Pore systems in the Middle Permian Phosphoria Rock Complex (PRC), Rocky Mountain Region, USA, evolved with biotic and chemical dynamics in a shallow epicontinental seaway undergoing extreme environmental shifts. Biochemical responses to environmental changes directly affected pore systems and controlled diagenetic pathways through burial. Petrographic methods and spatially resolved measurements of δ 18 O in sequence stratigraphic context allow characterization of pore systems and their evolution in heterogenous biochemical sediments. Pore systems vary regionally and across systems tracts on second-order (9–10 million years [MY]) and third-order (2–5 MY) timescales. Minimal porosity occurs in transgressive mudrocks rich in organic matter (OM), phosphorites, and carbonates. Cool, acidic, low-oxygen, nutrient-rich basinal waters interacted with warm open to restricted shelfal waters in transgressions. This resulted in accumulation and microbial decay of S-rich OM, phosphatization, carbonate precipitation, silicification, as well as deposition of calcitic-biotic debris (bryozoans, brachiopods, and crinoids) and micrite. Relative to landward and highstand marine components, transgressive basinal marine carbonates and silica are δ 18 O depleted due to microbial decay of OM. Extensive cementation coupled with near-surface compaction and recrystallization of micrite occluded large portions of porosity in transgressive phosphorites and carbonates. Porosity in these rocks is dominated by interparticle and, to a lesser degree, intraparticle microporosity in microbored phosphatized and micritized grains. Phosphorites are compacted where cements are not pervasive. OM-rich sediments host minimal volumes of interparticle nanoporosity due to mechanical compaction and incursion of secondary OM (bitumen) during burial. PRC OM is S-rich, due to sulfate-reducing bacterial enrichment, and locally abundant. This drove early generation of secondary OM and inhibited OM-hosted porosity development through thermal maturation. Large volumes of porosity accumulated in highstand sediments and varied with transitions from silicisponge spicule cherts and calcitic-biota carbonates to pervasively dolomitized micritic, peloidal, aragonitic mollusk, and peritidal microbial sediments. These biochemical transitions, and ultimately pore-system evolution, were driven by interaction between oxygenated open marine waters, eolian siliciclastic debris, and increasingly restricted shelfal waters. Marine carbonate and silica δ 18 O are consistent with Middle Permian open marine waters but are enriched landward and through highstands with evaporative fractionation. This δ 18 O-enriched authigenic silica in carbonates and evaporite replacements, as well as δ 18 O enrichment through silica precipitation, suggest dolomitization and silicification were driven by evaporitic processes. In spiculitic cherts and siltstones, silicification and carbonate diagenesis resulted in small volumes of intraparticle, interparticle, and moldic porosity, as well as increased susceptibility to fracturing and associated permeability enhancement. Chalcedony in spiculites and silicified carbonates host minor volumes of porosity where moganite crystallites dissolved during hydrocarbon migration. Highstand dolomites host abundant intercrystalline, moldic, fenestral, and interparticle macroporosity and microporosity, especially in peloidal wackestones, mollusk debris, ooid grainstones, and peritidal microbialites. Dolomitization resulted in dissolution of aragonitic mollusk and ooids, cementation, and preservation of primary porosity. Porosity loss through burial in dolomites occurs through mechanical compaction, and to a lesser degree, precipitation of zoned carbonate cements that are δ 18 O depleted relative to earlier dolomite. Compaction strongly decreases intercrystalline porosity in dolomitized peloidal wackestones. Secondary OM related to hydrocarbon migration coats surfaces and fills small pore volumes, inhibiting burial cementation.

