Marine geology and geologic evolution of the Northern Channel Islands Platform west of San Miguel Island

An investigation of the northwestern Channel Islands Platform, using geological and geophysical data including seismic reflection profiles, dart core descriptions, and borehole information, was completed. The primary purpose of the study was to determine the Cenozoic geologic history of the western portion of the northern Channel Islands Platform and surrounding continental shelf. Seismic stratigraphic units, generally corresponding to onshore formations, were established and the stratigraphic relationships, thickness variations, and sea floor distribution of these units were defined. Formations studied range in age from Late Cretaceous through Recent and represent deep basin to subaerial environments. From Late Mesozoic through Oligocene time, marine deposition occurred in a shoaling trench environment, associated with subduction tectonics. During Oligocene time a plate/spreading ridge collision resulted in the extensive subaerial erosion and deposition, which occurred during this period of maximum uplift. In early Miocene time, deep sediment starved basins formed as the result of plate boundary readjustments. These basins received dominantly biogenic sediments (the Monterey Formation) until the end of Miocene time. Clastic deposition (the Sisquoc Formation) followed renewed deformation and uplift. Additional clastic facies occur in the “Pico" and younger units in the study area. Pliocene to Recent depositional features include the large scale on-lap of the north margin of the Channel Islands Platform by units originating to the north. Other depositional features include buried and modern submarine channels, terrace deposits, and modern channel levee deposits. Structural features of the study area include: the major anticlinal fold belt and its associated faults, which trend approximately north 45 degrees west, the Ferrelo Fault zone with Cretaceous to late Miocene (or younger) offset, and the Santa Rosa Fault with Quaternary offset. Dip separation along the Ferrelo Fault zone is evidenced by uplift at the southwestern margin Channel Islands Platform in the study area. Strike-slip motion is evidenced on the Santa Rosa Island Fault where it offsets Quaternary units on Santa Rosa Island and the sea floor. Numerous unnamed faults occur in the region and can be placed in two major groups: those that do not cut the late Miocene unconformity and those that cut the unconformity and in some cases offset the sea floor. Geophysical profiles and dart core descriptions from the Miocene and younger units, particularly in the immediate vicinity of the northern Channel Islands Platform, permit the assignment of seismic reflectors to geologic formations. Unconformities between the Monterey, Sisquoc, and “Pico" seismic stratigraphic units are well defined and may be correlative over a large portion of the study area. The investigation has shown that: 1) deformation in the study area was contemporaneous with known regional tectonic events and (2) that it is possible to correlate onshore formations to seismic stratigraphic units. The investigation has also shown that the northern portion of the Channel Islands Platform is structurally related to the Peninsular Ranges Province. The boundary of the Transverse Ranges and Peninsular Ranges Provinces may be along the north margin of the Channel Islands Platform in the study area.