EARLY FLUVIAL (CATHEDRAL BLUFFS) DEPOSITION INTO A LACUSTRINE BASIN (GREEN RIVER) AND THE RESULTING PRESERVATION OF AN UNUSUAL TERRESTRIAL VERTEBRATE ASSEMBLAGE, HONEYCOMB BUTTES, WYOMING

In the Jack Morrow Hills-South Pass region of the northeastern Green River Basin, lake margin and fluvial deposits of Cathedral Bluff Tongue of the Wasatch Formation interfinger with and overlie lacustrine deposits of the Tipton Shale, Farson Sandstone, and Wilkins Peak members of the Green River Formation. These deposits record the deposition of a clastic wedge derived from the Wind River Uplift into the Paleolake Gosiute basin. In most places, the transition from shallow-lake shales and shoreline carbonates to mudflat, clastic, deltaic and fluvial deposits is gradual and laterally consistent. However, at the eastern end of Honeycomb Buttes (the most proximal-to-source part of the outcrop belt), the upper 26 meters of the shale-carbonate facies of the Green River is replaced by the lake margin and fluvial sandstone and mudstone facies of the Cathedral Bluff Tongue, indicating precocious fluvial deposition into the lake.

Detailed biostratigraphic analyses of the South Pass section show that the basal Cathedral Bluffs deposits along the outcrop belt are uniformly early Bridgerian (Br1a, Lower Palaeosyops fontinalis Assemblage Zone) in age. In addition, the preserved vertebrate assemblages have a distinctive, Basin-Margin nature to them in that they are characterized by high taxonomic diversity, unique species, and the co-occurrence of presumed ancestor-descendant species couplets.

A modest vertebrate assemblage (125 specimens) has been recovered from the early Cathedral Bluffs deposits at the eastern end of Honeycomb Buttes. The assemblage differs in a number of ways from the typical Br1a Basin-Margin assemblages of South Pass, most notably the occurrence of Basin-Center species of turtles, crocodilians, and Brachianodon. These difference may be due to the earlier deposition of the Cathedral Bluffs in this area leading to the preservation of a basal Bridgerian (Br0, Eotitanops borealis Assemblage Zone) assemblage and/or the preservation of a Basin-Center Br1a assemblage before typical Basin-Margin conditions developed on the flank of the still-active Wind River Uplift. Since the two brontothere specimens collected thus far cannot be assigned with certainty to either Palaeosyops or Eotitanops, the precise age determination of this assemblage awaits collection of more diagnostic material.