Description
The Upper Cretaceous Rosario Formation, locally exposed between La Mision and Ensenada, is composed of nearly 1,200 meters of marine mudstones, sandstones, and conglomerates. It rests on a basement of undifferentiated Mesozoic volcanic rocks and intrusive granodiorite and is overlain by Tertiary andesites and Quaternary deposits. Abundant sedimentary structures were studied to determine the paleocurrent directions and environment of deposition. Sedimentary structures are divided into two groups: (1) Non-directional – graded bedding, rip-up clasts, and worm burrows; and (2) Directional – cross-bedding, ripple marks, convolute laminations, flame structures, slump structures, and sole markings. Each structure is individually discussed in regard to definition, description, types, origin, and significance. Current rose diagrams of individual structures, formational members, and areas are compared. Measurements of 477 oriented sedimentary structures indicates a predominant southwest current direction. Ripple marks, micro-cross-bedding, and flute casts show a close relationship formed by the same west-southwest moving current. Flame structures, convolute bedding, and cross-bedding show a similar current direction toward the south. Current directions indicate a change from southerly in the north part to northerly in the south part of the study area. Turbidity currents are substantiated by the combined occurrence of graded bedding, sole markings, rip-up clasts, convolute laminations, flame structures, poor sorting, and lack of a fauna. Slope deposits are indicated by the presence of slump structures, sole markings, convolute laminations, rip-up clasts, and local unconformities. A nearshore shelf environment is implied by the location of the paleoshoreline, interbedded conglomerates, abundant worm burrows, and common occurrences of current ripple marks. The sedimentary structures of the Rosario Formation apparently were formed in a nearshore shelf environment, probably a local embayment, with sediment derived from the east-northeast.
