FACIES MODEL OF CRETACEOUS-TERTIARY VIZCAíNO FORE-ARC BASIN, BAJA CALIFORNIA, FROM INTERPRETATION OF WELL-LOG AND SEISMIC REFLECTION IMAGES

The Pacific margin of Baja California contains Cretaceous-Tertiary fore-arc basins developed during subduction of Farallon plate. Seismostratigraphic, litostratigraphic and electrofacies analysis of seismic data and four marine wells drilled by PEMEX allow integration of a chronostratigraphic evolution of the Vizcaíno basin and its depositional environment. The Vizcaíno basin extends for 250 Km northwestward from the Vizcaino desert in the south of Punta Canoas in the north. This forearc basin contains four units separated by three regional unconformities or sequence boundaries. Unit 1 overlies the volcanic-plutonic basement and is characterized by basal conglomerate with subordinated sandstone, shale and limestone reef-coquina. Unit 2 is >1900 m-thick at its depocenter and is composed by fine-grained sandstone, shale and minor interbedded conglomerate, which represent a Cenomanian to Turonian marine transgression. Unit 3 overlies a local unconformity but is lithologically similar to Unit 2. Units 2 and 3 represent neritic conditions to the east and upper bathyal depths to the west, this is best displayed in the southern part of the basin. Low-amplitude NNW-SSE folds affect units 2 and 3, and control its thickness. Clinoforms on discrete intervals in units 2, 3 and 4 indicate the progradation of the continental shelf to the west. We interpret that units 2 and 3 constructed large part of the continental shelf to the east of the subduction accretionary prism during the late Cretaceous and Paleogene. The unconformity at the top Unit 3 indicates basin inversion and erosion of the continental shelf from middle Eocene to early Miocene. This hiatus probably resulted from a flat subduction of the young Farallon plate. Unit 4 is a >350 m thick sandstone-siltstone unit of Miocene age. Unit 4 thickens to the west and its distinctive horizontal reflectors are cut by Neogene faults at the edge of the modern continental shelf.