ABSTRACT

Three large submarine mass-movement deposits at the base of the Venado Formation (Cenomanian-Turonian) in the Sacramento Valley sequence consist of allochthonous igneous and sedimentary boulders up to 90 meters long embedded in a matrix of pebbly mudstone. The sedimentary boulders were derived from a shallow water marine sequence which overlay plutonic and metamorphic basement on the Cretaceous continental shelf. These rocks are now deeply buried beneath younger Cretaceous and Cenozoic sediments to the east. During the late Cenomanian and early Turonian, tectonism affecting the shelf initiated at least three episodes of large scale submarine mass-movement. Debris originating on the western edge of the shelf moved westward across the Cretaceous continental slope as a series of slides a d mudflows. The main avenue of sediment movement was a submarine canyon cut into the outer shelf and slope. The larger boulders were deposited on the upper continental rise near the canyon's mouth while smaller debris moved within mudflows as far as 25 kilometers out onto the rise. Below the canyon's mouth the rise was probably fan shaped, but a marked southerly dipping paleoslope resulted in a highly asymmetric fan, sharply bounded to the north and extending as a broad plume of sediment to the south and southwest.

Each major episode of mass-movement was followed by an interval of grain flow and turbidity current movement through the canyon. The same earthquakes which triggered sediment movement in the canyon also initiated turbidity currents on the continental slope away from the canyon. North of the canyon, these currents moved westward off the slope, turned south, and merged with the larger flows debouching from the canyon.