SEDIMENTARY RECORD OF MIOCENE RIFTING ALONG THE EASTERN SIDE OF THE SAN JOSE DEL CABO BASIN, BAJA CALIFORNIA SUR, MEXICO: CRITICAL EARLY EVIDENCE OF THE PROTO GULF OF CALIFORNIA

Miocene (<~12 Ma) rifting separated Baja California from mainland Mexico and created the early (proto) Gulf of California. The oldest known marine strata in the Gulf of California are ~8 Ma in the San Jose del Cabo basin and the Tres Marias islands on the west and east side of the mouth of the Gulf of California. In these locations, we can not be sure that these basins are not recording an early embayment rather than a true proto Gulf. However, preliminary interpretations of seismic reflection lines in the southern Gulf suggest that many basins within the mouth and southern Gulf contain older, syn-rift sequences that will be undated until drilling, but are likely to be proto Gulf age. At present, the exposed older strata of the Cabo and Tres Marias (and Santa Rosalia) basins are the best record of the southern proto Gulf. The Cabo basin contains a complete record of rifting, and like a classic rift basin, the oldest exposed strata are along the eastern hanging wall. The undated La Calera Formation consists of conglomerate formed in alluvial fans and sandstone and conglomerate deposited in braided streams; the fining upward strata unconformably overlie Cretaceous granite to the east. The Trinidad Formation conformably overlies the La Calera Formation in the main bay, but unconformably lies on the granite along the north bay. The Trinidad Formation consists of a heterolithic assemblage of beach, shoreface, near shore, and restricted lagoon facies of sandstone and mudstone that abruptly interfinger laterally with each other and the La Calera Formation. Many types of fossils including foraminifera confirm the nearshore environments of the lower Trinidad Formation. An east-dipping, oblique-slip normal fault in the bay repeats the section from east to west and allows 3-D analysis of the complex geometry of the Cabo basin. The La Calera and Trinidad Formations represent one cycle of basin deepening and later shoaling. The strata of the study area are a key to test if the bay formed from syn-sedimentation faulting, segmentation of the hanging wall margin, or complex rotations and tilting in the hanging wall basement. Ultimately, the comparison of the Cabo basin to offshore basins and the Tres Marias basin, all of which formed near one another, will provide critical new information on the early formation of the Gulf of California.