QUATERNARY TEPHROCHRONOLOGY AND STRATIGRAPHIC CORRELATION IN THE SACRAMENTO-SAN JOAQUIN DELTA, NORTHERN CALIFORNIA

The Sacramento-San Joaquin Delta is the hub of a major water transport infrastructure that supplies a large part of California. Such infrastructures are vulnerable to levee failure and liquefaction hazards associated with the subsurface Delta stratigraphy. However, stratigraphy of Quaternary Delta deposits is partially constrained, especially for pre-Holocene strata. Three tephra have been identified thus far in 26 boreholes in the northern Delta and underpin chronostratigraphic correlations and paleoenvironmental interpretations. The U.S. Geological Survey Tephrochronology Project Laboratory analyzed three samples obtained from California Department of Water Resources (CDWR) boreholes near Hood and Walnut Grove, California. The major element compositions of the glass shards cluster tightly for the samples, indicating that these layers are composed of tephra from discrete volcanic eruptions. We correlate these samples to dated Cascades eruptions based on glass chemistry. The Rockland ash, which erupted from the southern Cascades near Lassen Peak ca. 0.575 Ma, was identified at depths between 132.25 and 143.5 feet from a borehole near Hood. Unanalyzed tephra have also been observed at similar depths from 11 boreholes drilled nearby. The Loleta ash, which erupted from the central Cascades, Oregon ca. 0.40-0.37 Ma (probably near the Three Sisters), was encountered at depths between 174.1 and 174.8 feet in a borehole along the Sacramento River southwest of Walnut Grove. A third unnamed tephra recovered at a depth of 197.3 feet from a borehole near the Mokelumne River northeast of Walnut Grove correlates geochemically with a volcanic ash layer from the Ferndale area, California, that is stratigraphically younger than the Rio Dell ash (1.45 Ma) and older than the ca. 0.78 Ma Brunhes/Matuyama paleomagnetic boundary. These ages and subsurface depths suggest that sedimentation rates are not consistent, and deposits may thicken in some areas. We use these ages to interpret stratigraphy and map chronostratigraphic surfaces in three dimensions. Tephra units in cores retained by CDWR provide evidence for environments of deposition and mechanisms of transport. This chronologic framework could provide additional insight into distribution of subsurface deposits in the Delta.