ABSTRACT

The Miocene along the Louisiana Coast and the inner part of the continental shelf is one of the thickest known wedges of terrigenous clastic sediments. It was deposited in a rapidly subsiding marginal sea basin by the ancestral Mississippi River which built deltas along a 300 mile segment of the coast. During times of abundant sediment supply, the shoreline was built seaward. When the rate of sedimentation decreased or subsidence increased, the sea moved inland and neritic shales were deposited over coarser deltaic deposits. Thus, this thick wedge is composed of interbedded sand, silt and shale, and there is alternation of shallow marine and deltaic deposits. Seaward, the entire section is composed of prodeltaic clay and silt which may be very thin at the outer edge of the continental shelf.

Along the upper Texas coast and in the adjacent offshore region, the Miocene is also quite thick and is composed entirely of terrigenous clastic sediments. However, this was an interdeltaic area during Miocene time; the thickest section is composed of fine grained neritic deposits much of which was brought there by longshore currents from the large deltas farther east; there is rapid transition from all-continental to all-marine deposits; and there are relatively few important fluctuations of the shoreline.

Most of the lower Texas coast was also an interdeltaic region during the Miocene, with a depositional history similar to that of the upper Texas coast. However, the ancestral Rio Grande was a large stream in Miocene time which did build deltas north of the present Rio Grande delta.

Eastward from southern Louisiana, the Miocene section thins markedly and only about 500 feet of section, mostly terrigenous clastics, is present in the Florida Panhandle. Farther east, in northern peninsular Florida and southern Georgia, there is a thin section of Lower Miocene carbonates and a thin section of Upper Miocene restricted marine carbonates and alluvial and transitional clastic sediments. The Miocene in southern peninsular Florida is about 900 feet thick and is composed of limestone of very shallow marine origin.

The depositional history of the western Gulf Coast terrigenous clastic Miocene and the eastern Gulf Miocene carbonates is described and illustrated. The upper and lower boundaries of the Miocene are discussed; the geographic extent, thickness, and volume of this important group of sediments are pointed out; and correlation of the outcrop and subsurface stratigraphic units is indicated.

Special attention is given the very thick Miocene deposits in southern Louisiana and adjacent offshore. The wedge of regressive, mainly deltaic, sand, silt and shale and transgressive marine shale is highly productive of oil and gas. Ten Divisions, based on local top ranges of species and on assemblages of benthonic Foraminifera in the more widespread transgressive marine sections, are proposed, and a representative section is presented for each Division. The factors responsible for making the Miocene of this area one of the most favorable sections known for hydrocarbon occurrences are pointed out.