Abstract

The lithofacies of Barataria Basin, an interlobe basin of the Mississippi Delta Plain, were established from analyses of 115 boreholes. Five organic lithofacies are distinguished by organic matter content, botanical origin and degree of decomposition. Organic-rich facies accumulated in three stages (A, B, and C) during two depositional events. Stage A developed locally, but stages B and C developed as blankets over abandoned delta lobes, protected from marine influx by a shoreline. Stage B lasted from 2,300-1,000 yr BP and ended when clastic sediments from renewed deltaic sedimentation blanketed the peats, expanding the basin about 50 km seaward. Peat accumulation of stage C lasted from 900 yr BP to the present and is currently being terminated by subsidence-induced transgression. High-quality stage B peats accumulated beneath herbaceous, freshwater-floating and emergent marshes, mainly above the footwall of a strike-oriented, reactivated growth fault. Above the footwall, the basin was shallower than on the hanging wall, where the deeper basin invited more overbank sedimentation, which increased ash content and diluted peats. Thus, both tectonic and sedimentary factors influenced organic facies. The quality and quantity of Barataria Basin peats differ from those of many deltaic coals. However, these peats accumulated in only 2,300 yr immediately after a rapid sea-level rise. Frequent delta-lobe switching increased organic matter preservation potential but did not provide sufficient time for the accumulation of thick, low ash peats. The geometries of organic and clastic facies are similar to those of some coal-bearing strata.

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