Cross-platform architecture of a sequence boundary ...

Sedimentology (1996) 43, 197-217

Cross-platform architecture of a sequence boundary in mixed siliciclastic-carbonate lithofacies, Middle Cambrian, southern Great Basin, USA DAVID A . O S L E G E R a n d I S A B E L P. M O N T A N E Z Department of Earth Sciences, University of California, Riverside, CA 92521 -0423, U S A ABSTRACT

Stratigraphic analysis of mixed siliciclastic-carbonate lithofacies within the Middle Cambrian Bonanza King Formation of the southern Great Basin reveals three distinct facies associations that record a range of depositional environments from semi-arid tidal flats to deeper subtidal, restricted lagoons. Stratigraphic trends, cross-platform facies variations and correlation of individual surfaces across 250 km of the study area suggest that these mixed lithofacies were deposited in three temporally distinct phases. (1)Extensive progradation of mixed peritidal environments culminated in a prolonged episode of subaerial exposure marked by an areally extensive intraclast breccia (0.5-1-2 m thick) that we interpret to be a major Type 1 sequence-bounding disconformity. (2) Abrupt flooding of the exposed platform resulted in the deposition of mixed deeper subtidal lithofacies, including a condensed interval of fissile, fossiliferous shale. (3) Progressive shallowing and aggradational accumulation was accompanied by a decrease in siliciclastics and a shift to pure carbonate deposition. Deep-water siliciclastics and megabreccias record deposition along the base-of-slope off the Middle Cambrian shelf-edge, and are interpreted to represent lowstand deposits emplaced during the prolonged episode of subaerial exposure of the shallow shelf. The presence of fine siliciclastics in both peritidal facies and sharply overlying deeper subtidal facies of the study interval within the Bonanza King suggests a variable, but relatively continuous, influx of terrigenous material throughout an extended period of accommodation change, apparently asynchronous with respect to the predictive model of reciprocal sedimentation. We suggest that the primary siliciclastic source changed with relative sea-level position. During lowered sea level, aeolian processes acting upon the unvegetated Cambrian craton transported fine siliciclastics onto peritidal and shallow-subtidal environments. During higher sea level, coastal siliciclastic reservoirs supplied sediment that was transported for long distances by geostrophic currents flowing along the submerged platform. As opposed to many Cambro-Ordovician grand cycles that are commonly interpreted to consist of a transgressive shaly half-cycle grading upward into a regressive carbonate half-cycle, the sequence boundary within this Middle Cambrian succession occurs within siliciclastic-rich, mixed lithofacies rather than in adjoining purer carbonates, implying that some ‘grand cycles’ should not be considered synonymous with ‘sequences’. Interbasinal correlations of the Type 1 sequence boundary within the mixed unit are speculative, primarily because of the inherent imprecision of available trilobite biostratigraphy. However, there is evidence that an extended episode of subaerial exposure may have been continent-wide during the Ehmaniella trilobite biochron. 0 1996 International

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198 D. A. Osleger and I. P. Montaiiez INTRODUCTION

In stratigraphic successions of mixed siliciclastic-carbonate lithofacies, fundamental questions exist regarding criteria for the identification of sequence boundaries and systems tracts, controls on the timing of siliciclastic influx and the role of relative sea-level in determining the physical architecture of depositional sequences. These questions are particularly acute for mixed depositional systems that characterize the large Cambro-Ordovician passive-margin platforms that surround the North American craton. Traditional sequence stratigraphic techniques that depend upon seismic-scale stratal patterns to decipher the genetic history are difficult to apply to Lower Palaeozoic platforms because of the common lack of extensive lateral outcrop, the typical absence of easily identifiable marker beds, the broad temporal resolution of biostratigraphic control and structural dislocations. Recent studies of Cambro-Ordovician rocks have correlated stacking patterns of metrescale, upward-shallowing cycles from individual outcrop sections to unravel the genetic stratigraphy and long-term accommodation history (Hardie, 1989; Kerans & Lucia, 1989; Koerschner & Read, 1989; Montafiez & Read, 1992; Goldhammer et al., 1993; Montafiez & Osleger, 1993; Osleger & Read, 1993). This approach has proven useful for identifying sequence boundaries and systems tracts in individual stratigraphic sections, but detailed descriptions of the cross-platform characteristics of sequence boundaries on broad Lower Palaeozoic platforms are typically lacking, especially those developed i n mixed siliciclasticcarbonate lithofacies. The Middle Cambrian Bonanza King Formation of the southern Great Basin contains a wellexposed, areally extensive interval of mixed carbonates and siliciclastics that exhibits discrete surfaces of inferred chronostratigraphic significance. We have systematically correlated these surfaces between 10 sections across the 250-300-km-wide platform in order to establish criteria for the recognition of sequence boundaries, transgressive surfaces and maximum flooding surfaces developed in CambroOrdovician mixed depositional systems. The study interval displays evidence for a major sealevel fall during Middle Cambrian time that may have been at least continent-wide in extent, based on correlations with coeval intervals in the western US, the southern Canadian Rockies and the

southern Appalachians. The mixed lithofacies of the study interval also reveal a pattern of siliciclastic influx that is asynchronous with relative sea-level change, an apparent exception to the principle of reciprocal sedimentation. The goals of this paper are to: (i) document and interpret mixed siliciclastic-carbonate lithofacies surrounding a single Type 1 sequence boundary developed in the Bonanza King Formation, (ii) suggest a model for siliciclastic influx onto this Middle Cambrian carbonate platform and (iii) discuss implications for the interpretation of Cambrian grand cycles. STRATIGRAPHIC A N D TECTONIC FRAMEWORK

