ABSTRACT

On the eastern half of the Edwards Plateau, central Texas, caliche occurs on various Precambrian, Paleozoic, Cretaceous, and Quaternary formations.There is an increase in the variety of caliche horizons, macrofeatures, and biotic macrofeatures as the caliche matures.Caliche forms within outcrops that contain carbonate-rich and basaltic alluvial gravel, marl, indurated limestone, siliciclastic shale, sandstone, terra rossa, and schist. Variations in caliche development are primarily a function of bedrock lithology and, in some cases, additional characteristics of the host strata; that is, percent gravel content, degree of exposure to the ground surface, fabric of the shales, and degree of brecciation.

Low-magnesium calcite and lesser amounts of quartz are the two dominant minerals present within the non-clay fraction of the caliche, with trace amounts of lithogenic and/or detrital feldspar, hematite, dolomite, and ilmenite.Both abiotic and biotic processes play a role in the development of these caliches, and the combination of both processes produces many of the macrofeatures and caliche horizons (e.g., platy horizons).The degree of biotic activity increases both upward within caliche profiles, and as the degree of caliche development increases.Abiotic features include recrystallized mottled fabrics, desiccation-related fractures, and grain replacements, whereas biotic features include rhizoliths, calcified filaments, and rod-shaped and filamentous bacterially induced precipitates.

The ¹³C values reveal a mixed C₄ to C₃ biomass during caliche development.Hardpans at the ground surfaces show the most positive 13C values when compared with the lower caliche horizons, which could be indicative of either: 1) isotopic mixing with atmospheric CO₂, 2) the degassing of ¹²C-enriched CO₂ closer to the ground surfaces, or 3) an increase in the abundance of C₄ plant biomass.The ¹⁸O values within the caliches, when compared with ¹⁸O values within the local rivers and streams, show that uniform temperature conditions occurred during caliche development.