Origin of Lower Triassic microbialites in mixed carbonate-siliciclastic successions: Ichnology, applied stratigraphy, and the end-Permian mass extinction

Palaeogeography, Palaeoclimatology, Palaeoecology 300 (2011) 158–178

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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o

Origin of Lower Triassic microbialites in mixed carbonate-siliciclastic successions: Ichnology, applied stratigraphy, and the end-Permian mass extinction Scott A. Mata ⁎, David J. Bottjer Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA

a r t i c l e

i n f o

Article history: Received 7 October 2009 Received in revised form 7 December 2010 Accepted 20 December 2010 Available online 27 December 2010 Keywords: Early Triassic Wrinkle structures Trace fossils Stromatolites Biotic recovery

a b s t r a c t Studies aimed at understanding the recovery from the end-Permian mass extinction have focused on the distribution of faunas, trace fossils, and microbialites across depositional environments, but few have examined how these individual ecologic components relate to one another, especially the latter relationship between microbialites and trace fossils, which appear to be mutually exclusive. Microbialites occur throughout Lower Triassic strata primarily in two different forms depending on the depositional system in which they were preserved in. In carbonate-dominated settings, microbialites can commonly take the form of large stromatolitic patch reefs, while in siliciclastic settings microbialites are found primarily as wrinkle structures. This study focuses on the palaeoenvironmental distribution of microbial structures and trace fossils from an uppermost Lower Triassic deposit, the Virgin Limestone Member of the Triassic Moenkopi Formation, which is comprised of mixed carbonate-siliciclastic sedimentary rocks, and is known to contain both stromatolites and wrinkle structures, in addition to trace fossils. Results show that the highest trace fossil diversities are found in lower shoreface environments, while offshore environments contain the lowest diversity. Strata deposited in the offshore transition, separating lower shoreface and offshore environments, contain moderate diversity trace fossil assemblages. Microbialites, in the form of either stromatolites or wrinkle structures, appear to emerge only following transgression, and are commonly found across and following marine flooding surfaces. Wrinkle structures appear to have formed as low-oxygen waters encroached upon nearshore settings, suppressing bioturbation and allowing for microbial mat development. Stromatolites likely formed due to the upwelling of anoxic alkaline waters during transgression, which may have generated a firm substrate for colonization, as well as fostering cementation of the microbialite. While most studies examining the aftermath of the end-Permian mass extinction focus on either carbonate- or siliciclastic-dominated settings, this study reports the environmental distribution of microbialites in both carbonate and siliciclastic facies, and unites them into a single depositional model that accounts for stratigraphic distribution, relation to trace fossils, and the unique environmental conditions present during the aftermath of the end-Permian mass extinction. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The largest mass extinction of the Phanerozoic occurred during the Permian–Triassic transition and has been associated with the development of extensive anaerobic and dysaerobic facies that reflect a bout of anoxia and euxinia that spread across many marine basins during the Late Permian and earliest Triassic (e.g., Wignall and Hallam, 1992; Isozaki, 1997; Wignall and Twitchett, 2002), and lingered throughout the duration of the Early Triassic (e.g., Isozaki, 1997; Woods et al., 1999; Wignall and Twitchett, 2002). The onset and withdrawal of these oceanic stresses during this interval do not appear to have been synchronous globally, and may have waxed and waned in shallow marine environments diachronously from region to

⁎ Corresponding author. Tel.: +1 213 821 6290; fax: +1 213 740 8801. E-mail address: [email protected] (S.A. Mata). 0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2010.12.022

region during the aftermath of the extinction event (e.g., Woods et al., 1999; Wignall and Newton, 2003; Twitchett et al., 2004; Powers and Bottjer, 2007), with additional lingering hypercapnic stresses (e.g., Fraiser and Bottjer, 2007; Knoll et al., 2007). Indeed, anoxia and euxina appear to be major factors controlling the timing and regional distribution of the end-Permian mass extinction in the benthic environment (e.g., Wignall et al., 1996; Wignall and Newton, 2003), with the main phase of die-offs occurring contemporaneously with the onset of anoxic conditions at different times in different regions (Wignall et al., 1996; Wignall and Newton, 2003). These marine stresses also appear to affect the rate of recovery following the biotic crisis, with the rapid recovery of benthic faunas in the absence of environmental stress (Wignall et al., 1998; Twitchett et al., 2004; Beatty et al., 2008; Zonneveld et al., 2010a). Many recent studies have strongly emphasized the importance of depositional setting in analyzing the recovery from the end-Permian mass extinction, finding that not all benthic marine environments

S.A. Mata, D.J. Bottjer / Palaeogeography, Palaeoclimatology, Palaeoecology 300 (2011) 158–178

experience a single synchronous recovery (e.g., Powers and Bottjer, 2007; Beatty et al., 2008; Zonneveld et al., 2010a). The end-Permian mass extinction and its aftermath appear to exhibit a great deal of environmental disparity in terms of recovery, and deep-water offshore benthic environments seem to be the first hit by the extinction (Powers and Bottjer, 2007), as well as the last to recover following the event (Powers and Bottjer, 2007; Beatty et al., 2008; Zonneveld et al., 2010a). The use of biogenic structures as a proxy for the recovery from the end-Permian mass extinction has great utility. This is because, unlike body fossils, biogenic structures such as trace fossils are not susceptible to transport, and can be readily and accurately ascribed to specific depositional environments. In general, with recovery from an extinction event there is an increase in extent and depth of bioturbation, as well as increases in diversity (e.g., Twitchett and Wignall, 1996; Pruss and Bottjer, 2004a; Twitchett and Barras, 2004), with trace fossils occupying deeper tiers (Twitchett, 1999). Indeed, trace fossils are a robust proxy for assessing the timing and rate of recovery from a mass extinction event; however, there are also biogenic structures that can speak to the inhibition of recovery. During the aftermath of the end-Permian mass extinction there is a marked increase in subtidal microbial features such as stromatolitic patch reefs (e.g., Schubert and Bottjer, 1992; Lehrmann, 1999; Pruss and Bottjer, 2004b) and wrinkle structures (Pruss et al., 2004; Mata and Bottjer, 2009a, 2009b), both of which form primarily in the absence of pervasive bioturbation, and thus indicate intervals of environmental stress (Pruss et al., 2004, 2005). Throughout Earth's history, stromatolites form almost exclusively in carbonate-dominated environments, while wrinkle structures are restricted primarily to siliciclastic environments (Hagadorn and Bottjer, 1997). Wrinkle structures are comprised of a series of low-amplitude ridges with interspersed pits and sinuous troughs and are believed to form through primary microbial mat growth (Hagadorn and Bottjer, 1997) or through the liquefaction of a mat during burial (Noffke et al., 2002). Both stromatolitic patch reefs and wrinkle structures are part of a suite of anachronistic facies (sensu Sepkoski et al., 1991) that emerged following the end-Permian mass extinction event (e.g., Pruss et al., 2005, 2006) and are features more characteristic of the poorly-mixed Precambrian and Early Cambrian seafloors than those of the Phanerozoic, which were typically well-mixed and thoroughly bioturbated (Sepkoski et al., 1991). Wrinkle structures have been far less documented than carbonate microbialites in the wake of the endPermian mass extinction, but appear to be the only anachronistic facies found in siliciclastic-dominated environments during this interval (Pruss et al., 2004, 2005; Mata and Bottjer, 2009a, 2009b). Few studies have applied high-resolution palaeoenvironmental analysis to examine the recovery from the end-Permian extinction across depositional environments (e.g., Twitchett and Wignall, 1996; Wignall et al., 1998; Powers and Bottjer, 2007; Beatty et al., 2008; Zonneveld et al., 2007, 2010a, 2010b). Fewer still have documented how the environmental distribution of trace fossils relates to the distribution of microbial structures (i.e., wrinkle structures), and if they are mutually exclusive or can exist contemporaneously within a single environment. The upper Olenekian (Spathian) Virgin Limestone Member of the Triassic Moenkopi Formation is a mixed carbonate-siliciclastic succession that has been well documented with respect to trace fossils (Pruss and Bottjer, 2004a) as well as microbial structures (e.g., Pruss and Bottjer, 2004b; Pruss et al., 2004, 2005; Mata and Bottjer, 2009a, 2009b), and provides an ideal opportunity to examine the relationship between the two, as well as to place each of these types of biogenic structures into a highresolution palaeoenvironmental and sequence stratigraphic context. The purpose of this study is to document the environmental distribution of biogenic structures across depositional environments of the Virgin Limestone to provide insight into the nature of disparity in recovery from the end-Permian mass extinction for different