The Middle Cambrian Bonanza King Formation is exposed in several tilted fault block mountains in the Great Basin of southern Nevada and eastern California (Fig. 1). The formation is divided into the Papoose Lake Member and the Banded Mountain Member, both dominantly composed of cyclical, shallow-marine carbonates (Barnes & Palmer, 1961; Kepper, 1972; Montaiiez & Osleger, 1993) (Fig. 2). The basal lithology of the Banded Mountain Member is an areally extensive, orange- to red-weathering interval of intermixed carbonates and fine siliciclastics (herein called the ‘mixed unit’) that forms a distinct slope between cliff-forming carbonates. The mixed unit ranges from < 1 0 m near the craton edge (Sheep Mountain) to 73 m on the mid-platform (Funeral Mountains) (Fig.1) and forms a widespread marker throughout the study area. Barnes & Palmer (1961) called these calcareous siltstones, silty limestones and dolomites, and fissile shales the ‘brown-weathering siliceous carbonate’. Kepper (1972, 1976) also described this interval (his Unit ‘G’) in the context of a broader-scale project on the Middle to Upper Cambrian of the Great Basin. Trilobites of the Ehmaniella biozone have been found in the mixed unit (Palmer & Hazzard, 1956; Palmer, 1971). The Bonanza King Formation was deposited on the Cordilleran passive margin which developed in response to breakup of a Late Proterozoic supercontinent around 600-545 Ma (Stewart & Suczek, 1977; Bond et al., 1984; Levy & Christie-Blick, 1991). The continental platform edge of western North America during the Middle Cambrian extended essentially east-west at about 10-15”N latitude (Scotese & McKerrow, 1990). Palinspastic estimates of the platform width

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Sequence-boundary architecture, Great Basin, U S A 199

Fig. 1. Location map of the study area in the southern Great Basin of Nevada and eastern California. Black dots denote locations of measured sections; the asterisk marks the location of off-platform equivalents to the Bonanza King Formation. TJ=Teakettle Junction; ETS=Eureka Thrust System. Sections extend across depositional dip for about 300 km (palinspastically estimated) from the early Palaeozoic hinge line neaI Las Vegas to the platform edge in the Last Chance Range. Adapted from Snow & Wernicke (1989) and Snow (1992).

during the Middle Cambrian range from 250 to 300 km from the craton margin hinge line near Las Vegas to the outer platform edge near the Last Chance Range (Fig. 1; Levy & Christie-Blick, 1989, 1991). Deposition of the Bonanza King Z BIOMERE

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Formation occurred on a flat-topped, fully aggraded shelf, as indicated by the widespread, dominantly shallow-subtidal to peritidal lithofacies (Montafiez & Osleger, 1993). The actual shelf-edge is probably buried beneath the Eureka thrust sheet, but base-of-slope lithofacies of the coeval Emigrant Formation crop out in the northern Last Chance Range (Fig. 1) and record deposition at the base of a high-relief shelf-margin composed of thrombolite bioherms and carbonate sands (Kepper, 1981).

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Fig. 2. Biostratigraphic framework of the Middle to Late Cambrian of the southern Great Basin. The focus of this study is the uppermost Papoose Lake Member, the Mixed Unit and overlying carbonates of the Banded Mountain Member. Generalized lithologies denoted by standard geological symbols. Biostratigraphic control from Palmer (1971) and Robison (1976).

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Ten sections of the uppermost Papoose Lake Member, the mixed unit and the lowermost carbonate portion of the Banded Mountain Member were measured and logged on a decimetre scale in locations spread out across depositional dip of the Middle Cambrian passive margin (Figs 1 and 3). Genetically related lithofacies were grouped into three facies associations based on stratigraphic relationships that reflect deposition in adjacent and gradational palaeoenvironments. Table 1 provides the raw descriptive data for each of the component lithofacies that make up the three facies associations. The following sections are primarily interpretative, using the observations from Table 1 to infer depositional environments and processes.

Peritidal facies association The peritidal facies association characterizes the lower two-thirds of the mixed unit (Fig. 3);

Sedimentology, 43, 197-217

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Sequence-boundary architecture, Great Basin, U S A 201 lithofacies in this association (Table 1; Fig. 4) were deposited in environments ranging from supratidal to very shallow subtidal. The component lithofacies are typically arranged into asymmetric upward-shallowing cycles (0-4-7.0 m thick) that record abrupt flooding to shallow subtidal water depths followed by gradual shoaling to intertidal or supratidal flats. The depositional environments of the component lithofacies that comprise peritidal carbonate cycles have been exhaustively reviewed in the literature (e.g. Fischer, 1964; Wilson, 1975; Wright, 1984; Hardie & Shinn, 1986; Osleger & Read, 1991; Pratt et a]., 1992). Rather than re-review, we prefer to focus on the environmental interpretation of two of the peritidal lithofacies, the silty dololaminites and the intraclast breccias, that are critical to the sequence stratigraphic interpretation developed later in the text.

Silty dololaminites The silty dololaminite lithofacies that caps most peritidal cycles in the mixed unit (Fig. 5a) represents a mixed facies in the strictest sense; siliciclastic and carbonate material is texturally intermingled as well as interlaminated. These silty dololaminites are interpreted to have been mechanically deposited on tidal flats as storm-tide couplets of fine peloidal silts and mud drapes, as suggested by the centimetre-scale lamination and low-angle cross-lamination (cf. Hardie & Ginsburg, 1977). The abundant interlaminated quartz-muscovite silt is interpreted to be aeolian in origin, blown onto the carbonate flats much as they are today on the sabkhas of the southern Persian Gulf where detrital dolomite and quartz silt are derived from the Zagros Mountains several hundreds of kilometres away (Shinn, 19861 The aeolian origin is based on the associated presence of detrital dolomite, the excellent degree of sorting and the subangular shapes of quartz silt grains (cf.

Mazzullo et al., 1992). It is probable that once the siliciclastic silts were windblown as dust onto the carbonate tidal flats they were reworked by tidal action (‘eolo-marine’ deposition of Fischer & Sarnthein, 1988) and redeposited as planar laminae and low-angle cross-laminae. The lack of land plants on the nearby Cambrian craton probably contributed to the generation of large volumes of siliciclastic sediment that was sorted by wind and redistributed as aeolian dust onto the prograding tidal flats of the adjacent passive margins (Dalrymple et a]., 1985). Stratigraphically adjacent cryptalgal laminites that typically underlie silty dololaminites contain negligible amounts of siliciclastic silt. This may reflect a greater distance from aeolian sources on the continental interior or perhaps is a function of changes in predominant wind direction. Silty dololaminites typically overlie cryptalgal laminites in individual peritidal cycles, are more common than cryptalgal laminites in more cratonward sections and also predominate toward the top of the overall upward-shallowing trend within the peritidal facies association of the lower mixed unit (Fig. 3). These observations suggest that the silty dololaminites occupy a ‘diffuse border between contrasting facies’ (Mount, 1984), in this case between carbonate tidal-flats and continental aeolian siliciclastics.

