Shelf-slope Break Research Articles

Miocene Stratigraphy and Sand Distribution in East and West Cameron OCS Areas, Gulf of Mexico

ABSTRACT This study indicates that (1) prograding complexes comprise most of the lowstand systems tracts (LST); (2) sequences and systems tracts can be traced across the entire area with little change in thickness; (3) dramatic changes of lithofacies occur only when crossing large displacement faults; (4) shelf-slope breaks tend to coincide with growth faults; and (5) faults generate accommodation space for sands on down thrown sides. The study is an example for integrated sequence analysis in a tectonic setting with abundant growth faults and complex salt structures. This approach improves predictability of regional sand distribution in the area.

Large Illinoisan Shelf-Margin Channel in the South Timbalier and Ewing Bank Areas, Offshore Gulf of Mexico

During an Illinoisan lowstand, a major channel became entrenched on the shelf and deposited deltaic sediment of the shelf-slope break in the South Timbalier and Ewing Bank areas. This shelf-edge delta overloaded the margin, creating a major slope failure. This 'channel,' or slump canyon, which was originally deep and narrow, evolved through retrogradational wasting of the shelf into a broader feature during back-filling and abandonment. Mapping with integrated seismic and well data indicates a feature that is 6 mi wide and has 2300 ft of relief, from the base of the channel to the highest margin. Gravel lag fills the deepest portion of the scour and constitutes an excellent, though restricted, gas reservoir. The limits of the reservoir are defined by mapping of a seismic amplitude anomaly. The balance of the back-fill is comprised of thinly bedded silstone and sandstone sequences that generally thin and fine upward; each is capped by marine shales. The fill geometry is primarily nested or stacked channels. Positive topographic features on the outer shelf tower above the channel floor, either as uneroded remnants or as part of channel margin slump mounds. Sand-prone sediment onlapped these highs, thereby creating numerous stratigraphic traps. As sea level rose,more » the locus of deposition within the channel shifted progressively shoreward, and the fill was capped by a clean, transgressive shale. Further work is necessary to ascertain the location of the submarine fan far down-dip of this shelf-edge channel. Laterally extensive reservoirs can be expected in more distal depositional environments.« less

A Preliminary Study of Seismic Stratigraphy of the Late Cenozoic Sequences in the Tainan Basin off Southwestern Taiwan

This study investigates the late Cenozoic strata in the Tainan basin off southwestern Taiwan. The nature and development history of late Cenozoic sequences are discussed in terms of seismic stratigraphy. Multichannel seismic profiles more than 330 km long were acquired and processed, including 4 dip profiles and 3 strike lines. The quality of the seismic profiles range from fair to good. Seismic profiles show that late Cenozoic strata are relatively conformable stratigraphic successions from. which Pleistocene and Pliocene seismic sequences are defined. The seismic reflection configurations of the late Cenozoic sequences are characterized by parallel, subparallel and divergent patterns. The parallel pattern suggests that sediments accumulated at a relatively uniform rate on a shallow shelf within the study area. The divergent reflection pattern and the external wedge- shape of the sequences indicate lateral changes in depositional rates and progressive tilting of the shelf facies. Seismic facies analysis indicates that late Cenozoic sediments were deposited mainly in the fluvial-nearshore environments. The regional trends of the Pliocene and Pleistocene sequences represent relatively smooth parts of the shelf without the characteristic shelf-slope breaks. The shelf breaks in the Pliocene and Pleistocene sequences are probably situated farther seaward than today’s shelf edge. Velocity models indicate an interval velocity ranging from 1,800 m/s to 4,000 m/s suggesting that the late Cenozoic sequences are composed mainly of sands and shales. Vertical distribution of the interval velocity shows that the yelocity of the sequences increases with depth as a result of compaction (i.e. depth of burial). Analyses of seismic sequences, seismic facies and velocity suggest that the Pleistocene and Pliocene sequences developed into a clastic wedge. The sequences form parts of the South China continental margin which has undergone tilting and subsiding during the past ten or more million years.

