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Sequence stratigraphy and platform evolution of Lower–Middle Devonian carbonates, eastern Great Basin Maya Elrick Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 ABSTRACT Lower–Middle Devonian carbonates (270– 400 m thick) of the eastern Great Basin were deposited along a low-energy, westward-thickening carbonate platform. Six regional facies representing peritidal, shallow subtidal, stromatoporoid biostrome, deep subtidal, slope, and basin environments are recognized. Four third-order ('1.5–2.5 m.y. durations), transgressive-regressive sequences are identified across the platformto-basin transition based on deepening and shallowing patterns in regional facies, intensity and stratigraphic distribution of subaerial exposure features, and stacking patterns of fourth- to fifth-order, upwardshallowing peritidal and subtidal cycles. Transgressive systems tracts along the basin/slope are characterized by upwarddeepening successions of proximal through distal turbidites overlain by fine-grained, hemipelagic deposits. Shallow-platform transgressive systems tracts are composed of stacks of thicker-than-average peritidal cycles overlain by subtidal cycles or noncyclic deep subtidal facies. Maximum flooding zones along the shallow platform are composed of stacked peritidal cycles dominated by subtidal facies, noncyclic deep subtidal facies, or distinct deeper subtidal units within successions of restricted shallow subtidal or peritidal facies. Highstand systems tracts along the basin/slope are composed of hemipelagic deposits overlain by distal through proximal turbidites. Highstand systems tracts along the shallow platform are characterized by upward-shallowing succession of cyclic peritidal through shallow subtidal facies. Sequence boundary zones (2–16 m thick) along the shallow platform are composed of exposure-capped peritidal and subtidal cycles that exhibit upsection increases in the proportion of tidal-flat subfacies and increases in the intensity of cycle-capping subaerial exposure features. Sequence boundary zones along the basin/slope (6 –20 m thick) are composed of upward-shallowing successions of proximal turbidites or by platform-margin peloid shoal deposits; the absence of exposure features and meterscale cycles within basin/slope sequence boundary zones indicates that the combined rates of third- through fifth-order sea-level fall rates were less than tectonic subsidence rates. Sequence stratigraphic correlations between contrasting facies belts of the basin/slope (section NA) and the edge of the shallow platform (section TM) were independently verified with high-resolution conodont and brachiopod biostratigraphy. Correlation of sequences 1– 4 with transgressive-regressive sequences of similar age in the western, midwestern, and eastern United States, western Canada, and Europe indicates they are eustatic in origin. Systems-tract scale correlations across the study area indicate that the platform evolved from a homoclinal ramp to a distally steepened ramp, then into a flattopped platform (sequences 1–2). An incipiently drowned, intraplatform basin developed during sequence 3 as the result of third-order sea-level rise and differential sediment accumulation rates between the platform margin and intraplatform basin. During deposition of highstand systems tract 3, progradation infilled the intraplatform basin, resulting in a flat-topped platform. A distally steepened ramp developed during transgressive systems tract/maximum flooding zone 4 and evolved into a flattopped platform during highstand systems tract 4 deposition. The four sequences stack in an aggradational to slightly progradational pattern (‘‘keep-up’’ style sedimentation) and are bound by sequence boundary zones rather than unconformities, suggesting that greenhouse climate modes and second-order accommodation gains related to the lower portion of the second-order Kaskaskia sequence controlled sequencescale stacking patterns. GSA Bulletin; April 1996; v. 108; no. 4; p. 392– 416; 12 figures; 1 table. 392 INTRODUCTION Sequence stratigraphic concepts and terminology were originally defined from geometric relationships of seismic reflectors in siliciclastic systems (Vail et al., 1977). These original concepts have been refined and integrated with data from well logs, cores, and outcrops, and have incorporated the effects of high-frequency sea-level oscillations (104–105 yr) to understand the internal architecture of depositional sequences (e.g., Wilgus et al., 1988; Goldhammer et al., 1990, 1993; Montañez and Osleger, 1993). These stratigraphic techniques have been successfully applied to outcrops across carbonate platform-to-basin transitions where exposures are laterally continuous and permit tracing of stratal geometries and boundaries physically or by photo mosaics (Sarg, 1988; Franseen et al., 1993; Sonnenfeld and Cross, 1993). In structurally complex regions or where exposures are limited, such as the Great Basin, laterally continuous, undeformed outcrops are rare, and stratal geometries cannot be traced over long distances. This paper discusses the sequence stratigraphy and evolution of a Lower–Middle Devonian carbonate platform-to-basin system that developed in a passive-margin setting in the eastern Great Basin of the western United States (Fig. 1). The Lower–Middle Devonian deposits are present in blockfaulted mountain ranges characteristic of the Basin and Range Province; consequently, tracing of stratal geometries and critical horizons is not possible between isolated mountain ranges. In addition, biostratigraphically diagnostic fossils are present only in the deeper-water deposits, thus correlations based on high-resolution biostratigraphy are limited. The main objectives of this paper are (1) to illustrate how one-dimensional stratal stacking patterns at individual stratigraphic sections are used to correlate depositional sequences across a full platform-to-basin transition; (2) to interpret platform evolution at the systems- SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES ably below Middle–Upper Devonian platform carbonates (Guilmette Formation and coeval deposits) (Fig. 2). These Devonian deposits form the top of a 4- to 7.5-kmthick succession of uppermost Precambrian through Devonian passive-margin carbonates and siliciclastics (Stewart and Poole, 1974; Bond and Kominz, 1984). During the latest Devonian–Early Mississippian, eastward-directed thrusting related to the Antler orogeny juxtaposed lower–middle Paleozoic deep-water deposits over coeval shallow-water deposits along the Roberts Mountain thrust (Fig. 1; Roberts et al., 1958; Johnson and Pendergast, 1981; Speed and Sleep, 1982). Eight stratigraphic sections were measured along the platform-to-basin transition in Nevada (Fig. 1); six of the sections are autochthonous with respect to the Antler orogenic thrust belt, whereas the two more basinal sections were transported eastward by Antler-related and younger compressional events. REGIONAL FACIES AND DEPOSITIONAL INTERPRETATIONS Figure 1. Map of study area with location of measured sections and line of cross section shown in Figure 3. Regional extent of intraplatform basin modified from Johnson et al. (1989). PR 5 Pahranagat Range; SC 5 Schell Creek Range; SP 5 Sheep Pass, southern Egan Range; S 5 Sunnyside, southern Egan Range; CC 5 Cherry Creek Range; NM 5 Newark Mountain, Diamond Range; TM 5 Table Mountain, Mahogany Hills; NA 5 northern Antelope Range. tract scale, which alternated between a distally steepened ramp and a flat-topped platform; and (3) to interpret controls on sequence-scale stacking patterns. In an earlier paper (Elrick, 1995), the stratigraphy of the Lower–Middle Devonian deposits was discussed and interpreted in the context of understanding the spatial and temporal distribution and origin of high-frequency, upward-shallowing cycles or parasequences. For brevity in this paper, basic facies are summarized in table format (Table 1) and only a brief discussion of interpreted depositional environments is provided to facilitate sequence stratigraphic interpretations. The reader is referred to Elrick (1995) for detailed discussions of facies, subfacies, meter-scale cycles, and cycle-generating mechanisms. GEOLOGIC SETTING The uppermost Lower–Middle Devonian Simonson Dolomite and stratigraphic equiv- alents (Fig. 2) were deposited along a westward-thickening carbonate platform that extended along depositional strike from southern Canada to southeastern California, and '400 km across strike (Fig. 1; Osmond, 1954; Johnson et al., 1989, 1991). Oceanic deposits lay to the west of the platform, and the partially emergent Transcontinental Arch lay to the east (Sandberg et al., 1982; Johnson et al., 1989, 1991). Throughout the Early and Middle Devonian, the regional trend of the platform-to-basin slope break was roughly north-south through central Nevada (Fig. 1; Johnson and Murphy, 1984; Johnson et al., 1989). This relatively stable slope-break position had existed since the Late Silurian (Llandovery) when faultinduced(?) downdropping caused the slope break to backstep eastward '65 km (Johnson and Potter, 1975). Middle Devonian deposits lie disconformably above Lower Devonian platform dolomites (Sevy Dolomite) and conform- Six regional facies are recognized across the platform-to-basin transition; in order of increasing water depths these facies are peritidal, shallow subtidal, stromatoporoid buildup, deep subtidal (including intraplatform basin), slope, and basinal; all but the basinal and slope facies have been dolomitized. Meter-scale, upward-shallowing cycles (or parasequences) are present in peritidal through deep subtidal facies, but cannot be discerned in basinal, slope, and intraplatform basin facies. As a result of extensive dolomitization and recrystallization, meter-scale cycles are obscured within the coarse crystalline dolomite subfacies. Detailed descriptions of the regional facies and meter-scale cycles are given in Elrick (1995) and Table 1; brief environmental interpretations are given below. Four, large-scale (tens to hundreds of meters thick), transgressive-regressive sequences (sequences 1– 4) are recognized and correlated across the platform-to-basin transition using deepening and shallowing patterns in regional facies, the intensity and stratigraphic distribution of subaerial exposure features, and changes in meter-scale, cycle stacking patterns. Middle Devonian time scales of Odin et al. (1982), Palmer (1983), and Harland et al. (1982, 1989) indicate the average duration of the sequences Geological Society of America Bulletin, April 1996 393 M. ELRICK is '1.5–2.5 m.y.; as such, they are third-order in scale (Goldhammer et al., 1993). In this paper, the term platform is used in a general sense for a thick succession of mostly shallow-water carbonates and does not imply any specific morphology (e.g., Wilson, 1975; Read, 1985). The term ramp refers to a gently dipping surface (,18 slope) where shallow-water carbonates pass gradually into deeper-water carbonates (Ahr, 1973; Read, 1985). As a result of longterm changes in accommodation space, the Middle Devonian carbonate platform changed from a ramp to flat-topped platform, and the position of the shoreline migrated; consequently, to simplify discussions, the terms ‘‘inner’’ and ‘‘outer’’ ramp/ platform refer to present geographic positions. The inner ramp/platform includes stratigraphic sections in eastern Nevada (sections PR, SC, S, and SP), and the outer ramp/platform refers to sections in central 394 Nevada (CC, NM, and TM) (Figs. 1 and 3). The term shallow ramp or platform refers collectively to inner through outer ramp/ platform sections because, at these locations, the deposits are composed dominantly of shallow subtidal and peritidal facies. Basinal and slope facies occur only at section NA; consequently this section is referred to as ‘‘basinal.’’ When interpreting the regional cross section shown in Figure 3, it is important to realize that inner platform sections are oriented parallel to depositional strike, whereas basinal and outer platform sections are dip-oriented sections. Because the magnitude and direction of Tertiary Basin and Range extension and Mesozoic contraction are similar between sections along the inner platform (sections PR, SC, SP, S, and CC; Levy and Christie-Blick, 1989), the relative distance between these sections does not change appreciably when palinspastically restored. Consequently, the inner platform portion of the cross section was constructed using present (or actual) distances. Distances between dip-oriented sections CC, NM, TM, and NA were restored using data from Levy and Christie-Blick (1989) and Dobbs et al. (1993). Peritidal Facies Peritidal facies include thin and thick laminite beds, laminated-wavy beds, and monomict and polymict breccias (Table 1). These subfacies are arranged into upwardshallowing, meter-scale peritidal (capped by tidal-flat subfacies) and, less commonly, subtidal cycles (capped by subtidal subfacies). Thin to thick laminites subfacies are present across the shallow platform and form caps (0.1–2.8 m thick) to peritidal cycles and thin, transgressive bases (,0.1 m thick) to some cycles. Thin laminites repre- Geological Society of America Bulletin, April 1996 SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES sent full aggradation of the seafloor to highintertidal and supratidal water depths (Hardie and Shinn, 1986). Thick laminites were deposited in lower intertidal to restricted, shallowest subtidal environments as indicated by the conformable stratigraphic position below thin laminites, rare skeletal and trace fossils, and rare interbedding with thrombolites or LLH stromatolites. The abundance of planar laminae indicates deposition from suspension settling and traction transport as subtidal sediment was washed onto the tidal flats during storms and high tides (i.e., mechanical sedimentation dominated algal binding). Both thin and thick laminites display abundant evidence of subaerial exposure including sediment-filled dissolution cavities, horizontal and vertical desiccation cracks, rubble and solution-collapse breccias, microkarst erosion surfaces, textural homogenization resulting from in- cipient pedogenesis, and alveolar structures (Elrick, 1995). Laminated-wavy beds (0.2–3.3 m thick) are present at the base or in the middle part of peritidal cycles across the shallow platform. Ripple cross-laminae attest to traction transport and graded planar laminae indicate deposition from waning-flow, tidal, or storm currents. Restricted, shallow subtidal conditions are indicated by the paucity of skeletal and trace fossils, the stratigraphic position below tidal-flat subfacies, and by the lack of subaerial exposure features. Shallow subtidal environments have been interpreted for similar facies (‘‘ribbon rocks’’) by Demicco (1983), Osleger and Read (1991), and Cowan and James (1993). Monomict and polymict breccias are present at the tops of some peritidal cycles along the inner and outer ramp. Cycle-capping polymict breccia beds (0.08 – 4.0 m thick) represent solution-collapse breccias (e.g., Kahle, 1988; Knight and James, 1991) that developed during high-frequency (104– 105 yr) sea-level falls related to meter-scale cycle development (Elrick, 1995). No evidence of former evaporite minerals is observed, thus it is likely that more soluble phases such as aragonite and/or high-Mg calcite were preferentially leached. The variety of clast compositions indicates that at least several subfacies were involved in solution-collapse events. At section PR, sequence 4 is composed of an '60-m-thick polymict breccia unit (Fig. 3). The origin of this thick breccia unit is well understood; however, it is not likely related to high-frequency sea-level fluctuations, rather to events postdating Simonson Dolomite deposition. Monomict rubble breccias represent the initial breakdown of bedrock and protosoil formation during periods of subaerial expo- Geological Society of America Bulletin, April 1996 395 M. ELRICK ramp of sequence 1 (sections NA and TM). The abundance of open-marine fossil types (crinoids and brachiopods) and stratigraphic relationships with adjacent facies indicate deposition in normal marine, shallow subtidal environments. Stromatoporoid Buildup Facies Figure 2. Chronostratigraphic and biostratigraphic chart for Lower and Middle Devonian formations from shallow-water inner-platform (eastern Nevada) to deeper-water outer-platform and basin (central Nevada). Vertical lined pattern indicates unconformity. Modified from Kendall et al. (1983), Johnson et al. (1989), and Hurtubise (1989). sure (e.g., Meyers, 1988; Knight and James, 1991). Fitted fabrics and host-attached clasts represent in situ fragmentation and dissolution of lithified bedrock by downward-flowing, undersaturated fluids. Shallow Subtidal Facies Shallow subtidal facies include burrowmottled mudstone/wackestone, peloid-ooid packstone/grainstone, Amphipora wackestone/grainstone, coarse crystalline dolomite, and crinoidal packstone/grainstone subfacies (Table 1). They are arranged into peritidal and subtidal cycles. Burrow-mottled dolomudstone/wackestone (0.3- to 5.5-m-thick intervals) is common along the shallow platform and forms the base or the middle of peritidal cycles, and the tops of subtidal cycles. Deposition in low-energy, moderately restricted, shallow subtidal environments is indicated by the abundance of Thalassinoides burrows, the paucity of skeletal fossils, the stratigraphic position below tidal-flat subfacies, and the lack of subaerial exposure features. Peloid-ooid packstone/grainstone is rare and forms the base (,4 m thick) of some peritidal cycles along the shallow platform or forms thick, noncyclic intervals (.20 m thick) along the outer platform (Fig. 3). The deposits represent current- or wave-agitated, shallow subtidal environments as indicated by coarse grain size, degree of sorting, sedimentary structures, stratigraphic 396 position below tidal-flat subfacies, and analogies with modern oolitic and peloid grainstone deposits. Peloidal-ooid beds directly overlain by tidal-flat deposits represent fringing shoals, whereas thick, noncyclic intervals represent amalgamated shoals that developed along the outer platform. Amphipora dolowackestone/grainstone subfacies (0.7–3.0 m thick) is most common along the outer platform and forms the base or the middle of peritidal cycles. The deposits are defined by the high abundance of Amphipora stromatoporoids. The presence of unabraded beds containing only Amphipora fossils and the conformable association with overlying tidal-flat subfacies indicate deposition in relatively low-energy, restricted, shallow subtidal environments (Krebs, 1971; Wong and Oldershaw, 1980). Local patches of current-aligned fossils and grainstone layers suggest episodic, high-energy conditions. Coarse crystalline dolomite (1- to 50-mthick intervals) is present along the inner through outer ramp of sequence 1. Recrystallization and dolomitization have obscured most primary features; however, the presence of planar laminae, fenestral fabrics, cross-bedding, stromatolites, and typical shallow-water fossils, and stratigraphic relationships with adjacent facies suggest deposition in shallow-subtidal to tidal-flat environments. Crinoidal packstone/grainstone (1- to 11m-thick intervals) is present along the outer Stromatoporoid floatstone/boundstone (0.3– 4.0 m thick) caps subtidal cycles along the outer and inner platform, and isolated stromatoporoid heads occur locally in deep subtidal (intraplatform basin facies) and peloid-ooid packstone/grainstone subfacies. The diversity of skeletal material in the matrix between stromatoporoid heads (brachiopods, corals, gastropods, rare bryozoans, and crinoids) and the stratigraphic association with deep subtidal facies indicate deposition in deep subtidal, openmarine environments. The relatively finegrained texture of the matrix and the lack of current- or wave-generated features suggest the buildups did not act as wave-resistant barriers or reefs; rather the buildups formed low-relief biostromes. Deep Subtidal Facies (Including Intraplatform Basin Facies) Deep subtidal facies are composed of bioturbated skeletal mudstone/wackestone and are present mainly as thick, noncyclic intervals (12– 47 m thick) along the inner and outer platform of sequence 3, and at the base of sequence 1 (section NA; Fig. 3). In particular, these facies form the base of subtidal cycles developed along the inner platform. The fine-grained texture, lack of wave- or current-generated sedimentary structures, and the presence of typical, open-marine fauna indicate deposition below storm-wave base in normal marine environments. These deep subtidal facies lie both seaward and landward of coeval shallow subtidal/peritidal facies (Fig. 3), indicating that a deeper-water intraplatform basin developed during sequence 3 (discussed below). Slope Facies Slope facies are present only in the basinal section NA and are not arranged into meter-scale cycles. Coarse crinoidal packstone/grainstone beds (,0.5 cm thick) are present in the uppermost part of each sequence and are interpreted as upper slope or proximal turbid- Geological Society of America Bulletin, April 1996 SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES Figure 3. Palinspastically restored cross section of the platform-to-basin transition. Location of measured sections is shown in Figure 1. Note that distances shown in Figure 1 differ from the cross section because of palinspastic restoration using data of Levy and Christie-Blick (1989) and Dobbs et al. (1993). Note also that inner platform sections lie parallel to depositional strike, and outer platform and basinal sections are dip oriented. Stratigraphic datum of cross section is the maximum flooding zone of sequence 3. Because a single datum within sequence 3 was used to ‘‘hang’’ the entire succession, overlying and underlying sequences are slightly distorted. Only 5 of the 11 formation and/or member boundaries coincide with sequence boundaries. ites derived from outer platform and/or upper-slope crinoid thickets. Laminated pellet-peloid packstone/grainstone lenses (,0.4 m thick, less than a few meters wide) are present in the basal and middle parts of sequence 2 and represent more distal turbidites deposited along the lower slope and toe of slope as indicated by the fine-grained texture and intercalation with basinal facies. Lime-clast conglomerates (0.3- to 5.0-mthick beds) contain tabular clasts composed of slope and basinal rock types, and individual beds commonly grade laterally into folded, slumped, or undeformed basin/slope facies indicating only minor downslope transport. This subfacies is interpreted to represent debris-flow beds deposited along Geological Society of America Bulletin, April 1996 397 M. ELRICK the slope and toe of slope (e.g., Cook and Taylor, 1977). Basinal Facies Basinal facies are present only at section NA. Limestone-marl rhythmites (5- to 80-mthick intervals) were deposited below stormwave base in poorly oxygenated basinal environments. Individual, submillimeter-thick, graded laminae within limestone and marl layers represent suspension deposition from either single storm events or distal turbidity currents with the lime mud and pelleted material being derived from the adjacent shallow platform region. The rhythmic alteration of limestone and marl layers likely reflects climatically controlled changes in (1) fluvial and/or eolian influx of terrigenous material into the marine environment; (2) storm tract location, which controlled the transport of carbonate material derived from the shallow platform; or (3) turbidity current generation, which controlled the transport of siliciclastic and carbonate material to the basin (Elrick, 1993; Elrick and Hinnov, in press). Platy argillaceous wackestone (0.2- to 30m-thick intervals) represents deposition below storm-wave base; the presence of small burrows and sparse skeletal material indicates an increase in bottom-water oxygen levels relative to the limestone-marl rhythmites. The graded laminae are interpreted as the result of distal storm or distal turbidite deposition. METER-SCALE UPWARD-SHALLOWING CYCLES Peritidal through deep subtidal facies are arranged into meter-scale upward-shallowing peritidal cycles (capped by tidal-flat laminites) and subtidal cycles (capped by shallow subtidal subfacies). Detailed descriptions and interpretations of the cycles are given in Elrick (1995) and are briefly outlined below to aid in illustrating how cycle stacking patterns and intracycle facies composition are utilized to identify and correlate sequences, systems tracts, and sequence boundaries. Peritidal cycles (90% of the observed cycles) are present within each of the four shallow platform sequences. They are composed of shallow subtidal subfacies gradationally overlain by tidal-flat facies. Approximately 80% of the peritidal cycle caps show evidence of subaerial exposure (desiccation 398 cracks, sediment- and spar-filled dissolution cavities, rubble and solution-collapse breccias, and incipient pedogenic features). Nearly half of the peritidal cycles have a basal transgressive unit (upward-deepening facies trends) overlain by a typical regressive unit (upward-shallowing facies trends); these are termed transgressive-prone cycles. Subtidal cycles are limited to the basal and middle portions of sequences and are laterally equivalent to updip peritidal cycles. Two types of subtidal cycles are observed. Exposed subtidal cycles are composed of an upward-shallowing succession of shallow subtidal subfacies, and cycle caps display evidence of subaerial exposure (sediment- and spar-filled dissolution cavities and brecciation). Submerged subtidal cycles are characterized by deep subtidal facies gradationally overlain by shallow subtidal subfacies; these subtidal cycle caps show no