Abstract The consistent low porosity and permeability of reservoirs in the Bakken petroleum system, in the Williston Basin, have increased the need for fracture studies. Although situated in an intracratonic setting, the Williston Basin displays evidence of deformation enabling the presence of regional and local fracturing. In this study, applicable fracture models are utilized to delineate regional and local fracture orientations within the Williston Region. Northwest and northeast regional fracture trends have been determined by integrating results from previous fracture studies, collecting field data at outcrop locations in the Williston Basin Region, and from subsurface three-dimensional (3-D) seismic data in the Williston Basin. A right-lateral wrench fault strain ellipse model is offered to explain these regional trends. Fracture orientations acquired from outcrop sites (Little Rocky Mountains, Big Snowy Mountains, and Beartooth Mountains) also reveal local, structurally controlled, conjugate fracture trends that are parallel or perpendicular to the structural axis. Using curvature analysis on the interpreted 3-D seismic data, local fracture patterns are also observed within the Williston Basin. When regional and local trends are compared, overlap occurs in fracture orientation showing preference to fractures produced from local structures. Regional and local trends are also incorporated into a mechanical stratigraphy study using field observations of outcropping Bakken age equivalent and lithologically similar strata from the Bighorn Basin. Dense fracturing occurs within the middle Bakken equivalent member of the Cottonwood Canyon Formation. Extensive fractures that are perpendicular to bedding are also observed and cut through the lower bounding Three Forks Formation, Cottonwood Canyon Formation, and overlying Lodgepole Formation.

Abstract Standard carbonate facies models are widely used to interpret paleoenvironments, but they do not address how carbonate platforms are affected by relative changes in sea level. An understanding of how the subtidal carbonate "factory" responds to relative sea-level changes and the role played by other environmental factors towards influencing the formation of carbonate platforms allows one to differentiate platform types and it helps establish a basis for constructing depositional sequence and systems tract models. The combination of in-situ production of carbonate sediment, which is also subject to transport, and local variations in depositional processes result in the formation of a wide variety of stratal patterns, some of which are unique to carbonate systems. Fundamental carbonate-depositional principles and geologic-based observations were used to construct depositional sequence and systems tract models for a variety of rimmed shelves and ramps. The models show how, for example, depositional sequences made up of (1) carbonate, (2) carbonate-siliciclastic, or (3) carbonate-evaporite-siliciclastic facies are produced by depositional systems responding to lowstand, transgressive, and highstand conditions. Lowstand: Carbonate sediment production is reduced on rimmed shelves because a relatively small area of shallow seafloor is in contact with the carbonate "factory." Reduced sedimentation and subaerial exposure foster the retreat of shelf edges and slopes by erosion and slope failure during lowstands. As a result, thick debris-flow deposits may form. Karst development is important in humid climates and can affect large areas of a subaerially exposed platform. If siliciclastic sediments are available, they are delivered to the shelf edge and slope by fluvial-deltaic systems or, in arid climates, by wadis and advancing ergs. Under arid conditions, lowstand evaporites may fill an isolated or completely silled basin. Transgression: Carbonate sedimentation initiates in restricted environments and later as more open conditions develop, open marine facies, including patch reefs, may locally develop atop flooded platforms and ramps. Retrogradational parasequences comprising shallow-water carbonates form and subsequently drown, and shelf edges tend to aggrade, backstep, and drown if the rate of sea-level rise is high. Highstand: Seaward-prograding carbonate or siliciclastic coastal sediments and landward-prograding carbonate rimmed shelf edges may partially fill inner to outer shelf seas. Under arid conditions, evaporites and red beds commonly fill wide and shallow salinas. These strata onlap subaerially exposed rimmed shelf edges and prograding grain-stone islands in ramps. Shelf edges and shorelines tend to prograde under the influence of high rates of carbonate sedimentation across the shelf and shelf edge. Slope and basinal environments receive excess shelf- and shelf-edge-derived sediment. Factors listed above must be integrated with established facies models in order to arrive at comprehensive sequence and systems tract models. As should be the case with all models, however, they are not meant to serve as rigid templates within which all carbonate sequences must fit. Modification may be needed to accommodate each case. Once they are deemed applicable to a specific case, they function as working hypotheses to help geologists visualize how and why carbonate strata were laid down and fit together as they do. As a general predictor of facies, carbonate depositional sequence and systems tracts models may be used in conjunction with seismic records to identify depositional systems and to locate reservoir-, seal-, and source-prone facies.

