and definitions for reefal carbonate-platform systems with emphasis on margin and slope depositional realms.

Slope

Margin

System Scale

Term Type

Terminology after Playford et al. (2009)

Equivalent terminology used herein (after Playton et al. 2010; Playton 2008)

reef complexes

reef-rimmed/reefal platform

carbonate-platform system with in situ bound margins

no equivalent

reef-rimmed/reefal

[margin] dominated by autochthonous boundstone accumulation

reef

margin

transition from platform-top to foreslope depositional environments, including encrusted upper-slope, reef, and reef-flat deposits

upright scarp margin

aggradational-escarpment margin

vertical-upright evolution; coeval reefal and foreslope deposits are disconnected (onlap)

upright rollover margin

aggradational-accretionary margin

vertical-upright evolution; coeval reefal and foreslope deposits are connected (interfingering)

advancing margin

progradational-accretionary margin

lateral-seaward evolution; coeval reefal and foreslope deposits are connected (interfingering)

backstepping margin

backstepping-escarpment margin

lateral-landward evolution; coeval reefal and foreslope deposits are disconnected (onlap); laterally offset margins

collapsed margin

erosional-escarpment margin

margin partially removed from collapse, resulting in a truncation surface and subsequent foreslope onlap

backreef

platform top

depositonal environments landward of the margin; interfingers seaward with the reef-flat deposits

basin

basin

depositonal environments seaward of the foreslope that dip at 1u or less

platform margin

platform edge

most abrupt inflection at the margin

reef margin

reef

reefal deposits usually centered at the platform edge

reef scarp

erosional-escarpment surface

truncation surface of an erosional-escarpment margin

platform margin unconformity

erosional-escarpment onlap

relationship of younger foreslope deposits onlapping an erosional-escarpment surface

Definition used herein

reef flat

reef flat

reefal deposits landward of the reef that interfinger updip with platform-top deposits

marginal slope

slope or slope profile

inclined portion of the depositional profile between the platform edge and toe of slope inflections

reefal slope

encrusted upper slope

reefal deposits seaward of the reef that interfinger downdip with foreslope deposits; generally located on upper third of slope profile

forereef

foreslope

dominantly allochthonous deposits that interfinger updip with encrusted upper-slope deposits and downdip with basinal deposits at the toe of slope inflection

no equivalent

upper, middle, and/or lower slope

arbitrary subdivisions of the slope profile based on depositional and architectural characteristics (e.g., facies tracts); unique for every system

toe of slope

toe of slope

inflection point where foreslope transitions into basinal deposits; marked by dips of 1u or less

end-member variations. Here we focus on Late Devonian margin and foreslope settings of the Lennard Shelf, Canning Basin, Western Australia, an outcrop example with extensive strike- and dip-view exposures that allow detailed examination of carbonate shelf-to-basin development in response to a variety of drivers. In addition to excellent preservation of primary depositional fabrics and architecture in Middle–Upper Devonian platform-top, margin, and slope environments (Playford 1980; Kerans et al. 1986; Playford et al. 2009), outcrops of the Lennard Shelf are also unique in that reefal platform development was recorded 1) throughout a . 30 Ma succession from Givetian–middle Frasnian platform backstepping and aggradation to late Frasnian–Famennian platform progradation (Playford et al. 2009), 2) across a global extinction interval where reefs changed from skeletal–calcimicrobial assemblages in the Frasnian to calcimicrobially dominant assemblages in the Famennian (Frasnian–Famennian (F-F) extinction; Playford 1980; Playford et al. 2009), and 3) over a long-term

climatic change from greenhouse conditions in the Givetian–middle Frasnian to more transitional conditions with intermittent glaciations in the late Frasnian–Famennian (e.g., Markello et al. 2008). In terms of subsurface applications, these exposures are age-equivalent and stratigraphically similar to producing fields in the Alberta Basin, western Canada (e.g., Wendte 1992; van Buchem et al. 2000; Potma et al. 2001; Atchley et al. 2006), as well as exhibit comparable long-term margin architectural evolution to Devonian–Carboniferous giant reservoirs in the Caspian region, Kazakhstan (Tengiz and Karachaganak Fields; Weber et al. 2003; Collins et al. 2006; Katz et al. 2010; Collins et al. 2013). Previous Work and Terminology Seminal outcrop studies of Playford (1980, 1984) presented the foundational framework for Lennard Shelf carbonate research, including a composite stratigraphic architecture, depositional-profile models, faciestract subdivisions, and recognition of important depositional and

1336

T.E. PLAYTON AND C. KERANS

JSR

FIG. 1.—Simplified maps of outcrop exposures and locations of the Lennard Shelf, Canning Basin, northern Western Australia (modified after Playford et al. 2009 and Frost and Kerans 2010). Subtle regional structural tilt exposes progressively younger strata from southeast to northwest. Outcrop belt marks northeast fringe of northwest–southeast-trending normal-fault system that defines the Fitzroy Trough.

diagenetic processes. These studies also established terminology for the Lennard Shelf reefal platforms that subdivides major facies tracts of the depositional profile and describes margin characteristics; however, herein we will apply the generalized (non-Lennard Shelf-specific), carbonate margin, slope, and basinal nomenclature of Playton (2008) and Playton et al. (2010) (Table 1). Within Playford’s framework, subsequent work relevant to this study focused on specific localities, sedimentology of facies belts, sequence stratigraphic interpretation, and diagenetic aspects including syndepositional fracturing (e.g., Kerans 1985; Hurley 1986; Kerans et al. 1986; Playford et al. 1989; Ward 1999; Copp 2000; Frost and Kerans 2009, 2010). Critical accompanying studies unraveled the structural evolution of the basin (e.g., Begg 1987; Drummond et al. 1991; Do¨rling et al. 1996) and generated conodont- and goniatite-defined biostratigraphic zonations consistent with other global localities (e.g., Klapper 1989; Ziegler and Sandberg 1990; Becker et al. 1993; Becker and House 1997; Klapper 1997; Girard et al. 2005; Klapper 2007). In particular, detailed discussions of foreslope deposits, allochthonous and autochthonous processes, margincollapse triggers, and sequence stratigraphic timing of resedimentation are available for specific localities or time intervals (George et al. 1994, 1995, 1997; George 1999; George and Chow 2002; Adams and Hasler 2010). Recently, Playford et al. (2009) comprehensively summarized more than five decades of dedicated research and meticulously encapsulated the breadth of knowledge extracted from works ranging from regional and conceptual syntheses to detailed data for particular outcrop localities. Outcrop Dataset and Scope Middle to Late Devonian reefal platforms of the Lennard Shelf are exposed along a northwest–southeast-trending, 350-km-long outcrop belt

of low-relief ranges that mark the northeastern margin of the Canning Basin in northern Western Australia (Fig. 1). Cenozoic erosion exhumed the largely undeformed Givetian through Famennian carbonates and an overall , 5u regional northward tilt allows examination of progressively younger parts of the Lennard Shelf system (in general) from southeast to northwest. Field data for the comprehensive study consists of twentyone measured sections (15–500 m thick), 150 samples with thin sections and slabs, and . 25 km2 of detailed photomosaic and plan-view mapping from the South Lawford Range (Bugle Gap area), Dingo Gap, Windjana Gorge, and other localities in the Napier Range (Fig. 1). The existing biostratigraphic framework (e.g., Becker et al. 1993; Becker and House 1997; Klapper 2007) was leveraged in linking the various outcrop localities together to generate conceptual models and composite depictions that synthesize the development of Lennard Shelf foreslopes and margins:

N N N N

within a long-term backstepping and aggradational setting and peak greenhouse climate in the early Frasnian, from the South Lawford Range area; across a low-frequency transition from platform backstepping and aggradation to progradation around the middle–upper Frasnian boundary, from Windjana Gorge; across a global biotic crisis and extinction surrounding the Frasnian– Famennian boundary, from the Windjana Gorge, Dingo Gap, and Casey Falls areas; and within a long-term progradational setting and interpreted transitional climate in the Famennian, from Windjana Gorge, Dingo Gap, and Casey Falls areas.

JSR


Based on observations from these high-quality exposures, we here present 1) conceptual depositional models of the Upper Devonian of the Lennard Shelf, 2) descriptions of margin–slope–basin variations within sequence stratigraphic, ecological, and climatic contexts, and 3) predictive relationships that link sediment composition, fabric, and stratal architecture to the intrinsic and extrinsic controls of the system. We have chosen to present these results in two stand-alone parts: 1.

2.

Part A (this paper) focuses on early, middle, and late Frasnian margin and foreslope variations as related to long-term platform backstepping and the transition from aggradation to progradation, highlighting the effects of low-frequency accommodation changes; and Part B (Playton and Kerans 2015b) describes upper Frasnian and Famennian margins and foreslopes within a long-term highstand (HST) progradation that encompasses a global biotic crisis (F-F extinction) and shift in global climate, stressing the impacts of environmental conditions (ecological and climatic) on margin-toslope development in addition to accommodation drivers. GEOLOGIC BACKGROUND

Structural Evolution of the Lennard Shelf Middle to Late Devonian crustal extension produced northwest– southeast-trending troughs and structural highs that segmented the eastern Canning Basin into deep basins and structurally positive terraces that served as nucleation sites for carbonate platforms (Begg 1987; Drummond et al. 1991). The Fitzroy Trough developed along the northeastern margin of the Canning Basin and was fringed to the northeast by relatively uplifted blocks defining the Lennard Shelf, a preferred site for accumulation of Middle to Upper Devonian reefal carbonate land-attached and isolated platforms. Uplifted Proterozoic hinterland terrane of the Kimberley Block confined the Lennard Shelf to the northeast, and structural complexity related to transfer zones produced irregular basement paleogeography and differential subsidence along the terraces (Begg 1987; Do¨rling et al. 1996). Active, yet episodic, rifting in the Fitzroy Trough region persisted until the late Frasnian, resulting in pulses of subsidence that undoubtedly influenced carbonateplatform development and backstepping (Playford 1980; Begg 1987). In the late Frasnian, active rifting waned and subsidence rates decreased, but syndepositional tectonism related to reactivation along transfer zones is documented throughout the Late Devonian. The spatial structural complexity and temporal subsidence patterns associated with Late Devonian rifting along the Lennard Shelf affected the loci of carbonate nucleation, the long-term evolution of margin trajectories, and the input points for siliciclastic fan-delta complexes (Playford et al. 2009). Likewise, the pre-carbonate and syn-carbonate differential structural movements introduced considerable stratigraphic and depositional variability from one locality to another and triggered particular types of local deposition resulting in reefal platform heterogeneity along the shelf. Sequence Stratigraphic Framework Low-Frequency Supersequence Architecture.—Playford (2002) and Playford et al. (2009) subdivided carbonate-platform development along the Lennard Shelf into the Givetian–Frasnian Pillara Sequence and the Famennian Nullara Sequence (Fig. 2). Reefal platform development ceased in the uppermost Famennian when the system was locally exposed, eroded, and ultimately draped by Tournaisian (Lower Carboniferous) non-reefal carbonate deposits with apparent cool-water faunal assemblages (Playford 1980, 2002; Playford et al. 2009). Givetian, lower

1337

Frasnian, and middle Frasnian margins of the Pillara Sequence display retrogradational configurations with intervening aggradational pulses, whereas margins in the upper Frasnian portion of the Pillara Sequence and the Famennian Nullara Sequence are characterized by progradation. Thus, the shift from backstepping and aggradation to progradation does not occur at the Pillara–Nullara contact (F-F boundary), but instead around the middle–late Frasnian contact. Using sequence stratigraphic terminology, the Givetian through Famennian succession comprises a low-frequency supersequence, where the retrogradational and aggradational Givetian to middle Frasnian section represents a long-term TST, the progradational upper Frasnian to Famennian section represents a long-term HST, and the MFS is located near the middle–late Frasnian contact. This succession is classified as second-order (e.g., Goldhammer et al. 1991; Sarg et al. 1999) with a duration of 20–30 Ma (Gradstein et al. 2012). The combined platform-top thickness of the Givetian through Famennian succession approaches 2000 m, but it varies across the Lennard Shelf due to irregular basement topography and non-uniform local subsidence rates (Playford 1980; Do¨rling et al. 1996). The retrogradational and aggradational (supersequence TST) Givetian to middle Frasnian succession accounts for most of the accumulated thickness (. 1500 m) as late Frasnian to Famennian (supersequence HST) platform-top accumulation is approximately 100–300 m (e.g., Frost 2007; Playford et al. 2009). The total extent of Givetian to middle Frasnian TST retrogradation is variable, also related to localized structure and subsidence patterns, but it can be estimated at 2–3 km (Playford et al. 2009), yielding a progradation:aggradation (P:A) ratio of (–) 1.0 to (–) 2.0 (sensu Tinker 1998; Kerans and Tinker 1999). Late Frasnian through Famennian HST progradational extent can be approximated locally at 1–3 km, yielding a P:A ratio of (+) 5 to (+) 15. Givetian–Frasnian Sequences.—Playford (2002) and Playford et al. (2009) defined six sequences within the Givetian–Frasnian succession (Fig. 2A) that successively record: 1) initial carbonate growth and building of topography through aggradation (Givetian–lower Frasnian Sequence 1); 2) multiple phases of platform aggradation punctuated by pronounced backstepping events (lower–middle Frasnian Sequences 2–4); 3) platform aggradation at the turnaround from retrogradational to progradational configurations (middle Frasnian Sequence 5); and 4) initial platform progradation (upper Frasnian Sequence 6). This study modifies Playford’s framework as exposures in Windjana Gorge reveal that the upper Frasnian succession can be subdivided into two progradational units (Sequences 6 and 7; Fig. 2) based on stratal architecture and facies changes observed in the upper to middle slope. Within the low-frequency supersequence architecture, Sequences 1–4 signify the TST, Sequence 5 marks the long-term MFS, and Sequences 6 and 7 represent the onset of progradation in the early HST. Playford et al. (2009) defines these sequences as third-order based on their subordinate nesting within the lower-frequency supersequence architecture, but their absolute durations are uncertain. Playford’s sequence boundaries (SBs) for Givetian to middle Frasnian Sequences 1–4 are defined by pronounced platform backstepping events, which are hypothesized to have resulted from tectonic tilting or rearrangement followed by subsequent pulses of elevated subsidence (similar interpretations in George et al. 2002, 2009a, 2009b). Playford et al. (2009) reported karstification along interpreted backstep surfaces in two outcrop localities to support this framework. Transgressive and highstand systems tracts in Sequences 1–6 are not interpreted by Playford (2002) or Playford et al. (2009). This study documents a conspicuous internal complexity within the net aggradational margin architecture between pronounced backstepping events in Givetian through middle Frasnian Sequences 1–4 (Fig. 2); these observations complicate the tectonically controlled sequence interpretation, which invokes a rapid increase in accommodation (post-tectonic subsidence) followed by a gradual decrease in accommodation where

1338


JSR

JSR


carbonate factories “caught up” and “kept up” with background subsidence. Based on interpretations of accommodation from margin stratal evolution, we present an alternative sequence model for Givetian to middle Frasnian Sequences 1–4 (Figs. 2B, 3).

