Age-dating implications from the morphologic, petrologic, and isotopic investigations of a calcic soil, Terrell County, Texas

Abstract

A fine grained calcic soil has developed upon Pliocene-Miocene alluvial gravels derived from Cretaceous limestone of the Stockton Plateau and Paleozoic limestone, chert, and novaculite from the Marathon uplift. This soil is located in an area classified by the Soil Conservation Service as having "marginal" conditions for calcic soil development. Establishing pedogenesis as the mechanism of carbonate accumulation will allow the use of a calcic soil developmental index to obtain age-dates in this "marginal" area. The soil has a 109 cm thick Stage IV K horizon at depths of 68-80 cm. In gravelly areas of the soil a 15-20 cm thick laminar Km horizon caps the K horizon. Microprobe analyses on portions of the Km horizon show that CACO, is the dominant material present and displaced the clays forming laminar and pisolith features. The carbonate chemistry is dominated by CaCO3 (98% by weight) with minor amounts of MgCO3 (0.7% by weight). Two stable C and 0 isotope transacts sampled at I and 2.5 mm intervals across the laminated Km horizon were analyzed, The comparison of individual laminae within two transacts shows that individual laminae have a distinct isotopic signature and correlate with depth. Bulk isotopic analyses were run on the K horizon and local Cretaceous (Segovia) limestone. 8"O differences between the K horizons (-3.0 to-1.7%o) and local limestone (-6.1 to-4.9[ ]) indicate the Km and K horizon carbonates are the result of soil processes. Transects across limestone clasts within the Km horizon show a 8"O shift of 2-49voo across the soil-rock interface. After ruling out atmospheric mixing and isotopic inheritance from limestone as factors influencing isotopic composition, the 8"C results (-3.4 to 1.4%o) are interpreted as reflecting carbonate precipitation in a C4 vegetative environment. This C4 environment remained dominant throughout Km horizon deposition. These results imply pedogenesis is responsible for the carbonate in the soil. Thus, a calcic soil development index can be response applied to obtain an age-date. The index analysis of the K horizon yields an average amount of secondary carbonate of 92 g/CM2 in the soil resulting in an average soil development age of 180 ︢20 Kyr.