New Mexico Geological Society Annual Spring Meeting — Abstracts

The Nacimiento Formation has been divided into three members based largely on exposures of the unit at Mesa de Cuba and Torreon and Escavada Washes. These members are, in ascending order, the Arroyo Chijuillita Member, the Ojo Encino Member, and the Escavada Member. Study of the Nacimiento Formation on outcrop and in the subsurface to the north as far as the San Juan River indicates that the Escavada Member is present, but the lower two members become increasingly indistinct northward. A possible new member consisting of a thick, sheet sandstone underlies the Escavada Member in the Kutz Canyon area. Correlation to the time scale using magnetostratigraphy and biostratigraphy based on fossil mammals indicates that accumulation rates of the Nacimiento Formation decreased through the Paleocene, concurrent with a general increase in average grain size upsection. Changes in thickness of the Nacimiento Formation, which varies from about 260 m to 470 m in the study area, can be attributed primarily to intraformational thinning toward the basin margin. Much of the rapid variations in thickness of the Escavada Member in the Mesa de Cuba area can be explained by erosional scouring at the base of the San Jose Formation on local structural highs rather than a more regional angular unconformity at the basin margin. The hiatus separating the Nacimiento Formation and the basal San Jose Formation in the southern San Juan Basin is significant and estimated to be between 3 and 6 million years.