TECTONIC SUBSIDENCE AND CYCLIC QUATERNARY DEPOSITION CONTROLLED BY CLIMATE VARIATION, SANTA CLARA VALLEY, CALIFORNIA

Eight alluvial sequences that are correlated with climate cycles back to marine oxygen isotope stage 18 (~710 ka) define remarkably regular and rapid subsidence with a long-term rate of 0.38 mm/yr.

Sedimentary layering determined from geophysical logs and cores in two deep wells (CCOC and GUAD) defines upward-fining cycles 22-54 m thick. The erosive bases of the cycles are sequence boundaries that we correlate with sea-level lowstands to yield an average cycle duration of 100 ka. The correlation reference here is the topographic surface prior to each depositional cycle, which in the valley axis was controlled by stream gradients responding to falling sea level. Deposits of the cycles would have formed as local climate warmed and crossed the threshold between stable hillside weathering and change in vegetation that released sediment rapidly from the slopes. Initial supply was high, as is indicated by the 22 m of Holocene section that has already accumulated in only 20% of a typical cycle. Correlation of the sequence boundaries with sea-level lowstands defines the subsidence curve; the simplicity of that curve helps confirm the correlation. The erosive base of the uppermost sequence is bracketed between C14 ages of 12,400 and 32,850 yrs, indicating that the Holocene pulse of alluviation began about 12,500 ybp and that the preceeding unconformable hiatus represents less than 20,000 yrs, only 20% of typical cycles. The only direct age control for the rest of the section comes from identification of the 780 ka Bruhnes/Matuyama boundary (Mankinen and Wentworth, this session) just below the eighth cycle in CCOC (305 m) and GUAD (291 m).

The early geomorphic setting in each cycle must have involved rapid denudation of hillsides and rapid downstream progradation and accumulation of sediment in response to the abundant supply. Later, as supply slackened, streams entrenched and sediment accumulated more gradually as subsidence continued. CCOC and GUAD are located in the fluvial axis of the valley, the position of which has persisted throughout accumulation of the section (Wentworth and others, this session) thus making erosive bases of the cycles probable. Estuarine influence during high sea stands did not penetrate deeply into the valley, indicating that continued deposition kept pace with subsidence.