ORIGIN OF KAOLINITE IN PALEOCENE-EOCENE ROCKS (SIMSBORO, CARRIZO, MANNING) OF EAST-CENTRAL TEXAS, USA

Diverse textures of kaolinite formed in fluvial and coastal sediments in several stratigraphic units during extended weathering, leaching by acid swamp water, and long-term meteoric flushing of sand aquifers. Kaolinite abundance ranges from a few percent to 70% of sandstones (Simsboro, Carrizo), 80-90% of ash beds (Manning) and 95⁺% of claystones (Simsboro, Carrizo). Elongate grains have vermicular shape typical of authigenic kaolinite, and many grains have one or more grooves that run their length.

Discrete sand-sized kaolinite grains have unit extinction under the polarizing microscope and direct or indirect evidence that they are, or could be, pseudomorphs of biotite or sanidine or other feldspars. Direct evidence of precursor volcanic grains includes partially kaolinized biotite grains (Simsboro, Carrizo); indirect evidence includes grain shapes similar to euhedral biotite and sanidine, ghost 001 cleavage of biotite, ghost twinning of feldspar, and association with volcanic quartz and ash. On the other hand, sand-size grains with microcrystalline “chert-like” texture, abundant in the Simsboro, are claystone rip-ups derived from overbank deposits. Except in ash beds (Manning Fm.), where discrete kaolinite grains are angular and have not seen abrasion, most discrete kaolinite grains are rounded from abrasion during fluvial transport. Compactionally deformed kaolinite grains indicate kaolinization was largely a surficial process and occurred before significant burial.

In nearly all elongate kaolinite grains, the 001 cleavage is perpendicular to grain elongation. Thus, the 001 crystal faces of most kaolinite grains are perpendicular to 001 faces of the biotite precursor, which records an unusual replacement process. SEM imaging shows that kaolinite grains with unit extinction under a polarizing scope have a complex internal architecture (Folk et al., this meeting).

Evidence of extensive leaching of kaolinite host rocks includes grain molds, oversize pores, skeletal chert and skeletal polycrystalline quartz grains. The pervasive distribution of kaolinite in some Simsboro and Manning samples indicates kaolinite also formed as pore-filling cement.