A paleotropical carbonate-dominated archive of carboniferous icehouse dynamics, Bird Spring Fm., Southern Great Basin, USA

Palaeogeography, Palaeoclimatology, Palaeoecology 329-330 (2012) 64–82

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A paleotropical carbonate-dominated archive of carboniferous icehouse dynamics, Bird Spring Fm., Southern Great Basin, USA Lauren G. Martin a,⁎, Isabel P. Montañez a, James W. Bishop b a b

University of California, Davis, Geology Dept., 1 Shields Ave, Davis, CA 95616, USA Chevron North America Exploration & Production Company, 1500 Louisiana Street, Houston, TX 77002, USA

a r t i c l e

i n f o

Article history: Received 15 October 2011 Received in revised form 9 February 2012 Accepted 14 February 2012 Available online 19 February 2012 Keywords: Carboniferous sea level Paleotropical paleoclimate Carbonate sequence stratigraphy Glacioeustacy Carboniferous glaciation

a b s t r a c t Much of our far-field knowledge of glaciation and climate during the late Paleozoic ice age is built on decades of study of mixed carbonate–siliciclastic cyclothemic successions from paleotropical Euramerica. Far fewer carbonate-dominated successions have been studied despite their high sensitivity to changes in accommodation space and environmental conditions. Here we present a sequence stratigraphic framework for the carbonatedominated Bird Spring Formation, southern Great Basin, U.S.A. and infer from it a relative sea-level history for the latest Mississippian through latest Pennsylvanian. Correlation of four successions across the paleoplatform documents changes in the lithofacies composition of meter-scale cycles, their cycle stacking patterns, and cycle bounding surfaces that record substantial variation in relative sea-level at several temporal scales during the Pennsylvanian. Ten genetic sequences bounded by intervals of restricted peritidal lithofacies, karst, terra rossa, rooted caliche and/or Protosols are recognized. Within sequences, 126 parasequences stack into parasequence sets that in turn define lowstand, transgressive and highstand systems tracts. The stratigraphic cycle stacking patterns and across-platform architecture of systems tracts suggest a mid-Carboniferous lowstand in relative sea level followed by a middle Morrowan (middle Bashkirian) highstand as previously suggested (Bishop et al., 2010). Subsequent progradation of the platform occurred during a gradual long-term fall through the late Morrowan and Atokan (late Bashkirian–early Moscovian), upon which repeated higher frequency events were superimposed, culminating in widespread exposure across the platform proximal to the Atokan– Desmoinesian boundary. Long-term sea level increased into the mid-Desmoinesian (end-Moscovian) after which progradation (late Desmoinesian) and aggradation (Missourian–Virgilian) of shallow-water and peritidal carbonates occurred across the Bird Spring platform. The progradational-to-aggradational phase involved an abrupt shift from ramp to rimmed shelf geometry in the Missourian (Kasimovian) likely in response to progressively increasing sea level, dampened high-frequency sea level fluctuations and/or to regional faulting along the Bird Spring platform margin. Inferred magnitudes of short-term (105 yr) fluctuations superimposed on the longer-term rises and falls range from 20 to >70 m during the early to mid-Pennsylvanian and are substantially reduced (≤20 m) during the late Pennsylvanian. The early to middle Pennsylvanian sea-level history inferred from the carbonate-dominated Bird Spring platform corresponds well with a recently published Pennsylvanian onlap–offlap history defined from the cyclothemic succession of the Donets Basin and with independent near-field reconstructions of glaciation during the Pennsylvanian. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Recent field-based and modeling studies of the late Paleozoic icehouse provide evidence for a series of discrete (1–8 my duration) ice ages characterized by multiple ice centers and separated by equally long periods of glacial minima (Fielding et al., 2008a,b, 2010; Isbell et al. 2008; Gulbranson et al., 2010). The degree to which late Paleozoic ice sheets retreated during glacial minima

⁎ Corresponding author at: Apache Corporation, 2000 Post Oak Boulevard, Houston, TX 77056, USA. Tel.: + 1 713 296 6000. E-mail address: [email protected] (L.G. Martin). 0031-0182/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2012.02.018

remains controversial given the potential for preservational bias in glacigenic deposits and the predominance of cyclothems throughout paleotropical Permo-Carboniferous successions that have long been inferred as glacioeustatically driven (e.g., Wanless and Shepard, 1936; Heckel, 1986; Gibling and Rygel, 2008). Moreover, the complexities of cyclothem depositional systems, such as their sensitivity to sediment supply and distribution, superposition of incised valleys, and uncertainties in estimating paleo-water depths from siliciclastic fine-grained deposits, have led to inferred magnitudes of glacioeustasy that range from a few tens of meters to well over 100 m (e.g., Heckel, 1977; Soreghan and Giles, 1999a,b), with estimates for particular intervals (e.g., late Pennsylvanian) varying up to an order of magnitude (Rygel et al., 2008).

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Shallow water carbonates are excellent archives of past sea-level and climate change given that they tend to fill accommodation space created by relative sea-level change and are highly responsive to changing environmental conditions (e.g., Osleger and Read, 1991; James, 1997; Read, 1998). Despite their potential, far fewer studies of Permo-Carboniferous carbonate systems than cyclothemic ones have contributed to our understanding of the late Paleozoic icehouse (Goldhammer et al., 1991; Saller et al., 1994; Soreghan and Giles, 1999b; Barnett et al., 2002; Bishop et al., 2009, 2010; Koch and Frank, 2011). The carbonate-dominated Bird Spring Formation, southern Great Basin, U.S.A. (Fig. 1), provides an excellent opportunity to evaluate relative sea level and regional climate for western equatorial Euramerica throughout the Pennsylvanian, given mountain-front exposures of thick (~500–750 m) uppermost Mississippian through Lower Permian carbonates that capture the platform interior through upper slope environments, including the paleo-platform margin (Stevens and Stone, 2007). Here we develop a cycle and sequence stratigraphic framework for the Upper Carboniferous Bird Spring Formation that builds on the work of Bishop et al. (2009, 2010) in Arrow Canyon, Nevada. We use this chronostratigraphic framework to further evaluate the hypothesis that large-scale, million-year variations in ice sheet extent led to sealevel changes that are recorded in the facies patterns and hierarchy of cyclicity of the Bird Spring carbonate succession and to place constraints on the magnitude of high-frequency (cyclothemic-scale) glacioeustatic fluctuations. 1.1. Geologic setting and chronostratigraphy The Bird Spring platform, presently located in southeastern California and southern Nevada, Great Basin province (Fig. 1), was the southern extension of the Ely carbonate platform during the Permo-Carboniferous (Stevens and Stone, 2007). This broad, shallow-water carbonate platform, located at ~10 to 15°N latitude (Blakey, 2008), either faced the

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eastern Panthalassan Ocean (Stevens, 1986; Stevens and Stone, 2007) or formed the eastern margin of an interior seaway enclosed to the west by the Antler orogenic belt (Poole and Sandberg, 1991; Ross 1991). Antler orogenesis began in the late Devonian (Johnson and Pendergast, 1981; Goebel, 1991) and may have influenced long-term accommodation rates as the timing of end-deformation is not well constrained (Dickinson et al., 1983; Stevens et al., 1997; Trexler et al., 2004; Cashman et al., 2010). During the early Pennsylvanian (Morrowan to Atokan Stages), carbonate deposition extended westward into the Tihvipah ramp of Stevens and Stone (2007) where deeper water cherty micritic limestones were deposited (Fig. 1). Platform geometry during this time was that of a distally steepened ramp (Yose and Heller, 1989; Miller and Heller 1994; Bishop et al., 2010). Beginning proximal to the middle-to-late Pennsylvanian transition (Desmoinesian–Missourian) through Early Permian (Sakmarian), the Bird Spring platform prograded westward and underwent a major change from a distally steepened ramp to a steep-rimmed shelf (Yose and Heller, 1989; Miller and Heller, 1994; Stevens and Stone, 2007; Bishop et al., 2010). West of the platform, carbonate sediment gravity flows were deposited in the deeper-water Keeler Basin, interpreted to have formed in part by transtension along a continental truncation fault on the western margin of North America (Stone and Stevens, 1988; Stevens et al., 1997; Stevens and Stone, 2007). Paleozoic strata in the Great Basin experienced up to 135 km of shortening during the Mesozoic to early Cenozoic and approximately 250 km of extension during the mid- to late Cenozoic (Levy and Christie-Blick, 1989), resulting in the creation of excellently exposed canyon and mountain-front successions. The tectonic history of the Bird Spring platform is not well constrained and may have influenced long-term accommodation and stratigraphic patterns. The change from a distally steepened ramp to a flat-topped shelf in the early Missourian (early Kasimovian) may record mid-Pennsylvanian faulting, recognized to date, however,

A

B

C

D

Fig. 1. (A) Interpreted Pennsylvanian locations of studied successions (gray squares) based on palinspastic reconstruction of Great Basin Proterozoic to Early Cambrian successions (shown in black) by Levy and Christie-Blick (1989). MC—Marble Canyon; LC—Lee Canyon; AC—Arrow Canyon; MSP—Mountain Springs Pass. Dotted gray lines extrapolate MC and AC to cross-section A–A′ (see Figs. 5 and 6). Present-day locations shown in inset. SB—Striped Butte. (B) Inferred palinspastic locations of the sections (as in 1A), on the Bird Spring platform, Tihvipah Ramp, and Keeler Basin during the early Pennsylvanian and (C) late Pennsylvanian. (D) Generalized Bird Spring stratigraphic column; correlation of stage boundaries after Davydov et al. (2010) and M. Schmitz and V. Davydov (pers. comm. 2011). M—Mississippian; S—Serpukhovian; C—Chesterian.

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only in northern Nevada (Trexler and Cashman, 1997; Trexler et al., 2003, 2004). This study examines the high-frequency stratigraphic record to assess the influence of eustasy and tectonically driven subsidence on the long-term relative sea-level history. 1.2. Methods Three successions were measured and described at the decimeterscale and lithofacies were interpreted according to their Dunham classification, sedimentary structures, and fossil assemblages. Over 50 samples per measured section were collected, thin sectioned, stained with Dickson's solution (Dickson, 1965), and examined under transmitted and cathodoluminscent light to refine lithofacies interpretations. The three measured sections, along with the previously published Arrow Canyon succession (Bishop et al., 2009, 2010), were used to define a ~105 km transect across the Bird Spring platform (Fig. 1). A fifth succession, Striped Butte, palinspastically located along the midinner platform, was measured and described in detail (Greene, 2010) but due to lack of chronostratigraphic constraint is not included in this paper. Chronostratigraphy is based on fusulinids (C. Stevens, pers. comm., 2009–2010), conodonts (Stone, 1984; B. Wardlaw, pers. comm., 2010) and Amoco biostratigraphic data collected from Arrow Canyon (Table 1; Bishop et al. 2010 and references therein). The Arrow Canyon (AC) succession includes the Global Stratotype Section and Point for the midCarboniferous boundary and provides excellent biostratigraphic control for early to mid-Pennsylvanian N. American stages and moderate control for late Pennsylvanian stages (e.g., Cassity and Langenheim, 1966; Brenckle et al., 1997; Lane et al., 1999; Barnett and Wright,

Table 2 Decompacted thickness and long term sediment accumulation rate by stage and locality.

