1 CHAPTER 1 INTRODUCTION 1.1 Background ...

CHAPTER 1 INTRODUCTION

1.1 Background Mass-transport deposits (MTDs) are increasingly recognized as being important components of Earth’s modern and ancient deepwater stratigraphic record. These deposits, and the processes that create them, may significantly affect human activity as they can present hazards by triggering tsunamis or by destabilizing coastal or subsea infrastructure such as port installations, drilling platforms, seafloor petroleum collection and delivery structures, and telecommunications cables. They also may be significant factors in petroleum exploration, as MTDs may be reservoirs, may be top and lateral seals, or may have acted as paleobathymetric constraints on deposition of overlying reservoir deposits. MTDs have been the subject of considerable study, especially within the past decade. However, there is no widely accepted nomenclature for defining or characterizing MTDs. In this study, a mass-transport deposit (MTD) is defined as sediment deposited by any process that transports it by gravitational force, in which grains generally remain in contact with one another during transport. These processes include creep, sliding (gliding), mudflow, and debris flow, with variable amounts of internal deformation developing within the MTD. Noncohesive flows, such as turbidity currents, concentrated density flows, and hyperconcentrated density flows (Mulder and Alexander, 2001), are not included in this category. The term MTD is considered generic and nonhierarchal. It does not imply the number of separately emplaced deposits that make up the MTD, the number of separate mass movements that may have brought the given sediment volume to its current depositional location, or whether strata not deposited by mass transport are present within (and thus subdividing) the MTD. A masstransport event (MTE) initiates mass transport of a sediment volume and results in emplacement of that volume. An MTE body is an MTD for which final emplacement

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occurred in a single MTE. Polyphase MTD is a volume of sediment that can be demonstrated to have been transported as a whole by more than one MTE. MTD interval refers to a stratigraphic interval composed mostly or entirely of MTE bodies. Much of the recent work on MTDs has focused on the external geometry and/or the internal architecture of MTE bodies (e.g., Lucente and Pini, 2003; Strachan and Alsop, 2006). Figure 1.1a shows a “classic” model of a hypothetical MTD with an updip extensional zone and downdip shortening zone. Other studies have analyzed the paleobathymetry at the top of MTE bodies and its control on sedimentation patterns in overlying strata (e.g., Moore and Shannon, 1991; Moscardelli et al., 2006). Figure 1.1b shows hypothetical sediment gravity flows focused in the extensional zone and diverted away from the shortening zone of the hypothetical MTD shown in Figure 1.1a. Few studies have considered the cumulative effect of stratigraphic stacking of multiple MTE bodies on the ultimate paleobathymetry at the top of such a succession, the effect of inherited bathymetry on the internal architecture of MTE bodies, or the relationship between the distribution of MTDs within a basin and the evolution of the basin (Figure 1.1b).

1.2 Present Study The purpose of this study is to analyze the internal architecture and stratigraphic relationships of MTDs in successions formed in two different deepwater depositional settings (Table 1.1): the Permian (Cisuralian–Guadalupian) Cutoff Formation and the overlying Guadalupian Brushy Canyon Formation (BCF) in the Delaware Basin of west Texas, USA, exposed in the Guadalupe and Delaware Mountains (“Texas study area”); and the Late Cretaceous to possibly earliest Paleogene (late Santonian to possibly Danian) Upper Gosau Subgroup, exposed in the Muttekopf area of the Northern Calcareous Alps, Austria (“Austria study area”). The Texas study area comprises a relatively fine-grained (mostly mud–medium sand) carbonate–siliciclastic depositional system in a large (~35,000 km2)(Baptista Brito, 2004), roughly circular foreland basin that experienced little or no syndepositional tectonism. The Austria study area comprises

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a relatively coarse-grained (mud to >400,000 m3 megaclast), carbonate-rich siliciclastic depositional system in a relatively small (~16 km2), elongate piggyback basin that underwent significant syndepositional tectonism (Ortner, 2001). Chapters 2 and 3 discuss the Texas study area, and chapter 4 discusses the Austria study area. In chapter 2, stacked MTE bodies in the Williams Ranch Member of the Cutoff Formation are described in detail in a ~10 km2 area in the Delaware Mountains representing a basin floor depositional setting. This location was selected because of excellent 3-D exposures and because of local thinning in the lower BCF, suggesting the presence of a paleobathymetric high (Carr and Gardner, 2000). The primary goals of this study were to 1) describe systematically the internal soft-sediment structure and stratigraphy of the MTD; 2) use the distribution of soft-sediment structures and MTD thickness variations to infer the paleobathymetry of the top of the Cutoff Formation and its control on overlying deepwater sedimentation patterns; 3) determine the number, transport directions, and changes over time of MTEs; 4) determine the distribution and character of soft-sediment shortening and/or extensional domains; and 5) determine the relationship between small-scale (millimeter–decimeter) and large-scale (decimeter–10s of meters) soft-sediment deformation. In chapter 3, these relationships are analyzed within the complete exposed thickness of the Cutoff Formation (Shumard, El Centro, and Williams Ranch Members) and the BCF exposed within an approximately 9x1 km area on the Western Escarpment of the Guadalupe Mountains and from reconnaissance and a previous study (Amerman et al., in press; chapter 2). These strata were deposited over an inherited and drowned slope and basin floor depositional profile and are discussed in the context of previous studies (Harris, 1982; 1987; 1988b; Gardner and Sonnenfeld, 1996; Gardner and Borer, 2000; Harris, 2000; Gardner et al., 2003; Amerman et al., in press). The primary goals of this study are to 1) describe the internal stratigraphy and MTD structure of the Cutoff Formation deposited on the drowned slope and basin floor settings; 2) describe sedimentological and stratigraphic changes in the Cutoff Formation from the drowned shelf to basin floor positions and relate these observations to similar changes in the BCF over the same profile; 3) describe and map members of the Cutoff Formation (Shumard, El Centro, and Williams Ranch Members and their informal subdivisions) in the drowned

