Abstract: Structure and Stratigraphy of Late Quaternary Deposits on Outer Louisiana Shelf

Lamar B. Roemer, William R. Bryant

Regional investigation of late Quaternary deposits on the outer Louisiana shelf indicates two principal structural provinces: (1) a western province between the West Cameron and Ship Shoal areas which is highly faulted and structurally controlled by salt tectonics; and (2) an eastern province, occupying the South Timbalier and Grand Isle areas, which is greatly influenced by the effects of tectonics and sedimentation associated with the old Mississippi Canyon complex. Vertical forces caused by a mobile salt layer at depth, continual basin subsidence, and the incompetence of the overlying sedimentary strata dominate the western structural province, the main locus of deposition during Pleistocene time. Growth faults, postdepositional faults, salt-dome faults (radial and per pheral), grabens, horsts, and collapse faults are present in the western province; movement along several of these faults is occurring at present. In the eastern province little deformation is apparent in the late Quaternary section where thick accumulations of recent sediments are deposited.

Thick regional sands are widely distributed across the outer Louisiana shelf. These sands represent fluvial-deltaic sediments that initially were deposited in a series of coalescing deltas during low stands of sea level. As sea level rose these sands subsequently were transported to the north and spread laterally across large areas. Transgressive bars formed during intermittent halts in sea-level rises.

Shallow gas accumulations and natural gas seeps are prevalent in the near-surface sediments. Many of these gas zones are associated with older formations and the gas has been brought to the surface via faults and/or sediment layers uplifted by recent salt movement. Variations in sand thickness associated with channel cuts and transgressive bars, carbonate banks, and oyster reefs produce shadow zones on high-resolution profiles that previously have been confused with gas zones. The correct identification of these phenomena together with shallow unconformities, active faults, and pipelines is important to the oil industry in order to avoid potential drilling hazards, which might cost millions of dollars and possible loss of life.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas