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Upper Plio-Pleistocene salt tectonics and seismic stratigraphy on the lower continental slope, Mississippi Canyon OCS Area, Gulf of Mexico
Abstract
This study is based on approximately 1400 line miles of 32- fold migrated data provided by CGG. Paleontologic data from nine wells are incorporated in the study for correlation and age controls. The study includes the central part of the Mississippi Canyon and the northern part of the Atwater OCS (outer continental shelf) areas. The study area covers Shell's Mars prospect (M.C. Block 763) and Conoco's discovery (M.C. Block 243). From Lenticulina 1 (-2.2 Ma) to present, eight seismic sequences have been identified (A-H, A being the oldest sequence). Bottoms of three sequences (A, B, and C) correspond to benthic foraminifera tops (Lenticulina 1, Angulogerina B, and Trimosina A). Base of sequence F is tied to Walters' (1 985) base of sequence E, which represents the late Wisconsinan glacial. Salt generally occurs as tongues or sheets, and forms continuous masses in the basinward part of the canyon at water depths of about 1300 m (4300 ft). Areas without salt are near the "spur" and in the canyon axis. Time-structure maps show that in sequence A-B sediment distribution is strongly disturbed by salt movement. A salt diapir near the canyon axis pierces sequence F. Isochron maps indicate that salt movement was active in the eastern part for sequences A and B and moved to the central portion of the study area in sequences C and D. Near the canyon axis, salt has been active for the last 2.2 m.y. Sediment sources are from the northeast at sequences A-B, and from updip of the canyon for the whole 2.2 m.y. Seismic facies mapping shows four prevailing types: parallel reflectors (PR), divergent wedge (DW), mounded reflectors (MR), and chaotic reflectors (CH). PR shows sheet drapes external form which is interpreted as hemipelagic/pelagic sediments or turbidites overlying a filled basin. DW suggests lateral variations in the rate of deposition, subsidence, or progressive depositional surface uplift caused by salt intrusion. MR with a hummocky or chaotic internal configuration is interpreted as mass transport deposits, or it may represent local channel deposits on the slope. Disordered reflectors (CH) usually indicate salt movement/faulting caused disorganized strata or channel fills. Sedimentation rates derived from two wells suggest low rates (70 m/m.y. and 230 m/m.y.) between A and B and high rates (960 m/m.y. and 2,190 m/m.y.) from C to H. Low depositional rates produced turbidity current dominated tilted turbidites in sequences A-B. The high sedimentation rates from sequences C-H resulted from mass transport. Isochron pinch outs toward the northeast between sequences C-D and F-G indicate significant lowstand deposition. Channels in bases of sequences F and G, occurring in the northeast two strike lines and near the canyon axis, coincide with major sea-level drops in sequences F-G. Depositional units inferred from seismic records appear to be mainly lowstand deposits, although in areas close to the canyon axis, highstand mass transport deposits may occur. Major factors controlling seismic facies and sediment distribution are sediment flux, salt activity, and sea-level fluctuation. The slope gradient is considered a relatively insignificant factor, as it ranges only from -0.3' near the canyon axis to -0.8' in the northeastern part of the study area.
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Citation
Liu, Jia-Yuh (1993). Upper Plio-Pleistocene salt tectonics and seismic stratigraphy on the lower continental slope, Mississippi Canyon OCS Area, Gulf of Mexico. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1993 -THESIS -L783.
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