Submarine Canyon in a Forearc Basin, Upper Cretaceous Rosario Group, Baja California, Mexico: ABSTRACT

Abstract

Late Cretaceous marine and nonmarine sedimentary rocks of the Rosario Group, deposited in one or more forearc basins, are exposed discontinuously from San Diego to Punta Canoas. Near the southern end of this belt, at San Carlos, mexico, a well-exposed, 6-km wide submarine canyon cuts at least 150 m into a fluvial section. Paleocurrent data indicate a west-southwest-directed sediment transport in the canyon. Large blocks of limestone 3-4 m in size, derived from the Early Cretaceous arc basement complex of the Alisitos group, were transported at least 20 km from the source area in the submarine canyon. The canyon fill consists of a basal and upper conglomerate to sandstone unit separated by a 20 to 45-m thick shale and sandstone unit, and capped by a second shale and sandstone unit of unknown thickness. The lower conglomerate-sandstone unit ranges from 20 to 140 m thick, and cuts into fluvial deposits in a curved or steplike pattern. The upper conglomerate-sandstone unit fills deep channels incised into a sandstone-shale unit and is 35-60 m thick. The conglomerates represent high-density turbidites and debris flows. Sandstones are massive (ungraded or graded), or display Bouma AB or ABC divisions. Shaly slump blocks, derived from channel marginsmore » or terraces, were injected by clastic sills and dikes as they were incorporated into the conglomeratic flows. Slump blocks increase in frequency and size near the northern canyon wall. This pattern has been attributed to a meandering channel with slumping on the outer bend of a meander. The abrupt contact between the conglomerate-sandstone units and the overlying shale-sandstone units indicates that on two occasions the canyon head was abruptly denied access to sources of coarse-grained material. This cyclic pattern may be the result of tectonic events or eustatic sea level changes.« less