| Caliches occur extensively in Texas along a climatic gradient from the humid east to the arid west, and on different types of host rocks (e.g., limestone, gravelly and non-gravelly alluvium, eolian deposits, and non-sedimentary rocks). They range in thickness from a few centimeters up to 10 meters. The development of caliche profiles varies from stage I (young) to stage VI (mature); the major facies include nodular, massive, platy, laminar, and pisolitic caliches. Four types of idealized caliche profiles and their development have been differentiated and established based on host rock characteristics and weathering intensity.;Morphologically, caliches in Texas exhibit both alpha-type (abiogenic) as well as beta-type (biogenic) features. The alpha-type features include nodules and massive caliches that are characterized by dense micritic to microsparitic groundmass of rhombohedral calcite crystals, desiccation cracks, and sparry crack-filling cement. Beta-type features, mostly in rhizoliths, laminar crusts, and coated grains, include root structures, alveolar septal fabrics, calcified filaments, needle calcite fibers, spherulites, micro-rods, and nano-spheres. The unusual crystal forms and the recognition of abundant calcified soil organisms strongly indicate the important role of soil biota (e.g., plant roots, fungi, algae, and eubacteria) in the formation of these biotic caliches.;The delta18O values of caliches range mostly between -6‰ and -2‰ V-PDB, and the delta13C values are mostly distributed evenly between -9‰ and -1‰ V-PDB. From east to west, the delta18O values of caliches decrease gradually, mimicking the areal variation of delta18O values of modern meteoric water, whereas the delta13C values reflect the distribution of regional vegetation zones. Temporal vegetative and climatic shifts were also recognized from compound profiles with significantly different delta 13C values in Mission Bay and the Southern High Plains.;The caliches are predominantly composed of low-Mg calcite. The concentrations of minor and trace elements (e.g., Mg, Fe, Mn, Sr, and Na) in the carbonate components are low (a few ppm to < 1%). These, combined with the morphological features and stable isotope values, indicate a pedogenic origin. The three most important factors controlling the formation of caliche in Texas are climate, parent material, and vegetation and soil organisms. |
