Print this pageABSTRACT: Tectonic Significance of Upper Cretaceous Alluvial-Fan Deposits in the Peninsular Ranges Forearc Basin Complex, Baja California (Mexico)
Madeleine Fulford, Cathy Busby-Spera
Recent evidence suggests that forearc basins, previously depicted as broad downwarps, may experience syndepositional faulting and/or folding. The upper Campanian El Gallo Formation, which was deposited along the northern margin of the Rosario embayment of the peninsular Ranges forearc basin complex, records tilting of the underlying arc massif basement contemporaneous with dissection of the arc source terrane.
The La Escarpa member at the base of the El Gallo Formation consists of a 100-180-m thick upward-fining sequence, interpreted as a retrogradational proximal to distal alluvial-fan deposit. Overlying the La Escarpa member is the 1000-m thick El Disecado member, which lies at the top of the El Gallo Formation and consists of sandy fluvial deposits. Conglomerate clast counts from the La Escarpa member and point-count data from sandstones throughout the El Gallo Formation show an upsection increase in granitic and metasedimentary rock fragments relative to volcanic/metavolcanic rock fragments, reflecting unroofing and progressive headward erosion of the source terranes in the Peninsular Ranges. Paleocurrent data from the La Escarpa and El Disecado members suggest a rotation of approximate y 110° with time, from west-northwesterly to southerly, reflecting tilting of the basin floor. This may have been accomplished by downdropping along an east-west-trending fault that divided the Rosario embayment into separate subbasins during the Cretaceous.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990