Abstract

The Merced Formation is a Pleistocene sedimentary succession of shallow marine and coastal deposits exposed just south of San Francisco, California. Merced Formation strata record a change in sedimentary provenance from local Franciscan Complex sources to detritus from the relatively distant Sierra Nevada batholith. This provenance change is interpreted to represent the establishment of the modern Sacramento–San Joaquin river system, which today drains into the San Francisco Bay estuary. The provenance change was originally identified in a rich unpublished heavy-mineral data set, which we have digitized and reanalyzed using modern statistical approaches. We further confirm this provenance shift with targeted heavy-mineral data developed through quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN). While this provenance shift is striking in heavy-mineral data, it is more subtle in detrital-zircon data due to extensive overlap between zircon age populations of the Franciscan Complex and the Sierra Nevada batholith. New magnetostratigraphic data place constraints on the position of the Brunhes–Matuyama reversal in the succession. Using these magnetostratigraphic data and the 598 ± 13 ka Rockland ash above the provenance shift to develop a Bayesian age model gives an estimate of 633 ka for the age of the provenance shift with a 95% highest density interval between 708 and 608 ka. This timing matches the prediction that the establishment of the Sacramento–San Joaquin river system occurred following the draining of a large paleolake (Lake Clyde) in the California Central Valley, allowing Sierran detritus to reach the California coast.