New Mexico Geological Society Annual Spring Meeting — Abstracts

The piedmont facies of the Palomas Formation (santa Fe Group) exposed on the western shore of Elephant Butte Reservoir at Monticello Point (NE1/4, sec. 35, T11S, R4W, Sierra county) contains two intervals of lacustrine deposits. The older interval lies above 26+ m of grayish orange and yellowish gray, laminar and trough-crossbedded sandstone and basalt-cobble conglomerate that represent alluvial-fan deposits. This lacustrine deposit is 6 cm of very pale orange to pale yellowish orange charophytic limestone that extends more than 0.5 km on strike. It is overlain by another 4.5 m of alluvial-fan deposits followed by 0.75 m of grayish yellow and grayish orange, laminar clay and very finegrained
quartz sand. These deposits represent the onset of the second lacustrine episode and are overlain by 4-7 m of white to very pale orange, clayey diatomite. This diatomite crops out over an area of 0.6 square km, and is also exposed on the eastern shore of Elephant Butte Reservoir, 6 km to the east, in sec. 28, T11S, R3W.

The thin, charophytic limestone of the lower lacustrine interval consists mostly of two types of Chara and also contains foraminiferans, ostracodes and gastropods. Foraminifera, unusual in lacustrine deposits, pertain to the calcareous species Ammonia beccarii. The ostracode taxa are Candona truncata, C. bretzi, Physocypria pustulosa, Cyprideis salebrosa and Limnocythere staplini. These taxa have a temporal distribution of Pleistocene- Recent, prefer water of high salinity and are typical of permanent bodies of water. The gastropod Physa cf. P. virgata from the charophytic limestone is an aquatic taxon that generally prefers ponds, swamps or slow-moving water.