Depositional Topography Research Articles

High-Resolution Sequence Stratigraphy of Lower-Middle Guadalupian Outcrops, Western Guadalupe Mountains, Texas and New Mexico: ABSTRACT

The authors describe parts of three third-order depositional sequences and their associated system tracts within seismic-scale outcrops of the Lower-Middle Guadalupian Northwestern shelf-to-Delaware basin transition. These sequences are well exposed along a 20-km dip-oriented transect in the western Guadalupe Mountains, Texas and New Mexico. Systems tracts are defined on the basis of lithofacies distribution, stratal geometry, and bounding (stratal termination) surfaces. These Lower-Middle Guadalupian sequences record (1) carbonate platform retrogradation, basin margin erosion, and sediment starvation followed by platform aggradation and progradation (lower San Andres Formation-Cutoff Formation; transgressive and highstand systems tracts), (2) major basin infill by terrigenous clastics bypassed over the underlying lower San Andres highstand carbonate bank and development of a shelf-margin-restricted carbonate bank and coeval base-of-slope apron (Brushy Canyon Formation, Cherry Canyon Tongue, and basal upper San Andres Formation; lowstand and transgressive systems tract), (3) progradation of a mixed carbonates-terrigenous clastic bank (upper San Andres Formation; Highstand systems tract), followed by subaerial exposure of the highstand bank top, and (4) deposition of interbedded terrigenous clastics and carbonates in a shorezone that onlaps the subaerial exposure surface at the top of the upper San Andres Formation highstand bank (lower Grayberg Formation; lowstand and transgressive systems tracts). Systems tractsmore » developed in response to the interaction of rate and direction of change of relative sea level; timing, rate and location of terrigenous clastic supply; and depositional topography.« less

Setting of and Mixed Carbonate-Siliciclastic Facies Associated with Holocene Nearshore Sabellid Worm Reefs, Northern Belize: ABSTRACT

Communities of sabellid worms (Polychaeta) in northern Belize (mouth of Northern River Lagoon) occur as areally discontinuous, unlithified patch reefs cresting positive features on an irregular depositional topography of Holocene and older sediments. They are found in nearshore marine, moderate energy, tidally influenced environments (intertidal to 60-cm depths) of normal salinity (36%) adjoining subtidal deposits. These colonies, as much as 30 cm thick, are composed of dense thickets of agglutinating worm tubes (1.0 mm diameter, 3.0 cm long) that trap and bind sand to silt-size bioclastic debris and micrite. Worm population density in these communities averages 30 tubes/cm{sup 2}. The antecedent depositional topography beneath the worm reefs and subjacent to adjoining Holocene deposits is characterized by ridges oriented normal to the present shoreline. These ridges are held up by pre-Holocene (latest Pleistocene ) terrigenous sands of probably fluvial distributary origin, occurring at approximately 2.0 m in the subsurface. The overlying Holocene deposits on these ridges are 2.0 m thick and consist of a general upward-coarsening section of terrigenous sandy, carbonate sandy muds to muddy sands punctuated by mangrove peats and capped by the worm reefs. Low areas on the antecedent topography are presently the sites of deposition of deeper subtidalmore » (1.0 m), muddy carbonate sands with some admixed, reworked terrigenous sand. The Holocene section records drowning due to recent (approximately 6 Ka) transgression of the older, lowstand distributary sands on the northern Belize shelf. This mixed carbonate-siliciclastic section and the worm reefs presently are being reshaped and modified by littoral processes.« less

Concepts for Future Exploration for Submarine Fan and Turbidite System Reservoirs: ABSTRACT

