BIOSTRATIGRAPHY AND PALEOTECTONIC SETTING OF PALEOZOIC ROCKS NORTH OF ESTACIÓN SAHUARO, NORTHWESTERN SONORA, MEXICO

Conodont and lithofacies studies of previously poorly dated carbonate rocks, 12–19 km north of railway Estación Sahuaro and ~60 km ESE of Puerto Peñasco in the Altar Desert, elucidate a complexly deformed, metamorphosed post-lower Paleozoic sequence. Our conodont dating reveals the presence of Lower Mississippian (Osagean), Lower Pennsylvanian (Morrowan and Atokan), and Lower and Middle Permian marbleized carbonate rocks that are interbedded with subordinate cherty, sandy, and silty carbonate rocks, quartzite, schist, and phyllite. The sequence is exposed primarily along two north-trending, en echelon ridges, 5 to 10 km NW of the Mina La Herradura gold deposit. The rocks were previously studied by Calmus and Sosson (1995), who reported only supposedly early Paleozoic corals from the northern ridge where we recovered younger Paleozoic conodonts. The studied sequence rests on undated, probably Devonian and older Paleozoic carbonate rocks, underlain by marble, quartzite, and phyllite of probable Neoproterozoic age and older schist, quartzite, gneiss, and amphibolite basement rocks.

Conodonts occur in shallow-marine, subtidal, bioclastic, partly dolomitized carbonate beds associated with intertidal nonfossiliferous possible algal laminites. The conodont-bearing beds contain deformed pelmatozoan debris, some brachiopods, and other invertebrate fossils. Conodont color-alteration index values range from CAI 6.5 in Permian rocks to CAI 7 in Mississippian and Pennsylvanian rocks, indicating that host rocks reached temperatures of 500–600°C. Most of the carbonate beds are marbleized and plastically deformed, and the associated quartzites contain stretched fine to very coarse grains. Deformation and metamorphism probably occurred during Mesozoic tectonic, plutonic, and volcanic events.

The studied Paleozoic strata were deposited on the outer cratonic platform of southwest Laurentia, south of the mid-Paleozoic Transcontinental arch. The Mississippian and Pennsylvanian sequence is probably correlative to the Escabrosa, Black Prince, and Horquilla Limestones in SE Arizona and NE Sonora.