PALEOGEOGRAPHY AND STRATIGRAPHY OF PALEOZOIC STRATA IN THE RANCHO EL BÍSANI AREA, NORTHWESTERN SONORA, MEXICO

Laurentian Upper Ordovician to Upper Mississippian marine strata are exposed in seven isolated hills rising abruptly above the alluvial plain within the 9-km² El Bísani area 65 km east of the Gulf of California and 22-25 km west of Caborca. Biofacies and lithofacies studies of the previously insufficiently dated strata indicate moderately deep, shallow-subtidal, and intertidal deposition on the inner carbonate shelf of southwestern Laurentia. The strata are more similar to medial-shelf strata near Rancho Placeritos, 175 km to the southeast in west-central Sonora, than to coeval shelf strata in the Western Mojave Desert-Death Valley region, 550-700 km to the northwest in southern California. Distribution of miogeoclinal facies along the southwest margin of Paleozoic North America indicate a continuous Laurentian continental shelf from California to Sonora without large left-lateral offset of facies trends. Conodonts and megafossils as well as bedding features reveal that most sections are overturned as a result of late Mesozoic and/or early Cenozoic tectonism. Conodont color-alteration index values averaging CAI 2 to 3 indicate that host-rock temperatures reached 50° to 200°C, revealing a relatively low thermal history.

The stratigraphic succession composited from Cerros Bísani and Los Murciélagos and Cerritos El Represo and La Berrenda comprises the following informal map units: Upper Ordovician “La Huerta Formation” (>125 m), Upper Silurian-Lower Devonian “Cerro Bísani Dolostone” (>73 m), Lower and Middle Devonian “La Ventana Formation” (>122 m), Lower, Middle, and Upper Devonian “Los Murciélagos Formation” (>450 m), Lower Mississippian “El Represo Limestone” (>58 m), and Upper Mississippian “La Venada Limestone” (>80 m). The dolostone and limestone units are locally interbedded with secondary chert, silty and sandy carbonate, sandstone, and quartzite. The only exposed formational contact separates Upper Ordovician and Upper Silurian strata and represents a major unconformity. These map units will be formally named in a report now in progress.