ABSTRACT We present a tephrochronologic/chronostratigraphic database for the Mount Diablo area and greater San Francisco Bay region that provides a spatial and temporal framework for geologic studies in the region, including stratigraphy, paleogeography, tectonics, quantification of earth surface processes, recurrence of natural hazards, and climate change. We identified and correlated 34 tephra layers within this region using the chemical composition of their volcanic glasses, stratigraphic sequence, and isotopic and other dating techniques. Tephra layers range in age from ca. 65 ka to ca. 29 Ma, as determined by direct radiometric techniques or by correlation to sites where they have been dated. The tephra layers are of Quaternary or Neogene age except for two that are of Oligocene age. We correlated the tephra layers among numerous sites throughout northern California. Source areas of the tephra layers are the Snake River–Yellowstone hotspot trend of northern Nevada, southern Idaho, and western Wyoming; the Nevadaplano caldera complex of central Nevada; the Jemez Mountains–Valles Caldera in northwestern New Mexico; the Southern Nevada volcanic field and related source areas in eastern California and west-central Nevada; the Quien Sabe–Sonoma volcanic centers of the California Coast Ranges; and the young Cascade Range volcanic centers of northeastern California and Oregon.

ABSTRACT Mohawk Valley lies in northeastern California on the margin of the northernmost Sierra Nevada and was occupied by Mohawk Lake during much of the middle and late Pleistocene. Throughout that time, the Sierra Nevada ice cap repeatedly extended northward into Mohawk Lake, and ice-contact deltaic sediments were deposited along the valley margins and in the valley bottom. Nearly 200 m of lacustrine and deltaic sediments are now well exposed along streams draining the Sierra Nevada. Tephra beds deposited within the deltaic sediments allow correlation of stratigraphic sections around the valley margin and, together with geomorphic evidence of former lake levels, permit interpretation of a Mohawk Lake history as far back as 740 ka. Mohawk Valley changed from a through-flowing fluvial setting to an intermittent closed basin sometime before 740 ka. After this change occurred, relatively small lakes intermittently formed in Mohawk Valley until ca. 600 ka, when the lake dramatically deepened. Mohawk Lake fluctuated in size over the next ~400,000 yr and increased in size to its highest levels after ca. 200 ka, possibly due to drainage integration with the upstream Lake Beckwourth. After this time, Mohawk Lake spilled over its westward sill, incrementally eroding and lowering lake levels until Mohawk Lake was emptied by ca. 7 ka.