Abstract Post-Ordovician carbonate buildups and buildup plays from around the world have been evaluated to determine the distinctive aspects of hydrocarbon-productive buildups. Ninety percent of the approximately 40 billion barrels of recoverable oil equivalent found within carbonate buildup reservoirs exists within strata deposited during just 15% of post-Ordovician geologic time. These time windows correspond to periods of extensive source rock deposition and, with the exception of the late Paleozoic and late Miocene, to the rising sea-level portions of long-term, second-order eustatic highstands. Nearly three-fourths of buildup reserves are found in buildups deposited during the early phases of periods of rapidly increasing rates of relative sea-level rise. These buildups are found in the basal, transgressive portions of thick sedimentary wedges and display progressive areal restriction through time until they are eventually unable to keep up with rapid increases in accommodation (total space available for sedimentation). A younger regressive wedge progrades over the buildups during a later period of more limited increases in accommodation and seals the mounded reservoir in fine-grained basinal sediment. Buildups deposited during this regressive phase carry higher exploration risk due to leaky top and lateral seals, and they generally have smaller trap sizes. These cycles of basin fill are clearly recognizable on seismic and log data and have a distinctive character on geohistory plots. Although the origin of these long-term accommodation changes is difficult to assess, we conclude that basin tectonism is responsible for these changes in most of our examples. Long-term eustasy plays a secondary role, according to our analysis. The model illustrated here can be helpful in both the geologic and risk assessment of new buildup plays and prospects.

Abstract The Ammonitico Rosso of the western Venetian Alps is a 10- to 25-m-thick, red nodular limestone that overlies thick Late Triassic to Middle Jurassic shallow-water carbonates that form the South Trento Platform. Deposition of the Ammonitico Rosso is thought to represent a Middle-Late Jurassic drowning event whereby the South Trento Platform became a deeply submerged plateau. The Ammonitico Rosso is problematic in that it: (1) overlies a platform-wide unconformity that contains complex brecciated fabrics filled by pelagic-rich "Posidonia alpina" sediment and cement; (2) is rich in ammonites and other pelagic fauna; and (3) contains stromatolites, oncolites, and wave-rippled coquinas. Based on new data from the eastern margin of the South Trento Platform, the drowning succession is interpreted to have a shallow origin. These data include: (1) the discovery of two sponge-coral-stromatoporoid patch reefs within oolitic and peloidal grainstone below the unconformity; (2) cavities and fill associated with the unconformity; and (3) diagenetic fabrics and transition from platform interior to platform margin facies in both the Lower and Intermediate members of the Ammonitico Rosso. Faunal and lithologic similarity of sponge-coral-stromatoporoid reefs with other Lower Jurassic reef complexes suggest that these reefs are Late Pliensbachian in age. Cavities and neptunian dikes within back-reef, reef, and fore-reef sediments are filled by P. alpina sediment, rounded lithoclasts, fibrous cement, and crystal silt. Pendant cement and crystal silt found within reef cavities and neptunian dikes overlap deposition of internal sediment. In a west-to-east transect above the unconformity, stromatolitic and oncolitic mudstone/wackestone in the Lower Ammonitico Rosso grades first into thrombolites and stromatolites, then into nodular burrowed wackestone and packstone/grainstone. Packstone and grainstone contain well-preserved ammonites, pelagic bivalves, peloids, belemnites, gastropods, solitary corals, and fibrous cement. In the Intermediate Member, thin-bedded chert-rich limestone grades into event strata (i.e., tempestites) composed of limestone gravel and well-sorted sand, and pelagic-dominated mudstone/wackestone. Gravels are poorly sorted, sometimes imbricated and contain lithoclasts derived from underlying sediments. These lithologies overlie truncation surfaces that include deep irregular excavations, rounded gutters, and gently scoured surfaces and grade upward into sands that possess hummocky, low-angle, and planar cross-stratification. Sands are composed of coarse- to fine-size