evidence of subaerial exposure. Both subtidal cycle types indicate incomplete infilling of accommodation space, before sea-level fall in the case of exposed subtidal cycles, and before the succeeding transgression for submerged cycles. The average calculated duration of peritidal and subtidal cycles using the Odin et al. (1982), Palmer (1983), and Harland et al. (1989) time scales is between '50 and 130 k.y.; as such, they are fourth- or fifth-order in scale (Goldhammer et al., 1993). The mechanism that best explains the abundance of cycle-capping exposure features, transgressive-prone peritidal cycles, and subtidal cycles correlative with updip peritidal cycles is high-frequency (104–105 yr) sea-level fluctuations (see Elrick, 1995, for detailed discussion). Exposure-capped peritidal and subtidal cycles are interpreted as the result of high-frequency sea level falling below the platform surface for one, or more likely, several sea-level oscillations (‘‘missed beats’’; Goldhammer et al., 1990). Transgressive-prone cycles indicate that subtidal sediment was being produced and deposited during transgressions; that is, sedimentation lag times were minimal during initial flooding of the platform. Subtidal cycles correlative with updip peritidal cycles are interpreted to reflect incomplete shallowing and infilling of accommodation space before the succeeding high-frequency rise in sea level. Noncyclic intervals in the intraplatform basin, slope, and basin are interpreted as the result of the seafloor lying too deep to resolve high-frequency sea-level oscillations (subtidal missed beats). The autogenic tidal-flat progradation model of Ginsburg (1971) is precluded be- cause it requires a period of nondeposition combined with gradual basin subsidence to cause transgression and relative deepening, and the presence of abundant transgressiveprone cycles indicates deposition during transgressions. Episodic subsidence (faultinduced downdropping; Cisne, 1986) is also precluded as a cycle-generating model because of the abundance of transgressiveprone cycles, which indicate gradual deepening from tidal flat to subtidal water depths rather than abrupt deepening, which would be expected from fault-induced subsidence. The preservation of upward-deepening facies trends at cycle bases also indicates that sediment redistribution by waves or currents during transgressions was minor (low-energy transgressions). DEPOSITIONAL SEQUENCES AND SYSTEMS TRACTS Methods used to define depositional sequences in the Middle Devonian carbonate deposits differ from those used to define seismic-scale sequences, because onlapping and downlapping stratal geometries are difficult to recognize along low paleoslopes and where stratigraphic sections are widely separated between isolated Basin and Range fault blocks. Instead, the Middle Devonian sequences are identified from vertical and lateral changes in regional facies, from changes in meter-scale cycle stacking patterns (systematic vertical changes in cycle subfacies and cycle thickness), and from the stratigraphic distribution and intensity of cycle-capping subaerial exposure features. Sequences were initially identified from vertical facies changes and cycle stacking patterns in individual stratigraphic sections, then similar patterns were correlated between adjacent sections to identify lateral or two-dimensional facies relationships of retrogradation, aggradation, and progradation. The bed-by-bed resolution available from this outcrop study reveals that the changes between retrogradational and aggradational/progradation facies patterns (maximum flooding surface) and progradational to retrogradational patterns (sequence boundaries) are gradational over meters to tens of meters and are composed of stacked meterscale cycles along the shallow platform and noncyclic intervals along the basin/slope. These gradational or transitional maximum flooding zones and sequence boundary zones reflect the effects of repeated fourth- to fifth-order sea-level fluctuations superposed upon third-order relative sea-level events Geological Society of America Bulletin, April 1996 SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES (see discussions in Goldhammer et al., 1993, and Montañez and Osleger, 1993). These transitional zones contrast with boundaries identified seismically, because typical seismic data has the spatial resolution of a few tens of meters; consequently, boundaries between retrogradational and aggradational/progradational facies patterns will appear as surfaces rather than stratigraphic zones. In the following sections, the stratigraphic, sedimentologic, and biostratigraphic criteria used to identify and correlate systems tracts and their boundaries are described for shallow platform, platform margin, and basin/slope regions for each of the sequences. Figure 4A (and Figures 6A, 8A, and 10A below) illustrates lateral and vertical relationships of regional facies, meter-scale cycle thickness trends, and systems tracts for sequences 1– 4. Detailed stratigraphic columns of portions of each sequence are provided to illustrate the meter-scale details of specific boundaries and systems tracts across the platform-to-basin transect. Sequence Boundaries The concept of sequences and sequence boundaries were originally defined from seismic-scale siliciclastic systems deposited on shelves with recognizable shelf-slope breaks (Vail et al., 1977). Defining type 1 versus type 2 sequence boundaries based on the seaward extent and intensity of unconformity development was, as a consequence, relatively straightforward. More recent sequence stratigraphic studies along carbonate ramps that lack distinct slope breaks and in systems recording various orders of sealevel oscillation highlight the fact that type 1 versus type 2 sequence boundaries are difficult to define (Burchette and Wright, 1992; Goldhammer et al., 1993; Montañez and Osleger, 1993). Instead, the boundaries along these types of platforms are typically conformable stratigraphic intervals representing transitions between transgressive (retrogradational) and regressive (progradational) depositional patterns (transitional sequence boundaries of Montañez and Osleger, 1993). These transitional boundaries are several meters to tens of meters thick and are composed of stacked, high-frequency cycles rather than discrete, laterally traceable surfaces. Along the Middle Devonian platform, no evidence of major penetrative karstification or truncation related to prolonged (several hundreds of thousands to millions of years) subaerial exposure is recognized. Instead, sequences are bounded by stratigraphic intervals (meters to tens of meters thick) of exposure-capped cycles. Within these intervals, cycle caps typically exhibit progressive upsection increases in the intensity of subaerial exposure features (positive chronosequences of Wright, 1994). For example, shallow platform sequence boundary zones are typically composed of three to six peritidal cycles; the caps to all of the cycles exhibit textural homogenization (laminite recrystallization of Elrick, 1995) and alveolar structures related to incipient pedogenesis. In addition to pedogenic features, the cycles toward the top of the succession also display dissolution cavities filled with yellow-weathering argillaceous dolomite (cavities may extend 1 m below cycle cap). The cycles at the top of the succession may be capped by monomict rubble breccias that reflect extensive dissolution and fragmentation of the exposed surface. Each exposure-capped cycle is interpreted to represent a drop in fourthto fifth-order sea level below the platform surface and exposure for time periods on the order of 103–105 yr; successions of exposurecapped cycles indicate that the shallow platform was affected by multiple episodes of subaerial exposure during a long-term accommodation minimum rather than a single, long-lived exposure event. These stratigraphic and diagenetic relationships indicate that the rates of third-order sea-level fall were less than tectonic subsidence rates along the shallow platform, resulting in the development of third-order sequence boundary zones rather than third-order sequence bounding unconformities. Physical tracing of shallow platform sequence boundary zones into downdip basin/slope regions is not possible because of discontinuous outcrop exposures; however, basin/slope deposits show distinct shallowing-then-deepening facies changes over stratigraphic intervals of tens of meters, and these can be lithologically and biostratigraphically correlated to similar facies patterns observed along the shallow platform. Depositional Sequence 1 Sequence 1 is 30 –75 m thick and includes the Denay Limestone, upper Coils Creek Limestone, middle and upper members of the Sadler Ranch Formation, the coarse crystalline members of the Oxyoke Canyon Formation, and Simonson Dolomite (Fig. 3). Along the shallow ramp, deposits are so strongly dolomitized and recrystallized that meter-scale cycles are difficult to discern; however, large-scale (several tens of meters), transgressive-regressive facies trends can be identified and correlated to patterns observed in undolomitized deposits along the ramp margin and basin/slope. The base of sequence 1 is a widespread unconformity that separates the Lower Devonian Sevy Dolomite from the uppermost Lower–Middle Devonian Simonson Dolomite (Johnson and Murphy, 1984); traced basinward (west), the unconformity becomes a conformable succession in the Oxyoke Canyon and Sadler Ranch Formations (Kendall et al., 1983; Fig. 2). At sections NA and TM, an upsection gradation from crinoidal packstone/grainstone (shallow subtidal subfacies) to overlying darkgray, skeletal wackestone (deep subtidal facies) represents upward-deepening of the transgressive systems tract (sections NA and TM, Fig. 4B). Along the rest of the shallow ramp, the basal Lower Devonian sequence boundary is directly overlain by a succession of sandy coarse crystalline to coarse crystalline dolomite (coarse crystalline members of the Oxyoke Canyon Formation and Simonson Dolomite). This subfacies contains local planar laminae, cross-stratification, and stromatolitic horizons indicating deposition in shallow subtidal to peritidal environments; as such, they represent a transgression over the unconformity. At section NA, shallowing related to the overlying highstand systems tract is indicated by dark-gray, skeletal wackestone (deep subtidal facies) grading into laminated pellet-peloid pack-/grainstone (distal turbidites), and capped by coarse-grained, graded crinoidal packstone/grainstone (proximal turbidites) (section NA, Fig. 4B). Shallowing at section TM is represented by dark-gray, skeletal wackestone (deep subtidal facies) gradationally overlain by medium-bedded, crinoidal wackestone/packstone (shallow subtidal subfacies). Updip along the shallow ramp, the transition between retrogradational (transgressive systems tract) and aggradational/progradational (highstand systems tract) deposition cannot be discerned in the coarse crystalline dolomite; however, late highstand systems tract 1 is not as strongly recrystallized as the underlying early highstand systems tract 1. Consequently, a 6- to 28-m-thick interval of stacked peritidal cycles is recognized (sections PR and CC, Fig. 4B). Sequence boundary zone 2 along the shallow ramp is identified as a 6- to 13-m-thick interval of exposure-capped peritidal cycles. Geological Society of America Bulletin, April 1996 399 Figure 4. (A) Cross section of sequence 1 illustrating systems tracts, boundaries, and changes in meter-scale cycle thickness. Symbol explanations shown in Figure 3. Dolomitization and recrystallization along the majority of the inner and outer ramp obscures meter-scale cycle recognition except during the late highstand systems tract 1. (B) Partial stratigraphic columns illustrating details utilized to interpret systems tracts and boundaries. Scale in meters. 