Climate Driven Depositional Events in the Middle to Late Pleistocene Manix Formation, Central Mojave Desert, California

G. T. Jefferson
Colorado Desert District Stout Research Center, California Department of Parks and Recreation Anza- Borrego Desert State Park, Borrego Springs, CA

The Manix Formation is comprised primarily of fluvial and lacustrine sediments that were deposited within and adjacent to pluvial Lake Manix. The lake, covering approximately 236 km² at high stands, was probably the largest along the Mojave River drainage system, which headed on the north slope of the Transverse Ranges of southern California and ran northeast to Death Valley.

The Formation, with an exposed thickness of 40 m, is geologically mappable as four laterally equivalent members. Generally from oldest to youngest these were deposited as: basin margin fanglomerates, alluvial and fluvial deposits, lacustrine and paralimnic deposits, and fluvial/deltaic deposits. The interfingering relationships of these sediments, especially between the lacustrine and fluvial systems, documents at least four major transgressive/regressive lacustrine events within the last 0.5 m.y. These are largely climatically driven with highest lake levels, 543 m above mean sea level, the result of increased flow along the ancestral Mojave River. Transgressive lacustrine events are in phase with relatively cool/moist regional climatic conditions inferred to be present during even-numbered marine oxygen isotope stages. The chronology of the deposits is relatively well constrained by paleomagnetic data, U/Th series ages, tephra chronology, and ¹⁴C ages. Given the available dates and stratigraphic placement, it is suggested that such events occurred during marine oxygen isotope stages 14, 12, 8, and 6.

Late in lake history, between about 60 and 19 ka, the ancestral Mojave River deposited a fan delta across the west-central margin of the clover-shaped lake basin, dividing it into northern Coyote, southern Troy and eastern Afton arms. Pulses of deposition along the prograding delta front were not necessarily climate driven or the result of changes in the upper drainage system. Lake Manix breached the east end of the Afton sub-basin about 18 ka ago, possibly during isotope event 2.2, initiating incision of the Mojave River and erosion of the Manix Formation to its present badlands topography.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California