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NARROW
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all geography including DSDP/ODP Sites and Legs
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Sequence and Seismic Stratigraphy of the Bossier Formation (Tithonian; Uppermost Jurassic), Western East Texas Basin
Abstract Sequence and seismic stratigraphic analysis of well logs and 2-D seismic lines from Freestone, Anderson, Leon, Houston, Madison, Robertson and Limestone Counties, Texas, demonstrates that the Bossier Formation of the western East Texas basin can be subdivided into two recognizable sequences separated by a major sequence boundary (SB-2). Similarly, the Bossier Formation is also bracketed by a basal (SB-1) and upper (SB-3) sequence boundary separating it from the Gilmer (Cotton Valley) Lime of the Haynesville Formation below, and the Cotton Valley Sand above, respectively. In seismic sections, the SB-2 boundary in the middle of the Bossier Formation was identified by tracing mounded basal reflectors and sigmoid basal reflectors representing basin floor and slope fans. This boundary was correlated onto the shelf below deltaic sands. In well log sections, basin floor fan log shapes were traced laterally into slope fan and stacked delta log patterns to identify SB-2. These basin floor and slope fans immediately above the SB-2 boundary represent a lowstand systems tract, whereas the lower Bossier (below the SB-2 Sequence Boundary) represents a transgressive systems tract and the upper Bossier (above the SB-2 boundary) represents a prograding complex. Burial history analysis suggests that the lower Bossier accumulated during a time of rapid mechanical subsidence when the East Texas basin was underfilled. A drop in sea level associated with the SB-2 boundary represents a major climate shift from tropical to cooler conditions, favoring rapid influx of sands from the ancestral Mississippi, Ouachita, and Red River systems. These sands developed inner shelf prograding deltaic packages, outer shelf and incised valley fill stacked deltas, and basin submarine fan systems. The stacked deltas and basin fan sand systems all represent prospective gas plays.
What do campus oil-company recruiters look for?
Abstract Research on tidalites, sediments deposited by tidal currents, evolved through four phases during the last half century: PHASE I, Facies mapping of Holocene tidalites in Germany, Holland, the United Kingdom, and Canada identified the seaward-coarsening pattern of sediment distribution, a distinct zonation of sedimentary structures, and provided a fining-upward facies model used to recognize ancient counterparts. Mapping in subtidal areas showed that extensive sheets of tidally molded-and-deposited sand accumulations characterized continental shelves that were both wide, and funnel-shaped in plain view. Similarly, extensive work was completed on carbonate tidalites, although it is not discussed herein. PHASE II. Study of sedimentary structures was followed by a detailed analysis of sediment transport dynamics on intertidal sand bodies in Canada, where time-velocity asymmetry is the major factor controlling sand body geometry, orientation of bedforms, grain size distribution, sediment dispersal, and sand body orientation. Parallel work in tide-dominated continental shelves of the Yellow Sea of Korea and the southern North Sea showed similar patterns. These studies confirmed that tidal sand bodies are likely to be preserved in the rock record and provide a counterpart facies that is likely to dominate ancient cratonic seas. PHASE III. In tide-dominated estuaries of The Netherlands, cross-bedded units were observed to be organized into discrete bundles that were correlated to neap and spring tides. These observations were replicated in ancient counterparts. PHASE IV. Detailed analysis of the Schelde Estuary, The Netherlands, demonstrated that parallel-bedded couplets of sand and mud (tidal bedding) could be correlated directly to neap-spring tidal cycles. Recognition of such couplets, particularly in Mississippian and Pennsylvanian sediments of the midcontinent of North America, can be correlated to lunar dynamics and tidal patterns. ALL of these studies demonstrated that tidalites accumulated rapidly and were preserved widely. Where preserved in the stratigraphic record, tidalites represent accumulation during very short time intervals. Consequently, in many sequences where such facies are preserved, the time gaps in the stratigraphic record were far longer than previously interpreted.
Tectonic subsidence analysis in the characterization of sedimentary ore deposits; examples from the Witwatersrand (Au), White Pine (Cu), and Molango (Mn)
Depth Determination and Quantitative Distinction of the Influence of Tectonic Subsidence and Climate on Changing Sea Level During Deposition of Midcontinent Pennsylvanian Cyclothems
Abstract New sedimentological determinations of the water depth and associated sea-level change of midcontinent Pennsylvanian cyclothems shows that they accumulated in water depths ranging from as low as 32 m to as high as 160 m, depending on which model is used to establish the deepest water facies. These depth determinations also indicate that, regardless of model, depth variations existed for different cyclothems both laterally and in time. Average water-depth determinations and sea-level change for models of Heckel (1977) and Gerhard (1991) are 96.4 m and 86.0 m respectively. Analysis of tectonic subsidence permits calculation of the magnitude of tectonic processes and associated climatic effects, which controlled changes in sea level during deposition of Pennsylvanian cyclothems. Far-field tectonic effects, in response to regional orogenic movements, partially influenced Pennsylvanian sea-level change in the midcontinent. Organization of Virgilian and Missourian midcontinent cyclothems into four-to fivefold bundles shows that sea-level changes in midcontinent platform areas were influenced both by Milankovitch orbital parameters and longer-term climate change, whereas Desmoinesian sea-level change apparently was influenced more strongly by tectonic subsidence controlled by foreland-basin tectonism. The magnitude of tectonically-contributed change in sea level varied laterally. In the midcontinent, tectonic subsidence accounts for approximately 5 to 20% of the total sea-level change in platform areas, and perhaps as much as 20% in basin depocenters. The remaining change in sea level is controlled by both short-term glacial eustasy (Milankovitch orbital forcing; approximately 70% of sea-level change) and long-term climate change (approximately 15% of sea-level change). These findings suggest that, away from orogenic belts, climatic change is the principal driving mechanism controlling sea level change, whereas within orogenic belts, climate becomes somewhat more subordinate as a driving mechanism for Pennsylvanian sea-level change, even though indicators of climatic change itself are preserved. Methods discussed herein permit calculation of magnitudes of both tectonic and climatic-eustatic components of sea-level change influencing Pennsylvanian cyclothem deposition, and may be applicable to other cyclic sequences.