400 Geological Society of America Bulletin, April 1996 The two to six cycles within this interval show upsection increases in the proportion of tidal-flat subfacies, and each cycle cap displays evidence of prolonged subaerial exposure. For example, at the innermost ramp section PR, the caps of the upper four peritidal cycles defining sequence boundary zone 2 display sediment-filled dissolution cavities that extend 0.5–1.2 m below cycle tops; these features are absent in the underlying peritidal cycle caps (section PR, Fig. 4B). At each of the shallow ramp locations, sequence boundary zone 2 is directly overlain by a succession of peritidal cycles dominated by subtidal subfacies (section PR, Fig. 4B). At sections TM and NM, the stratigraphic interval corresponding to sequence boundary zone 2 is covered by Quaternary colluvium. At section NA (basin/slope), minimum accommodation related to sequence boundary zone 2 development is marked by the upsection transition from distal turbidites (laminated pellet-peloid pack-/grainstone) to proximal turbidites (coarse-grained, graded crinoidal packstone/grainstone). This '20-m-thick upward-shallowing succession is abruptly overlain by basinal facies intercalated with debris flows composed of slope-derived clasts (section NA, Fig. 4B). There is no evidence for subaerial exposure within this interval, implying that the combined rate of third-order and fourth- to fifth-order sea-level fall was less than tectonic subsidence along the basin/slope. Platform Evolution During Sequence 1 Figure 4. (Continued). The early transgressive systems tract of sequence 1 is characterized by peritidal to shallow subtidal facies passing seaward into shallow subtidal facies with no detectable break in slope; that is, a homoclinal ramp morphology (Fig. 5A). During maximum flooding (maximum flooding zone 1), a slight break in slope developed between sections NM and TM, evidenced by the deposition of shallow subtidal subfacies deposited at section NM, while sub-wave-base facies were deposited at the adjacent section TM (Fig. 5B). The lack of sediment gravity flow deposits within deep subtidal facies at sections TM and NA indicates that the gradient along this newly formed slope was not steep enough to generate allochthonous deposits. During deposition of highstand systems tract 1, the slope break migrated seaward to between sections TM and NA, and the slope gradient steepened enough to generate abundant distal to proximal turbidites; that is, a distally steepened ramp (Fig. 5C). Geological Society of America Bulletin, April 1996 401 M. ELRICK Figure 5. Schematic depositional profiles of ramp evolution during sequence 1. Symbols as in Figure 7; TM 5 approximate location of section TM. (A) During early transgressive systems tract 1, a homoclinal ramp morphology developed. (B) During maximum flooding zone 1, a slight break in slope developed between sections NM and TM (incipient distally steepened ramp). (C) During deposition of highstand systems tract 1, the slope break prograded seaward to between sections TM and NA, and a distally steepened ramp morphology developed. To recapitulate, during sequence 1 the platform evolved from a homoclinal ramp during transgressive systems tract 1, to a ramp with a slight break in slope between sections NM and TM during the maximum flooding zone (incipient distally steepened ramp), to a distally steepened ramp with the slope break between section TM and NA during highstand systems tract 1. During the late transgressive systems tract 1, when the rate of third-order accommodation gain was the greatest, relatively rapid shallow-water sedimentation rates along the shallow ramp kept pace with the accommodation gain, while deeper subtidal accumulation rates lagged behind the increase in accommodation. As a result, the gradient between shallow and deeper water environments steepened to generate a ramp with a slight break in slope. Progradation during highstand sys- 402 tems tract 1 resulted in the westward migration of the slope break and a concurrent steepening of the slope gradient. Depositional Sequence 2 Sequence 2 is 95–180 m thick and includes part of lower Denay Limestone, Sentinel Mountain Dolomite, and the lower alternating member of the Simonson Dolomite (Fig. 3). The sequence is characterized by deepening and retrogradation of basinal, slope, and subtidal through peritidal facies, followed by shallowing and progradation of shallow subtidal through peritidal facies (Fig. 6A). Isolated stromatoporoid buildups developed along the platform margin throughout sequence development. An upward-deepening succession (transgressive systems tract 2) along the basin/ slope (section NA) is recorded by an abrupt vertical change from coarse crinoidal turbidites (underlying sequence boundary zone 2) to platy, argillaceous wackestone (basinal facies) intercalated with lime-clast conglomerates (slope-derived debris flow beds) (section NA, Fig. 4A). Continued deep-water conditions (late transgressive systems tract 2) is recorded by a thick, monotonous succession of limestone-marl rhythmites (basinal facies). Updip at section TM, a succession of peritidal cycles overlain by exposed subtidal cycles reflects an increase in accommodation space related to transgressive systems tract 2 development. Along the outer and inner platform, evidence for upward-deepening is more subtle than that displayed in downdip regions, but is indicated by successions of peritidal cycles that display upsection increases in the proportion of subtidal subfacies and an associated upsection increase in the abundance and diversity of skeletal fossils within the subtidal units (section CC, Fig. 4B); both characteristics suggest an increase in third-order accommodation space. The maximum flooding zone separating transgressive systems tract 2 from highstand systems tract 2 is not recognized along the basin/slope because of poor outcrop exposure. At section TM, facies representing maximum water depths occur within a single peritidal cycle whose base is composed of dark-gray, stromatactis-bearing dolowackestone with in-growth-position colonial corals (1.5-m-thick interval); in comparison, underlying (transgressive systems tract) and overlying (highstand systems tract) cycles contain subtidal units composed of bioturbated Amphipora wackestone (restricted shallow subtidal subfacies). Along the inner and outer platform, maximum deepening is represented by a 7- to 20-m-thick succession of subtidal and peritidal cycles that are dominated by shallow subtidal units (burrowmottled wackestone); in contrast, underlying and overlying cycles are composed dominantly of tidal-flat subfacies (section PR, Fig. 6B). Within maximum flooding zone 2, a thin marker bed (,0.5 m) composed of Stringocephalus brachiopod– bearing wackestone can be correlated between four shallow platform stratigraphic sections or '140 km along depositional strike (Fig. 6A); the presence of this marker bed attests to the isochronous nature of the maximum flooding zone. Shallowing along the basin/slope (highstand systems tract 2) is subtle but is represented by thin-bedded limestone-marl Geological Society of America Bulletin, April 1996 SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES rhythmites (basinal facies) grading upsection into medium-bedded, nodular rhythmites interbedded with laminated peloid packstone/grainstone (distal turbidites) (section NA, Fig. 6B). Along the platform margin, shallowing is evidenced by a succession of upward-thinning, exposed subtidal cycles reflecting a decrease in third-order accommodation space. Shallowing related to the highstand systems tract along the outer and inner platform is evidenced in individual sections by successions of peritidal cycles dominated by tidal-flat subfacies (sections NM and SP, Fig. 6B), and regionally by the progradation of a thick peritidal wedge over shallow subtidal facies (Fig. 6A). Sequence boundary zone 3, along the shallow platform, is recognized as a 7- to 16-m-thick interval of peritidal cycles whose caps display sediment-filled dissolution cavities, rubble and solution-collapse breccias, and microkarst erosion surfaces. These exposure-capped cycles are immediately overlain by noncyclic, intraplatform basin facies or a succession of submerged subtidal cycles (sections NM and SP, Fig. 6B). Minimum accommodation along the platform margin is difficult to recognize from regional facies patterns alone; however, the highstand systems tract succession of exposed subtidal cycles is immediately overlain by an upward-thickening succession of subtidal and peritidal cycles that are dominated by deeper-water subtidal subfacies (section TM, Fig. 6B). Along the basin/slope, the third-order accommodation minimum is subtle, but is recorded as an 8-m-thick interval of mediumbedded, nodular limestone-marl rhythmites (with sparse fossils) that are capped by a thin crinoidal packstone/grainstone bed. This succession is abruptly overlain by a thick interval of thin-bedded limestone-marl rhythmites (basinal facies) of the overlying transgressive systems tract 3 (section NA, Fig. 6B). Platform Evolution During Sequence 2 Transgressive systems tract 2 inherited the distally steepened ramp morphology of the underlying sequence (Fig. 7A). A slight seaward dip along the shallow ramp during transgressive systems tract 2, maximum flooding zone 2, and early highstand systems tract 2 is indicated by a thicker accumulation of shallow subtidal facies (composed of peritidal cycles dominated by shallow subtidal units) along the outer ramp than the inner ramp (Fig. 6A). A flat-topped platform mor- phology evolved during late highstand systems tract 2, indicated by the presence of cyclic peritidal facies across the entire shallow-platform region (Fig. 7B). A flat-topped morphology is also indicated by the fact that during the succeeding transgressive systems tract 3, deepening is expressed by similar facies deposited over the entire region (Fig. 3). The development of peritidal cycles across the entire shallow platform throughout sequence 2 deposition indicates that water depths in this region never exceeded shallow subtidal water depths (,20 m), and the platform remained nearly aggraded to sea level throughout sequence development (‘‘keep-up’’ style deposition of Schlager, 1981). The flat-topped or aggraded nature of the platform, particularly during highstand systems tract 2, indicates that sediment accumulation rates kept pace with accommodation space gains. As a consequence of the shallow platform remaining nearly aggraded to sea level throughout sequence development, large portions of the shallow platform were subaerially exposed during fourth- to fifth-order sea-level lowstands. Until additional accommodation space was generated by continued subsidence or by succeeding higher amplitude fourth- to fifth-order sea-level oscillations, the position of the shallow platform remained above the effects of the smaller amplitude oscillations, and the sedimentary record of one or more high-frequency sealevel oscillations was not recorded (‘‘missed beats’’; Elrick, 1995). Depositional Sequence 3 Sequence 3 is 90 –130 m thick and includes the Denay Limestone, Woodpecker Limestone, basal Bay State Dolomite, and the brown cliff and basal part of the lower alternating members of the Simonson Dolomite (Fig. 3). The sequence is characterized by abrupt and extensive deepening atop the underlying sequence boundary zone 3 and by the formation of a deep subtidal, intraplatform basin along the former flattopped platform. During deepening, stromatoporoid buildups developed along the inner platform (as far landward as section PR), while deep subtidal facies of the intraplatform basin (Woodpecker Limestone and parts of the brown cliff member) were deposited as far landward as section S (Fig. 8A). This deepening was followed by relatively abrupt shallowing and prograda- tion by peritidal, shallow subtidal, and slope facies (Fig. 8A). Initial deepening marking transgressive systems tract 3 along the basin/slope is indicated by a 50-m-thick interval of limestone-marl rhythmites (basinal facies) immediately overlying the underlying sequence boundary zone 3 (section NA, Fig. 6B). Deepening along the platform margin (section TM) is recorded by a succession of thicker-than-average subtidal and peritidal cycles, which are dominated by subtidal units reflecting the increase in accommodation space (section TM, Fig. 6B). At sections NM and CC, abrupt deepening is indicated by noncyclic intraplatform basin facies (Woodpecker Limestone) lying immediately above peritidal cycles of the underlying sequence boundary zone 3 (section NM, Fig. 6B). At sections SP, S, and SC, a succession of submerged subtidal cycles grading up into noncyclic intraplatform basin facies (brown cliff member) indicates an increase in third-order accommodation or transgressive systems tract 3 deposition (section SP, Fig. 6B). At the innermost platform section (section PR), exposure-capped peritidal cycles of the underlying sequence boundary zone are abruptly overlain by stromatoporoid buildup facies, indicating an increase in accommodation space (section PR, Fig. 8B). Distinct facies changes signaling maximum flooding along the basin/slope is not observed because of poor exposure. Maximum water depths at section TM are recorded in a single peritidal cycle containing a 5-m-thick unit of platy and bulbous stromatoporoid floatstone/boundstone (moderately deep subtidal facies) (section TM, Fig. 8B). Along the intraplatform basin, maximum water depths are represented by a 5- to 17-m-thick interval of thin-bedded, noncyclic, deep subtidal facies that contain a greater abundance of open-marine fossils (crinoids, whole brachiopods, tentaculitids, and bryozoans) than surrounding intraplatform basin facies (section SP, Fig. 8B). At the most landward section PR, maximum flooding zone 3 is similar to that developed at section TM; maximum water depths occur within a single subtidal cycle containing a 1.5-m-thick unit composed of platy stromatoporoid boundstone; underlying and overlying cycles deepen only to shallow subtidal water depths represented by Amphipora packstone and burrow-mottled wackestone subfacies. Shallowing related to highstand systems tract 3 is indicated along the basin/slope by an upward-shallowing succession of thin- Geological Society of America Bulletin, April 1996 403 Figure 6. (A) Cross section of sequence 2; symbols as in Figure 3. (B) Partial stratigraphic columns illustrating details utilized to interpret systems tracts and boundaries. Symbols as in Figure 4B. Scale in meters. 