Exploration and Development of the Offshore Gulf of Mexico

Technology has driven exploration and development of oil and gas resources in the offshore Gulf of Mexico. Since drilling barges first probed open waters south of Louisiana, exploration has been governed by the water depth capacity of drilling equipment. In early wildcatting, extension of exploration concepts developed for south Louisiana Miocene trends governed the direction of offshore operations. Explorationists were initially drawn to deeper water by gravimetrically identified salt structures. As shallow-water salt targets were exhausted, improved marine seismic techniques located Miocene fault traps between salt-related discoveries. In the late 1950s and 1960s, Pliocene exploration was also initially motivated by salt targets, which stimulated advances in the water depth capacity of mobile drilling rigs and platforms. While salt targets in the Pleistocene provided important oil discoveries, most discovered Pleistocene hydrocarbons are gas, the largest volume of which was found through the introduction of 'bright spot' technology at the end of the 1960s and very successfully applied in the 1970s. In the 1980s, further advances in drilling rigs and platforms provided access to water depths greater than 1,000 ft, extending Pleistocene exploration to the flexure trend corresponding to the modern shelf-slope break and to even deeper water targets on the slope.more » Major integrated oil and gas companies have dominated exploration and production in the offshore Gulf of Mexico. However, the composition of industry leadership has changed over the last 45 years as have the performances of companies within this group of active firms. Changes in exploratory and development success can be related to access to technology as well as exploration strategies.« less

Late Ordovician stratigraphy northwest of the Sugarloaf, mid‐Wales — a discussion

AbstractSeveral modifications are proposed to the Ordovician stratigraphy recently proposed by Mackie and Smallwood (1987), based in particular on the recognition of a major Mynydd Trawsnant anticline on the northwest flank of the Tywi structure and on field relationships indicating the Foel Formation directly to overlay the Caradoc black shale succession and directly to underlay the Craig Las Formation. The Clyn Glas Formation is suggested to have accumulated at a slope base rather than a shelf–slope break.

Balancing of Stratigraphic Cross Sections of the Delaware Basin and the New Jersey Continental Shelf to Determine Tectonic and Eustatic Controls: ABSTRACT

Balancing of stratigraphic cross sections from offshore New Jersey and the Delaware basin of New Mexico results in an improved understanding of the stratigraphic and structural elements that controlled their development. Cross section balancing is accomplished in three stages: interpretation of the sequence stratigraphy, backstripping, and simulation of the history of the section with a forward model. Interpretations of lithostratigraphic and biostratigraphic data from outcrops, seismic data, and well-log cross sections define the distribution in time and space of stratal surfaces, slope angles on depositional interfaces, and sediment flux. Backstripping coupled with biostratigraphic data defines the subsidence history in time and space. The simulation incorporates rules governing the behavior of sequences in response to the distribution of accommodation space. The simulation also incorporates rules such as deltaic sediments that prograde basinward filling the available space below a predefined depositional profile, and sediment bypassing that occurs when the coastline falls below the shelf slope break of the previous highstand and relative sea level is falling. The quantitative model constrains the interpretation by limiting process variables to values consistent with observed rates. The two-dimensional model assumes that progradation occurs in the plane of the cross section, and lithospheric flexure can be approximatedmore » by the convolution of point loads on an elastic plate. The stratal patterns in the two stratigraphic cross sections can be reproduced by third-order (1.0-3.0 Ma) changes of eustasy. The Delaware basin is characterized by 5-10 Ma flexure loading cycles that are driven by tectonically induced variations in subsidence and clastic sediment delivered to the depocenter. Offshore New Jersey is characterized by exponentially declining thermal subsidence punctuated by a period of volcanism and uplift.« less

High-energy, inner shelf carbonate facies along a tide-dominated non-rimmed margin, northwestern Australia

