San Martin Formation near Cucurpe, Sonora,
Mexico: A record
of Middle Jurassic arc magmatism and continental
William Leggett, New Mexico State University Department of
Geological Sciences, Las Cruces, New Mexico, [email protected]
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.