Genesis of soils and carbonate enriched horizons in a climo-sequence developed over cretaceous limestone in Central and West Texas

Abstract

The Edwards Plateau covers 10 million ha in Central and West Texas (nearly 14% of the state) and is an important agricultural rangeland. A strong climo-gradient extends across the area with annual precipitation decreasing westward from 800 to 300 mm. There is a paucity of information on the shallow and stony soils derived from Cretaceous limestone. Following reconnaissance investigations of 34 sites, 15 pedons were sampled and analyzed for routine physical, chemical, mineralogical and micromorphological data. Four pedons were selected for detailed mineralogical, elemental, and SEM analyses. These data serve for developing pedogenic models. Carbonate levels in surface horizons were significantly correlated with Thornthwaite's P-E (moisture) index. Soils in the western part of the area commonly had calcic or petrocalcic horizons. Argillic horizons were common in the easternmost part of the study area although illuvial argillans were difficult to verify except in protected areas such as pores within chert fragments. Euhedral, prismatic quartz grains were identified by SEM to be a prominent component of residues from hard crystalline limestones. These grains were used as marker minerals in identifying parent material discontinuities. Quartz grain morphology, particle size distribution, elemental assay, and mineralogical data indicate a discontinuity between the soil and subjacent limestone. The underlying rock should not, therefore, be considered as the soil parent material. Airborne dusts of uniform quantity and composition are deposited to the surface at the approximate rate of 1 mm/100 yrs. Marker minerals indicate, however, that these dusts are not accumulating on stable land surfaces because erosion presumably exceeds accretion.Differentiation of lithogenic (limestone) from pedogenic forms of carbonates in soils was accomplished by applying microfabric and stable carbon isotope methodologies. Both methods confirmed that massive indurated carbonate zones and much of the disseminated carbonates were pedogenically derived. Petrocalcic horizons occurring over limestone have formed through in situ pedogenic alteration and reconstitution of limestone. This is distinctly different from the 4-stage model of Gile et al. (1966). A new 3-stage model is proposed to describe the genesis of limestone derived petrocalcic horizons.