Intraclast breccias Two intraclast breccias in the mixed unit can be correlated for >250 km across the shelf from the craton edge (Frenchman Mountain) to the mid-shelf (Teakettle Junction) (Figs 3 and 5). These breccias vary from 0.1 to 1 - 2 m in thickness, range from 2 to 1 6 m apart depending on platform position and are readily identifiable within the peritidal facies association in that almost all other peritidal cycles are capped by silty dololaminites or cryptalgal laminites with

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Fig. 3. Dip-orientated cross-section spanning 300 km (palinspastic) of the Middle Cambrian passive margin (during the Ehmaniella biozone) in the southern Great Basin, USA. The datum for the cross-section is an areally widespread contact within the lower Banded Mountain Member that separates underlying, slope-forming, mixed siliciclastics and carbonates from overlying, cliff-forming carbonates. Lower correlation line marks the top of the noncyclical, shallow-subtidal, massive cliff of the Papoose Lake Member. Other correlation lines are based on key surfaces that separate major breaks between facies associations. Wavy lines correlate two inferred exposure breccias, representing erosional unconformities, recognized in eight of the sections. Intervals labelled A-F designate component subunits of the mixed unit and are linked to the discussion on sequence stratigraphy in the text. PFA=peritidal facies association; SSFA= shallow subtidal facies association; DSFA= deeper subtidal facies association. The ‘inner’, ‘middle’ and ‘outer’ shelves are based on geographical position rather than lithological characteristics; ‘outer’ shelf shallow-marine lithofacies are very similar to ‘inner’ shelf lithofacies, reflecting deposition on a fully aggraded, flat-topped shelf.

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Association of Sedimentologists, Sedimentology, 43, 197-217

202 D. A.

Osleger and I. P. Montaiiez

Table 1. Lithofacies characteristics. Peritidal facies association Intraclast breccia. Thin (10-120 cm] beds and irregular veneers of angular, disorientated, poorly sorted clasts of chert and dolomite floating in a coarse dolomitic matrix; breccias typically overlie laminite lithofacies along an irregular, corroded contact; common red discoloration and lenses of nodular chert; intraclasts are composed of chert and dololaminite, are commonly matrix-supported, range in size from cobble to pebble, and exhibit corroded rims and sharp, angular edges that ‘fit’ with adjacent clasts; matrix is composed of poorly sorted dolomitized intraclast grainstone containing quartz silt, detrital dolomite grains and muscovite. Silty dololaminite. Centimetre- to millimetre-scale laminations of dolomitic peloidal silt and mud couplets containing abundant laminae of haematite-coated quartz silt; planar and low-angle cross-lamination dominant; usually cap upward-shallowing cycles and gradationally overlie cryptalgal laminites; exhibit mudcracks, prism cracks, low-relief tepees, mud-chip rip-ups, silicified evaporite nodules and abundant quartz silt laminae; subangular quartz silt is very well sorted and comprises 5-30% of the lithofacies. Cryptalgal laminite. Typically dolomitized, millimetre-scale planar and crinkly laminations exhibiting mudcracks, prism cracks, tepees and silicified evaporite nodules; commonly overlie peloidal wackestone-packstone and underlie silty dololaminites; individual laminae are composed of peloid-to-lime mud couplets with common laminar fenestrae. Microbial boundstone. Morphologies range from massive thrombolitic bioherms to low-relief, stacked hemispheroid and laterally linked stromatolites; mounded thrombolites with clotted digitate fabrics commonly occur near the bases of upward-shallowing cycles and may grade up into low-relief stromatolites. Peloidal wackestone-packstone. Thin beds of peloidal carbonates that commonly form interhead fill between thrombolite bioherms; bedding within peloidal wackestones is commonly obscured by burrowing but bedding is better expressed in packstones, along with scour features, low-angle cross-bedding and multiple hardgrounds; typically grades up into laminite cycle caps. Shallow subtidal facies association Lenticular-bedded silty dolomite and lime wackestone. Wavy, centimetre-scale couplets of grey peloidal wackestone that grades up into orange-weathering silty dolomite; scoured bases and low-angle cross-lamination common; discrete dolomitized burrows that trend oblique or vertical to bedding; dolomitic interbeds are finely crystalline and contain 10-30% quartz silt and 2-5% muscovite flakes. Massive bioturbated wackestone-packstone. Dark grey, massive cliff-former composed of heavily burrow-mottled peloidal wackestone-packstone; burrowing is pervasive and destroys any original bedding; burrows are preferentially dolomitized whereas peloidal mud matrix remains limestone; may contain 5-10% quartz silt randomly disseminated throughout matrix. Deeper subtidal facies association Mixed calcareous siltstone and silty limestone. Platy thin beds of red-weathering quartz siltstones and fine sandstones interbedded with nodular to lenticular-bedded lime mudstones; siltstone-to-limestone ratio ranges from 90% siltstone to250 km from the craton margin through the mid-shelf. The two exposure breccias merge into a single sharp bedding plane in the most cratonward section at Sheep Mountain. In the outer shelf section in the Last Chance Range, the breccias are apparently absent, although a critical 1-5-m interval toward the top of the unit is entirely covered along the length of its outcrop. The stratigraphic proximity of several marker beds and surfaces (Fig. 3) make for confident correlations of these two exposure breccias between measured sections, strongly suggesting that they are regional in extent across the southern Great Basin and are not local phenomena. Platform-wide peritidal deposition is abruptly terminated across a single surface that separates the peritidal part of the mixed unit from the overlying deeper subtidal part of the mixed unit. On the inner to mid-shelf, this surface of abrupt deepening is separated from the upper exposure breccia by 1-9 m of tidal-flat laminites. Near the craton-margin hinge line (Frenchman Mountain), the surface of abrupt deepening merges with the upper exposure breccia (Fig. 3). Maximum apparent water depths were attained during deposition of the fissile green-brown shales within the lower

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half of the deeper subtidal mixed unit. The proportion of fine siliciclastics to carbonates gradually decreases upward toward the shallowsubtidal cycles of the Banded Mountain Member. The study interval exhibits three long-term stratigraphic trends that can be summarized as follows: (i) progressive shallowing, accompanied by an influx of siliciclastic silt, which culminated in two discrete episodes of prolonged subaerial exposure and surface karstification; (ii)the abrupt superimposition of significantly deeper-water lithofacies above peritidal facies marks an apparently rapid flooding event that peaked with the deposition of fossiliferous, fissile shales; (iii) renewed shoaling to shallow-subtidal conditions is accompanied by a gradual decrease in siliciclastics. It is important to note that fine siliciclastics are intermixed and interstratified through the entire succession of initial shallowing, abrupt deepening and subsequent renewed shallowing within the mixed unit.