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depositional environments, as well as examine what sequence stratigraphy reveals about the distribution of these biogenic structures in relation to relative sea level change. 1.1. Previous Lower Triassic studies in the southwestern United States The Early Triassic is a very unusual time in Earth's history and the sedimentary rock record representing this Phanerozoic interval is often referred to as ‘anachronistic’ due to the fact that many of the sedimentary features observed are reminiscent of the poorly-mixed seafloors of the Precambrian and earliest Cambrian (e.g., Pruss et al., 2005, 2006), prior to the development of extensive and deeppenetrating bioturbation. The Virgin Limestone Member of the Moenkopi Formation records the occurrence of these anachronistic facies (sensu Sepkoski et al., 1991) during the Early Triassic, and the extensive nature of these features has been well documented (e.g., Schubert and Bottjer, 1992; Pruss and Bottjer, 2004b; Pruss et al., 2004, 2005, 2006). Offshore facies of the Virgin Limestone have been shown to exhibit many characteristics that hint at a severe limitation of infaunal activity and bioturbation during the Early Triassic (Schubert and Bottjer, 1992; Pruss and Bottjer, 2004a, 2004b; Pruss et al., 2005). Subtidal stromatolites were initially recognized by Schubert and Bottjer (1992) and were subsequently further documented and reinterpreted as stromatolitic patch reefs by Pruss and Bottjer (2004b). These stromatolites have been interpreted to form during an interval of relaxed ecological constraints in which deep infaunal bioturbation was severely reduced, allowing for the development of macroscopic microbial mats (Schubert and Bottjer, 1992; Pruss and Bottjer, 2004b). In addition to microbial patch reefs, the Virgin Limestone also exhibits direct indications of low levels of infaunal bioturbation. Thin-bedded limestones are a very distinctive facies in deeperwater deposits of the Virgin Limestone and are comprised of mm- to cm-thick beds of micritic mudstone that range from parallel laminated, exhibiting no signs of bioturbation, to highly contorted, with bioturbation consisting entirely of the trace fossil Planolites (Pruss et al., 2005). While bioturbation may appear quite pervasive in this facies, it is usually very shallow-penetrating. Studies of the Virgin Limestone have been commonly tied to those of the Lower Triassic Union Wash Formation of east-central California (e.g., Corsetti, 2004; Pruss et al., 2005, 2006; Woods et al., 2007; Woods, 2009), as each unit was likely deposited across different portions of the same carbonate ramp during the latest Early Triassic, making up what has been termed the ‘Moenkopi Platform’ (Woods, 2009). This platform has been interpreted as featuring carbonate seafloor precipitates (Union Wash Formation) in an outer ramp to slope environment and stromatolites (Virgin Limestone) in a middle to inner ramp environment (Woods et al., 1999, 2007; Woods, 2009), both likely the result of carbonate supersaturation across the ramp (Corsetti, 2004; Woods et al., 2007; Woods, 2009). 1.2. Geologic background and study sites The Triassic Moenkopi Formation is an extensive deposit within the southwestern United States that is comprised of continental and marginal marine reds beds that intertongue with marine carbonates, siliciclastics, and evaporites (McKee, 1954; Larson, 1966; Shorb, 1983). It occurs in southern Nevada, southern Utah, and throughout much of Arizona (McKee, 1954). The Moenkopi Formation has been subdivided into six members (McKee, 1954), although only the lower three are known to be Lower Triassic in age (Poborski, 1954; Marenco et al., 2008). These Lower Triassic units include the Timpoweap Member, the Lower Red Member, and the Virgin Limestone Member of the Moenkopi Formation (Fig. 1A). Early Triassic sedimentation in the southwestern United States began near the start of the Olenekian with deposition of the marginal

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A

Nevada and SW Utah

Age

B 120°W

115°W

Spathian

Moenkopi Formation

Lower Triassic

Smithian

Olenekian

40°N Virgin Limestone Member

Nevada

Utah

California

Lower Red Member

35°N Timpoweap Member

Arizona

Pacific Ocean

Hurricane

110°W

Dienerian Griesbachian

Induan

Ute Lost Cabin Spring

15

Las Vegas

N Lake Mead 50 km

Fig. 1. (A) Age and stratigraphy of the Lower Triassic portion of the Moenkopi Formation in southern Nevada and southwestern Utah. Modified from Pruss and Bottjer (2004b). (B) Map showing the locations of measured sections within the studied regions, including the Lost Cabin Spring locality (36°05′01″N, 115°39′15″W) and Ute locality (36°32′57″N, 114°36′50″W) in southern Nevada, and the Hurricane locality (37°08′21″N, 113°14′54″W) in southwestern Utah.

marine Timpoweap Member, which filled in relict valleys and channels that were formed during a depositional hiatus that began in the Middle Permian and lasted through the earliest Triassic (Larson, 1966; Marzolf, 1993). The Timpoweap Member is overlain by the Lower Red Member, which is comprised of interbedded red mudstones and evaporites that have been interpreted to represent sedimentation across an extensive muddy tidal flat (Reif and Slatt, 1979). The Virgin Limestone Member overlies the Lower Red Member and is a marine tongue of interbedded carbonates and siliciclastics that can be found cropping out at many localities within southern Nevada and southwestern Utah. It represents the final marine incursion in the southwestern United States during the Early Triassic and is upper Olenekian (Spathian) in age based upon ammonoid biostratigraphy (Poborski, 1954) and strontium isotope stratigraphy (Marenco et al., 2008). The Virgin Limestone is comprised of a westward-thickening wedge of mixed carbonate-siliciclastic sedimentary rocks that were deposited in environments ranging from supratidal to middle shelf across a gently-dipping, distally-steepened carbonate ramp (Larson, 1966; Shorb, 1983; Stevens et al., 1997; Woods et al., 2007; Woods, 2009). Siliciclastic material was likely sourced from the east, and is found to be less abundant in deeperwater facies to the west (Larson, 1966), which are dominated by thick successions of marine carbonates. For the purposes of this study, 3 localities for the Virgin Limestone were examined (Fig. 1B). Within southern Nevada, two sections were measured. The first locality is the Lost Cabin Spring locality, which is located in the Spring Mountains. It is comprised primarily of carbonates with subsidiary siliciclastics and occupies the most distal ramp position of the three localities (Larson, 1966). The second locality is the Ute locality, which is located along the western edge of the Muddy Mountains. It consists of interbedded carbonate and siliciclastic sedimentary rocks, and was located on a more proximal ramp position than at Lost Cabin Spring (Shorb, 1983). Within southwestern Utah, one section was examined and is located southeast of Hurricane, Utah. This locality records deposition in the

most proximal setting of the ramp and is dominated primarily by siliciclastics and bioclastic carbonates.

2. Methods To assess the palaeoenvironmental distribution of biogenic structures within the Virgin Limestone, stratigraphic sections were measured and detailed data were acquired on sedimentary facies, body fossils, trace fossils, overall levels of bioturbation, microbial features (i.e., wrinkle structures and stromatolites), as well as any anachronistic facies present. For the Ute locality and the Hurricane locality, only partial stratigraphic sections were measured, and were restricted to intervals that contained biogenic structures for analysis or sufficient physical sedimentary structures to confirm the presence or absence of bioturbation. To document the levels of bioturbation, the ichnofabric index was used (Droser and Bottjer, 1986). Ichnofabric indices grade the amount of bioturbation observed within sedimentary layers in cross section, and essentially reflect the degree to which primary physical sedimentary structures were disrupted by bioturbation. Values for the ichnofabric index range from 1 to 6, with 1 representing no bioturbation and 6 indicating complete destruction of primary sedimentary structures by bioturbation. For bedding surfaces, the bedding-plane bioturbation index (Miller and Smail, 1997) was used, and records the degree of bioturbation observed across a single bedding plane. The bedding-plane bioturbation index has values that range from 1 to 5, with 1 indicating no disruption of the bedding surface by bioturbation, and 5 indicating high levels of bioturbation or complete disruption of the bedding plane. Values reported for ichnofabric and bedding-plane bioturbation indices were only taken from well-exposed outcrops and bedding planes where sufficient primary sedimentary structures or sufficient bedding-plane exposure allowed for determination of bioturbation levels. Where these values could not be adequately determined due to an absence of a bioturbate texture and primary sedimentary structures, no data is given.

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3. Sedimentary facies and depositional environments For the studied regions, 12 sedimentary facies were defined for the sections examined (Table 1). These facies were based upon grain size, sedimentary structures, biogenic structures, and body fossils. Closely related facies were then grouped into facies associations, which reflect a similar depositional setting. Four facies associations are defined within this study and are as follows: facies association 1, offshore ramp; facies association 2, lower offshore transition; facies association 3, upper offshore transition and lower shoreface; facies association 4, tidal inlet complex. The offshore transition (sensu Howard and Reineck, 1981)—the zone between fair-weather wave base and storm wave base—was divided into a lower and an upper portion because storm activity appears to be prevalent across a broad area of the carbonate ramp during this interval and therefore this depositional region requires subdivision. This wide range of storm-dominated facies within the Virgin Limestone is likely a result of the very gentle dip of the continent toward the sea during this interval (Larson, 1966; McKee, 1954), resulting in a broader zone of intense storm activity. 3.1. Facies association 1 (offshore ramp): description Facies association 1 (offshore ramp) consists of facies A, which is found at the Lost Cabin Spring and Ute localities; facies B, which is found at the Ute locality; facies C, which is found at the Lost Cabin Spring locality and Ute locality; and facies D, which is found at the Lost Cabin Spring locality. Facies A is comprised of micritic limestones and silty micritic limestones and has been previously described by Pruss et al. (2005) as the ‘thin carbonate bed’ facies of the Virgin Limestone. The sedimentary fabric of this facies is dominated by thin bedding (b2 cm thick) and wispy parallel laminae (b1 cm thick) (Fig. 2A). Bioturbation within this facies is highly variable, and ichnofabric indices range from 1 to 5, although most are usually ii 2–4. Trace fossils are restricted to Planolites. Bedding-plane bioturbation is difficult to ascertain due to the fact that this facies tends not to develop discrete bedding planes. Facies A is commonly associated with several features that are found within the facies or are a modification of it, including intraclastic limestones (i.e., flat pebble conglomerates) and stromatolites. Intraclastic limestones can be found locally developed, usually pinching-out laterally, and individual layers may be up to 50 cm thick. In some instances thin-bedded limestone can be seen to grade seamlessly into intraclastic limestone, and these intraclastic limestones commonly form the uppermost

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portion of packages of facies A. Stromatolites, which are found only at the Lost Cabin Spring locality occur sparsely throughout this facies and are commonly underlain or overlain by thin-bedded micritic limestone (Fig. 2B). Each horizon is typically comprised of several stacked decimeter-scale levels of stratiform to domal mounds with stromatolitic and thrombolitic textures (for descriptions of stromatolite macrostructure and microstructure see Schubert and Bottjer, 1992; Pruss and Bottjer, 2004b; McCoy and Woods, 2009; Woods, 2009). Facies B is comprised of finely laminated shale to massive mudstone. This is differentiated from other shales and mudstones found in other facies due to the fact that it can make up meter or more thick successions of purely mudstone, with no coarse fraction or other sediments present. No body fossils have been found in association with this facies. Bioturbation levels appear to be low, and no apparent ichnofabric has been developed. Facies C consists of bivalve wackestone and packstone that can be found interbedded with shale at the meter scale (Fig. 2C). The shale of this facies is largely obscured due to cover, but can be seen locally beneath resistant ledges that reveal its presence. Wackestones, which occur in equal proportions to shale, are commonly found forming massive beds of 0.5 to 2.5 m thick, with bivalves found in association exhibiting little to no preferred orientation, aside from rare occurrences of cm-scale convex-up shell packstone horizons. The only trace fossil found within this facies is Thalassinoides, which only occurs within a single horizon. Due to the massive nature of this facies coupled with a general absence of trace fossils, it is inconclusive whether the lack of structure is due to bioturbation or other processes. Facies D is comprised of crinoidal packstone and wackestone interbedded with shale. Much like in facies C, the shale is largely obscured by cover, but is exposed in parts beneath resistant ledges. Crinoid remains, assignable to Holocrinus smithi (Schubert et al., 1992; Schubert and Bottjer, 1995), are found as isolated ossicles or 5–10 mm long segments of articulated columnals. Fragmentary bivalve debris is commonly associated with crinoidal debris, but generally makes up a lesser component within beds in which both are found. Most crinoidal limestones are massively bedded; however, in certain horizons, weak low-angle cross-stratification and trough cross-stratification are developed, highlighted by fossil lags along foresets (Fig. 2D). In parts, these crinoidal packstones display a well-developed thickening-upward trend, which is best exemplified within the lowermost meters of exposed section at the Lost Cabin Spring locality in which crinoidal limestones thicken from 5 cm to 50 cm upsection over the span of 150 cm (Fig. 2E). Trace fossils include rare Planolites and Thalassinoides.