lithoclasts, peloids, and skeletal grains. Peloidal mud-stone and wackestone contain protoglobigerinids, radiolarians, ammonites, pelagic bivalves, belemnites, crinoids, and solitary corals. Solitary corals are found in growth position on ammonite and belemnite substrates. The drowning succession of the South Trento Platform correlates with long-term eustatic rises and falls of sea level and includes: (1) Upper Pliensbachian deposition and tectonism; (2) a transgressive systems tract and high-stand systems tract (Toarcian); (3) a small-scale type 1 sequence boundary (late Toarcian-Lower Bajocian?); (4) a drowning sequence (Aalenian-Upper Bajocian); and (5) a composite condensed section (Upper Bajocian-Tithonian). The appearance of pelagic organisms on the South Trento Platform and a biotic succession from (1) sponge-coral-stromatoporoid reefs to (2) bioeroded sponge and hermatypic coral reefs to (3) grainstone composed of ahermatypic suspension/detrital feeders and planktic organisms to (4) "stunted" pelagic and benthic faunas to (5) microbial mat (stromatolite) structures indicates progressive paleoecologic deterioration of shallow-water environments. Analogy of these transitions with those observed on modern "drowned" platforms suggests that the demise of carbonate producing benthos was caused by increasing amounts of nutrients and organic matter (i.e., trophic resources) and establishment of oxygen-deficient environments. Faunal transition is coincident with the breakup of Pangea during the Lower-Middle Jurassic, the deposition of organic-rich shale and manganese-rich limestone in periplatform and basinal settings, and eustatic sea-level rise. This suggests that influx of trophic resources was associated with changes in regional circulation patterns and upwelling. Drowning is interpreted to have occurred gradually over time through a combination of eustatic sea-level rise and environmental change.

Abstract Sequence stratigraphic interpretations of carbonate platform margins are based to a large degree on concepts of variable timing and nature of deposition relative to fluctuations in sea level. Quaternary platform margins, such as those found in the Bahamas, provide a unique opportunity to calibrate the sedimentary record because of the well-constrained nature of sea-level history during this period. Detailed observations and sampling from a research submersible combined with high-resolution radiocarbon dating in the Tongue of the Ocean, Bahamas, have enabled us to document variations in deposition along the upper parts of the marginal slope during the most recent rise in sea level. We have found that the steep marginal slopes around the Tongue of the Ocean record deposition during the early rise of sea level following the last lowstand some 18–21 Ka. Coarse-grained skeletal sands, gravel, and boulders derived from reefs growing along the overlying escarpment were deposited on slopes of 35–45º and cemented in place within a few hundred years. Deposition by rockfall and grainflow resulted in a series of elongate lenses oriented parallel to the slope. These lenses are generally less than 0.5 m thick and pinch out downslope within tens of meters. Repeated deposition and cementation produced slope deposits that are both laterally discontinuous and internally heterogeneous. Radiocarbon dating of skeletal components and cements indicate that active deposition on the slopes ceased approximately 10,000 years ago as sea level rose above the escarpment and began to flood the top of the Great Bahama Bank. Fine-grained, nonskeletal sands and muds derived from the platform are presently bypassing these slopes and are deposited downslope as a wedge of sediment with slope declivities of 25–28º. Cracks and slide scars are a common feature of the steep-cemented slopes. The cracks are a few centimeters wide and may extend for tens of meters across the slope with an arcuate, convex-up expression. The slide scars are generally a few meters wide by several meters long and cut back into the slope a few meters to less than 1 m, although one large example is 30 m wide, extends downslope for 75 m, and has exposed 10 m of the interior of the slope. Transects downslope from slide scars show that large blocks of the slope, some in excess of 10 m across, have been transported for tens or hundreds of meters downslope. The release and transport of such blocks may be one mechanism by which turbidity currents are initiated in deeper slope environments.

