Rb-Sr dating of a paleosol, Llano Uplift, Texas

A red, oxidized, clay-rich paleosol is locally developed upon 1.2 Ga old Packsaddle Schist in the eastern Llano Uplift, central Texas. This soil, which differs from organic-rich soil of recent origin, evidently formed under moist tropical conditions. Bounding the area on the east side is a retreating escarpment of early cretaceous limestone resting directly upon basement; thus the age of the paleosol must be late Cretaceous or younger. Ultra-fine fractions (<0.05 micrometers) from the paleosol consist of clay high in Al2O3 (30 wt%) with very low K2O (0.7%). Aliquots were treated with solutions of ammonium acetate, silver thiourea, or dodecyl- or hexadecylammonium chloride to displace the loosely-bound Rb and Sr present in clay interlayers. These reagents remove all readily exchanged ions immediately, and their attack on the clay structure continues to an extent that depends upon the reagent and duration of the application. The result, after chemical treatment, is a Rb-Sr isochron based upon data from a single sample. The isotopic data argue for a consistent interpretation. Initial 87Sr/86Sr at different sample sites is high and variable, as expected in a soil that weathered from ancient, inhomogeneous bedrock. The age of ultrafine material from a locality 8 km form the Cretaceous escarpment is 51 +/- 3 Ma, whereas at a site 3 km from the escarpment the age is 27 +/- 1 Ma. However, the apparent age of the 0.2-0.1 micrometer size fraction from the latter sample is 50 +/- 4 Ma, suggesting inheritance of radiogenic Sr from the substrate