Shelf-edge morphology Exposures of the ‘true’ Middle Cambrian shelfedge have not been recognized in the study region and it is probable that the actual shelf-edge is buried beneath the Eureka thrust system. Snow (1992) estimates about 30 km of total shortening by movement of the Eureka allochthon. The morphology and composition of the shelf-margin can be determined conjecturally, however, from the composition of coeval base-of-slope lithofacies of the Emigrant Formation exposed in the northern Last Chance Range (Fig. 1) described by Kepper (1981). Large Epiphyton boundstone megabreccia blocks (>6 m across), thrombolitic clasts within polymict breccias and allodapic grainstones suggest deposition from debris flows emanating from a relatively steep shelf-margin composed of patchy algal bioherms and interhead carbonate sands (Kepper, 1981). Peloidal intraclasts (algal in origin) and peloids derived from the fragmentation of branching Epiphyton (Coniglio & James, 1985) provide additional evidence for algal bioherms rimming the Bonanza King shelf-edge. Abundant micritized grains and oolitic grains within the calcarenitic matrix between breccia blocks were probably derived from lime sand shoals that formed lateral to the algal bioherms. This high-energy, biohermal shelf-rim may have provided a protective barrier that attenuated wave energy (Osleger, 1991), resulting in lowenergy, restricted conditions necessary for the accumulation of muddy, shallow subtidal and Association of Sedimentologists, Sedirnentology, 43, 197-217

Sequence-boundary architecture, Great Basin, U S A 207

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B. Fig. 7. Three phases of platform development of the mixed unit and bounding units. Distances between sections were estimated based on the palinspastic maps of Levy & Christie-Blick (1989). The actual position and composition of the Middle Cambrian shelf-margin biohermal belt is speculative; the composition of the raised marginal rim is based on base-of-slope lithofacies of the Emigrant Formation exposed in the northern Last Chance Range. HST=highstand systems tract; SB = sequence boundary; LST=lowstand systems tract; ts=transgressive surface; mfs=maximum flooding surface; TST=transgressive systems tract. Dashed lines in the HST are interpretative time lines that reflect inferred progradational clinoform geometries. The eustatic sea-level curve is hypothetical and simply serves as a guide to the chronological order of depositional events (A, B, C) as they relate to changes in superimposed orders of sea-level fluctuations.

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peritidal lithofacies across the broad, flat Bonanza King shelf. The probable existence of a raised shelf-edge barrier is also suggested by the sparse, low-diversity faunas that characterize the Ehmaniella biozone, reflecting restricted conditions and physical separation from open-marine settings (Robison, 1976).

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work. Platform development during deposition of the mixed unit and adjacent units can be divided into three distinct phases (Fig. 7) that are interpreted to reflect changing rates of relative sea-level rise and fall. Intervals critical to the discussion are labelled A-F on Fig. 3.

Phase 1: progradation and platform exposure SEQUENCE STRATIGRAPHY Building upon the previous section on crossplatform trends and shelf-edge morphology, the intent of this section is to interpret the cross-platform changes and relative sea-level history within a sequence stratigraphic frame-

Progressive shallowing across the entire shelf is indicated by the transition from the shallowsubtidal Papoose Lake Member into mixed carbonate-siliciclastic peritidal lithofacies of the lower mixed unit (Figs 3 (interval A) and 7A). Abundant intermixed siliciclastic silts within tidal-flat caps of cycles were episodically

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208 D. A. Osleger and I. P. Montaiiez windblown out onto the prograding carbonate tidal flats as the cratonal margin was exposed during high-frequency lowstands or stillstands of sea-level. Long-term progradation culminated with extended episodes of subaerial exposure as represented by the two platform-wide intraclast breccias. This interval up to the uppermost exposure breccia (top of interval A) constitutes the upper highstand systems tract (HST) of the underlying sequence and a ‘keep-up’ phase of sedimentation. The uppermost breccia is volumetrically the more significant of the two breccias (0.5-1.2 m thick) and appears to be the culminating exposure event in the large-scale, upwardshallowing trend. Thus we interpret the top of the upper breccia as a Type I sequence boundary that formed during a ‘forced regression’ (Posamentier et al., 1992), a lowering of relative sea-level independent of variations in sediment production or influx. Seaward thickening within the peritidal mixed unit (A) is primarily a consequence of increasing tectonic subsidence rates toward the outer platform, but thickening may also reflect progradational stratal geometries generated as the locus of sedimentation shifted basinward in response to decreasing accommodation. The ubiquitous peritidal cycles (considered synonymous with parasequences) within this interval suggest that high-frequency (104-105 year) sealevel fluctuations were superimposed upon the long-term relative sea-level signal (Koerschner & Read, 1989; Goldhammer el al., 1993). Amplitudes of Middle to Late Cambrian highfrequency sea-level fluctuations have been estimated by Osleger & Read (1991) to have been relatively low, perhaps 20 m ( & 5), based upon computer simulations of Cambrian metre-scale peritidal and subtidal cycles. Depending on the tectonic subsidence rate, the rate of third-order sea-level fall and the magnitude of the highfrequency oscillations, it seems likely that a few of the superimposed rapid sea-level events will expose the platform tap for extended periods of time (Fig. 7), generating a transitional zone of repeated small-scale erosional unconformities leading to a major unconformity at the top (cf. Christie-Blick, 1991).

