Abstract The Yellow Bank creek complex (YBCC) is a large, upper Miocene injectite complex, one of numerous injectites northwest of Santa Cruz, California. The feeder for these injectites is the Santa Margarita Sandstone, a shelfal sandstone unit that is also the reservoir rock in several exhumed oil fields. The impermeable cap rock for these oil fields, the Santa Cruz Mudstone, was breached by sand injectites, some of which reached the sea floor. Located near the edge of one of these oil fields, the YBCC is a dike-sill complex that shows evidence for multiple phases of injection by fluidized sand that was initially gas or water saturated and later possibly oil bearing. Vertical injection of a large sand dike along a fracture was followed by lateral injection of a sill from the dike along bedding planes in the Santa Cruz Mudstone. Flow differentiation during injection of fluidized sand into the sill formed centimeter-scale layering in its lower part. Subsequent emplacement of oil into this sand may have occurred by injection and by seepage that displaced pore water, producing sand masses that became preferentially cemented by dolomite. Some evidence suggests that the injection and cementation occurred at relatively shallow burial depths beneath the sea floor, with the injection resulting from a combination of possible seismic shaking and migration of overpressured fluids from more deeply buried parts of the Santa Margarita Sandstone. A pervasive lamination marked by limonite staining developed following uplift and subaerial exposure of the complex, possibly in a groundwater environment.

ABSTRACT Sandstone intrusions are widespread west of Santa Cruz, California and were emplaced during late Cenozoic tectonic deformation of this region. Among these is a very large and complex intrusion which is well exposed along the coastline at Yellow Bank Creek. Here, fluidized sands from the Miocene Santa Margarita Sandstone were injected upward into fractured biosiliceous rocks of the Santa Cruz Mudstone, probably due to faulting and seismic shaking. The complicated internal structure of this intrusion includes sedimentary xenoliths, fluidization structures, and secondary limonite staining. The latter likely occurred during oxidation by groundwater and produced conspicuous, complicated layering which serves to mask and confuse interpretations of the earlier-formed features. Among the earlier formed features are fluidization structures, comprising (1) flow banding which records injection of sands horizontally in silllike areas of the intrusion, and (2) heave structures which reflect mainly vertical injection of hydrocarbons and sands partially saturated with hydrocarbons into water-saturated sands. This latter type of injection appears to have occurred at a hydrocarbon front that was derived from either a localized petroleum accumulation or else from remnants of hydrocarbons that had mostly migrated updip prior to the clastic intrusion event. Dolomitic cementation occurred preferentially in the hydrocarbon-saturated sands due to degradation of the hydrocarbons. Paleotemperature estimates of the intrusive sandstones (by apatite fission track analysis) and of the host Santa Cruz Mudstone (by vitrinite reflectance) indicate maximum temperatures of about 60°C for the former, 50°C for the latter. Our data suggests that initial fluidization began in water-saturated sands of the bioturbated facies in the Santa Margarita Sandstone; following upward intrusion of these sands, fluidization and injection expanded into hydrocarbon-bearing sands within the cross-bedded facies of the same unit.