Table 1 Sedimentary facies and environmental interpretations of the Virgin Limestone Member of the Moenkopi Formation at Lost Cabin Spring and Ute, Nevada, and Hurricane, Utah. Ar = Arenicolites; At = Asteriacites; As = Asterosoma; Co = Conichnus; Cy = Cylindrichnus; Di = Diplocraterion; Gy = Gyrochorte; He = Helminthopsis; Lo = Lockeia; Pa = Palaeophycus; Pl = Planolites; Rh = Rhizocorallium; Th = Thalassinoides. Facies Lithology

Physical sedimentary structures

Body fossils

Biogenic structures

Depositional environment

A B C D

Micritic mudstone Shale Shale and wackestone Shale and packstone Wackestone and shale

Distal offshore Various environments Proximal offshore Proximal offshore / offshore transition Lower offshore transition

F G H

Sandstone and shale Packstone and shale Sandstone and silty shale Sandstone

Bivalve fragments None observed Bivalve fragments Crinoids: Holocrinus; bivalves; echinoid spines Bivalves: Eumorphotis, Promyalina, Leptochondria None observed Bivalves: Eumorphotis, Promyalina None observed

Pl, stromatolites None observed Th Pl, Th

E

Thin bedding, massive bedding, intraclasts Planar laminae, massive bedding Massive bedding, randomly oriented shells Trough cross-stratification and low-angle cross-stratification Graded bedding, convex-up shells

I J K L

Bioclastic silty sandstone Bioclastic sandstone Sandy packstone and grainstone

Cross-lamination, gutter casts Graded bedding, convex-up shells Hummocky cross-stratification and planar lamination Hummocky and swaley cross-stratification, None observed planar lamination, and oscillation ripples Low-angle laminae and oscillation ripples Crinoids: Holocrinus Trough and tabular cross-stratification Sigmoidal bedding, mud drapes, bi-directional cross-stratification

Shell fragments Shell fragments

None observed At, Pl None observed Ar, Di, He, Pl, Rh, wrinkle structures Ar, At, Di, Gy, Lo, Pl, Rh

Lower offshore transition Upper offshore transition Upper offshore transition Lower shoreface

As, Co, Cy, Pa, Pl, Rh

Lower shoreface

None observed None observed

Upper shoreface Tidal inlet

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Fig. 2. Outcrop photographs of facies within facies association 1. (A) Thin-bedded micritic mudstone of facies A, displaying cm-scale bedding, Lost Cabin Spring, 7.7 m. Coin is 2.4 cm in diameter. (B) Stromatolite patch reef overlying thin-bedded micritic mudstone of facies A, Lost Cabin Spring, 50.7–56.6 m. Handle of hammer marks the boundary between underlying thin-bedded limestone and overlying stromatolites. Hammer is 30 cm long. (C) Bioclastic wackestone of facies C displaying a horizon of loosely packed bivalve shells that are generally oriented convex-up (marked by arrows) grading into packstone near the top, Lost Cabin Spring, 166.5 m. Pen is 15 cm long. (D) Trough cross-stratified crinoidal packstone of facies D, Lost Cabin Spring, 41.2–41.5 m. (E) Thickening-upward succession of crinoidal packstones (facies D) at the base of the measured Lost Cabin Spring section, 0–1.5 m. Hammer is 30 cm long.

3.2. Facies association 1 (offshore ramp): interpretation Both facies A and facies B reveal evidence to support deposition within an offshore ramp, below mean storm wave base, or just at the lowest reaches of major storm activity. The thin-bedded micritic limestones of facies A were likely deposited out of suspension in a quiet-water environment. The presence of intraclastic limestones within this facies suggests that erosive bottom currents or wave energies were strong enough to rip-up the seafloor (e.g., Sepkoski, 1982; Sepkoski et al., 1991; Mount and Kidder, 1993), which may indicate that certain portions of this facies were deposited within the reach of major storms. Indeed, the transition from pure micritic mudstone to intraclastic limestone may represent the shoaling upward of this facies into the zone of storm reworking, as has been suggested previously by Osleger and Read (1991) for similar

successions of intraclastic limestone. Facies A is interpreted to have been deposited primarily in a distal offshore environment, largely below the reach of most storm wave and current activity. Facies B exhibits no evidence for storm reworking, but due to its ubiquitous nature throughout most other facies, it likely has several interpretations. In its most basic form, it represents quiet-water sedimentation below fair-weather wave base, with the sediment predominantly settling out of suspension. The common juxtaposition of shale with storm-influenced sediments such as in facies C and D, as well as deeper-water sediments such as facies A, suggests that this facies was likely deposited in a wide range of depositional environments from well below storm wave base to just below fair-weather wave base, in which fine-grained siliciclastic sediment could accumulate appreciably. Facies C contains little evidence that it was influenced by fairweather or storm conditions. The presence of very loosely packed and

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weakly concentrated shell debris suggests that this facies was deposited largely below the influence of most major storm activity, in which deposition of mud from suspension predominated (e.g., Calvet and Tucker, 1988; Burchette and Wright, 1992). Rare occurrences of convex-up shell lags indicate that this facies may have been subject to only weak, possibly storm-generated, currents that would have oriented the shells within these thin concentrations into a stable convex-up orientation (e.g., Seilacher and Aigner, 1991). This facies is therefore interpreted to have been deposited below storm wave base, but still under the influence of weak storm-generated currents; environmentally, this would be defined as proximal offshore. Facies D exhibits indications of being influenced by either storm waves or storm-generated currents. The shales that are interbedded with and often part crinoidal packstones suggest that this facies was deposited below fair-weather wave base, due to the accumulation of significant amounts of fine mud, while the cross-stratified nature of the packstones is likely due to reworking by storms. Crinoid grains are wellknown for their high intraskeletal porosity (e.g., Savarese et al., 1997), and therefore have a higher potential for entrainment and transport over other larger and denser skeletal particles. While the presence of trough cross-stratification and low-angle cross-stratification might suggest a depositional environment close to fair-weather wave base, the common association of this facies with thin-bedded micritic mudstones of facies A and shales of facies B imply that this is likely a deep-water facies, closer to storm wave base. The association of trough cross-stratified crinoidal limestones with micritic mudstones and shales is a commonly documented facies throughout the Phanerozoic, and has been interpreted previously as being deposited in an environment below fair-weather wave base with the crinoidal limestone being largely transported and reworked by storm activity, and in some cases strong tidal activity, forming a parautochthonous accumulation (Aigner, 1985; Ausich and Sevastopulo, 1994; Neumeier, 1998 Jach, 2005). In terms of depositional environment, these crinoidal packstones were likely deposited below fair-weather wave base, but within the distal reaches of major storm activity, therefore deposited within an offshore transition to proximal offshore environment. 3.3. Facies association 2 (lower offshore transition): description Facies association 2 consists of facies E and F which are found at the Ute locality. Facies E is comprised of siltstone and shale with 1–10 cmthick bioclastic packstone and wackestone interbeds (Fig. 3A). Bioclastic horizons are massively bedded with bivalves making up the dominant body fossils. Some beds exhibit normal grading whereby bivalve shells transition from randomly oriented near the base to convex-up at the uppermost bedding surface. Bivalves within this facies are comprised dominantly of the genera Eumorphotis and Promyalina. Gutter casts can be found in association with this facies; however, they are rare. Shale and siltstone interbeds make up more than half of this facies, and are typically laminated to massively bedded. No trace fossils have been found in association with this facies, and most bioclastic beds have very sharp upper and lower surfaces. Facies F is comprised of siltstone and shale with 1–4 cm-thick very fine sandstone beds (Fig. 3B). Shale makes up the dominant fraction of this facies, and is laminated or massively bedded. Sandstone beds are cross-laminated to massive, and are commonly associated with 3– 5 cm-thick gutter casts that can be found in isolation within the shale, or semi-attached to overlying sandstone beds. Bivalves are infrequent within this facies; however, when found, they occur as convex-up shells atop sandstone layers. Trace fossils are found only rarely within facies F, and are comprised solely of Asteriacites and Planolites. 3.4. Facies association 2 (lower offshore transition): interpretation Facies E and F are similar in their stratigraphic character and sedimentary structures, but differ primarily in composition, with

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Fig. 3. Outcrop photographs of facies within facies association 2. (A) Facies E comprised of bivalve wackestones interbedded at the decimeter-scale with shale, Ute, 15.5– 16.5 m. (B) Facies F consisting of shale with interbedded very fine sandstone layers and lenses with isolated gutter casts, Ute, 0.7–1.7 m. The hammer is 24 cm long.

facies E containing carbonate interbeds and facies F containing siliciclastic interbeds. Facies E and F are interpreted to have been deposited within the lower offshore transition, just above mean storm wave base. The presence of alternating beds of shale and crosslaminated sandstone beds or graded bioclastic beds suggests an environment subject to fluctuating hydrodynamic energy conditions— typical for the zone between fair-weather wave base and storm wave base. For facies E, it is interpreted that the shale settled out of suspension during fair-weather conditions, while the graded beds represent distal tempestites that were deposited under storm conditions. Distal tempestites are often graded, and can be found in association with gutter casts that form during highly erosional storm phases (Aigner, 1982, 1985). Bivalve grains within each tempestite are chaotically oriented and densely packed, suggestive of rapid deposition, while the convex-up bivalve shell accumulations atop bedding planes were likely reworked and oriented convex-up, into a stable configuration, during the waning stages of the storm (Seilacher and Aigner, 1991). Facies F likely also represents the product of fluctuating hydrodynamic energy conditions, with the shale being deposited during fair-weather conditions, punctuated by periodic storm deposition of distal cross-laminated sandy storm layers by tractional currents. 3.5. Facies association 3 (upper offshore transition and lower shoreface): description Facies association 3 consists of facies G, H, I, and J. Facies G, which is found at the Ute locality, is comprised of amalgamated beds of bivalve packstone with occasional siltstone and shale partings (Fig. 4A). Individual packstone beds are 10–20 cm thick with partings ranging from mm-scale to several centimeters in thickness. Body fossils are

