Reconnaissance Geology of the Boquillas and Ocuca Hills, Northern Sonora, Mexico

The Boquillas and Ocuca Hills lie approximately 5 km north of the inferred trace of the Mojave-Sonora Megashear, between Altar and Santa Ana in northern Sonora, Mexico. Recent geologic mapping indicates these hills are underlain by a succession of southwest-dipping, slightly metamorphosed sedimentary rocks with strike-parallel quartz veins. The lithologies include conglomerate, sandstone, and siltstone that are arranged in fining-upward cycles with northeast-directed paleocurrents. The basal intervals of these cycles are conglomeratic and composed of quartzite, quartz, chert, and limestone with clasts up to 16 cm in diameter. Sandstone intervals are very fine to coarse-grained and compositionally are feldspathic litharenites with rock fragments dominated by chert and volcanics with minor plutonics. Purple and olive siltstone intervals cap the fining-upward cycles. These fining-upward packages are interpreted as fluvial deposits and have previously been assigned to the Pozo Duro Formation of El Chanate Group (Upper Cretaceous). Conglomerate clast and sandstone composition indicates both cratonic and volcanic source areas.

Structurally, the Boquillas and Ocuca Hills are more difficult to interpret due to the repetitive nature of the stratigraphy and the lack of distinctive marker horizons. Examination of ASTER satellite imagery provides some clues to the local structure particularly when analyzing the near- to short-wave infrared range (bands 8, 6, 4). The imagery shows the presence of a northwest-southeast oriented, plunging, symmetrical fold. Field measurements indicate the fold is actually a northwest plunging overturned syncline with a steeper dipping, overturned, southwest limb and a shallower dipping, upright, northeast limb. The axis zone of the fold is dominated by highly fractured orange sandstone that contains abundant quartz veins. This fold appears to be an extension of a syncline mapped to the east in the Cerros Cabeza Colgada.