404 Geological Society of America Bulletin, April 1996 SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES Figure 6. (Continued). bedded, limestone-marl rhythmites (basinal facies) grading into medium-bedded, nodular rhythmites with intercalated lime-clast conglomerates (upper slope-derived debrisflow beds), and overlain by well-sorted peloid grainstones (platform-margin shoals) (section NA, Fig. 8B). Along the platform margin (section TM), highstand systems tract 3 is characterized by a succession of upward-thinning peritidal cycles that exhibit a concurrent increase in the proportion of tidal-flat subfacies, which reflects a decrease in third-order accommodation space. At sections CC and NM, intraplatform basin facies are overlain by a 4- to 25-m-thick interval of noncyclic, peloid-ooid packstone/ grainstone. On the landward (southeastern) side of the intraplatform basin, shallowing related to highstand systems tract 3 is indicated by peritidal, shallow subtidal, and stromatoporoid buildup facies prograding seaward over intraplatform basin facies (sections S and SP, Fig. 8B). Progradation from both sides on the basin margin resulted in infilling and shallowing of much of the accommodation space and allowed cyclic peritidal facies to prograde across the former intraplatform basin region. The development of an intraplatform basin during transgressive systems tract 3 and maximum flooding zone 3 is interpreted from the presence of noncyclic, deep subtidal facies lying landward (or east) of cyclic peritidal and shallow subtidal facies along the platform margin (section TM, Fig. 8A). Regionally, these intraplatform basin facies Geological Society of America Bulletin, April 1996 405 M. ELRICK tion cavities extending '0.3 m down from cycle tops, rubble breccias, and microkarst erosion surfaces (sections CC and S, Fig. 8B). Along the platform margin, sequence boundary zone 4 is represented by a 7-m-thick interval composed of a polymict breccia bed (1– 4 m thick) overlain by three exposurecapped peritidal cycles. Along the basin/slope, sequence boundary zone 4 is represented by a 12-m-thick interval of peloid grainstones (platformmargin shoals) that are abruptly overlain by platy, argillaceous wackestone (basinal facies) of the overlying transgressive systems tract 4 (section NA, Fig. 8B). Progradation of platform-margin shoals over former basin/slope environments indicates that the slope break now lay west of the study area; prior to this, the slope break lay between sections TM and NA. Platform Evolution During Sequence 3 Figure 7. Schematic depositional profile of platform morphology during sequence 2. TM 5 section TM. (A) A distally steepened ramp morphology developed during the late transgressive systems tract 1/maximum flooding zone 1 with shallow subtidal and stromatoporoid buildup facies deposited along the outer ramp. (B) Seaward progradation of cyclic peritidal facies during late highstand systems tract 2 generated a flat-topped platform morphology with a slope break lying between sections TM and NA. The gradient along the slope was steep enough to generate minor sediment gravity flow deposits. are present from southeastern California to the Idaho-Nevada border and in an '20- to 100-km-wide facies belt (Fig. 1). Intraplatform basin facies were deposited in deeper and more open-marine waters than coeval platform deposits as suggested by (1) the lack of coarse-grained textures or wave- or current-reworked sedimentary structures, (2) the lack of meter-scale cycles or subaerial exposure features, and (3) the presence of typical open-marine fossils (crinoids, colonial corals, tentaculitids, and bryozoans). While the platform margin was clearly top- 406 ographically higher than the intraplatform basin, it did not act as an energy barrier because platform margin deposits lack typical high-energy sedimentary structures/textures (i.e., cross-beds, winnowed grainstones, and wave-resistant reef structures). Sequence Boundary Zone 4. Along the shallow platform, minimum third-order accommodation is recorded in an 8- to 12-mthick zone of peritidal cycles whose caps show abundant evidence for prolonged subaerial exposure such as alveolar structures, desiccation cracks, sediment-filled dissolu- A profound change in platform morphology occurs at the base of sequence 3. The relatively flat-topped platform of the underlying sequence deepens abruptly landward of section TM and substorm–wave-base water depths were attained across the majority of the study area (Fig. 9A). During deposition of transgressive systems tract 3 and maximum flooding zone 3, sediment accumulation rates did not keep pace with accommodation gains resulting in the development of an incipiently drowned intraplatform basin. It is not clear why accumulation rates at section TM were able to kept pace with accommodation gains while rates landward of this region lagged behind accommodation gains. The decrease in accommodation space during highstand systems tract 3 resulted in the infilling of the intraplatform basin from both margins (Fig. 9B). Infilling of the intraplatform basin was complete by late highstand systems tract 3; subsequently, a relatively flat-topped platform morphology developed landward of section TM (Fig. 9C). Depositional Sequence 4 Sequence 4 (55–70 m thick) includes the Denay Limestone, the middle portion of the Bay State Dolomite, and the upper portion of the upper alternating member (Fig. 3). Regional facies patterns are characterized by minor deepening and retrogradation of basin/slope and shallow subtidal facies, followed by basinward progradation of peritidal through shallow subtidal facies Geological Society of America Bulletin, April 1996 SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES (Fig. 10B). The most landward section (section PR) is composed wholly of polymict karst breccia obscuring facies trends. Upward-deepening facies relationships related to transgressive systems tract 4 is well developed along the basin/slope, while platform-margin and shallow platform regions display only subtle changes in regional facies trends. Upward-deepening and retrogradation defining transgressive systems tract 4 along the basin/slope is indicated by platform-margin grainstones abruptly overlain by a 5-m-thick interval of platy, argillaceous wackestone (basinal facies) (section NA, Fig. 8B). Maximum flooding presumably occurs within this thin interval because it is immediately overlain by platform marginderived debris flow beds, then peloid grainstone shoal deposits (highstand systems tract 4). Upward-deepening facies trends at section TM are subtle; the stratigraphic interval above the underlying sequence boundary zone 4 is composed of a succession of peritidal cycles that have a higher proportion of subtidal facies than the underlying succession of peritidal cycles. Maximum water depths (maximum flooding zone 4) at section TM are recorded in a single, exposed subtidal cycle that contains a 0.5-m-thick brachiopod wackestone unit; subtidal units in underlying (transgressive systems tract) and overlying (highstand systems tract) cycles are composed of shallow and/or restricted subtidal deposits (Amphipora wackestone or wavy-laminated beds) (section TM, Fig. 10B). Along the shallow platform, increases in third-order accommodation (transgressive systems tract 4) are indicated by a succession of peritidal cycles that are dominated by subtidal units at sections SC, SP, and CC. Maximum flooding zone 4 is best developed along the outer platform where subtidal units within peritidal cycles contain more open-marine fossils (Stringocephalus brachiopods, colonial corals, and in-growth-position stromatoporoids) than underlying and overlying peritidal cycles (subtidal units composed of Amphipora wackestone and wavy-laminated subtidal subfacies) (section CC, Fig. 10B). Distinct shallowing and highstand systems tract development along the basin/slope are indicated by slope facies overlain by a 60m-thick interval of peloid grainstones with up to 10% quartz sand (section NA, Fig. 10B). Along the shallow platform (including section TM), highstand systems tract 4 is composed of stacked peritidal cycles that are dominated by tidal-flat subfacies (section TM, Fig. 10B). The upper 10 –13 m of the highstand systems tract at sections S and SC are composed of ooid grainstone– based peritidal cycles (section S, Fig. 10B). Sequence Boundary Zone 5. Minimum accommodation along the shallow platform is defined by the vertical change from exposure-capped peritidal cycles to an '15-mthick interval of noncyclic shallow subtidal facies (sections S and TM, Fig. 10B). Along the inner platform, this shallow subtidaldominated interval represents the basal Fox Mountain member of the Simonson Dolomite (Fig. 2). At section NM, the upper 7 m of highstand systems tract 4 is composed of lightbeige, coarse crystalline peloidal dolomite; this is abruptly overlain by platy, argillaceous wackestone (basinal facies) interbedded with platform margin– derived debris flow beds (slope facies) (section NA, Fig. 10B) and indicates a rapid increase in third-order accommodation space related to the overlying sequence. The upward-deepening succession above sequence boundary zone 5 represents the Taghanic onlap of Johnson (1970), a relatively long-lasting transgression recognized in the western United States, New York, Belgium, and Germany (Johnson et al., 1985). Platform Evolution During Sequence 4 During transgressive systems tract 4 and maximum flooding zone 4, a distally steepened ramp developed with the break in slope between sections TM and NA (Fig. 11A). A slight westward-dipping gradient along the outer ramp is indicated by thicker successions of shallow subtidal facies at sections TM and NM than updip section CC; in a landward direction, the transgressive systems tract 4/maximum flooding zone 4 is composed of cyclic peritidal facies, indicating that the inner part of the platform was relatively flat topped. During deposition of highstand systems tract 4, cyclic peritidal facies prograded seaward across the rest of the platform, and a flat-topped morphology was reestablished across the majority of the study area. Grainstone shoals developed between sections TM and NA and prograded westward over underlying basin/slope facies during highstand systems tract 4 (Fig. 11B); consequently, the location of the platform margin and the gradient along the slope cannot be discerned within the study area. The vertical succession of facies at section NA from highstand systems tract 3 to highstand systems tract 4 (platform-margin grainstones, basinal facies, overlain by platform-margin grainstones), indicates that third-order accommodation gains during transgressive systems tract 4/maximum flooding zone 4 were great enough to establish subwave-base water depths atop former grainstone shoals, and that the slope between the platform margin and basin during transgressive systems tract 4/maximum flooding zone 4 was gentle enough to permit the shoals to prograde seaward across the slope and infill the remaining topography. These relationships indicate that the volume of sediment produced at section NA was great enough to infill the underlying topography and permit progradation of the slope break. Slopes greater than a few degrees would likely have restricted seaward progradation of autochthonous grainstones because of the greater water depths and gradients developed along the slope. BIOSTRATIGRAPHIC CORRELATION OF SEQUENCES Biostratigraphically diagnostic (at the substage level) conodonts and brachiopods are present only in basin/slope (section NA) and some platform-margin deposits (section TM); intraplatform basin through peritidal facies, which compose the majority of the sequences, contain fossils that can only constrain the deposits to a Middle–Late Devonian age (i.e., Stringocephalus brachiopods). The high-resolution biostratigraphic control available in the basin/slope section and the adjacent platform-margin section permits the correlation between contrasting facies belts; that is, noncyclic subwave-base deposits (section NA) can be correlated biostratigraphically with cyclic, shallow subtidal and peritidal deposits (section TM). Correlation from the platform margin landward across the rest of the shallow platform is based on distinct vertical and lateral facies patterns, cycle stacking patterns, and subaerial exposure features described above for individual systems tracts. The transgressive systems tracts and highstand systems tracts of sequences 1– 4 along the basin/slope have been biostratigraphically dated using conodonts and/or brachiopods (Johnson et al., 1980, 1989, in press); transgressive systems tracts have been dated in sequences 1 and 4 at section TM (Fig. 3) (Johnson et al., 1980). The maximum flooding zone 1 at section NA is in the serotinus conodont Zone (late Emsian; Fig. 3; Johnson et al., 1980). At section TM, the maximum flooding zone 1 is represented by the Geological Society of America Bulletin, April 1996 407 Figure 8. (A) Cross section of sequence 3; symbols as in Figure 3. Note absence of cycles within the broad intraplatform basin. (B) Partial stratigraphic columns illustrating details utilized to interpret systems tracts and boundaries. Symbols as in Figure 4A. Scale in meters. 