TUFFACEOUS EPHEMERAL LAKE DEPOSITS ON AN ALLUVIAL PLAIN, MIDDLE TERTIARY OF CENTRAL CALIFORNIA

The Oligocene and Miocene Valley Springs Formation is one element in the middle to late Cenozoic alluvial fill of the Central Valley of California and represents a large fluvial depositional system that extended westward from sediment-filled palaeovalleys in the high Sierra Nevada to a piedmont alluvial plain under the present Central Valley. The Valley Springs Formation consists largely of tuffaceous mudrock, tuffaceous sandstone, polymict conglomerate and rhyodacitic tuff. The most prominent lithofacies in the formation is yellowish grey or greyish yellow, tuffaceous mudstone and claystone characterized by crude, commonly wavy layering or bedding, an irregular fracture, and common clay-lined partings, fissures, and small branching tubules. Petrographic and X-ray diffraction analyses of the composition, texture, fabric, mineralogy and organic content of an 8 m thick section of the tuffaceous mudrock lithofacies have provided evidence for an origin quite different from the earlier interpretations of this lithofacies as altered tuffs. Numerous rounded mudstone or claystone clasts in a detrital mud matrix, together with abundant freshwater microfossils (megaspores, chrysophyte cysts, diatoms and sponge spicules), indicates that the lithofacies is a mostly epiclastic sedimentary deposit. Root traces, fissuring, orientated clay skins lining pores and secondary surfaces, and mineralogical data are evidence of periodic exposure and desiccation. The tuffaceous mudrock lithofacies of the Valley Springs Formation, interpreted in the context of the middle to lower regions of a piedmont alluvial system, most probably represents a complex of ephemeral lake and marsh environments on a low gradient alluvial plain. The inferred abundance of shallow lakes, ponds and marshes implies a climate that was wetter than the semi-arid climate of the region today.