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Description
Santa Rosa Island, one of the northern Channel Islands off Santa Barbara, California, lies at the northern limit of the southern California Continental Borderland. Exposures of Middle Eocene strata on the southern coast of the island were examined in order to define vertical and lateral facies changes useful in the delineation of Middle Eocene depositional history. Two populations of granule- to pebble-sized "microclasts" were studied with regard to clast lithotype affinities. The results of these microclast counts allow comparison of Santa Rosa Island's Eocene clast populations to the characteristic Eocene conglomerate clast populations found on the mainland and the other islands of the Borderland. The preponderance of Poway rhyolites found in the Santa Rosa Island clast population corresponds to San Miguel Island, Santa Cruz Island, and San Diego. This Poway clast affinity supports the suggestion that the outer three of the northern Channel Islands, including Santa Rosa Island, lay offshore from San Diego during the Eocene, receiving clastic sediment input from the same Poway clast-dominated fluvial system. The South Point sandstones and Cozy Dell mudstones were deposited in bathyal submarine fan and lower slope/basin plain environments. Paleontological age dates, based on nannofossils and foraminifera, indicate that deposition occurred entirely within the Middle Eocene. Two retrogressive sedimentation cycles were defined on the basis of facies progression. The first retrogressive cycle started prior to the early Middle Eocene and terminated during the middle Middle Eocene. This cycle is seen in the gradual transition from stable middle fan channel deposition to outer fan lobe and finally fan fringe/starved basin deposition. The second cycle followed the reactivation of the submarine fan late in the middle Middle Eocene. Braided middle fan channel deposition was followed by outer fan lobe deposition and finally fan fringe/starved basin sediments during the late Middle Eocene. This final abandonment resulted in deposition of the Cozy Dell mudstones. Both retrogressive submarine fan cycles correspond with intervals of global eustatic sea-level rise. Submarine fan reactivation corresponds to abrupt drops in eustatic sea level. Therefore, eustatic sea level appears to have been the major factor influencing changes in marine deposition off San Diego during the Middle Eocene. The depositional history and microclast lithotype affinities described above serve to refine the positioning of the northern Channel Islands off San Diego during the Middle Eocene. Paleocurrent and, in part, paleomagnetic evidence suggests a 60° to 90° clockwise rotation of Santa Rosa Island and San Miguel Island since the Eocene. When this rotation is moved, San Miguel, Santa Rosa, and Santa Cruz Islands exhibit west to southwest Eocene paleocurrent trends. The resulting alignment is supported by lower Middle Eocene inner fan deposits on Santa Cruz Island, middle fan deposits on Santa Rosa Island, and outer fan to basin plain deposits on San Miguel Island.