High-energy, inner shelf carbonates occur within 1 to 2 km of the mainland along the southern part of the Northwest Shelf (a non-rimmed carbonate shelf) along northwestern Australia. Coarse-grained sands and gravels are deposited on and adjacent to a chain of subtidal and intertidal rock platforms which lies oblique to regional NE-SW directed tidal currents. Intertidal and subtidal sand sheets, less than 50 cm thick, predominate along the margins and interior of the chain, and subtidal linear to sinuous sand bodies up to 10 m thick and 4 km in length form banner banks to the northeast of rock platforms and reefs. Other features include narrow fringing and patch coral reefs, calcareous red-algal reefs, and intertidal through subtidal coral shingle ridges. Fleshy algae are abundant on open, sand-covered and bare rock platforms as well as forming a mantle on relict coral reef frameworks. Supratidal through subtidal sediments are comprised of skeletal (molluscs, foraminifera, bryozoans, corals, calcareous red algae) and rock fragments, the latter derived from rock platforms; calcareous chlorophytes (e.g. Penicillus and Halimeda) are not important sediment contributors. In general, sediments are not representative of the present-day turbid and fleshy algal-dominated subtidal environment. The predominantly coarse-grained lithofacies (<3 wt.% mud) and configuration of sand bodies are controlled by (1) vigorous tidal currents (up to 170 cm s −1) established via unimpeded across-shelf transport of oceanic waters, in the absence of a shelf margin rim, and tide ranges of 2–4 m; (2) interaction of shelf waters with rock platforms and reefs along the inner shelf creating strong currents; (3) passing cyclones; and (4) sediment reworking within normal strong wind, tide and wave regimes. High-energy carbonate sand shoals along this tide-dominated inner shelf are far removed from the shelf-slope break, facies distribution being similar to nearshore facies of a distally steepened carbonate ramp.

Sedimentology of the Neogene to Modern glauconite-goethite-phosphate system : east australian continental margin between 29° and 32° south latitude. Le système sédimentaire glauconite-goethite-phosphate du Néogène à l'Actuel : la marge continentale orientale de l'Australie entre 29° et 32° de latitude sud

Outer shelf and upper slope environments (250 to 525 m) on the East Australian continental margin off northern New South Wales, represent an important iron-rich sediment province. The dominant (up to 30 9c) authigenic-diagenetic components are glauconite and associated goethite, with minor concentrations of carbonate fluorapatite (mean P₂O₅ content = 0.37 %) within a muddy foraminiferal fine sand. The outer shelf and shelf-slope break (250 to 425 m depth) contain a series of small scarp features interpreted to be outcrops of indurated subsurface units. Sedimentologic and biostratigraphic data from these features suggest that they are Miocene hardgrounds composed of ferruginous-phosphate pavements and gravels. Plank tonic foraminiferal assemblages in unconsolidated surface sediments overlying the hardgrounds range in age from late Pliocene through Holocene and are draped across the continental margin as a veneer covering older strata which only locally crop out. Relative concentrations of glauconite and goethite change downslope and downsection as follows. (1 ) Surface sediments at depths from 250 to 340 m are dominated by goethite with subordinant amounts of weathered glauconite grains. (2) Sediments underlying (1) and constituting the surface sediments from 340 to 465 m are dominated by glossy dark green glauconite with subordinant amounts of goethite. (3) Sediments underlying both of the above and constituting the surface sediments from water depths of 465 to 525 m are dominated by glossy dark green glauconite with subordinant concentrations of light green glauconite within foraminiferal tests. Glauconite forms within foraminiferal tests as light green grains and is subsequently altered to glossy dark green grains with accompanying dissolution of the tests. Above the shelf-slope break, dark green glauconite is altered to goethite within the surface sediment. Goethite occurs at the top of multiple depositional cycles in some sediments between 375 and 465 m depth, suggesting either periods of upper slope oxidation or off-shelf sediment transport during sea-level lowstands. Two types of phosphate occur within the sediments of outer shelf and upper slope. Type I (dark reddish-brown, ferruginous phosphate) is found in water depths from at least 200 to 425 m where it forms hardground pavements and associated nodules and lag gravels. Type I phosphates contain 7 9c to 12 9c P205, 10 % to 50 % Fe₂O₃, and are in radioactive equilibrium indicating uranium-series ages in excess of 800 ka (O'BRiENand Veeh, 1980 ; O'Brien et al., 1986). Type II (gray phosphate) occurs in water depths from 340 to 525 m as irregularly shaped nodules and internal molds within macrofossils in muddy sediments with unweathered light and dark green glauconite. Type II phosphates contain 4% to 19 9c P205, 2 9c to 4 9c Fe₂O₃, negligible goethite, and yield uranium-series ages that range from 250 ka to 2.5 ka (O'Brien and Veeh, 1980 ; O'Brien et ai, 1986).

