AGE AND DEPOSITIONAL SETTING OF DEEP-MARINE ORDOVICIAN RÍO FUERTE AND SAN JOSÉ DE GRACIA FORMATIONS IN NORTHERN SINALOA, MEXICO: REMNANTS OF THE EARLY PALEOZOIC IAPETUS OCEAN

Field, paleontological, lithologic, and detrital-zircon studies of the Río Fuerte (RFF) and San José de Gracia (SJGF) Formations suggest different depositional settings within the Iapetus Ocean. Amorphognathus superbus Zone conodonts occur in a local lime-grainstone unit in the RFF. Graptolites in argillites of the SJGF suggest ages from the Nemagraptus gracilis through Pleurograptus linearis Zones. All these open-ocean faunas of Late Ordovician (460-450 Ma) age are indicative of the North Atlantic Faunal Realm. Intercalated quartzite, phyllite, schist, chert, and sparse turbiditic lime grainstone of the RFF are widespread near El Fuerte. Coeval intercalated quartzite, chert, and argillite of the SJGF are widespread to the southeast near San José de Gracia. The SJGF was named by Carrillo-Martínez, who assigned it a Carboniferous age on the basis of shallow-marine fossils in limestone outcrops or displaced blocks that we could not relocate. The SJGF is here re-dated as Ordovician from contained graptolites.

U-Pb (SHRIMP-RG) dating of detrital zircons in six feldspar-rich immature quartzite beds from the RFF yielded predominantly Cambrian and Neoproterozoic (Gondwanan?) ages (~520-960 Ma) with minor amounts of older Proterozoic (~1320-1460, 1500-2225 Ma) and late Archean (~2485-2855 Ma) ages. In contrast, analyses of zircons in four feldspar-poor mature quartzite beds in the SJGF yielded few Cambrian or Neoproterozoic ages, but are dominated by Mesoproterozoic (~1400 Ma), Paleoproterozoic (~1800-1990 Ma), and earliest Paleoproterozoic to late Archean (2390-2990 Ma) ages. Only one sample yielded a single analysis each of Late Cambrian and late Proterozoic age (~500 and 1000 Ma) zircons. Both RFF and SJGF quartzite beds received detrital zircon from Proterozoic and Archean-aged terranes; however, only RFF quartzites contained significant amounts of zircon from Gondwanan-age (450-800 Ma) terranes. One SJGF quartzite bed contained a few Gondwanan-age (530-1050 Ma) zircons, which may link the two formations. Composition and zircon-age spectra of RFF feldspathic quartzites suggest a sediment source closely associated with a Gondwanan-age magmatic-arc terrane whereas SJGF feldspar-deficient quartzites were deposited far from the magmatic arc or near the margin of Gondwana.