Facies Relationships and Systems Tracts in the Late Holocene Mississippi Delta Plain

Abstract

ABSTRACT Facies relationships in abandoned Holocene Mississippi Delta complexes are characteristic of both retrogradational transgressive systems tracts (TST) and progradational highstand systems tracts (HST). In the Barataria interlobe basin, delta-plain facies of the early Holocene Maringouin/Teche delta complex (TST), which accumulated from 7500 to 6000 yr BP, are overlain by a lagoonal facies 1-2 m thick (MFS) that accumulated during the maximum flooding event from 6000 to 3500 yr BP. Wave reworking transformed the distributary sands of retrogradational delta complexes into stratigraphically backstepping shoreline sand bodies. The most landward of these shorelines, the Teche shoreline, overlies the MFS and is, by definition, the shoreline of maximum transgression (SMT) (Penland et al. 1987 , 1987b; Kosters 1989). Relatively thick peats of high organic content, dating from 2400 to 1100 yr BP, are located immediately landward of this shoreline. Younger delta lobes, rapidly prograding since 1100 yr BP, have shifted the coastline seaward of the Teche shoreline, and form the first progradational HST parasequence. Thin, organic-poor salt marsh sediments are accumulating within this parasequence landward of the present shoreline. Rising relative sea level provides increased accommodation space while fresh water may be held within the delta plain, creating conditions of both groundwater and nutrients favorable to accumulation of high-quality organic facies of this type. In a subsequent progradational setting, stable relative sea level results in less accommodation space landward of the shoreline, while fresh water and nutrients are discharged into the Gulf of Mexico, forcing formation of brackish and salt marsh environments, unfavorable to accumulation of high-quality organic facies. These hypotheses may help explain the variability of some littoral high-quality coals vs. carbonaceous shales in the rock record.