With the emphasis of future petroleum exploration in deeper water and on frontier plays, submarine fan and turbidite systems will be major reservoir targets. Successful exploration and development of these reservoirs will be largely controlled by world oil prices, politics, markets, and drilling and producing technologies. Present economics require large reserves when targeting deep water or when drilling remote regions of countries that require large capital expenditures for pipelines and development. Future techniques for successful exploration and production of turbidite reservoirs will include (1) successful application of sequence stratigraphy in identifying reservoirs, (2) new play concepts for exploring synclinal low areas, and (3) the expanded use of three-dimensional seismic surveys. Lacustrine turbidites in rift, wrench, and successor basins are good potential targets because of their association with good source rocks/seals. Reservoirs occur at the base of deltaic clinoforms interbedded with source locks or along actively subsiding fault scarps. Many of these basins are extremely large, but the distribution of source rocks may be limited, making the potential reserves in these systems highly variable. Submarine fans associated with Cenozoic deltas remain an active deep water play. Turbidite reservoirs, deposited within bathymetric lows and localized basins on the slope created by saltmore » and shale diapirism, occur in areas such as the northern Gulf of Mexico, continental margin of Nigeria, and Mackenzie Delta/Canadian Beaufort Sea. Other prospective Cenozoic delta-related margins include those associated with the Amazon, Indus, Ganges-Bramapuhtra, and Mahakam Deltas. The tremendous exploration successes in the deep-water Campos basin illustrate the great potential in passive margins. Targets are subtle and influenced by depositional topography controlled by growth faults, and shale and salt diapirs.« less

High-Frequency Carbonate/Siliciclastic Reciprocal Sedimentation Within the Upper San Andres Depositional Sequence (Permian, Guadalupian), Last Chance Canyon, Guadalupe Mountains, New Mexico: ABSTRACT

The upper San Andres Formation and Cherry Canyon Sandstone Tongue in Last Chance Canyon form a third-order depositional sequence comprising at least 12 smaller scale, time-bounded depositional units. The intertidal through toe-of-slope facies associations, volumetric proportions of carbonate/siliciclastic sedimentation, and depositional topography change progressively throughout the sequence. These variations reflect the controls that produce the landward-stepping, vertically stacked, and seaward-stepping arrangements of units constituting the sequence. In Last Chance Canyon many of the small-scale depositional units have stratal geometries and carbonate/siliciclastic relationships that are analogous to third-order sequences. The oldest depositional unit is a point-sourced turbidite lobe that onlaps the underlying sequence boundary. Carbonate-rich slope sandstones form the next two units, which also onlap the sequence boundary. These three landward-stepping to vertically stacked units compose the transgressive systems tract (TST). Units within the TST record a progressive decrease in sedimentation rate, depositional energy, and siliciclastic content that reflects the sequence-scale transition from toe-of-slope siliciclastic turbidites to an increasingly carbonate-dominated ramp. Within each of the two youngest units, a basal sandstone hemicycle is overlain by a mounded fusulinid-rich carbonate hemicycle. The maximum flooding surface caps the TST and is overlain by the most carbonate-rich interval of the sequence. From basal tomore » upper slope, the facies in this unit are laminated mudstones and wackestones, crinoid-bryozoan bafflestone bioherms, brachiopod-sponge reefs, and fusulinid mounds. Seaward-stepping units compose the middle highstand systems tract (HST).« less

Tectonosedimentary Evolution of the Crotone Basin, Italy: Implications for Calabrian Arc Geodynamics: ABSTRACT

Analysis of outcrop, well, and offshore seismic data has allowed the Neogene tectonosedimentary evolution of an Ionian Sea satellite basin to be outlined. The Crotone basin contains a series of postorogenic sediments deposited since Serravallian time atop a complex nappe system emplaced in the early Miocene. The basin's evolution can be considered predominantly one of distension in a fore-arc setting punctuated by compressional events. The earliest sediments (middle-late Miocene) consist of conglomerates, marls, and evaporites infilling a rapidly subsiding basin. A basin-wide Messinian unconformity and associated intraformational folding mark the close of this sedimentary cycle. Reestablishment of marine conditions in the early Pliocene is documented by sediments which show a distinct color banding and apparent rhythmicity, which may represent the basin margin to lowermost Pliocene marl/limestone rhythmic couplets present in southern Calabria. A bounding unconformity surface of middle Pliocene age (3.0 Ma), which corresponds to a major northwest-southeast compressional event, closes this depositional sequence. The basin depocenter shifted markedly toward the southeast, and both chaotic and strong subparallel reflector seismic facies of wide-ranging thicknesses fill the depositional topography created during this tectonic episode. Basin subsidence decreases dramatically in the late Pliocene and cessates in response to basin margin uplift inmore » the early Pleistocene. The chronostratigraphic hierarchy of these depositional sequences allows them to constrain the deformational history of the basin. In addition, similar depositional hierarchies in adjacent basins (i.e., Paola, Cefalu, and Tyrrhenian Sea) allow them to tie the stratigraphy and evolution of the Crotone basin to the geodynamic evolution of the Calabrian arc system.« less

