MAPPING, STRATIGRAPHY, AND DEPOSITIONAL ENVIRONMENT OF THE PLIOCENE PUNTA COLORADA BASIN, SAN JOSÉ ISLAND, GULF OF CALIFORNIA, MEXICO

The Punta Colorada Basin (PCB) is one of two Pliocene subbasins, which are remnants of a larger basin formed during the formation of the Gulf of California. We mapped the PCB and conducted a stratigraphic, facies, and structural analysis. The basin is bounded against Cretaceous tonalite by faults on the southwest and west-central sides, and a buttress nonconformity along the western embayment. The PCB contains several facies: Conglomerate (C, >700 m) varies from para to orthoconglomerate formed in an alluvial fan. Facies C also includes talus deposits produced along a syn-depositional fault. Sandstone and conglomerate (SC, ~200 m) has interbedded shelly sandstones and para to orthoconglomerates interpreted as fan delta. Massive sandstone (MS, >30 m) is a homogenous, partly muddy, silty or shelly sandstone which formed in a protected shallow marine setting. Bedded sandstone (BS, >300 m) includes alternating silty and shelly sandstone produced in a shallow marine setting influenced by currents. Bedded shelly sandstone (BSS, ~50 m) is stratified sandstone with varying amounts of shell hash and shells. Includes sets of marine mega-foresets prograded to east and west. BSS reflects a very shallow marine environment influenced by strong bottom currents or Gilbert deltas. Mudstone (M, >60 m) includes mudstones and muddy sandstones. Benthic forams from this facies in the northern basin indicate upper bathyal as the maximum water depth. Calcarenite (CA, >35 m) is crudely stratified and reflects an inner shelf depositional environment. Conglomerate and sandstone (CS, ~5 m) includes conglomerates and lesser sandstone. Rhizoliths suggest a fluvial setting. The PCB exhibits complex vertical and lateral facies patterns, in addition to syn and postsedimentary structures. Initial subsidence of the PCB resulted in the deposition of facies C, SC, and MS. Synsedimentary deformation structures are common. The PCB reached its maximum water depth ~3.5-3.0 Ma (based on planktonic forams) with deposition of facies M, which interfingers laterally with facies BS and BSS. The strata up to facies M form an upward deepening succession as subsidence outpaced sediment influx. A sharp unconformity between facies M and superimposed facies CA (which interfingers laterally with facies BSS and CS) indicate subsequent abrupt uplift of the PCB.