We observed a simple characteristic log signature for the Bt horizon in outcrop geologic and geophysical observations that was readily correlated to logs in a nearby well at Sunland Park, New Mexico. The Bt horizon approaches 1 m. thickness and the high clay-size fraction is readily recognizable by a very low resistivity,resistance, or neutron log countrate. The distinguishing characteristic is a strong gradient in the total gamma ray (low at the top to high at the bed bottom) where resistivity or neutron logs indicate a uniform bed. This gradient in the total natural gamma ray is weakly supported by spectral gamma ray measurements on outcrop. The gradient is consistent with the leaching processes that form the soil horizon: transport of fines with water infiltration, dissolution and migration of potassium during feldspar alteration, and migration of uranium decay products.
This characteristic paleosol signature has been used in correlations in two Plio-Pleistocene environments. In a landfill site characterization near Sunland Park, New Mexico (near El Paso, Texas) stacked paleosols formed on overbank deposits were laterally correlated into coarse grained fluvial channels. In a site characterization study for Sandia National Laboratory, the paleosol signatures on logs aid correlation from fluvial to alluvial fan deposits, permit subdivision of alluvial fans into likely fluid flow units, and reflect the large-scale geometry of block movements along basin-bounding faults.