Late Pennsylvanian Storm-Dominated Shelf Sand Ridges, Sacramento Mountains, New Mexico

ABSTRACT Unlike previous researchers who concentrated on carbonate strata in the Sacramento Mountains, New Mexico, we conducted facies analysis on sandstone units within the Upper Pennsylvanian Holder Formation (Virgilian). Detailed work on several exposures in Laborcita Canyon revealed that the sandstone bodies have flat bases and upward-coarsening grain size sequences and are composed of three main subfacies. The sandstone subfacies in ascending vertical order are 1) thin-bedded, fine-grained sandstone with parallel-laminae and hummocky stratification with graded tempestite sequences; 2) fine-grained sandstone similar to subfacies 1, but occurring within medium to thick-bedded, large-scale foresets up to 5 m high; and 3) medium-grained sandstone with multidirectional cross-stratification, ipples and abundant marine trace fossils. The sandstones were deposited in a storm-dominated, inner shelf environment, and the preserved depositional topography, lenticularity, distribution of sedimentary structures and paleocurrent relationships suggest that the Laborcita Canyon sandstones represent sand ridges. The sand ridges migrated and accreted seaward during storms that generated large waves (estimated wave heights = 1.25-6 m) and downwelling pressure gradient currents, which combined to transport sand efficiently offshore. Storm-flow paleocurrent indicators trend roughly normal to the regional paleoshoreline, suggesting that the ridges were deposited in a friction-dominated or transitional shelf dynamic zone rather than in a fully-developed geostrophic zone. Sand ridges on the U.S. Atlantic inner shelf are modern proc ss analogues to the Laborcita Canyon sandstones because of the similarities in scale, internal bounding surfaces, grain-size sequences and storm processes that control deposition.

The late Permian palaeogeography of north-east England

SUMMARY All available information has been used in the preparation of a series of palaeogeographical maps showing the evolution of north-east England at the end of the early Permian and at intervals throughout the late Permian. Most of the area initially lay near the western fringe of the Southern North Sea tropical desert basin, much of which was flooded at the beginning of the late Permian so as to create the epicontinental Zechstein Sea. With continually oscillating relative sea levels and episodic differential subsidence, contemporary coastlines fluctuated widely whilst successive progradational carbonate shelves were constructed in the first two of four main cyclic sequences; the deposition of thick salt at the end of the second main cycle almost filled the basin, eliminating most of the earlier depositional topography and ensuring that carbonates and evaporites of the third and fourth cycles were formed in a less diverse range of coastal plain to shallow marine environments.

Three distinctive feeder‐lobe systems within one time slice of the Tortonian Tabernas fan, SE Spain

ABSTRACTExcellent exposures and the presence of lithological markers make it possible to reconstruct the fan facies pattern for a narrow (6000–60 000 yr), late Tortonian (7–8 Ma) time slice of a submarine fan complex that developed in the Tabernas basin. Unlike most classical fan models, which seem to refer to a single feeder‐lobe system, the reconstruction from the Tabernas basin reveals at least three distinctive juxtaposed feeder‐lobe systems: (I) a straight feeder valley terminating in a sandstone body consisting of stacked sand‐filled scours (sand‐rich system); (II) a straight feeder channel diverging in a broad front of nested scours in mudstones. These scours terminate in fine grained sheet‐like turbidite deposits (muddy system); (III) a sinuous channel complex extending far into the basin without ‘lobe’ deposits at its mouth (solitary system).Sedimentary features indicate that in the Tabernas basin, initial basin floor morphology and mass‐flow transport behaviour controlled and eventually stopped formation of the sand‐rich system (I), while depositional topography and slope instability controlled formation of the muddy system (II). The unusual narrowness (10–30 m) and length (8 km) of the solitary system (III) points to its confinement, possibly a result of an intrabasinal fault escarpment trending obliquely to the general direction of slope. Flow stripping (Piper & Normark, 1984) could then explain the absence of a lobe at the channel terminus.