408 Geological Society of America Bulletin, April 1996 basal Sadler Ranch Formation, which is in the serotinus Zone (Fig. 3; Kendall et al., 1983). The Sadler Ranch Formation interfingers with the Oxyoke Canyon Formation (Fig. 3; Kendall et al., 1983) which, in turn, interfingers landward (east) with the coarse crystalline member of the Simonson Dolomite (Johnson et al., 1985, 1989). The last two units directly overlie the regional Lower Devonian unconformity and show similar transgressive and regressive facies patterns as the biostratigraphically controlled portion of sequence 1 at section TM. The transgressive systems tract 4/maximum flooding zone 4 along the basin/slope occurs within the ensensis conodont Zone (Johnson et al., 1980, in press). At section TM, the maximum flooding zone 4 occurs within western United States Faunal Interval 20 (containing Geranocephalus truncatus brachiopods; Johnson, 1978); Faunal Interval 20 immediately overlies the ensensis Zone, indicating that initial deepening (early transgressive systems tract 4) is within the ensensis Zone. The biostratigraphic and sequence stratigraphic correlations of sequences 1 and 4 bracket the ages of intervening sequences 2 and 3. Consequently, conodont dates from the transgressive systems tract and/or maximum flooding zone of sequences 2 and 3 along the basin/slope indicate that sequence 2 is in the middle costatus Zone, and sequence 3 is in the middle kockelianus Zone (Fig. 3; Johnson et al., in press). Each of the biostratigraphically dated systems tracts at section NA has been recognized and dated in Idaho and Montana, the Figure 8. (Continued). Geological Society of America Bulletin, April 1996 409 M. ELRICK midwestern and eastern United States, western Canada, and Europe (Johnson et al., 1985, in press). These Early–Middle Devonian deepening events have been utilized along with other Devonian events by Johnson et al. (1985, 1989) to generate a thirdorder sea-level curve for the Devonian (Fig. 12). The third-order sea-level fluctuations of the latest Early–Middle Devonian, termed T-R cycles Ic, Id, Ie, and If by Johnson et al. (1985), correlate biostratigraphically with sequences 1, 2, 3, and 4, respectively; this intrabasinal and interbasinal correlation implies a eustatic control on the uppermost Lower–Middle Devonian sequences recognized in this study. DEPOSITIONAL CONDITIONS DURING PLATFORM DEVELOPMENT Several lines of evidence indicate that low-energy conditions prevailed across the Middle Devonian platform. First, typical high-energy deposits (peloid-ooid grainstones) are restricted to specific locations, were deposited during limited time intervals, and compose ,20% of the Lower– Middle Devonian succession. Second, stromatoporoid buildups are associated with fine-grained rather than coarse-grained deposits, indicating that they were not waveresistant structures that modified surrounding depositional environments. Third, .40% of the fourth- to fifth-order cycles preserve upward-deepening facies trends at their bases (transgressive-prone cycles), implying that high-frequency transgressions were low enough in energy that transgressive deposits were not redistributed offshore by wave or tidal processes. Fourth, transgressive-prone cycles lie directly above delicate, cycle-capping microkarst features that show no evidence of abrasion or reworking by shoreline currents and waves (see Elrick, 1995, for further discussion). The interpreted low-energy conditions may be explained by prevailing paleogeographic conditions. Middle Devonian paleogeographic reconstructions of Scotese and McKerrow (1990), Witzke (1990), and Van der Voo (1993, p. 263) place the western United States between paleolatitude 108S and 208S, with the paleoshoreline trending roughly northeast-southwest (present coordinates) (Fig. 1). Although these low paleolatitudes were likely influenced by hurricanes, it is unlikely that westward- or southwestward-moving, Southern Hemisphere hurricanes would have affected the western sides of continents (Marsaglia and 410 Figure 9. Schematic depositional profiles of platform morphology during sequence 3. Symbols as in Figure 7; TM 5 section TM. (A) During deposition of transgressive systems tract 3/maximum flooding zone 3, a deep subtidal intraplatform basin developed along former shallow platform. (B) During the early highstand systems tract 3, ooid shoal deposits prograded landward (east) and cyclic peritidal facies prograded seaward to infill the intraplatform basin. (C) The loss of third-order accommodation during late highstand systems tract 3 resulted in the progradation of cyclic peritidal and platform-margin facies across the intraplatform basin and basin/slope deposits, respectively. Because of late highstand systems tract 3 progradation, the location of the slope break and the gradient along the slope cannot be observed within the study area. Klein, 1983). In addition, during Devonian time, an eastward-migrating volcanic island arc presumably lay some distance seaward of western North America and collided with the continent during the latest Devonian– Early Mississippian Antler orogeny (Johnson and Pendergast, 1981; Speed and Sleep, 1982; Burchfiel and Royden, 1991). Prior to collision, the island arc likely acted as a barrier to open-ocean storm waves and winds, creating relatively low-energy conditions along the western continental margin of North America. SEQUENCE-SCALE STACKING PATTERNS Fourth- to fifth-order or high-frequency cycle stacking patterns (graphically illustrated using Fischer plots) have been used to estimate changes in third-order accommodation space (Read and Goldhammer, 1988). The premise for the use of cycle stacking patterns is that the combined effects of third-order through fifth-order sealevel rises generate successions of thickerthan-average cycles that exhibit evidence of Geological Society of America Bulletin, April 1996 SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES dominantly submergent conditions. The loss of accommodation space during relative third-order sea-level fall and superposed high-frequency sea-level fall generates successions of thinner-than-average cycles that are dominated by peritidal facies and exhibit evidence of prolonged subaerial exposure. Stacking patterns of cycles composed wholly of deeper subtidal facies should be used with caution, because the thickness of these cycles is controlled by sediment accumulation rates rather than sea-level– controlled accommodation space. Consequently, successions of thinner-than-average, deep subtidal– dominated cycles reflect slower sedimentation rates in deeper water environments rather than a relative fall in thirdorder sea level. Stacking patterns of third-order sequences can be used in a similar manner to estimate changes in second-order (longterm) accommodation space. Intrasequence facies distributions, facies-tract geometries, platform morphology changes, and the nature of sequence boundaries are emphasized more than variations in sequence thickness to estimate long-term accommodation changes. Similar to high-frequency cycle stacking patterns, sequences capped by highstand systems-tract peritidal facies reflect full aggradation of the platform to long-term sea level, thus they provide a more accurate estimate of long-term accommodation changes than do sequences dominated by deep subtidal facies. The comparison between basin/slope sequences (dominated by deep subtidal facies) and shallow platform sequences (dominated by peritidal and shallow subtidal facies) provides a means of evaluating the relative contributions of third-order eustasy, subsidence, and sediment supply to second-order accommodation trends. The following discussion of second-order accommodation changes is based on interpretations of comparisons between shallow platform and basin/slope sequence stacking patterns. During sequence 1, the evolution from a homoclinal ramp (transgressive systems tract 1) to a distally steepened ramp (highstand systems tract 1) reflects an increase in second-order accommodation space as the former shallow-ramp region aggraded to form a nearly flat-topped morphology (keep-up sedimentation). Slope steepening during sequence 1 is interpreted to reflect differential sedimentation rates along the shallow platform (relatively rapid) versus the deeper platform region (relatively slow). Increasing second-order accommoda- tion space is also indicated by the development of a sequence boundary zone (indicating third-order sea-level fall was less than tectonic subsidence across the study area) rather than a single, regional unconformity. During sequence 2, the generation of a thick wedge of shallow subtidal facies during late transgressive systems tract 2/early highstand systems tract 2, the development of an aggradational platform-margin geometry, and the development of a sequence boundary zone reflect continuing increases in second-order accommodation space. The aggradational facies patterns along the basin/ slope also support this interpretation (Fig. 12). The formation of an incipiently drowned, intraplatform basin during transgressive systems tract 3 and maximum flooding zone 3 suggests a rapid and/or large magnitude increase in the rate of second-order accommodation gain along the shallow platform, hence the interpreted inflection in the second-order accommodation curve, Figure 12. However, the basin/slope region does not record a similar magnitude or rate of accommodation gain. In fact, rock types and facies trends of the basin/slope transgressive systems tract 3 and early highstand systems tract 3 are very similar to transgressive systems tract 2 and highstand systems tract 2, despite the major morphologic and apparent accommodation differences between the equivalent shallow-platform regions. The difference between the apparent magnitude or rate of third-order accommodation gain along the shallow platform and basin/slope during transgressive systems tract 3/maximum flooding zone 3 indicates that the accommodation gain was not solely the result of third-order eustatic sea-level rise. The greater apparent magnitude or rate gain along the shallow platform suggests (1) differential subsidence rates existed between the shallow platform and platform margin, (2) apparent deepening was caused by sedimentation rates along the shallow platform lagging behind platform-margin rates, (3) the shallow platform was faulted and downdropped with respect to the platform margin, and/or (4) basin/slope environments were insensitive recorders of third-order accommodation changes. Given the regional extent of the intraplatform basin (.600 3 100 km; Fig. 1), the ability to correlate sequence 3 systems tracts across the platform, and the lack of evidence of Middle Devonian tectonism in Nevada, it is unlikely that fault-induced subsidence generated the intraplatform basin. Presently, it is not clear which of the remaining three mechanisms, in addition to third-order eustatic sea-level rise, generated the incipient drowning event; however, differential sedimentation rates combined with sea-level rise have been invoked for other Paleozoic intrashelf basins in North America (e.g., Markello and Read, 1982; Read, 1985). Transgressive systems tract 3 and early highstand systems tract 3 along the basin/ slope are very similar in lithology and facies patterns to transgressive systems tract 2 and early highstand systems tract 2; this is true despite the differences in shallow platform morphologies between the two sequences. This suggests that depositional processes along the shallow platform were decoupled with the basin/slope. This decoupling is also indicated by the fact that in each of the basin/slope sequences, there is no evidence of nondeposition or slow deposition (condensed intervals, hardgrounds, or other features) that would correspond to times of subaerial exposure on the shallow platform. In other words, the majority of the flattopped shallow platform was exposed during many high-frequency sea-level falls/lowstands, thus was unable to provide sediment to slope and basin