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slope and basin floor settings, and correlate these units to those described and mapped by Harris (1982; 1987; 1988b; 2000), thereby completing the mapping of these units in the Guadalupe Mountains; 4) describe cyclicity and alternating carbonate–siliciclastic sedimentation (Meissner, 1972) in the Cutoff Formation in the drowned slope and basin floor settings and correlate this cyclicity to that described Sarg and Lehmann (1986b), Sarg et al. (1999), Kerans et al. (1993), Sonnenfeld (1993), Kerans and Fitchen (1995), and Gardner et al. (2003); and 5) refine correlations and summarize the areal distribution of the Cutoff Formation and equivalent strata in outcrop and in the subsurface (King, 1948; King, 1965; Wood, 1965; Wilde and Todd, 1968; Kirkby, 1988; Hart, 1998; Harris et al., 2000; Hart et al., 2000; Lambert et al., 2000; Wardlaw et al., 2000; Baptista Brito, 2004; Amerman et al., in press). Chapter 4 analyzes deepwater deposits of the Upper Gosau Subgroup exposed in the Austria study area that contain numerous MTDs distributed laterally and vertically throughout an almost 1000-m-thick succession. These deposits fill an elongate piggyback basin that was syndepositionally deformed during nappe emplacement at an active, transpressional plate margin (Ortner, 2001; Ortner et al., 2007). Excellent pseudo-3-D exposures over a 2x8 km area likely comprise much of the original depositional extent of the Upper Gosau Subgroup in this area, allowing an interpretation of marginal-to-axial and proximal-to-distal changes within the strata and their contained MTDs. The purpose of this study is to use high-resolution stratigraphic and structural data in combination with internal MTD attribute data to determine the relationship between the 4-D distribution of MTDs and syndepositional tectonic activity in a superbly exposed outcrop example of this basin type. In chapter 5, results from chapters 2, 3, and 4 are synthesized and analyzed with respect to deepwater MTD attributes (dimensions and volume, transport path, runout distance, external geometry, types of internal mass-transport-related soft-sediment structures, coherence of pre-MTE stratal geometries and sedimentary structures, and intensity and internal distribution of soft-sediment deformation) and controls on these attributes (composition of the sediment available for mass transport, rheologic contrasts at stratal boundaries, intensity and frequency of seismic events, pre-MTE bathymetry, basin area and plan-view geometry, sedimentation rates, and rates and kinematics of

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syndepositional tectonism). Implications for greater understanding of deepwater masstransport processes in the context of basin evolution are discussed, as are implications for exploration of petroleum resources. Opportunities for further research are suggested within the two study areas, as well as for mass-transport processes in general.

1.3 Chapter Format

Chapters 2–4 were written in publication-ready format. Thus, some introductory material is repeated in these chapters. Chapter 2 has been accepted as a paper (Amerman et al., in press) in a special publication of the Society for Sedimentary Geology (SEPM); the expected publication date is 2009. Data and figures too detailed to be appropriate for inclusion in the papers intended for publication are provided as appendices on the DVD included with this dissertation. These data and figures include map data for the Texas study area (Appendix A), Cutoff Formation MTD structural data for the Texas study area (Appendix B), Cutoff Formation stratigraphic sections for the Texas study area (Appendix C), Upper Gosau Subgroup stratigraphic section thicknesses and related data for the Austria study area (Appendix E), Upper Gosau Subgroup MTD structural data for the Austria study area (Appendix F), Upper Gosau Subgroup stratigraphic sections for the Austria study area (Appendix G), and unannotated and annotated photopanels of the Austria study area (Appendix H). These appendices are only minimally referenced in the following chapters. Appendix D, comprising notes to Upper Gosau Subgroup thickness data in the Austria study area, is intended for inclusion in published version of chapter 4 and is appropriately referenced.