Phase 2: onlap and full platform flooding Subsequent to the final major episode of subaerial exposure, relative sea-level began to rise slowly, resulting in increased accommodation space. The gradual relative rise in sea-level and the

accompanying transgression of the shoreline across the exposed platform is recorded by a thin (1-9 m) veneer of silty dololaminites that apparently onlap the sequence-bounding unconformity, pinching out toward the craton margin (Figs 3, interval B , and 7B). These peritidal lithofacies are separated from overlying, distinctly deeper water lithofacies (C) by a very sharp transgressive surface (described below). The peritidal wedge (B) is temporally distinct from the underlying Type I sequence boundary and is interpreted to be a shelfal tongue of the lowstand systems tract, analogous to incised valley fill in siliciclastic systems. We recognize that this lowstand wedge extends further updip than published diagrams predict, but if the formal definitions (Van Wagoner et al., 1987; Sarg, 1988) require the lowstand systems tract to be defined at the base by a Type I sequence boundary and at the top by a transgressive surface, then this peritidal interval fits these criteria. The distance of onlap of this wedge is likely to be a function of the rate of relative sea-level rise (under conditions of constant sediment production), and the gradient of the foreslope and platform top; if the rate of rise is slow and the platform slope is gentle, then the wedge may potentially onlap as far inboard as the craton margin (as appears to be the case here). The peritidal facies association in the mixed unit (A, B) is juxtaposed against the overlying deeper subtidal facies association (C, D, E) by a very sharp contact that can be traced for more than 200 km across the platform (Figs 3 and 8). Kepper (1976) also noticed the sharpness of this surface separating underlying peritidal lithofacies from overlying subtidal lithofacies and suggested that it was a ‘diastemic’ boundary. We interpret this horizon to be a transgressive surface that marks the abrupt flooding of the flat-topped, peritidal platform. The transgressive surface merges with the uppermost sequence-bounding breccia near the craton margin. Five of the measured sections show this transgressive surface to be overlain by low-relief thrombolitic boundstones (Fig. 8) that probably grew in shallow waters after the initial transgression. Overlying thin-bedded to nodular lime mudstones were deposited as the rate of relative sea-level rise outpaced the rate of carbonate production and the platform entered a ‘catch-up’ phase (Neumann & Macintyre, 1985). Ultimately, burrowed and nodular carbonates were replaced by deep-water platy quartz siltstones and lenticular-bedded lime mudstones that were deposited in a shelf-lagoon, inferred to have been

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Sequence-boundary architecture, Great Basin, U S A 209

Fig. 8. Field photograph of sharp transgressive surface separating underlying mixed peritidal lithofacies (lowstand systems tract) from overlying mixed deeper-subtidal lithofacies [transgressive systems tract), Indian Ridge, Nevada. Thrombolitic bioherms are interpreted to have nucleated along the transgressive surface during the initial transgression before rapid deepening occurred. PFA=peritidal facies association; TS=transgressive surface (hammerhead rests on the surface);th= thrombolite bioherms; DSFA=deeper-subtidal facies association.

bounded along the seaward margin by thrombolitic bioherms and lime sand shoals. Fossiliferous, green-brown, fissile shales (1-3 m) (D on Fig. 3) can be traced for -200 km across the mid- to inner platform and are interpreted as a condensed interval that marks slow sedimentation during deepest water conditions, coincident with the most rapid rate of sea-level rise (Fig. 7B). Deeper subtidal lithofacies (C) between the transgressive surface and the shaly condensed interval range from 6 to 15 m thick and constitute the transgressive systems tract (TST). The thinness of the TST, relative to the underlying and overlying HSTs, may reflect either slow sedimentation rates associated with a rapid rate of sea-level rise or a short duration of sea-level rise.

Phase 3: renewed shallowing and aggradational accumulation Above the shaly condensed interval (D), lithofacies exhibit evidence for gradual shallowing accompanied by a progressive decrease in siliciclastic content (Figs 3 and 7C). Deeper subtidal lithofacies above the condensed interval (E) and cyclical shallow-subtidal lithofacies above the mixed unit (F on Fig. 3) are interpreted to represent aggradational accumulation of the early HST. Peritidal lithofacies and associated progradational geometries occur 40-70 m above the condensed interval. The remaining peritidal carbonates of the Banded Mountain Member (400-1000 m thick above the mixed unit) suggest that the Middle Cambrian passive margin in the study area maintained a flat-topped, fully

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aggraded morphology as carbonate production easily kept pace with relative sea-level change (Montaiiez & Osleger, 1993).

Off-platform siliciclastics and megabreccias Following the concept of reciprocal sedimentation (Wilson, 1967; Meissner, 1972), sequence stratigraphic models predict that during lowstands in sea-level, siliciclastics bypass the subaerially exposed platform to be deposited in basin-margin settings (Posamentier & Vail, 1988; Sarg, 1988; Handford & Loucks, 1993). Base-ofslope lithofacies of the Emigrant Formation exposed in the northern Last Chance Range exhibit a stratigraphic arrangement that may, arguably, be the depositional response to subaerial exposure of the Middle Cambrian shelf during Ehmaniella time. Kepper (1981) documented the structurally complicated stratigraphy of off-platform lithofacies of the Emigrant Formation in the northern Last Chance Range and recognized a rust-coloured dolomitic siltstone ( - 30 m thick) overlying thick-bedded, burrowmottled dolomite (which he called Papoose Lake Tongue). The rust-coloured dolomitic siltstone is overlain by 50-75m of megabreccias and calcarenites. These off-platform dolomitic siltstones show evidence for suspension settling or distal turbidity current deposition rather than deposition by traction currents, suggesting deeperwater sedimentation. Kepper (1981) correlated these deep-water dolomitic siltstones with the mixed unit on the shallow shelf and interpreted the Emigrant siliciclastics to have been deposited

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210 D. A. Osleger and I. P. Montaiiez on the deep basin-margin during a lowstand in sea-level. We agree with Kepper’s (1981) interpretation and further suggest that the Emigrant siliciclastics were deposited as a lowstand basin-floor fan (cf. Sarg, 1988) during the extended episode of subaerial exposure that generated the Type 1 sequence boundary in the mixed unit. Siliciclastics may have been transported to the shelf-edge by aeolian processes, ultimately to accumulate as hemipelagic deposits in the deep basin-margin. Off-platform facies of the Emigrant Formation directly overlying the dolomitic siltstones consist of (i) megabreccias composed of matrixsupported, large Epiphyton boundstone blocks, (ii) polymict breccias made up of cobble-size thrombolitic clasts and black, angular platy clasts of laminated peloidal silts probably derived from the upper foreslope, and (iii) amalgamated beds of massive carbonate sands consisting of micritized grains and oolitic and algal boundstone grains. These facies were deposited as debris flows and reflect episodic collapse of a submergent shelfedge composed of bioherms and lime-sand shoals. Based on stratigraphic position directly above the dolomitic siltstones, it is possible that these debris flow deposits record oversteepening of the shelf-edge shortly after the exposed platform was flooded (cf. Yose & Heller, 1989; Cook & Taylor, 1991). We interpret both the dolomitic siltstones and the carbonate megabreccias to express the downward shift in facies associated with formation of the Type I sequence boundary zone in the mixed unit on the platform top. TIMING A N D MECHANISMS O F SILICICLASTIC INFLUX

