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Description
Upper Cretaceous sandstone, mudstone, and conglomerate exposed as an eastward dipping homocline at the western end of San Miguel Island record deposition in the inner- and middle-fan regions of a submarine fan system. Outer-fan and basin-plain sedimentary rocks are absent while thick sections of mudstone-dominated strata deposited in fan-fringe areas abound. Because the basin into which these submarine fans accumulated was elongate perpendicular to the depositional dip, finer grained sediments normally bound for outer-fan and basin-plain environments were redirected laterally to fan-fringe areas. Fossiliferous mudstones exposed on San Miguel Island indicate that deposition began as long ago as Turonian and continued into the Maestrichtian. Campanian through Maestrichtian age rocks in San Diego County record deposition in inner- and middle-fan and fan-fringe areas of a submarine fan in addition to slope mudstone and shallow marine mudstone, sandstone, and conglomerate. These rocks afford a unique view of a tectonically formed basin in which submarine fan sediments accumulated. Down-dropping of the basin occurred during Middle Campanian time along a series of faults which lie in the approximate position of the Quaternary Rose Canyon Fault Zone. Comparison of sandstone petrologies of Upper Crestaceous submarine fan strata at San Miguel Island to those in San Diego County indicates that these rocks were deposited in two separate but closely adjacent fans which were receiving coarse clastic detritus from similar overlapping source terranes. Examination of the Upper Cretaceous rocks of San Miguel Island and San Diego County in relation to the Middle Eocene Poway clast bearing strata which overlies the Cretaceous section in both localities mandates the existence of post-Maestrichtian but pre-Middle Eocene right-lateral, offshore faulting. This faulting juxtaposed a portion of the Upper Cretaceous fan exposed at San Miguel Island against the inner portion of the Upper Cretaceous fan exposed in San Diego County prior to deposition of the Middle Eocene fan. Subsequent post-Oligocene borderland faulting associated with the initiation of transform faulting in western North America transported the rocks exposed on San Miguel Island and in San Diego County to their present positions.
