ALTERED MAFIC LAPILLI TUFFS FROM THE CRETACEOUS BALCONES IGNEOUS PROVINCE, CENTRAL TEXAS

The Cretaceous Balcones Igneous Province of central Texas is a suite of silica-undersaturated, alkalic igneous rocks including both intrusive and extrusive rocks. This study documents the post-eruptive alteration and porosity changes present in the extrusive rocks. The extrusives occur mainly as thick lapilli tuff mounds generated by submarine volcanoes. Mound thicknesses up to 260 m (850 ft) have been observed. Mounds occur within Cretaceous sedimentary rocks, primarily carbonates. Previous researchers have noted a broad range of compositions from alkali basalt to melilite olivine nephelinite to phonolite for the intrusives. It is suspected that the associated tuffs will show a similar range of compositions, although some previous researchers have mistakenly characterized them as being “serpentine.”

This study has focused on two subsurface cores from different tuff mounds in the northern part of the province. Core depths range from 457 to 718 m (1501 to 2356 ft). Samples from the cores have been analyzed for porosity, permeability, texture and mineralogy.

Volcanic textures are mostly preserved in the tuffs, despite widespread alteration. Minerals and volcanic glass are altered primarily to clay (smectite) and/or chlorite and in some cases titanite. Phenocrysts, presumably of olivine, are completely replaced. Microphenocrysts of titanaugite survive in some lapilli. Alteration style and intensity varies between adjacent lapilli suggesting heterogeneous origin for material. Sporadic layering within the cores is primarily defined by differences in cementation and less by changes in particle size. Overly steep layering at some depths suggests slumping of material.

Porosities up to 30% have been measured, highlighting the excellent hydrocarbon storage potential of these rocks. Pores are present in four locations; between lapilli, in partially cemented vesicles, in partially replaced phenocrysts, and between phyllosilicates replacing volcanic glass. Pore sizes extend from nanometers to millimeters in diameter. Pore space, both between ash grains and lapilli and in vesicles, is lined with combinations of smectite, chlorite, pectolite, xonotlite and rarely calcite. One core is more Na-rich and has more smectite and pectolite, the other core has less Na, and has more chlorite and xonotlite.