ROSSELIA ICHNOFABRIC IN THE MIOCENE PULLEN FORMATION, NORTHWESTERN CALIFORNIA: IMPLICATIONS FOR THE INTERPRETATION OF REGIONAL TECTONICS

The occurrence of the Rosselia ichnofabric in the basal sandstone beds of the late Miocene Pullen Formation (basal Wildcat Group) has the status of an “ichnologic silver bullet”—it is the one piece of irrefutable evidence that supports our interpretation of a littoral to inner shelf depositional setting for the lowest division of this formation. These sediments were deposited on a wave-cut platform developed along a steep coastline. Our study reconfirms the utility of the Rosselia ichnofabric in the identification of inner shelf depositional settings having high net deposition rates, represented by the basal Pullen sandstone beds; but beyond this we demonstrate how recognition of this unique ichnofabric can inform larger-scale tectonic interpretations. The correct interpretation of paleoenvironment in the case of the lowest Pullen beds confirms the view that rapid regional subsidence occurred shortly after their deposition. This allows us to eliminate a previously published alternate hypothesis that the initial accumulation of Pullen sediments occurred at bathyal depths on an inner trench slope.