Pliocene to Recent Infilling and Subsidence of Intraslope Basins Offshore Louisiana

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Aapg Bulletin-American Association of Petroleum Geologists
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Intraslope basins on the lower eastern Louisiana continental slope are modern analogs of large oil- and gas-charged basins infilled on the upper Louisiana slope and buried landward beneath the shelf. High-resolution, gridded multibeam bathymetry and single-channel seismic reflection profiles are used to (1) characterize the morphology and shallow stratigraphy of 46 intraslope basins within the outer slope region; and (2) look for surficial evidence among the basins of slope channelways that fed sediment flows to the Mississippi Fan during the Pliocene-Pleistocene. Computer algorithms, traditionally employed for automatically mapping river networks in gridded land topography, are used to extract morphologic measurements of the intraslope basins from the gridded bathymetry. The basins are found to average approximately 15 km in length, 10 km in width, and 200 m in depth and occur over a map area of about 50 km(2). These dimensions, however, can range over an order of magnitude.The intraslope basins examined exhibit (1) distinctive plots of basin area vs. relief (i.e., hypsometric curves) and (2) different near-surface (<2 seconds two-way travel time) stratal geometries, which together appear to reflect a continuum between two intraslope basin end-member morphologies. Analysis of the hypsometric curves suggests that the transformation between basin end members is a result of differences in amounts of basin subsidence relative to basin infilling. Comparison with intraslope basins along a canyon-channel system offshore Texas suggests that the transformation occurs as the basins are progressively infilled downslope. However seismic stratigraphic unconformities within basin strata indicate asymmetric subsidence, presumably along growth faults bounding the basins, which complicates this infilling by periodically deepening the intraslope basins and staving off their burial. Such subsidence appears to have occurred over the Quaternary deepening the intraslope basins and helping mask any regional bathymetric imprint of the slope pathways eroded by sediment flows during the Pliocene and Pleistocene.


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