DEPOSITIONAL AND DEFORMATIONAL HISTORY OF THE COLMA AND UPPERMOST MERCED FORMATIONS, SOUTHWEST SAN FRANCISCO

Age constraints for coastal exposures of the Pleistocene Colma and uppermost Merced Formations have previously been meager and primarily correlative, creating uncertainty about the depositional history of the units as well as the timing of fault activity responsible for lifting them to elevations up to 70 m along the northwestern coast of the San Francisco Peninsula. Recent studies have suggested that Merced and Colma deposits, as well as younger Holocene inset channel deposits, show fault-propagation fold growth and tilting related to activity on the nearby Serra fault. We dated sediments of the uppermost Merced Formation (sequences X and Y) and the Colma Formation using the optically-stimulated luminescence (OSL) technique in order to constrain the timing of onset of fold growth associated with the Serra fault and calculate late Pleistocene uplift rates for the area by correlating the deposits to specific paleo high stands of sea level. We also reassessed the stratigraphy of the uppermost Y-Sequence of the Merced Formation and comprehensively examined the stratigraphy of the Colma Formation in order to better understand the context of their deposition. Field investigations show that the Merced Y-sequence is a regressive sequence, deposited either during a time when sea level was decreasing or when the coastline was prograding due to increased sediment supply. The Colma Formation contains nearshore, foreshore, and backshore deposits repeated several times within its thickness. Most of these deposits form short regressive sequences, where either foreshore deposits transition upward into backshore deposits, or where nearshore deposits transition directly upward into backshore deposits. The main factor responsible for creating these minor regressive sequences was most likely episodic subsidence followed by depositional progradation. We will present new age data and refined uplift rates for the Colma and uppermost Merced Formations, and will use this information to further characterize the local tectonic environment and detail a history of deposition and deformation for the units.