ABSTRACT

Chemically precipitated alpha-cristobalite is a common matrix constituent of many fine-grained siliceous Tertiary rocks of the Atlantic and Gulf Coastal Plain. It forms as a low temperature authigenic mineral having a free growth morphology of fine blades arranged in spherulitic clusters 3 to 12 microns in diameter called "lepispheres." Cristobalite lepispheres have been observed via scanning electron microscopy of fracture surfaces through a wide variety of Lower to Middle Tertiary flint clays, "opal claystones," "buhrstones," and "pseudobuhrstones." Microspherulites of identical morphology also compose moderate to large percentages of off-shore Carribean-North Atlantic sediments of equivalent age including cherts, silicified limestones and chalks. The principal immediate source of silica for these deep water sediments is thought to be fossil diatom and radiolarian tests.

The immediate source of silica precipitated as low temperature cristobalite in coastal plain sediments has generally been ascribed to supposed volcanic sources in the Gulf Coast or Caribbean areas. Volcanic ash accumulations or direct precipitation from silica-rich bottom waters circulating above such ashes in restricted coastal environments have been variously postulated. Our scanning electron micrographs and petrographic studies of opaline claystones from South Carolina (Black Mingo Formation), however, have revealed numerous molds or tests of siliceous fossils preserved in a cristobalitic matrix of lepispheres similar, if not identical to, the matrix of deep sea cristobalitic cherts thought to be derived from the in situ dissolution and reprecipitation of siliceous microfossils. These findings suggest a biogenous source of silica for most of the cristobalitic matrix of these Atlantic Coastal Plain sediments, an observation supported by the paucity of zeolites in the units. Siliceous tests are also observed in the Tallahatta Formation of Alabama, an indication that a biogenous source was important in the silicification of opaline claystones of the Gulf Coastal Plain as well. The occurrence of the planktonic fossils in these units further indicates an unrestricted open marine environment (albeit at shelf depths) for the deposits. "An attempt to reconstruct the conditions under which the Tertiary faunas of the continental shelf lived and died and were buried requires the courage born of ignorance." Little Stave Creek, Alabama--Paleoecologic Study, Julia Gardner, 1957.