Abstract Basin-margin, Late Guadalupian carbonate strata contain units with alternating baselap patterns. Downlap occurs at the base of steeply dipping boulder-conglomerate lower slope strata; onlap occurs at the base of gently dipping wackestone-rich toe-of-slope strata. The downlap and onlap stratal patterns reflect differences in gravity-flow deposition in response to different sediment type supplied to the basin margin. The lower slope was too steep for deposition of peloidal carbonate muds transported by low-density turbidity currents, so gravity flows bypassed the slope and deposited carbonate mud-rich beds at the toe-of-slope, where they onlapped lower slope boulder conglomerates. Matrix-poor boulder conglomerates terminate by downlap in talus cones because the rock falls and low-matrix debris flows responsible for their transport reached a slope too gentle for continued transport. In contrast, steeply dipping, matrix-rich boulder-bearing debris-flow deposits interfinger downslope into finer grained debris-flow and turbidity current deposits concordant with underlying strata. The sediment type delivered to the basin margin varied systematically during the Late Guadalupian due to sea-level fluctuations. Relative sea-level stand was interpreted from correlation to the relative sea-level record of contemporaneous shelf strata and from analogy to the Bahamian Quaternary carbonate sediment history. Siliciclastic silts were deposited in the basin during lowstands as onlapping strata. Downlapping silt-matrix boulder conglomerates (units 1 and 7) were deposited during the early transgression, when fringing reefs supplied boundstone boulders and siliciclastic silt could be transported across the emergent shelf. Downlapping matrix-poor boulder conglomerates (unit 2) were deposited during the late transgression, when fringing reefs were still growing and the shelf was flooded enough to stop the basinward transport of quartz silt but was not flooded enough to pro-duce significant quantities of carbonate mud. Onlapping toe-of-slope wackestones (unit 3) were deposited during early highstand, when the flooded shelf was producing carbonate mud and the reefs either had not caught up with sea level rise or had stepped back onto the shelf. Boulder conglomerates with lime-mud matrixes (units 4, 6, and 7) formed after reefs had caught up with sea-level rise or had prograded to the shelf edge (late highstand). The stratal patterns in these Permian, basin-margin carbonates are different from those commonly interpreted in generalizations of siliciclastic sequences (e.g., Vail et al., 1984). The Late Guadalupian stratal patterns are caused by the change in carbonate sediment type and quantity with change of relative sea level, rather than by proximity to shoreline with changing relative sea level. This study demonstrates the importance of understanding the relationship between sea level, sediment supply, and depositional mechanisms before using stratal patterns to interpret relative sea levels in carbonate basin margins.

Abstract Integrated sequence analysis of seismic and well data of the Givetian to Tournaisian sedimentary succession on the outer margin of the Lennard Shelf and adjacent Fitzroy Trough has recognized 18 third-order stratigraphic sequences in a major transgressive-regressive facies cycle. A Givetian phase of crustal extension initiated the transgressive-regressive cycle. The cycle terminated in the Tournaisian, following a phase of slow thermal subsidence. The transgressive half-cycle comprises at least four third-order sequences; an initial Frasnian-Givetian sequence followed by three back-stepping Frasnian sequences. The regressive half-cycle comprises 14 basin-ward-advancing third-order sequences. A period of tectonic uplift and erosion immediately prior to the Frasnian-Famennian boundary resulted in a major basinward shift in coastal onlap in the initial stages of the regressive half-cycle. Systems tracts and facies are partitioned according to their position on the transgressive-regressive cycle; in the transgressive half-cycle, lowstand deposits are subdued and transgressive and highstand deposits accentuated. In the regressive half-cycle, lowstand deposits are accentuated and provide a foundation for the overlying transgressive and highstand deposits. The extent of basinward highstand progradation was limited by the break point of their underlying lowstand deposits. Sequence and systems tract geometries, their stacking patterns, and component facies define two distinct styles of sedimentation: (1) a Givetian to early Famennian, reef-rimmed platform complex; and (2) a late Famennian to Tournaisian mixed carbonate and siliciclastic ramp complex. The reef complex demonstrates marked reciprocal sedimentation. During lowstands, terrigenous sediments by-passed the exposed platform to be deposited in the basin as basin floor fans, slope fans, and prograding complexes. During transgressions and highstands, carbonate sediments were deposited on the platform, allodapic carbonate particles were shed into proximal marginal-slope settings, and clastics were trapped on the inner platform. Lowstand carbonate production occurred locally in areas starved of terrigenous clastic influx. The recognition of relative sea-level cycles in these strata has led to a subsurface model of reef development, which provides new insights into the third-order cyclicity within the larger Pillara and Nullara cycles recognized from previous outcrop studies. The outcrop model of backstepping and advancing reef complexes emphasizes transgressive and highstand depositional systems, and fails to recognize phases of lowstand deposition.