Temporal and Spatial Variability of Ooid Sand Shoals: Comparison of Mississippian of Kentucky and Quaternary of Bahamas: ABSTRACT

An examination of the lithology and topography of Andros Island, Bahamas, reveals it is a Pleistocene ooid sand shoal. A comparison with Joulters Cays (a modern ooid sand shoal directly to the north) shows that much of the original depositional topography is preserved through at least one cycle of sea level highstand and lowstand. Both the Pleistocene and the Holocene ooid sand bodies are a few kilometers to tens of kilometers wide. The total vertical relief of a single episode of Quaternary ooid sand deposition is more than 10 m and includes accumulation in tidal channels, shallow flat areas, and eolian dunes. Today, much of Andros Island is within 2 m of present sea level and is the site of a belt several kilometers wide consisting of muddy tidal flat sediments overlying an exposure surface. The site of ooid sand deposition and shoal complex formation is not continuous along shorelines, especially windward margins, but shifts abruptly along the margins of platforms as a result of minor fluctuations of sea level. Thus, it should be expected that ooid sand shoals (ancient and modern) should be in direct lateral and vertical contact with lagoons, tidal flats, and reefs. The Mississippian Slade Formationmore » contains many of the features of Quaternary ooid sand accumulation: abrupt vertical and lateral gradations between oolitic grainstones, packstones, and lime mudstones, vertical relief of individual oolitic sedimentary packages up to 30 m (perhaps with eolian dunes) and numerous exposure surfaces of varying intensities. These characteristics suggest that this formation represents a time of rapid fluctuations of relative sea level and abrupt shifts in the sites of ooid sand shoal complexes.« less

Significance of Loessite in the Maroon Formation (Middle Pennsylvanian to Lower Permian), Eagle Basin, Northwest Colorado

ABSTRACT Quaternary loess deposits are widespread on the earth's surface, yet pre-Quaternary loess deposits have rarely been reported. The Maroon Formation (Middle Pennsylvanian to Lower Permian) of the Eagle Basin, northwest Colorado, includes a siltstone-dominated facies interpreted as loessite (lithified loess) along its downwind basin margin. The section of inferred loessite in the Maroon Formation is locally at least 490 m thick and consists in large part of structureless and nearly structureless beds of homogeneous sandy siltstone. Bed contacts are generally planar to undulatory and are either horizontal or are characterized by gentle relief. Loessite beds are separated by common claystone drapes and weakly developed paleosols, and by rare pond deposits, channel deposits, and eolian-ripp e-laminated deposits. The loess interpretation is based on 1) the homogeneity and dominance of the sandy silt grain-size; 2) the relative lack of primary sedimentary structures; 3) the gentle character of most bedding contacts and the common mantling of irregular depositional topography; 4) the inferred paleogeographic setting; and 5) the absence of suitable alternative interpretations. The loessite grades laterally into mixed fluvial-eolian deposits of the Maroon Formation in the main part of Eagle Basin, which served as the loessite sediment source. Deposition of the Maroon Formation was probably strongly affected by cyclic climatic changes synchronous with fluctuations in late Paleozoic continental ice sheets. The paleogeography and paleoclimatology of the Maroon Formation depositional system are not unique, suggesting that there are probably many other ancient loessites that have gone unrecognized.

