The late Neogene section in the Salton Trough, California, and along the lower Colorado River in Arizona is composed of marine units bracketed by nonmarine units. Microfossils from the marine deposits indicate that a marine incursion inundated the Salton Trough during the late Miocene. Water depths increased rapidly in the Miocene and eventually flooded the region now occupied by the Colorado River as far north as Parker, Arizona. Marine conditions were restricted in the Pliocene as the Colorado River filled the Salton Trough with sediments and the Gulf of California assumed its present configuration. Microfossils from the early part of this incursion include a diverse assemblage of benthic foraminifers ( Amphistegina gibbosa , Uvigerina peregrina , Cassidulina delicata , and Bolivina interjuncta ), planktic foraminifers ( Globigerinoides obliquus , G. extremus , and Globigerina nepenthes ), and calcareous nannoplankton ( Discoaster brouweri , Discoaster aff. Discoaster surculus , Sphenolithus abies , and S. neoabies ), whereas microfossils in the final phase contain a less diverse assemblage of benthic foraminifers that are diagnostic of marginal shallow-marine conditions ( Ammonia , Elphidium , Bolivina , Cibicides , and Quinqueloculina ). Evidence of an earlier middle Miocene marine incursion comes from reworked microfossils found near Split Mountain Gorge in the Fish Creek Gypsum ( Sphenolithus moriformis ) and near San Gorgonio Pass ( Cyclicargolithus floridanus and Sphenolithus heteromorphus and planktic foraminifers). The middle Miocene incursion may also be represented by the older marine sedimentary rocks encountered in the subsurface near Yuma, Arizona, where rare middle Miocene planktic foraminifers are found.

During the late Miocene, 8.9 to 7.6 Ma, the Gabilan Range shed sediments northeast of the San Andreas fault as debris and turbidity flows depositing eight submarine-fan complexes in the southwestern San Joaquin basin encapsulated within the diatomaceous Antelope Shale Member of the Monterey Formation at Midway-Sunset Oil Field (a major mechanism causing these turbidity flows was probably movement along the San Andreas fault, in other words, the flows were seismically induced). The fans were generally timetransgressive northwestward, as the Gabilan Range was transported right laterally. The larger submarine fan members of the Monterey Formation in ascending stratigraphic order, are the Leutholtz-Metson, Williams, and Republic Sandstone Members. Minor submarine fans sourced from the southern Gabilan Range and deposited synchronously with Republic Sandstone Member deposition are the Sub-Moco, Moco T, Obispo, Uvigerina "C" and Rass. The fan morphology was controlled by 1) sand and gravel rich turbidites quickly depositing, thick sand lobes, 2) syndepositional growth folds with thicker sandstones being restricted to intraslope basin synforms, 3) lateral fan deposition, time- transgressively, due to convex fan shape and poor levees, 4) limited sediment supply between seismic and other triggering events, and 5) upper slope channel switching. The Antelope Shale sandstones are arkosic in composition, fine to coarse grained, and locally conglomeratic, depending upon whether distal, mid or proximal in original fan location. At Midway-Sunset Oil Field these submarine fan sandstones have produced more than 80 million barrels of Monterey Formation sourced oils from anticlinal, pinchout, truncation and fluid level traps.