Morrowan Thickness (m) Accumulation rate Atokan Thickness (m) Accumulation rate Desmoinesian Thickness (m) Accumulation rate Missourian Thickness (m) Accumulation rate Virgilian Thickness (m) Accumulation rate

Marble Canyon

Arrow Canyon

Lee Canyon

Mountain Springs Pass

(mm/kyr)

357.9 55.5

159.2 24.7

171.0 26.5

52.6 8.2

(mm/kyr)

401.8 84.6

489.9 103.1

525.0 110.5

293.1 61.7

(mm/kyr)

559.2 95.6

357.2 61.1

194.4 33.2

94.5 16.1

(mm/kyr)

– –

80.2 40.1

60.8 30.4

48.1 24.0

(mm/kyr)

– –

67.4 22.5

– –

81.1 27.0

2008; Bishop et al., 2010). Moderate biostratigraphic control (Table 1) exists for the Marble Canyon (MC), Lee Canyon (LC) and Mountain Springs Pass (MSP) successions and when integrated with the sequence stratigraphy permits sub-stage (~1 my) resolution correlation of these sections to the AC section. Sediments were decompacted according to lithofacies type and burial depth (Table 2; Goldhammer, 1997; Hillgärtner and Strasser, 2003). Color alteration indices (CAI) of conodonts from AC, considered a proxy of long-term burial temperatures (Epstein et al., 1977; Rejebian et al., 1987), are 2 to 2.5 (B. Wardlaw, pers. comm., 2010)

Table 1 Biostratigraphic constraints. Stage

Sample #a

Fauna

Source

Morrowan

C1

Idiognathoides sinuatus (Harris and Hollingsworth); Idiognathodus sp. (juvenile); Neognathodus symmetricus (Lane); Idiognathodus suberectus (Dunn) I. sinuatus (Harris and Hollingsworth); Ideognathodus convexus (Ellison and Graves); I. sulcatus parvus (Higgins and Bouckaert); Idiognathodus delicatus (Gunnell), sensu Webster (1969) Idiognathodus delicatus (Gunnell); Idiognathodus convexus (Ellison and Graves); I. sulcatus parvus (Higgins and Bouckaert); ?Neognathodus bassleri (Harris and Hollingsworth); ?Streptognathodus lanceolatus (Webster); Idiognathoides (thin); ?Hindeodus Neognathodus bassleri (Harris and Hollingsworth); Idiognathodus delicatus (Gunnell); Idiognathodus cf. I. convexus (Ellison and Graves); Idiognathoides (thin) Idiognathodus delicatus (Gunnell); Streptognathodus expandus (Igo and Koike)

Stone, 1984; Bruce Wardlaw, pers. comm. Stone, 1984; Bruce Wardlaw, pers. comm. Stone, 1984; Bruce Wardlaw, pers. comm.

C2 C3

C4 C5 Atokan

Desmoinesian

F1 F2 F3 F4 F5 F6 S1 C6 F7 C7 F8 F9 C8

Missourian

F10 F11 C9

Virgilian

F12 F13 F14 F15 F16

Fusulinella sp. Millerella marblensis Fusulinella devexa ?Fusulinella Fusulinella devexa (Thompson); Fusulinella juncea (Thompson); Schubertella sp. Beedeina rockymontana (Roth and Skinner); ?Fusulinella serotina (Thompson) Komia Neognathodus medexultimus (Merrill); Idiognathodus delicatus (Gunnell); Gondolella pulchra (Merrill); Neognathodus asymmetricus Wedekindellina sp.; Beedina sp.; Komia Hindeodus minutus (Ellison); Idiognathodus delicatus (Gunnell); Neognathodus medexultimus (Merrill); N. cf. N. bothrops (Merrill); Neognathodus asymmetricus; Idiognathodus expansus Wedekindellina sp.; Beedina sp.; Komia Beedeina retusa (Thompson and Thomas) Idiognathodus expansus; Neognathodus asymmetricus; Adetognathus lautus (Gunnell); Idiognathodus delicatus (Gunnell); Neognathodus bassleri (Harris and Hollingsworth); N. medexultimus (Merrill); N. roudyi (Gunnell) Triticites burgessae (Burma) Triticites sp. Idiognathodus cherryvalensis; Adetognathus lautus (Gunnell); Hindeodus minutus (Ellison); Idiognathodus sp.; Streptognathodus eccentricus (Ellison) Triticites muddiensis (Cassity and Langenheim) T. aff. T. muddiensis Triticites rhodesi Triticites cf. T. muddiensis (Cassity and Langenheim); Triticites cf. T. rhodesi (Needham); Pseudofusulinella sp. Triticites cf. T. muddiensis (Cassity and Langenheim); Triticites cf. T. rhodesi (Needham)

Stone, 1984; Bruce Wardlaw, pers. comm. Stone, 1984; Bruce Wardlaw, pers. comm. Calvin Stevens, pers. comm. Rich, 1961 Calvin Stevens, pers. comm. Calvin Stevens, pers. comm. Calvin Stevens, pers. comm. Calvin Stevens, pers. comm. Calvin Stevens, pers. comm. Stone, 1984; Bruce Wardlaw, pers. comm. Calvin Stevens, pers. comm. Stone, 1984; Bruce Wardlaw, pers. comm. Calvin Stevens, pers. comm. Calvin Stevens, pers. comm. Stone, 1984; Bruce Wardlaw, pers. comm. Calvin Stevens, pers. comm. Calvin Stevens, pers. comm. Stone, 1984; Bruce Wardlaw, pers. comm. Calvin Stevens, pers. comm. Calvin Stevens, pers. comm. Calvin Stevens, pers. comm. Calvin Stevens, pers. comm. Calvin Stevens, pers. comm.

a Sample numbers correspond to those in Figs. 5 and 6, except for C9, which is a spot sample whose location is not precisely known but is located between C8 and the top of the Marble Canyon succession. Data for Arrow Canyon is provided in Bishop et al. (2010) and references therein.

Table 3 Lithofacies of the Bird Spring Fm. Lithofaciesa

Bedding

Outermost platform-to-upper slope facies assemblage Conglomerate and breccia facies subassemblage Mud-supported conglomerate Meter scale bedding, dark gray

Meter scale bedding, gray to orange

Low-energy outermost platform-to-upper slope facies subassemblage Shale Fissile, purple to gray, Calcisiltite Decimeter to meter scale bedding, red-orange to gray

Interbedded calcisilt and marl Mid-platform facies assemblage Heterozoan Wkst/Pkst

Photozoan Wkst/Pkst

Centimeter scale bedding

Biotic components

Abiotic components

Depositional environmentb

Water depthc (m)

Slumped and contorted bedding, erosive bases with flame and loading structures and soft sediment deformation Calcisiltite matrix; monomictic clasts composed of calcisiltite and chert, polymictic clasts composed of calcisiltite and bioclastic Wkst/Pkst

Sponge spicules, crinoids, stick and fenestrate bryozoa, phosphatized grains

Centimeter to decimeter scale clasts, centimeter scale spherical black chert, peloids, ~ 40% qtz silt

Debris flow, upper slope, below SWB

> 40

Skeletal grains inside Wkst/Pkst clasts

Centimeter scale angular clasts

Debris flow, upper slope, below SWB

> 40

Whole brachiopods Sponge spicules, crinoids, brachiopods, echinoderm spines, ostracodes, bryozoa, rare foraminifera, skeletal fragments, phosphatized and oxidized grains, plant fragments

Quartz silt Subrounded to subangular coarse quartz silt, centimeter scale spherical black to nodular to lenticular black chert, mud rip-up clasts

Below SWB Below to just above SWB

> 50 40–70

Sponge spicules, ostracodes, foraminifera

Coarse quartz silt, sparse nodular chert

Below SWB

> 40

Fenestrate bryozoa, crinoid columnals, brachiopods, whole sponges, trilobites, uniserial foraminifera, echinoderm spines, solitary rugose coral, unidentifiable silt-sized carbonate grains Crinoids, bryozoa, brachiopods, rugose coral, fusulinids

Spherical to nodular to lenticular chert

Seaward of skeletal banks, between SWB and FWWB

15–40

Nodular to lenticular black chert

Seaward of skeletal banks, between SWB and FWWB

15–30

Massive to laminated to cross bedded, bioturbated, skeletal material filling scours

Crinoids, brachiopods, solitary rugose coral, echinoderms, skeletal fragments

Nodular black chert

High energy shoals and skeletal banks, above FWWB

5–20

Massive to high-angle planar cross-bedding, decimeter scale

Crinoids, gastropods, fusulinids, bryozoa, foraminifera, solitary

Micritic encrustations on grains, peloids, nodular to

Thinly laminated Massive to planar to low-angle cross laminated, lenticular skeletal material filling scours, little to no bioturbation, occasional normal grading, rare zoophycos traces Thinly laminated, soft sediment deformation

Decimeter to meter scale bedding, gray

Planar laminated to massive, skeletal material filling scours, moderately bioturbated

Decimeter scale bedding, gray

Skeletal material filling scours, bioturbated

Inner platform facies assemblage Inner platform high-energy heterozoan facies subassemblage Heterozoan Grst/Pkst Meter scale bedding, light gray to gray

Inner platform high-energy photozoan facies subassemblage Photozoan Grst/Pkst Decimeter to meter scale bedding, red-orange to gray

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Mono- and polymictic breccias

Sedimentary structures

5–20 (continued on next page)

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Table 3 (continued) Lithofaciesa

Bedding

Massive to meter scale bedding, red-orange to gray

Calcareous qtz siltstone

Decimeter to meter scale bedding, orange-brown to gray Massive to meter scale bedding, gray

Boundstone

Inner platform low-energy facies subassemblage Heterozoan Wkst/Pkst Decimeter to meter scale bedding, gray Photozoan Wkst/Pkst

Decimeter to meter scale bedding, gray

Restricted platform interior facies assemblage Dolomudstone Centimeter to decimeter scale bedding, orange to light gray

a b c

Biotic components

Abiotic components

Depositional environmentb

channels or interbeds of bioclastic wackestone variably present, skeletal material filling scours, vertical burrows, traction deposits Planar to cross-laminated, skeletal material filling scours

rugose coral, syringopora coral, echinoderm spines, skeletal fragments

spherical black chert, lithoclasts, quartz sand and silt

High energy shoals and shorelines, above FWWB

Brachiopods, gastropods, echinoderms, fusulinids, sponge spicules, skeletal fragments Skeletal fragments

