Genesis of soils and carbonate enriched horizons associated with soft limestones in Central Texas

Abstract

Soils associated with soft Cretaceous limestones occupy more than 3 million hectares in Central Texas and are important to the state's agricultural rangelands. Most of these soils occupy moderately to strongly sloping erosional landforms with interbedded weakly- to strongly-indurated limestone parent materials. Fourteen pedons from four hillslopes located in a north-south gradient across the study area were sampled to evaluate the effects of climate, topography, and parent material on pedogensis, with emphasis on calcic and petrocalcic horizons. The primary effect of hillslope position on genesis of the soils is through site stability. No consistent relationships were found between expression of carbonate-enriched horizons and climatic or topographic gradients. Geologic erosion truncates these soils and keeps them developmentally young. Radiocarbon dates indicate that soils on summit positions are <20,000 years old; therefore, soils on backslopes must be younger. While long-range dust is being added to the surface at a rate of 1-2 mm/100 years, there is little evidence for long-term accumulation based on quartz grain morphology observed with SEM. Soft white segregations, pendants on pebbles, films and threads, and petrocalcic horizons were identified as pedogenic by stable carbon isotope analysis, micromorphology and radiocarbon dating. Limestone lithics were differentiated by fossils of marine organisms. Stable carbon isotope analysis indicated that 4-90% of the carbonates were pedogenic. Most of these pedogenic carbonates were finely-divided and do not meet calcic horizon criteria. Carbonate microfabrics in Bk and Bkm horizons were convoluted, laminar, and granular. Proposed genesis of these microfabrics suggested that limestone dissolution and reprecipitation are more important to their development than translocation. From reconstruction analysis, there is a net carbonate loss in these soils relative to parent limestones. Indurated limestones within the wetting front are altered to petrocalcic horizons without substantial accumulation of translocated carbonates. Calcic horizons are formed by similar processes. Thus, litholohy serves as the dominant pedogenic factor in soils of this region.