RECONSTRUCTING THE HISTORY OF SACRAMENTO-SAN JOAQUIN RIVER DRAINAGE FROM THE MERCED FORMATION, CALIFORNIA

The Sacramento and San Joaquin Rivers make up the largest watershed in California, draining the Sierra Nevada and the Central Valley before converging and flowing into the San Francisco Bay. The Pleistocene Merced Formation south of San Francisco records the initiation of this modern drainage pattern as an abrupt provenance shift, from sediments with heavy mineral assemblages dominated by a local Franciscan source to sediments with assemblages dominated by a Sierran granitic source. An intriguing hypothesis links this provenance shift and establishment of the Sacramento-San Joaquin River system to rapid draining of a large lake (Lake Clyde) recorded by widespread lacustrine clays in the Central Valley. The presence of the Lava Creek B ash bed near the top of Lake Clyde sediments constrains the end of lacustrine deposition to ~630 ka, setting up a prediction of a roughly synchronous age for the Merced Formation provenance shift. However, evaluation of this hypothesis is limited by sparse chronostratigraphic constraints through the Merced Formation itself. In this study, we generated new paleomagnetic data which demonstrate that the Merced Formation preserves a primary detrital remanent magnetization that can be used to develop magnetostratigraphy. These data, along with maximum depositional ages developed through detrital zircon geochronology, provide new chronostratigraphic constraints that inform an updated age model for the deposition of the formation. This new age model provides a quantitative estimate for the age of the Merced Formation provenance change, allowing us to test the hypothesis that initiation of Sacramento-San Joaquin River drainage closely followed the draining of Lake Clyde. This work contributes to an improved timeline of the establishment of the Sacramento-San Joaquin River system and the development of the modern-day physiogeography of the San Francisco Bay region.