Genetic Sequence Relationships of Winnipegosis Platform Carbonates, Southern Elk Point Basin, North Dakota: ABSTRACT

Examination of cores and well log data from the Winnipegosis Formation (Givetian) within a study area of approximately 11,500 mi/sup 2/ (30,000 km/sup 2/) in northern North Dakota allows recognition of seven time-stratigraphic progradational units within the Winnipegosis Formation. Together with the underlying Ashern Formation, these units are arranged in landward-stepping, vertical stacking, and seaward-stepping geometric patterns, which reflect changes in relative sea level. Abrupt juxtaposition of shallow over deeper water lithologies, evidence for subaerial exposure, and onlap geometries further suggest that these progradational units form two larger, Vail-type sequences separated by regionally persistent unconformities or their correlative conformities. Sea level rise during the early Eifelian caused southeastward onlap of the Ashern Formation onto Middle Silurian carbonates of the Interlake Formation. Maximum flooding, expressed by deepest marine facies and a hardground surface, suggests the existence of a condensed section at the top of the Ashern Formation. This was developed during the maximum rate of sea level rise. A decrease in the rate of sea level rise resulted in aggradation of lower Winnipegosis units on a gently dipping ramp. These are represented by nodular and burrowed open marine limestones with scattered stromatoporoid patch reefs and grainstone shoals. During the subsequent seamore » level fall, represented by Temple units, a shelf margin with pronounced depositional topography and adjacent starved basin were developed. Temple strata include coral-brachiopod-stromatoporoid reefs and productive fore-reef talus deposits along the shelf margin rim.« less

Sedimentologic and geomorphic variations in storm-generated alluvial fans, Howgill Fells, northwest England

In June 1982, a storm with a return period greater than 100 yr, but lasting less than 2.5 hr, destabilized hillslopes and produced a suite of geomorphologically and sedimentologically diverse alluvial fans. Thirteen major fans were deposited at the tributary junctions between small ( 2 ) catchments and two north-flowing; headwater streams of the River Lune, northwest England. Storm-generated fans spread over or became inset into older stable fans and produced localized vertical accretion of as much as 3 m and lateral accretion of as much as 100 m. Sedimentary processes operating during deposition involved debris flow, transitional flow, and streamflow. Six facies types are recognized on the basis of depositional topography, sedimentary texture and fabric, and matrix content: viscous debris flow (D1), dilute debris flow (D2), transitional flow (T1), fluvial bars and lobes (S1, S2), and fluvial sheet gravels (S3). Regionally, streamflow deposition prevails over debris-flow deposition, and type S3 facies has the greatest areal extent. Temporal and spatial variations in facies deposition during the storm, however, resulted from water:sediment ratio variations. Fan deposition involved an early phase of debris flow to transitional flow due to large inputs of sediment from hillslope failures. This was followed by a systematic change to more dilute conditions, resulting in streamflow deposition and eventually in channel incision. A significant amount of geomorphic work and complex variations in sedimentary processes during the storm resulted, in part, from extensive overland flow and hillslope destabilization. Discriminant analyses indicate that catchment size, channel gradient, and percentage of area eroded during the storm controlled whether debris-flow or streamflow facies dominated a fan sequence. Smaller, steeper catchments had a greater percentage of the area yielding sediment and are dominanted by debris flows, whereas larger catchments produced more runoff resulting in dilution and streamflow. The results indicate that the facies sequences and fan entrenchment in the Howgill Fells, which are typically considered products of longer term climatic change or tectonics in other localities, are here primarily affected by thresholds related to catchment geomorphology, by the type of sediment available, and by the position within the storm cell.

