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

William Morris, Douglas P. Smith, Cathy J. Busby-Spera

Detailed studies of slope apron, fan-delta, submarine canyon, and submarine fan deposits from noncontemporaneous Cretaceous forearc subbasins in Baja California provide key criteria for recognizing ancient shallow-marine 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 marine graben floors at bathyal 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. A shelf-type fan delta at Punta Canoas, exposed continuously down-paleocu rent for a distance of 20 km includes distal alluvial fan deposits, shingle conglomerate and swash cross-laminated sandstone beach deposits, mouth-bar clast-supported conglomeratic mass-flow deposits, prodelta slumped mudstones and channelized conglomerates with local textural inversion, outer shelf channelized sandstone and conglomerate, and nonchannelized mudstone and HCS sandstone. Submarine canyon deposits at San Carlos consist largely of disorganized conglomerates bearing bathyal-depth benthic foraminifera that contain large (up to 100 m) mudstone slide blocks; these fill a 7-km wide canyon cut into fluvial red beds. A fault-controlled submarine canyon in a deep-sea half graben on Cedros Island shows (1) rapid decrease in locally derived avalanche blocks and soft-sediment deformatio structures away from the fault boundary and (2) funneling of arc-derived conglomerates along the deepest part of the half graben while fine-grained sediment accumulated on the shoulder. 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 at 7.5-km wide valley laterally bounded by a large levee deposit of interbedded sandstones and mudstones with slump blocks up to 80 m thick.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.