Volcanic Origin and Significance of Glauconite Grains in the Upper Cretaceous Austin Chalk Formation in the Balcones Igneous Province, South and Central Texas

Abstract

Abundant glauconite grains occur in the upper Austin Chalk A and B1 units within the confines of the Balcones Igneous Province (BIP) in the area of the Maverick Basin and San Marcos Arch in Central to South Texas. Controversy exists as to whether the origin of the grains is related to sedimentary hardgrounds or to alteration of volcanic ash and lapilli. The Austin Chalk A and B1 units were deposited contemporaneously with active volcanism (i.e., BIP), and glauconite grains are closely associated with and limited to the area of the BIP. Abundant ash and lapilli are noted near volcanic mounds, where many of these grains are converted to vermiculite and glauconite. It is well established that the BIP pyroclastics underwent alteration in seawater soon after deposition. An unambiguous transition from vermiculite and glauconite-altered volcanic fragments to sand-size glauconite is clearly demonstrated by thin-section and scanning electron microscopy analysis. Grains of glauconite (highly altered ash) deposited away from volcanic mounds have the same origin as those near the mounds. The sand-size glauconite grains can retain internal outlines of gas vesicles and other volcanic microstructures. The misidentification of these volcanic grains as sedimentary-formed glauconite, and not as altered volcanic ash, can lead to misinterpretation of the geologic history and sequence stratigraphy of the upper Austin Chalk section.