Ooids, coated grains, peloids, oncoids, evaporite pseudomorphs, nodular black chert Quartz sand and silt, minor (b1%) clays and micas, hematite stained Clotted micrite, gypsum laths

High energy shoals and shorelines, above FWWB

0–5

High energy, above FWWB

1–20

High to low energy, patch reefs and thrombolites, shallow subtidal in the photic zone, above FWWB

b1–15

Planar to low-angle cross lamination Centimeter to decimeter scale bindstones and framestones with Wkst/ Pkst/Grst forming channels between, laminae truncating against clotted mud

Colonial and solitary rugose coral, syringopora coral; ecrusting foraminifera, crinoids, and brachiopods in grainy deposits between boundstones

Massive to planar laminated, well bioturbated, skeletal material filling scours Massive, well bioturbated, skeletal material filling scours

Crinoids, whole brachiopods, gastropods, miliolid and biserial foraminifera, echinoderm spines Crinoids, bryozoa, syringopora coral, brachiopods, fusulinids, solitary and colonial rugose coral, gastropods, ostracodes

Peloids

Low energy, above FWWB

2–20

Peloids, black nodular chert, pseudomorphs after gypsum laths variably present

Low energy evaporative lagoon, above FWWB

2–20

Planar to low angle cross-laminations, sparse skeletal material filling scours, mudcracks, sparsely bioturbated

Thin walled brachiopods, crinoids

Peloids, nodular to continuous chert, popcorn chert, gypsum rosettes and laths, cauliflower chert, variably silicified, quartz silt, b 5% mica Micritic and peloidal encrustations on grains, clotted mud, mudchips, evaporite pseudomorphs, popcorn chert, gypsum laths Micritic and peloidal encrustations on grains, clotted mud, mudchips Chert rosettes, cauliflower chert, swallowtail pseudomorphs after gypsum, diamond shaped molds after dolomite, calcrete

Highly restricted lagoon, evaporative waters, above FWWB

0–5

Middle to high intertidal, lower to shallowest subtidal thick mechanical laminites Middle to high intertidal, crypalgal laminites Exposure surface, evaporite solution collapse breccia, evaporative waters and sabkha environment Terrestrial, aeolian dunes

0–2

Peloidal Grst/Pkst

Decimeter scale bedding tan to orange

Planar to crinkly lamination, low angle cross-lamination, mudchips, variably bioturbated

Crinoid fragments, skeletal fragments, grains extensively micritized

Bindstone

Centimeter scale bedding, tan to orange

Planar to crinkly mm lamination, mudchips and mudcracks

Skeletal fragments

Massive to brecciated chert

Massive, red to brown

None

Cross-bedded quartz siltstone

Decimeter scale bedding, orange, poorly outcropping

Breccia with dolomitized Wkst/Pkst or dolomudstone matrix, scalloped and corroded angular to circular clasts, root halos, ripped up laminites, erosive base, terra rossa infill Parallel lamination

Roots oriented parallel to bedding

Wkst/Pkst—wackestone and/or packstone; Grst/Pkst—grainstone and/or packstone. SWB—storm wave base; FWWB—fair weather wave base. SWB taken as 40 m, FWWB as 20 m, photic zone as 30 m. Estimates of paleo-water depths of deposition from Bishop et al. (2010).

Angular to subangular coarse quartz silt with sutured grain boundaries

0–2

0

0

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Coated grain Grst/Pkst

Water depthc (m)

Sedimentary structures

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and are interpreted to record maximum burial temperatures of 60– 140 °C and burial depths of 2.4 to 5.6 km (assuming a geothermal gradient of 25 °C/km). A decompaction value of 1.3 was used for carbonate grainstones to account for early cementation minimizing compaction during burial. Decompaction factors for grain/packstones (D = 1.5), packstones (D = 1.7), pack/wackestones (D = 2), and wackestones (D = 2.2) were extrapolated between grainstone and mudstone values according to Hillgärtner and Strasser (2003). Decompaction estimates presented here are minimum values given that they do not account for erosion, covered intervals, or chemical compaction. All thickness values are decompacted unless stated otherwise. 2. Lithofacies and depositional environments Nineteen lithofacies identified in the Bird Spring Formation are grouped into four major depositional environments (outermost platform-to-upper slope, mid-platform, inner platform, and restricted platform interior) based on fossil assemblages, Dunham classification, and sedimentary structures (Table 3). Lithofacies for all studied successions are detailed below with the exception of the AC succession, which is described in Bishop et al. (2010). Lithofacies are further subdivided into subassemblages based on heterozoan or photozoan assemblages according to James (1997) and depositional energy. Heterozoan lithofacies are indicative of cool- to temperate-water, eutrophic to mesotrophic, and/or sub-photic carbonates with muddy, sponge-spicule dominated outer-ramp environments. Photozoan lithofacies are indicative of warm-water, euphotic, oligotrophic environments dominated by photoautotrophs, corals, and abiotic carbonate precipitation. The facies assemblages were defined on the basis of interpreted depositional position, at any given time, on the platform relative to fair weather wave base (FWWB) and storm wave base (SWB).

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Through time the position of these facies migrated with relative sealevel change. 2.1. Outermost platform-to-upper slope facies assemblage Outermost platform-to-upper slope lithofacies include shales, calcisiltites, mud-supported conglomerates, mono- and polymictic breccias, and interbedded calcisiltites and marls (Table 3). Shales are very thinly planar-laminated and lack fossils. Thinly laminated to massive calcisiltites are characterized by dm- to m-scale bedding, abundant sponge spicules and silt-sized bioclasts, up to 40% subangular coarse silt-sized quartz, and minimal bioturbation (Fig. 2A). Abundant nodular, bedded lenticular, or spherical chert is present within calcisiltites. Where normally graded, calcisiltites are weakly crosslaminated, display mud rip-ups and granule- to pebble-sized clasts at their bases, and grade upward into massive or planar laminated calcisiltites (Fig. 2B). Conglomerates are mud-supported and polymictic, whereas breccias are predominantly clast-supported and are either polymictic or monomictic (Fig. 2C). Polymictic conglomerates and breccias occur stratigraphically intercalated with slumped and contorted calcisiltites and marls; clasts are rounded and abraded, pebble- to boulder-sized, and composed of calcisiltites, bioclastic wacke/packstones with shallow water skeletal grains, and bioclastic grain/packstones. Slumped and contorted beds of calcisiltites (Fig. 2D) grade laterally into monomictic breccias, which consist of cobble- to boulder-sized cherty calcisiltite clasts. These lithofacies are most common in the MC succession, less common in the LC and AC successions, and absent in the MSP succession. 2.1.1. Interpretation These lithofacies formed below SWB along the outermost platform and upper slope. Storm-induced currents transported bioclasts and

Fig. 2. Outermost platform-to-upper slope facies assemblage, Marble Canyon. (A) Thin-bedded marl-calcisiltite couplets. (B) Coarse grained carbonate sediment filling erosive base at base of turbidite deposit, scale bar = 1 cm. (C) Polymictic breccia, sighting device at top of jacob staff for scale. (D) Slumped and contorted monomictic breccia; note undisturbed basal thin-bedded fine-grained limestone beds at base of unit.

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silt-sized sediment to these environments from the inner and middle platform. Laminated shales were deposited in sub-SWB quiet, dysoxic waters as suggested by the lack of bioclasts or bioturbation. Normally graded to weakly cross-laminated calcisiltites are interpreted as carbonate turbidites. The presence of rip-up clasts and granule- to pebble-sized clasts (interval A of the Bouma sequence, Bouma, 1962) indicates turbidite deposition relatively proximal to the sediment source. Conglomerates and polymictic breccias with reworked clasts composed of inner- to middle-shelf lithofacies are interpreted as debris flows. Monomictic breccias formed in situ on the upper slope of a distally-steepened ramp as suggested by their lateral gradation into undisturbed to slumped sediment of similar composition. Turbidites and debris flows are interpreted to have formed during sea-level low stands when the carbonate platform became unstable (Yose and Heller, 1989; MacNeil and Jones, 2006; Catuneanu et al., 2009) and to be located basinward from the fine-grained sediments because they are located along the steeper part of the ramp or shelf slope. Quartz silt, along with siliceous sponge spicules, provided a local silica source for the abundant chert present in these lithofacies. The angular nature of the quartz silt and narrow size range indicates an eolian continental source (Johnson, 1989; Pye, 1995).

2.2. Mid-platform facies assemblage Heterozoan or photozoan wackestones and packstones define the mid-platform facies assemblage and are characterized by dm- to mscale bedding with minimal to moderate bioturbation, planarlamination, skeletal material filling scours and abundant nodular to lenticular chert (Table 3). Heterozoan wackestones and packstones contain crinoids, encrusting and fenestrate bryozoa, brachiopods,

sponges, trilobites, echinoderms, peloids and rare solitary rugose corals and small foraminifera. In contrast, photozoan wackestones and packstones contain fusulinids and algal-coated skeletal fragments, in addition to rugose coral, brachiopods, crinoids, and bryozoa. These lithofacies are common in the lower to middle Pennsylvanian (Morrowan–lower Desmoinesian) intervals of the platform successions (AC, LC, MSP) whereas they are limited to the middle to upper Pennsylvanian (Desmoinesian–Missourian) interval at MC. 2.2.1. Interpretation Mid-platform sediments are interpreted to have been deposited seaward of the skeletal banks between SWB and FWWB. Diverse fauna and common bioturbation suggests normal marine conditions, whereas skeletal material filling scours indicates periodic storm influence. 2.3. Inner platform facies assemblage Inner platform lithofacies occur as heterozoan and photozoan wacke/packstones and grain/packstones, microbial boundstones and skeletal framestones, coated grain/packstones, and calcareous quartz siltstones (Table 3). Wacke/packstones are dm- to m-scale bedded, extensively bioturbated, variably dolomitized, and contain sparse discontinuous nodular chert. Heterozoan facies are similar in composition to those of the mid-platform sediments, whereas photozoan facies include, in addition to the aforementioned grains, syringopora coral, chaetetes, gastropods, ostracodes, and occasionally evaporite pseudomorphs. Thrombolite boundstones and chaetetid, syringoporoid (Fig. 3A), and colonial rugose coral framestones are interbedded with photozoan wacke/packstones. Clotted mud in the form of upward-

Fig. 3. Inner platform and restricted platform interior facies assemblages, Lee Canyon. (A) Syringopora coral heads in growth position. (B) Clotted fine-grained carbonate with fenestral fabric and domal topography, interpreted as a thrombolite, scale bar = 1 cm. (C) Lateral accretion deposits composed of skeletal grain/packstone interpreted as migrating channel bars, jacob staff for scale. (D) Silicified pseudomorphs of gypsum laths within photozoan wacke/packstone, scale bar = 1 cm. (E) Popcorn chert with secondary calcite cement after evaporites within dolomitized mud/wackestone, scale bar = 1 cm. (F) Silicified pseudomorphs of gypsum rosettes within dolomudstone, scale bar = 1 cm.

