A new age constraint for the Early Triassic Alcova Limestone (Chugwater Group), Wyoming

The Alcova Limestone is a widely exposed but poorly studied Triassic unit found throughout Wyoming. Although it is used as a regional lithostratigraphic marker bed, the age of the limestone is not well constrained and contention regarding its depositional nature (marine vs. non-marine saline lake) has not been adequately resolved, although the marine hypothesis is more widely accepted. Currently, its age is based on the occurrence of a single basal pistosaurid, Corosaurus alcovensis. We provide the first Sr-87/Sr-86 analysis of the Alcova Limestone. The data strongly support a marine origin for this unit. Furthermore, the data provide an age range, based on their placement on the marine strontium curve, that limits the deposition of the Alcova Limestone to the latest Olenekian to earliest Anisian (Early Triassic-Middle Triassic boundary). This placement is consistent with the timing of the Spathian regression recorded in other transitional marine to non-marine strata of the western USA. (C) 2015 Elsevier B.V. All rights reserved.