Chemically precipitated alpha-cristobalite is a common matrix constituent of many fine-grained siliceous Tertiary rocks of the Atlantic and Gulf coastal plains. It forms as a low-temperature authigenic mineral having a free growth morphology of fine blades arranged in spherulitic clusters 3-12 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, claystones, "opal claystones," "buhrstones," and "pseudobuhrstones." Microspherulites of identical morphology also compose moderate to large percentages of offshore Caribbean-North Atlantic sediments of equivalent age including cherts, silicified limestones, and chalks. The source o silica for these deep-water sediments is thought to be fossil diatom and radiolarian tests.

The immediate source of silica precipated as low-temperature cristobalite in coastal plain sediments generally has 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 have been postulated. In major ocean basins explored by the Deep Sea Drilling Project, however, cristobalitic cherts are seldom associated directly with ash sequences although some diatom and radiolarian remains usually are preserved in these cherts. Scanning electron micrographs and petrographic studies of opaline claystones from South Carolina (Black Mingo Formation) have revealed many molds or tests of siliceous fossils preserved in a cristobalitic matrix of lepispheres similar, i 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 also are observed in the Tallahatta Formation of Alabama, an indication that a biogenous source was important in the silicification of some Gulf coastal plain sediments as well.

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