Abstract The separation of eustatic, tectonic, and other controls on the development of sedimentary cyclicity is difficult. In mixed carbonate-siliciclastic successions, the conventional interpretation of unconformity-bounded depositional sequences is that they are due to reciprocal sedimentation in response to relative changes of sea level. According to this view, transgressive and highstand systems tracts are composed primarily of carbonate rocks, and lowstands of siliciclastic rocks. The application of this model to the interpretation of cyclic carbonate and siliciclastic rocks in the Upper Devonian of the Canning basin, Western Australia, presents a paradox because expected evidence for subaerial exposure of the platform is not well developed. Sequence stratigraphic studies in outcrop at two localities along the northern margin of the Canning basin confirm a complex relation between carbonate and siliciclastic conglomerate. At Stony Creek, the carbonate rocks are interpreted to represent an assemblage of reef, foreslope floatstones, and backreef carbonate-conglomerate cycles that accumulated in a shallow marine environment along the margin of a fan-delta. Conglomerates and sandstones inferred to onlap the reefal foreslope are interpreted tentatively as fluvial, and the contact is interpreted as a sequence boundary. In the Van Emmerick Range, foreslope floatstones are onlapped along the margin of an incised valley with at least 10 m of relief by conglomerate and sandstone of probable marine origin, and overlain by a transgressive fossiliferous limestone. The age of the sequence boundary at Stony Creek is not well established, but probably early Frasnian. The age of the sequence boundary in the Van Emmerick Range is better constrained as late Frasnian, and this surface appears to correlate with the base of the Frasnian 4 sequence identified by Southgate et al. (1993) on the basis of subsurface data. Both surfaces are thought to have involved base-level lowering, but the evidence is equivocal. Evaluation of available data indicates that subaerial exposure is required for only one of eight potential sequence boundaries in the Frasnian-Famennian interval, a surface that corresponds locally to minor karstification, and which is dated as latest Frasnian. Several explanations are proposed as working hypotheses. The development of thick lowstand deposits coeval with flooding events on the platform is consistent with continued extension and tilting of a fault block. Alternatively, exposure may have been restricted to topographic highs remaining after extension had ceased. If subaerial exposure was widespread, diagenetic effects may have been limited or not preserved. The development of onlap surfaces within the basin may be related in part to variations in sediment flux, and the distribution of siliciclastic sediments influenced by the geologic structure, especially the configuration of accommodation or transfer zones. Further work is needed to resolve uncertainties in the existing sequence stratigraphic interpretation, to improve the calibration of individual boundaries, and to evaluate these ideas. Ultimately, comparisons with coeval successions on other continents will be needed to evaluate the possible role of eustasy in the development of the observed sequences.

Abstract Upper Pennsylvanian carbonate platform, bank, and reef-mound complexes of the Horseshoe atoll constitute major oil reservoirs within the northern Midland basin of west Texas. Analyses of over 200 mi of seismic data, constrained by fusulinid biostratigraphy, allow seismic sequences and facies to be identified for the eastern and southern portions of the atoll. The reef complex in these regions is composed of four third-order (1–10 m.y.) seismic sequences, including, from oldest to youngest: (1) Strawn (Desmoinesian) sequence; (2) Canyon A (early-early Missourian) sequence; (3) Canyon B (middle-early to early-middle Missourian) sequence; and (4) Canyon C/Cisco (late-middle Missourian-early Virgilian) sequence. The seismic sequences are composed of one to five parasequence sets, and display a retrogradational geometry in cross section and map view. Additional third-order sequences may be present in the Desmoinesian and Virgilian intervals, but are unresolved seismically in the study area. The Strawn seismic sequence is characterized by the occurrence of discontinuous, mounded reflectors interpreted to represent amalgamated phylloid-algal mound complexes. The lower Strawn sequence boundary represents the eroded surface of the Absaroka I cratonic subsequence, a type 1 sequence boundary. The upper Strawn sequence boundary appears conformable with the overlying Canyon A sequence, although the exact nature of the upper Strawn sequence boundary is equivocal. The Canyon A sequence is characterized by internal sigmoid geometries interpreted as prograding clinoforms, indicative of oolitic and skeletal