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Late Paleozoic Basin Evolution And Tectonism In East-Central California

Abstract

Prior to the latest Devonian to Early Mississippian Antler Orogeny the continental margin off east-central California was passive and marked by a long slope westward to the ocean floor. During that orogeny, an uplift was constructed in the Sierra Nevada which depressed the crust to the east forming the Late Mississippian Rest Spring Basin. This basin was only partially filled, primarily with shale. This was followed by deposition of calcareous turbidites in the successor Keeler Basin in Pennsylvanian to Early Permian time. Also in the Late Pennsylvanian, part of the older shelf to the southeast was depressed, possibly along a SE-trending fault parallel to a sinistral continental truncational fault farther west. This marked the first phase in development of the Darwin Basin which is exposed in the Darwin Hills, some 6 km west of the exposed shelf. Here a sequence of Gzhelian (Upper Pennsylvanian) calcareous turbidites > 225 m thick lies upon older shelf strata which have been cut into by a submarine canyon at least 275 m deep and 1 km wide. This canyon was filled by a basal sequence of limestone conglomerates and an overlying thick sequence of siltstone. Reconstruction of the paleogeography using an average of upper slopes adjacent to shelves ∼6 km to the east suggests that the water depth at the base of the submarine canyon was in the vicinity of 500 m. A second phase of development of the Darwin Basin followed emplacement of the Last Chance allochthon in the middle Sakmarian at the older Mississippian shelf margin. Folds and thrust wedges at the toe of the allochthon built up the Conglomerate Mesa Uplift which thereafter marked the western margin of the Darwin Basin. Emplacement of the allochthon apparently resulted in downdropping more of the previous shelf along a northeast trend parallel to the uplift. The resulting basin received approximately 2000 m of deep-water sediment, including debris-flow deposits derived from the shelf more than 30 km to the east, which piled up against the allochthon. The calculated maximum depth of this basin adjacent to the Conglomerate Mesa Uplift, assuming a slope of about 1.5,Å∞, was about 2000 m when the debris-flow deposits formed. The Darwin Basin was further enlarged eastward later in the Artinskian. Before the end of the Permian, however, basinal deposition ceased and all previous basinal rocks were buried by mostly terrestrial deposits probably derived from the west.