Subsurface Geology of Upper Tertiary and Quaternary Deposits, Coastal Louisiana and Adjacent Continental Shelf

E. McFarlan, Jr., D. O. Leroy

Upper Tertiary and Quaternary deposits thicken seaward from a feather edge on the outcrop in the uplands of southern Louisiana to more than 7,000 ft (2,134 m) beneath the middle continental shelf. Through a study of cores and cuttings from 100 control wells and electric-log pattern correlations from 350 water and petroleum industry wells with seismic corroboration in the offshore area, these deposits have been divided into six major time-stratigraphic units, four of which correlate to outcropping terraces. Using historical Gulf Coast nomenclature, the units are, oldest to youngest, as follows: Nebraskan-Aftonian = High terrace, Kansan-Yarmouthian = Intermediate terrace, Early Illinoian = (subcrop pinch-out), Late Illinoian-Sangamonian = (subcrop pinch-out), Early Wisconsi ian = Prairie terrace, Late Wisconsinian = Mississippi deltaic and chenier plains.

This investigation presents a regional stratigraphic framework of the major upper Tertiary and Quaternary units from their updip pinch-outs in and beneath the terraced uplands, into the subsurface, across the coastal plain to the Louisiana offshore area.

The five oldest time-stratigraphic units in the offshore area are recognized by their respective time-index microfossils, distinct lithologic sequences, electric-log patterns, seismic sequence boundaries, erosional unconformities, and cyclic paleobathymetric changes. Six major facies comprise each time-stratigraphic unit; fluvial, deltaic, and strand-plain facies grade seaward into littoral, shelf, and slope facies. Distinct faunal assemblages are associated with each major facies. The basal Nebraskan sands and gravels are not only defined by extensive updip to downdip log, sample, and seismic correlations, but also their stratigraphic positions are supported by the presence of an underlying ash layer dated at 3.10 ± 0.43 million years (Pliocene) by the fission-track method and t e extinction of Globoquadrina altispira dated at 2.9 million years. Inland, each time-stratigraphic unit rests unconformably upon a subaerial erosional surface locally mantled by an oxidized soil zone.

Sediments of the time-stratigraphic units were derived principally from the Mississippi River's sediment load, which accumulated in six broad shovel-shaped depocenters. Downwarping of these sediments in the coastal and offshore area occurred contemporaneously with their deposition. In the latitude of the modern Mississippi-Birdfoot delta, the basal strand-plain deposits of the oldest Nebraskan-Aftonian unit have subsided about 4,100 ft (1,250 m). The distribution of facies and faunal assemblages in each time-stratigraphic unit provides a clear record of the cylic nature of deposition in an alternating transgressive and regressive sea. These sea level fluctuations are related to eustatic changes brought about by waxing and waning of Pliocene and Pleistocene continental ice sheets.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.