EARLY MISSISSIPPIAN NORTH AMERICAN CARBONATE-SHELF MARGIN IN WEST-CENTRAL SONORA, MEXICO

Stratigraphic sections of the Lower Mississippian (Kinderhookian and Osagean) shelf-slope Placeritos Formation are important in defining the southwestern edge of paleo-North America in northwestern Mexico. These rocks were studied near Rancho Placeritos, 40 km northwest of Hermosillo, Sonora. The Lower Mississippian sequence is at least 385 m thick, but the maximum thickness is unknown because the base is faulted and the top is unconformably overlain by Lower Jurassic-Upper Triassic continental clastic deposits with minor interbedded marine deposits.

The lower member consists of 200 m (Kinderhookian) of laminated lime mudstone, interbedded lime mudstone-wackestone, and parallel layers to irregular lenses and wisps of chert; some graded interbeds consist of turbiditic crinoid-ossicle lime packstone-grainstone that contains corals, brachiopods, bryozoans, and pebbles and cobbles of limestone and sparse chert. Except for the basal 41 m, which is metamorphosed and undated, conodonts of the Siphonodella isosticha-Upper Si. crenulata Zone date the lower member. The upper member consists of 185 m of interbedded laminated lime mudstone-wackestone and chert layers, and is considered to be entirely Osagean in age. Conodonts of the middle Osagean Scaliognathus anchoralis-Doliognathus latus Zone occur in the top 85 m of the upper member, which contains conspicuous siliceous spheroidal concretions and sparse crinoid ossicles. Conodonts of the early Osagean Lower and Upper Gnathodus typicus Zones may occur, but have not yet been positively identified, in the lower 100 m of the upper member, including a 9-m chert unit at the base. Lithofacies and faunas of the Kinderhookian part indicate a middle to outer shelf setting whereas the Osagean part is a deeper water outer shelf or slope setting.

Fossil debris in the turbiditic limestones was derived from shallow-water environments of the carbonate platform and inner shelf to the east and north, and was transported west and south into deeper water of the shelf or slope by turbidity currents. The Kinderhookian-Osagean shelf-slope sequence at Rancho Placeritos is the most oceanward section of the Mississippian North American continental margin yet found in Sonora.