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Fig. 4. Outcrop photographs of facies within facies association 3. (A) Amalgamated bivalve packstone of facies G, Ute, 6 m. Hammer is 24 cm long. (B) Facies H comprised of interbedded hummocky cross-stratified very fine sandstone beds with erosive bases and shale, Ute, 17.5–18.5 m. The two beds marked within this photograph—lower (L) and upper (U)—both contain wrinkle structures on their upper bedding surfaces. Upper bed is shown in Fig. 10F. Hammer is 24 cm long. (C) Facies I displaying a thickening- and coarsening upward succession of fine and very fine sandstone with hummocky cross-stratification, Lost Cabin Spring, 83.2–85.3 m. Hammer is 30 cm long. (D) Mottled and heavily bioturbated bioclastic sandy siltstone of facies J, Hurricane, 4.7 m; Pa = Palaeophycus.

restricted primarily to bivalves. Within each packstone bed, bivalves are chaotically oriented; however, bedding-plane exposures reveal the presence of convex-up shell accumulations at the interface between beds. Like facies E, bivalves within this facies are primarily Eumorphotis and Promyalina. Symmetrical wave ripples and interference ripples can also be seen atop bedding surfaces. Trace fossils have not been found in association with this facies. Facies H is found at the Ute and Hurricane localities, and is comprised of siltstone and shale interbedded in equal proportions with isolated fine to very fine sandstone beds that range in thickness from 5 to 15 cm (Fig. 4B). Sandstones beds are hummocky crossstratified to parallel laminated, and are frequently erosive-based. Symmetrical wave ripples are commonly developed on exposed bedding surfaces and cap hummocky cross-stratified sandstone beds. Siltstone and shale are massively bedded to weakly laminated, and are often obscured by local weathering. Trace fossils found within this facies include Arenicolites, Diplocraterion, Helminthopsis, Planolites, and Rhizocorallium. Bioturbation levels are variable in cross section and ichnofabric indices range from ii 1 to 3, usually being concentrated near the uppermost portion of each bed. Along bedding surfaces, bioturbation appears to be far more intense, with bpbi averaging 2 to 4. Wrinkle structures are found exclusively within this facies, and have not been found in direct association with discrete trace fossils or bioturbation. Wrinkle structures are comprised of a series of low-amplitude ridges with interspersed pits and troughs, and have been found to extend across certain bedding surfaces for at least 10 m. Facies I is found at the Lost Cabin Spring and Ute localities, and is comprised of amalgamated fine to very fine sandstones with occasional mm-scale shale partings (Fig. 4C). Individual beds range in thickness from 10 to 15 cm and are usually truncated at the top and erosional along the base. Sandstone beds are swaley to hummocky

cross-stratified with symmetrical wave ripples in parts and occasional quasi-planar lamination (sensu Arnott, 1993). Upper bedding surfaces are often planar, or display symmetrical wave ripples or interference ripples. Trace fossils within this facies include Arenicolites, Asteriacites, Diplocraterion, Gyrochorte, Lockeia, Planolites, and Rhizocorallium. Planolites and Arenicolites are by far the most common and usually dominate the ichnofabric within this facies. Ichnofabric indices range from 1 to 4, and bpbi is usually 2 to 3. Facies J is found at the Hurricane locality, and is composed of massively to weakly bedded bioclastic silty very fine sandstone and sandy siltstone (Fig. 4D). This facies is observed to coarsen upward in parts from bioclastic sandy siltstone near the base to very fine bioclastic sandstone near the top, and is associated with cm-scale siltstone partings. Low-angle parallel laminae and wave ripples are weakly developed near the tops of these coarsening upward sequences, and are highlighted in portions where bioturbation is low. Crinoid ossicles and bivalves make up the dominant bioclasts and are highly abraded and fragmentary, usually no larger than 1–2 mm long. Trace fossils usually overprint a very mottled bioturbate texture. Common ichnogenera within this facies include Asterosoma, Conichnus, Cylindrichnus, Palaeophycus, Planolites, and Rhizocorallium. Beddingplane bioturbation is difficult to ascertain within this facies due to a lack of exposed bedding surfaces; however, ichnofabric indices typically range from ii 3 to 5. 3.6. Facies association 3 (upper offshore transition and lower shoreface): interpretation Most strata within facies association 3 are characterized by features typical of storm deposition within proximal settings. Facies G is comprised of amalgamated bivalve packstones in which bivalves are oriented chaotically within the bed, but are convex-up on bedding

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layers deposited within a lower shoreface environment. This is based upon the presence of alternating very fine sandstone and siltstone, coupled with the presence of low-angle laminae and ripples (Reinson, 1984; Arnott, 1993). The high degree of bioturbation and low preservation of physical sedimentary structures are similar to that of a low-energy lower shoreface (MacEachern and Pemberton, 1992). In addition, this facies is commonly overlain in shoaling-upward successions by facies deposited within upper shoreface and tidal inlet environments of facies association 4, which provides strong evidence for a lower shoreface interpretation (e.g., Reinson et al., 1988; Cheel and Leckie, 1990). 3.7. Facies association 4 (tidal inlet complex): description

Fig. 5. Outcrop photographs of facies within facies association 4. (A) Tabular and weakly trough cross-stratified bedsets of facies K, with foresets exhibiting tangential bases, Hurricane, 3.4–3.8 m. Staff is in decimeters. (B) Thick unit of bioclastic grainstone of facies L, with sigmoidal bedding, directly overlying facies J, Hurricane, 5.0–6.2 m. Staff is in decimeters.

surfaces. This arrangement is typical for bioclastic tempestites in which shells are deposited rapidly as a storm wanes—resulting in a weak orientation within the bed—and bivalves remaining at the surface are reworked by fair-weather waves and currents that re-orient the shells into a stable convex-up position (Emery, 1968; Clifton, 1971; Seilacher and Aigner, 1991). The thickness and frequency of these bioclastic storm beds, coupled with the fact that they are commonly amalgamated, is suggestive of proximal tempestites, likely from a local carbonate bank or shoal (Aigner, 1982, 1985). Facies H and I exhibit a similar lithology of hummocky, swaley, and quasi-planar laminated fine quartz sandstones, and differ primarily in the proportion of siltstone and shale to sandstone. Hummocky crossstratification and swaley cross-stratification are generated by strong oscillatory flow or by combined flow during storm events (e.g., Duke et al., 1991; Dumas and Arnott, 2006). These stratification types can form in a range of environments (e.g., Dott and Bourgeois, 1982; Yang et al., 2006), but are commonly preserved within the lower shoreface and in the offshore transition (e.g., Reinson, 1984; Dumas and Arnott, 2006). Based upon the frequency, thickness, and amalgamated nature of sandstone beds within facies I, coupled with the prevalence of symmetrical wave ripples, this facies is interpreted to represent proximal sandy tempestites (e.g., Aigner and Reineck, 1982) preserved within the lower shoreface. Facies H consists of approximately equal amounts of sandstone and shale, suggestive of an environment dominated more by fair-weather deposition than by storm activity, and is interpreted to represent proximal sandy tempestites interbedded with silts and muds deposited during fair-weather conditions (Aigner and Reineck, 1982). The environment of deposition for facies H would be the upper offshore transition (i.e., closer to fair-weather wave base than storm wave base). Facies J differs from others of this facies association in that many of the original primary physical sedimentary structures have been obliterated by pervasive bioturbation, leaving a strongly mottled fabric. This facies is interpreted to represent proximal sandy storm

Facies association 4 consists of Facies K and L, both of which are found at the Hurricane locality. Facies K is comprised of bioclastic packstone and grainstone with trough and tabular cross-stratification (Fig. 5A). Bioclasts are predominantly bivalves. Foresets commonly exhibit tangential bases, and individual bedsets are approximately 10 cm thick. Bioclasts are commonly fragmentary and are noted to highlight foresets. No apparent trace fossils have been found in association with this facies. At the Hurricane locality where this facies is found, it is underlain by heavily bioturbated bioclastic sandy siltstone of facies J. Facies L is comprised of bioclastic packstone and grainstone that form sigmoidal bedsets that occur as single sets or stacked sets of two (Fig. 5B). When stacked, bedsets thin upward, with the underlying bedset usually being thicker than the overlying bedset. Mud laminae occur frequently throughout this facies and commonly drape foresets, as well as the upper bounding surfaces of cross-strata. Individual bedsets are approximately 10–60 cm thick. Bioclasts are common within this facies and occur as lags along foresets. Bi-directional lowangle cross-stratification is found associated with this facies in a single horizon, but is not a distinguishing feature. This facies always overlies bioturbated bioclastic sandy siltstone of facies J. 3.8. Facies association 4 (tidal inlet complex): interpretation Facies K and L are strongly linked together and exhibit numerous indicators of strong wave and tidal influence. Facies I is comprised of trough cross-stratified bioclastic grainstone. Trough cross-stratification typically forms through the migration of dunal bedforms, and in the marine realm such bedforms are a common occurrence in the highenergy build-up and surf zone of modern coastlines in which shoaling waves lead to the development of subaqueous lunate dunes (Clifton et al., 1971). Based upon this, facies K is interpreted to have been deposited in the upper shoreface, which is generally defined as the zone between wave build-up (sensu Clifton et al., 1971) and mean low tide (e.g., MacEachern and Pemberton, 1992). The absence of trace fossils in this facies might also be a testament to the high-energy nature of the environment. Facies L is comprised of sigmoidal bedsets of bioclastic grainstones. Sigmoidal bedding is a unique form of cross-stratification that is comprised of tabular units of sigmoid-shaped lenses of cross-stratified to cross-laminated sediments that are bounded and separated from adjacent lenses by mud laminae (Visser, 1980; Kreisa and Moiola, 1986; Shanley et al., 1992). Similar structures have been observed in modern tidal inlets and estuaries that are dominated by tidal processes, and are termed tidal bundles (Visser, 1980; Kreisa and Moiola, 1986; Shanley et al., 1992). Sigmoidal bedding forms due to the alternation of asymmetric ebb and flood tidal currents separated by slack water conditions (Visser, 1980). The presence of these sigmoidal bedsets in facies L suggests that it was deposited in an environment characterized by strong tidal currents, but with low wave energy. Based upon these criteria, facies L was likely deposited within a tidal inlet.

