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Description
An allochthonous Lower Jurassic marine sedimentary and andesitic volcanic sequence 2,360 m thick is exposed in the Sierra de Santa Rosa in northwest Sonora, Mexico. The Jurassic sequence is thrust over older Precambrian regionally metamorphosed rocks and autochthonous younger Precambrian dolomites. The Lower Jurassic Sierra de Santa Rosa Formation (new name) consists of 1,460 m of alternating andesitic lithic wackes, calcareous arkoses, sandy biomicrites and argillites, with lesser thicknesses of calcareous shales, sandy oosparites and andesitic conglomerates. The formation is interpreted to have been deposited in a shallow, nearshore marine environment with tidal flat, lagoon and barrier bar facies. The source area for the formation is possibly an island arc previously proposed to exist along the western edge of Sonora. The Lower (?) Jurassic Cerro San Luis Formation (new name) lies conformably on the Sierra de Santa Rosa Formation, and consists of 0 to 70 m of a conglomerate containing quartz arenite and dolomite clasts. The conglomerate grades upward into approximately 860 m of andesitic agglomerates, lapilli tuffs, lavas and interbedded channel conglomerates and arenites. The Cerro San Luis Formation is interpreted to have been deposited subaerially. The tectonic event necessary for the deposition of the conglomerate at the base of the formation and the andesite volcanism could also be related to the presence of an island arc. The Precambrian and Jurassic rocks are intruded by a Late Mesozoic or Early Tertiary post-thrust rhyolite porphyry. The structure is dominated by a series of north-south striking, westward dipping Cretaceous (?) thrust faults defining four major allochthons. The direction of thrusting was to the east, but the distance of movement is not known. The thrusting could also be related to back-arc tectonics. Inclined beds of the Miocene (?) Baucarit Formation indicate that the central portion of the range has been tilted approximately 25 degrees to the west, but normal faulting has not altered the configuration of the allochthons in a major way.