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Study Area (Subbasin)

Delaware Basin

Regional Basin

Permian Basin.

Mountain Belt Geographic Location

Guadalupe and Delaware Mtns. West Texas, USA. Rio Grande Rift (easternmost Basin and Range) extensional province. Relict foreland basin of Marathon thrust belt with restricted southeasterly access to the Pacific Ocean. ~35,000 km2. Cutoff and Brushy Canyon Fms. Permian. Cutoff Fm.: Cisuralian (Leonardian)– Guadalupian (Roadian), Brushy Canyon Fm.: Guadalupian (Roadian). 270.6 (±0.7) Ma (base of upper Cutoff Fm.)–268.0 (±0.7) Ma (7.6 m above base of Getaway Limestone Mbr. of Cherry Canyon Fm.) (Gradstein et al., 2004). 1.2–4.0 my. >522 m [59 m (upper Cutoff Fm.) + 463 m (BCF, inc. GTS)] (Baptista Brito, 2004). Thickness of lowermost Cherry Canyon Fm. and Getaway Limestone Mbr. not available. Based on subsurface isochore data (Fig. 3.9).

Modern Physiographic Setting Depositional Setting Subbasin Size Units Under Study Age

Absolute Age Depositional Time Interval Maximum Measured Thickness Time-Averaged Sedimentation Accumulation Rate Synsedimentary Tectonism Tectonic Depositional Expression Gross Composition of Basin Fill Overall Character of Clastics Grain Size

Muttekopf Area Northern continental margin of Austroalpine paleoplate. Northern Calcareous Alps. Tirol, western Austria. Inntal Nappe. Piggyback basin above contractional continental margin below southern Penninic Ocean. ~16 km2 present exposure extent. Upper Gosau Subgroup, megasequences 1 and 2. Late Cretaceous (late Santonian–late Maastrichtian). 85.8 (±0.7 m)–83.5 (±0.7 m) (base of megasequence 1) 70.6 (±0.6)–69.6 Ma (top of megasequence 2) (Table 4.1). 11.6–16.9 my. 944m. Based on stratigraphic sections (Figs 4.11a and b).

>13.1–43.5 cm/kyr.

5.6–8.1 cm/kyr.

Little to none. Seismicity may have been trigger for Cutoff mass failures. Weak late-stage tectonic pulses mildly influencing intra- and post-Cutoff bathymetry are a remote possibility. Alternating carbonate–siliciclastic. Sand-rich. Mostly mud (silt in siliciclastics)–medium sand, coarse sand–boulder uncommon.

Significant. Progressive unconformities and provenance changes. Changing bathymetry with depocenter shifts. Seismicity and steepening slope gradients are likely causes of mass failures. Clastics with significant carbonate component. Sand-rich. Mud–megaclast (megaclasts up to >400,000 m3).

Study Area (Subbasin) Sedimentary Architecture Coarse-Grained Debris and Hyperconcentrated Flow Deposits Megabreccias Nature of Synsedimentary Mesoscopic Deformation Scatter of Mass-Transport Indicator Trend Data MTD Fold Types MTD Mesoscopic Features MTD Microstructures (mm to cm Scale)

Delaware Basin Erosional and depositional channels, overbank deposits, lobes.

Muttekopf Area Mostly sheets, erosive channels restricted to far western end of basin.

Mainly on drowned slope in Cutoff Formation.

Common across basin.

At base of Cutoff Fm. and isolated within proximal Brushy Canyon on drowned slope. Mass-transport related and primary sedimentary structures. Uncommon, small offset (few meters) brittle reverse faults may or may not be synsedimentary.

Common within sequences 2B and 2C with clast sizes up to >400,000 m3.

More scattered.

Less scattered.

Mostly gentle upright and isoclinal recumbent folds, with some cascade and S/Z folds. Extensional (slump scars) and shortening (folds and faults). Extensional and shortening. More common than in Muttekopf area. Overprinting is usually contractional over extensional. Often clustered in ZAMs (zones of abundant microstructures).

TABLE 1.1

Mass-transport related, primary sedimentary structures, soft-sediment tectonic folds (Ortner, 2007), and brittle tectonic structures.

Many types. Shortening. Extensional features may have been located outside outcrop area. Extensional and shortening. Less common than in Delaware basin. Clustered in ZAMs (zones of abundant microstructures). Overprinting not observed.

a)

Normal faults

Extensional strain

Slump scar

e Br

a ak

wa

on yz

Symmetric folds

e

Basal detachment

Asymmetric folds

M as

stra

MTD

ns

po

rt d

ire c

tio

Marker bed Tear fault

b)

Sheath folds

n

na actio r t n Co

e l zon

Paths of sediment gravity flows and location of resulting deposits What is the cumulative effect of vertically stacking MTDs?

What is the relationship between the distribution of MTDs within a basin and the evolution of the basin?

What is the effect of inherited bathymetry underlying MTDs?

FIG. 1.1