Robert D. Winn

Stratigraphy and Rifting History of the Mesozoic-Cenozoic Anza Rift, Kenya

Lithological and compositional relationships, thicknesses, and palynological data from drilling cuttings from five wells in the Anza rift, Kenya, indicate active rifting during the Late Cretaceous and Eocene-Oligocene. The earlier rifting possibly started in the Santonian-Coniacian, primarily occurred in the Campanian, and probably extended into the Maastrichtian. Anza rift sedimentation was in lacustrine, lacustrine-deltaic, fluvial, and flood-basin environments. Inferred synrift intervals in wells are shalier, thicker, more compositionally immature, and more poorly sorted than Lower Cretaceous(?)-lower Upper Cretaceous and upper Oligocene(?)-Miocene interrift deposits. Synrift sandstone is mostly feldspathic or arkosic wacke. Sandstone deposited in the Anza basin during nonrift periods is mostly quartz arenite, and is coarser and has a high proportion of probable fluvial deposits relative to other facies. Volcanic debris is absent in sedimentary strata older than Pliocene-Holocene, although small Cretaceous intrusions are present in the basin. Crtaceous sandstone is cemented in places by laumontite, possibly recording Campanian extension. Early Cretaceous history of the Anza basin is poorly known because of the limited strata sampled; Jurassic units were not reached. Cretaceous rifting in the Anza basin was synchronous with rifting in Sudan and with the breakup and separation of South America and Africa; these events likely were related. Eocene-Oligocene extension in the Anza basin reflects different stresses. The transition from active rifting to passive subsidence in the Anza basin at the end of the Neogene, in turn, records a reconfigured response of east African plates to stresses and is correlated with formation of the East Africa rift.

Shallow-Water and Sub-Storm-Base Deposition of Lewis Shale in Cretaceous Western Interior Seaway, South-Central Wyoming

Shallow-and deep-water (below storm base) clastics of the Lewis Shale were deposited in south-central Wyoming during a part of the Late Cretaceous (Maestrichtian). Core, log, and outcrop data indicate that the Lewis Shale consists of a thin, transgressive, marine shale overlain by a thicker, coarser, progradational interval. The initial transgression of the Lewis sea occurred during the Maestrichtian (Baculites eliasi range), and the maximum westward and northward extent of submergence was in areas of the Rock Springs and Wind River uplifts. The sea in which the Lewis Shale was deposited opened eastward into the North American Western Interior seaway. Shale in the lowermost Lewis is black, carbonaceous, bioturbated, and commonly contains shell debris and was deposited in hallow water. The central basin floor changed from shallow to sub-storm-wave and sub-storm-current depths, seemingly in response to eustatic sea level rise and coeval tectonic subsidence. Deepening of the basin began at the time of dominance of Baculites eliasi to B. grandis (Maestrichtian). Sandstone and shale of the upper Lewis Shale are mostly related to distributary systems that entered the basin first from the northeast (during B. grandis to B. clinolobatus range) and later possibly from the south (during B. clinolobatus). Southwest progradation from a northeastern delta suggests significant uplift in the Wind River upthrust area concurrent with Lewis sedimentation. Many hydrocarbon-producing sandstones in the Lewis Shale are thickly bedded and massive, and some show fluid-escape structures. Thin sandstones are typically characterized by grading, Bouma sequences, and sole markings. Turbidity currents are believed to be responsible for deposition of all these sandstones. Unburrowed shale and shale containing burrows made by a restricted infauna are interbedded with reservoir sandstones at Wamsutter field. The shale is interpreted to indicate anaerobic to dysaerobic conditions and water depths of 500-650 ft (150-200 m) or more.

Latest Quaternary deposition on the outer shelf, northern Gulf of Mexico: Facies and sequence stratigraphy from Main Pass Block 303 shallow core

Sedimentologic, biostratigraphic, and isotopic geochemical data from a nearly continuous 91.4 m core and high-resolution seismic-reflection data from the middle to outer shelf east of the Mississippi River delta document deposition during changes in latest Quaternary sea levels. The data suggest that the shelf edge of the northern Gulf of Mexico is constructed from deltas deposited during falling sea level and lowstands, from sediment deposited in valleys during rising sea level, and from highstand clay. The Main Pass Block 303 core records sedimentation from the late middle Pleistocene (before ca. 135 ka) to the present. A lower delta-front interval was sampled at the core base (78.5–91.4 m). Delta sediment is truncated by a sequence boundary probably eroded during maximum glaciation corresponding to oxygen isotope stage 6 (late Illinoian?) and then modified during the following transgression. Above the sequence boundary is a transgressive shelf sand (77.2–78.5 m), overlain by burrowed, hemipelagic clay (∼63.4–77.2 m). The sand and clay were deposited during postglacial sea-level rise and a highstand. The rise and highstand correspond to oxygen isotope stage 5 and Ericson zone X and part of Ericson zone Y Sangamonian-“Eowisconsinan”). The clay interval from ∼51.8 to 63.4 m depth, in turn, was likely deposited during oxygen isotope stages 3 and 4 during the early and middle Wisconsinan. Overlying, with a transitional contact, is a relatively thick mud and sand (16 to ∼51.8 m) deposited during the subsequent sea-level fall to the maximum late Wisconsinan lowstand (oxygen isotope stage 2). The mud and sand interval correspond to steeply dipping clinoforms on seismic-reflection records. An interpreted sequence boundary, formed during fluvial incision of the shelf during the maximum late Wisconsinan lowstand, separates deltaic sediment from overlying fluvial, bay, and marsh deposits, which fill an incised valley. Thin, regressive delta and shelf facies overlie the valley fill. Sediment above the sequence boundary (above 16 m) was deposited during the rise in sea level from the maximum late Wisconsinan lowstand to the present. Late Wisconsinan deltaic and overlying incised-valley-fill sediment was derived from an ancestral river system that drained the southeastern United States.

