ABSTRACT: Tectonic Significance of Large-Scale Chaotic Deposits in a Cretaceous Fore-Arc Basin: Valle Formation, Cedros Island (Mexico)

Douglas P. Smith, Cathy J. Busby-Spera

A mappable, deep-marine slide deposit (olistostrome) within medial-Cretaceous fore-arc basin strata (Valle Formation), located on Cedros Island, Baja California Norte, records the initiation of intrabasinal faulting. Studies of both modern and ancient olistostromes show that olistostromes can form in all physiographic provinces (including shelf and abyssal plain) and tectonic settings of the marine environment. A variety of triggering mechanisms have been suggested for olistostromes, including tectonism, sea level changes, diapirism, rapid sedimentation that overloads steep slopes, migration of gas hydrates, or combinations of the above. The olistostrome in the Valle Formation ranges in thickness from 0 to at least 180 m, and extends areally for at least 34 km² It can be divided into two parts. The basal 30-40 m contains large (up to 8 m) angular blocks (allolistoliths) derived from the Jurassic substrate. The allolistoliths decrease in abundance upsection, whereas internally coherent intraformational slide blocks (endolistoliths), which reach tens of meters in width, increase. Beds composing the endolistoliths are alternating mudstone and sandstone turbidites that were deposited on a tectonically stable basin plain or rise setting before catastrophic failure of the sedimentary pile produced the olistostrome. Intrabasinal faulting is invoked as a cause of the sediment failure because of the presence of allolistoliths, which must have been shed into the basin from uplifted(?) basin floor scarps. Allolistoliths occur sporadically throughout at l ast 400 m of coarse-clastic sediment gravity-flow deposits that cap the olistostrome, suggesting that intrabasinal faulting continued to affect the basin long after the olistrostrome formed.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990