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branching fingers or domal structures with cm- to dm-scale relief are flanked by wacke/packstones and occur in upper Pennsylvanian (Desmoinesian–Virgilian) intervals of platform successions (LC, MSP) (Fig. 3B). Grain/packstones, which are cross-bedded or form traction deposits filling scours, consist of abundant broken or abraded skeletal grains, commonly with micritic coatings (Fig. 3C). Non-skeletal grains include ooids, coated grains, peloids, and oncoids. Calcareous quartz siltstones are planar to low-angle cross-laminated. Chert, discontinuous and nodular where present, is less common than in mid- and outerplatform lithofacies. These inner platform lithofacies are present throughout the platform successions but are limited to the middle Pennsylvanian (Desmoinesian) in the outermost platform-to-upper slope succession (MC); muddier inner-platform lithofacies are found in all but the MC succession.

deposited in the mid- to high-intertidal zone. Cauliflower cherts, gypsum pseudomorphs, and calcite nodules are interpreted as pseudomorphs after displacive evaporites that precipitated in carbonate sediments from evaporatively concentrated pore waters. Brecciated cherts likely formed as bedded evaporites during arid and/or restricted conditions and were subsequently partially dissolved and brecciated during exposure to meteoric or normal salinity seawater. Evidence for subaerial exposure includes caliche crusts, rooting structures, and regoliths. Terra rossa fills on irregular surfaces of dolomudstones and scalloped carbonate clasts infilled with sediment from the overlying deposits are interpreted as karst. Cross-bedded quartz siltstones likely formed as eolianites deposited during lowstands that were subsequently reworked during marine transgressions (Johnson 1989; Soreghan, 1992; Pye, 1995; Soreghan et al., 2007).

2.3.1. Interpretation Grain/packstones and calcareous quartz siltstones were deposited as high-energy skeletal banks and shoals above FWWB as indicated by their common cross-bedding, lack of carbonate mud, and the presence of ooids and abundant broken or abraded grains. These lithofacies are interpreted as the highest energy deposits on the platform and acted as wave breaks for the protected shallow subtidal environments landward of the banks. Wacke/packstones and boundstones formed near FWWB in shallow low-energy waters landward of skeletal banks. This is indicated by the presence of lime mud, intensive bioturbation, and the predominance of photozoan skeletal grains including oncoids, many of which are unabraded. Abraded fossils were likely transported landward from skeletal banks by storm currents. Chaetetes, colonial rugose coral, and Syringopora coral formed small patch reefs and bioherms within the lagoon. Massive clotted muds, many of which show textures typical of thrombolite boundstones and are flanked by wackestones and channelized skeletal grain/packstones, formed in shallow subtidal waters.

3. Stratigraphic correlations and sequence stratigraphy Sequences are genetically related strata composed of parasequences and parasequence sets bounded by unconformities or their

2.4. Restricted platform interior facies assemblage Restricted platform interior lithofacies include dolomudstones, planar to crinkly laminated bindstones, planar to cross-laminated skeletal peloidal grainstones and packstones, massive brecciated cherts within wackestones and packstones, and low-angle crossbedded quartz siltstones (Table 3). These lithofacies are extensively dolomitized, with the exception of the cross-bedded quartz siltstones, and are minimally bioturbated. Crinoid fragments and thin-walled brachiopods are abraded and extensively micritized. Dolomudstones are thinly laminated with sparse skeletal material filling scours; bindstones have planar to crinkly millimeter-scale laminations with mudchips and mudcracks. Low-angle cross-bedded siltstones consist of well-sorted angular to subangular coarse silt-size quartz grains with sutured boundaries and calcite cements. Cauliflower cherts and calcite nodules replacing evaporites are common in all lithofacies; pseudomorphs of gypsum laths observed in the field and petrographically are less common (Fig. 3D–F). Scalloped angular to rounded clasts, terra rossa infill, rooting structures, and massive to brecciated chert up to a meter thick occur at the tops of many restricted platform interior beds. These lithofacies are abundant as cycle caps in the inner platform succession (MSP), common at sequence boundaries in the mid–outer platform AC and LC successions, and are extremely rare in the MC succession. 2.4.1. Interpretation Restricted platform interior lithofacies formed in highly restricted, evaporative settings between shallowest subtidal to supratidal conditions. Skeletal peloidal grainstones and packstones were deposited by shallow currents, whereas crinkly laminated bindstones with mudchips and mudcracks (i.e., cryptalgal laminites) are interpreted as stromatolites

Fig. 4. Representative examples of parasequence set types, shown correlated between mid-to-outer platform sections Arrow Canyon and Lee Canyon. Facies as in Fig. 6. ‘A’ numbers (as in Bishop et al., 2010) for Arrow Canyon correspond to 1.5 m stratigraphic increments defined for the Amoco standard reference section. (A) Progradational parasequence set 6.4. (B) Aggradational parasequence set 10.2. (C) Retrogradational parasequence set 3.1. See Figs. 5 and 6 for placement within sequences. Thickness shown is not decompacted.

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correlative conformities (Mitchum and Van Wagoner, 1991). The Bird Spring across-platform-to-slope transect permits reconstruction of the complete geometry of the Pennsylvanian sequences. Stratigraphic and geographic lithofacies patterns, coarsening or fining upwards trends, parasequence stacking patterns and evidence of crossplatform development of subaerial exposure or flooding surfaces were used to identify sequences and their internal components (i.e., systems tracts and associated surfaces). Parasequences in the Bird Spring Fm. are identified as shallowingupwards cycles bounded by flooding surfaces (locally also with subaerial exposure) and are more completely recorded in the mid-toouter platform successions where accommodation was highest (AC and LC). A total of 126 parasequences, ranging in thickness from 0.7 to 29.5 m (avg. of 8.8 m) are recognized in the mid-to-outer platform successions (AC and LC). In the outermost platform-to-upper slope succession (MC), parasequences are limited to the upper Desmoinesian

through lower Missourian interval, whereas few parasequences are recognized in the inner platform succession (MSP), reflecting missed beats due to a much lower accommodation rate (see Section 5.1). Stacking of parasequences defines thirty-four parasequence sets ranging in thickness from 3 to 85 m (avg. of 33 m; Fig. 4). Parasequence sets are regionally traceable across the outer to inner platform and define lowstand, transgressive and highstand systems tracts (Figs. 5 and 6). Three parasequence set types are recognized: progradational, retrogradational, and aggradational (Fig. 4). In progradational (retrogradational) parasequence sets, facies capping each successive parasequence become shallower (deeper), and a larger proportion of each successive cycle is composed of shallower (deeper) water sediments. In aggradational parasequence sets, facies capping each parasequence set are similar and no marked change in stacking patterns occurs within sets. Progradational parasequence sets dominate the lower to middle Pennsylvanian interval (Morrowan–Desmoinesian),

Fig. 5. Interpretive cross-sections of sequences 1–5, hung on the top boundary surfaces of sequences 1, 2, and 5. Parasequence sets are numbered according to their placement within each sequence. Lee Canyon section is a composite based on two measured sections, Lee Canyon A and B; both sections are shown correlated at the top of sequence 5. Conodont and fusulinid biostratigraphic picks (white circles with labels) as in Table 1; vertical black lines between ‘pick’ symbols indicate temporal uncertainty. Distance between sections after palinspastically restored cross-section line A–A′ in Fig. 1. Thickness shown is not decompacted. See Fig. 6 for key.

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Fig. 6. Interpretive cross-sections of sequences 6–10, hung on the top boundary surfaces of sequences 6 and 10. Numbering of parasequence sets, biostratigraphic picks as in Fig. 5. The uppermost portion of the mid platform succession at Lee Canyon (parasequence sets 10.2 and 10.3) was not measured due to faulting at the top of the section (Fault B in Rich, 1961). The outermost platform-to-upper slope section (Marble Canyon) is truncated by an erosional unconformity underlying the Permian Osborne Canyon Fm. (Stone, 1984). Thickness shown is not decompacted.

whereas aggradational parasequence sets dominate the upper Pennsylvanian (Virgilian; Figs. 5 and 6). This trend influences the evolving architecture of the Bird Spring platform (see below). Ten sequences (Figs. 5 and 6) are recognized and bounded by stratigraphic intervals, 1 to 10 m thick, dominated by restricted peritidal lithofacies (Table 3) and exposure surfaces. Sequences range in thicknesses from 52 to 194 m (avg. of 120 m) and their relative ages are based on available biostratigraphy (Table 1). Sequences can be correlated across the platform, but are more difficult to recognize in the outermost platform-to-upper slope succession (MC) due to cycle amalgamation and fewer exposure surfaces. Sequence boundaries are tentatively correlated to MC using debris flows as markers of low relative sea level given the evidence for reworked clasts of

shallow water origin in polymictic breccias (Yose and Heller, 1989; MacNeil and Jones, 2006; Catuneanu et al., 2009; Whalen et al., 2000). The following discussion is a description of the ten sequences and their internal components. Sequences are grouped according to component facies, parasequence set stacking patterns, and depositional geometry across the platform. 3.1. Sequence 1 Sequence 1 includes uppermost Mississippian (Chesterian or upper Serpukhovian) through mid-Morrowan (mid-Bashkirian) strata and ranges in decompacted thickness from 1 m on the inner platform (MSP) to 164 m at the outermost platform-to-upper slope