N

N

N

We interpret platform drowning and backstepping as signals of highest-accommodation conditions such that margins were forced to step landward and reinitiate. These phases of highest accommodation are reflective of maximum flooding, and thus backstepping surfaces are here interpreted as MFSs, rather than SBs (Figs. 2B, 3A). Platform-top stacking patterns corroborate this interpretation with deepening-upward cycle successions leading up to backstepping events. Moreover, we have found no evidence for karstification at backstepping surfaces in this study; instead abrupt vertical changes from shallow-water carbonate environments to toe of slope facies are observed. Following backstepping events, accretionary margins re-established, constructed relief, and weakly prograded, implying enough infill of accommodation to force basinward advance. Progradational extents were minimal (hundreds of meters maximum), and margins never advanced far enough to reach the position of the older, underlying former margin. However, this subtle indication of slightly lower accommodation is here interpreted to represent a HST setting that is poorly developed due to its positioning within the overall low-frequency TST. By this interpretation, SBs are placed at the tops of the weakly progradational phases, just prior to the onset of the accretionaryescarpment profile transition (sensu Playton et al. 2010, Figs. 2B, 3A–C). This subsequent transition into escarpment configurations reflects accommodation increase where euphotic reefal margins kept pace with rapidly rising relative sea level, but contributions of material to the foreslope were not adequate to keep the slope profile filled. This record of accommodation increase is here interpreted to represent a TST setting (Figs. 2B, 3C, D), characterized by the evolution into escarpments from vertical reefal growth and well-developed aggradational platform tops with deepening-upward facies successions. The terminus of escarpment development was coincident with the succeeding backstepping event, and thus marks the next MFS (Fig. 3E).

For the purposes here, margins and foreslopes will be described within the alternative systems tract framework presented above for Sequences 1– 4. Sequences 5–7 surface and systems-tract interpretations are either consistent with that of Playford et al. (2009) or defined for the first time here based on stratigraphic and facies relationships. Formation Terminology.—Nomenclature for the major mapped lithological and/or biostratigraphic units of the Lennard Shelf (Fig. 2A; Table 2) was established by Playford (1980). As the nomenclature was developed for large-scale, regional mapping of the outcrop belt, the terms are based chiefly on combinations of location, age, and depositional environment, and are loosely in accordance to Playford’s (2002) thirdorder sequence framework. For describing margin and slope characteristics within sequence stratigraphic contexts, we here choose to present

1339

observations in terms of age (e.g., lower Frasnian) and depositional environment (e.g., encrusted upper slope). Frasnian Overview Lower Frasnian.—Exposures in the Emanuel Range, Paddy’s Valley, Bugle Gap, and the South Lawford Range area (Nardji Cave, McWhae Ridge, and Kelly’s Pass; Figs. 1, 4) allow detailed observation of lower Frasnian facies, margin styles, and stratal architecture. Data and interpretations herein are dominantly derived from outcrops of lower Frasnian Sequence 2 as it is best exposed (Fig. 2B), but Sequences 1, 3, and 4 are assumed to have developed under similar conditions with comparable resulting architectures. Early Frasnian reefal platforms of the Lennard Shelf were flat-topped, and margins were dominated by coral–calcimicrobial–stromatoporoid assemblages that produced framestone fabrics, and substantial euphotic components restricted the bulk of reefal growth to shallow water (Playford 1980, 2002; Playford et al. 2009). Frame-building skeletal constituents of the bound margins decrease seaward of the platform edge where encrusted calcimicrobial terrains, dominated by Renalcis–Sphaerocodium assemblages (sensu Riding 1988; James and Gravestock 1990; Riding 1991), extend down the upper slope for a few tens of meters. Centered around the platform edge, bulbous and tabular forms of Actinostroma stromatoporoids gradually transition landward into thickets of Stachyodes and Amphipora stromatoporoids with more delicate branching forms (producing bafflestone to floatstone fabrics) of the reef-flat environment. In situ reef-flat tongues interfinger landward with outer platform-top bioclastic rudstone and fenestral peloid–skeletal packstone–grainstone. Interior portions of the platform-top exhibit lower-energy, more restricted facies including burrowed peloidal mudstone–wackestone and occasional laminated to fenestral mudstone. Quartzose to carbonate silty and argillaceous sediments are often intermixed into various lithologies or can occur as discrete beds that vertically punctuate carbonate successions. Between backstepping events, platforms built relief from flat nucleation surfaces to thicknesses of 100–200 m (Playford et al. 2009), producing slope heights (vertical distance from platform edge to toe of slope) of similar dimensions as basinal deposition was relatively starved. Slope widths (horizontal distance from platform edge to toe of slope) are reconstructed to be 1 km or less, with maximum detrital slope gradients up to 35u. Middle–Upper Frasnian.—Intact outcrop walls in Windjana Gorge (Fig. 1) exquisitely expose the flat-topped reefal platforms around the middle to upper Frasnian contact that record the long-term transition from platform aggradation (following several backstepping events) to progradation across the supersequence MFS (Sequences 5 and 6; Fig. 2B) (Playford 2002; Playford et al. 2009). Reefal margin assemblages, platform-top depositional environments, facies belts, and their transitions are similar to those described for early Frasnian platforms, but the proportions of fine-to-coarse siliciclastic material in all environments along the depositional profile are locally greater in the middle–upper

r FIG. 2.—Idealized composite cross sections showing alternative sequence architecture interpretations of the Lennard Shelf (modified after Playford et al. 2009). Although variable along the strike of the Lennard Shelf, backstepping distances and Frasnian sequence thicknesses are within the ranges of the type localities provided by Playford et al. (2009). Frasnian Sequence 4 architecture is hypothesized and not constrained by data from this study. Frasnian Sequence 6 through Famennian evolution constrained to southeast Windjana Gorge exposures (brown hatched line; see Fig. 11). Condensed drape of distal slope sediments over Frasnian backstepping topography is based on McWhae Ridge and Casey Falls outcrops in the South Lawford Range area. Thin black lines denote internal stratal architecture. Green line above Frasnian– Famennian boundary indicates top of “lowermost Famennian” interval. Individual Famennian high-frequency sequences are not shown. HST, highstand systems tract; TST, transgressive systems tract; MFS, maximum flooding surface. A) Sequence architecture with formation terminology after Playford et al. (2009), where lower to middle Frasnian sequence boundaries are shown at platform backstepping surfaces and third-order systems tracts are not defined. B) Alternative sequence architecture defined in this study and used herein, where backstepping events are interpreted as MFSs and systems tracts for lower to middle Frasnian sequences are defined.

1340


JSR

JSR


Frasnian (Hocking and Playford 2001; Playton et al. 2011). At the final phase of reefal margin aggradation and escarpment development immediately before the supersequence MFS (Sequence 5 TST; Fig. 2B), slope heights are estimated at several hundreds of meters (locally approaching 500 m; Playford et al. 2009), and slope widths can be reconstructed accordingly at up to 1 km using observed depositional dips of 30–35u. SLOPES AND MARGINS OF BACKSTEPPING PLATFORMS OF THE LOWER FRASNIAN

HST Accretionary Margins and Associated Foreslopes Early HST Nucleation.—Immediately after platform backstepping events (interpreted as MFSs) in the early Frasnian, former platform tops became the nucleation sites for younger margins, reef spines, pinnacle buildups, and small isolated platforms that are characterized by aggradational accretionary margins, here interpreted as an early HST setting of initial accommodation infill (Figs. 3A, 5A). Slope profiles of the early HST can be subdivided into upper-slope and lower-slope sub-environments based on sedimentary characteristics. Maximum slope heights and widths are projected to be 50 m and 300 m, respectively. Upper-slope deposits of the early HST are dominated by macroskeletal floatstone–bafflestone that exhibits upright growth fabrics, whole-fossil preservation, early marine cementation, and calcimicrobial encrustation (Fig. 5B). The primary skeletal constituents include brachiopods, laminar Actinostroma stromatoporoids, platy sponges, crinoids, and receptaculitids. Steep bedding dips (up to 40u) are confirmed as primary by abundant fine sediment geopetal infill of cavities and intra-fossil voids (Fig. 5B) (after Playford 1980). Bedding is characteristically tabular to slightly lenticular, 10–50 cm thick, and forms laterally extensive biostromal sheets. Secondary deposits of the upper slope include bioclastic rudstone, peloid–skeletal packstone–grainstone, and calcimicrobial boundstone, often occurring as discontinuous lenses. Lower-slope deposits are dominated by peloid–skeletal packstone– grainstone with variable amounts of admixed silt and/or bioclasts (Fig. 5C). Normal grading with silty bed caps and low-angle to planar lamination are common. Beds are typically 5–20 cm thick, dip at 2–10u, and form tabular sheets. Lower-slope secondary deposits include thin silty skeletal wackestone–packstone and lenses of bioclastic rudstone. The transition between upper-slope and lower-slope environments is not well exposed, but gradational and interfingering relationships are likely as deposits appear to fine basinward. Basinal deposits and their relationship with lower-slope deposits are not well exposed. In general, the upper slopes of the early HST are characterized by autochthonous (in situ) colonization by macroskeletal communities and their early stabilization through cementation and encrustation. Lowerslope packstone–grainstone is interpreted to be derived from platformtop or marginal settings, implying bypass across upper-slope skeletal communities via concentrated to turbulent flow processes (after Mulder and Alexander 2001). The paucity of observed breccias with either reefal

1341

margin- or slope-derived clasts indicates these were periods with relatively stable margins and slope profiles, despite steep depositional dips and aggradational growth. HST Weak Progradation.—Following the early HST, re-established land-attached margins continued to aggrade and constructed shelf-tobasin relief such that outboard pinnacles and isolated platforms drowned (Fig. 3B). Margins remained accretionary and eventually evolved from aggradational to weakly progradational configurations (Fig. 6A, B), interpreted here to represent a HST setting of accommodation infilling. Based on reconstructions, slope heights and widths are estimated up to 100 m and 500 m, respectively, and available sedimentologic data supports the subdivision of the slope profile into upper-slope and lowerslope sub-environments. Upper-slope deposits of the HST consist chiefly of bioclastic rudstones and coarse, skeletal-dominated grainstones that exhibit pervasive fragmentation and abrasion of grains (Fig. 6C, D). Bioclasts are composed dominantly of Stachyodes stromatoporoids, solitary rugose corals, crinoids, brachiopods, and cephalopods. Silty matrix material forming geopetals between bioclasts confirms steep depositional dips (up to 35u), and early marine cement is common—but evidence for calcimicrobial encrustation is rare to absent. Bedding is tabular to slightly lenticular, forming laterally extensive sheets, and is 10–50 cm thick. Secondary deposits on the upper slope include peloid–skeletal packstone–grainstone and thin silty drapes at bedding partings. Upper-slope bioclast-rich deposits interfinger and grade downdip into lower-slope peloid–skeletal packstone–grainstone beds which are similar to those of the early HST lower slopes (Fig. 5C), and similarly, basinal deposits are not well exposed. The fragmented nature of grains in the HST upper slopes suggests predominant reworking of platform-top and marginal skeletal communities from shallow current and wave action, and subsequent downslope transport as short-lived hyperconcentrated flows (after Mulder and Alexander 2001). The apparent downdip fining of deposits from upperslope to lower-slope settings reflects either upper-slope bypass of finer sand fractions via concentrated-to-turbulent flow processes (after Mulder and Alexander 2001) or separation of grain-size populations during downslope transport. As in early HST slopes, the lack of reefal debris deposits indicates a relatively stable configuration despite margin outbuilding. Upper-slope deposits throughout the HST conformably interfinger updip with encrusted reefal facies around the platform edge, producing accretionary-margin configurations (Figs. 3A, B, 5A, 6A, B). Downdip-interfingering boundstone tongues are fairly thin (,1 m) and do not extend far downslope from the platform edge (10– 20 m maximum), with exception to thin encrusted lenses that are present on the upper slopes during the early HST. These accretionary configurations indicate that the volume of sediment accumulated on the slope was sufficient to fill the slope profile in pace with reefal growth (much of which had an aggradational component) around the platform edge.

r FIG. 3.—Schematic generalized models showing sequential depositional and architectural evolution between early Frasnian platform backstepping events based on outcrops in the South Lawford Range area. See text for details on foreslope and margin deposits, architecture, and discussion of accommodation setting. dom’d, dominated; bndstn, boundstone; rudstn, rudstone; fltstn, floatstone; gnstn, grainstone; pkstn, packstone; wkstn, wackestone; skel, skeletal; pel, peloidal. A) Early HST nucleation. Former platform tops are drowned during the prior MFS and transform into the basinal equivalents of backstepped platforms, hosting fine-grained hemipelagics and serving as nucleation sites for pinnacles, small isolated platforms, and younger accretionary margins. B) HST weak progradation of accretionary margins. Backstepped platforms construct synoptic relief, develop robust bioclastic factories, and locally prograde, while pinnacles and atolls drown. C) Early TST aggradation and evolution from accretionary margin to escarpment configurations. Shelf-to-basin relief continues to increase, requiring greater volumes of bioclastic sediment to fill the slope profile to the coeval margin. D) TST aggradational escarpment and local collapse. Margin aggradation rate eventually relatively outpaces the coeval foreslope accumulation rate, resulting in an escarpment margin that is sensitive to gravitational collapse triggers and prone to failure. E) MFS margin backstepping. Subsequent margin backstepping event results in a condensed drape over the former depositional profile and pinnacle nucleation on the former platform top.

