ONCOLITE-ASSOCIATED LITHOFACIES SEQUENCES AND CLIMATE FLUCTUATIONS OF THE LACUSTRINE LATE PLEISTOCENE MANIX FORMATION, CALIFORNIA

Oncoids in ancient lake systems are important sources of information about lake depositional systems. They provide insights into regional climate, tectonics, and physiography, and they provide a barometer of lake system dynamics and chemistry. Two oncolite units occur within the Late Pleistocene Manix Formation, a predominantly lacustrine sequence that formed in pluvial Lake Manix, east of present day Barstow, California. The oncolites marked the initiation of two major lake transgressions. The lake transgressions deposited lithofacies successions composed of fanglomerate, oncoids in a sandstone matrix, sandstone, and claystone. This sequence was deposited in response to the initiation of a wetter climate phase and concurrent lake deepening.

The oncoids are associated with a high-energy, near-shore environment as indicated by well-sorted, well-rounded quartz sand that forms the matrix of the oncoids. Oncoids were buried by beach and long shore sand as the lake deepened. The sand facies was succeeded by claystone as a deeper, quieter lake depositional regime climaxed the sequence.

The angularity of the clasts encrusted by oncoidal carbonate suggests that transgressions occurred fairly rapidly. Lack of cobbles in the succeeding sand indicates that the lake was deepening without the local input of conglomerates.

Conditions favorable for oncoid formation were related to the high calcium-bicarbonate input associated with the increase in precipitation and supersaturated ground waters that moved down and through the fans to emerge at the lake edge. However, the lake water was not saturated enough to move it's chemistry into a carbonate depositional regime. Additional carbonate sediments such as ooids and calcimicrite are uncommon.

Minor regressions occurred during the major transgressive phases as indicated by reworking of some oncoids into channels, and vugs and corrosion surfaces in the oncoids.

Lake Manix is one of a number of lacustrine examples in which oncoids and stromatolites mark the base of climatically induced lake transgressive sequences (e.g., Eocene Green River Formation).