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4. Biogenic structure distribution 4.1. Lost Cabin Spring, Nevada At the Lost Cabin Spring locality, 8 shallowing-upward flooding surface-bound units were defined based upon facies relationships and ichnofabric index trends, which will be described below (Fig. 6). These units may equate to parasequences, but in the absence of a means of regional correlation this cannot be clearly demonstrated. The boundary separating an underlying unit from an overlying unit was drawn at the marine flooding surface—a stratigraphic surface in which there is an abrupt increase in water depth upsection (Van Wagoner et al., 1988)—that marks the change from the regressive to the transgressive portion of a succession. Within carbonate facies at Lost Cabin Spring, some units are often floored by 1–4 m of strata exhibiting a minor transgressive phase prior to the main shallowing trend shown by the remainder of the unit. These transgressive phases are marked by retrogradational facies stacking patterns or by progressive decreases in ichnofabric index. To accommodate these transgressive phases, this study follows the methodology suggested by Arnott (1995) and combines these thin transgressive deposits with the overlying regressive deposits to form a single unit comprised of a transgressive–regressive couplet. In these instances the maximum flooding depth represented within the unit would likely be several meters above the marine flooding surface. Unit 1 (0 to 1.5 m above the base) is the first exposed at the Lost Cabin Spring locality, and strata below are largely covered. No biogenic structures have been found in this unit. Unit 2 (1.5 to 43.6 m above the base) is floored by ~15.5 m of thinbedded micritic mudstone with an ichnofabric dominated by the trace fossil Planolites. There is a general trend in ichnofabric index developed within this facies, proceeding from ii = 2 near the base, generally increasing upsection to ii = 3, followed by ii = 4, and reaches a maximum of ii = 5 near the top portion of this facies, although some variability is present. The next 5 m is largely covered, but contains isolated beds of bivalve wackestone and crinoid wackestone. Thin-bedded micritic mudstone dominate the next 12 m with ii = 5 in the lower portion and ii = 3 making up the majority of the upper portion; once again, with some variability. Unit 3 (43.6 to 85.3 m above the base) begins above the last significant crinoidal packstone bed of unit 2. The lowermost 9.5 m of unit 3, where exposed, consist of thin-bedded micritic mudstone, with a minor 40 cm-thick crinoid and bivalve wackestone. Ichnofabric index shows a general decreasing trend throughout this interval and proceeds from ii = 4 to ii = 2. The next 3.5 meters is made up entirely of stacked stromatolitic domes that were originally described by Schubert and Bottjer (1992) as level-bottom stromatolites and subsequently re-interpreted as stromatolitic microbial patch reefs (Figs. 2B, 7A) (Pruss and Bottjer, 2004b). These stromatolites are capped by 20 cm of shale that fills the antecedent topography of the uppermost domes. The next 23 m is comprised of wavy laminated intraclastic wackestone with thin bedding (cm-scale) occurring in parts that transitions upward into crinoidal and bivalve packstone with echinoid spines scattered throughout (Moffat and Bottjer, 1999). The top of the unit is capped by 2.1 m of amalgamated fine sandstone beds that exhibit hummocky cross-stratification and symmetrical wave ripples. Ichnofabric index is low throughout this sandstone, ii = 2, with the trace fossils Gyrochorte and Planolites being found. Unit 4 (85.3 to 102.7 m above base) starts at the uppermost exposed surface of the sandstone capping unit 3. The lowermost stratum of unit 4 consists of a 20 cm-thick intraclastic wackestone

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that is welded to the uppermost sandstone of unit 3. The next 3.7 m is largely covered except for an isolated 40 cm-thick bed of crinoidal and bivalve packstone from 86.8 to 87.2 m above the base of the section. Above this interval is a 60 cm-thick stromatolite horizon that is comprised of isolated stromatolitic domes (Fig. 7B). A 70 cm-thick layer of thin-bedded micritic mudstone with ii = 3 separates this underlying stromatolite horizon from a second horizon containing stratiform to weakly domal stromatolites that form a continuous stromatolitic mudstone layer (Fig. 7C). Overlying these stromatolites are several meter-scale beds of bivalve wackestone. The unit is capped by 2.4 m of thin-bedded micritic mudstone with an ichnofabric index averaging ii = 3 that leads upward into crinoid and bivalve wackestone at the top. The unit ends at the base of a covered interval that is placed within unit 5. Unit 5 (102.7 to 121.7 m above base) begins at the aforementioned covered interval, but the first exposed strata of the unit are comprised of a 1.7 m-thick layer of isolated stromatolitic domes (Fig. 7D). Overlying this stromatolite horizon is a 3.5 m succession of thinbedded mudstone with a general decreasing ichnofabric index trend that transitions from ii = 4 to ii = 2 upsection. The next 8.6 meters are largely covered except for a 40 cm-thick intraclastic wackestone horizon at 114 m above the base of the section and a massively bedded crinoid and bivalve wackestone at 115 m above the base of the section. The unit is capped by a 30 cm-thick hummocky crossstratified medium-grained sandstone bed (Fig. 7E). This sandstone contains no evident trace fossils, has an ii = 1, and displays wrinkle structures on its uppermost bedding surface (Fig. 7F). Unit 6 (121.7 to 146.7 m above base) starts at the uppermost surface of the sandstone capping unit 5, directly overlying the wrinkle structure horizon. The unit begins with an 80 cm-thick bed of crinoidal wackestone that is massively bedded. Overlying this bed is a 7.9 m-thick succession of thin-bedded micritic mudstone with an average ichnofabric index of 2 in the lower 6 m, with an increasing trend from ii = 3 to ii = 4 in the upper 1.9 m. This is overlain by a 1.2 m bed of intraclastic limestone. The unit is capped by 12.9 m of crinoid and bivalve packstone that occurs as 30-60 cm-thick beds. Unit 7 (146.7 to 166.5 m above base) begins directly above the uppermost crinoidal packstone of unit 6. The lowermost 7.6 m of the unit is comprised of thin-bedded mudstone with a general increasing trend in ichnofabric index, transitioning gradually from ii = 2 to ii = 4 throughout the succession. Several intraclastic wackestone beds are found within this interval and appear to thicken with each occurrence upsection. At approximately 152.5 m above the base of the section, it is observed that thin-bedded mudstone with an ii = 4 transitions seamlessly into intraclastic wackestone (Fig. 8). Overlying this succession is 12.2 m containing several discrete beds of bioclastic limestone containing bivalves and crinoids. Beds are 40–80 cm in thickness and transition upward from wackestones to packstones. The lowermost of these beds contains the trace fossil Thalassinoides. Unit 8 (166.5–172.0 m above base) is the final unit exposed at the Lost Cabin Spring locality and is floored by a 2.9 m-thick covered interval that separates it from the underlying unit. The first exposed strata of the unit are comprised of 1.2 m of thin-bedded micritic mudstone that transitions upward from ii = 2 to ii = 3. This interval is overlain by a densely packed intraclastic packstone horizon. The uppermost 1.7 m of the unit consists of thin-bedded mudstone with an ii = 4. The section is not well exposed above this interval at this locality. Based upon the units that are found at the Lost Cabin Spring locality, a composite idealized shallowing-upward succession can be inferred. The facies representing the deepest-water environment is

Fig. 6. Measured section of the Virgin Limestone at Lost Cabin Spring; FS = flooding surface; ii = ichnofabric index. When not specified, all bioturbation recorded within facies A, thin-bedded micritic mudstone, consists of Planolites. Arrows mark stratigraphic intervals in which transgressive deposits are present. Boxed numbers label units 1–8. Flooding surfaces (FS) are dashed when approximated due to cover.

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Fig. 7. Biogenic structures from the Virgin Limestone at Lost Cabin Spring. (A) Close-up of one domal stromatolite from the stromatolitic patch reef found near the base of unit 3, 54.0 m (see Fig. 2B). (B) Outcrop photograph of domal stromatolites near the base of unit 4, 89.2–89.8 m. These stromatolites are from the lower of the two horizons found within this unit. (C) Second horizon of stromatolites near the base of unit 4, 90.5–91.2 m. Stromatolites are predominantly stratiform. Pen is 15 cm long. (D) Hand sample containing domal stromatolites from the base of unit 5, 108 m. (E) Hummocky cross-stratified medium sandstone that caps unit 5. Wrinkle structures are preserved on the uppermost surface of this bed, 121.4–121.7 m. Hammer is 30 cm long. (F) Wrinkle structures from the uppermost surface of hummocky cross-stratified sandstone seen in Fig. 7E, 121.7 m. These wrinkle structures are found across the marine flooding surface that separates units 5 and 6.

facies A, which is comprised of thin-bedded micritic mudstone. The typical manifestation of this facies within units at Lost Cabin Spring is a succession of thin-bedded micritic mudstone dominated by the trace fossil Planolites, with ichnofabric index increasing up section from ii = 2 to ii = 4. This facies with the given ichnofabric index trend is found at the base of units 2, 6, 7, and 8. Facies A then commonly transitions into intraclastic wackestones and packstones upsection, as seen in units 3, 5, 6, 7, and 8. Above these intraclastic limestone intervals are typically bioclastic wackestones and packstones dominated by crinoidal elements, with secondary bivalves, as displayed in units 3, 5, 6, and 7. The facies deposited in the shallowest environment at the Lost Cabin Spring locality is facies G, which is comprised of amalgamated beds of hummocky cross-stratified and wave-rippled sandstones; this facies caps units 3 and 5. Two types of microbialites are found within the units at the Lost Cabin Spring locality, stromatolites and wrinkle structures, and each

has stratigraphic significance. Four stratigraphic horizons within the units are observed to contain stromatolites, and each occurs near the base of a unit in association with thin-bedded micritic mudstone of facies A. In unit 3 (41.7 m thick), a 3.5 m-thick interval of stromatolites is found 9.5 meters above the defined marine flooding surface at the base of the unit. In unit 4 (17.4 m thick), a 60 cm-thick and a 70 cm-thick stromatolite horizon are found at 3.9 m and 5.2 m, respectively, above the defined marine flooding surface. In unit 5 (19.0 m thick), a 1.4 m-thick stromatolite unit occurs 5.2 m above the defined marine flooding surface, and represents the first exposed strata above this stratal surface. Wrinkle structures are found only once within the examined units and are found on the uppermost bedding plane of a 30 cm-thick hummocky cross-stratified medium-grained sandstone bed that caps unit 5. These wrinkle structures occur across the marine flooding surface that separates unit 5 from unit 6.