Pillara Sequence Pillara Sequence Pillara Sequence Nullara Sequence Nullara Sequence Pillara and Nullara sequences Nullara Sequence

Middle Devonian; Givetian

Middle–Upper Devonian; Givetian–Frasnian

Middle–Upper Devonian; Givetian to lower Frasnian

Middle–Upper Devonian; Givetian to Lower Frasnian

Upper Devonian; Famennian

Upper Devonian; Famennian

Upper Devonian; Frasnian– Famennian

Upper Devonian; mostly middle– upper Famennian

Cadjebut Formation

Pillara Limestone

Sadler Limestone

Gogo Formation

Nullara Limestone

Windjana Limestone

Napier Formation

Bugle Gap Limestone

Nullara Sequence n/a

Upper Devonian; middle–upper Famennian

Lower Carboniferous; Tournaisian

Piker Hills Formation

Fairfield Group

Pillara and Nullara sequences

Pillara Sequence

Period/Stage (approximated)

Mappable Unit

Virgin Hills Formation Upper Devonian; middle Frasnian to Famennian

Playford et al. (2009) Low-Frequency Sequence

n/a

n/a

Sequences 4–6

n/a

Sequences 1–6

n/a

n/a

Sequences 1–3

Sequences 1–3

Sequences 1–6

Sequence 1

Givetian–Frasnian 3rd-Order Sequence

Southern exposures (e.g., Bugle Gap area, Pillara Range, Horseshoe Range)


across outcrop belt


Northern exposures (e.g., Napier Range, Oscar Range)

across outcrop belt

across outcrop belt



across outcrop belt


Position Within Outcrop Belt

reefal margin and platform top

Environment of Deposition (this study)

n/a

marginal slope and basin


marginal slope


reef

backreef

basin

marginal slope

n/a

foreslope and basin

foreslope and basin

encrusted upper slope

encrusted upper slope, foreslope, and basin

reefal margin

platform top

distal foreslope and basin

foreslope

reef, backreef and bank reefal margin and platform top

bank

Playford et al. (2009) Subfacies

TABLE 2.—Lithostratigraphic mapping nomenclature and associated subdivisions for Devonian through Carboniferous outcropping mappable units along the Lennard Shelf (modified after Playford et al. 2009). Givetian–Frasnian, Frasnian-Famennian, and Famennian–Tournaisian boundaries are dated at 385.3 Ma, 374.5 Ma, and 359.2 Ma, respectively (Gradstein et al. 2012).

1342 T.E. PLAYTON AND C. KERANS

JSR

JSR

CANNING BASIN CARBONATE SLOPES AND MARGINS, PART A TST Escarpment Margins and Associated Foreslopes

Systems-Tract Setting.—Following the HST, where margins exhibit accretionary configurations and weakly prograde (Fig. 3B), a threshold was reached where carbonate factories that contributed sediment volume to the slope were no longer able to fill the slope profile in pace with a coeval margin that had shifted into vertical aggradation (Fig. 3C, D) (sensu Schlager 1981). This threshold was the point when accretionary margins begin to transition into escarpment-margin configurations and foreslope deposits onlapped beneath their reefal equivalents against slightly older surfaces. These transitions are interpreted to represent an early TST setting and accommodation increase, as reflected by a net aggradational margin trajectory. Thus, a sequence boundary (SB) separates the weakly prograding HST from the overlying early TST; however, evidence of exposure is unapparent and margin configurations offer the best supporting criteria for the interpretation. After the transition into escarpment configurations was complete, vertical aggradation continued, reflecting further accommodation increase of the TST (Fig. 3D). Foreslope Facies.—Including the early TST margin transition and the TST escarpment phase together, slope profiles can be subdivided into upper-slope and lower-slope sub-environments based on sedimentary characteristics, and maximum slope heights and widths are estimated at 200 m and 1 km, respectively (Fig. 3C, D). Upper-slope deposits during the TST are dominated by intraclast–bioclast rudstone (to fine breccia) and coarse, skeletal-dominated grainstone with moderate proportions of void-filling silty sediment (Fig. 7A). Skeletal fragments are similar in composition to those described for upper slopes of the HST, and intraclasts are typically peloid–skeletal packstone–grainstone or skeletal rudstone. As elsewhere in the dataset, steep depositional dips (up to 30u) are corroborated by geopetal sediment fill in large pores that predates or is contemporaneous with early marine cementation. Beds are characteristically tabular to slightly lenticular, 10–50 cm thick, and display a range in style from laterally extensive sheets to discontinuous scourforms. In these strike-limited deposits with basal scours, intraclasts are commonly concentrated. Secondary deposits of the upper slope include peloid– skeletal packstone–grainstone (Fig. 7B), silty drapes at bedding partings, and rare yet conspicuous (mega)breccias with reefal blocks up to 3 m thick. Lower-slope and basinal deposits are poorly exposed but typically consist of silty, peloid–skeletal wackestone, packstone, and grainstone. However, locally present and well exposed are thick, chaotic megabreccia deposits that can be reconstructed to reside in lower-slope or toe of slope positions, and they contain abundant meter-scale blocks of reefal boundstone (Fig. 7C, D). Megabreccia beds are up to 4 m thick, lenticular in shape, and form laterally discontinuous complexes tens to hundreds meters across with internal compensational architecture. Similarly to the HST, the evidence for macrofossil reworking suggests fragmentation of platform-top and marginal skeletal communities and subsequent downslope transport via hyperconcentrated flow (after Mulder and Alexander 2001) as the dominant process feeding the upper slopes during TSTs. Intraclastic scour fills (interpreted as slope-derived) and local reefal megabreccias indicate that slope readjustment and smallscale margin failure, respectively, were secondary processes contributing to upper-slope sedimentation. The large-scale megabreccia deposits found in distal settings reflect periods or events where the margin experienced severe instability and failed catastrophically (Fig. 7D)— a process and product not observed during the HST, where margins are interpreted to have remained stable. The processes of such collapse events include rockfall, avalanche, and debris flow (after Mulder and Alexander 2001), and the distal position of the deposits infers slope bypass.

1343

Accretionary-to-Escarpment Margin Transition and TST Evolution.— Intact exposures at Nardji Cave, South Lawford Range (Fig. 4), display alternations of reefal margin-to-foreslope interfingering and onlapping, recording in detail the complexity associated with the early TST transition from accretionary to escarpment configurations subsequent to the weakly prograding HST (Figs. 3C, D, 8, 9). The margin evolution and architectural succession recorded at Nardji Cave is as follows: 1.

2.

3.

4.

weakly progradational accretionary margin with reefal boundstone tongues interfingering downdip with foreslope rudstones (Sequence 1 latest HST, Figs. 8A, 9A); minor margin backstepping followed by aggradation, and consequent onlap of foreslope deposits onto slightly older surfaces to form an escarpment configuration (Sequence 2 early TST, Figs. 8B, 9B); minor margin progradation and downslope interfingering of reefal boundstones with foreslope rudstones, producing an accretionary configuration (Sequence 2 early TST, Figs. 8C, 9C); and margin aggradation forming a subvertical reefal wall, resulting in foreslope onlap and a persistent escarpment configuration (Sequence 2 TST, Figs. 8D, 9D).

These pulses of margin backstepping and progradation were smallscale spatially (10–20 m) and high-frequency within context of the longerterm net aggradational succession, and reinforce the architectural heterogeneity associated with long-lived escarpment development. The development of the youngest margin exposed (Figs. 8D, 9D) represents the onset of a sustained TST aggradational escarpment architecture, where foreslope deposits were no longer able to accumulate to the point of their reefal equivalents, and consequently onlapped beneath, providing no immediate substrate. Reconstructions project the following backstepping event (Sequence 2 MFS) above the exposure limit; thus the TST subvertical reef wall was at least 40 m in relief. Less than 200 m along strike from the intact TST aggradational escarpment at Nardji Cave is a 100-meter-wide area where the reefal margin is no longer present (hundreds to thousands of cubic meters of removed material; Fig. 7D). Lines of evidence, such as truncated bedding and sediment-filled dikes around the periphery of this feature, suggest that it is an erosional reentrant generated from large-scale, brittle failure of the lithified reefal margin. The large volumes of material that evidently collapsed away from this position are not exposed in the Nardji Cave area, and presumably were transported downslope into the present-day subcrop. Based on observed truncational relationships, the earliest timing of the collapse can be constrained to the youngest phases of TST escarpment development (just prior to backstepping at the Sequence 2 MFS; Fig. 2B). This timing suggests that severe margin instability developed during the later phases of TST escarpment aggradation. SLOPES AND MARGINS OF THE MIDDLE–LATE FRASNIAN AGGRADATION-TOPROGRADATION TRANSITION

Margin Evolution and Systems-Tract Overview The supersequence MFS is placed at the top of the Sequence 5 TST (coincident with the MFS of Sequence 5) and marks the end of platform backstepping, significant margin aggradation, and growth-escarpment formation characteristic of the supersequence TST (Figs. 2B, 10A). Following the supersequence MFS, subregional large-scale collapse events locally removed and mobilized substantial volumes of the margin downslope (after Ward 1999 and Playford et al. 2009). These events are interpreted to have occurred during the Sequence 5 HST, but the exact timing and number of failures within the interval are unclear (Fig. 10B). After this period of instability, a phase of pervasive upper-slope and margin calcimicrobial encrustation occurred, reflecting a relative starvation of portions of the profile, pause in detrital sediment accumulation in

1344


JSR

FIG. 4.—Geologic map of the Bugle Gap–South Lawford Range area showing outcrop localities, measured sections, photograph, and interpreted photopanel locations (modified after Playford 1981). See Figure 1 for inset legend. McIntyre Knolls locality is not shown but is located northeast of McWhae Ridge in the Lawford Range.

upper-slope positions, and post-collapse margin recovery period (after Ward 1999). This phase is interpreted as the Sequence 6 early TST, as it appears equivalent with platform-top bioherm development (after Frost 2007) and reef-flat backstepping that immediately postdates and overlies strata truncated from the collapse events (Fig. 10C). The rest of the Sequence 6 TST exhibits platform aggradation, a vertically stacked

margin directly over the Sequence 5 HST erosional escarpment surface, and very little to absent upper-slope equivalent. The Sequence 6 HST is characterized by attempted progradation of the reefal margin over the inherited and collapsed escarpment profile (Fig. 10D, E). Initially, the reefal margin lacked a substrate to advance over and consequently collapsed frequently, producing onlapping debris wedges, until the slope

JSR


1345

FIG. 5.—Slope deposit types and margin architecture for Sequence 2 early HST nucleation phases of lower Frasnian reefal platforms subsequent to major backstepping events. See Figure 3A for associated depositional model and Figures 1 and 4 for locations. A) Outcrop photograph of aggradational–retrogradational accretionary (rollover) margin showing downdip interfingering transition along narrow, elongate reef spine, McWhae Ridge, South Lawford Range. B) Outcrop photograph of calcimicrobial–brachiopod floatstone with silty peloidal matrix in upper-slope setting, Emanuel Range. Stromatactoid, cemented cavities are associated with calcimicrobial encrustation of autochthonous brachiopods. Geopetal fills compared with bedding dip corroborate primary depositional dips (after Playford 1980). C) Outcrop photograph of succession of skeletal–stromatoporoid bioclastic rudstone and stratified peloid–skeletal grainstone in lower-slope setting, east of Emanuel Range.

profile was infilled and regraded to the angle of repose (Sequence 6 early HST; Fig. 10D). Once a substrate was in place to support progradation, the margin advanced basinward and was able to sustain an accretionary configuration (Sequence 6 late HST; Fig. 10E). These sequence stratigraphic interpretations are constrained by platform-top and reef-flat cycle-stacking criteria and margin observations either projected into Windjana Gorge from nearby measured sections (Playton et al. 2011) or observed directly from the canyon walls (Fig. 11). Margins and Foreslopes of Middle Frasnian Sequence 5 and Upper Frasnian Sequence 6 Sequence 5 TST.—The margins and foreslopes of the Sequence 5 TST (Fig. 10A) are not well exposed and thus their characteristics are inferred

from better-exposed lower Frasnian examples that are presumed similar. Playford et al. (2009) reconstructs Sequence 5 to be locally up to 500 m in shelf-to-basin relief, and measured sections north of Windjana Gorge (Playton et al. 2011) document aggradational cycle stacking in equivalent platform-top to reef-flat strata. Based on this and assuming lower Frasnian models to be representative, we infer that the Sequence 5 TST is characterized by high-relief, aggradational escarpment margins with onlapping, grain-dominated, bioclast-rich foreslopes (Fig. 3D). By definition, the MFS of Sequence 5 is placed at the top of the Sequence 5 TST, coincident with the supersequence MFS (Figs. 2B, 10A). Sequence 5 HST.—Subvertical, erosional escarpment surfaces, reflecting large-scale gravitational failures of the margin, are observed at multiple sites in Windjana Gorge and exhibit truncation well into Sequence 5 TST and

1346


JSR

FIG. 6.—Slope deposit types and margin architecture for lower Frasnian Sequence 2 HST weakly progradational accretionary margins, Kelly’s Pass, South Lawford Range. See Figure 3B for associated depositional model and Figure 4 for location. A) Interpreted photomosaic of aggradational and weakly progradational accretionary (rollover) margin showing encrusted margin deposits interfingering downdip into allochthonous foreslope deposits and updip into bedded reef-flat deposits. These margins are slightly younger than the drowned reef spine at McWhae Ridge (see Fig. 5A) (after Playford et al. 2009). B) Outcrop photograph of accretionary (rollover) margin showing changes in depositional dip at the margin inflection. C) Outcrop photograph of Stachyodes stromatoporoid bioclast rudstone (talus) deposit with siliciclastic-rich silt pore-fill in upper-slope setting.Stachyodes bioclasts display early marine cement pore lining (white rims) prior to silt infill. D) Photomicrograph of Stachyodes stromatoporoid bioclast rudstone in Part C displaying multiple generations and types of pore-lining early marine cement (mc) prior to siliciclastic-rich silt pore-fill (si). Intraclasts, i; br, brachiopod bioclasts.