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(MC) (Fig. 5). The latest Mississippian stratigraphic boundary between the Indian Springs and Bird Spring formations was used in addition to biostratigraphic constraints (Table 1) for correlation. During this overall lowstand sequence, the mid-Carboniferous boundary falls within the progradational parasequence set 1.1 of the LST. The basal unconformable boundary of sequence 1 exhibits incipient soil development (Protosol; Bishop et al., 2010) including terra rossas at the mid-to-outer platform successions (AC, LC) and a regolith at MC. Systems tracts are best preserved along the mid-to-outer platform (AC, LC) and are difficult to correlate seaward (MC) and landward (MSP), presumably due to cycle amalgamation (on the outermost platformto-upper slope, MC) and missed beats (on the innermost platform, MSP) during the first half of the Morrowan (Fig. 5). The lowstand systems tract (LST) and the transgressive systems tract (TST) are primarily composed of a condensed section of calcisiltites and marls along the outermost platform-to-upper slope (MC), massive to cross-bedded grain/packstones along the mid-to-outer platform (AC, LC), and are not preserved on the inner platform (MSP). The maximum flooding surface (MFS) is picked at the base of parasequence set 1.4 and occurs as calcisiltites on the mid-toouter platform (AC, LC) grading upslope to grain/packstones on the inner platform (MSP). The highstand systems tract (HST) is composed of carbonate turbidites at MC passing landward into offshore wacke/packstones and inner platform photozoan grain/packstones (AC), which in turn grade updip (LC) into inner platform photozoan grain/packstones. Sequence 1 is interpreted as a lowstand wedge formed during a lowstand in relative sea-level in the early Morrowan (early Bashkirian). Multiple exposure surfaces and abundant shallow water carbonates along the mid-to-outer platform (AC, LC) suggest major seaward progradation of the platform with maximum progradation in the earliest Morrowan. Instability of the margin during this lowstand led to turbidite and debris flow deposition along the upper slope (MC). The landward-most section (MSP) was a locus of non-deposition and erosion during the overall lowstand. Estimates of magnitudes of shortterm (10 5 yr) sea-level change associated with this lowstand are between 40 and >70 m as inferred by the stratigraphic juxtaposition of calcisiltites, wacke/packstones, cryptalgal laminites and paleosols (Table 3). 3.2. Sequence 2 Sequence 2 incorporates mid-Morrowan through lowermost Atokan (mid–upper Bashkirian) strata and varies in decompacted thickness from 80 m on the inner platform (MSP) to 220 m at the outermost platform-to-upper slope (MC). Conodont and fusulinids from the outermost platform-to-upper slope (MC, Table 1; AC, Bishop et al., 2010) provide biostratigraphic control. The Morrowan–Atokan boundary is well constrained in the outermost two successions and was correlated to the mid-to-inner platform using sequence stratigraphy (Fig. 5). The basal boundary of sequence 2 (Fig. 5) is defined by paleosols and regoliths developed on outer platform carbonates (AC, LC) and calcareous sandstones on the inner platform (MSP). Systems tracts are best defined along the outer platform (AC, LC) where accommodation rates were most sensitive to relative sea-level changes. The TST consists of calcisiltites along the outermost platform-to-upper slope (MC) that grade platform-ward into heterozoan and photozoan grain/packstones (AC, MSP) or wacke/packstones (LC). The MFS is picked at the base of progradational parasequence set 2.2 and occurs as calcisiltites on the mid-to-outer platform (AC, LC). The HST is defined by progradational parasequence sets 2.2 through 2.4 that exhibit progressively greater progradation across the platform. These sets consist of calcisiltites along the outermost platform-to-upper slope (MC) grading upslope to grain/packstones on the outer platform and inner platform (AC, MSP), which flank wacke/packstones of the mid-platform (LC). Sequence 2 is interpreted to record maximum flooding of the platform during the mid-to-late Morrowan (mid-to-late Bashkirian) due

to a long-term rise in relative sea level. Calcisiltites deposited as far inboard as the mid-platform (LC) are the thickest of their kind recorded in the Pennsylvanian succession at this location. The lack of exposure surfaces along the platform obscure the recognition of sequence boundaries. Although parasequences are poorly developed, facies offsets within them record progradation of high-energy innerplatform facies toward the outer platform alternating with incursion of the deepest-water facies onto the mid-platform, indicating the occurrence of moderate- to high-magnitude, parasequence-scale sealevel fluctuations during this period. Inferred water depths of juxtaposed facies within parasequences indicate magnitudes of short-term (105 yr) fluctuations of 20 to >65 m (Table 3). Seaward progradation of shallow-water carbonates toward the end of sequence 2 (latest Morrowan to earliest Atokan) indicates the onset of renewed fall in relative sea-level. 3.3. Sequences 3, 4 and 5 Sequences 3 through 5 incorporate lower Atokan (upper Bashkirian) to lowermost Desmoinesian strata (middle Moscovian) as indicated by Komia from MC (Table 1) and conodonts and fusulinids from AC (Bishop et al., 2010), LC and MSP (Table 1). The Atokan-Desmoinesian boundary is well constrained in the mid-to-outer platform sections (AC, LC) and was correlated to landward successions using sequence stratigraphy (Fig. 6). Decompacted thickness estimates for the sequences range from 69 m at the inner platform (MSP) in sequence 3 to 216 m at the outermost platform-to-upper slope (MC) in sequence 4. Sequence 3 is defined at the base by a 1.4 m thick unit of slumped and contorted bedding and cm-scale angular skeletal clasts overlying an erosive contact that is interpreted as a debris flow on the outermost platform-to-upper slope (MC). This debris flow likely formed in response to instability on the outermost platform and upper slope with the inferred relative sea-level fall between sequences 2 and 3 (Fig. 5). On the platform, the sequence boundary is distinguished by cryptalgal laminites and grain/packstones. The TST consists of heterozoan grain/packstones that deepen upward into calcisiltites or offshore wacke/packstones (AC), and photozoan grain/packstones and wacke/ packstones are present further inboard (LC, MSP). The MFS is picked at the base of progradational parasequence set 3.2 and consists of calcisiltites along the mid-to-outer platform (AC, LC). The HST consists of calcisiltites or offshore wacke/packstones capped by cross-bedded calcareous quartz siltstones (MC) or grain/packstones (AC, LC). Middle to inner platform HST facies consist of large-scale cross-bedded grain/ packstones interpreted as migrating channel bars with gypsum precipitates (LC, Fig. 3C–F) and lagoonal wacke/packstones capped by massive chert interpreted as replacements of bedded evaporites (MSP). The basal boundary of sequence 4 (Fig. 5) is defined by cross-bedded calcareous quartz siltstones (MC), paleosols (AC), dolomudstones with evaporite pseudomorphs (LC), and massive chert replacing bedded evaporites (MSP). The TST consists of calcisiltites farthest seaward (MC), calcisiltites, offshore wacke/packstones, and grain/packstones on the mid-to-outer platform (LC, AC). The MFS is picked at the base of progradational parasequence set 4.2 and consists of marls on the outer platform (AC) grading landward into calcisiltites (LC). The HST is defined by calcisiltites along the outermost platform-to-upper slope (MC) and calcisiltites grading upward into mixed heterozoan–photozoan grain/packstones, karsted horizons, or restricted wacke/packstones with evaporite pseudomorphs (AC, LC), whereas restricted platform facies dominate the inner platform succession (MSP). Sequence 5 is defined at the base by paleosols and dolomudstones with evaporite pseudomorphs on the mid-to-outer platform (AC, LC). Systems tracts are best defined along the mid-to-outer platform (AC, LC). The TST is characterized by calcisiltites on the outermost platform-to-upper slope (MC), calcisiltites and grain/packstones on the outer platform (AC), and photozoan grain/packstones and lagoonal wacke/packstones on the middle to inner platform (LC, MSP). The MFS

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is picked at the base of progradational parasequence set 5.3 and consists of calcisiltites extending into the mid-platform (LC). The HST records deposition of calcisiltites and ‘mid-platform’ wackestones along the outermost platform-to-upper slope (MC). These grade landward into calcisiltites coarsening upward into grain/packstones and wacke/packstones on the mid- to outer platform (AC, LC) and grain/packstones coarsening upward into restricted platform interior facies on the inner platform (MSP). Sequences 3, 4, and 5 are interpreted to record the long-term fall in relative sea-level that commenced in the latest Morrowan and continued through the Atokan (late Bashkirian and early Moscovian). A thin retrogradational interval (R3.1) records the last of several major flooding events onto the middle platform that characterized the latter part of the Morrowan and early Atokan. Eleven short-term (10 5 yr) sea-level fluctuations of parasequence set-scale are superimposed on the long-term fall and are inferred to be of relatively high magnitude (40 to >70 m) based on the presence of well-developed paleosols and eolianites juxtaposed against deep-water marls and calcisiltites (Fig. 5). Platform-wide progradation occurs toward the tops of each sequence (see parasequence sets 3.3, 4.4, and 5.4 in Fig. 5). 3.4. Sequence 6 Sequence 6 was deposited during the early–mid Desmoinesian (mid–late Moscovian), as indicated by conodonts and fusulinids from the MC, LC, and MSP (Table 1) and AC (Bishop et al., 2010) successions. The decompacted thickness of sequence 6 varies from 21 m at the inner platform (MSP) to 331 m at the outermost platform-toupper slope (MC). Sequence 6 is bounded at the base by offshore wacke/packstones on the outermost platform-to-upper slope (MC), dolomudstone with evaporite pseudomorphs on the mid-to-outer platform (AC, LC), and laminites and massive chert interpreted as replaced bedded evaporites on the inner platform (MSP). The LST is a lowstand wedge (Fig. 6) composed of calcisiltites, debris flows, and offshore wacke/packstones along the outermost platform-to-upper slope (MC) coarsening upward to grain/packstones and dolomudstones along the outer platform (AC). The TST consists of calcisiltites along the outer platform-to-upper slope (MC, AC) coarsening updip into grain/ packstones or lagoonal wacke/packstones along the mid-to-outer platform (AC, LC). The MFS is picked at the base of parasequence set 6.4 and consists of marls (AC) grading landward into lagoonal wacke/ packstones on the mid-platform (LC). Notably, facies of the HST on the outermost platform-to-upper slope (MC) shallow upwards from cherty calcisiltites to wacke/packstones with pseudomorphs of gypsum laths, recording the only occurrence of shallow water facies with evidence of restricted circulation at this location. Elsewhere, the HST consists of calcisiltites, grain/packstones, wacke/packstones, and restricted platform interior facies (AC, LC) that grade landward into restricted platform interior facies (MSP). The earliest Desmoinesian lowstand wedge (LST) of sequence 6 is interpreted to record the culmination of the long-term relative sealevel fall that characterized the Atokan (latest Bashkirian and early Moscovian). Retrogradational parasequence set 6.2 (Fig. 6) records the onset of transgression in the early Desmoinesian (mid-Moscovian) that terminated with renewed progradation of inner and mid-platform facies onto the outer platform and upper slope (progradational parasequence sets 6.4 and 6.5) in the middle Desmoinesian (late Moscovian). Magnitudes of short-term fluctuations of 35 to >70 m are inferred from the juxtaposition of dolomudstones and restricted lagoonal facies against calcisiltites in the TST and early HST of sequence 6 (Table 3). 3.5. Sequences 7, 8, 9 and 10 Sequences 7 through 10 incorporate upper Desmoinesian (uppermost Moscovian) and Virgilian (Gzhelian) strata as indicated by