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Fig. 8. (A) Intraclastic limestone horizon occurring within a succession of thin-bedded micritic mudstone of facies A with an ichnofabric index of 4, 151.4–152.2 m. Note the irregular shape of the intraclasts, marked by arrows. Pen marks the base of the intraclastic zone. (B) Close-up on the boundary between thin-bedded micritic mudstone and intraclastic limestone. Intraclasts closely resemble thin-beds of the underlying lithology, but can also form irregular clast morphologies, marked by arrow.

4.2. Ute, Nevada

4.3. Hurricane, Utah

At the Ute locality, trace fossils are found within several intervals of the section examined (Fig. 9). The lowermost ~6.5 m of section is comprised of meter-scale alternations of distal sandy tempestites of facies F with amalgamated proximal bioclastic tempestites of facies G. The trace fossils Asteriacites and Planolites are found within this interval and appear in facies F at a single horizon. From 6.5 to 13.7 m above the base of the section there is a general thickening and coarsening upward succession transitioning from hummocky crossstratified fine and very fine sandstones interbedded with shale (facies H) of the offshore transition to amalgamated units of hummocky cross-stratified fine sandstone (facies I) of the lower shoreface near the top. Trace fossils change throughout this interval with facies H containing only Arenicolites and Planolites, while facies I displays Arenicolites, Asteriacites, Diplocraterion, Gyrochorte, Lockeia, Planolites, and Rhizocorallium (Fig. 10A–D). Ichnofabric index is generally low throughout this interval, generally ii = 1 or ii = 2. Bedding- plane bioturbation reaches levels as high as 4, but is on average 3. From 13.7 to 17.5 m above the base of the section is a series of thinning upward bioclastic tempestites of facies C that is capped by a firmground horizon marked by sharp-walled Thalassinoides burrows that were passively filled with skeletal debris from above (Fig. 10E). Directly overlying this firmground horizon is a thickening-upward succession of non-amalgamated hummocky cross-stratified very fine sandy tempestites interbedded with shale (facies H) that transitions into amalgamated sandstones (facies I) of the lower shoreface. The lowermost two sandstone beds of this succession, found within facies H, contain wrinkle structures (Fig. 10F), while the overlying amalgamated units of facies I contain generally monospecific occurrences of Asteriacites lumbricalis, an ophiuroid resting trace (e.g., Twitchett and Wignall, 1996), with Planolites being found rarely (Fig. 10G). Ichnofabric index is 1 throughout most of this interval, while bedding-plane bioturbation index averages 2. The uppermost ~ 11 m of the measured section is comprised of thin-bedded micritic mudstone with an ichnofabric displaying only the trace fossil Planolites. Despite being dominated by Planolites, the resultant ichnofabric index is extremely low; ii = 1 throughout most of the interval, with ichnofabric index reaching 2, and rarely 3, within the lower ~8 m.

The Virgin Limestone at the Hurricane locality is comprised of bioclastic sandy siltstones and grainstones that form several shallowing-upward successions (Fig. 11). Each is floored by heavily bioturbated bioclastic sandy siltstone (facies J) that is overlain by either trough cross-stratified and tabular cross-stratified bioclastic grainstone (facies K), interpreted as upper shoreface, or is overlain by bioclastic grainstone that is arranged into sigmoidal bedsets (facies L) interpreted as tidal inlet deposits. Trace fossils are found exclusively within the lower shoreface deposits at this locality and are absent from the upper shoreface and tidal inlet deposits. The first shallowing-upward succession, from the base of the section to 3.8 m above the base, is floored by bioturbated sandstones of the lower shoreface containing Asterosoma, Conichnus, Cylindrichnus, Palaeophycus, Planolites, and Rhizocorallium, with an ichnofabric index of 3 (Fig. 12A–C). The second shallowing-upward succession, from 3.8 to 4.8 m above the base of the section, contains the trace fossils Asterosoma, Palaeophycus, Planolites, and Rhizocorallium within its lower shoreface deposits, resulting in an ichnofabric index of 5 (Fig. 4D). The third shallowing-upward unit, from 4.8 to 8.4 m above the base of the section, contains lower shoreface deposits that are moderately bioturbated, ii = 3, and display the trace fossils Planolites and Palaeophycus overprinting a generally mottled fabric. The remainder of the section is comprised of mottled lower shoreface deposits that transition upward into interbedded shale and hummocky cross-stratified very fine sandy tempestites of the offshore transition (facies H). Lower shoreface deposits are moderately bioturbated, ii = 3, with only Planolites being found. Overlying sandy tempestites, interbedded with shale, contain the trace fossils Arenicolites, Diplocraterion, Helminthopsis, Planolites, and Rhizocorallium, with resulting ichnofabric indices of 3 to 4 and bedding-plane bioturbation indices of 2 to 4 (Fig. 12D–G). 5. Discussion 5.1. Ramp architecture The majority of the ramp architecture present during deposition of the Virgin Limestone is revealed through shallowing-upward, flooding

F

23 22

E

21

F

LSF

44

H

43

E 15

A

42

Lower offshore transition

16

41 40

14

39 38 37

11 Upper offshore transition

36

10 9

H

8 7 6

G B

4

G

3 2

34 33

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1 G

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32 31

B

30

Lower offshore transition

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B

35

29 28

A

27 F 26 C

Biogenic Structures

Lithology Massive mudstone Thin-bedded mudstone with Planolites

LOT

I

12

C

Lower shoreface

13

6 5 4 3 2 1 5 4 3 2 1

46

I

Firmground

bpbi

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UOT

18

ii

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45

19

Mudstone

Depositional Environment

PS WS

Facies

VC C M F VF Silt GS

Shale

49

20

17

Meters

6 5 4 3 2 1 5 4 3 2 1

Lithology

Offshore

24

bpbi

Biogenic Structures

PS WS Mudstone

ii

Lower offshore transition

GS

Shale

Facies

VC C M F VF Silt

Meters

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Bioclastic limestone

Sandstone and siltstone

Shale

Bivalves

Arenicolites

Planolites

Asteriacites

Rhizocorallium

Diplocraterion

Thalassinoides

Gyrochorte

Wrinkle Structures

Lockeia Fig. 9. Measured section of the Virgin Limestone at Ute. When not specified, all bioturbation recorded within facies A, thin-bedded micritic mudstone, consists of Planolites. LOT = lower offshore transition; UOT = upper offshore transition; LSF = lower shoreface; ii = ichnofabric index; bpbi = bedding-plane bioturbation index.

surface-bound units at the Lost Cabin Spring locality, and is supplemented by additional nearshore facies associations present at the Ute and Hurricane localities. Units at the Lost Cabin Spring locality are dominantly shoaling-upward successions, with only a few being floored by minor transgressive phases, such as in units 3, 4, and 6. Shoaling-upward successions reveal the facies distribution on a ramp or shelf, and begin with the most distal facies and trend upward into proximal facies. The presence of thin-bedded micritic mudstone within the lowermost portion of all units at the Lost Cabin Spring locality

suggests that it is the most distal facies of the Virgin Limestone. The common occurrence of an increasing trend in ichnofabric index upsection within these intervals may suggest an increase in oxygenation from a poorly-oxygenated distal expression of the thinbedded micritic mudstone facies to a better-oxygenated proximal expression of this facies, a phenomenon documented for similar deep ramp parasequences deposited adjacent to an anoxic basin (e.g. Calvet and Tucker, 1988). The most proximal expression of the thin-bedded micritic mudstone facies typically contains an ichnofabric index of

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Fig. 10. Biogenic structures from the Virgin Limestone at Ute. (A–E) Examples of some trace fossils from lower shoreface deposits below the wrinkle structure horizons: (A) Asteriacites lumbricalis on the top of a bed in concave epirelief; (B) Lockeia and Planolites preserved on the sole of a bed in convex hyporelief; Lo = Lockeia; Pl = Planolites; (C) Rhizocorallium; and (D) Arenicolites and Planolites; Ar = Arencolites; Pl = Planolites; 12.7–13.7 m. (E) Sole of bed showing branching Thalassinoides passively filled with skeletal debris, marking firmground below wrinkle structure horizons, 17.2 m. (F) Wrinkle structures from bed directly overlying firmground horizon; lower bed in Fig. 4B, 17.7 m. (G) Astericiates lumbricalis in lower shoreface deposits, preserved in convex hyporelief, directly overlying wrinkle structure horizons, 18.5 m.

4–5, and it is this ichnofabric that is most commonly associated with intraclastic limestones, as is the case in units 6, 7, and 8. Indeed, it has even been observed that micritic mudstone with an ichnofabric index of 4 can transition seamlessly into intraclastic limestone, as was noted for unit 7 (Fig. 8). The transition from thin-bedded micritic mudstone to intraclastic limestone may have palaeoenvironmental significance, and it has been suggested previously that such a transition from mudstone to intraclastic limestones in shoaling-upward successions may mark the shallowing from below, to within a zone of storm reworking (Osleger and Read, 1991). The highly bioturbated nature of the proximal expression of the micritic mudstone facies may have even facilitated the production of intraclasts during storm events. Lee and Kim (1992) noted a close association between nodular limestones and intraclastic limestones in storm-influenced Ordovician strata, and found that many of the intraclasts present resembled isolated nodules from underlying limestones. For strata of the Virgin Limestone, it may be that the bioturbated and contorted nature of the thin-bedded micritic mudstones, coupled with early cementation at the seafloor, may have made the sediment far more easy to erode than it would have been had it been purely parallel laminated with no irregularities to exploit.

