Abstract: Plio-Pleistocene Geology, Outer Continental Shelf, Louisiana and Texas

By

H. O. Woodbury¹, I. B. Murray, Jr.¹, W. H. Akers¹, and P. J. Pickford^2
1 Chevron Oil Company, N. O., La. 70112
² Chevron Oil Company, Houston, Texas

Pliocene and Pleistocene deposition on the Outer Continental Shelf and upper Continental Slope offshore from Louisiana and Texas was a continuation of the process of prograding deltaic sedimentation with associated hydrocarbon accumulation that has been active in the northern Gulf of Mexico since the end of the Cretaceous. However, this more recent phase of the geologic history of the northern Gulf of Mexico Basin differs from the earlier Tertiary history of the area in several significant aspects:

1. The rate of sediment deposition was high, and the rate of seaward progradation of the continental shelf edge was rapid. The Plio-Pleistocene embraced only 5 million years, whereas the Miocene lasted 17 million years, yet the volume of sedimentation deposited during each epoch is comparable.

2. The center of deposition moved northeast, from southwest Louisiana in the lower Miocene to southeast Louisiana in the upper Miocene - lower Pliocene, and

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then shifted westward again in the Pleistocene. This shifting in depocenters was accompanied by a progradation of the continual shelf edge to its present position near the 600 ft. isobath. Hydrocarbon productive trends follow these shifting depocenters.

3. The bulk of the Plio-Pleistocene sediments was deposited upon substrata which is made up of several thousand feet of mobile salt plus a comparable thickness of mobile, deep water pro-delta clay. The weight of the accumulating sediments has caused movement of the underlying mobile material so that today the structural condition of the sediments in the Plio-Pleistocene depocenters is complicated by a great number of large piercement salt domes and ridges; by domes and ridges of diapiric shale; and by many normal faults with displacements up to thousands of feet with omnidirectional strike and dip.

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