HST reef to reef-flat environments (Figs. 10B, 11, 12A, 13). These features have been recognized and mapped subregionally (Playford 1980; Ward 1999; Playford et al. 2009). Relationships with younger Sequence 6 deposits constrain the timing of the latest collapse episodes to the HST of Sequence 5, although the exact placement of all events in the HST is unclear as they were ultimately recorded as composite erosional escarpment surfaces. These surfaces are all that remain of Sequence 5 in marginal and upper-slope positions; thus the Sequence 5 MFS and SB merge along them and delineate the truncated Sequence 5 reef to reef-flat remnants from younger Sequence 6 slope deposits (Figs. 12A, 13). Little else is known about the Sequence 5 HST margin or upper slope other than evidence for large-scale failure, but the resulting types of basinal deposits generated through such catastrophic events can be observed at other age-equivalent localities along the Lennard Shelf (Fig. 12B). The McIntyre Knolls locality in the Lawford Range (Fig. 4) displays margin-derived olistoliths up to 40 m across that came to rest in distal toe of slope or basinal settings (kilometers from the collapsed margin; Fig. 12B). The underlying deposits have been dated as middle Frasnian (Becker et al. 1993; Becker and House 1997). Thus, an assumption can follow that the McIntyre Knolls olistoliths (although at a different position along strike) are age-equivalent and reflective of the types of deposits generated from the Sequence 5 HST collapse events recorded in Windjana Gorge (Figs. 12B, 13). According to this interpretation, the bases of the McIntyre Knolls olistoliths would approximate the supersequence MFS in basinal positions (Figs. 2B, 10B, 12B). Sequence 6 TST.—Margins and the upper slopes of the Sequence 6 TST are well exposed in Windjana Gorge (Figs. 12A, 13). The relict erosional

escarpment surfaces generated from Sequence 5 HST margin collapse are consistently encrusted by stromatolitic to stromatactoid calcimicrobial boundstone (Ward 1999) (Figs. 10C, 12A, 13, 14A), reflecting a postcollapse recovery period where sediment produced from platform-top and marginal factories was not accumulating in upper-slope settings. Along erosional escarpment surfaces, accumulations of calcimicrobial boundstone occur as dipping lenses, veneers, mounded tongues, and margincentered wedges that can accrete horizontally for tens of meters, extending the subvertical profile basinward (Figs. 12A, 13). Exposures on the south side of Windjana Gorge (Fig. 13B) display a backstepped calcimicrobial boundstone bioherm above the Sequence 5–6 boundary (after Frost 2007) that is interpreted to be the early TST of Sequence 6 in a reef-flat position. The bioherm interval can be tracked laterally and linked to the calcimicrobial encrustations along the erosional escarpment surface, making them roughly age-equivalent and both Sequence 6 early TST deposits (Fig. 13B). As such, the erosional escarpment surfaces define the Sequence 5–6 SB in margin to upper-slope positions upon which the Sequence 6 early TST calcimicrobial communities encrust (Figs. 12A, 13). Bioherm-equivalent platform-top deposits are present in a landward direction but are thin and poorly developed—thus it follows that margin encrustation and local bioherm development in reef-flat settings appear to be the dominant styles of carbonate deposition during the early TST of Sequence 6. Considering this, the remainder of the Sequence 6 early TST slope profile is interpreted to have been relatively starved of margin- or platform-derived sediment, and accordingly is characterized by mud-dominated background deposits instead of grainstones and/or breccias (Fig. 10C). Directly above the calcimicrobial encrustation and underlying Sequence 5 HST failed margin is an aggradational-reef to reef-flat transition that vertically stacks, constructing tens of meters of relief, but with a thin

JSR


1347

FIG. 7.—Slope deposit types and margin character for lower Frasnian Sequence 2 TST aggradational escarpment margins, Nardji Cave area, South Lawford Range. See Figure 3D for associated depositional model and Figure 4 for locations. A) Photomicrograph of intraclast–bioclast rudstone (talus) with pore-lining radial–fibrous (rf) and radiaxial (ra) marine cement, silty sediment infill (si), and micritic encrustation (m) of pores postdating cementation in upper-slope setting. I, packstone–grainstone intraclasts; cr, crinoids; bz, bryozoans; St, Stachyodes stromatoporoids; br, brachiopod valves; brs, brachiopod spines; g, gastropods. B) Photomicrograph of peloid– micritized skeletal packstone–grainstone in upper-slope setting. C) Outcrop photograph of bulbous Actinostroma framestone-clast megabreccia with silty intraclast– bioclast floatstone matrix in toe of slope setting. Larger clasts are outlined in white. D) Strike-view (looking landward) interpreted photomosaic of collapse reentrant formed from large-scale gravitational failure of the reefal margin. Timing of margin failure is interpreted at the approximate terminus of escarpment aggradation during the latest TST, just prior to subsequent platform backstepping at the MFS. Megabreccia from younger large-scale collapse event funneled through and backfilled the reentrant.

veneer of equivalent slope (Fig. 13B). The slope veneer facies are similar to those described for the early TST encrustations of Sequence 6 (Fig. 14A), consisting of calcimicrobial boundstone fabrics but with greater proportions of incorporated skeletal material. It is difficult to determine whether the outboard surface of the slope veneer is truncational or constructional; however, the aggradational nature of the equivalent margin and reef-flat strata indicates continued transgressive conditions postdating early TST bioherm development. Thus, this interval is interpreted as the remainder of the Sequence 6 TST (Fig. 13B), but its full vertical extent is unknown because the upper bounding surface (Sequence 6 MFS) in reef-flat positions is not exposed in Windjana Gorge. The equivalent Sequence 6 TST deposits in more distal, middle-slope to lower-slope settings are also not exposed, but they are presumed to consist of grain-dominated onlapping wedges because there was a robust shallow-water carbonate source. Sequence 6 HST.—Onlapping the encrusted subvertical scarps that developed from Sequence 5 HST collapse and subsequent Sequence 6 TST encrustation are wedges of debris dipping 20–30u (Figs. 10D, 12A, 13). These upper-slope wedges consist dominantly of thick-bedded, lenticular reefal margin-derived talus or megabreccia, reefal boulder complexes, and bioclastic–intraclastic rudstone, with lesser platformtop-derived peloid–skeletal packstone–grainstone and thin drapes of quartzose, micro-peloidal siltstone background sediment (Figs. 12A, 13, 14B, C). The coarser reefal debris intervals exhibit compensational

backfilling geometries, where 1) lenticular deposits formed depositional topography along the slope profile, 2) subsequent deposits ponded and stacked updip of the previously generated depositional topography, and 3) upslope accretionary architecture with internal downlap and onlap patterns resulted as the process repeated to ultimately fill the slope profile (Figs. 12A, 13). Meter-scale lenses up to wedges tens of meters thick of packstone–grainstone and rudstone infill depositional inflections and depressions between lenticular debris deposits and complexes (Figs. 12A, 13). Equivalent lower-slope deposits are not exposed but are assumed to follow a general fining of sediment down the slope profile, and as such, a packstone–grainstone-dominated lower slope is inferred (Fig. 10D, E). In this case, the dominance of debris on the slope implies that reefal margin failure was a dominant and quasi-continuous process contributing sediment downslope—a scenario that likely develops from basinward advance of reefal boundstones without adequate underlying substrate. Furthermore, the onlapping architecture of the wedges indicates escarpment margin configurations (Fig. 13). North of Windjana Gorge (Fig. 11), slightly younger (but still mapped as Sequence 6) outcrops show progradational, accretionary margin configurations where platform-top, reefal, and encrusted upper-slope facies pass conformably from one to the other (Fig. 14D); this suggests a transition from escarpment to accretionary margin architecture in Sequence 6 that postdates TST margin encrustation. Tying these observations together, an interpreted progression is as follows that explains margin and slope evolution after Sequence 6 TST encrustation and margin aggradation: 1) attempted

FIG. 8.—Interpreted photomosaic showing the detailed margin evolution from the latest HST of lower Frasnian Sequence 1 and a portion of the TST of Sequence 2 (Margins a–d, respectively), Nardji Cave, South Lawford Range. See Figure 3B–D for associated depositional models and Figure 4 for location. Black dashed lines are measured sections. Yellow lines separate different margin development phases (Margins a–d, see also inset model). White lines are foreslope bedding planes. White dotted lines are interpreted margin timelines. White dashed lines are interpreted interfingering reef-to-foreslope transitions of accretionary margins. Red arrows denote foreslope onlap against escarpment surfaces. dom’d, dominated; rudstn, rudstone; TST, transgressive systems tract; HST, highstand systems tract; SB, sequence boundary. Margin (a) is accretionary with an interfingering reef-to-foreslope transition and weakly progradational representing the latest HST of Sequence 1. Margin (b) backsteps relative to Margin (a), thus defining the SB at the top of Sequence 1, and displays an aggradational escarpment configuration with an onlapping foreslope representing the onset of the early TST accretionary-escarpment transition of Sequence 2. Margin (c) progrades relative to Margin (b), and displays an accretionary configuration with an interfingering reef-to-foreslope transition, still marking the early TST margin transition of Sequence 2. Margin (d) displays an aggradational-escarpment configuration with an onlapping foreslope, and marks the TST of Sequence 2 where accretionary margins no longer develop. A large-scale margin failure event crosscuts strata equivalent to Margin (d) along strike (see Fig. 7D), suggesting that the subsequent MFS platform backstepping event (Kelly’s Pass Event) and onset of the Sequence 2 HST projects above the vertical exposure limit (see Fig. 2A, B). See Figure 9 for cross section.


JSR

JSR


1349

FIG. 9.—Collapsed cross section of photomosaic in Figure 8, showing the detailed margin evolution from the latest HST of lower Frasnian Sequence 1 and a portion of the TST of Sequence 2 (Margins a–d, respectively), Nardji Cave, South Lawford Range. See Figure 3B–D for associated depositional models and Figure 4 for location. Vertical black lines are measured sections. Thick lines separate different margin development phases (Margins a–d, keyed to Fig. 8). Thin black lines represent foreslope bedding architecture. Thin black dotted lines are interpreted margin timelines. Thick black arrows denote foreslope onlap against escarpment surfaces. Rudstn, rudstone; gnstn, grainstone; pkstn, packstone.

progradation of the reefal margin over inherited escarpment profiles initially developed from Sequence 5 HST collapse; 2) due to insufficient underlying substrate to support progradation, the margin failed, producing onlapping debris deposits, and escarpment configurations persisted; 3) the process repeated quasi-continuously forming backfilled, onlapping debris wedges that eventually infilled the slope profile to a graded equilibrium (Fig. 10D); and 4) upon complete slope filling, a substrate was made available to support margin progradation and accretionary configurations (Fig. 10E). As this evolution is characterized by margin outbuilding (whether progradation was achieved or not), we here interpret it as the Sequence 6 HST—where the initial phase of slope infilling with escarpment geometries was the early HST of Sequence 6, and the progradational phase with accretionary margins was the late HST of Sequence 6 (Fig. 10D, E). By this delineation, the Sequence 6 MFS in upper-slope positions is defined by the onlap surface that separates the older encrusted collapse scarp from younger backfilled debris (Figs. 12A, 13). The SB that represents the top of Sequence 6 is defined by an abrupt

change in slope facies, but it is not discussed here (upper Frasnian Sequence 7; Playton and Kerans 2015b). The deposit types of the Sequence 6 late HST are similar to those of the early HST (reefal debrisrich), still reflecting a collapse-dominated margin related to progradation, however, indicating an achieved balance between the rates of slope filling and basinward advance of the margin. DISCUSSION

Lower Frasnian Foreslopes and Margins of the Supersequence TST Grain-Dominated Foreslopes.—Lower Frasnian examples here from Sequence 2 can be classified as grain-dominated foreslopes (after Playton et al. 2010) (Figs. 3, 15A), with exception to early HST slopes that are related to an incipient and post-backstepping, recovering carbonate factory. Grain-dominated foreslopes are composed of 50% or greater grain-dominated deposits, which include packstones, grainstones, and pebbly rudstones that are generally sourced from platform-top sand or

1350


JSR

JSR


marginal skeletal factories and transported via concentrated or highdensity turbulent flow (after Mulder and Alexander 2001). Lower Frasnian foreslopes of the Lennard Shelf are composed chiefly of bioclastic to intraclastic coarse grainstones and pebbly rudstones (60% of foreslope deposits; Fig. 15A), representing the coarser end of the spectrum of grain-dominated deposits that originated from mechanical reworking of skeletal factories around the margin (stromatoporoid–coral framestone reefs in this case; Figs. 6C, D, 7A). Lower Frasnian foreslopes also clearly exhibit the tabular bedding character and stacking of sheetstyle architectural elements to form strike-extensive complexes that are distinctive of grain-dominated slopes (Figs. 6A, 8, 9, 16A). Playton et al. (2010) attribute the development of grain-dominated foreslopes and basins to two key controls: 1.