Examples of reciprocal sedimentation, wherein siliciclastics invade the basin during lowstands and carbonate sedimentation dominates during highstands (Wilson, 1967; Meissner, 1972), are very common throughout the Phanerozoic (e.g. Palmer & Halley, 1979; Driese & Dott, 1984; Mack & James, 1986; Sonnenfeld & Cross, 1993; Southgate et al., 1993). A less common pattern has been recognized, particularly in Cambrian strata, where the input of siliciclastics into the basin bears no particular relationship to relative sea-level history (Cooper, 1989; Cowan & James, 1993). The Middle Cambrian mixed siliciclasticcarbonate system discussed in this paper appears to be another example of an out-of-phase relationship between siliciclastic input and sea-level history.

The presence of fine siliciclastics in both peritidal facies and sharply overlying deeper subtidal facies of the mixed unit indicates a variable, but relatively continuous, influx of terrigenous material throughout a period of changing accommodation. Well-sorted, fine siliciclastics occur in three stratigraphically adjacent, lithologically distinct facies associations: (i) progradational tidal flats (lower mixed unit-A & B on Fig. 3), (ii) deeper-water facies (upper mixed unit-C-E), and (iii) burrowed shallow-subtidal facies (Banded Mountain Member lower carbonates-F). The initial introduction of inferred aeolian siliciclastics into what was once a pure carbonate system occurred during a relative sea-level fall and thus appears to fit the reciprocal sedimentation model. The continued occurrence of siliciclastic silt and fine sand in overlying deeper water units, however, does not fit with model predictions; siliciclastics evidently continued to invade the platform during a long-term episode of flooding and retrogradation. In the absence of any apparent base-level control, siliciclastic influx onto the platform may have switched on and off in response to processes occurring in the cratonic hinterland, such as source-terrane tectonism or climate change. Alternatively, the presence of siliciclastics may not solely be a function of changes in base level or in source-area dynamics, but rather may be controlled by changes in the rate of carbonate production on the platform (J. Mount, personal communication, 1994). We can use the deeper subtidal facies association (C-E on Fig. 3) to illustrate the problem. Siliciclastics dominate over carbonates in this unit, but is this a result of (i) decreasing rates of carbonate productivity in response to an influx of siliciclastics or (ii) decreasing rates of carbonate productivity with no change in background siliciclastic influx, essentially increasing the relative proportions of siliciclastics? Considering the first scenario, an influx of continentally derived waters rich in nutrients and entrained fine siliciclastics may have partially ‘poisoned’ the carbonate-producing factory (Hallock & Schlager, 1986), forcing a reduction in sediment accumulation rate and allowing for progressive flooding to occur. The abundant siliciclastic silt and fine sand did not shut down the entire carbonate system, however, because intimately interbedded peloidal lime muds had to have been generated somewhere nearby on shallower portions of the platform. Considering the second option, carbonate sedimentation may have decreased because of a rapid relative sea-level rise

1996 International Association of Sedimentologists, Sedimentology, 43, 197-217

Sequence-boundary architecture, Great Basin, U S A 2 1 1

Fig. 9. Schematic model for the influx of siliciclastic sediment onto the Bonanza King carbonate platform during the Ehmaniella biochron. Hypothetical relative sea-level curve shows a higher frequency signal superimposed upon one long-term [ 1-3 Myr?) fall and rise. Numbered events on the relative sea-level curve correspond to portions of the generalized stratigraphic column of the mixed unit.

-

Relative Sea-Level

of silidclastics onto deeper shelf by geostrophic

Eolian transport of fine silidc,astics tidal-flats and shallow shelf

that deepened the platform beyond the photic zone of optimal carbonate production, resulting in a relative increase in the percentage of siliciclastics. Both scenarios are tenable. The important point is that changes in the rate of carbonate productivity may exert an important control on siliciclastic content as do changes in base level or discharge from the source area.

Model of mixed deposition Acknowledging that several of the above factors may be involved, we suggest the following model for the deposition of mixed siliciclastic-carbonate facies in the mixed unit of the Bonanza King Formation of the southern Great Basin (Fig. 9). During long-term progradation and formation of the upper HST and the sequence-bounding exposure breccia within the peritidal facies association, siliciclastics were blown across the exposed, semi-arid carbonate platform by aeolian processes acting upon the unvegetated Cambrian craton. The desert source terrane may have been far-removed from the study area, based on the extremely long distance of transport of aeolian dust from the modern Sahara (Sarnthein & Koopman, 1980). Siliciclastic silts appear to be most prevalent within tidal-flat laminites that cap peritidal cycles, suggesting that siliciclastic deposition occurred primarily during high-frequency (104-105 year) lowstands or stillstands that were superimposed upon the longer term (10" year) relative sea-level fall. During the early phases of the subsequent long-term relative sea-level rise, siliciclastic

Trapping of &'pass of si/iciclastics silicidastics in coastaf across reservoirs exposed plarform

source areas were pushed back toward the craton margins. Migrating deserts on the Cambrian craton continued to supply aeolian dust onto the surrounding shallow shelves (Dalrymple et al., 1985). In some areas along the coast of Middle Cambrian western North America, siliciclastics may have become trapped in low-energy coastal reservoirs along the newly submergent craton edges. With increasingly rapid sea-level rise and associated higher wave energies, these coastal siliciclastics were remobilized by storms, with attendant geostrophic currents transporting these fine siliciclastics for long lateral distances. The siliciclastics of the deeper subtidal facies association, deposited on the low-energy, restricted shelf, may have been supplied to the study area in this fashion. Subtle metre-scale fluctuations in siliciclastic content within the deeper subtidal facies association may reflect a higherfrequency sea-level signal, or perhaps associated fluctuations in carbonate production related to terrigenous influx. As relative sea-level crested and began to fall, 'catch-up' deeper subtidal mixed lithofacies gradually gave way to 'keep-up' shallow-subtidal carbonates and associated intermixed aeolian silts of the Banded Mountain Member above the mixed unit. This model has previously been suggested in various forms for Cambrian rocks throughout North America (Aitken, 1978; Palmer & Halley, 1979; Mount & Rowland, 1981; Chow & James, 1987) and provides a scenario for the continuous influx of siliciclastics into a basin, seemingly out-of-phase with respect to reciprocal sedimentation models hut still controlled by