Storm deposition in marine sand sheets; Wall Creek Member, Frontier Formation, Powder River basin, Wyoming

ABSTRACT The Turonian (Cretaceous) Wall Creek Member, Frontier Formation and equivalent Turner Sandy Member, Carlile Shale, Powder River Basin, Wyoming were deposited on the western margin of the North American Seaway. The Seaway occupied the foreland of the Sevier Orogenic belt. The Wall Creek-Turner interval is up to 67 m thick and consists of shale, sandstone, and minor conglomerate and bentonite. Deposition occurred during rising sea level following an approximately 90 Ma lowstand. Sandstone and mudstone of the Wall Creek were deposited in thin shelf sand sheets 6-15 m thick up to approximately 120 km from equivalent shorelines. Shelf sand sheets dip gently seaward ( Core and outcrop show shelf sandstone of the Wall Creek in the Powder River Basin to consist of burrowed to bioturbated, medium-scale cross beds, near-horizontal laminated beds, and ripple-laminated beds. Reactivation surfaces and mud drapes are common. The lack of consistent periodicity across cross sets in foreset angle and thickness and unperiodic spacing of reactivation surfaces, mud drapes, and burrowed intervals suggest that tides were not an important transport process over most of the depositional area. The dominance of storm currents over tides is also indicated by variable bed thickness, structure type, grain size, and degree of burrowing vertically. Much of the mud separating beds and draping foresets likely represents sedimentation from river floods following storm events. /P>

Seismic Facies of Shelf, Slope, and Submarine Fan Environments of the Lewis Shale, Upper Cretaceous, Wyoming

The Maestrichtian Lewis Shale of south-central Wyoming was deposited on the west side of the Sevier foreland basin. Well-log, core, and outcrop data document a vertical succession consisting of a condensed shelf shale overlain by bioturbated, probable turbidite sheet sandstone, deeper water sandy turbidites, and a progradational sandstone-shale interval of turbidite and deltaic deposits. Basin deepening and turbidite deposition were controlled by tectonic subsidence during the Sevier orogeny or early stages of the Laramide orogeny, and was possibly affected by eustatic sea-level rise.

Stratiform lead-zinc sulfides, mudflows, turbidites: Devonian sedimentation along a submarine fault scarp of extensional origin, Jason deposit, Yukon Territory, Canada

Lead-zinc sulfides were deposited on the sea floor in a deep-water setting during the Middle to Late Devonian at the Jason deposit, Yukon Territory, Canada. Galena, sphalerite, and barite, among other minerals, precipitated from hydrothermal fluids expelled along faults. Stratiform mineralization is laminated to thickly bedded and massive. Underlying the stratiform deposits in two areas, there are zones of silicified, carbonatized, and brecciated rock containing veins with hydrothermal minerals (stockwork); the stockwork represents alteration in and around hydrothermal conduits. Mineralization is interstratified with, and cuts across, clastic rocks of the Lower Earn Group. Lower Earn Group sedimentation was locally controlled by the graben and by movement on the graben-bounding faults. The graben center is represented by a thick sequence (as much as 1,500 m) of conglomerate, sandstone, and shale that was derived from a western provenance. These clastics were deposited by sediment gravity flows in channels and in channel-flanking areas, mostly within the graben. Graben-floor channels are estimated at being up to a few tens of metres deep. Conglomerates are lenticular, commonly imbricated, and locally normally or inversely graded. A very few beds have faint parallel stratification. Most sandstones are more evenly bedded, many show flutes and grooves, and almost all can be described using Bouma-sequence terminology. The lack of burrows in the Lower Earn Group at Jason suggests anoxic conditions on the sea floor. Abrupt stratigraphic thinning and the abundance of laterally discontinuous, locally derived, mass failure deposits indicate the existence of syndepositional faults in the mineralized area. The fault zone formed the graben margin and appears to have focused discharge of metalliferous fluids. The local tectonic setting during the Late Devonian was extensional and apparently related to transform motion between the North American cratonic plate and an ocean plate to the west.