While the presence of bioturbation in association with intraclasts may appear at odds with conventional interpretations of subtidal intraclast generation (e.g., Sepkoski, 1982; Sepkoski et al., 1991; Wignall and Twitchett, 1999), the very shallow-penetrating nature of burrowing, possibly coupled with early cementation at the seafloor due to elevated alkalinity, may have facilitated the requisite firming of the seafloor necessary for intraclast formation. A mixture of bivalve wackestones and crinoidal packstones commonly overly intraclastic limestones within the observed units, such as in units 3, 5, 6, and 7; however, thin-bedded micritic mudstone can also transition directly into bioclastic wackestones and packstones, as in units 2 and 4. Both the crinoidal packstones and bivalve wackestones exhibit little evidence for storm reworking, although crinoidal limestones have been observed to exhibit trough crossstratification and low-angle cross-stratification very infrequently, and only within thick amalgamated beds. Each of these facies may be just within the reach of major storms, and were likely deposited only slightly landward of the intraclastic limestones of facies A. They may also represent an equivalent weakly storm-influenced depositional setting to intraclastic limestones in successions where thin-bedded micritic mudstone transitions directly into bioclastic limestones.

GS

PS

WS

Shale

Facies

VC C M F VF Silt

Meters

Lithology

Mudstone

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ii

bpbi

6 5 4 3 2 1 5 4 3 2 1

19 18 17

H

Offshore transition

J

Lower shoreface

L

Tidal inlet

J

Lower shoreface

16

K J

USF LSF

L

Tidal inlet

J

LSF

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

Lithology Sandstone and siltstone

Bioclastic sandstone and siltstone

Bioclastic limestone

Biogenic Structures Arenicolites

Helminthopsis

Asterosoma

Planolites

Conichnus

Palaeophycus

Cylindrichnus

Rhizocorallium

Diplocraterion

Shale Bivalves Fig. 11. Measured section of the Virgin Limestone at Hurricane. LSF = lower shoreface; USF = upper shoreface; ii = ichnofabric index; bpbi = bedding-plane bioturbation index.

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Fig. 12. Biogenic structures from the Virgin Limestone at Hurricane. (A–C) Examples of some trace fossils found associated with bioclastic sandy siltstones of facies J: (A) Asterosoma, marked by arrow, 3.0 m; (B) Cylindrichnus; and (C) Rhizocorallium and Conichnus; Rh = Rhizocorallium; Co = Conichnus; 0–1.0 m. (D–G) Examples of some trace fossils found associated with proximal sandy tempestites of facies H: (D) Outcrop photograph of Helminthopsis in cross section, 9.8 m. (E) Polished slab showing dark mud-cored burrows of Helminthopsis. (F) Outcrop photograph of Diplocraterion preserved in cross section within a hummocky cross-stratified sandy tempestite, 13.3 m. (G) Bedding-plane view showing several Diplocraterion burrows within a sandy tempestite. Coin is 2.4 cm in diameter.

Lying landward of the bioclastic limestones of facies C and D is likely interbedded sandstones and shales of facies F and H, both of which represent deposition in the offshore transition—between fairweather wave base and mean storm wave base. While these facies are not observed at the Lost Cabin Spring locality, they are present at Ute and Hurricane, and likely exist at Lost Cabin Spring based upon facies models of modern storm-influenced coastlines (e.g., Howard and Reineck, 1981; Aigner and Reineck, 1982), but are possibly obscured. At the Ute locality, heterolithic units of hummocky cross-stratified sandstone interbedded with shale (facies C and D) commonly coarsen and thicken upward into amalgamated units of hummocky crossstratified sandstone of facies I, which is interpreted as a lower shoreface environment deposited above fair-weather wave base. At the Hurricane locality, lower shoreface deposits of facies J were likely deposited under lower hydrodynamic energy conditions, indicated by an increase in trace fossil preservation and a decrease in grain size and physical sedimentary structure preservation relative to facies I at Ute, but the environmental setting is likely equivalent. The shallowest depositional environments of the Virgin Limestone are represented by facies K and L at the Hurricane locality, and represent deposition in upper shoreface and tidal inlet environments, respectively. These facies always overlie extensively bioturbated lower shoreface deposits of facies J, and commonly occur as sharpbased units. Facies L is the only facies of the Virgin Limestone that shows evidence for tidal influence, which is emphasized by sigmoidal tidal bundles and bi-directional cross-stratification. 5.2. Origin of microbialites and relation to trace fossil distribution With the interpreted ramp architecture, there appears to be a strong disparity in trace fossil diversity between nearshore—primarily shoreface—environments and offshore environments. The offshore environment—below mean storm wave base—typified by thin-bedded micritic mudstone, is dominated by monospecific occurrences of Planolites that generate variable ichnofabric indices, usually from 2 to 4, with ii = 1 and ii = 5 only occurring rarely. The shoreface, specifically the lower shoreface, exhibits much higher trace fossil diversities shown at both the Ute and Hurricane localities, with sediments of the offshore transition—separating the shoreface from offshore—showing fairly low to moderate trace fossil diversities of

usually 2 or 3 ichnogenera. This distribution of high nearshore diversity contrasted with low offshore diversity is similar to that found for faunal assemblages (Wignall et al., 1998) and trace fossils (Beatty et al., 2008; Zonneveld et al., 2010a) for lowermost Triassic strata deposited in the immediate aftermath of the end-Permian mass extinction. Wignall et al. (1998) examined offshore to nearshore uppermost Permian to lowermost Triassic strata in Spitsbergen, deposited in the Boreal Ocean, and noted that earliest Triassic recovery of marine faunas and ichnofossils appears to begin first at high palaeolatitudes, specifically occurring within nearshore sandstone bodies. Beatty et al. (2008) and Zonneveld et al. (2010a) examined Lower Triassic (Induan) trace fossil assemblages from the northwest margin of Pangaea and found that diverse suites of trace fossils, along with thoroughly bioturbated ichnofabrics, occurred primarily in environments that experienced frequent to periodic wave reworking—the lower shoreface and offshore transition—while those environments deeper than storm wave base were characterized generally by laminated, unbioturbated shales. This distribution suggests that shallow-water environments in which frequent to periodic wave reworking kept the overlying water column oxygenated were able to host a diverse infauna during the post-extinction interval, while many offshore environments, which likely experienced persistent oxygen stress, are essentially devoid of benthic macroscopic life (Beatty et al., 2008). Beatty et al. (2008) have termed these shallow-water environments characterized by diverse and thoroughly bioturbated ichnofabrics the “habitable zone”, which corresponds generally to the lower shoreface and offshore transition. Surprisingly, lower shoreface and offshore transitional environments examined by Beatty et al. (2008) and Zonneveld et al. (2010a) from the Induan of the northwestern margin of Pangaea display much higher diversities than those of the upper Olenekian Virgin Limestone—deposited along the western margin of Pangaea at a lower palaeolatitude. This is despite the fact that those strata deposited at higher palaeolatitudes represent intervals much closer to the extinction interval, whereas those of Virgin Limestone were deposited at almost the very end of the Early Triassic (Marenco et al., 2008), several million years after the actual mass extinction event. This distribution gives strong credence to the notion that benthic recovery may have palaeolatitudinal constraints, with higher palaeolatitudes

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recovering quicker than lower palaeolatitudes (Wignall et al., 1998; Beatty et al., 2008; Zonneveld et al., 2010a). Indeed, it appears as though the moderately diverse lower shoreface trace fossil assemblages of the Virgin Limestone represent a stressed expression of the habitable zone, further evidenced by the fact that microbialites such as wrinkle structures are commonly found in offshore transitional facies of the Virgin Limestone (Mata and Bottjer, 2009a, 2009b), but are not reported from strata deposited along the northwestern margin of Pangaea. Wrinkle structures are, however, present at other lower palaeolatitude Lower Triassic localities, such as within northern Iraq, northern Italy, and northern Pakistan (Mata and Bottjer, 2009b). Previous studies of wrinkle structures from the Virgin Limestone and other Lower Triassic successions have shown that wrinkle structures are typically found within strata comprised of shale interbedded with sandy proximal tempestites of the offshore transition (Mata and Bottjer, 2009a, 2009b), and are also found across marine flooding surfaces separating underlying sandstone of the lower shoreface from overlying shale and sandstone of the offshore transition (Mata and Bottjer, 2009a, 2009b). In nearly all instances wrinkle structures are preserved on an underlying resistant sandstone bed and are capped by less resistant shale or mudstone. The presence of wrinkle structures across marine flooding surfaces indicates that while they may be found at the top of a succession of shoreface sandstones—found below the marine flooding surface—they likely formed under deeper-water conditions better approximated by strata overlying the marine flooding surface. These marine flooding surfaces have been interpreted to represent rapid deepening events during which the lower shoreface was submerged below fair-weather wave base (Mata and Bottjer, 2009a). Stromatolites are found at several discrete horizons within the thin-bedded micritic mudstone facies at the Lost Cabin Spring locality and in all instances occur in strata almost directly overlying a marine flooding surface. The fact that they don't occur directly across the flooding surface may be due to the presence of transgressive deposits at the base of units 3, 4, and possibly 5. Evidence for the transgressive origin of these strata are the retrogradational stacking pattern of facies found in units 3 and 4 in which facies become progressively deeper environmentally, such as the thinning upward crinoidal beds between the top of unit 2 and the lower portion of unit 3, and the progressively decreasing trends in ichnofabric index within the lower portions of units 3 and 5. These decreasing ichnofabric index trends are likely marking a progressive shift from the proximal, more bioturbated, expression to the distal, less bioturbated, expression of the thinbedded micritic mudstone facies that is likely tracking oxygenation. While the thin-bedded micritic mudstone facies is quite ubiquitous at this locality, it is interesting that only a few horizons actually contain stromatolites. Stromatolites therefore do not appear to represent an ever-present feature on the seafloor and their presence might be explained by some short-lived process that allowed for their emergence and persistence for a brief interval prior to their disappearance and a return to deposition of thin-bedded micritic mudstone. A similar inference can be made for wrinkle structures, as they are not an ever-present feature, even in the facies in which they are most commonly found. Marine flooding surfaces result from relative sea level rise, which typically leads to clastic sediment starvation on the shelves due to the trapping of river-derived sediment in newly formed nearshore accommodation space, such as estuaries (Galloway, 1989; Cattaneo and Steel, 2003). These periods would likely slow sedimentation rates and possibly improve water clarity due to less sediment being present within the system. Increased water clarity might allow photoautotrophic microbial mats to occupy depths that would be otherwise below the euphotic zone during more turbid conditions, while reduced inputs of sediment would allow for the microbial mats to build upward, outpacing rates of sedimentation. An additional requirement that needs to be met to allow for the development of

