Miocene-Pliocene biochronology of the Obregón Basin and it's bearing on the evolution of the Proto-Gulf of California

Abstract The content of dinoflagellates, foraminifera, and calcareous nannofossils in cuttings samples from the Cajeme-1 well defines the age and depositional environments of ∼1500 m of marine sediments drilled in the Obregon Basin, in the south of Sonora. The base of the drilled column is composed of actinolite hornfels dated as 43 ± 2 Ma, which underlies ∼500 m of pyroxene basalts, correlatable with local late Miocene extrusives. The sedimentary strata are deposited unconformably on top of the basalts, and its paleobathymetric evolution indicates that the sedimentary column represents a marine stratigraphic sequence that starts with late Miocene marine evaporites that grade upwards to upper bathyal shales. The upper part of the sequence represents a general shallowing to inner neritic, transitional, and even continental environments in the Pliocene to Holocene interval. Marine microfossil assemblages represent a mixed provenance, from subtropical and temperate forms, and dinoflagellate assemblages indicate deposition in a marine basin with a significant terrigenous influx and no evidence of upwellings. The Obregon Basin represents marine deposition on the Proto-Gulf starting sometime between 11 and 8 Ma and, reaching bathyal depths (150–500 m) at approximately 8 Ma. Marine sedimentation in the Proto-Gulf, however, seems to have started earlier to the north, in the Tiburon and Wagner basins, where marine microfossils indicate an age older than 11.2 Ma. By late Miocene (∼6 Ma), there were marine environments to the north, from the Obregon Basin to the Salton Sea area. The marine Obregon Basin represents the oldest marine deposits in the central and southern parts of the Gulf of California.