Middle Pleistocene IRSL age of the upper Blackwater Draw Formation, Southern High Plains, Texas and New Mexico, USA

A pilot study has yielded the first infrared stimulated luminescence (IRSL) age from the upper part of the eolian Blackwater Draw Formation near its type locality in the Southern High Plains of Texas. The age on potassium feldspar is 294 ± 32 ka (50°C IRSL) and 347 ± 40 ka (290°C post-IR IRSL) and averaged 300 to 350 ka. Both ages are significantly earlier than previously reported thermoluminescence (TL) ages. A Middle Pleistocene age of the upper Blackwater Draw Formation is consistent with its mature argillic soil Bt horizon and the overprint of a well-developed calcic soil Bk horizon. The deposition of the Blackwater Draw Formation also pre-dates the Mescalero sand sheet of southeastern New Mexico, a late Pleistocene component of which has been miscorrelated with the Blackwater Draw. The tendency to regard all red eolian sands in the region as the Blackwater Draw Formation may be mistaken, overlooking younger red eolian sand bodies with less mature soil development.