grainstone-bearing units. The Canyon B and Canyon C/Cisco sequences, which are generally restricted to the topographically highest portions of the atoll, are characterized by coherent mound facies that are interpreted to represent heterogeneous reef complexes. Adjacent to many of the larger reef masses, the presence of reef-debris facies of late Canyon age is indicated by reflector packages that show offlaping geometries and basinward downlap. The upper Canyon B sequence boundary shows evidence of significant erosion associated with mass wasting of the atoll bank margin, perhaps related to an atoll-wide exposure event. The top of the atoll appears to be a maximum-flooding surface that is unconformably overlain by onlapping Upper Pennsylvanian and Lower Permian shales. The documentation of third-order seismic sequences and facies of the Horseshoe atoll is important, not only because it provides an internally consistent stratigraphic framework for stratigraphic analysis, but also because it serves as an ancient example of a detached, retrogradational carbonate system. Seismic analysis of the atoll illustrates that the geometries of these types of systems differ substantially from that of attached, prograding systems. The arrangement of systems tracts within retrogradational systems may also differ. The stratigraphic architecture of third-order seismic sequences and facies displayed by the Horseshoe atoll may represent a recurring depositional pattern that arose during Late Pennsylvanian time, a consequence of the geographic, climatic, oceanographic, and tectonic setting of the developing Pangea supercontinent. If so, such a pattern may be anticipated in other time-equivalent carbonate regions.

Abstract The morphology of platform margins as depicted on high-resolution seismic data and the development of outer shelf, eutrophic ecologic assemblages can be used as keys in understanding the evolution of carbonate platforms. The Baltimore Canyon platform, offshore United States East Coast, and the Liuhua platform, Pearl River Mouth Basin, People's Republic of China, exhibited the effects of environmental collapse prior to their extinction by drowning. Reprocessed seismic data, lithologic data, and biostratigraphic data from the Baltimore Canyon area show that environmental deterioration of the platform immediately preceded or was coincident with deltaic progradation. This implies that slope-front fill seaward of the platform is probably coeval with platform deposits, and that a previously identified carbonate sequence boundary may actually be an older drowning sequence. A seismic sequence boundary should be placed at the top of the youngest drowning sequence. Late-growth reefs appear to be discontinuous along both the Baltimore Canyon and Liuhua platform margins. The proximity of prograding deltas appears to be the main control on the location of late-growth reefs on both platforms, though tectonic subsidence (local faulting) may govern their distribution along the southern part of the Liuhua platform margin. The horizontal-planar onlap of basinal shales onto the Liuhua platform margin could be misinterpreted as always representative of an unconformable contact with the platform sequence. In reality, local differences in highstand off-bank transport of platform and platform margin sediments may have produced a progradational fore-slope (a wedge of fore-slope debris) in some areas, and only horizontal-planar onlap in others.

Abstract Four major Oligocene carbonate sequences were studied in the Teweh area of Central Kalimantan, Indonesia, to better understand how they might serve as reservoirs for hydrocarbons in the area. Each sequence (200–500 m thick) was delineated in outcrops and/or on seismic lines: (1) early Oligocene (34.0–36.5 Ma); (2) middle Oligocene (29.7–32.0 Ma); (3) early late Oligocene (28.0–29.7 Ma); and (4) middle to late late Oligocene (N3; >24–28.0 Ma). In landward areas to the south, sequence 1 consists mainly of sandstones and shales with thin limestone beds. Isolated carbonate buildups and shales occur in basinal areas to the north in sequence 1. An erosional unconformity separates sequences 1 and 2. During deposition of sequences 2–4, carbonate shelves developed in the southern part of the Teweh area, while shales were deposited in basinal environments to the north. The carbonate shelf margin of sequence 2 was established along a structural hinge line. Boundaries between sequences 2–4 do not show onlap or erosional truncation in this area. On seismic lines, boundaries between carbonate sequences 2–4 are defined by surfaces of renewed carbonate growth (mounding and/or downlap) on the shelf immediately above the sequence boundary. Subaerial unconformities were not found in or between sequences 2–4 on outcrop, so boundaries between sequences 2, 3, and 4 were placed where strata first indicated a substantial deepening of depositional environments. Rapid