Jeffry D. Grigsby¹

(1) Ball State University, Muncie, IN

ABSTRACT: Analcime in the Fluvial Frio Formation, South Texas

Volcanic glass, occurring as subangular detritus and as angular shards with undistorted bubble-wall texture, is common in the medial third of the fluvial middle Frio Formation of South Texas. Within this interval, volcanic-glass detritus averages 8% and ranges from 1% to 26% of the total rock volume, and its presence has lead to a complex diagenetic history. Of particular interest is the formation of analcime. Petrographic analysis (SEM) and electron probe microanalysis (EPM) have found that analcime occurs as both a replacement mineral and as a pore filling cement. Replacement analcime replaces volcanic glass (no unaltered volcanic glass has been identified) and is chemically homogeneous. It has an average composition of Na11.5Al14.8Si34.0O96nH2O, with Si/Al ratios averaging 2.31 (± 0.01). Pore-filling analcime averages 6% and ranges from 0% to 18% of the total rock volume. Pore-filling analcime has an average composition of Na9.2Al14.9Si34.4O96nH2O, with Si/Al ratios averaging 2.31 (±0.11). Pore-filling analcime commonly has a lower Na content and a broader range in the Si/Al ratio than replacement analcime. Pore-filling analcime cementation was the first major lithification event in much of this interval of the middle Frio Formation, although petrographic relationships indicate an antithetic relationship with early Fe-poor calcite. The early formation of analcime suggests that Na-rich, alkaline porewaters developed very early in the burial history of the nonmarine Frio Formation.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado