ABSTRACT

The low coastal marshland of southwestern Louisiana is characterized by numerous relict beach ridges, or cheniers, which give the region its name. From analyses of cores along a north-south line of section through Creole, Louisiana, it has been possible to identify 4 faunal zones and 7 distinct sedimentary facies in the post-Pleistocene deposits which underlie the plain. The faunal zones, named after the dominant foraminifers, are: the Streblus, Streblus-Elphidium, Quinqueloculina and Trochammina zones. The sedimentary facies consist of varying proportions of sand, silt, and clay which, together with their contained faunas, clearly reflect the environments of deposition, including marsh, bay, mudflat, open gulf, and beach.

The relict beaches or cheniers are lenticular sand and shell bodies similar to certain shoestring sands which commonly serve as oil reservoirs in older rocks; thus, they hold special interest for the petroleum geologist. The cheniers, which extend as uninterrupted coastwise ridges up to 30 miles in length, are biconvex in cross section and average about 600 feet across and about 7 feet in thickness. They rise from a few inches to more than 10 feet above the marsh. They are characterized by a smooth, generally arcuate seaward front and an irregular landward margin. These deposits rest on shallow gulf bottom sand and silty clay and are overlapped by organic marsh silt and clay. Should the ridges eventually be buried by such marsh deposits, they will form potential stratigraphic traps. A knowledge of the characteristic shape, trend, and dimensions of the chenier sands, and of their distinctive sedimentary and faunal properties should aid materially in identifying similar potential reservoirs in the subsurface.