Abstract: Sedimentation Response to Earthquake-Related Events, Middle Eocene Ventura Basin, California

Michael S. Clark

Middle Eocene strata in the Topatopa Mountains, northeastern Ventura Basin, California, were deposited as five unconformity-bounded depositional sequences in a seismically-active basin characterized by rapid, episodic subsidence. Analysis of stratigraphic geometries, detailed facies analyses, and backstripping-derived subsidence rates indicate that tectonically-induced differential subsidence caused basin-wide migrations of the Topatopa depocenter at least every 2 m.y.

Unusually abundant convolute laminations and a slumped interval may record the influence of earthquakes. The convolute laminations do not grade downward into ripple laminations and are not associated with dewatering dikes and pipes as would be expected if the laminations were formed by shear from an overlying current or by dewatering during rapid burial. Thus, formation during shock-induced liquefaction is more likely. Also, a 20-m thick slumped interval in the uppermost Cozy Dell Formation underlies a sequence boundary interpreted as a surface across which depocenter migration took place. Association of this slumped interval with a tectonically-formed surface is consistent with deposition in a seismically-active environment.

Rapid, differential subsidence in the Topatopa depocenter was probably episodic and associated with seismic events. Rapid, episodic subsidence with attending earthquakes is recorded in Holocene strata of the Humboldt basin, an analog to the Topatopa depocenter. Also, a Middle Tertiary slumped interval in the southernmost San Joaquin basin is attributed to a seismic event. Similar earthquake-related events are recorded by sedimentation patterns in the Middle Eocene Ventura Basin and may be evident in the strata of other active-margin basins as well.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994