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212 D. A. Osleger and I. P. Montafiez

long-term sea-level change. In essence, this model emphasizes the effect of total sediment supply on the basic process of reciprocal sedimentation; the greater the supply of available siliciclastics, the more out-of-phase the siliciclastic input may appear relative to sea-level lowstands and highstands. The apparent asynchronous relationship between relative sea-level history and influx of siliciclastics onto the Middle Cambrian platform emphasizes the importance of variable sediment supply as an influence on the composition and architecture of depositional sequences (Schlager, 1991, 1993). Variations in both siliciclastic and carbonate sediment supply were probably not the dominant cause for the architectural arrangement of lithofacies across the Cordilleran passive margin during the Middle Cambrian because the evidence for subaerial exposure along the sequence-bounding disconformity clearly demonstrates relative sea-level fall. However, variations in siliciclastic sediment supply, perhaps acting in response to source-terrane tectonism or possibly to changes in carbonate productivity, exerted an important overprint on the composition of lithofacies within the mixed unit.

DISCUSSION Implications for the interpretation of Cambrian grand cycles Grand cycles, 100-600 m thick and spanning two or more fossil zones, consist of a lower shaly half-cycle that gradationally passes upward into a carbonate half-cycle (Aitken, 1966, 1978). The shaly half-cycle is commonly interpreted to reflect transgressive deepening whereas the carbonate half-cycle represents progressive shoaling fe.g. Aitken, 1978; Markello & Read, 1982; Chow &James,1987; Osleger & Read, 1993; Srinivasan & Walker, 1993) and the natural inclination is to search for sequence boundaries near the top of the carbonate half-cycle. This pattern may hold true for many grand cycles, but it is unlikely, however, that they all can be interpreted as individual depositional sequences. The internal complexity of many grand cycles, both stratigraphically and laterally (Aitken, 1978, 1993), suggests that some grand cycles may exhibit sequence boundaries and systems tracts independent of position within a half-cycle. The mixed unit of this study can be considered a siliciclastic half-cycle sandwiched between carbonate half-cycles of the remaining Banded

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Mountain Member above and Papoose Lake Member below. We have demonstrated that the late highstand systems tract and sequencebounding disconformity occurs in the middle of the mixed unit, rather than at the top of the underlying Papoose Lake Member. Cowan & James (1993) have recognized a similar motif in Upper Cambrian rocks of western Newfoundland. They interpret sequence boundaries beneath desiccated peritidal laminite cycles positioned near the top of the ‘ribbon’half-cycle (‘shaly’half-cycle of Chow & James, 1987) and below the ‘oolitic’ half-cycle (‘carbonate’ half-cycle of Chow & James, 1987). The interpretation of Cowan & James (1993) is entirely different from that of Chow & James (1987) who called the shale-tocarbonate lithological change a transgressive-tohighstand transition. Mount et al. (1991) have likewise identified possible sequence boundaries within the shaly half-cycle of Lower Cambrian grand cycles in the southern Great Basin; they interpret two depositional sequences per grand cycle, each bounded by type 2 sequence boundaries. Similar to this study, Mount et d . (1991) interpreted a transgressive systems tract and overlying maximum flooding surface to occur within the uppermost portion of the shaly half-cycle. Other workers have described internal characteristics of Cambrian grand cycles that contradict the conventional transgressive-shale to regressive-carbonate pattern; this variability was initially recognized by Aitken (1966, 1978, 1993). Cooper (1989) documented large-scale siliciclastic-to-carbonate relationships in Upper Cambrian strata in the southern Great Basin and concluded that differences in lithofacies and metre-scale cyclicity are very minor between the siliciclastic and carbonate half-cycles. He attributed the siliciclastic-to-carbonate transition not to major environmental shifts related to long-term sea-level change but rather to autogenic controls on the terrigenous sediment budget. Spencer & Demicco (1993) likewise identified sedimentary features in the shaly half-cycle of the ArctomysWaterfowl grand cycle of the southern Canadian Rockies that clearly indicate deposition in very shallow-water settings. Aitken (1966) originally identified this grand cycle as ‘aberrant’ relative to other grand cycles whose shaly half-cycles typically exhibit deep-water characteristics. The above discussion emphasizes the point that ‘grand cycles’ and ‘depositional sequences’ should not be considered synonymous units; grand cycles appear to be lithostratigraphic units and would define depositional sequences only if Association of Sedimentologists, Sedimentology, 43, 197-217

Sequence-boundary architecture, Great Basin, U S A 213 sequence boundaries occur near the carbonate-toshale transition at the upper and lower contacts of grand cycles. Stratigraphers working on Cambrian and Ordovician grand cycles should actively search for sequence-bounding disconformities within the lower siliciclastic half-cycle as well as near the top of the carbonate half-cycle, as recognized by Mount et al. (1991), Cowan & James (1993) and this study.

Continent-scale correlations Regional evidence suggests that the lower peritidal portion of the mixed unit and the overlying sequence boundary zone were part of an overall regression in western North America during Ehrnaniella time. Robison (1964, 1976) recognized a westward expansion of ‘inner detrital belt’ (Palmer, 1960; Robison, 1960) calcareous siltstones and silty limestones across what had previously been a pure carbonate platform. Kepper (1972, 1976) also identified the mixed unit (his ‘Unit G’, including two ‘intraformational breccias’) in the Bonanza King and equivalent formations throughout the Great Basin and concluded that the influx of terrigenous sediments and associated emergence of the carbonate platform was evidence for the final phases of a long-term regressive event. Additional evidence is provided by the Whirlwind Formation (and equivalents) of western Utah and eastern Nevada which is an Ehmaniella-age succession composed of mixed siliciclastic-carbonate lithofacies that have been interpreted (Kopaska-Merkel, 1988; Sundberg, 1991) to have been deposited during an overall westward regressive progradation of a peritidal to shallow-subtidal mixed system. On a broader scale, the upper Stephen Formation of the southern Canadian Rockies is coeval with the mixed unit in the Great Basin (Robison, 1976) and records the seaward migration of a low-energy, terrigenous ‘inshore basin’ across underlying carbonate lithofacies (Aitken, 1978, 1993). Near the Cathedral escarpment in eastern British Columbia, a thin interval of the upper member of the Stephen Formation (Ehmaniella burgessensis faunule) disconformably overlies thick massive carbonate of the Cathedral Formation comprising the near-vertical shelf-margin (McIlreath, 1977; Fritz et al., 1991; Aitken, 1993). It is tempting to speculate that this disconformity and the Ehmaniella-age disconformity of the southern Great Basin are coeval features. Further to the north in Alberta, the Stephen Formation grades into fine siliciclastics