microbial mats is the suppression of pervasive bioturbation, which, unless curbed, can be detrimental to mat growth and preservation (e.g., Fenchel, 1998). The Lower Triassic sedimentary rock record is often characterized by anaerobic and dysaerobic facies that reflect offshore anoxic and euxinic conditions that periodically impinged upon the continental shelf, likely during intervals of transgression (Wignall and Hallam, 1992; Wignall and Twitchett, 1996, 2002; Wignall et al., 2005), and their development can be tied to flooding surfaces and the intervals directly following them (Wignall and Hallam, 1992; Mata and Bottjer, 2009a). In the case of wrinkle structures—which are commonly found at the interface between lower shoreface sandstones and interbedded shales and sandstones of the offshore transition—marine flooding likely led to the drowning of the shoreface below fair-weather wave base (Mata and Bottjer, 2009a). Wrinkle structures would then be preserved on underlying relict shoreface sands, but would have actually developed during the interval in which the shoreface sediments were submerged below fair-weather wave base, and thus below the zone of constant wave reworking. This would have allowed for the development of low-oxygen conditions and a suppression of bioturbation. Indeed, it has even been shown that biomarkers for anaerobic photoautotrophic bacteria (i.e., green sulfur bacteria), suggesting photic zone euxina, have been found within Induan lower shoreface deposits, and occur within intervals of unbioturbated sediments, ii = 1, that are interstratified with discrete thoroughly bioturbated intervals, ii = 5 (Hays et al., 2007; Beatty et al., 2008). These unbioturbated intervals associated with biomarkers for green sulfur bacteria have been interpreted as being deposited during upward excursions of the euxinic chemocline into the environment (Beatty et al., 2008). A similar mechanism might explain the common association of wrinkle structures with marine flooding surfaces, whereby the establishment of dysoxic conditions coincides with transgression and an upward expansion of low-oxygen waters. While previous studies have attributed Early Triassic upwelling of anoxic water to climatic fluctuations (e.g., Algeo et al., 2007), the common association of these microbialites with flooding surfaces implies in this instance the movement of the chemocline with transgression, rather than an upward chemocline expansion at a steady relative sea level. Wrinkle structures are known to be strongly tied to marine flooding surfaces; however, they also can occur following transgressive deposits. At the Ute locality, a series of thinning upward bioclastic tempestites—interpreted as a deepening event (Mata and Bottjer, 2009a)—underlies the two beds containing wrinkle structures and separates them from a diverse lower shoreface trace fossil assemblage. Contrasted with the low diversity lower shoreface trace fossil assemblage that overlies the wrinkle structure horizons—containing monospecific beds of Asteriacites with rare Planolites horizons—it's possible that the deepening event that resulted in wrinkle structure formation also had devastating effects on lower shoreface trace fossil diversity. The connection of stromatolites with marine flooding surfaces may be similar to that of wrinkle structures, but certain aspects are likely different. Clastic sediment starvation associated with marine flooding would certainly aid in microbialite growth, but there does not appear to be a complete suppression of bioturbation in thin-bedded micritic mudstone intervals that bracket-in stromatolite horizons. This persistence of low levels of bioturbation in deep-water facies of the Virgin Limestone may be due to the tracemakers associated with the thin-bedded micritic mudstone facies being more tolerant of dysoxic conditions, while those tracemakers of the shoreface were adapted to higher oxygen levels and could not tolerate the periodic upward excursions of low-oxygen waters associated with marine flooding. Perhaps the two most necessary parameters that govern stromatolite growth are a firm substrate for colonization, which has been shown to be nearly essential in modern stromatolites (Ginsburg and

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Planavsky, 2008), as well as increased alkalinity in order to allow for upward propagation and cementation of the microbial structure (e.g., Reid et al., 2000). Upwelling of alkaline waters associated with transgression might allow for both of these requirements, and would explain why the stromatolites of the Virgin Limestone are found in association with marine flooding surfaces. A similar mechanism has been proposed previously by Kershaw et al. (2007) for earliest Triassic microbialites (ETMs) primarily from the Tethys Ocean in which upwelling of anoxic carbonate-rich waters led to the development of extensive cemented microbialites. A similar mechanism seems warranted for the uppermost Olenekian stromatolites of the Virgin Limestone, especially considering that the Panthalassic Ocean in which they formed appears to have experienced periodic anoxic conditions through the late Olenekian and into the Anisian (Woods et al., 1999; Takahashi et al., 2009; Wignall et al., 2010). Also, previous studies have suggested a strong connection between the stromatolites of the Virgin Limestone and a period of increased carbonate saturation represented by seafloor carbonate precipitates from the upper member of the Union Wash Formation (Corsetti, 2004; Pruss and Bottjer, 2004b; Pruss et al., 2005; Woods et al., 2007; Woods, 2009), which likely formed the outer ramp to slope equivalent of the Virgin Limestone in the southwestern United States (Woods et al., 2007; Woods, 2009). The additional component that this study adds to the mechanism outlined by Kershaw et al. (2007) is the association of microbialite horizons with marine flooding surfaces, connecting transgressive events with potential periods of upwelling.

A

Tidal Inlet/ Upper Shoreface

Lower Shoreface

175

6. Conclusions All the lines of evidence presented herein suggest that there is a strong disparity in recovery between nearshore and offshore environments following the end-Permian mass extinction, a conclusion reached by previous studies (e.g., Wignall et al., 1998; Powers and Bottjer, 2007; Beatty et al., 2008; Zonneveld et al., 2010a), with nearshore environments recovering much quicker than offshore environments—for a given region—presumably in the absence of environmental stress. This present study shows that for uppermost Lower Triassic strata of the southwestern United States, trace fossil diversity is highest (4–7 ichnogenera per assemblage; 11 total observed) in facies interpreted as a lower shoreface environment, with moderate diversity (2–3 ichnogenera per assemblage; 6 total observed) in offshore transitional environments—between fairweather wave base and storm wave base—and the lowest diversity (mostly 1 ichnogenus per assemblage; 2 total observed) in offshore environments—below mean storm wave base (Fig. 13A). This appears to be the biogenic structure distribution during steady relative sea level; however, directly following sea level rise new biogenic structures emerge that are distinctly microbial (Fig. 13B). In Lower Triassic strata of the southwestern United States wrinkle structures are found primarily on marine flooding surfaces separating lower shoreface sandstones from interbedded sandstone and shale of the offshore transition, being preserved at the interface between underlying sandstone and overlying shale (Mata and Bottjer, 2009a,

Offshore Transition

Proximal Offshore

Distal Offshore

High diversity Moderate diversity Low diversity

B

Tidal Inlet/ Upper Shoreface

Lower Shoreface

Offshore Transition

Proximal Offshore

Wrinkle structures across marine flooding surface

Distal Offshore

Stromatolites in strata overlying marine flooding surface

Upwelling of alkaline water

Facies A: thin-bedded micritic mudstone

Facies F & H: sandstone and shale

Facies A: intraclastic limestone

Facies I & J: sandstone

Facies C & D: bioclastic limestone

Facies K & L: bioclastic grainstone

Fig. 13. Biogenic structure model for the Virgin Limestone. (A) Under steady sea level conditions the highest trace fossil diversity is found in lower shoreface deposits, with moderate diversity in the offshore transition, and the lowest trace fossil diversity in offshore environments. (B) Following sea level rise, wrinkle structures develop across the marine flooding surface separating lower shoreface sandstones from offshore transition shales and sandstones, as well as on overlying proximal sandy tempestites. Stromatolites emerge several meters above the marine flooding surface, which in instances where the units are floored by transgressive deposits, may correspond to maximum flooding prior to shallowing within the unit. Lower shoreface and tidal inlet/upper shoreface are above fair-weather wave base; offshore transition is between fair-weather wave base and storm wave base; proximal offshore is below storm wave base, but influenced by storm-generated currents; distal offshore is largely not affected by storm waves or storm-generated currents.

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2009b). These wrinkle structures likely emerged as the lower shoreface was drowned below fair-weather wave base, allowing for the development of dysoxic conditions and a suppression of bioturbation; this in addition to transgression-associated sediment starvation on the shelves, which may have fostered microbial mat growth. Stromatolites are found within thin-bedded micritic mudstone and occur several meters above defined marine flooding surfaces, but not necessarily across them. Stromatolites are predominantly associated with a wide range of ichnofabric indices, but never ii = 1, which might be an expected requirement for the growth of microbial mats; one that appears to be a certainty for wrinkle structures. Rather, stromatolites may have developed in response to changes in alkalinity, with transgression allowing for the upwelling of deep carbonate-rich waters and fostering the precipitation of calcium carbonate, perhaps in addition to generating a firm, cemented seafloor for colonization. The common occurrence of stromatolites within the proximal, more extensively bioturbated, expression of the thinbedded micritic mudstone facies may possibly be due to phototrophic requirements, leaving the distal, less bioturbated, expression of the facies more accommodating of microbial mats, but possibly out of the euphotic zone. This study supports the “habitable zone” model of Beatty et al. (2008) and Zonneveld et al. (2010a), with the highest trace fossil diversities occurring in lower shoreface and offshore transition environments. Trace fossils in these environments, however, appear to represent a stressed expression of the habitable zone, a point further supported by the presence of wrinkle structures in offshore transition environments, as well as across marine flooding surfaces in which the lower shoreface was drowned below fair-weather wave base; features not found in similar environments at higher palaeolatitudes during the Early Triassic. This uppermost Lower Triassic assemblage of biogenic structures reveals that although shoreface and offshore transition environments of the southwestern United States appear to be well recovered, contemporaneous offshore environments exhibit persistent environmental stress and little to no recovery up through the very end of the Early Triassic. Further studies aimed at reconciling the relationship between microbialite and trace fossil distributions, as well as benthic body fossils and small metazoan bioherms and reefs (Pruss et al., 2007; Griffin et al., 2010; Pietsch et al., 2010), will further add to the picture of the seemingly disparate recovery observed across depositional environments during the aftermath of the end-Permian mass extinction.

Acknowledgements We thank Sarah Greene, Kathleen Ritterbush, and Lydia Tackett for field assistance and thought-provoking discussions. We also thank Stephen Kershaw, Paul Wignall, and John-Paul Zonneveld for their helpful comments on this manuscript. This research was supported by grants from the American Association of Petroleum Geologists, the Geological Society of America, the International Association of Sedimentologists, and the University of Southern California.

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