ABSTRACT

Few thin-section studies have been published of post-middle Eocene formations along the Gulf Coast. One purpose of this paper, therefore, is to summarize such a study of the outcropping late Oligocene(?)-early Mioceue(?) Catahoula Formation of western Louisiana. In outcrops, this formation is continental, mainly fluviatile, and in part lacustrine in origin. Thus a second purpose of this study is to enumerate thin-section criteria for identifying strata of similar origin in the subsurface.

The Catahoula Formation, where studied, is a 52- to 550-foot-thick sequence of sandstone, siltstone, shale, claystone, and bentonite. A large proportion of the detrital material and matrix is of volcanic origin. Quartz is the most common framework mineral. It is of several types though volcanic quartz is the most abundant. Volcanic rock fragments, bentonite clasts, qnartzite, orthoclase, and plagioclase are next in abundance among framework grains. Lesser amounts of chert, muscovite, biotite, opaques, tourmaline, zircon, epidote, apatite, and other minerals are present. A characteristic of the sandstone is the opaline and montmorillonite cement which imparts a quartzitelike hardness in some outcrops.

Thin-section criteria are used for identifying the fluviatile, nonmarine environment of deposition in which the Catahoula formed. These criteria are neither unequivocal nor unique, and some of them are negative. Where these criteria are used collectively, however, they provide strong evidence for the depositional environment of this formation. They include (1) immature textural appearance and scarcity of samples with mature texture, (2) the presence of numerous beds with poorly sorted framework grains, (3) bimodal character of many samples where matrix is allogenic, (4) low degree of grain roundness, (5) low degree of grain sphericity, (6) moderately abundant flat-grain alignments, (7) absence of unabraded marine fossils, (8) presence of fossil wood and leaves, some bone fragments, an fresh-water mollusks, (9) scarcity of glauconite, and (10) general absence of authigenic, primary and secondary cements or grains of calcite, dolomite, and siderite.

Although these characteristics may also be found in other types of sediments which are transported in channels and "dumped" (for example, turbidites), other criteria--such as mixed deep-water and shallow-water faunas --can be used to separate fluviatile from turbidity-type deposits. The petrographic characteristics listed here cannot, and should not, be used to the exclusion of other criteria (that is, paleontological).