The Rancho San Martin Formation near Cucurpe, Sonora, Mexico: A record of Middle Jurassic arc magmatism and continental deposition

William Leggett, New Mexico State University Department of Geological Sciences, Las Cruces, New Mexico, [email protected]

Approximately 700 meters of volcaniclastic conglomerate, eolian quartz arenite, lacustrine limestone and interbedded basaltic, andesitic and dacitic flows, provisionally termed the Rancho San Martin Formation, underlies Upper Jurassic marine shale and sandstone 10 kilometers northwest of Cucurpe, north-central Sonora, Mexico. A lower succession, 250 meters thick, consists of well sorted, fine-grained cross-bedded quartz arenite interbedded with moderately sorted, fine- to medium-grained volcanic litharenite and feldspathic litharenite. This succession is overlain by a 50 meter thick interval of dacitic ash-flow tuffs. Analysis of 15 zircon grains from this unit at the Stanford/USGS SHRIMP facility yielded a U/Pb age of 168.4±1.2 Ma (2σ, 1.47 MSWD). The middle 250 meters of the section consists of interbedded pebble to cobble volcaniclastic conglomerate and andesitic to basaltic flow breccias. Overlying these is 40 meters of limestone characterized by laminated algal boundstone which grades upward into bedded ostracod and bivalve wackestone. The uppermost 110 meters of the section is composed of pebble to cobble volcaniclastic conglomerate and discontinuous outcrops of fine-grained quartz arenite.  SHRIMP analyses of 14 zircon grains from a dacitic flow 20 m below the unconformity with the Upper Jurassic strata yielded a U/Pb age of 170.1±1.2 Ma (2σ, 1.67 MSWD). The overlap of this age with that of the lower dacite indicates deposition occurred in a rapidly subsiding basin during the Bajocian. I interpret the Rancho San Martin Formation to record eolian, alluvial fan and lacustrine deposition in an extensional basin within or adjacent to the Late Triassic to Middle Jurassic Nazas Arc.