Tectonic insights from an Upper Jurassic–Lower Cretaceous stretched-clast conglomerate, Caborca–Altar region, Sonora, Mexico

The ranges directly northeast of Caborca and Altar preserve a 2000+ m-thick folded section of the Altar Formation, composed of ductilely deformed Upper Jurassic–Lower Cretaceous polymict conglomerate, sandstone, mudstone, and andesite. Stratigraphic relationships indicate that this section, which accumulated in a 60 km-long northwest-trending basin, is broadly correlative with the Glance Conglomerate. Coarse (boulder–cobble) detritus in the lower part of the section was shed from prominent highlands consisting of the Middle Jurassic magmatic arc and the Neoproterozoic–Paleozoic miogeocline. In the pebbly upper part of the section, abundant sandstone, mudstone, lacustrine(?) carbonate, and felsic tuff interstratified with andesite flows, and breccias record lower-energy depositional conditions interrupted locally by volcanism. By the time of Bisbee Sea incursion into northwestern Mexico, the Jurassic arc source had disappeared and the miogeoclinal source was topographically subdued. The Lower Cretaceous Morita Formation, which conformably overlies the Altar Formation along a gradational contact, records marine transgression that accompanied regional subsidence. icant northeast–southwest contraction affected the Altar Formation, Bisbee Group, and Upper Cretaceous strata between 71 and 51 Ma. Characteristics of this Laramide event include asymmetric, northeast-verging folds and thrusts accompanied by penetrative southwest-dipping stretched-clast foliation or cleavage and greenschist-facies metamorphism. Mid-Tertiary normal faults overprint the compressional structures such that deeper levels of the stratigraphy are juxtaposed against shallower levels, and axial planes of major range-scale folds are rotated about horizontal axes relative to one another.