ALTAR-CUCURPE BASIN, CENTRAL SONORA: KEY TO MID-MESOZOIC TECTONICS OF NORTHWESTERN MEXICO

The Altar-Cucurpe basin, a SE-trending Jurassic sub-basin within the more widespread Bisbee Basin of northern Sonora and southern Arizona, lies adjacent to the northern flank of the Caborca block for at least 250 km in central Sonora. Although poorly known, a thick Upper Jurassic (Oxfordian-Tithonian) deep-marine section defines a narrow basin keel not more than 50 km wide. The Jurassic strata are overlain by a Lower Cretaceous section correlative with the Bisbee Group. The Bisbee section, although everywhere incomplete, reaches a thickness of nearly 3000 m above the basin keel. Lower Cretaceous strata onlap and thin southward onto Proterozoic rocks of the Caborca block, where Jurassic strata are absent. The Bisbee Group was deposited in shallow marine and fluvial environments, and resembles its regional counterparts throughout the Southwest. Upper Jurassic basinal strata are tightly folded about northwest-trending axes and are unconformably overlain by the Bisbee Group, which is locally less tightly folded than the Jurassic. Preliminary sandstone petrology of the Jurassic-Cretaceous section indicates initial unroofing of a volcanic and basement terrane, likely the Caborca block, followed by a major influx of intermediate to silicic volcanic detritus that accompanied initial progradation of continental facies in the basin. With data currently available, this volcanic influx may be interpreted as either: (1) longitudinal transport of paleovolcanic detritus from Triassic-Jurassic arc rocks to the northwest during transcurrent movement of the Caborca block or (2) northeastward delivery of syn-collisional neovolcanic detritus from the Guerrero oceanic volcanic terrane to the southwest of the Caborca block.