ABSTRACT: A Petrologic Analysis of Caliche Within the Central Texas Region

JAYNES, RUSSELL S., and HENRY S. CHAFETZ

Caliche occurs on various Precambrian, Paleozoic, Cretaceous, and Quaternary formations on the Edwards Plateau. Variations in caliche development are primarily a function of bedrock lithology and, in some cases, additional characteristics of the host strata (e.g., degree of brecciation, percent gravel content). Caliche forms within outcrops that contain carbonate-rich and basaltic alluvial gravel, marl, indurated limestone, siliciclastic shale, sandstone, terra rossa, and schist. Specifically within formations that contain marl, caliche forms in situ and the degree caliche development is a function of their degree of exposure to the ground surfaces. For example, where marls are overlain by resistant limestones, immature caliche development is observed. However, where marls are exposed at the ground surfaces, the calichification process occurs more rapidly, and more mature caliche profiles occur.

Both abiotic and biotic processes play a role in the development of the caliches, and the combination of both processes produces many of the macrofeatures and caliche horizons (e.g., platy horizons). Abiotic features include desiccation fractures and grain replacements, whereas biotic features include rhizoliths and calcified filaments.

Delta¹³C values reveal a C4 dominant biomass during caliche development. Hardpans at the ground surfaces show the most positive delta¹³C values, indicative of either degassing and/or isotopic mixing with atmospheric CO₂. delta¹⁸O values are fairly consistent throughout vertical profiles, indicative of either isotopic homogeneity with local meteoric waters and/or uniform climatic conditions throughout the caliche development.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana