Description
Paleogene marine and fluvio-deltaic strata are exposed on San Miguel and Santa Cruz Islands, in the Santa Ana Mountains, in San Diego, and near Valle de Las Palmas, Mexico. Many of these strata are conglomeratic and contain pebbles, cobbles, and boulders of rhyolitic to dacitic composition. These distinctive, exotic clasts exhibit genetically-controlled textural, color, and mineralogic characteristics which are unique to specific lithologic varieties. Using these genetically-controlled features, four families of siliceous meta-volcanic clasts are recognized: Poway, Black Dacites, Owl Creek, and Las Palmas. Poway and Black Dacite clasts are most abundant in conglomerate assemblages from San Miguel Island, Santa Cruz Island, and San Diego. This similarity of conglomerate clast assemblage supports previous suggestions that San Diego and the Northern Channel Islands were once part of a continguous conglomerate terrain and depositional system. Poway and Black Dacite clasts occur together in abundance; however, all the remaining clast families are almost mutually exclusive. Thus, conglomerate assemblage and depositional system are closely related in all the areas studied. Owl Creek clasts are found primarily in Paleogene strata exposed in the Santa Ana Mountains and are well exposed in the Owl Creek Quarry. The remaining clast assemblage, Las Palmas clasts, are found in greatest numbers in the Eocene Las Palmas Gravels of northern Baja California, Mexico. Petrologic features, primarily conglomerate clast assemblages, are combined with lithostratigraphic correlations in reconstruction of a late Paleocene through Eocene depositional system. Tectonically dismembered remnants of this system are found on San Miguel and Santa Cruz Islands and in coastal San Diego. On San Miguel Island, latest Paleocene basin- starved mudstones rest disconformably on latest Maestrichtian middle submarine fan sandstones. Overlying the Paleocene sequence in apparent conformity are Lower Eocene basin-starved mudstones, fan-fringe siltstones and mudstones, and middle submarine fan depositional and channel conglomerate. These facies are in turn overlain by latest Lower Eocene through lower Middle Eocene shale-filled channels, levees, fan-fringe and starved basin-mudstones. The remainder of the Middle through Upper Eocene strata are braided, middle submarine fan sandstones and mudstones. Paleogene sedimentation on Santa Cruz Island began in the late Paleocene and continued uninterrupted through the entire Eocene. Upper Paleocene strata were deposited on an inner shelf which received detritus washed from paralic environments to the east-northeast. Overlying this facies is a progradational sequence of shelf storm flood-ebb deposits. This sequence of siltstone and fine sandstone is capped by latest Paleocene-Earliest Eocene Poway clast- bearing conglomerate. The remainder of the depositional facies records sedimentation under progressively deepening marine environments These environments include outer shelf, passive slope, fan fringe, outer submarine fan depositional lobes, and inner submarine fan channel, Poway clast-bearing conglomerate. In San Diego, the Mount Soledad Conglomerate represents six sedimentary environments: (1) lagoonal, (2) deltaic, (3) alluvial fan and fluvial channels, (4) submarine canyon head, (5) inner fan channel, and (6) slope. Deltaic facies of the Mount Soledad Formation are proximal equivalents to delta-fringe sedimentary rocks on Santa Cruz Island. Alluvial fan, fluvial channel, submarine canyon head, and inner submarine fan conglomerates are part of the Mount Soledad sequence and represent shelf-edge equivalents to Lower Eocene middle submarine fan conglomerates on San Miguel Island. Comparison of age and bathymetry of the Mt. Soledad Formation and equivalent facies on San Miguel and Santa Cruz Islands, with Tertiary marine cycles suggests that the sequences of marine environmental facies at all these localities developed synchronously in response to global changes in sea level. Late Paleocene and Early Eocene paleogeography is inferred from lithologic and paleontologic analyses of strata and palinspastic adjustments for: 160 to 200 kilometers of right slip on the East Santa Cruz Basin Fault, 90 kilometers of left-slip on the Malibu Coast Fault System, and 4 kilometers of right-slip on the Mount Soledad Fault.
