PALEOTECTONIC AND PALEOGEOGRAPHIC SETTING OF PALEOZOIC AND MESOZOIC ROCKS AT RANCHO SAN MARCOS, EAST-CENTRAL SONORA, MÉXICO

Field studies at Rancho San Marcos provide evidence for widespread Paleozoic shelf and foredeep deposits, Sonora-allochthon, and Mesozoic-overlap deposits. Mapping confirms that the Sonora allochthon overrides synorogenic foredeep deposits of the Mina México Formation of the outer shelf. Detrital-zircon age distributions indicate that Mina México sediments were derived from the Laurentian craton to the north and the Sonora allochthon and possible Gondwana terranes to the south. Following late Paleozoic emplacement of the Sonora allochthon, rocks equivalent to the dominantly continental lower Mesozoic Barranca Group were deposited on the allochthon.

Rancho San Marcos area geology includes Paleozoic rocks correlative with those of the Minas de Barita, ~50 km to the west in central Sonora. Some facies changes occur in allochthonous rocks between Rancho San Marcos and Minas de Barita. The thick Laurentian carbonate-shelf sequence consists of Cambrian to Permian rocks, the foredeep Mina México sequence consists of Lower and Middle Permian rocks, and the superjacent late Paleozoic Sonora allochthon consists of Lower Ordovician to Upper Pennsylvanian oceanic rocks. The farthest northeast outcrops of rocks correlative with the Barranca Group are in the San Marcos area.

Interpretations are supported by geologic mapping, fossil data, and newly determined detrital zircon dates.