The highly fossiliferous Barstow Formation, of medial Miocene age, crops out in the Mud Hills, north of Barstow, in the northwestern part of the Mojave Desert, San Bernardino County, California. The Barstow Formation is composed of a sequence of fluviatile and lacustrine sediments, and water-laid air-fall tuff beds; it is about 1,000 m thick, interfingers with the lower Miocene Pickhandle Formation, and is unconformably overlain by Pliocene basalt and Quaternary alluvium. The sediments have been folded into a syncline and broken by several faults that generally trend northwest-southeast and show right-lateral separation.

In this report, the Barstow Formation has been divided into three members. A newly expanded Owl Conglomerate Member is at least 200 m thick. It is exposed on both limbs of the syncline and is composed mainly of gray-green granitic conglomerate on the north limb and of conglomerate and conglomeratic sandstone of reddish hues on the south. Near the base of both sequences, the distinctive Red Tuff has been dated isotopically at about 19.3 Ma, and on the north limb of the syncline, the uppermost unit of the Pickhandle Formation stratigraphically overlies this tuff. Owl Conglomerate clasts in the unit indicate deposition from the north and south to southwest sides of the basin. Although the actual limits of the basin are not known, present outcrops of potential source terranes suggest that it may have been about 8 km wide from north to south. Fossil mammals, termed the "Red Division Fauna," that occur in the upper part of the Owl Conglomerate Member are of late Hemingfordian age, probably range in isotopic age from 16.5 to 163 m.y., and correlate to the upper part of the reversed magnetozone of magnetic chron C5C. The uppermost breccias of the Pickhandle Formation are younger than 19.3 m.y. and are likely older than 16.3 m.y., the age of the Rak Tuff that locally caps the Owl Conglomerate Member.

The middle member of the Barstow Formation lies stratigraphically above the Owl Conglomerate Member and below the Skyline Tuff. The lower contact is locally unconformable. The unit is about 570 m thick and is composed of predominantly fluviatile conglomerate and conglomeratic sandstone beds on the west and southwest that pass laterally into a sequence of predominantly lower-energy sandstone and claystone beds on the east. Facies distribution and sparse paleocurrent data suggest that the conglomeratic sequence was derived from sources to the south and southwest and that the finergrained beds were deposited in the more distal part of the basin to the east and northeast.

The Oreodont Tuff, situated about 270 m stratigraphically above the base of the middle member, has been dated at about 15.8 Ma, and the strata that occur in the interval from the top of the Owl Conglomerate Member to a level 40 m below this tuff produce a previously unreported assemblage, herein named the "Rak Division Fauna." This fauna is biologically and stratigraphically intermediate between the Red Division and Green Hills Faunas, is assigned a latest Hemingfordian age, ranges in isotopic age from about 16.3 to 15.9 m.y., and correlates to the upper half of magnetic chron C5C and the lowermost part of Chron C5B and Chron C5AD.

Strata about 40 m below the position of the Oreodont Tuff and extending 270 m higher than the Oreodont Tuff, in the middle member of the Barstow Formation, contain fossil mammals of the Green Hills Fauna, of early Barstovian age. The Cupidinimus lindsayi Assemblage Zone also begins about 40 m stratigraphically below the Oreodont Tuff. Beds that contain the Green Hills Fauna range in isotopic age from about 15.9 to 15.3 m.y. and correlate to all but the lowest and uppermost parts of magnetic chron C5B.

The upper 60 m of the middle member of the Barstow Formation yields a transitional assemblage, herein named the "Second Division Fauna," that contains some elements of the Green Hills Fauna associated with other taxa typical of the succeeding Barstow Fauna. Beds that contain the Second Division Fauna range in isotopic age from about 15.3 to 14.8 m.y. and correlate to the reversed magnetozone of magnetic chron C5AC.

The upper member of the Barstow Formation begins at the base of the Skyline Tuff and extends to the top of the formation, a stratigraphic interval of about 270 m. In the eastern half of the outcrop area and on the north limb of the syncline in the western outcrop area, the deposits are largely beds of fine-grained lacustrine shale, mudstone, and claystone, with lesser amounts of interbedded sandstone and conglomeratic sandstone. To the west and southwest, the equivalent stratigraphic interval is represented by beds of sandstone and conglomeratic sandstone. The interval bearing the Lapilli Sandstone (isotopically dated at about 13.4 Ma) and 30 m of capping beds of limestone and equivalent beds of mudstone appears to be the geologically youngest part of the Barstow Formation in the Mud Hills.

This part of the Barstow Formation also seems to have been deposited in a basin that was filled from the west and southwest by fluviatile deposits, whereas lacustrine sedimentation took place mainly on the north and east. A distinctly northern source may have been responsible for an ∼30-m-thick unit of hornblende-rich sandstone including the Lapilli Sandstone.

Fossil mammals from the upper member of the Barstow Formation pertain to the Barstow Fauna, of late Barstovian age, defined by the first appearance of proboscideans (just below the Dated Tuff). The Barstow Fauna thus is found in beds just below the Dated Tuff to the top of the Barstow Formation in the Mud Hills, ranges in isotopic age from about 14.8 to 133 m.y., and correlates to most of the normal magnetozone of Chrons C5AC and C5AB. Three rodent assemblage zones (Pseudadjidaumo stirtoni, Copemys longidens, and Copemys russelli, from oldest to youngest) have been described from the interval that begins just below the Dated Tuff and extends upward for about 250 m.

The evidence from the Barstow Formation provides local calibration of important biochronological events: the Hemingfordian-Barstovian boundary at 15.9 Ma, and the local first appearance of proboscideans at about 14.8 Ma.

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