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conodonts from MC (Table 1) and AC (Bishop et al., 2010) and fusulinids from across the platform (MC, AC, LC, and MSP; Table 1). Decompacted thickness estimates for the sequences range from 45 m on the inner platform (MSP) in sequence 9 to 156 m on the outermost platform-to-upper slope (MC) in sequence 8. The Desmoinesian–Missourian boundary is identified at AC and was correlated across the platform using sequence stratigraphy constrained by biostratigraphy. The Missourian–Virgilian and Permo-Carboniferous boundaries are not well constrained in the Bird Spring Fm. due to the predominance of restricted inner platform facies, sparse conodonts, and low fusulinid diversity (Amoco: Groves and Miller, 2000; Bishop et al., 2010). The Missourian–Virgilian boundary in the Arrow Canyon succession has been reassigned to A456 (Groves and Miller, 2000), constraining the top boundary of sequence ten at Arrow Canyon as mid–late Virgilian. Sequence 7 is bounded at the base by wacke/packstones with gypsum pseudomorphs on the outermost platform (MC), karst along the outer platform (AC), dolomudstone with evaporite pseudomorphs on the mid-platform (LC), and low-angle calcareous quartz siltstone on the inner platform (MSP) (Fig. 6). In contrast to all previous sequences, parasequence sets and systems tracts can be straightforwardly correlated across the entire platform due to progradation of platform facies to the outermost location (MC) and minimal subaerial exposure features as far inboard as the inner platform (MSP). Heterozoan facies markedly decrease in abundance and photozoan facies dominate for the remainder of the succession. The TST is characterized by calcisiltites or offshore wacke/packstones coarsening upward to grain/packstones. Abundant above-SWB fossiliferous wacke/pack/ grainstones at MC, interpreted as bioherms, are unique to this sequence. The MFS occurs at the base of parasequence set 7.2 and is composed of calcisiltites at MC that grade landward into lagoonal wacke/packstones. The HST consists of calcisiltites or offshore wacke/packstones coarsening upward into tidal laminites and dolomudstones with evaporite pseudomorphs. Sequence 8 is bounded at the base by a thin (1.5 m) mudsupported conglomerate interpreted as a debris flow on the outermost platform-to-upper slope (MC), dolomudstones with evaporite pseudomorphs on the outer platform (AC), and laminites on the inner platform (MSP; Fig. 6). A thin TST and the MFS are recognizable on the outer platform (MC, AC). The TST consists of calcisiltites and offshore wacke/packstones along the outermost platform (MC) that grade upslope into grain/packstones and lagoonal wacke/packstones. Calcisiltites and offshore wacke/packstones define the MFS, which is picked near the base of parasequence set 8.2. The HST consists of offshore wacke/packstones and grain/packstones grading upsection into deeper water calcisiltites along the outermost platform-to-upper slope (MC) that grade landward into grain/packstones, lagoonal wacke/packstones, and restricted platform interior facies. Sequence 9 is bounded at the base across the platform by dolomudstones with evaporite pseudomorphs. Parasequence sets and systems tracts are easily identified except on the outermost platform (MC), which is dominated by calcisiltites and capped by an erosional unconformity (Fig. 6). Progradational parasequence set 9.1 forms the LST and is characterized by calcisiltites along the outermost platformto-upper slope (MC) grading upslope into photozoan pack/grainstones, lagoonal wacke/packstones, and restricted platform interior facies (AC, LC, MSP). The thin TST (parasequence set 9.2) consists of photozoan pack/grainstones that also define the MFS. The HST consists of photozoan pack/grainstones, lagoonal wacke/packstones, and restricted platform interior facies platform-wide. Sequence 10 (Fig. 6) is bounded at the base by dolomudstones with evaporite pseudomorphs across the mid-to-outer platform (AC, LC) to inner platform (MSP). The TST is dominated by photozoan pack/grainstones and lagoonal wacke/packstones, and the MFS is characterized by photozoan pack/grainstones. Aggradational parasequence sets 10.2 and 10.3 form the HST and consist of photozoan

Avg. thicknesses and durations were not calculated for parasequences and parasequence sets in seq. 10 due to lack of age control at top of sequence. Thicknesses and durations were averaged between Arrow Canyon and Lee Canyon to minimize the influence of local variations. Durations approximated within biostratigraphic constraints using thickness as an estimate of time; absolute ages for N. Amer. stages taken from Davydov et al. (2010) and M. Schmitz and V. Davydov (pers. comm., 2011). d Error estimates for basal boundaries of Morrowan, Atokan and Desmoinesian stages were assigned values of ± 0.5 My (Davydov, pers. comm. 2010); ±0.4 My was assigned for the base of the Missourian and Virgilian by using longeccentricity tuning of correlated cyclothems between the Bird Spring platform, Donets basin, and N. Amer. Midcontinent cyclothems (Heckel et al., 2007; Heckel, 2008; Davydov et al., 2010). e Parasequence and parasequence set durations were estimated by dividing the amount of time each stage or sequence represents by the number of depositional units within each stage or sequence. Error estimates were determined by dividing the max. and min. stage durations calculated using error estimates for those boundaries by the # of parasequences within each stage. c

b

a

3.1 8.4 12.2 11.5 8.4 7.8 11.3 18.1 14.3 485 ± 75 1230 ± 250 422 ± 89 349 ± 73 499 ± 250 562 ± 86 1079 ± 166 894 ± 450 1194 ± 400 13.0 40.1 44.9 37.5 48.4 28.0 45.4 36.2 42.8 52 160 135 150 194 140 91 72 86 1 2 3 4 5 6 7 8 9

1.9 ± 0.3 4.9 ± 1.0 1.3 ± 0.3 1.4 ± 0.3 2.0 ± 1.0 2.8 ± 0.4 2.2 ± 0.3 1.8 ± 0.9 2.4 ± 0.8

Decompacted thickness (m)b

Durations of sequences and their internal components were estimated in order to evaluate possible drivers of cyclicity and to test whether cycle durations fall within Milankovitch-band parameters (Table 4). Integration of available biostratigraphy (Table 1) with the sequence stratigraphic framework provides sub-stage (~1 my) resolution. Sequence durations range between ~1.3± 0.3 and 2.8 ± 0.4 my (Table 4) and fall within the range of the long-period eccentricity modulation of both obliquity (~1.2 my) and eccentricity (~2.4 my) (Lourens and Hilgen, 1997; Laskar et al., 2004) with the exception of sequence 2 (~4.9 my). This sequence may be a composite with a bounding sequence boundary between two sequences masked by the prevalence of deep-water sediments in this interval. Parasequence set durations range from ~349±73 to 1230±250 ky and fall within the range of both long-term eccentricity (400 kyr) and the long-period eccentricity modulation of obliquity (~1.2 my) (Lourens and Hilgen, 1997; Laskar et al., 2004). Average parasequence durations range from ~87 ± 43 to 398 ± 133 ky and fall within the range of both long- and short-period eccentricity that have been widely recognized in Permo-Carboniferous strata (e.g., Heckel, 1986; Rasbury et al., 1998; Eros et al., 2012). The large number of parasequences recorded in Atokan sequences 3 through 5 likely reflect the high rates of accommodation during this time (see following section). We interpret this interval to record the fewest “missed beats” in short-term sea level and to provide the best estimate of the durations of short-term fluctuations. Parasequence durations inferred from the Atokan interval are 87 ± 44 to 115 ± 24 ky (avg. of ~103 ky) with durations of parasequence sets between 349 ± 73 and 499 ± 250 ky (avg. of ~423 ky). The duration of sequences into which these are bundled range between 1.3 ± 0.3 and 2.0 ± 1.0 my.

Table 4 Sequence, parasequence set, and parasequence thicknesses and durations.

4. Duration of sequences, parasequences, and parasequence sets

Estimated sequence duration (my)c,d

Avg. parasequence set thickness (m)

Estimated parasequence set duration (ky)d,e

Avg. parasequence thickness (m)

Estmated parasequence duration (ky)d,e

wacke/pack/grainstones grading upsection into restricted platform interior facies. The upper sequence boundary is defined by dolomudstones with evaporite pseudomorphs on the outer platform (AC) and massive chert after bedded evaporites on the inner platform (MSP). Sequences 7 and 8 record the continuation of the progradation initiated in Sequence 6 through the late Desmoinesian (latest Moscovian– early Kasimovian). This period of overall progradation and inferred fall in relative sea-level is interrupted by two periods of retrogradation (sea-level rise) in the late Desmoinesian. Deposition of debris flows and bioherms on the outermost platform-to-upper slope during maximum seaward progradation at the base of Sequence 7 and middle of Sequence 8 (Desmoinesian–Missourian boundary interval; Fig. 6) suggests maximum offlap on the Bird Spring platform at these times. The stratigraphic juxtaposition of photozoan grain/packstones and sub-SWB calcisiltites in these two sequences further suggest magnitudes of short-term (10 5 yr) sea-level fluctuations of 20 to > 65 m. In contrast, the consistent juxtaposition of peritidal facies and other shallow-water photozoan wacke/pack/grainstones in sequences 9 and 10 coupled with the loss of evidence of exposure surfaces suggest that the magnitudes of short-term fluctuations were substantially dampened (≤20 m) in the late Pennsylvanian (Missourian–Virgilian) relative to those inferred from older sequences. This, coupled with the abrupt return to sub-SWB deposition on the outer platform-to-slope and the dominance of aggradational parasequence sets in sequences 9 and 10, suggests that part of the late Pennsylvanian recorded by the Bird Spring succession was a period of gradually increasing relative sea-level. The shift in facies and sequence style at the Desmoinesian–Missourian boundary (i.e., abrupt shift to sub-SWB deposition at MC, laterally continuous shallow water facies across the platform) also suggests a change in platform geometry from a gently sloping ramp to a steep rimmed flat-topped shelf.

114 ± 18 224 ± 45 115 ± 24 107 ± 23 87 ± 44 156 ± 24 360 ± 55 358 ± 180 398 ± 133

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Sequence #a

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5. Discussion Facies and cycle stacking patterns define retrogradational, aggradational, and progradational trends across the Bird Spring platform during the Pennsylvanian from which a relative sea-level history can be inferred. Comparison of this relative sea-level history to other far- and near-field records permits further evaluation of the glaciation history during this time, an issue which remains incompletely understood (Fielding et al., 2008b; Eros et al., 2012).

5.1. Long-term accommodation history Estimation of long-term sedimentary accumulation rates (Fig. 7; Table 2) for the Bird Spring Fm. reflects how long-term accommodation in this region varied throughout the Pennsylvanian given the in situ production and proximal source of sediment supply in carbonate systems. Average long-term accommodation across the Bird Spring platform was low during the early Pennsylvanian (avg. long-term sediment accumulation rates of 29 mm/ky for the Morrowan), peaked during the early middle Pennsylvanian (avg. of 90 mm/ky for the Atokan), and decreased during the later middle and late Pennsylvanian (avg. of 52 to 25 mm/ky during the Desmoinesian and Virgilian, respectively). Similar accommodation trends are defined across the platform, with the exception of the outermost platform-to-upper slope (MC) succession where long-term accumulation rates differed due to the additional allochthonous source (debris flows) of carbonate sediment.

A

B Fig. 7. Long-term accommodation history of the Bird Spring platform evaluated through (A) cumulative decompacted thickness and (B) long-term sediment accumulation rates presented by stage. Biostratigraphic tie-points (circles) from Table 1; correlated stage boundary tie-points (squares) from Figs. 5 and 6. Numeric ages interpolated using stratigraphic thickness within stages and the assigned ages of corresponding North American stage boundaries from Davydov et al. (2010) and M. Schmitz and V. Davydov (pers. comm., 2011). Rates not calculated for the Missourian–Virgilian and Virgilian stages for MC and LC, respectively, given the termination of the records below the Permo-Carboniferous boundary. M.—Missourian.

