THE STRATIGRAPHY AND SEDIMENTOLOGY OF UPPER CRETACEOUS SEDIMENTS OF SOUTHWESTERN CALIFORNIA AND BAJA CALIFORNIA, MEXICO

The Late Cretaceous sedimentary record is preserved in a series of scattered outcrops along the coast of southwestern California to central Baja, Mexico. Virtually identical stratigraphic sequences and interpreted depositional histories suggest that regional sedimentation was controlled primarily by eustatic sea level changes, with a strong tectonic overprint. Sediments derived from the uplifted Peninsular Ranges granitic-metamorphic terrain (arc system) were implaced in a Great Valley-type forearc basin as a westerly-thickening clastic wedge of continental to deep-marine facies. The stratigraphic record within the main part of the basin shows evidence of only a single large transgressive-regressive cycle. By contrast, the stratigraphic record along the eastern basin margin is complex and more complete, characterized by numerous small transgressive-regressive hemi-cycles within one major eustatic cycle. During the Cenomanian and Turonian, the shoreline was located several kilometers west of the present coastline, and fluvial/alluvial fan sediments were deposited on arc basement. During a major eustatic sea level rise that continued through the Early Campanian, a transgressive sequence of beach to offshore coastal deposits was onlapped over continental sediments. In the Late Campanian/Early Maestrichtian, sea level peaked then began to fall. Large volumes of eroded material were emplaced in nearshore fan-delta sequences. Offshore mass-flow transport across the steep narrow shelf produced submarine fan systems that prograded into the basin from several major embayments. The size and morphology of these fans appears to be comparable to the present-day La Jolla system. In each case, the sedimentary section is characterized by an upward transition from outer fan to middle fan to inner fan facies associations. From the Early Maestrichtian to the Tertiary, the shoreline prograded west in response to falling sea level, possibly marked by a minor stage of transgression during the Early Maestrichtian. A large erosional hiatus marks the Cretaceous boundary