ABSTRACT

High-frequency sea-level fluctuations during the late Pleistocene-Holocene have caused Louisiana's distal shelf and upper slope to be alternately flooded with and starved of terrigenous sediments. During periods of falling to low sea level, fluvial systems entrench the shelf and deposit abundant sediments at or near the shelf edge, developing relatively thick or expanded sections. Under rising to high sea-level conditions, sediment sources retreat landward and leave the shelf and slope to slowly accumulate hemipelagic and other calcareous deposits. These normally thin or condensed sections are widespread, have a high-amplitude acoustic response, and are the stratigraphic marker horizons that separate expanded sections, which can vary greatly in thickness and sediment characteristics.

Condensed sections accumulate at rates of 5.0-30.0 cm/1000 yr and are highly calcareous. High-relief areas of the slope are dominated by carbonate sediments and moundlike buildups. Carbonates in these areas include cemented hemipelagics, shell hashes, hardgrounds, isolated nodules, and large mounds. Many of the diagenetic carbonates, including the mounds, are isotopically light (-20 to -48 ppt PDB) and are directly linked to methanogenesis. Aside from the above diagenetic products, condensed-section carbonates are typically unstratified, highly burrowed, and pelleted.

Expanded sections can accumulate at rates of up to two orders of magnitude greater than condensed ones. Individual delta lobes can vertically accrete at rates of 100 m/1000 yr, whereas upper-slope environments have average accumulation rates of 40-60 cm/1000 yr. Rapidly deposited sediments of expanded sections are generally well stratified, can display textural variations from clay to sand, and only rarely contain significant structures or inclusions associated with biogenic or diagenetic activity. Late Pleistocene-Holocene sea-level cycles of 20-30 kyr have caused condensed and expanded sections to be stacked at the shelf edge and on the upper slope.