2.

the presence of a shallow skeletal reef system that produces large volumes of sand- to pebble-size particles due to mechanical reworking from wave and current energy, such as the Miocene of Mallorca (Pomar et al. 1996) and the Lower Cretaceous of Poza Rica Field, Mexico (Janson et al. 2011); and particular styles of platform morphology conducive to the formation of robust platform-top sand factories, such as narrow wave-swept shelves and high-energy outer platforms without energy barriers, such as the Lower Jurassic of the High Atlas, Morocco (Merino-Tome et al. 2012).

Another factor influencing the development of grain-dominated slopes is margin trajectory, where aggradational or retrogradational margins tend to not fail as often as progradational margins due to inherent stability from vertical reefal growth over a solid underlying foundation (versus the less stable setting of an outbuilding margin). It follows that development of grain-dominated foreslopes are more likely during phases of margin aggradation or backstepping when debris deposition is at a minimum, given that the other controls outlined above are also acting in concert. Lower Frasnian grain-dominated foreslopes of the Lennard Shelf result from a combination of these drivers.

N

N

N

Source factory: The dominant sediment type of the foreslopes consists of reworked skeletal material from the margin (Figs. 6C, D, 7A), which is linked directly to the presence of a shallow stromatoporoid– coral reefal assemblage which was subject to continual fragmentation from shallow wave and current energy. Platform-top morphology: Significant margin barriers are not documented around the Frasnian Lennard Shelf system, but instead are reconstructed as flat-topped platforms (Playford 1980) that allowed for platform-top sand sources to easily resediment downslope, resulting in peloid–skeletal packstone–grainstone secondary deposits intercalated within bioclastic rudstones (Figs. 7B, 16A). Lower Frasnian margins are net aggradational, thus they were less prone to collapse as reflected by minimal proportions of slope debris deposits (10%; Fig. 15A); grain shedding was not “competing” with other resedimentation processes during foreslope deposition.

1351

Growth-Escarpment Margins.—Lower Frasnian margins and their evolution between platform backstepping events are excellent examples of “growth escarpments” (after Playton et al. 2010) (Figs. 3, 15A), along with age-equivalent cases documented in the Alberta Basin of Canada (e.g., Workum and Hedinger 1989; Whalen et al. 2000a). Growth escarpments are defined as margin successions that nucleate on flat surfaces, build relief over time through net aggradation, and eventually generate enough shelf-to-basin relief (slope height) that the volume of sediment required to fill the slope profile to coeval reefal environments is too great for the contributing sediment factories to produce in step with vertical margin growth (sensu Schlager 1981). In other words, the reefal margins “relatively outpace” their coeval foreslope accumulations in a vertical sense as the rate of margin upbuilding exceeds the rate of slope net deposition. As this evolution begins from relatively flat surfaces, margins in the earlier phases with lesser slope heights are accretionary— thus, a definitive characteristic of growth escarpments is the transition from accretionary to escarpment configurations over time (see McIlreath and James 1978 and Playton et al. 2010 for definitions). Outcrop exposures of lower Frasnian Sequence 2 at Nardji Cave (Fig. 4) clearly display the accretionary-margin to escarpment-margin transition and its complexities, where alternations of interfingering accretionary and onlapping escarpment relationships are observed in an overall net aggradational stack of margins (Figs. 8, 9, 16A). As growth escarpments have characteristically net aggradational configurations, they by definition require a constant increase in accommodation to develop. In the case of the lower Frasnian of the Lennard Shelf, growth escarpments were a direct function of the supersequence TST high-accommodation setting coupled with a shallow, euphotic reef assemblage that tracked rising relative sea level (Fig. 15A). In the earlier stages of growth-escarpment development, where accretionary margins weakly prograded (Figs. 3B, 6A, 9), accommodation increase had evidently slowed enough for margins to “catch up” (Schlager 1981) and temporarily advance with a basinward trajectory. Conversely, during TST escarpment development, the rate of accommodation increase was high, where margins were “keeping up” (Schlager 1981) and building vertically such that they relatively outpaced their coeval foreslope deposits (Figs. 3D, 9). Thus, the entire growth-escarpment evolution is a reflection of accommodation increase over time, leading up to backstepping events where platforms temporarily drowned and reconfigured landward in response to maximum accommodation conditions. While the overall development of growth escarpments between backstepping events was a function of continuous high-accommodation conditions during the low-frequency TST, the internal complexities are linked to higher-frequency, superimposed changes in the rate of accommodation increase. As the latter stages of growth escarpments represent periods of nearmaximum accommodation, subvertical reef walls with tens to 100+ meters of relief formed in response, and accordingly maximum separation between coeval margins and onlapping foreslopes was

r FIG. 10.—Schematic generalized models showing depositional, architectural, and systems-tract evolution across middle–upper Frasnian Sequences 5 and 6, based on outcrops in the Windjana Gorge area. MFS, maximum flooding surface; SB, sequence boundary; TST, transgressive systems tract; HST, highstand systems tract; dom’d, dominated; intra, intraclast; rudstn, rudstone; gnstn, grainstone; pkstn, packstone; wkstn, wackestone; skel, skeletal. A) Supersequence MFS and aggradational escarpment (coincident with middle Frasnian Sequence 5 MFS and TST, respectively). Upon long-term maximum accommodation conditions, margins became most disequilibrated and sensitive to collapse triggers. The composition of foreslope deposits is inferred from older escarpment systems. B) Large-scale margin failure and basinal megabreccia (middle Frasnian Sequence 5 HST). Attempted basinward advance of disequilibrated margin over underfilled slope results in substantial instability and gravitational collapse, producing basinal megabreccia deposits (observed in Bugle Gap area and southwest of Horseshoe Range; see Fig. 1) and margin reentrants. C) Slope starvation and margin encrustation (upper Frasnian Sequence 6 early TST). Period of platform backstepping (Classic Face Event; see Fig. 2A) and decreased downslope shedding results in extensive calcimicrobial encrustation of the margin and interpreted drape of the slope profile by silt-dominated hemipelagic background sediment (approximately Sadler–Virgin Hills formations contact; see Table 1). D) Sustained margin collapse and slope regrading (upper Frasnian Sequence 6 early HST). Attempted margin progradation without sufficient basinward substrate results in sustained failure and eventual regarding of slope profile by breccia-dominated foreslope deposits. E) Equilibrated accretionary margin and progradation (upper Frasnian Sequence 6 late HST). Upon regrading of the slope profile, basinward substrate is available for margin progradation and sustained accretionary configurations.

1352


JSR

FIG. 11.—Geologic map of the Windjana Gorge area showing outcrop localities, measured sections, and interpreted photopanel locations (modified after Playford 1981 and Playford et al. 2009). “Classic Face” locality is marked by Figure 13B. See Figure 1 for location along the Lennard Shelf.

achieved—slope profiles became significantly underfilled (Figs. 3D, 9, 16A). As slope profiles became progressively underfilled, they increasingly departed from stable, graded equilibrium profiles, which are built to the detrital angle of repose along the entire slope height and provide a downdip substrate for the margin. Escarpment margins are inherently associated with underfilled slope profiles, and during early

Frasnian late TSTs, this configuration resulted in an elevated sensitivity to collapse triggers, such that phenomena like small-scale progradational pulses, storms, or seismic tremors could result in large-scale, catastrophic failures of the margin (large-scale, infrequent collapse events of Playton et al. 2010). Evidence for this setting, process, and timing are apparent in the Nardji Cave area (Fig. 4), where an interpreted margin reentrant

JSR


1353

FIG. 12.—Features and deposits of transition between middle–upper Frasnian Sequences 5 and 6, coincident with maximum flooding surface (MFS) and earliest highstand (HST) of the supersequence. See Figure 2B for stratigraphic context, Figure 10B–E for associated depositional models, and Figures 4, 11, and 14 for locations. MFS, maximum flooding surface; SB, sequence boundary; TST, transgressive systems tract; HST, highstand systems tract. A) Interpreted photomosaic (following Ward 1999) of truncated reefal boundstone of Sequence 5 defining an erosional escarpment surface also coincident with Sequence 5–6 SB, east mouth of Windjana Gorge (see text for discussion of timing). Sequence 6 early TST deposition is here preserved as an encrusted calcimicrobial veneer along erosional escarpment surface. B) Interpreted photomosaic of reef and reef-flat olistolith overlying toe of slope packstones and grainstones, representing large-scale, regional margin failure around the Frasnian 5–6 SB and subsequent bypass of debris to toe of slope settings, McIntyre Knolls, Lawford Range. Middle Frasnian age constrained by Becker et al. (1993) and Becker and House (1997). Rotated geopetals in reef fabrics corroborate allochthonous origin. fx, fracture planes. Base and top of olistolith approximate supersequence MFS (also Sequence 5 MFS) and Sequence 5–6 SB, respectively (see text for discussion of timing).

generated from gravitational failure can be observed (Fig. 7D). Such events introduce highly anomalous deposits, consisting of chaotic megabreccias and olistoliths, within an otherwise grain-dominated slope stratigraphy or background-dominated toe of slope to basin floor (Fig. 3D). These occurrences also present a caveat to the association that aggradational margins are typically fairly stable due to vertical growth overtop solid underlying foundations (Playton et al. 2010), the exception here being cases of maximum escarpment-profile development with highly underfilled slopes. Middle–Upper Frasnian Foreslopes and Margins of the Supersequence MFS and Early HST Debris-Dominated Foreslopes.—Upper Frasnian examples in this study (Sequence 6 HST; Fig. 2B) show that debris-dominated foreslopes (after Playton et al. 2010) (Figs. 10D, E, 12A, 13) developed subsequent to periods of large-scale margin failure around the supersequence MFS. Debris-dominated foreslopes are composed of 50% or greater debris

deposits, which are allochthonous blocks and breccias that originate from brittle failure of early lithified marginal material (dominantly reefal) and are transported downslope through rockfall, hyperconcentrated-flow, and/or debris-flow processes (after Mulder and Alexander 2001). Debris deposits are lenticular to block-shaped and laterally discontinuous, thus compensational or shingled bed stacking architectures are common; a distinctive style of debris stacking is known as backfilling, where older deposits produce topography along the slope profile that younger deposits “pond” or stack behind or updip of, resulting in upslopeaccreting geometries (see fig. 16 of Playton et al. 2010). Upper Frasnian Sequence 6 HST foreslopes are debris-dominated aprons, as multiple exposures along Windjana Gorge (Fig. 11) show that deposits are chiefly reefal margin-derived allochthonous blocks and a variety of reef-rich breccias (50% of foreslope deposits; Figs. 14B, C, 15B, 16B), with lesser intercalations of skeletal rudstone and grainstone. Furthermore, these foreslopes clearly exhibit internal backfilled bedding geometries and their continuity along strike indicates laterally extensive apron morphology instead of an isolated complex or channel fill (Figs. 12A, 13, 16B).

FIG. 13.—Interpreted photomosaics of middle-slope to upper-slope successions of middle–upper Frasnian Sequences 5 and 6, displaying facies associations, stratal architecture, and interpreted systems tracts, Windjana Gorge. See Figure 10 for associated depositional models and Figure 11 for locations. MFS, maximum flooding surface; SB, sequence boundary; TST, transgressive systems tract; HST, highstand systems tract; rudstn, rudstone; gnstn, grainstone. Aspects of stratal subdivision and architecture interpretation are modified after Playford et al. (2009) and Frost and Kerans (2010).


JSR

JSR


1355

FIG. 14.—Slope deposit types and margin character for upper Frasnian Sequence 6, Windjana Gorge area. See Figure 10C–E for associated depositional models and Figures 11, 12, and 13 for locations. A) Outcrop photograph of skeletal–calcimicrobial boundstone encrustation along erosional escarpment surface of Sequence 6 early TST. sr, stromatactoid cavities, R, receptaculitids; n, nautiloids; s, silty matrix. B) Outcrop photograph of calcimicrobial boundstone-clast (white outlines) talus deposit with pore-rimming radiaxial marine cement (ra) and silty sediment pore infill (si) in upper-slope setting of Sequence 6 HST. C) Outcrop photograph of clast-supported megabreccia with Renalcis boundstone clasts (cl) and silty matrix (s) within upper-slope backfilled debris complexes of Sequence 6 HST. D) Interpreted photomosaic of progradational accretionary (rollover) margin of Sequence 6 HST showing interfingering transitions between reef-flat, reef, and encrusted upper-slope environments.