regions; contributions from pelagic calcareous microfossils is not considered because they did not evolve until the Mesozoic. The lack of evidence of nondeposition or slow deposition in basinal deposits implies that some of the deposits were derived locally from the continuously submerged slope and/or were derived from along-slope sediment sources via geostrophic or contour currents. Relatively high accumulation rates for basinal facies is indicated by the fact that sequences 2, 3, and 4 are thicker along the basin/slope than along the shallow platform. If sedimentation rates were significantly slower along the basin/ slope, in part resulting from slow deposition or nondeposition during high-frequency exposure events, then basinward-thinning geometries or basin starvation would be observed (e.g., Upper Permian Capitan–Bell Canyon complex of west Texas; Garber et al., 1989). Rapid shallowing during highstand systems tract 3, development of a flat-topped platform, and the concurrent seaward progradation of the platform margin over the former basin/slope (catch-up style deposition) signal a decrease in the rate of secondorder accommodation gain (Fig. 12). Again, the formation of a sequence boundary zone indicates that relative third-order sea-level Geological Society of America Bulletin, April 1996 411 M. ELRICK fall rates were less than tectonic subsidence rates across the study area. A decrease in the rate of second-order accommodation gain during sequence 4 development (Fig. 12) is reflected by the decrease in sequence thickness relative to underlying sequences, development of a thin transgressive systems tract and maximum flooding zone (,5 m thick) along the basin, and seaward progradation of the platform margin. The rate decrease was not great enough, however, to generate a type 1 sequence-bounding unconformity. Although not described in this study, the extent and apparent magnitude of deepening recorded in the overlying sequence along the basin/ slope and shallow platform (Fox Mountain member of Simonson Dolomite) indicate a significant increase in the rate of secondorder accommodation gain related to the Taghanic onlap (Fig. 12; Johnson, 1970). Except for incipient drowning during transgressive systems tract 3 and maximum flooding zone 3, sediment accumulation rates along the Middle Devonian shallow ramps/platforms kept pace with fifth- through secondorder accommodation gains, thus generating aggradational to slightly progradational sequence-scale stacking patterns or keepup style deposition. These sequence-scale stacking patterns and the development of sequence boundary zones rather than sequence-bounding unconformities reflect the effects of second-order accommodation gains related to the lower part of the Kaskaskia sequence of Sloss (1963) (Fig. 12). These long-term, keep-up style sedimentation patterns are similar to those observed in many Cambrian–Ordovician and Mesozoic rimmedshelf systems (e.g., Read, 1989; examples in Simo et al., 1993). However, the Middle Devonian deposits lack features that characterize rimmed shelves such as platform-rimming reef complexes or grainstone shoals, and gradients seaward of the platform margins are not steep enough to generate thick accumulations of coarse-sediment gravityflow deposits. The aggraded nature of the Middle Devonian shallow ramp/platform deposits suggests that fourth- to fifth-order sea-level oscillations were of low magnitude (,10 m), which allowed sediment accumulation rates to keep pace with the combined high-frequency and third-order sea-level rise rates. These low-magnitude, fourth- to fifth-order sea-level oscillations support interpretations for Middle Devonian greenhouse climate conditions that are characterized by globally warm temperatures, high mean ocean temperature, and as a result, 412 Figure 10. (A) Cross section of sequence 4. Symbols as in Figure 3. Note that sequence 4 is relatively thin, deepening during transgressive systems tract 1 is minor, and facies along the shallow platform are dominated by peritidal deposits. (B) Partial stratigraphic columns illustrating details utilized to interpret systems tracts and boundaries. Symbols as in Figure 4B. Scale in meters. minor continental ice sheet development (Fischer, 1981; Frakes et al., 1992; Wright, 1992) (see discussion in Elrick, 1995). Low-magnitude or slow rise/fall rates of third-order sea-level oscillations are also suggested from the Middle Devonian deposits because each sequence boundary is gra- dational over meters to tens of meters, indicating tectonic subsidence rates were greater than sea-level fall rates, and fourthto fifth-order cycles are absent in intraplatform basin, slope, and basinal facies. If third-order magnitudes or rise/fall rates were greater, then during third-order low- Geological Society of America Bulletin, April 1996 Figure 10. (Continued). Geological Society of America Bulletin, April 1996 413 M. ELRICK Figure 11. Schematic depositional profiles of platform morphology during sequence 4. Symbols as in Figure 7; TM 5 section TM. (A) During deposition of late transgressive systems tract 4/maximum flooding zone 4, a gentle dipping ramp with a distally steepened edge developed between sections NA and TM. Landward of section TM, a wide region of shallow subtidal and peritidal facies was deposited, indicating the platform was relatively flat topped. (B) Seaward progradation of cyclic peritidal facies and peloid-ooid grainstones during late highstand systems tract 3 resulted in the generation of a flat-topped platform across the entire study area. Because of late highstand systems tract 3 progradation, the location of the slope break and the gradient along the slope cannot be observed within the study area. stands, superposed high-frequency oscillations would have affected sedimentation patterns in more of the deeper water environments. For example, along the intraplatform basin during high-frequency sea-level fall and lowstand, peritidal facies would have prograded over intrashelf basin facies to generate meter-scale peritidal cycles, while along the slope, platform-margin grainstones would have prograded over basin and slope facies to generate meter-scale, grainstone-capped cycles. CONCLUSIONS (1) Uppermost Lower–Middle Devonian carbonates (270 – 400 m thick) of the eastern Great Basin were deposited along a westward-thickening, carbonate platform. Six regional facies representing peritidal, shallow subtidal, stromatoporoid buildup, deep subtidal, slope, and basinal environments are recognized. Four third-order, transgressive- 414 regressive sequences ('1.5–2.5 m.y. durations) are identified based on shallowing and deepening patterns in regional facies, intensity and stratigraphic distribution of subaerial exposure features, and stacking patterns of fourth- to fifth-order, upwardshallowing cycles. (2) Depositional sequences are 30 –180 m thick and can be correlated across the entire platform-to-basin transition. Transgressive systems tracts along the basin/slope are characterized by upward-deepening successions of proximal through distal turbidites overlain by fine-grained hemipelagic deposits (sequences 2, 3, and 4). Transgressive systems tracts along the shallow platform are composed of successions of peritidal cycles overlain by subtidal cycles, successions of thicker-than-average peritidal cycles, or by noncyclic deep subtidal facies. Along the shallow platform, maximum flooding zones are defined by succession of peritidal cycles dominated by subtidal units, intervals of noncyclic deep subtidal facies, or distinct deeper subtidal units within a single cycle. Highstand systems tracts along the basin/ slope are composed of fine-grained hemipelagic deposits overlain by distal through proximal turbidites, which may be capped by platform-margin grainstones. Along the shallow platform, highstand systems tracts are well defined by upward-shallowing successions of cyclic shallow subtidal through peritidal facies. Sequence boundary zones (2–16 m thick) along the shallow platform are composed of stacked, exposure-capped peritidal and subtidal cycles that exhibit upsection increases in the proportion of tidal-flat subfacies and concurrent increases in the intensity of cycle-capping subaerial exposure features. These cycle-capping exposure features indicate that the shallow platform was affected by multiple episodes of subaerial exposure rather than a single, long-lived exposure event. Sequence boundary zones along the basin/slope (6 –20 m thick) are composed of upward-shallowing successions of proximal turbidites or platform-margin grainstone shoal deposits, which are immediately overlain by basinal facies. The lack of exposure features in these sequence boundary zones indicates that third- through fifth-order sealevel fall rates were less than tectonic subsidence rates along the basin/slope. (3) High-resolution conodont and brachiopod biostratigraphy permits correlation across contrasting facies belts of the basin/ slope and platform margin, while stratal stacking patterns and exposure features enable correlation across the rest of the shallow platform where biostratigraphically diagnostic fossils are absent. The four sequences are biostratigraphically correlated with previously dated, uppermost Lower– Middle Devonian transgressive-regressive sequences in the western and eastern United States, western Canada, and Europe, indicating they are eustatic in origin. (4) Subsequence-scale correlations across the study area indicate that platform morphologies alternated between distally steepened ramps during transgressive systems tract development and flat-topped platforms during highstand systems tract development. Except for incipient drowning during transgressive systems tract 3 and maximum flooding zone 3, the sequences stack in an aggradational to slightly progradational pattern (keep-up style sedimentation) and are bounded by sequence boundary zones rather than unconformities, suggesting that greenhouse climate modes Geological Society of America Bulletin, April 1996 SEQUENCE STRATIGRAPHY OF DEVONIAN CARBONATES Figure 12. Devonian third-order, eustatic sea-level curve (or more correctly termed, a relative paleobathymetric curve) modified from Johnson et al. (1985). The second-order accommodation curve represents the envelope of third-order highstand positions and reflects the increase in accommodation associated with the lower part of the Kaskaskia sequence of Sloss (1963). Note that incipient drowning associated with sequence 3 is interpreted as the second-order maximum flooding zone. Third-order segments labeled Ia–IIf are previously recognized deepening events (T-R cycles of Johnson et al., 1985, in press) in the western, midwestern, and eastern United States, western Canada, and Europe. Biostratigraphic and sequence stratigraphic correlations recognized in this study indicate that sequences 1, 2, 3, and 4 correlate with T-R cycles Ic, Ib, Ie, and If, respectively. and second-order accommodation gains related to the lower part of the Kaskaskia sequence controlled sequence-scale stacking patterns. ACKNOWLEDGMENTS Funding for this project was provided by the American Chemical Society (PRF #25593-G8) and a Research Allocations Committee grant from the University of New Mexico. Field assistance was gratefully received from Tom Oesleby, Chris Andronicus, Bob Goldhammer, Gabriella Savarese, Sue Rotto, Katerina Petronotis, and Mark Boslough. My gratitude goes to the late J. G. ‘‘Jess’’ Johnson for our invaluable discussions about the Devonian stratigraphy/biostratigraphy of the Great Basin. The manuscript benefited from reviews by Todd LaMaskin, Katie Giles, Brian Pratt, Carl Drummond, Charles Kahle, Jim Markello, Steve Reid, and Steve Dorobek. REFERENCES CITED Ahr, W. M., 1973, The carbonate ramp—An alternative to the shelf model: Gulf Coast Association of Geological Societies Transactions, v. 23, p. 221–225. Bond, G. C., and Kominz, M. A., 1984, Construction of tectonic subsidence curves for the early Paleozoic miogeocline, southern Canadian Rocky Mountains: Implications for subsidence mechanism, age of breakup and crustal thinning: Geological Society of America Bulletin, v. 95, p. 155–173. Burchette, T. P., and Wright, V. P., 1992, Carbonate ramp depositional systems: Sedimentary Geology, v. 79, p. 3–57. Burchfiel, B. C., and Royden, L. H., 1991, Antler orogeny: A Mediterranean-type orogeny: Geology, v. 19, p. 66– 69. Cisne, J. L., 1986, Earthquakes recorded stratigraphically on carbonate platforms: Nature, v. 323, p. 320–322. Cook, H. E., and Taylor, M. E., 1977, Comparison of continental slope and shelf environments in the Upper Cambrian and Lowest Ordovician of Nevada, in Enos, P., and Cook, H. E., eds., Deep-water carbonate environments: Society of Economic Paleontologists and Mineralogists Special Publication 25, p. 51– 81. Cowan, C. A., and James, N. P., 1993, The interactions of sea-level change, terrigenous-sediment influx, and carbonate productivity as controls on Upper Cambrian