DEPOSITIONAL SETTING OF THE DINOSAUR-BEARING EL DISECADO MBR., EL GALLO FM., LATE CRETACEOUS, BAJA CALIFORNIA, MEXICO: EPISODIC CLASTIC SEDIMENTATION IN THE DOTT TRADITION

The late Campanian (Late Cretaceous) El Disecado Member of the El Gallo Formation, Baja California, México, is a rich source of a distinctive flora and bimodal vertebrate fauna from the western side of Laramidia. There are two taphonomic filters: the vertebrate material is allochthonous, while the plant material is likely autochthonous. This dichotomy results from the depositional system.

The El Disecado is dominated by three facies: (1) La Amarilla, 2) GF; and (3) a paleosol. La Amarilla is a 2 – 10 m multistoried, lg-scale, cross-stratified, Fe₂O₃-stained medium – coarse, lenticular, channelized sandstone with erosive bases. It is moderately well-sorted, angular, and compositionally immature. Although in an active tectonic setting, petrographically, the facies evinces little evidence of volcanism. Petrified trunks and disarticulated large vertebrates - commonly hadrosaurs - line the bases of scours. This unit reflects high-energy channelized flows, likely within a braided fluvial system. Flow directions were scattered, but largely north and west.

GF is a dark-grey, 0.5 – 1 m thick siltstone, without visible sedimentary structures. Moderately well-rounded chert granules are distributed chaotically, in matrix support. GF produces a disarticulated microvertebrate fauna. This facies represents shallow non-Newtonian sheet flows.

Facies (3) is a carbonate nodule-bearing paleosol. Key features include weakly developed blocky ped structure, Fe₂O₃ mottling, and vertically aligned carbonate nodules, suggestive of root-soil matrix interactions. Some beds show extensive burrowing. The facies appears to present mainly sub-surface horizons, modified by compound pedogenesis. Facies 3 produces plant material at several localities, including roots, pollen, and the compound fruit Operculifructus. Subsurface wet/dry cyclicity is strongly suggested in this paleosol, likely reflecting climate.

The combination of braided and sheet flow deposits, and disarticulated fossils in apparent hydraulic equivalence with their settings, suggests very high-energy landscape instability, although the paleosols suggest some inter-channel land surface stability. The entire package beautifully epitomizes R.H. Dott’s prescient recognition of the episodicity of clastic sedimentation.