Playton et al. (2010) highlight an association between debris-dominated foreslopes and deep, oligophotic boundstone upper slopes that generally involve a significant calcimicrobial component. As these encrusted upper-slope settings often extend down the slope profile for hundreds of meters (e.g., Della Porta et al. 2003), most of the in situ growth is well below the influence of relative-sea-level changes, shallow currents, light penetration, and other factors that might interrupt or modify autochthonous accumulation. Thus, these upper-slope boundstone systems are capable of accreting, failing, and producing large volumes of slope debris quasi-continuously (forming debris-dominated slopes), and do so somewhat unaffected by phenomena that typically impact shallow carbonate factories (e.g., slope shedding or allstand shedding of Kenter et al. 2005 and Adams and Kenter 2013, respectively) The debris-dominated foreslopes of upper Frasnian Sequence 6 HST, however, do not appear to fit the above model. The upper Frasnian Sequence 6 margins, although with some calcimicrobial component, were instead dominated by skeletal–stromatoporoid framestone reefs (as evidenced by slope-debris composition), with encrusted upper-slope settings that extended down the profile for only tens of meters (not hundreds of meters). Thus, Sequence 6 margin styles are best categorized as shallow euphotic skeletal reef systems rather than deep oligophotic boundstone factories (after Playton et al. 2010), suggesting another mechanism behind the development of debris-dominated foreslopes— attempted progradation over high-relief, underfilled escarpment profiles. By the supersequence MFS in middle Frasnian Sequence 5 (Fig. 2B), highrelief aggradational growth escarpments had formed with significantly underfilled slope profiles that were sensitive to collapse (Fig. 10A). The onset of attempted progradation immediately after the supersequence MFS

(Sequence 5 HST; Fig. 10B) provided the collapse trigger that resulted in large-scale, catastrophic collapse episodes that erosionally modified the margin and exported olistolithic material to distal slope and basin settings (Figs. 12, 13). These underfilled profiles and severe instability persisted throughout most of Sequence 6 as margins continuously were driven to prograde in response to the supersequence HST forcing, but lacked the underlying substrate; the result was quasi-continuous margin failure and the emplacement of backfilled, debris-dominated foreslopes (Figs. 10D, 13, 16B). Once slope debris had sufficiently backfilled the profile to the coeval margin, a substrate was then available to support progradation and accretionary margin configurations were possible (Fig. 10E, 16B). Debris-dominated foreslopes continued to develop for a short while in the accretionary phase as progradational stability (balance of the rates of margin advance versus emplacement of downdip substrate) was established, but overall this stage marked the point where margin collapse was no longer the dominant process and other sediment factories could contribute in greater relative proportions to the slope. Transition from Escarpments to Accretionary Margins.—The middle to upper Frasnian margins of Sequences 5 and 6 (Fig. 2B) show an evolution from erosional escarpments to accretionary configurations (Figs. 10B–E, 13, 14D, 15B). Erosional escarpments (after Hooke and Schlager 1980 and Schlager and Ginsburg 1981) are margins with escarpment configurations that exhibit pervasive truncation along the escarpment surface (often truncated platform-top deposits), indicating net erosion of the margin and a likelihood that brittle failure was a substantial component in the generation of the escarpment itself. Large-scale margin-collapse events occurring at and just subsequent to the middle

1356


JSR

FIG. 15.—Summary diagram of lower to upper Frasnian (Sequences 6 and older) margins and slopes along the Lennard Shelf highlighting variations in margin architecture, stratal anatomy, facies associations, deposit types and proportions, and dominant processes across supersequence TST to MFS accommodation settings. dom’d, dominated. Model shown in Part A is representative of older and younger backstepping platforms. Simplified framework for “Stratigraphic Context” from Figure 2B. “Deposit Proportions” breakout is after Playton et al. (2010). “Relative Processes” are modes of sedimentation that contribute material to the slope profile. “Encrustation” depths are average reconstructed water depths for the downdip limit of encrusted upper-slope boundstones that remain connected to updip reefal facies.

Frasnian shift from long-term aggradation to progradation transformed the Sequence 5 margin from a high-relief growth escarpment (developed from vertical growth) to an erosional escarpment that persisted into Sequence 6 with truncational surfaces separating older reef-flat deposits from younger slope deposits (Figs. 12A, 13, 16B). This Sequence 5 and Sequence 6 TST composite escarpment profile was high-relief with significantly underfilled slopes; thus an infilling phase was required to regrade the profile before progradation could occur (early HST of Sequence 6; Fig. 10D). These configurations are coined “inherited escarpments” by Playton et al. (2010), where antecedent topography and large slope heights themselves are dominant controls on stratal patterns and evolution. Only when profile infilling has regraded the slope profile to the angle of repose for its entire slope height, and complete emplacement of sediment substrate required for margin progradation achieved, can progradational accretionary margins develop (late HST of Sequence 6; Fig. 10E, 14D, 16B). There are excellent examples of inherited escarpments in the Cretaceous of Italy that exhibit a long-term escarpment-to-accretionary-margin evolution (e.g., Bosellini et al. 1993; Eberli et al. 1993; Borgomano 2000; Eberli et al. 2004); however, these suggest tectonic influences on escarpment development. Many modern isolated platforms in the Caribbean are most likely inherited escarpments as well, but they have not undergone the transition into long-term progradation yet, except locally due to oceanographic effects (i.e., leeward offbank shedding; e.g., Eberli and Ginsburg 1989). The evolution from inherited-erosional escarpments to accretionary margins exposed in the Windjana Gorge area along the Lennard Shelf (Figs. 1, 11), however, contains no suggestions of tectonic influence or factors that could have locally driven progradation—syndepositional fault zones and other evidence for tectonism are not observed at that locality specifically, and late Frasnian to Famennian southwest progradation is consistent across the shelf. Additionally, considering that reefal assemblages remained fairly constant from middle to late Frasnian time (eliminating the potential faunal variable), this margin evolution is here interpreted to directly reflect changes in the longterm accommodation setting (Fig. 2B). Inherited and erosional escarp-

ments developed from vertical aggradation during high-accommodation conditions of the supersequence late TST and subsequent large-scale collapse around the middle Frasnian supersequence MFS, where accommodation conditions were high but also beginning to relatively decrease. As accommodation conditions continued to decrease into the supersequence HST, margins responded by adding volume to underfilled slope profiles and constructing a substrate to facilitate progradation— upon which, accretionary configurations ensued and basinward advance persisted. Therefore, steep reefal margins that, due to a shift in long-term accommodation setting, undergo a change from aggradational to progradational trajectories and are likely to evolve from inherited and/ or erosional escarpments to accretionary-margin configurations—and, associated with this evolution, is the probable presence of olistolithic material in distal slope and basinal positions and the development of debris-dominated foreslopes. Summary of Accommodation Control and Predictive Associations Position within the Supersequence.—The examples presented herein exhibit distinctive variations in carbonate margin and slope composition, architecture, and sediment distribution patterns depending on position within the low-frequency supersequence evolution of the system. Climatic and ecological conditions remain overall constant throughout the early to late Frasnian (Sequences 2 through 6 as presented here), and tectonic evidence is not observed as a driver for margin and slope characteristics in the localities studied. Thus, long-term (supersequence) accommodation setting can be isolated in this dataset as a key extrinsic control on foreslope grain composition, deposit characteristics and proportions, margin morphology, and stratigraphic expression. These associations provide predictive capability as they link seismic-scale margin trajectory patterns to sub-seismic margin morphology and evolution, stratal architecture, and foreslope composition. Supersequence TST: During the Givetian to middle Frasnian, reefal margins backstepped with intervening pulses of aggradation. After each backstepping event, growth escarpments developed where margins

JSR


1357

FIG. 16.—Diagrams of key foreslope architectural elements and margin transitions of the lower to upper Frasnian along the Lennard Shelf. Strike and dip views are at the same scale, and the line of intersection is denoted by bold black dashed line. eros., erosional; escarp., escarpment; dom’d, dominated. A) Evolution of the growth escarpment margin and grain-dominated foreslope architecture of the lower Frasnian supersequence TST, based primarily on exposures of the Nardji Cave area (Figs. 4, 8, 9, 10). B) Transition from erosional-escarpment to accretionary-margin configurations and debris-dominated foreslope architecture around the middle to upper Frasnian supersequence MFS, based primarily on exposures in Windjana Gorge (Figs. 11, 13).

constructed relief from flat foundations and gradually relatively outpaced their coeval foreslope deposits over time through net aggradation (Fig. 15A). Growth escarpments characteristically exhibit accretionaryto-escarpment margin evolutions and associated foreslopes are graindominated due to net stability during vertical margin growth (Fig. 16A). Infrequent, large-scale collapse may locally occur during maximum escarpment development just prior to backstepping events, when slope profiles were substantially underfilled and sensitive to failure triggers. This overall pattern and set of associations is attributed to sustained highaccommodation conditions of the supersequence TST; however, it should not be overlooked that the shallow euphotic skeletal-dominated reefal assemblage, which builds to and tracks relative sea level, is also an important coupled variable. Supersequence MFS: In the middle–late Frasnian, high-relief aggradational growth escarpments catastrophically failed upon attempted progradation over an underfilled slope profile. Margins continued to mass waste until sufficient slope substrate was emplaced and a foundation was available for progradation. This evolution is characterized by the transition from erosional escarpment-to-accretionary margin transition with associated debris-dominated, backfilled

foreslopes (Fig. 16B), and is a direct function of the change from high to relatively decreasing accommodation conditions, and the turnaround from aggradation to progradation, at the low-frequency supersequence MFS. Devonian Outcrops of Canada.—Other outcrop and subsurface examples around the world exhibit similar relationships (Fig. 17). An age-equivalent outcrop dataset with similar reefal assemblages, paleolatitude, and arid climate from the Alberta Basin in western Canada documents aggradational growth escarpments during long-term TST high-accommodation settings (Fig. 17A, B) (Workum and Hedinger 1989; Whalen et al. 2000b). Margin architecture and evolution are quite similar to that exposed along the Lennard Shelf; however, associated foreslopes in the Canadian analog contain substantially more clay-size sediment than documented here (Whalen et al. 2000a; classified as muddominated slopes of Playton et al. 2010). This is attributed to differences in source terrane and proximity to the hinterlands; however, despite the contrasting foreslope compositions, both outcrop analogs exhibit the development of growth escarpments with debris-poor foreslopes in response to high-accommodation low-frequency TST conditions. Anoth-

1358


JSR

FIG. 17.—Diagram comparing large-scale stratal architecture, growth-escarpment development, and timing of large-scale margin failure of the Lennard Shelf system (this study), the Upper Devonian of the Alberta Basin, Canada (b; modified after Whalen et al. 2000b), the Lower Carboniferous of Tengiz Field, Kazakhstan (c; modified after Collins et al. 2006, 2013), and the Lower Carboniferous of Karachaganak Field (d; modified after Katz et al. 2010). Inset shows examples at the same scale. Hatched lines indicate interpreted extent of failure surfaces (lower surfaces; “c” in legend) and subsequent re-established accretionary-margin surface that marks the end of the infilling and profile regrading phase (upper surfaces; “h” in legend) across the long-term transition from aggradation to progradation. dom’d, dominated.

er key similarity between the two Devonian examples is margin instability around the long-term transition from backstepping and aggradation to progradation. Whalen et al. (2000b) document anomalously extensive, slope-to-basin megabreccia deposition (relative to the older backstepping phases) around the low-frequency MFS of the system (Fig. 17B), similar in timing and process to that observed along the Lennard Shelf. Carboniferous Subsurface of Kazakhstan.—Other comparisons to the Lennard Shelf system of note are Carboniferous subsurface examples from the Pricaspian Basin, Kazakhstan (Fig. 17C, D). Tengiz Field (Weber et al. 2003; Collins et al. 2006, 2013) and Karachaganak Field (Katz et al. 2010) are carbonate isolated platform reservoirs that display a backstepping to progradational evolution defining supersequences and systems tracts (or portions of them). Between backstepping events during long-term TST conditions, margins constructed relief from a flat surface and developed growth-escarpment architectures, although seismic imaging cannot fully resolve the detailed interfingering or onlapping characteristic patterns (Weber et al. 2003). However, seismic does reveal onlapping slope packages equivalent to the aggradational margins that appear to be debris-poor in composition based on log signature and

limited core coverage. Thick and extensive, onlapping debris wedges can be confidently mapped around the peripheries of Tengiz and Karachaganak fields based on distinctive seismic facies and integration of core and image logs for facies control (Collins et al. 2006; Katz et al. 2010). Seismic stratigraphy and biostratigraphic designation constrain these debris wedges as equivalent to the turnaround from long-term backstepping and aggradation to progradation, suggesting large-scale margin failure and development of erosional escarpments at the low-frequency MFS (Fig. 17C, D) (Collins et al. 2013; Katz et al. 2010). Debris-dominated foreslopes with accretionary margins are documented by core and image logs following these major collapse events; however, they persist throughout progradation, whereas the Lennard Shelf example predicts debris-rich slopes only during the slope profile infilling phase immediately postdating failure. This difference is due to the presence of deep oligophotic boundstone factories (after Kenter et al. 2005; Playton et al. 2010) on the upper slopes of the Tengiz and Karachaganak buildups, which extend downslope for hundreds of meters and are capable of overwhelming a slope profile with debris indefinitely. Thus, in these Carboniferous cases an ecological factor resulted in debris-dominated foreslopes independent of long-term accommodation forcing, likely related to cooler global temperatures, different continental configurations

JSR


and circulation patterns, wetter climates, and/or slightly higher, northern paleolatitudes—all potential controls affecting the development of deep oligophotic boundstone factories (discussed in Della Porta et al. 2003 and Kenter et al. 2005). However, margin instability, debris-wedge deposition, and erosional escarpment-to-accretionary margin evolution at the lowfrequency MFS is a reflection of accommodation and compares well with the observations collected from the Lennard Shelf.

1359

accretionary evolution), and significant processes (i.e., large-scale margin failure) are organized systematically within a low-frequency supersequence, and can be predicted based on seismic-scale architecture. These predictive associations allow characterization and interpretation of carbonate margin to foreslope heterogeneity from a variety of data types and scales of observation, and not only advance our academic understanding of these systems but also have utility in applied settings.