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The overall low accommodation of the earliest Pennsylvanian and anomalously high accumulation rates on the outermost platformupper slope (avg. of 56 mm/kyr) due to stacked gravity-driven slope deposits most likely record the eustatic sea-level lowstand of the mid-Carboniferous recorded globally (summarized in Rygel et al., 2008). In contrast, the Atokan maximum accommodation, previously noted by Bishop et al. (2010) for the region, likely reflects increased regional subsidence during that time given independent evidence for a contemporaneous eustatic low (Ross and Ross, 1987; Eros et al., 2012) and development of glaciation in most southern Gondwanan basins in the Early Pennsylvanian (summarized in Fielding et al., 2008b). The reduced accommodation on the middle to inner platform during the later part of the Pennsylvanian permitted widespread progradation of shallow water carbonates across the Bird Spring platform and thrombolitic mound formation on the outermost platform (MC section, this study; Nevada Test Site, northern margin; Miller and Heller, 1994).

5.2. Relative sea-level history A relative sea-level history for the Pennsylvanian inferred from the sequence stratigraphy and constrained by the long-term accommodation history of the Bird Spring succession is shown in Fig. 8. The globally recognized, mid-Carboniferous lowstand is recorded in the Bird Spring Formation by the seaward progradation of shallowwater carbonates to the outer platform, platformwide development of exposure features including karst, and repeated deposition of gravitydriven turbidites and debris flows on the outermost platform-upper slope. A subsequent rise in relative sea level in the middle to late Morrowan (later half of the Bashkirian) is indicated by the onlap of sub-SWB calcisiltites onto the mid-platform and overall loss of evidence for subaerial exposure across the Bird Spring platform. A protracted relative sea-level fall through the latest Morrowan and Atokan (latest Bashkirian and early Moscovian; Fig. 8) is suggested by the overall progradational nature of this interval of the Bird Spring succession. Short-lived episodes of retrogradation of deeper-water calcisiltites onto the middle platform are superimposed on the latest Morrowan and earliest Atokan portions of the longerterm sea-level fall, after which time the platform was dominated by seaward progradation in response to falling relative sea level through to the earliest Desmoinesian. The high accommodation rates of the Atokan make this interval of the Bird Spring succession sensitive to recording the full magnitude of superimposed short-term sea-level changes (parasequence-scale of ~ 0.1 my) as previously suggested by Bishop et al. (2010). Magnitudes of short-term fluctuations for the Atokan range from minimally 20 m to well over 70 m, the latter constrained by the assumed depth of storm wave-base (Fig. 8). Notably, the average duration (103 ky) of these short-term sea-level fluctuations suggests a short-eccentricity forcing of eustasy at this time. The early to middle Desmoinesian (middle to late Moscovian) is characterized by a long-term rise in relative sea level (Fig. 8). Renewed seaward progradation of inner and middle platform carbonates in the later part of the Desmoinesian (late Moscovian) is interpreted to record the onset of a long-term fall in sea level that continued through to the end of the Desmoinesian (early Kasimovian). Maximum offlap occurred in the late Desmoinesian and at the Desmoinesian–Missourian boundary interval as indicated by the most basinward extension of shelf facies (e.g. bioherms, wacke/packstones with evaporite pseudomorphs) on the outermost platform-to-upper slope. Two periods of shorter-term retrogradation and inferred rise in relative sea level interrupt the longer-term late Desmoinesian fall. A shift in the estimated average duration of parasequences from 103 to 373 ky (Table 4) occurs in the mid-Desmoinesian and could indicate a shift from short- to longterm eccentricity forcing of high-frequency eustasy. This hypothesis

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moderate to high sea-level falls during the late Pennsylvanian period of low accommodation would have exposed the platform and stranded tidal flats landward (Read et al., 1986). The termination of the hypothesized long-term rise is unknown given that the Bird Spring sequence stratigraphic record in this study does not extend past the mid-to-late Virgilian. 5.3. Comparison to other far- and near-field records

Fig. 8. Long-term relative sea-level curve (black) for the Bird Spring platform defined using parasequences and parasequence sets; gray curve defined using sequences. Lithofacies were assigned positions on the horizontal axis based on relative water depth; curve was estimated using dominant lithofacies of the two mid-to-outer platforms sections (Arrow Canyon and Lee Canyon). Time-scale after Davydov et al. (2010) and M. Schmitz and V. Davydov (pers. comm. 2011). See Fig. 9 caption for details of temporal calibration of the Bird Spring curve. Range of magnitudes of inferred sea-level change during deposition of high frequency parasequences shown by black bars on far right.

requires further testing by spectral analysis. Inferred magnitudes of short-term (105 ky) fluctuations remain in the ~20 to >70 m range (Fig. 8). An abrupt shift from a dominance of progradational to aggradational parasequence sets in the Missourian to mid-Virgilian interval of the Bird Spring succession is interpreted to record a period of gradually increasing relative sea level despite the overall low accommodation indicated by the widespread shoaling of facies through this interval (Fig. 8; see following section). A steady slow increase in long-term relative sea level would have been required for the sustained aggradation of these predominantly shallow-water carbonates given that low accommodation coupled with higher rates of carbonate production would have favored extensive seaward progradation of inner platform facies and platform-wide development of exposure surfaces, which are not observed in this interval of the Bird Spring succession. Furthermore, the near lack of exposure features on the late Pennsylvanian Bird Spring platform but widespread development of tidal flat facies within each parasequence suggests substantially lowered (≤20 m) magnitudes of short-term sea-level fluctuations at this time (Fig. 8). This reflects that

Comparison of the Bird Spring relative sea-level curve with onlap– offlap records for other paleotropical basins indicates an overall similar sea-level history across tropical Pangaea during much of the Pennsylvanian. The Bird Spring record reveals two periods of major sea-level fall across the mid-Carboniferous boundary and in the early Pennsylvanian (latest Morrowan through earliest Desmoinesian or latest Bashkirian through early Moscovian) that coincide with lowstands delineated by the onlap–offlap history of the cyclothemic Donets Basin (Fig. 9; Eros et al., 2012) and Moscow Basin (Alekseev et al., 1996). These lowstands are coincident with major glaciations in northwestern Argentina and eastern Australia and possibly in the Parana Basin, Brazil (Fig. 9; Fielding et al., 2008a; Gulbranson et al., 2010; Holz et al., 2010), and the more common occurrence of heterozoan-dominated facies during the Morrowan to Atokan is interpreted to reflect overall cooler ocean temperatures and/or high nutrient levels (James, 1997). The intervening highstand in the middle to late Morrowan (later half of the Bashkirian) inferred from the Bird Spring succession corresponds to a period of overall stable sea level in the Donets Basin. This interval of sea-level rise, however, is contemporaneous with a period of mid-Bashkirian onlap inferred from the carbonate-dominated Moscow Basin succession but not observed in the Donets record (Alekseev et al., 1996). The magnitude of the latest Morrowan to earliest Desmoinesian relative sea-level fall in the Bird Spring record is dampened relative to that defined by the degree of time-equivalent offlap (early Moscovian) in the Donets Basin (Fig. 9). This difference may reflect the substantial increase in regional subsidence on the Bird Spring platform at this time. A rise in relative sea level beginning in the middle Pennsylvanian (early Desmoisnesian or middle Moscovian) is defined by both the Bird Spring and Donets records, and coincides with the end of the middle Pennsylvanian interval of glaciation in several high-latitude Gondwanan basins (Fig. 9). In the Bird Spring record, this rise is terminated by the onset of two long-term falls in sea level, punctuated by a shortterm rise, in the later part of the Desmoinesian (late Desm. to earliest Missourian or latest Moscovian to early Kasimovian). These two periods of offlap likely correspond to two intervals of lowstand (late Moscovian and early Kasimovian) superimposed on the middle to late Pennsylvanian long-term rise of the Donets Basin, (Fig. 9; Eros et al., 2012). The younger of these two lowstands also corresponds to a cyclothemic interval in the Midcontinent and Appalachian Basin that is interpreted to record the largest regressive events of the mid-to-late Pennsylvanian (Belt et al., 2011; Falcon-Lang et al., 2011). These shorter-term regressive events may correspond to discrete glacial cycles in the KalahariKaroo and Parana basins although the paucity of chronostratigraphic constraints on their ages precludes further evaluation of this hypothesis (summarized in Eros et al., 2012). The overall similarity in relative sea level history inferred from the Bird Spring carbonate succession to those inferred from other paleotropical cyclothemic successions suggests a predominant influence of eustasy on the early and middle Pennsylvanian (through to the early Kasimovian or late Desmoinesian) portion of the Bird Spring stratigraphic record. The Bird Spring relative sea-level curve, however, diverges from the Donets and Midcontinent inferred sea level histories beginning in the Missourian (early Kasimovian) with the onset of long-term gradual shoaling of facies suggesting low relative sea level (Fig. 9). By comparison, the Donets and Midcontinent inferred sea level histories reveal a long-term stepwise rise through to the early Gzhelian upon which shorter-term lowstands are superimposed. This

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A

B

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C

Fig. 9. Comparison of the (A) relative sea-level curve inferred from a cross-platform reconstruction of the Bird Spring Fm. to (B) a recently published onlap–offlap curve for the Donets Basin, Ukraine, and (C) a synthesis of glacial records from mid-to-high latitude Gonwanan basins (Eros et al., 2012). Relative sea-level curve is calibrated to the timescale by stage boundaries for which chronostratigraphic uncertainty in the Bird Spring Fm. is limited and by pinning of sequence boundaries and major progradational or retrogradational events to time-equivalent offlap or onlap events defined by the radiometrically calibrated Donets curve. Position of Bird Spring Fm. sequence boundaries on relative sealevel curves indicated by dashed gray lines. Shaded portion of curve corresponds to inferred shift in platform geometry to a steep-rimmed shelf during the late Pennsylvanian.

