Mineralogy and Stratigraphy of Lower Tertiary Clays and Claystones of Alabama

Abstract

ABSTRACT Several stratigraphic units within the Lower and Middle Tertiary of Alabama contain numerous clay beds found to be composed of sedimentary zeolites, montmorillonite, and cristobalite. Mineral assemblages found in the clay strata of the Clayton Formation and the upper portion of the Nanafalia Formation (Grampian Hills Member) include montmorillonite and cristobalite combined with zeolites of the heulandite family, principally clinoptiloite. The thick Porters Creek clays are composed primarily of montmorillonite with minor amounts of heulandite. A prominent lower Tallahatta clay found in western Alabama is composed only of clinoptilolite. Massive claystones of the upper Tallahatta Formation in western Alabama are monomineralic rocks composed only of cristobalite, whereas they contain bo h montmorillonite and cristobalite in the eastern part of the state. Stratigraphically equivalent claystones over the central portion of the Tallahatta outcrop are interlaminated with clionoptilolite-rich clays. The clay strata of the Clayton, Porters Creek, Nanafalia, and Tallahatta Formations are considered to be diagenetic by virtue of their mineral content. On the other hand, montmorillonite is found in the clays of the Tuscahoma Sand and Hatchetigbee Formation not associated with the sedimentary zeolite or cristobalite but with terrigenous materials, and is considered to be detrital. It is suggested that clinoptilolite, as a part of the diagenetic mineral assemblages is an alteration product of rhyolitic volcanic ash, and that the heulandite and montmorillonite found in the same assemblages developed from clinoptilolite through a desilification process whereby crislobalite evolved as a byproduct. Conversely, it seems evident in some cases that montmorillonite developed directly from volcanic ash. This could he due to either the chemistry of the reacting system or the composition of the parent ash being different from that which clinoptilolite developed. Cristobalite, where it occurs in claystone strata and not associated with the sedimentary zeolites or montmorillonite, is believed to be a primary mineral locally precipitated in coastal environments. The diagenetic mineral suites found in many of the clay units of the Paleocene and Lower Eocene sections of Alabama seem to indicate periods of volcanic activity over the Gulf Coastal region during early Tertiary time. During these periods, fine-grained pyroclastic materials accumulated and was altered under silica-rich waters in the various coastal environments along the Alabama shoreline.