rises in relative sea level (subsidence + eustatic sea level) resulted in drowning and "backstepping" of carbonate shelf margins in some locations, and stacking of shelf margins in other locations. Internally, the carbonate shelves of sequences 2 and 3 are characterized by vertically building shelf margins with landward-dipping (south-dipping), shingled clinoforms indicating progradation of shallow carbonate environments from the shelf margin into the lagoon. Sequences 2 and 3 have well-developed transgressive systems tracts overlain by highstand systems tracts. In outcrop, the transgressive systems tracts contain interbedded large-foram wackestones/packstones and coral wackestones/packstones with poorly defined facies belts. The highstand systems tracts are characterized by well-developed facies belts which include from the basin shelfward: (1) shale and carbonate debris flows deposited on the lower slope; (2) argillaceous large-foram wackestones on the upper slope; (3) discontinuous coral wackestones and boundstones in bioclastic packstones on the shelf edge; (4) coralline-algae large-foram packstones and grainstones on back-reef flats and shelf-margin shoals; and (5) thin-branching coral and foraminiferal wackestones and packstones in the lagoon. Seismic lines show the carbonate shelf of sequence 4 as a massive buildup which thins substantially into the basin. Our interpretation suggests that in some circumstances, the definition of sequences requires more flexibility than that given in Van Wagoner et al. (1988). In carbonate systems during times of rapid subsidence and low-amplitude sea-level fluctuations, sea level may not drop below the shelf, and subaerial unconformities will not be present on the shelf to separate different sequences of deposition. The Haq et al. (1987) sea-level curve may also require modification, at least with regard to magnitudes and rates of eustatic sea-level rise. Deposition of carbonate shelves in Central Kalimantan spans the large mid-Oligocene (29.5–30.0 Ma) eustatic sea-level drop of Haq et al. (1987). Shallowing and subaerial exposure of these deposits might be expected during that large eustatic sea-level drop, however none was observed. Instead, deepening and local drowning of the carbonate shelf were observed at 29.5–30.0 Ma.

Abstract Establishment of a sequence stratigraphic framework, based on an integration of seismic, well-log, core, and biostratigraphic data, indicates that the Tertiary-aged Gunung Putih carbonate complex in the East Java Sea comprises an asymmetric buildup. This asymmetry was inferred to occur in response to paleo-oceanographic circulation patterns that favored aggradation on the north face and progradation on the south face. The aggradational pattern was associated with carbonate reefal buildups, whereas the progradational patterns were associated with grain-rich forestepping deposits. Widespread carbonate buildups were initiated on a beveled Cretaceous to early-Tertiary platform during the late Eocene. These buildups exhibit pronounced stratigraphic asymmetry; the northern side is inferred to lie on the paleowindward side and is characterized by bulwark-like framestone-rich buildups. Though the stacking pattern is predominantly aggradational, forestepping and backstepping can occur locally. The southern side is inferred to lie on the paleo-leeward side and is characterized by continuous forestepping buildups with clinoform reflection geometries. Mounded, toe-of-slope, lowstand deposits of several sequences form an apron around the entire Miocene complex. From the late Eocene through the Miocene, differential subsidence progressively changed the style of carbonate buildup from widespread distribution across the entire platform to more restricted distribution associated with smaller structural highs that occurred on the platform. Eventually, as the platform continued to founder, carbonate deposition ended completely during the late Miocene. The carbonate complex subsequently was buried in terrigenous mudstones and siltstones. Based on the evolution of the Gunung Putih carbonate complex, a general approach for the exploration for hydrocarbons within carbonate buildups has been developed. Initially, a structural high that caused the sea floor to be sufficiently raised so as to establish a carbonate factory must be identified. Subsequently, evidence for deposition at the top and margins of the structure should be identified. The occurrence of backstepping (i.e., transgressive) deposits, in particular, is suggestive of an active carbonate factory on the platform. Further evidence for carbonate buildups is the re-establishment on the structural crests of shallow-water conditions through the development of stacked, mounded, and clinoforming seismic reflections. Shallow-water conditions also can be inferred through the identification of erosionally truncated or toplapping seismic reflections.