Productive Lower Wilcox Distributary Channel Sands of Hallettsville Embayment, Lavaca County, Texas: ABSTRACT

Productive Lower Wilcox Distributary Channel Sands of Hallettsville Embayment, Lavaca County, Texas: ABSTRACT

Mixed Siliciclastic and Carbonate Sedimentation Within Spar Mountain Member of Ste. Genevieve Limestone, Hamilton County, Illinois: ABSTRACT

The Spar Mountain Member of the Ste. Genevieve Limestone (middle Mississippian) in Hamilton County, Illinois, consists of 40-60 ft (12-18 m) of interbedded limestones, shales, and sandstones. Five cores and 1,400 electric logs were used to delineate two shallowing-upward carbonate cycles and 2 major clastic pulses within the Spar Mountain. Eight lithofacies representing 6 depositional environments were identified. They are: (A) echinoderm-brachiopod dolomicstone to packstone (outer ramp), (B) ooid-peloidal grainstone (intermediate ramp), (C) skeletal grainstone (intermediate ramp), (D) ooid-molluscan-intraclastic wackestone to grainstone (inner ramp), (E) pelletal-skeletal wackestone (inner ramp), (F) quartzarenite (channelized nearshore), (G) quartz-sublithic arenite to acke (delta platform), and (H) quartz mudstone (prodelta, delta platform). Deposition occurred on a southwest-dipping carbonate ramp, with siliciclastic sediments originating from the northeast. The sequence of facies and their inferred depositional environments record 2 major progradational episodes. Oolitic facies are interpreted to be of tidal-bar belt origin and quartzarenite facies are interpreted to be of delta-distributary channel origin. Their distribution is partially controlled by antecedent and syndepositional topography. Many of these paleotopographic highs are positive features today and yield pinch-out stratigraphic relationships. Paleogeographic reconstructions demonstrate that the primary control on facies distribution was the position of the delta proper along strike. However, depositional topography also influenced sedimentation, particular y in the sand-sized fraction. Using this concept, better prediction of underlying porous buildups (ooid shoals) is possible if thickness of the overlying siliciclastic is known. Within buildups, a complex diagenetic history complicates the distribution of porosity. End_of_Article - Last_Page 301------------

Stratigraphy of Blair Formation, an Upper Cretaceous Slope and Basin Deposit, Eastern Flank of the Rock Springs Uplift, Wyoming: ABSTRACT

The Blair Formation (Upper Cretaceous) is the lowermost unit of the Mesaverde Group in southwestern Wyoming. Outcrop study of the Blair exposures on the east flank of the Rock Springs uplift reveals 1,100 ft (330 m) of sandstone, siltstone, and shale. The formation has a sharp conformable to locally erosional basal contact and contains intraformational channeling, syndepositional slumping, and high-energy sedimentary structures. Facies relationships indicate the Blair represents a slope and basinal deposit laterally equivalent to the shelf and delta complex of the lower Rock Springs Formation to the north. Overall stratification types, textures, and southeast paleotransport directions recorded within the Blair, which are normal to the southwest-trending Rock Springs shore ine, support this interpretation. High sedimentation rates in excess of subsidence rates during the early Campanian, possibly related to early movement on the Absaroka thrust and a eustatic lowering of sea level approximately 81 Ma, caused rapid shoreline progradation and favored the development of a narrow shelf. These conditions enabled sand-sized material to bypass the shelf and be deposited in slope and basin environments. A present-day example of these relationships is the modern Mississippi delta located near the shelf-slope break of the Gulf of Mexico. Recognition of a narrow shelf in southwestern Wyoming during the early Campanian requires a modification of Late Cretaceous paleogeography to incorporate the concept of depositional topography. The occurrence of slope and basin sandstones in the Blair suggests that new interpretations may be needed to explain sandstone distribution for other stratigraphic intervals within the Cretaceous of the Western Interior. End_of_Article - Last_Page 866------------

Depositional Models, Intrabasin Tectonics, and Sea Level Changes in Petroleum Exploration: ABSTRACT

Recurrent movement on Precambrian-age basement-fault systems has influenced the origin, thickness, and distribution of Phanerozoic strata in the interior of the North American continent. In this structural setting, tectonic and sedimentation patterns are well known for major orogenies of the late Paleozoic, late Mesozoic-early Cenozoic (Laramide orogeny), and post-Oligocene. The influence of basement-fault movement on sedimentation and erosion during anorogenic times is generally not as well known. Subtle structural movement controlled topography in the depositional basins, which in turn influenced the distribution of high energy (reservoir) and low energy (nonreservoir) deposits. In addition, extensional fractures in strata overlying basement faults provided pathways for petroleum migration, either vertically or horizontally. Inversions of structural movement along fault zones are commonly observed, and they sometimes mask the recognition of exact time of sporadic fault movement. The most common petroleum reservoirs are deposited during rising or high sea level stands and are fluvial-deltaic, estuarine, shoreline, shelf, and lacustrine sandstones, and shallow marine and tidal-dominated carbonates. Depositional topography, which formed within the depositional basin during highstands, controls the distribution of unconformities during lowstands. Associated lowstand reservoirs are either base of slope or shelf sandstones, or secondary porosity in carbonates associated with paleokarst or dolomitization. Porosity in sandstones deposited during highstands may be infilled with diagenetic clays because of subaerial exposure during lowstands. Thus, recognition of the lowstand unconformity is of vital importance in exploration. All rock systems record sea level changes, but those most important to petroleum exploration in western cratonic basins are in the Ordovician, Devonian, Permian-Pennsylvanian, Jurassic, Cretaceous, and Tertiary. A major problem is to determine whether structural topography, or depositional topography and eustatic changes, exerted the primary control on sedimentation. Petroleum occurrences in well-documented sequences related to sea level changes are discussed for the Rocky Mountain and Mid-Continent regions. End_of_Article - Last_Page 1838------------

Alluvial Model for Eocene Wasatch Formation Coal, Powder River Basin, Wyoming: ABSTRACT

The Eocene Wasatch Formation in the Powder River basin, Wyoming, consists of a conglomerate facies (Kingsbury Conglomerate Member) on the western margin of the basin and a coal-bearing facies near the center of the basin. The conglomeratic facies consists of abundant, basally scoured, pebble to boulder conglomerates and sandstones, and minor rooted siltstones. The conglomerates contain abundant sedimentary and subordinate crystalline rock fragments derived from the adjoining Bighorn uplift. The coal-bearing facies comprises dominant coarse to conglomeratic sandstones and rooted siltstones and claystones. Minor constituents are fossiliferous limestones, carbonaceous shales, and coals. A thick, widespread coal bed (Felix coal) ranges from 10 to 28 ft (3 to 8.5 m) thick with n a 400 mi2 (1,035 km2) area and splits outward from this area into several beds. Where the coal is thick, it is underlain by sandstones and the coal splits are underlain by finer grained deposits. The conglomeratic facies represents wet alluvial-fan deposits consisting of graded gravel bars, channel sands, and finer overbank detritus. These sediments grade eastward into the coal-bearing facies that represents deposits of meandering streams and their adjoining flood plain and backswamp. The locations of the thickest, most widespread coal body and its splits in this facies are governed by depositional topography controlled by differential compaction of the substrate. Where the substrate is poorly compactible channel sandstones, the swamp surface was relatively high and free of sediment influx. Where the underlying deposits are fine grained and more compactible, the resulting low-lying swamp attracted water-borne sediments that interrupted peat accumulation. End_of_Article - Last_Page 953------------

Paleotectonic Controls on Carbonate Reservoir Development in Central Williston Basin: ABSTRACT