Geologic Model of a Small, Intraslope Basin: Garden Banks 72 Field, Offshore Louisiana: ABSTRACT

Garden Banks 72 field is 115 mi off the Louisiana coast and lies near the shelf-slope break in water depths ranging from 450 to 800 ft. During the middle Pleistocene, the area was the site of a small, restricted basin on the upper slope, into which turbidite sandstones were deposited. These sandstones have been slumped, uplifted, and faulted, forming oil and gas traps in the field. Mobil and partners AGIP and Kerr-McGee leased block 72 in 1984. Three wells and two sidetracks have been drilled, discovering oil and gas in middle Pleistocene sandstones. A total of 650 ft of core was cut in two wells. Geologic data in the block have been supplemented by 2-D and 3-D seismic surveys. Trapping mechanisms in the field are both structural and stratigraphic. The structural high is on the southwest flank of a northwest-southeast-trending shale/salt ridge. The middle Pleistocene reservoir sandstones trend northeast, and their seismic signature consists of discontinuous, hummocky reflections; the presence of hydrocarbons in these sandstones causes anomalous seismic responses. Amplitude terminations often cross structural contours, implying stratigraphic pinch-outs. Data from electric logs, seismic, and cores demonstrate that the middle Pleistocene reservoir sandstones are the result of deposition by turbidites intomore » a small, restricted basin. Associated facies identified include channels, levees, and possible sheet (lobe) sandstones. Postdepositional activity has included slumping and reworking by bottom currents (contour currents ). The resulting depositional model for this field can be applied to many of the recent discoveries in the Flexure trend.« less

The sedimentology and development of Terravecchia Formation carbonates (Upper Miocene) of North Central Sicily: Possible eustatic influence on facies development

On at least two occasions during the Late Tortonian, siliciclastic deposition ceased and carbonates accumulated at the northern margin of the “Caltanissetta Basin” in north-central Sicily. Bioclastics dominated in the shallower proximal areas, while coral reef facies developed where shallow-water sand and cobble substrates existed or where sea-floor highs has created local shallows. The genetic model proposed is that siliciclastic deposition was repeatedly terminated during the Late Tortonian by rises in sea level which transgressed fluviatile and deltaic environments. Carbonate deposition was widespread, but, later erosion and reworking removed much of the sediments except within abandoned delta distributary channels and minor sea-floor depressions. Reef developments, associated with channel mouths and the shelf slope break, have been preserved in situ, whereas channels in more proximal locations are filled with siliciclastic, bioclastic and intraclastic rubble from elsewhere on the delta top. The wide lateral extent of the truncated tops of the carbonate beds argues for eustatic, rather than tectonic controls of erosion and subsequent reworking. These events may be closely related in time to other recorded Late Tortonian eustatic events in the foreland zones, such as the Hyblean Plateau and the Maltese Islands further south. Collectively they give some support to a model involving multiple eustatic fluctuations associated with the onset of the Messinian desiccation of the Mediterranean.

Aspects of Franklinian shelf, slope and trough evolution and stratigraphy in North Greenland

The Early Palaeozoic Franklinian basin of North Greenland was initiated in early Cambrian times with the onset of siliciclastic and mixed carbonate shelf deposition on a substratum of Proterozoic sediments and Precambrian crystalline basement. The basin rapidly differentiated into shelf, slope, and trough environments. The boundary between these major depositional regimes follows major roughly east-west trending lineaments. The inner-outer shelf transition fluetuated widely in position and was controlled by the interplay between sea-level changes and periodic northwards downwarping. In Ordovician-Silurian times the margin of the carbonate platform migrated northwards to coincide with the shelf-slope break and had a steep scarp configuration with an abrupt transition to the shales and turbidites of the deep-water basin. In this paper we pay special attention to the initial basin sequence (Skagen Group), the collapsed and broken platform carbonates of the Lower Cambrian Portfjeld Formation, the Middle - Late Cambrian inner-outer shelf transition and to the nature of the Silurian platform margin scarp.

