New Mexico Geological Society Annual Spring Meeting — Abstracts

Several thin carbonate and chert units of unknown origin occur within fifteen to twenty meters of section in a clastic succession in the Popotosa Formation near CaC*oncito de las Cabras. The two most continuous units are 20 to 60 cm thick and can be followed over several kilometers. They have not been studied in detail and have the potential to provide significant information about local depositional environments. A preliminary study was made of the upper, most fossiliferous unit in order to determine depositional environment. Data was collected by examining features of the unit in outcrop, hand sample and thin section, with study concentrated at three type locations representing different ratios of chert to limestone. Numerous fossils were found, including thinshelled ostracods, pelecypod fragments, root casts, plant debris, and unidentified large and small tube shaped fossils, which are suggestive of some type of encrusted aquatic plant. Other features observed include small-scale compositional layering and solution breccias. The presence and distribution of these features suggest deposition in a lacustrine environment of moderate salinity, with littoral and profundal environments represented, although other environmental influences (e. g. springs) may have been present.