Sorting and Wave Abrasion: Controls on Composition and Diagenesis in Lower Frontier Sandstones, Southwestern Wyoming

Lower Frontier sandstones on the Moxa arch were deposited in a wave-dominated, multi-river delta plain which prograded eastward into the Cretaceous Interior seaway; headwaters were in the Sevier thrust belt. Sands were deposited as fluvial units, as marine shoreline sequences, and as offshore sand ridges. Fluvial sandstones, some pebbly, are dominated by trough cross-stratification and by climbing ripple lamination. These sandstones are associated with flood-basin mudstones that contain abundant root traces and lack burrows. Marine shoreline sediments show a transition from burrowed lower shoreface parallel- and hummocky-stratified and ripple-laminated sandstones with mudstones, to surf-generated cross-stratification, to foreshore parallel stratification, and finally to s ructureless backshore sandstones. Offshore sandstones are mostly burrowed, but storm-generated parallel and hummocky stratification is present. Depositional hydrodynamics largely controlled diagenesis. Fluvial sandstones contain a greater percentage of rock fragments and have less quartz than equivalent marine sandstones owing to the combined effects of sorting and wave abrasion. The coarsest sands and gravels, which were trapped on the deltaic plain, were richer initially in rock fragments than finer grained sands that were passed to the shoreline. These latter sands had a significant portion of unstable grains, including chert, destroyed by wave abrasion. Composition significantly influenced diagenesis. Quartz-rich, permeable marine sandstones were cemented early by silica (because quartz overgrowths preferentially form on monocrystalline quartz), whereas primary porosity in quartz-poor fluvial sandstones was largely preser ed through this stage. Fluvial sandstones were affected more by grain dissolution, calcite replacement, calcite precipitation in open pores, and by subsequent carbonate dissolution. Fluvial sandstones generally have the best porosity and permeability mostly due to creation of secondary porosity. Compaction in finer grained sandstones destroyed permeability by squeezing clays into open spaces. Other major diagenetic processes are late-stage growth of kaolinite and continued precipitation of silica.

Lewis Shale Reservoirs, Southern Wyoming: Turbidite Sandstones Deposited in Delta-Toe Settings: ABSTRACT

Depositional history of the Lewis Shale, Maestrichtian, south-central Wyoming, consists of a transgressive period followed by an interval of delta infill. Clastics are up to 760 m thick. The Lewis sea transgressed westward to areas of the Rock Springs and Wind River uplifts and opened eastward into the main part of the Interior Seaway. Transgressive shales are black, high in organic material, relatively silt-poor, and totally bioturbated. Depositional setting was a shallow, well-oxygenated shelf. Lewis clastics above the transgressive shale are coarser and are related mostly to deltas that entered the basin first from the northeast and later from the south. Sediments were deposited in delta-front, prodelta, deep-basin, and interdistributary areas. Lewis sandstones produce at Wamsutter and Hay Reservoir fields in addition to other areas. Wamsutter and Hay Reservoir sandstones were deposited beneath storm wave base by sediment gravity flows at the toe of the northern delta system. Sandstones may be thickly bedded and are commonly massive. Presence of fluid escape structures indicates that many thick beds underwent liquefaction during the final stages of deposition. Thinner sandstones are graded and many show Bouma sequences. Interbedded shales are unburrowed or are slightly burrowed by restricted faunas, indicating anaerobic to dysaerobic conditions and water depths of 150-200 m or more at times. End_of_Article - Last_Page 317------------

Diagenesis of Frontier Formation, Moxa Arch, Wyoming--Function of Sandstone Geometry, Texture, and Composition of Fluid Flux: ABSTRACT

The lower Frontier Formation, Moxa arch, Wyoming, consists of sandstones and mudstones deposited in a wave-dominated delta and strand-plain system which prograded into the western margin of the interior Cretaceous seaway. Depositional environments in this system were offshore marine, marine sand ridges, marine shoreline, distributary channel, fluvial, and flood basin. Sediment from the Sevier orogene was sorted into different compositional and textural assemblages in depositional environments with different energies. Because of original differences in physical and compositional characteristics, diagenesis proceeded along different paths in different facies. The most important facets of original detrital composition affecting diagenesis are original clay content and the mo ocrystalline quartz/chert ratio. Diagenesis was also very sensitive to fluid flux. Sedimentary textures and sand-body geometries were important controls of fluid flow during dewatering. In the southern part of the study area, erosion eliminated the shoreline interval leaving only thin fluvial sandstones overlying offshore marine rocks. To the north, the shoreline facies was not eroded and the sandstone interval is thicker. On the assumption that sandstones received the discharge from equal volumes of shale, the thinner fluvial sandstones to the south experienced a far greater throughput per unit volume of sand and a higher rate of flow than did the thicker combined fluvial and shoreline sequence to the north. As a result, the southern sandstones are extremely porous and permeable and show signs of extensive leaching, whereas in the north there is more evidence of in-situ reactions resulting in the development of abundant authigenic and neomorphosed clays. Here, porosity and permeability are poorer. End_of_Article - Last_Page 635------------