ORIGIN OF RELICT THICK SOILS ON THE EDWARDS PLATEAU, CENTRAL TEXAS

Thick soils on isolated uplands of the Edwards Plateau in central Texas are interpreted to be relicts of a formerly more extensive soil cover that was eroded during the late Pleistocene to middle Holocene. The relict, thick, clay-rich soils are most commonly found on the relatively pure Cretaceous Edwards Limestone, which suggests that these soils did not form from the weathering of the underlying limestone bedrock. Instead, these thick soils may have formed from silicate material derived from eolian dust, alluvium, or from a locally-eroded, stratigraphically-higher, more clay-rich unit.

In order to better constrain the parent material for these soils, we investigate the geographic distribution of the relict soils and compare the grain size, clay-sized mineralogy, and the neodymium (Nd) isotope composition of the soils with potential silicate sources. Our results indicate that the modern thin soils and the relict thick soils are distinct in their geographic distribution, texture, mineralogy, and Nd isotope composition. Thus we interpret that the relict and modern soils formed from a different parent material.

We propose that the silicate material in the relict thick soils was derived from the Del Rio Clay, a stratigraphically-higher, but locally-eroded clay-rich strata. Evidence for this interpretation includes: 1) the thick soils have a Nd isotope composition, mineralogy, and texture, that is most similar to the Del Rio Clay compared to other potential silicate sources, and 2) independent of elevation, the thick soils occur mainly over the upper Edwards Limestone and are relatively absent on the statigraphically-lower Glen Rose Limestone. This soil-bedrock stratigraphy is best explained by in situ weathering of the Del Rio Clay, along with partial weathering of the upper portion of the chert-rich Edwards Limestone, to form thick, clay- and chert-rich soil accumulations on the more resistant Edwards Limestone. These results provide new insights into soil forming processes and landscape evolution in the Edwards Plateau and other karst terranes.