Geology west of the Canal de Las Ballenas, Baja California, Mexico

Mapping along the eastern coast of Baja California adjacent to the Canal de Las Ballenas (lat. 29 degrees 30' to 29 degrees 40') reveals over 4,000 m of complexly folded and faulted, metamorphosed sedimentary and volcanic strata, locally intruded by tonalite and gabbro of Cretaceous age. Protoliths include lime mudstone, coarse-grained bioclastic packstone, bedded black chert and shale, thin-bedded flysch-type sandstone-shale, thick boulder and cobble conglomerates, minor quartz arenite, and pillowed alkaline basalt. The depositional environment was anoxic slope to basin. The entire sequence is here named the Canal de Las Ballenas Group. Conodont fragments and favositid corals indicate a Devonian age, suggesting correlation with rocks in the Sierra Las Pinta to the northwest, and to southern Sonora to the east. The recognition of these rocks adds an important link to our understanding of the southwestern edge of North America in mid-Paleozoic time. The area has been pervasively deformed, first by tight, isoclinal folding and shearing, which produced large recumbent folds with axes dipping gently to the northeast and verging east-southeast; and second by large synforms with steep, east-trending axes.