Petrologic Trends in a Mid-Cretaceous Forearc Basin: Valle Formation, Cedros Island, Baja California, Mexico

Douglas P. Smith, Cathy J. Busby-Spera

A study of petrologic variability in Albian to Turonian-age deep marine sandstone specimens from the Valle Formation on Cedros Island indicates that one episode of magmatic-arc unroofing is recorded in these forearc basin deposits. Previous studies of similarly aged rocks on the adjacent Vizcaino Peninsula suggest that two separate episodes are recorded there. This apparent disparity makes correlation of the Valle Formation from those two locations difficult.

Uplift and unroofing of the Peninsular Ranges batholith in middle Cretaceous time apparently controlled the petrologic trends in the Valle Formation on Cedros Island. Normalized petrologic parameters that increase with stratigraphic height include total quartzose grains, potassium feldspar, and detrital mica. The proportion of polycrystalline quartz grains increases upsection at the expense of volcanic lithic fragments within the rock fragment subpopulation.

The continuity of petrologic trends across previously defined formational boundaries on Cedros Island lends evidence that one mid-Cretaceous-age formation exists on Cedros Island (Valle Formation) rather than three (Lower Valle or Los Chapunes, Upper Valle, and Morro Redondo Formations). Petrologic variation in post-Albian samples of the fine-grained or lower member of the Valle Formation appears to parallel variation in the coarse-grained or upper member, suggesting that the two members were adjacent lithofacies that coexisted from Cenomanian through Turonian time. Our field data indicate that coarse-grained sediments were deposited in the axis of a fault-controlled submarine canyon, while fine-grained sediments accumulated contemporaneously on the shoulder of the graben structure.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.