Supersequence Drivers Syndepositional tectonic activity equivalent to reefal platform growth along the Lennard Shelf has been well documented and accepted (e.g., Playford et al. 2009), but it remains to what extent these events affected various frequencies of carbonate development versus acting as local overprints superimposed on extrinsic global signals. Evidence in favor of the latter is that similar backstepping to prograding patterns in reef margins, constrained to the same absolute ages through well-accepted biostratigraphy, are observed in other localities around the world in the Late Devonian, including the Alberta Basin of Canada (Whalen 2000a, 2000b; Potma et al. 2001; Atchley et al. 2006) and the Guilin region of South China (Shen et al. 2008). Both examples document backstepping in the Frasnian and progradation in the Famennian, despite highly different structural settings and degrees of syndepositional activity (intracratonic sag versus active rift basin, respectively). Due to intense present-day tectonic overprint in the South China dataset, the exact timing of margin evolution patterns is difficult to further decipher, but the more-intact Canadian example constrains the long-term transition from aggradation to progradation to middle–late Frasnian time (see fig. 10 in Whalen et al. 2000b for ties to conodont zones)—a remarkable match to the supersequence architecture observed along the Lennard Shelf (Fig. 17A, B) (see fig. 20 in Playford et al. 2009 for ties to conodont zones). At higher-frequency scales, mismatches arise across the datasets in the number of sequences defined for the supersequence systems tracts (i.e., three versus five backstepping events documented in Canada and the Lennard Shelf, respectively), reflecting local to regional phenomena that superimpose complexities unique to each system onto global signals. However, considering the similarities in long-term margin evolution in datasets across the world and in different tectonic settings, global eustasy should be considered as a possible dominant control on the supersequence architecture expressed along the Lennard Shelf. Changes in subsidence patterns along strike and tectonic activity can augment sequence architecture locally, but these mechanisms are more likely to result in variable platform thickness, backstepping distance, and shelf width for example, rather than dictate the overall configuration of a supersequence. CONCLUSIONS

We here investigate variations in Late Devonian (Frasnian) reefal margins and foreslopes of the Lennard Shelf, Canning Basin, Western Australia, as a function of extrinsic accommodation, specifically 1) within a long-term backstepping and aggradational setting (supersequence TST) in the early Frasnian; and 2) across a long-term transition from platform backstepping and aggradation to progradation (supersequence MFS and early HST) around the middle–late Frasnian boundary. Assuming uniformity in ecological and climatic conditions, we find that accommodation changes largely dictate 1) the propensity, scale, frequency, and position along the slope profile of debris deposits that originate from gravitational collapse of the reefal margin, and 2) whether growth-escarpments, erosional-escarpments, or accretionary-margin styles are likely to develop. More specifically, we propose that particular slope types (i.e., grain-dominated versus debris-dominated), margin transitions (i.e., erosional escarpment to

ACKNOWLEDGMENTS

Charles Kerans is especially appreciated for lead supervision of this research and Phil Playford for introduction to the outcrop belt and logistical support. Mitch Harris, Jeroen Kenter, Scott Tinker, Ron Steel, Xavier Janson, Ned Frost, and Roger Hocking are thanked for committee support and/or research discussion. Special thanks to Editors Leslie Melim, Michael Grammer, Melissa Lester, and John Southard of the Journal of Sedimentary Research, and external reviewers Noel James and Erwin Adams, for insightful feedback that undoubtedly greatly improved the quality of this manuscript. Reservoir Characterization Research Laboratory at the Bureau of Economic Geology, the Jackson School of Geosciences Geology Foundation at The University of Texas at Austin, Chevron Australia (Paul Montgomery and Michael McLerie), the Geological Survey of Western Australia, and Chevron Energy Technology Company are thanked for financial and/or in-kind support. Meghan “Nutmegh” Playton is thanked for field assistance. Bunaba and Gooniyandi people are thanked for access onto indigenous lands, and Rosemary and Ronnie Nugget in particular for access to Mimbi lands. Rod O’Donnell and the Australian Department of Parks and Wildlife are thanked for access to the Windjana Gorge area. Napier Downs and Fairfield–Leopold Stations are thanked for access to various localities along Napier Range. REFERENCES

ADAMS, E.W., AND HASLER, C.-A., 2010, The intrinsic effect of shape on the retrogradation motif and timing of drowning of carbonate patch reef systems (Lower Frasnian, Bugle Gap, Canning Basin, Western Australia): Sedimentology, v. 57, p. 956–984. ADAMS, E.W., AND SCHLAGER, W., 2000, Basic types of submarine slope curvature: Journal of Sedimentary Research, v. 70, p. 814–828. ADAMS, E.W., AND KENTER, J.A.M., 2013, So different, yet so similar: comparing and contrasting siliciclastic and carbonate slopes, in Verwer, K., Playton, T.E., and Harris, P.M., eds., Deposits, Architecture, and Controls of Carbonate Margin, Slope, and Basinal Settings: SEPM, Special Publication 105, p. 14–25. ATCHLEY, S.C., WEST, L.W., AND SLUGGETT, J.R., 2006, Reserves growth in a mature oil field: the Devonian Leduc Formation at Innisfail Field, south-central Alberta, Canada: American Association of Petroleum Geologists, Bulletin, v. 90, p. 1153–1169. BECKER, R.T., AND HOUSE, M.R., 1997, Sea-level changes in the Upper Devonian of the Canning Basin, Western Australia: Courier Forschungsinstitut Senckenberg, v. 199, p. 129–146. BECKER, R.T., HOUSE, M.R., AND KIRCHGASSER, W.T., 1993, Devonian goniatite biostratigraphy and timing of facies movements in the Frasnian of the Canning Basin, Western Australia, in Hailwood, E.A., and Kidd, R.B., eds., High Resolution Stratigraphy: Geological Society of London, Special Publication 70, p. 293–321. BEGG, J., 1987, Structuring and controls on Devonian reef development on the northwest Barbwire and adjacent terraces, Canning Basin: Australian Petroleum Exploration Association, Journal, v. 27, p. 137–151. BORGOMANO, J.R.F., 2000, The Upper Cretaceous carbonates of the Gargano–Murge region, southern Italy: a model of platform-to-basin transition: American Association of Petroleum Geologists, Bulletin, v. 84, p. 1561–1588. BOSELLINI, A., NERI, C., AND LUCIANI, V., 1993, Platform margin collapses and sequence stratigraphic organization of carbonate slopes: Cretaceous–Eocene, Gargano Promontory, southern Italy: Terra Nova, v. 5, p. 282–297. CLAYTON, J.L., AND KERANS, C., 2013, Reservoir compartmentalization of a deep-water ooid fan, Happy Field, Permian Basin, in Verwer, K., Playton, T.E., and Harris, P.M., eds., Deposits, Architecture, and Controls of Carbonate Margin, Slope, and Basinal Settings: SEPM, Special Publication 105, p. 359–383. COLLINS, J.F., KENTER, J.A.M., HARRIS, P.M., KUANYSHEVA, G., FISCHER, D.J., AND STEFFEN, K.L., 2006, Facies and reservoir quality variations in the late Visean to Bashkirian outer platform, rim, and flank of the Tengiz Buildup, Pricaspian Basin, Kazakhstan, in Harris, P.M., and Weber, L.J., eds., Giant Hydrocarbon Reservoirs of the World: from Rocks to Reservoir Characterization and Modeling: American Association of Petroleum Geologists, Memoir 88, p. 55–95. COLLINS, J.F., NARR, W., HARRIS, P.M., PLAYTON, T.E., JENKINS, S., TANKERSLEY, D., AND KENTER, J.A.M., 2013, Lithofacies, depositional environments, burial diagenesis, and dynamic field behavior in a Carboniferous slope reservoir, Tengiz Field (Republic

1360


of Kazakhstan), and comparison with outcrop analogs, in Verwer, K., Playton, T.E., and Harris, P.M., eds., Deposits, Architecture, and Controls of Carbonate Margin, Slope, and Basinal Settings: SEPM, Special Publication 105, p. 50–83. COOK, H.E., MCDANIEL, P.N., MOUNTJOY, E.W., AND PRAY, L.C., 1972, Allochthonous carbonate debris flows at Devonian bank (“reef”) margins, Alberta, Canada: Bulletin of Canadian Petroleum Geology, v. 20, p. 439–497. COPP, I.A., 2000, Subsurface facies analysis of Devonian reef complexes, Lennard Shelf, Canning Basin, Western Australia: Geological Survey of Western Australia, Report 58, 127 p. DELLA PORTA, G., KENTER, J.A.M., BAHAMONDE, J.R., IMMENHAUSER, A., AND VILLA, E., 2003, Microbial boundstone dominated carbonate slope (Upper Carboniferous, N Spain): microfacies, lithofacies distribution and stratal geometry: Facies, v. 49, p. 175–208. DO¨RLING, S.L., DENTITH, M.C., GROVES, D.I., PLAYFORD, P.E., VEARNCOMBE, J.R., MUHLING, P., AND WINDRIM, D., 1996, Heterogeneous brittle deformation in the Devonian carbonate rocks of the Pillara Range, Canning Basin: implications for the structural evolution of the Lennard Shelf: Australian Journal of Earth Science, v. 43, p. 15–29. DROXLER, A.W., AND SCHLAGER, W., 1985, Glacial versus interglacial sedimentation rates and turbidite frequency in the Bahamas: Geology, v. 13, p. 799–802. DRUMMOND, B.J., SEXTON, M.J., BARTON, T.J., AND SHAW, R.D., 1991, The nature of faulting along the margins of the Fitzroy Trough, Canning Basin, and implications for the tectonic development of the trough: Exploration Geophysics, v. 22, p. 111–115. EBERLI, G.P., AND GINSBURG, R.N., 1989, Cenozoic progradation of northwestern Great Bahama Bank: a record of lateral platform growth and sea-level fluctuations, in Crevello, P., Wilson, J.L., Sarg, J.F., and Read, J.F., eds., Controls on Carbonate Platform to Basin Development: SEPM, Special Publication 44, p. 339–352. EBERLI, G.P., BERNOULLI, D., SANDERS, D., AND VECSEI, A., 1993, From aggradation to progradation: the Maiella Platform, Abruzzi, Italy, in Simo, J.A.T., Scott, R.W., and Masse, J.P., eds., Cretaceous Carbonate Platforms: American Association of Petroleum Geologists, Memoir 56, p. 213–232. EBERLI, G.P., ANSELMETTI, F.S., BETZLER, C., VAN KONIJNENBURG, J.H., AND BERNOULLI, D., 2004, Carbonate platform to basin transitions on seismic data and in outcrops: Great Bahama Bank and the Maiella platform margin, Italy, in Eberli, G.P., Massaferro, J.L., and Sarg, J.F., eds., Seismic Imaging of Carbonate Reservoirs and Systems: American Association of Petroleum Geologists, Memoir 81, p. 207–250. FROST, E.L., III, 2007, Facies heterogeneity, platform architecture and fracture patterns of the Devonian reef complexes, Canning Basin, Western Australia [unpublished PhD dissertation]: University of Texas at Austin, Austin, Texas, 156 p. FROST, E.L., III, AND KERANS, C., 2009, Platform-margin trajectory as a control on syndepositional fracture patterns, Canning Basin, Western Australia: Journal of Sedimentary Research, v. 79, p. 44–55. FROST, E.L., III, AND KERANS, C., 2010, Controls on syndepositional fracture patterns, Devonian reef complexes, Canning Basin, Western Australia: Journal of Structural Geology, v. 32, p. 1231–1249. GEORGE, A.D., 1999, Deep-water stromatolites, Canning Basin, northwestern Australia: Palaios, v. 14, p. 493–505. GEORGE, A.D., AND CHOW, N., 2002, The depositional record of the Frasnian/ Famennian boundary interval in a forereef succession, Canning Basin, Western Australia, in Racki, G., and House, M.R., eds., Late Devonian Biotic Crisis: Ecological, Depositional, and Geochemical Records: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 181, p. 347–374. GEORGE, A.D., PLAYFORD, P.E., AND POWELL, C.M., 1994, Carbonate breccias and quartzo-feldspathic sandstones of the marginal slope, Devonian reef complex, Canning Basin, Western Australia, in Purcell, P.G., and Purcell, R.R., eds., Proceedings of the West Australian Basins Symposium: Petroleum Exploration Society of Australia, Perth, p. 727–738. GEORGE, A.D., PLAYFORD, P.E., AND POWELL, C.M., 1995, Platform-margin collapse during Famennian reef evolution, Canning Basin, Western Australia: Geology, v. 23, p. 691–694. GEORGE, A.D., PLAYFORD, P.E., POWELL, C.M., AND TORNATORA, P.M., 1997, Lithofacies and sequence development on an Upper Devonian mixed carbonate–siliciclastic forereef slope, Canning Basin, Western Australia: Sedimentology, v. 44, p. 843–867. GEORGE, A.D., CHOW, N., AND TRINAJSTIC, K.M., 2002, Integrated approach to platformbasin correlation and deciphering the evolution of Devonian reefs, northern Canning Basin, Western Australia, in Keep, M., and Moss, S.J., eds., The Sedimentary Basins of Western Australia 3: Proceedings of the West Australian Basins Symposium, Petroleum Exploration Society of Australia, Perth, v. 3, p. 817–835. GEORGE, A.D., CHOW, N., AND TRINAJSTIC, K.M., 2009a, Syndepositional fault control on lower Frasnian platform evolution, Lennard Shelf, Canning Basin, Australia: Geology, v. 37, p. 331–334. GEORGE, A.D., TRINAJSTIC, K.M., AND CHOW, N., 2009b, Frasnian reef evolution and palaeogeography, SE Lennard Shelf, Canning Basin, Australia, in Konigshof, P., ed., Devonian Change: Case Studies in Palaeogeography and Palaeoecology: Geological Society of London, Special Publication 314, p. 73–107. GIRARD, C., KLAPPER, G., AND FEIST, R., 2005, Subdivision of the terminal Frasnian Linguiformis conodont zone, revision of the correlative interval of Montagne Noire Zone 13, and discussion of stratigraphically significant associated trilobites, in Over, J.D., Morrow, J.R., and Wignall, P.B., eds., Understanding Late Devonian and Permian–Triassic Biotic and Climatic Events: Towards an Integrated Approach: Amsterdam, Elsevier, Developments in Palaeontology and Stratigraphy, v. 20, p. 181– 198.

