CONVERSION OF SILICA-UNDERSATURATED MAFIC VOLCANIC GRAINS TO GLAUCONITE: EXAMPLES FROM THE LATE CRETACEOUS BALCONES IGNEOUS PROVINCE, TEXAS

Glauconite grains are common in the upper part of the Late Cretaceous Austin Chalk Group in south and central Texas. This part of the section is coincident with one peak in eruptive events in the Balcones Igneous Province (BIP). The BIP is made up primarily of over 200 small, marine, Surtsey-style volcanoes. The dominant lithology of the volcanic rocks is silica-undersaturated mafic lapilli tuff which has been heavily altered to clay, primarily vermiculite and smectite.

Lapilli and ash eroded off volcanic centers are commonly found in the Austin Chalk facies that were deposited near the volcanoes, particularly in deep-water debris flows that were sourced in the shallow-water carbonate factories developed on and around the volcanoes. Examination of these deep-water deposits has noted a correlation between proximity to a volcano and the presence of glauconite grains. Examination of the volcanic tuffs, volcanic rock fragments, and glauconite using optical and scanning electron microscopy has shown textural clues that the glauconite originated as altered volcanic grains. Despite being altered to clay minerals, the volcanic grains show original volcanic textures including the outlines of phenocrysts, vesicles, and layering. A progression can be seen between clearly volcanic vermiculite grains and altered glauconite grains. In some glauconite grains, the evidence is textural, with outlines of filled vesicles being common. In some grains, addition of potassium has only partially converted vermiculite to glauconite. The tuffs of the BIP are generally low in potassium, so it is thought to be sourced in seawater.

Regional study of the Austin Chalk Group has shown that abundant glauconite is found only in the area that overlaps with the BIP. In addition, glauconite is concentrated in parts of the Austin Chalk section that correspond to a peak in volcanism. Although glauconite in some other units may or may not be volcanic in origin, there is significant evidence that glauconite in the Austin Chalk originated as volcanic grains.