Evolution of the Cape Fear Terrace: A complex interaction between the Gulf Stream and a paleo-shelf edge delta

A high-resolution seismic stratigraphic study of the Cape Fear Terrace (outer continental shelf off Cape Fear, North Carolina) combined with bio-lithostratigraphic data have yielded a complex chronostratigraphic framework of the Quaternary sequences that comprise this portion of the North American continental margin. The Cape Fear Terrace is one of six terraces/protuberances that form significant topographic irregularities along the South Carolina and North Carolina shelf-slope break. The Cape Fear Terrace is situated upon an anomalous, point-source, prograding, shelf margin feature which has protruded out onto the upper slope throughout much of the Quaternary. This upbuilding/outbuilding followed a period of active submarine erosion in which the ancestral Gulf Stream and/or associated meanders cut an erosional path beneath the present shelf margin during the early Pliocene. The protuberance was originally constructed during a relative lowstand of sea level as a shelf-edge delta. Severe modification of this deltaic lobe occurred during a relative highstand of sea level as Gulf Stream activities began to more effectively modify the margin. The anomalous thick accumulation of shelf-edge sediments acted as a barrier to flow inducing complex flow patterns of the Gulf Stream causing preferential erosion on its downstream side. Subsequent deposition may have resulted during more elevated highstands of sea level, as evidenced by the presence of active sand waves in the terrace protuberance region today. Once this shelf-edge bathymetric irregularity had been established, the Gulf Stream acted as a dynamic force inducing cellular flow structures within the shelf environment and enabling sediments to be transported seaward along the paleo-shoals complex.

The benthic amphipod fauna of the west-norwegian continental shelf compared with the fauna of five adjacent fjords

Abstract Twentytwo epibenthic sled samples from the shelf and eleven from five fjords were investigated. The faunal patterns were analysed by means of hierarchical clustering, ordination, and ‘Pseudo F-test’. Three major habitats were distinguished, each characterized by a set of amphipod species: Deep localities on the shelf (247–520 m), shallow localities on the shelf (147–272 m), and fjord localities. Multiple stepwise regression analysis indicated that the distribution pattern of the amphipods is correlated with the ‘bathymetric isolation’ of the localities and sediment factors. Shannon-Wiener diversity index (H') and rarefaction analysis were used to investigate diversity patterns. Shelf sites showed higher diversity than fjord sites. Diversity (H') is significantly correlated with bathymetric isolation, distance from shelf-slope break, depth, and mean grain size in phi-scale. Three hypotheses are suggested as explanations to the lower diversity in the fjords than on the shelf.

Frontier seismic geologic techniques and the exploration of the Miocene reefs in offshore Palawan, Philippines

An 88 km grid of Seislog sections (synthetic sonic log traces) was used to supplement the conventional seismic interpretation to assess the hydrocarbon potential of Lower Miocene reefs of offshore Palawan, Philippines. The porosity distribution in the zone of interest as well as the oil-water contact in and around the Tara and Libro patch reefs were mapped. The structural mapping was also fine tuned, including mapping of faults with small displacements. The paradox of pooling of hydrocarbons is evident in the maximum fill in the front line of traps near the shelf-slope break that are in the lowest structural position. Progressively dwindling amounts of fill in the structurally higher updip basin positions are interpreted to be due to the migration of hydrocarbons from the hydrocarbon kitchen in the Palawan Trough. The front line of traps had the benefit of maximum fill, the traps at the rear only of residual fill that spilled past the frontal traps. Three play types corresponding to the front line of traps, the rear line of traps and the most remote line of traps have been identified. Frontal reefs are considered to have the highest hydrocarbon potential. Diminishing hydrocarbon potential is inferred for closures in the updip direction. A thorough screening of all available seismic control for mapping all the patch reefs that might compensate for their dimensions by their numbers as well as for detecting possible pinnacle reefs in the downdip basin position has been suggested.

