Biogenic caliches in Texas: The role of organisms and effect of climate

Abstract

Biogenic constituents are ubiquitous and abundant in the caliches of Texas. Investigation of 51 caliche profiles on various host strata (alluvium, limestone, igneous rocks, etc.) across approximately 900 km of Texas from subhumid east to arid west has shown that 43 of these profiles exhibit prominent biogenic constituents. These profiles exhibit significant differences in thickness (varying from centimeters to meters) and maturity (varying from I to VI). All of the different caliche facies are composed of low-Mg calcite. Biogenic features generally occur in the upper part of the profiles, including the uppermost portion of massive caliche horizons, platy horizons, laminar crusts, and pisoids. The main biogenic caliche facies include rhizoliths (calcified root structures), stromatolite-like laminar crusts, and coated grains. Compared to the abiogenic massive micritic to microsparitic calcite groundmass, biogenic constituents are morphologically distinct. These biogenic constituents are composed of several microscopic mineral components, including calcified filaments, needle fiber calcite (e.g., single crystalline needles and needle pairs, triangular crystals, and polycrystalline chains of rhombohedrons), spherulites, micro-rods, and nano-spheres. A large number of calcified root cellular structures and micro-organisms, e.g., fungal filaments, actinomycetes, and rod-like bacteria, are also present. Plant roots as well as soil biota produce distinctive structures and also enhance lithification by inducing calcite precipitation in the caliches, i.e., biologically controlled or influenced processes. Host strata did not significantly influence the abundance nor type of biogenic features in the caliches. In contrast, climate had an evident effect on the development of biogenic constituents in these caliches in terms of the amount as well as type. The thickness of laminar crusts and grain coatings and the abundance of biotic constituents within those facies decrease as the climate shifts from subhumid and subarid, in southeast and central areas, to subarid and arid, in west and northwest Texas. In addition, root structures and micro-rods diminish significantly from the subhumid east to the arid west.