Deep Marine Conglomerate Facies and Processes in Cretaceous Forearc Basins of Baja California, Mexico

Detailed studies of slope apron, fan delta, submarine canyon and submarine fan deposits from noncontemporaneous Cretaceous forearc sub-basins in Baja California provide key criteria for recognizing ancient shallowmarine to deep-marine conglomerate depositional environments. Slope apron deposits in the southern Vizcaino Peninsula represent scree cones that built directly from coastal fault scarps onto graben floors at bathyal marine water depths. The lower half of each slope apron deposit consists of rockfall/ avalanche and debris flow deposits at the base of the fault scarp, whereas the upper half consists of an upward-fining sequence of turbidites that reflect gradual erosion of the adjacent horst block. Submarine canyon deposits at San Carlos consist largely of disorganized conglomerates, bearing bathvaldepth benthic foramimfera, that contain large (up to 100 m) mudstone slide blocks; these fill a 7 km wide canvon cut into fluvial redbeds. On Cedros Island a fault-controlled submarine canyon in a deep-sea half-graben received avalanche blocks shed from the uplifted basement and funnelled more distally derived gravelly mass flows along the deepest part of the halfgraben. Rapid facies changes occur laterally away from the canyon axis, from channelized conglomerate to lenticular thickbedded sandstone to laterally continuous turbidites. An inner fan valley-levee complex at Arroyo San Fernando consists of channelized organized conglomerate-sandstone bodies with fining-upward sequences separated by mudstone-sandstone non-channelized deposits; these fill a 7.5 km wide valley that is laterally bounded by a large levee deposit of interbedded sandstone and mudstone with slump blocks up to 80 m thick.