Ramp-Style Deposition of Oligocene Marine Vedder Formation, San Joaquin Valley, California: ABSTRACT

The Oligocene Vedder formation consists of well-sorted medium to fine-grained marine sand and shale in the subsurface of the eastern San Joaquin Valley. Updip, this formation interfingers with nonmarine/lagoonal facies known as the Walker Formation. This relationship appears to be transgressive because the marine Vedder generally overlies the Walker Formation. Downdip, the Vedder sands interfinger with middle to lower bathyal shale in a progradational manner, forming upward-coarsening patterns in well logs. Depositional water depths for the shale were determined from benthic foraminifera assemblages. The Vedder formation is approximately 750 ft thick along its updip part, and gradually thickens to 1500 ft downdip. Overall deposition geometry, determined from well-log correlations and seismic data, is generally parallel and downlapping. A prominent shelf-slope break is not evident. Rather, depositional surfaces are tabular or broadly lobate, with a depositional slope of 5/sup 0/-10/sup 0/. This geometry of constant slope between nonmarine and deep marine water depth is termed a ramp. The depositional style and geometry are similar to that of the Oligocene upper Pleito Formation, which crops out in the San Emigdio Mountains on the southern margin of the San Joaquin Valley. The Vedder formation was deposited subsequent to a period of rapid subsidencemore » (about 50 cm/1000 years), as determined from geohistory analysis of well data on the Bakersfield arch. This rapid subsidence may have induced deposition in a ramp geometry, rather than a shelf-slope configuration.« less

Sequence Architecture and Lithofacies Assemblages of Submarine Fan Deposits in Los Molles Formation (Jurassic), Neuquen Basin, Argentina: ABSTRACT

The Neuquen basin is a remnant of the Mesozoic back-arc basin trend that developed along the western margin of South America. It contains a thick, diverse sequence of Jurassic sedimentary strata, whose facies distribution was strongly influenced by syndepositional tectonism. The predominantly dark, laminated shales and siltstones of the Los Molles Formation range from Pliensbachian to Callovian in age and record the progradation of the outer shelf, slope, and basin-plain sediments deposited during the shoaling phase of the lowermost, or Cuyan, Jurassic cycle. Based on outcrop, well, and seismic data, several thick packages of sandstone and conglomerate within the Los Molles are interpreted to represent submarine fan deposits that developed during periods of relative sea level lowstand. Sea level falls were probably related to local synchronous tectonic pulses rather than true eustatic fluctuations. The distribution of coarse-grained fan deposits was apparently strongly controlled by the location of major Jurassic fault trends that stabilized the position of the shelf-slope break through time. Based on the geometry and sequence architecture, two distinct styles of fan development are recognized in the Los Molles sandstones. The most common style (type A) is characterized by sequences that have poorly defined, sand-poor lobes with well-developed channel-leveemore » complexes. Some channels exhibit large-scale accretion surfaces, probably resulting from lateral migration. Thick-bedded arenite facies are limited to amalgamated channel fills, whereas thin-bedded classical turbidites are present as overbank deposits. Type A fans were built by turbidity and fluxoturbidity currents from submarine canyon point sources. The less common fan sequences (type B) lack channeling; they are dominated by thick, massive beds of internally featureless sandstone that are bounded by chaotic slump deposits.« less

Evolution of the Ordovician through Middle Pennsylvanian carbonate shelf in east-central California