of the upper Snake Indian Formation (Mountjoy & Aitken, 1978);the uppermost recessive shale of the Snake Indian Formation exhibits many of the same Iithological features as the mixed unit within the Bonanza King Formation and good exposures may reveal evidence for a disconformity. Correlations of the sequence-bounding disconformity in the mixed unit with coeval strata of the Appalachian orogen are tenuous. This is due to (i) the lower total subsidence rates and consequently thinner Cambrian sections in eastern North America [especially for the western Newfoundland sections that are located on the inner flexural wedge (Cowan & James, 1993)1, and (ii)the inherent lack of high-resolution trilobite control in the restricted lithofacies of the Appalachian passive margin. In the Conasauga intrashelf basin of Tennessee, however, a prominent subaerial exposure surface is developed above subtidal lithofacies of the Craig Limestone Member of the Rogersville Shale (Rankey et al., 1994). Ehmaniella zone trilobites are recognized within the Rogersville both above and below the Craig Limestone Member (Resser, 1938), supporting a correlation with the disconformity in the mixed unit in the southern Great Basin. Rankey et al. (1994) attributed this erosional disconformity to nonlinear pulses in subsidence along the Iapetan margin of North America, but the apparent synchroneity with the mixed unit disconformity suggests that the ultimate control may have been continent-wide in extent. The continent-scale correlations discussed above are speculative primarily because of the inherent imprecision of available trilobite biostratigraphy, which in turn is related to the high provinciality of Cambrian trilobites (Aitken, 1993). Intercontinental-scale correlations will ultimately need to be attempted once further refinement of Middle Cambrian biostratigraphy and sequence stratigraphy is accomplished worldwide. Precise global correlations are the only true method to prove a eustatic control on sequenceboundary formation and, until intercontinental correlations can be demonstrated, the Ehmaniellaage disconformity will remain at best simply a continent-wide lowstand in sea-level. As an alternative to eustasy, epeirogenic movement of the Middle Cambrian continent may be a plausible mechanism to generate sequence-bounding disconformities, such as the Ehmaniella-age disconformity. However, modelling of vertical motion driven by large-scale mantle convection (Gurnis, 1990) shows that epeirogenic movements occur at longer time scales (i07-i08 years) and higher

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amplitudes (tens to hundreds of metres) than suggested by the shorter-term, lower-amplitude Ehmaniella-age disconformity. SUMMARY AND CONCLUSIONS 1 Based on stratigraphic trends, cross-platform facies variations and correlation of individual surfaces across the study area, deposition of the Middle Cambrian mixed unit of the Bonanza King Formation in the southern Great Basin can be interpreted to have occurred in three phases: (i) extensive progradation of mixed peritidal environments culminated in a prolonged episode of subaerial exposure (upper HST and Type 1 sequence boundary); (ii) deepening and retrogradation of mixed deeper-subtidal lithofacies climaxed with the deposition of fissile, fossiliferous shale (TST and maximum flooding): (iii) renewed shallowing and aggradational accumulation was accompanied by a decrease in siliciclastics and a shift to pure carbonate deposition (lower HST). We speculate that deep-water dolomitic siltstones and debris-flow megabreccias recognized in offplatform settings were deposited during subaerial exposure and bypass of the shallow platform. 2 The Middle Cambrian mixed siliciclasticcarbonate system discussed in this paper illustrates an apparent exception to the principle of reciprocal sedimentation, based on an asynchronous relationship between siliciclastic influx and sea-level history. Our model for siliciclastic influx suggests that, during times of lowered sea-level, aeolian processes acting upon the unvegetated Cambrian craton transported fine siliciclastics onto the Cordilleran passive margin where they were intermixed with peritidal and shallow-subtidal carbonates. During times of higher sea-level, coastal siliciclastic reservoirs supplied sediment that was transported by geostrophic currents for long lateral distances along the subrnergent platform. These fine siliciclastics were intermixed and interbedded with peloidal lime mudstones on a low-energy, restricted shelf-lagoon. 3 Identification of a sequence-bounding disconformity within mixed siliciclastic-carbonate lithofacies of this Middle Cambrian ‘shaly half-cycle’ from the southern Great Basin illustrates the point that ‘grand cycles’ and ‘depositional sequences’ should not be considered synonymous units. 4 The peritidal lithofacies of the lower mixed unit of the Bonanza King Formation appear to record a regional regressive event throughout the western US during the Middle Cambrian.

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Broad-scale correlations with the southern Canadian Rockies and the southern Appalachians further suggest that the sequence-bounding disconformity within the mixed unit may be continent-wide in extent, within the limits of available trilobite biostratigraphy. Until accurate global correlations can be attempted, however, the ultimate control on Middle Cambrian sequence deposition will remain speculative. ACKNOWLEDGMENTS

Editorial reviews by C. Kahle, K. Srinivasan, K. Walker, J. F. Read and C. Lehmann improved an early version of the typescript. Sedimentology reviewers J. Mount, G. Plint and C. Cowan are thanked for their significant contributions. This study benefited from field discussions with Gerard Bond, Jim Markello, Rick Sarg and Eric Mountjoy. Information about biostratigraphic control in the Emigrant Formation in the northern Last Chance Range was provided by M. McCollum. Funding for research on the Middle Cambrian was provided by NSF grant EAR9205839, and acknowledgment is made to the donors of The Petroleum Research Fund, administered by the ACS, for partial support of this research.

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Manuscript received 20 September 2 994; revision accepted 20 February 1995

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