The Characteristics and Origins of Biogenic Caliches In Texas

Biogenic features are ubiquitous and abundant in the Texas caliches. However, the role of soil biota and other environmental factors (e.g., climate, bedrock) in caliche formation is not well understood. Investigation of 51 caliche profiles on various bedrocks (e.g., limestone, igneous, etc.) across Texas has shown that 43 of these profiles exhibit prominent biogenic features. Compared to abiogenic features, which are characterized by massive groundmass composed of micritic to microsparitic low-Mg calcite, biogenic features are mineralogically identical but morphologically distinct. The main biogenic caliche facies include rhizoliths (calcified root structures), stromatolite-like laminar crusts, and coated grains. These biogenic features are composed of several unusual 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. These peculiar crystal (or polycrystal) forms of constituent minerals strongly indicate the biogenic origin of the caliches. A large number of calcified root cellular structures and micro-organisms, e.g., fungal filaments, actinomycetes, and rod-like bacteria, were also recognized. Plant roots as well as soil biota not only produced intriguing structures, but also enhanced lithification by inducing calcite precipitation in the caliches, i.e., biologically controlled or influenced processes associated with organism metabolism.

There is not much difference in the biogenic features of caliches developed on different bedrocks. In contrast, climate has an evident effect on the development of biogenic features in these caliches in terms of the amount as well as type. The thickness of laminar crusts and grain coatings and the intensity of organic activities 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. Also, root structures and micro-rods diminish significantly from east to west.