Carbonate shelf rocks marked by various sedimentary features suggesting intertidal to supratidal deposition or bearing rich, benthic faunas and their corresponding slope and basin rock sequences containing planktic fossils and sediment-gravity-flow deposits have been identified in the Inyo Mountains–Argus Range region of east-central California for all of the Paleozoic from Ordovician through Middle Pennsylvanian. Four major depositional sequences, each beginning with a marine transgression and all but the third ending with a progradation, were developed representing (1) Early Ordovician into Early Silurian time, (2) Early Silurian into earliest Middle Devonian time, (3) Middle to Late Devonian time and (4) Early to middle Late Mississippian time. A period of basinal infilling in the Late Mississippian was followed by a fifth major marine transgression in the Early Pennsylvanian. The carbonate shelf itself evolved from a homoclinal ramp in the Ordovician to an accretionary rimmed margin by the Early Devonian. This margin was drowned, probably in the Middle Devonian; by the Early Mississippian, a new homoclinal ramp had been developed. Before the end of the Mississippian, this ramp became distally steepened and then was transformed into an accretionary rimmed margin. The Early to Middle Pennsylvanian marine transgression gave rise to still another homoclinal carbonate ramp. Subsidence of ∼2,300 m occurred between Early Ordovician and Middle Pennsylvanian time in the area immediately east of the Inyo Mountains. It increased westward to probably >3,500 m at the edge of the Owens Valley as the continental margin subsided and bent downward ∼1° between Early Ordovician and middle Late Mississippian time. As a result of subsidence and relatively low rates of sedimentation, the water depth in the western part of the study area increased from close to sea level at the beginning of the Ordovician to as much as 3,000 m in the Late Mississippian. Because of higher rates of sediment accumulation and lower rates of subsidence in the eastern part of the region, shallow-water environments were maintained throughout most of this time interval; even so, the shelf-slope break retreated at least 45 km southeastward from Middle Ordovician to middle Late Mississippian time and at least 35 km farther southeastward by Middle Pennsylvanian time.

Seismic Stratigraphy and Cenozoic Evolution of West Sumatra Forearc Basin

Late Tertiary (Neogene) sediments deposited in the Sunda forearc reveal the stratigraphic and structural evolution of this active plate margin. A dense grid of multichannel seismic reflection data and information from 17 exploratory wells were used to establish a detailed seismic-stratigraphic framework of the forearc region from 0° to 6°N. Several important tectonic cycles are recognized: Paleogene orogeny, Neogene subsidence, and late Tertiary tectonism. Superimposed on these regional tectonic events are three major transgressive-regressive cycles of sedimentation related to changes in sea level and provenance. Paleogene and older metasedimentary and metamorphic rocks comprise basement beneath the landward (inner) margin of the forearc basin. Both basement rocks and lower Tertiary sedimentary rocks were deformed and eroded approximately 25 to 30 Ma. The continental shelf was exposed to subaerial erosion, and basin deposits were restricted offshore, coincident with a worldwide lowstand of sea level in the Oligocene. The Paleogene orogeny probably occurred prior to the erosional event that cut the prominent angular unconformity on the shelf. The Neogene history of forearc basin development is characterized by subsidence and nearly continuous sedimentation. A basal transgression began in latest Oligocene time and culminated in the middle Miocene. Alternating sequences of limestone and shale comprise two second-order cycles of sedimentation that are superimposed on an overall transgressive event. A major regressive sequence followed in the Pliocene, owing to an influx of siliciclastic clay, silt, and sand derived from Sumatra. Sedimentation rates were high, and large volumes of terrigenous material were deposited in deltaic systems on the shelf. The shelf-slope break prograded basinward nearly 10 km (6 mi) through lateral accretion and aggradation during a relative highstand or stillstand of sea level.

Submarine Ramp Facies Model for Delta-Fed, Sand-Rich Turbidite Systems

Some sandy turbidite successions contain facies that differ in significant ways from those predicted by the canyon-fed submarine fan depositional model. The key differences are the absence of a master slope channel or canyon through which sediment is transported to the basin, and the lack of facies segregation into distinct channel and overbank or interchannel facies associations within the turbidite sequence. These types of sequences can be better described using a delta-fed submarine ramp depositional model. The primary components of this model are: (1) a sandy deltaic system that has prograded to the shelf-slope break; (2) an abbreviated section of mud-rich slope deposits traversed by multiple shallow channels that transport sand from the delta front to the deeper basi ; (3) very sandy proximal ramp deposits composed dominantly of laterally continuous sheets of Facies B turbidites; and (4) less sandy distal ramp deposits characterized by an increase in the abundance of Facies C and D turbidites. Ramp turbidites characteristically display statistically random patterns of bed thickness. Submarine ramp development requires rapid sediment accumulation (> 800 ft or 250 m/m.y.) in turbidite basins of shallow to moderate depth where deltaic progradation is rapid enough to mask the structural relief along basin margins. The delta-fed submarine ramp facies model may be useful in describing short-lived sandy depositional episodes in some rapidly aggrading and prograding basinal sequences. As such, they represent one member in a spectrum of submarine fan depositional styles.

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------------