divergence coincides with a shift in geometry of the Bird Spring platform (recorded between sequences 7 and 8 in Fig. 6) from a distally steepened ramp (Fig. 10A) to a flat-topped shelf (Fig. 10B). Gently sloping carbonate ramps, including distally steepened ramps, can evolve into flat-topped shelves with tectonic reactivation of normal faults; differential sediment accumulation rates on the highly productive shelf edge versus the lower productive, sediment starved slope and basin further enhances the shelf-to-slope geometry (Read, 1985, 1998). A shift in platform geometry from a gently sloping ramp to a flat-topped shelf has been previously interpreted to record middle Pennsylvanian activation of a transform fault system along the southwestern margin of the North American continent (Stone and Stevens, 1988; Stevens et al., 1997, 2005; Stevens and Stone 2007). MidPennsylvanian faulting recognized in northern Nevada could have also potentially contributed to the changing platform geometry (Trexler and Cashman, 1997; Trexler et al., 2003, 2004). We propose here an alternative hypothesis for the abrupt change in platform geometry — that the long-term shoaling of the Bird Spring platform synchronous with the shift in platform geometry may record a gradual eustatic rise coincident with a notable drop in magnitude of superimposed short-term sea-level fluctuations. However, the earlier hypothesized structural

controls may have influenced the ultimate position of the aggradational shelf margin. In this model, shallow water platform carbonates would have rapidly built to sea-level with each short-term sea-level rise, with peritidal facies easily prograding across the flat-topped platform with each subsequent short-term sea-level fall. Aggradational fill of accommodation space would have occurred within a few short-term sea-level cycles, thus requiring continued but gradual sea-level rise to allow the accumulation of such a thick package of shallow water carbonates (as observed in sequences 8–10) to accumulate (Read et al., 1986; Read, 1995; Emery et al., 1996). The repeated deposition of parasequences that kept up with such a prolonged sea level rise would have ultimately led to thick packages of aggradational to progradational cycles with tidal flat caps spanning across the platform, as observed in the Missourian and Virgilian sequences in the Bird Spring succession (sequences 8–10 in Fig. 6). The return of deeper water calcisiltites on the outermost platform-to-upper slope during the late Pennsylvanian further supports the notion of increased relative sea-level at this time and suggests the less productive outer margin was unable to keep up with the rapid onset of a long-term rise. The disparity in production rates between the inner and outer

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A

B

Fig. 10. Depositional models for the Bird Spring platform: (A) early–middle Pennsylvanian distally steepened ramp geometry and (B) late Pennsylvanian steep-rimmed shelf geometry.

platform would have led to the development of a well-defined slope fronting a flat-topped platform in a few short-term sea-level cycles. This hypothesis is supported by the development of stacked meterscale phylloid algal bioherms with bypass channels on the outer platform to the north (Nevada Test Site) and east (Death Valley) of the study area during the late Pennsylvanian (Miller and Heller, 1994; Stevens et al., 2001). An analagous transition from a ramp to flat-topped shelf geometry during sea level rise is documented in carbonate successions throughout geologic history, including the Silurian western U.S. (Hurst et al., 1985), the Devonian Canadian Rocky Mountains (Whalen et al., 2000), and the Cretaceous in northern Oman (Hillgärtner and Strasser, 2003). If this interpretation is correct, then the late Pennsylvanian sea-level rise inferred from the Bird Spring stratigraphy is contemporaneous with the time-equivalent long-term rise defined by the Donets onlapofflap curve (Fig. 9). Moreover, in this context, the shorter-term lowstands and intervening rises of the later half of the Desmoinesian Bird Spring record would be part of the longer-term stepwise rise of the mid-to-late Pennsylvanian suggested by the Donets onlap–offlap curve (Eros et al., 2012). This interval coincides with a time of inferred global warming and glacial retreat prior to the onset of the Early Permian glaciations (Isbell et al., 2003; Fielding et al., 2008a; DiMichele et al., 2009; Gulbranson et al., 2010). High latitude glaciation at this time may have been restricted to discrete glacial cycles in the KalihariKaroo and Parana basins, although a paucity of unequivocal chronostratigraphic constraints precludes more detailed evaluation of the temporal relationship to the sea level history inferred from paleotropical basins (summarized in Eros et al., 2012). Although faulting along the outermost platform margin of the Bird Spring platform cannot be dismissed as a possible origin for the inferred shift in platform geometry, it cannot account for the substantial decrease in inferred magnitudes of parasequence-scale (10 5 yr) fluctuations in the late Pennsylvanian portion of the Bird Spring record. The inferred high magnitudes of high frequency sea-level changes of

the early to mid-Pennsylvanian require both rapid forced regressions because parasequence thicknesses are much less than inferred maximum water depths (8.8 m compared to >40 m) and rapid forced transgressions in order to account for the abrupt shift in water depths inferred from stratigraphically juxtaposed facies (see Section 3). Such sea-level behavior is inconsistent with structural models attributing carbonate parasequence formation to episodic tectonic activity (Cisne, 1986; De Benedictis et al., 2007). A structural origin for late Pennsylvanian high frequency fluctuations of dampened magnitude, however, cannot be dismissed given that fault-related subsidence or uplift superimposed on longer-term regional subsidence have been shown to develop stacked peritidal cycles (Burgess, 2001; De Benedictis et al., 2007; Bosence et al., 2009). Nevertheless, facies stacking patterns and the paucity of subaerial exposure features in the upper Pennsylvanian interval of the Bird Spring succession precludes the occurrence of high-magnitude short-term sea-level fluctuations during the late Pennsylvanian. 6. Conclusions The carbonate-dominated paleotropical successions of the Bird Spring Formation preserve a highly sensitive record of glacioeustacy and shifting environmental conditions during the late Paleozoic. Inferred periods of sea-level maxima and minima correspond well to independent far- and near-field reconstructions of Pennsylvanian sealevel, although regional variation in subsidence and possible tectonic activity related to post-Antler deformation could have contributed to longer-term changes in relative sea-level in this region of western equatorial Pangaea. Inferred magnitudes of short-term (105 yr) sea-level fluctuations superimposed on the longer-term rises and falls range from 20 to >70 m during the early–mid Pennsylvanian and are substantially reduced (≤20 m) during the late Pennsylvanian. The shift coincides with a platform geometry change from a distally steepened ramp to a steep-rimmed platform, which is interpreted to result from

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long-term transgression and a decrease in short-term relative sea-level magnitudes that enabled the carbonate platform to keep up with sealevel rise. This record corroborates a period of glacial minimum during the late Pennsylvanian and agrees with high-latitude records that suggest the late Paleozoic icehouse was most likely characterized by discrete, short-lived ice ages separated by glacial minima. The Bird Spring record demonstrates low-latitude cyclothemic deposits are faithful records of high-latitude ice sheet fluctuations and confirms a dynamic glaciation history during the late Paleozoic. Acknowledgments This work was supported by NSF grant EAR0545701 to Isabel P. Montañez and research grants from ExxonMobil and the Geological Society of America. We thank two anonymous reviewers for their thoughtful reviews and the National Park Service and the Bureau of Land Management for permission to visit the outcrops and collect data. Paul Stone provided field expertise and extensive discussion regarding the Bird Spring Formation in eastern California, and Calvin Stevens and Bruce Wardlaw provided biostratigraphic data. We thank Mara Brady, Kyle Meyer, Erik Gulbranson, Joshua Garber and Pascal Martin for field assistance. References Alekseev, A.S., Kononova, L.I., Nikishin, A.M., 1996. The Devonian and Carboniferous of the Moscow Syneclise (Russian Platform): stratigraphy and sea-level changes. Tectonophys 268, 149–168. Barnett, A.J., Burgess, P.M., Wright, V.P., 2002. Icehouse world sea-level behaviour and resulting stratal patterns in late Visean (Mississippian) carbonate platforms; integration of numerical forward modelling and outcrop studies. Basin Research 14 (3), 417–438. Barnett, A.J., Wright, V.P., 2008. A sedimentological and cyclostratigraphic evaluation of the completeness of the Mississippian–Pennsylvanian (Mid-Carboniferous) Global Stratotype Section and Point, Arrow Canyon, Nevada, USA. Journal of the Geological Society of London 165, 859–873. Belt, E.S., Heckel, P.H., Lentz, L.J., Bragonier, W.A., Lyons, T.W., 2011. Record of glacial– eustatic sea-level fluctuations in complex middle to late Pennsylvanian facies in the Northern Appalachian Basin and relation to similar events in the Midcontinent basin. Sedimentary Geology 238, 79–100. Bishop, J.W., Montañez, I.P., Gulbranson, E.L., Brenckle, P.L., 2009. The onset of MidCarboniferous glacio-eustasy; sedimentologic and diagenetic constraints, Arrow Canyon, Nevada. Palaeogeography, Palaeoclimatology, Palaeoecology 276 (1–4), 217–243. Bishop, J.W., Montañez, I.P., Osleger, D.A., 2010. Dynamic Carboniferous climate change, Arrow Canyon, Nevada. Geosphere 6 (1), 1–34. Blakey, R.C., 2008. Gondwana paleogeography from assembly to breakup: a 500 myr odyssey. In: Fielding, C.R., Frank, T.D., Isbell, J.L. (Eds.), Resolving the Late Paleozoic Ice Age in Time and Space: GSA Special Paper, 441, pp. 1–28. Bosence, D., Procter, E., Aurell, M., Kahla, A.B., Boudagher-Fadel, M., Casaglia, F., Cirilli, S., Mehdie, M., Nieto, L., Rey, J., Scherreiks, R., Soussi, M., Waltham, D., 2009. A dominant tectonic signal in high-frequency, peritidal carbonate cycles? A regional analysis of Liassic platforms from Western Tethys. Journal of Sedimentary Research 79, 389–415. Bouma, A.H., 1962. Sedimentology of Some Flysch Deposits: A Graphic Approach to Facies Interpretation. Elsevier, Amsterdam. 168 p. Burgess, P.M., 2001. Modeling carbonate sequence development without relative sealevel oscillations. Geology 29, 1127–1130. Cashman, P.H., Villa, D.E., Taylor, W.J., Davydov, V.I., Trexler, J.H., 2010. Late Paleozoic contractional and extensional deformation at Edna Mountain, Nevada. GSA Bulletin. doi:10.1130/B30247.1. Cassity, P.E., Langenheim Jr., R.L., 1966. Pennsylvanian and Permian fusulinids of the Bird Spring Group from Arrow Canyon, Clark County, Nevada. Journal of Paleontology 40, 931–968. Catuneanu, O., Abreu, V., Bhattacharya, J.P., et al., 2009. Towards the standardization of sequence stratigraphy. Earth-Science Reviews 92, 1–33. Cisne, J.L., 1986. Earthquakes recorded stratigraphically on carbonate platforms. Nature 323, 320–322. Davydov, V.I., Crowley, J.L., Schmitz, M.D., Poletaev, V.I., 2010. High-precision U–Pb zircon age calibration of the global Carboniferous time scale and Milankovitch band cyclicity in the Donets Basin, eastern Ukraine. Geochemistry, Geophysics, Geosystems 11, 1–22. De Benedictis, D., Bosence, D., Waltham, D., 2007. Tectonic control on peritidal carbonate parasequence formation: an investigation using forward tectono-stratigraphic modeling. Sedimentology 54, 587–605. Dickinson, W.R., Harbaugh, D.W., Saller, A.H., Heller, P.L., Snyder, W.S., 1983. Detrital modes of upper Paleozoic sandstones derived from Antler Orogen in Nevada; implications for nature of Antler Orogeny. American Journal of Science 283 (6), 481–509.

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