PALEOENVIRONMENTS, VERTEBRATE FAUNAS, AND TAPHONOMY OF THE EL GALLO FM., LATE CRETACEOUS, BAJA CALIFORNIA, MÉXICO

Paleoenvironments and vertebrate faunas from the western margin of the Western Cordillera during Late Cretaceous time are relatively poorly known by comparison to their eastern counterparts. Here, we present a distinctive Late Cretaceous (75 – 74 Ma) western-margin Cordilleran vertebrate fauna in a striking taphonomic setting, from the region around El Rosario, Baja California, México. There, a sequence of interbedded fine and coarse sandstones of the Disecado Member of the El Gallo Fm. preserves a variety of macro- and microfossils, including elasmobranchs, osteichthyans, turtles, lizards, crocodylians, ornithischian and saurischian dinosaurs, and four mammal genera. Pollen, plant macrofossils, and petrified wood, are also preserved.

Paleobiogeographic analysis has revealed that the vertebrate fauna is mixed, containing northern and southern elements of younger and older N. Am. faunas. This may reflect its unique geographic position; it is one of the southernmost of the few Late Cretaceous localities in the Pacific coast.

Work by Busby, Fulford, Schile, as well as our own, indicates that the depositional environment was an extremely active, braided fluvial system, composed of interbedded channel and sheet-flood deposits. Channels averaged about 40 m in width; these are preserved in 5 – 15 m thick bodies, composed trough cross-stratified, compositionally immature, but texturally mature coarse sandstones.

Interbedded and contrasting with the channel-bearing coarse sandstone deposits are compositionally and texturally immature very fine- to fine grey – black sandstones in stacked ~1 m beds, totaling 2 – 10 m. Sedimentary structures are uncommon in this facies.

Fossils are uniformly disarticulated, generally (but not always) isolated and appear in association with sedimentary structures when preserved. Large fossils (dinosaur bones and petrified logs) are preserved exclusively in the coarse cross-stratified sandstone facies. By contrast, microvertebrates (including mammals), plant, and pollen are exclusively found in the grey – black fine-grained sandstone facies. Close association with depositional features suggests that the distribution of these fossils was controlled by their having been clasts in the aggrading fluvial system.