Proposed Microbial Origin of Kaolinite in Simsboro Sandstone (Upper Paleocene), Limestone County, Texas
Robert L. Folk¹, Earle F. McBride¹, and Thomas E. Yancey²
¹Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, Texas 78712–0254
²Department of Geology and Geophysics, Texas A&M University, MS 3115, College Station, Texas 77843–3115
The fluvial Simsboro Sandstone contains large amounts of intergranular kaolinite books that are easily imaged using the scanning electron microscope. Nascent flakes (0.1–0.4 μm wide) grew into “books” when flake width exceeded 1 μm. We propose here a probable biogenic origin for the kaolinite based on these observations: 1) Individual sheets of kaolinite are strikingly uniform, like stripes in a convict suit, ranging in different specimens from 45–60 nm (0.045–0.06 μm) thick, with a variation in any one specimen of about 7–10 nm. 2) Tiny spheroids (nannospheroids) occur on the edges of sheets; the spheroids match the thickness of the sheets. We propose that these spheroids are probably kaolinized microbial cells, and that microbes predestined the thickness of the kaolinite sheets by forming a monolayer of individuals. This explains why the kaolinite sheets have rounded edges in side view. Seen face on, the sheets range from euhedral pseudohexagons to flakes with scalloped or spheroid-trimmed edges. We propose that once precipitation of kaolinite was initiated by microbes, inorganic precipitation took over to form euhedral flakes. The size of the microbial cells is in the range of “nannobacteria.” 3) We also find discrete, dense clusters of spheroids that have not formed sheets; such clusters are typical of microbial colonies. Acid waters from soils and marshes developed in Texas during the hot, humid climate of Paleocene/Eocene time. The marshes developed a diverse stew of microbes and humic and other acids that percolated through Simsboro channel and overbank sands, dissolved feldspar and other silicates and permitted nannobacteria and other microbes to precipitate kaolinite.
AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012