JSR

GOLDHAMMER, R.K., OSWALD, E.J., AND DUNN, P.A., 1991, The hierarchy of stratigraphic forcing: an example from middle Pennsylvanian shelf carbonates of the Paradox Basin, in Franseen, E., and Watney, L., eds., Sedimentary Modeling: Computer Simulation of Depositional Sequences: Kansas Geological Survey, Subsurface Geology Series, v. 12, p. 361–413. GRADSTEIN, F.M., OGG, J.G., SCHMITZ, M., AND OGG, G., 2012, The Geologic Time Scale 2012: Amsterdam, Elsevier, 1176 p. GRAMMER, G.M., AND GINSBURG, R.N., 1992, Highstand versus lowstand deposition on carbonate platform margins: insight from Quaternary foreslopes in the Bahamas: Marine Geology, v. 103, p. 125–136. HOCKING, R.M., AND PLAYFORD, P.E., 2001, Cycle types in carbonate platform facies, Devonian reef complexes, Canning Basin, Western Australia: Geological Survey of Western Australia, 2000–2001 Annual Review, p. 74–80. HOOKE, R.L., AND SCHLAGER, W., 1980, Geomorphic evolution of the Tongue of Ocean and the Providence Channels, Bahamas: Marine Geology, v. 35, p. 343–366. HURLEY, N.F., 1986, Geology of the Oscar Range Devonian reef complex, Canning Basin, Western Australia [unpublished PhD dissertation]: University of Michigan, Ann Arbor, 269 p. JAMES, N.P., AND GRAVESTOCK, D.I., 1990, Lower Cambrian shelf and shelf margin buildups, Flinders Ranges, South Australia: Sedimentology, v. 37, p. 455–480. JANSON, X., KERANS, C., LOUCKS, R., MARHX, M.A., REYES, C., AND MURGUIA, F., 2011, Seismic architecture of a Lower Cretaceous platform-to-slope system, Santa Agueda and Poza Rica Fields, Mexico: American Association of Petroleum Geologists, Bulletin, v. 95, p. 105–146. KATZ, D.A., PLAYTON, T.E., BELLIAN, J.A., HARRIS, P.M., HARRISON, C., AND MAHARAJA, A., 2010, Slope heterogeneity and production results in a steep-sided upper Paleozoic isolated carbonate platform reservoir, Karachaganak Field, Kazakhstan: Society of Petroleum Engineers, Caspian Carbonates Technology Conference Paper, Atyrau, Kazakhstan, no. 139960, p. 1–7. KENTER, J.A.M., 1990, Carbonate platform flanks: slope angle and sediment fabric: Sedimentology, v. 37, p. 777–794. KENTER, J.A.M., HARRIS, P.M., AND DELLA PORTA, G., 2005, Steep microbial boundstone-dominated platform margins: examples and implications: Sedimentary Geology, v. 178, p. 5–30. KERANS, C., 1985, Petrology of Devonian and Carboniferous Carbonates of the Canning and Bonaparte Basins: Western Australian Mining and Petroleum Research Institute, Report, v. 12, 203 p. KERANS, C., AND TINKER, S.W., 1999, Extrinsic stratigraphic controls on development of the Capitan reef complex, in Saller, A.H., Harris, P.M., Kirkland, B.L., and Mazzullo, S.J., eds., Geologic Framework of the Capitan Reef: SEPM, Special Publication 65, p. 15–36. KERANS, C., HURLEY, N.F., AND PLAYFORD, P.E., 1986, Marine diagenesis in Devonian reef complexes of the Canning Basin, Western Australia, in Schroeder, J.H., and Purser, B.H., eds., Reef Diagenesis: Berlin, Springer-Verlag, p. 357–380. KLAPPER, G., 1989, The Montagne Noire Frasnian (Upper Devonian) conodont succession, in McMillan, N.J., Embry, A.F., and Glass, D.J., eds., Devonian of the World: Canadian Society of Petroleum Geologists, Memoir 14, p. 449–468. KLAPPER, G., 1997, Graphic correlation of Frasnian (Upper Devonian) sequences in Montagne Noire, France, and western Canada, in Klapper, G., Murphy, M.A., and Talent, J.A., eds., Paleozoic Sequence Stratigraphy, Biostratigraphy, and Biogeography: Studies in Honor of J. Granville (“Jess”) Johnson: Geological Society of America, Special Paper 321, p. 113–129. KLAPPER, G., 2007, Frasnian (Upper Devonian) conodont succession at Horse Spring and correlative sections, Canning Basin, Western Australia: Journal of Paleontology, v. 81, p. 513–537. MARKELLO, J.R., KOEPNICK, R.B., WAITE, L.E., AND COLLINS, J.F., 2008, The carbonate analogs through time (CATT) hypothesis and the global atlas of carbonate fields: a systematic and predictive look at Phanerozoic carbonate systems, in Lukasik, J., and Simo, J.A., eds., Controls on Carbonate Platform and Reef Development: SEPM, Special Publication 89, p. 15–45. MCILREATH, I.A., AND JAMES, N.P., 1978, Carbonate slopes, in Walker, R.G., ed., Facies Models: Geological Association of Canada, Geoscience Canada Series, p. 245–257. MERINO-TOME, O., DELLA PORTA, G., KENTER, J.A.M., VERWER, K., HARRIS, P.M., ADAMS, E.W., PLAYTON, T.E., AND CORROCHANO, D., 2012, Sequence development in an isolated carbonate platform (Lower Jurassic, Djebel Bou Dahar, High Atlas, Morocco): influence of tectonics, eustasy, and carbonate production: Sedimentology, v. 59, p. 118–155. MOUNTJOY, E.W., COOK, H.E., PRAY, L.C., AND MCDANIEL, P.N., 1972, Allochthonous carbonate debris flows: worldwide indicators of reef complexes, banks or shelf margins: International Geological Congress, Stratigraphy and Sedimentology, Report of the Session, v. 24, p. 172–189. MULDER, T., AND ALEXANDER, J., 2001, The physical character of subaqueous sedimentary density flows and their deposits: Sedimentology, v. 48, p. 269–299. MULLINS, H.T., AND COOK, H.E., 1986, Carbonate apron models: alternatives to the submarine fan model for paleoenvironmental analysis and hydrocarbon exploration: Sedimentary Geology, v. 48, p. 37–79. PLAYFORD, P.E., 1980, Devonian “great barrier reef” of the Canning Basin, Western Australia: American Association of Petroleum Geologists, Bulletin, v. 64, p. 814–840. PLAYFORD, P.E., 1981, Devonian Reef Complexes of the Canning Basin, Western Australia: Geological Society of Australia, 5th Australian Geological Convention, Field Excursion Guidebook, 64 p.

JSR


PLAYFORD, P.E., 1984, Platform-margin and marginal-slope relationships in Devonian reef complexes of the Canning Basin, in Purcell, P.G., ed., Canning Basin Symposium, Proceedings, Perth: Geological Society of Australia and Petroleum Exploration Society of Australia, p. 189–214. PLAYFORD, P.E., 2002, Paleokarst, pseudokarst, and sequence stratigraphy in Devonian reef complexes of the Canning Basin, Western Australia, in Keep, M., and Moss, S.J., eds., Petroleum Exploration Society of Australia Symposium, Proceedings, Perth, Western Australia, p. 763–793. PLAYFORD, P.E., HURLEY, N.F., KERANS, C., AND MIDDLETON, M.F., 1989, Reefal platform development, Devonian of the Canning Basin, Western Australia, in Crevello, P.D., Wilson, J.L., Sarg, J.F., and Read, J.F., eds., Controls on Carbonate Platform and Basin Development: SEPM, Special Publication 44, p. 187–202. PLAYFORD, P.E., HOCKING, R.M., AND COCKBAIN, A.E., 2009, Devonian reef complexes of the Canning Basin, Western Australia: Geological Survey of Western Australia, Bulletin 145, 444 p. PLAYTON, T.E., 2008, Characterization, variations, and controls of reef-rimmed carbonate foreslopes [unpublished Ph.D. dissertation]: The University of Texas at Austin, Austin, Texas, 283 p. PLAYTON, T.E., AND KERANS, C., 2015b, Upper Devonian carbonate margins and foreslopes of the Lennard Shelf, Canning Basin, Western Australia, Part B: development during progradation and across the Frasnian–Famennian biotic crisis: Journal of Sedimentary Research, v. 85, p. 1362–1392. PLAYTON, T.E., JANSON, X., AND KERANS, C., 2010, Carbonate slopes, Chapter 18, in James, N.P., and Dalrymple, R.W., eds., Facies Models 4: Geological Association of Canada, St. Johns, Newfoundland, GEOtext 6, p. 449–476. PLAYTON, T.E., MONTGOMERY, P., YAN, M., TOHVER, E., RATCLIFFE, K., WRIGHT, M., SANO, J., HILLBUN, K., KATZ, D., HAINES, P., HOCKING, R., HARRIS, P.M., AND JONES, G., 2011, High resolution chronostratigraphy of a carbonate platform-top to reef transition, Upper Devonian, Canning Basin, Western Australia [Abstract]: American Association of Petroleum Geologists, Annual Convention and Exhibition, April 10– 13, Houston, Texas. POMAR, L., WARD, W.C., AND GREEN, D.G., 1996, Upper Miocene reef complex of the Llucmajor area, Mallorca, Spain, in Franseen, E.K., Esteban, M., Ward, W.C., and Rouchy, J.M., eds., Models for Carbonate Stratigraphy from Miocene Reef Complexes of Mediterranean Regions: SEPM, Concepts in Sedimentology and Paleontology, v. 5, p. 191–225. POTMA, K., WEISSENBERGER, J.A.W., WONG, P.K., AND GILHOOLY, M.G., 2001, Toward a sequence stratigraphic framework for the Frasnian of the Western Canadian Basin: Bulletin of Canadian Petroleum Geology, v. 49, p. 37–85. RIDING, R., 1988, Classification of microbial carbonates: benthic microbes and reefs [Abstract]: 6th International Coral Reef Symposium, Townsville, Australia. RIDING, R., 1991, Classification of microbial carbonates, in Riding, R., ed., Calcareous Algae and Stromatolites: New York, Springer, p. 21–51. SARG, J.F., MARKELLO, J.R., AND WEBER, L.J., 1999, The second-order cycle, carbonateplatform growth, and reservoir, source, and trap prediction, in Harris, P.M., Saller, A.H., and Simo, J.A., eds., Advances in Carbonate Sequence Stratigraphy: Application to Reservoirs, Outcrops, and Models: SEPM, Special Publication 63, p. 11–34. SCHLAGER, W., 1981, The paradox of drowned reefs and carbonate platforms: Geological Society of America, Bulletin, v. 92, p. 1197–1211. SCHLAGER, W., AND GINSBURG, R.N., 1981, Bahamian carbonate platforms: the deep and the past: Marine Geology, v. 44, p. 1–24.

1361

SHEN, J., WEBB, G.A., AND JELL, J.S., 2008, Platform margins, reef facies, and microbial carbonates: a comparison of Devonian reef complexes in the Canning Basin, Western Australia, and the Guilin region, South China: Earth-Science Reviews, v. 88, p. 33–59. TINKER, S.W., 1998, Shelf-to-basin facies distribution and sequence stratigraphy of a steep-rimmed carbonate margin: Capitan depositional system, McKittrick Canyon, New Mexico and Texas: Journal of Sedimentary Research, v. 68, p. 1146–1174. VAN BUCHEM, F.S.P., CHAIX, M., EBERLI, G.P., WHALEN, M.T., MASSE, P., AND MOUNTJOY, E.W., 2000, Outcrop to subsurface correlation of the Upper Devonian (Frasnian) in the Alberta Basin (W. Canada) based on the comparison of Miette and Redwater carbonate buildup margins, in Homewood, P.W., and Eberli, G.P., eds., Genetic Stratigraphy on the Exploration and Production Scales: Case Studies from the Upper Devonian of Alberta and the Pennsylvanian of the Paradox Basin: Bulletin des Centres de Recherches Exploration-Production Elf-Aquitaine, v. 24, p. 225–267. WARD, W.B., 1999, Tectonic control on backstepping sequences revealed by mapping of Frasnian backstepped platforms, Devonian reef complexes, Napier Range, Canning Basin, Western Australia, in Harris, P.M., Saller, A.H., and Simo, J.A., eds., Advances in Carbonate Sequence Stratigraphy: Application to Reservoirs, Outcrops and Models: SEPM, Special Publication 63, p. 47–74. WEBER, L.J., FRANCIS, B.P., HARRIS, P.M., AND CLARK, M., 2003, Stratigraphy, lithofacies, and reservoir distribution, Tengiz Field, Kazakhstan, in Ahr, W.M., Harris, P.M., Morgan, W.A., and Somerville, I.D., eds., Permo-Carboniferous Carbonate Platforms and Reefs: SEPM, Special Publication 78, p. 351–394. WENDTE, J.C., 1992, Evolution of the Judy Creek complex, a late Middle Devonian isolated platform-reef complex in west-central Alberta, in Wendte, J.C., Stoakes, F.A., and Campbell, C.V., eds., Devonian–Early Mississippian Carbonates of the Western Canada Sedimentary Basin: A Sequence Stratigraphic Framework: SEPM, Short Course Notes 28, p. 89–125. WHALEN, M.T., EBERLI, G.P., VAN BUCHEM, F.S.P., MOUNTJOY, E.W., AND HOMEWOOD, P.W., 2000a, Bypass margins, basin-restricted wedges, and platform-to-basin correlation, Upper Devonian, Canadian Rocky Mountains: implications for sequence stratigraphy of carbonate platform systems: Journal of Sedimentary Research, v. 70, p. 913–936. WHALEN, M.T., EBERLI, G.P., VAN BUCHEM, F.S.P., AND MOUNTJOY, E.W., 2000b, Facies models and architecture of Upper Devonian carbonate platforms (Miette and Ancient Wall), Rocky Mountains, Alberta, Canada, in Homewood, P.W., and Eberli, G.P., eds., Genetic Stratigraphy on the Exploration and Production Scales: Case Studies from the Upper Devonian of Alberta and the Pennsylvanian of the Paradox Basin: Bulletin des Centres de Recherches Exploration-Production Elf-Aquitaine, v. 24, p. 139–175. WORKUM, R.H., AND HEDINGER, A.S., 1989, Burnt Timber and Scalp Creek margins, Frasnian Fairholme reef complex, Alberta, in Geldsetzer, H.J., James, N.P., and Tebbutt, G.E., eds., Reefs: Canada and Adjacent Areas: Canadian Society of Petroleum Geologists, Memoir 13, p. 552–556. ZIEGLER, W., AND SANDBERG, C.A., 1990, The Late Devonian Standard Conodont Zonation: Courier Forschungsinstitut Senckenberg, v. 121, 115 p. Received 22 August 2014; Accepted 24 July 2015.