THE PLIO-PLEISTOCENE MERCED FORMATION IN NORTHERN CALIFORNIA: A WORLD-CLASS EXAMPLE OF INTEGRATED SEA LEVEL AND TECTONIC CONTROLS

The Plio-Pleistocene Merced Formation was deposited in a tectonically dynamic basin and is presently uplifted and exposed in the coastal bluffs between San Francisco and Daly City, California. The unit comprises over 1700 m of marginal marine sediments packaged into over 40 unconformity-bounded, generally progradational sequences. In marginal marine basins, accommodation space is controlled by eustatic sea-level change, vertical motion of the basin floor and sedimentation rate. To investigate the control of eustasy, sedimentation rate and localized tectonic subsidence on sequence formation, we examined the coastal stratigraphy and compared it with available age data for the formation and attempted to match sequence boundaries to the ages of Quaternary sea level high stands indicated by the most recent oxygen isotope curve (OIS) curve. Using a 400 ka and 500 ka age for an ash marker bed in the formation, we created two time-deposition models in which we attempted to fit transgressional sequences to Quaternary sea-level highstands. In the upper part of the Merced, where mineralogical data indicate an increased sedimentation rate, the transgressional sequence boundaries match well with sea level high stands in both models. However, in the lower parts of the section, there are more sequence boundaries than high stands in both models indicating that sequence formation may have been tectonically driven. During deposition of the lower part of the formation, the Merced basin was likely more sensitive to minor sea-level fluctuations because of tectonically driven subsidence, whereas transgressional sequence boundaries in the upper section, during the time of increased sedimentation appear to have been driven by eustatic changes. Throughout this time period, subsidence within the basin was also beginning to slow and, during deposition of the uppermost sequences, contraction was beginning to change the basin into the uplifted block that is currently exposed to erosion along the sea cliffs.