The Williston basin has been portrayed as the archetype intracratonic basin, wherein tectonics have played only a minor role in geologic history. This may be demonstrated in the subtle manner in which tectonics have controlled the locus and development of most of the producing reservoirs in the Paleozoic sedimentary sequence. The pattern of the initial sedimentary sequence covering the crystalline basement is that of a westerly to southwesterly thickening wedge. Limited penetration of the Deadwood sequence has restricted full understanding of its hydrocarbon potential. However, it forms the foundation for overlying Upper Ordovician Red River production focussed along the North Dakota-Montana border. The structural features on which most Red River production occurs show near classic effects of active structural development during deposition of the reservoir beds. Peritidal facies represent minute variations in depositional topography and, therefore, respond to the smallest increments of uplift. This causes sharp variations in distribution of reservoir and nonreservoir facies within a given structural featur . Major unconformities mark tectonic episodes at the close of the Silurian and near the close of the Devonian. Each of these events is marked by uplift and leveling around the periphery of the basin and significant shifts in depositional patterns within the basin. Late Ordovician through Silurian sedimentation is reflected in basin-centered patterns of thickening. Facies patterns, including the distribution of reservoirs and seals, reflect this. The Devonian onlaps across an unconformable surface and has formed a wedge with significant northward thickening. Facies patterns of both Middle and Upper Devonian units reflect a tectonic and sedimentary connection into the Alberta basin. Mississippian seas flooded across the late Devonian unconformity and deposited thick sequences of carbonates and evaporites again in a predominantly basin-centered pattern. Rapid subsidence and rapid sedimentation caused a filling of the basin, and a strong regressive pattern of facies developed. Peritidal sedimentary facies dominate in development of reservoir and seal. Traps are thus influenced by very subtle tectonic-topographic variations throughout the basin. Late Paleozoic to early Mesozoic uplift, tilting, and erosion, followed by deposition of early Mesozoic seals has had a profound effect on the distribution of hydrocarbons in the previously developed traps. A more complete understanding of this final episode and its relation to source rock maturation and migration holds the key to much of the future development of the basin. End_of_Article - Last_Page 1346------------

Brachiopod Community Paleoecology, Paleobiogeography, and Depositional Topography of Devonian Onondaga Limestone in Eastern North America: ABSTRACT

The lower Middle Devonian Onondaga Limestone was deposited in a northwest-southeast elongated topographic basin and on the surrounding carbonate platform. Two sedimentary cycles are present in the Onondaga. The Edgecliff represents a transgression which spread epeiric seas over much of eastern North America. During the Nedrow-Lucas regression, the interior of the platform became restricted resulting in the deposition of evaporites. The Moorehouse transgression continued through the deposition of the Tioga Bentonite, followed by pre-Speeds-Dundee regression from the craton. Onondaga brachiopod communities, arranged from nearshore to offshore, include the Atrypid-Megakozlowskiella, Atrypid-Levenea, Chonetid, Atlanticocoelia, Ambocoeliid, and Truncalosia communities. The Onondaga-age Eastern Americas Realm is divided into the Appohimchi province in the Appalachian basin and the Michigan basin-Hudson Bay Lowland province in the midwest. The provincial assignment of the James Bay region of Ontario is uncertain; the eastern townships of Quebec are near the boundaries both of the two provinces of the Eastern Americas Realm, and of the Eastern Americas Realm and the Old World Realm. End_of_Article - Last_Page 1170------------

Brachiopod community paleoecology, paleobiogeography, and depositional topography of the Devonian Onondaga Limestone and correlative strata in eastern North America

The Emsian? through early Eifelian Onondaga Limestone of the Appalachian Basin was deposited in a topographic basin and on the carbonate platform which surrounded the basin on the west, north, and northeast. Onondaga strata thin from the platform into the basin. Two sedimentary cycles are present in the sub-Tioga Onondaga of eastern North America. The Edgecliff-Amherstburg represents an interval of transgression, in which epeiric seas spread over much of eastern North America. During the Nedrow-Lucas regression, the interior of the carbonate platform became restricted, resulting in the deposition of evaporites. The Moorehouse-Anderdon transgression continued through the deposition of the Tioga Bentonite, followed by the pre-Speeds-Dundee regression from the craton. Early Eifelian Appalachian Basin Onondaga brachiopod communities, arranged from nearshore to offshore, include the Atrypid-Megakozlowskiella, Atrypid-Levenea, Chonetid, Atlanticocoelia, Ambocoeliid, and Truncalosia Communities. The Onondaga-age Eastern Americas Realm is divided into the Appohimchi Province in the Appalachian Basin and the Michigan Basin-Hudson Bay Lowland Province in the Midwest. The provincial assignment of the James Bay region of Ontario is uncertain; the Eastern Townships of Quebec are near the boundaries both of the two provinces of the Eastern Americas Realm, and of the Eastern Americas Realm and the Old World Realm, the latter realm being probably in the Canadian Maritime Provinces.