GROUNDWATER-DISCHARGE DEPOSIT IN FENNER WASH, EASTERN MOJAVE DESERT

A 1.6-m-thick sequence of basal limestone overlain by silty, locally crossed-bedded calcareous sand, topped by organic-rich mud formed in a groundwater-fed wetland system that existed in the eastern Mojave desert in late Pleistocene(?) time. The poorly and patchily exposed sequence has a minimum wide of 1 km and is located on a broad alluvial surface in Fenner Wash, in the mouth of a narrow junction with a ~3200-km² drainage area. The chalky, white, ~35-cm-thick carbonate units contain locally abundant gastropods, ostracodes, and diatoms. Calcified roots and stems of vascular plants also occur in these rocks. The gastropod assemblage, Fossaria sp., Gyraulus sp., and Pupillidae, is typical of those found in wetlands dominated by groundwater discharge. The d¹⁸O (-6 to –8 ‰) and d¹³C (-2 to –4.5 ‰) values of the micrite and d¹⁸O (-6 to –8 ‰) and d¹³C (-7 to -9 ‰) values of the gastropods differ from those of Quaternary pedogenic and lacustrine carbonates in nearby locations in the Mojave region. The modern water table in area ranges from 50 to hundreds of meters deep. The Fenner Wash sequence formed when the water table was at or near the land surface. Carbonate deposits formed in small, perennial ponds during the period of maximum effective moisture. The water table lowered slightly as aridity increased, and silty sands accumulated in shallow stream channels, possibly spring-fed, that crossed a phreatophyte flat. The organic-rich clayey mud that caps the sequence formed in a marsh or wet meadow when the water table rose again to a level near the surface.