NEW DETRITAL ZIRCON DATA FROM A QUARTZ ARENITE NORTHWEST OF CUCURPE, NORTH-CENTRAL SONORA, MEXICO: IMPLICATIONS FOR MIDDLE TO LATE JURASSIC BASIN DEVELOPMENT ALONG THE SOUTHWESTERN MARGIN OF NORTH AMERICA

Eolian quartz arenite, provisionally termed the Cerro Agua Grande Sandstone, is exposed in close association with Upper Jurassic strata northwest of Cucurpe, north-central Sonora, Mexico. This unit was previously assigned a Cambrian age based on similarities with the Provedora Quartzite of northern Sonora. Preliminary observations indicate that the Cerro Agua Grande Sandstone unconformably overlies presumably Early to Middle Jurassic arc-related rhyolites. Overlying the Cerro Agua Grande Sandstone are interbedded basalt, volcaniclastic conglomerate and lacustrine limestone, which, in turn are overlain by Upper Jurassic marine shales and sandstones. We used multicollector laser ablation ICP-MS to date 100 detrital zircons from this unit. The youngest grain from this sample is 169±6 Ma (1 sigma), and the largest peak on a relative probablity plot is the youngest peak at 176 Ma indicating that the unit has a maximum age of Middle Jurassic. Other significant peaks are Devonian, Ordovician, and Mesoproterozoic. The sample had six Archean grains. Zircon age distributions within the Cerro Agua Grande Sandstone closely resemble those from recently published data on Jurassic eolian sandstones of the Colorado Plateau. This indicates a correlation of the Cerro Agua Grande Sandstone with one or more of the Jurassic eolian units and that a significant portion of these grains was transported from the Early to Middle Jurassic ergs of the Western Interior of North America. Furthermore, a significant population of grains ranging from 169±6 Ma to 218±7 Ma indicate that some grains were derived from the Jurassic arc itself and indicate probable equivalence with the Entrada Sandstone of Bajocian age. These data, along with tentative stratigraphic relationships, suggest that the Cerro Agua Grande Sandstone is part of a suite of eolian sandstones interbedded with the low-standing Middle Jurassic volcanic arc. Overlying basalt, lacustrine limestone and volcaniclastic conglomerate suggest a transition toward an extensional tectonic setting, eventually leading to marine deposition in the Late Jurassic.