grand cycles of western Newfoundland, Canada: Geological Society of America Bulletin, v. 105, p. 1576–1590. Demicco, R. V., 1983, Wavy and lenticular-bedded carbonate ribbon rocks of the Upper Cambrian Conococheague limestone, central Appalachians: Journal of Sedimentary Petrology, v. 53, p. 1121–1132. Dobbs, S. W., Carpenter, J. A., and Carpenter, D. G., 1993, Structural analysis from the Roberts Mountains to the Diamond Mountains, Nevada: Estimates on the magnitude of contraction and extension, in Gillespie, C. W., ed., Structural and stratigraphic relationships of Devonian reservoir rocks, east-central Nevada: Reno, Nevada Petroleum Society, 1993 Field Conference Guidebook, p. 51–57. Geological Society of America Bulletin, April 1996 415 M. ELRICK Elrick, M., 1993, Millennial-scale paleoclimate fluctuations in Paleozoic deep-water rhythmites, western U.S.A.: State College, Pennsylvanian, SEPM Annual Meeting Abstracts, p. 26. Elrick, M., 1995, Cyclostratigraphy of Middle Devonian carbonates of the eastern Great Basin: Journal of Sedimentary Research, v. B65, p. 61–79. Elrick, M., and Hinnov, L. A., in press, Millennial-scale climate origins for stratification in Cambrian and Devonian deepwater rhythmites, western U.S.A.: Palaeogeography, Palaeoclimatology, Palaeoecology. Fischer, A. G., 1981, Climatic oscillations in the biosphere, in Nitecki, M. H., ed., Biotic crisis in ecological and evolutionary time: New York, Elsevier, p. 103–131. Frakes, L. A., Francis, J. E., and Syktus, J. I., 1992, Climate modes of the Phanerozoic: Cambridge, Cambridge University Press, 274 p. Franseen, E. K., Goldstein, R. H., and Whitesell, T. E., 1993, Sequence stratigraphy of Miocene carbonate complexes, Las Negras area, southeastern Spain: Implications for quantification of changes in relative sea level, in Loucks, B., and Sarg, J. F., eds., Recent advances and applications of carbonate sequence stratigraphy: AAPG Memoir 57, p. 409– 434. Garber, R. A., Grover, G. A., and Harris, P. M., 1989, Geology of the Capitan shelf margin—Subsurface data from the northern Delaware Basin, in Harris, P. M., and Grover, G. A., eds., Subsurface and outcrop examination of the Capitan shelf margin, northern Delaware Basin: SEPM Core Workshop 13, p. 3–273. Ginsburg, R. N., 1971, Landward movement of carbonate mud: New model for regressive cycles in carbonate mud [abs.]: American Association of Petroleum Geologists Bulletin, v. 55, p. 340. Goldhammer, R. K., Dunn, P. A., and Hardie, L. A., 1990, Depositional cycles, composite sea level changes, cycle stacking patterns, and the hierarchy of stratigraphic forcing—Examples from platform carbonates of the Alpine Triassic: Geological Society of America Bulletin, v. 102, p. 535–562. Goldhammer, R. K., Lehmann, P. J., and Dunn, P. A., 1993, The origin of high-frequency platform carbonate cycles and third-order sequences (Lower Ordovician El Paso Gp., west Texas): Constraints from outcrop data and stratigraphic modeling: Journal of Sedimentary Petrology, v. 63, p. 318–359. Hardie, L. A., and Shinn, E. A., 1986, Carbonate depositional environments, modern and ancient, part 3: Tidal flats: Colorado School of Mines Quarterly, v. 81, p. 59–74. Harland, W. B., Cox, A. V., Llywellyn, P. G., Picton, C. A., Smith, A. G., and Smith, D. G., 1982, A geologic time scale: New York, Cambridge University Press, 131 p. Harland, W. B., Armstrong, R. L., Cox, A. V., Craig, L. E., Smith, A. G., and Smith, D. G., 1989, A geologic time scale: Cambridge, England, Cambridge University Press, 263 p. Johnson, J. G., 1970, Taghanic onlap and the end of North American Devonian provinciality: Geological Society of America Bulletin, v. 81, p. 2077–2105. Johnson, J. G., 1978, Devonian, Givetian age brachiopods and biostratigraphy, central Nevada: Geologica et Palaeontologica, v. 12, p. 117–134. Johnson, J. G., and Murphy, M. A., 1984, Time-rock model for Siluro–Devonian continental shelf, western United States: Geological Society of America Bulletin, v. 95, p. 1349–1359. Johnson, J. G., and Pendergast, A., 1981, Timing and mode of emplacement of the Roberts Mountains allochthon, Antler orogeny: Geological Society of America Bulletin, v. 92, p. 648– 658. Johnson, J. G., and Potter, E. C., 1975, Silurian (Llandovery) downdropping of the western margin of North America: Geology, v. 3, p. 331–333. Johnson, J. G., and Sandberg, C. A., 1989, Devonian eustatic events in the western United States and their biostratigraphic responses, in McMillan, N. J., Embry, A. F., and Glass, D. J., eds., Devonian of the world: Canadian Society of Petroleum Geologists Memoir 14, v. 3, p. 171–182. Johnson, J. G., Klapper, G., and Trojan, W. R., 1980, Brachiopod and conodont successions in the Devonian of the northern Antelope Range, central Nevada: Geologica et Palaeontologica, v. 14, p. 77–116, 4 pl. Johnson, J. G., Klapper, G., and Sandberg, C. A., 1985, Devonian eustatic fluctuations in Euramerica: Geological Society of America Bulletin, v. 96, p. 567–587. Johnson, J. G., Sandberg, C. A., and Poole, F. G., 1989, Early and Middle Devonian paleogeography of western United States, in McMillan, N. J., Embry, A. F., and Glass, D. J., eds., Devonian of the world: Canadian Society of Petroleum Geologists Memoir 14, v. 1, p. 161–182. Johnson, J. G., Sandberg, C. A., and Poole, F. G., 1991, Devonian lithofacies of western United States, in Cooper, J. D., and Stevens, C. H., eds., Paleozoic paleogeography of the western United States—II: Los Angeles, California, Pacific Section SEPM, v. 67, p. 83–105. Johnson, J. G., Klapper, G., and Elrick, M., 1996, Devonian transgressive-regressive cycles and biostratigraphy, northern Antelope Range, Nevada: Establishment of reference horizons for global cycles: Palaios, v. 11, p. 3–14. Kahle, C. F., 1988, Surface and subsurface paleokarst, Silurian Lockport, and Peebles Dolomites, western Ohio, in James, N. P., and Choquette, P. W., eds., Paleokarst: New York, Springer-Verlag, p. 229–255. Kendall, G. W., Johnson, J. G., Brown, J. O., and Klapper, G., 1983, Stratigraphy and facies across Lower Devonian–Middle Devonian boundary, central Nevada: American Association of Petroleum Geologists Bulletin, v. 67, p. 2199–2207. Knight, I., and James, N. P., 1991, The Ordovician St. George unconformity, northern Appalachians: The relationship of plate convergence at the St. Lawrence Promontory to the Sauk/Tippecanoe sequence boundary: Geological Society of America Bulletin, v. 103, p. 1200–1225. Krebs, W., 1971, Devonian reef limestones in the eastern Rhenish Schieferbirge, in Muller, G., ed., Sedimentology of parts of central Europe: Germany, Verlag Waldemar Kramer, p. 45– 81. Levy, M., and Christie-Blick, N., 1989, Pre-Mesozoic palinspastic reconstruction of the eastern Great Basin (western United States): Science, v. 245, p. 1454–1462. Markello, J. R., and Read, J. F., 1982, Carbonate ramp to deeper shale–shelf transitions of an Upper Cambrian (Dresbachian) shelf embayment, Nolichucky Formation, southwest Virginia Appalachians: American Association of Petroleum Geologists Bulletin, v. 66, p. 860– 878. Marsaglia, K. M., and Klein, G. de V., 1983, The paleogeography of Paleozoic and Mesozoic storm depositional systems: Journal of Geology, v. 91, p. 117–142. Meyers, W. J., 1988, Paleokarst features in Mississippian limestone, New Mexico, in James, N. P., and Choquette, P. W., eds., Paleokarst: New York, Springer-Verlag, p. 306–328. Montañez, I. P., and Osleger, D. A., 1993, Parasequences stacking patterns, third-order accommodation events, and sequence stratigraphy of Middle to Upper Cambrian platform carbonates, Bonanza King Formation, southern Great Basin, in Loucks, B., and Sarg, J. F., eds., Recent advances and applications of carbonate sequence stratigraphy: American Association of Petroleum Geologists Memoir 57, p. 305–326. Odin, G. S., Curry, D., Gale, N. H., and Kennedy, W. J., 1982, The Phanerozoic time scale in 1981, in Odin, G. S., ed., Numerical dating in stratigraphy, part I: New York, Wiley-Interscience, p. 957–960. Osleger, D. A., and Read, J. F., 1991, Relation of eustasy to stacking patterns of meter-scale carbonate cycles, Late Cambrian, U.S.A.: Journal of Sedimentary Petrology, v. 61, p. 1225–1252. Osmond, J. C., 1954, Dolomites in Silurian and Devonian of eastcentral Nevada: American Association of Petroleum Geologists Bulletin, v. 38, p. 1911–1956. Palmer, A. R., 1983, The Decade of North American Geology 1983 geologic time scale: Geology, v. 11, p. 503–504. Read, J. F., 1985, Carbonate platform facies models: American Association of Petroleum Geology Bulletin, v. 66, p. 860– 878. Read, J. F., 1989, Controls on evolution of Cambrian–Ordovician passive margin, U.S. Appalachians, in Crevello, P., Wilson, J. L., Sarg, J. F., and Read, J. F., eds., Controls on carbonate platform and basin development: SEPM Special Paper 44, p. 147–166. Read, J. F., and Goldhammer, R. K., 1988, Use of Fischer plots to define 3rd order sea level curves in peritidal cyclic carbonates, Early Ordovician, Appalachians: Geology, v. 6, p. 895– 899. Read, J. F., Osleger, D. A., and Elrick, M., 1991, Two-dimensional modeling of carbonate ramp sequences and component cycles, in Franseen, E. K., Watney, W. L., Kendall, C. G. St. C., and Ross, W. C., eds., Sedimentary modeling: Computer simulations and methods for improved parameter definitions: Kansas Geological Survey Bulletin 233, p. 231–251. Roberts, R. H., Hotz, P. E., Gilluly, J., and Ferguson, H. G., 1958, Paleozoic rocks of north-central Nevada: American Association of Petroleum Geologists Bulletin, v. 42, p. 2813–2857. Sandberg, C. A., Gutschick, R. C., Johnson, J. G., Poole, F. G., and Sando, W. J., 1982, Middle Devonian to Late Mississippian geologic history of the overthrust belt region, western United States, in Powers, R. B., ed., Geologic studies of the Cordilleran thrust belt, v. 2: Denver, Colorado, Rocky Mountain Association of Geologists, p. 691–719. Sarg, J. F., 1988, Carbonate sequence stratigraphy, in Wilgus, C. K., ed., Sea-level changes: An integrated approach: SEPM Special Publication 42, p. 56– 81, 155–182. Schlager, W., 1981, The paradox of drowned reefs and carbonate platforms: Geological Society of America Bulletin, v. 92, p. 197–211. Scotese, C. R., and McKerrow, W. S., 1990, Revised world maps and introduction, in McKerrow, W. S., and Scotese, C. R., eds., Palaeozoic palaeogeography and biogeography: Geological Society of London Memoir 12, p. 1–24. Simo, J. A., Scott, R. W., and Masse, J., 1993, Cretaceous carbonate platforms: AAPG Memoir 56, 478 p. Sloss, L. L., 1963, Sequences in the cratonic interior of North America: Geological Society of America Bulletin, v. 74, p. 93–114. Sonnenfeld, M. D., and Cross, T. A., 1993, Volumetric partitioning and facies differentiation within the Permian upper San Andres Formation of Last Chance Canyon, Guadaloupe Mountains, New Mexico, in Loucks, B., and Sarg, J. F., eds., Recent advances and applications of carbonate sequence stratigraphy: American Association of Petroleum Geologists Memoir 57, p. 435– 474. Speed, R. C., and Sleep, N. H., 1982, Antler orogeny and foreland basin: A model: Geological Society of America Bulletin, v. 93, p. 815– 828. Stewart, J. H., and Poole, F. G., 1974, Lower Paleozoic and uppermost Precambrian Cordilleran miogeocline, Great Basin, western United States, in Dickinson, W. R., ed., Tectonics and sedimentation: SEPM Special Publication 22, p. 27–57. Vail, P. R., Mitchum, R. M., and Thompson, S., 1977, Seismic stratigraphy and global changes of sea-level, part 4 —Global cycles of relative changes of sea-level, in Payton, C. E., Seismic stratigraphy—Applications to hydrocarbon exploration: AAPG Memoir 26, p. 83–97. Van der Voo, R., 1993, Paleomagnetism of the Atlantic, Tethys and Iapetus Oceans: Cambridge, Cambridge University Press, 411 p. Wilgus, C. K., Hastings, B. S., Kendal, C. G. St. C., Posamentier, H. W., Ross, C. A., and Van Wagoner, J. C., 1988, Sea level changes: An integrated approach: SEPM Special Publication 42, 407 p. Wilson, J. L., 1975, Carbonate facies in geologic history: New York, Springer-Verlag, 470 p. Witzke, B. J., 1990, Palaeoclimatic constraints for Palaeozoic palaeolatitudes of Laurentia and Euroamerica, in McKerrow, W. S., and Scotese, C. R., eds., Palaeozoic palaeogeography and biogeography: Geological Society of London Memoir 12, p. 57–73. Wong, P. K., and Oldershaw, A. E., 1980, Causes of cyclicity in reef interior sediments, Kabob Reef, Alberta: Bulletin Canadian Petroleum Geology, v. 28, p. 411– 425. Wright, V. P., 1992, Speculations on the controls on cyclic peritidal carbonates: Ice-house versus greenhouse eustatic controls: Sedimentary Geology, v. 76, p. 1– 4. Wright, V. P., 1994, Paleosols in shallow marine carbonate sequences: Earth Science Reviews, v. 35, p. 367–395. MANUSCRIPT RECEIVED BY THE SOCIETY MAY 23, 1995 REVISED MANUSCRIPT RECEIVED SEPTEMBER 19, 1995 MANUSCRIPT ACCEPTED SEPTEMBER 20, 1995 Printed in U.S.A. 416 Geological Society of America Bulletin, April 1996