PALEONTOLOGICAL CONSTRAINTS ON EARLY EOCENE LANDSCAPES AND PALEOENVIRONMENT OF NORTHWESTERN ARIZONA

An understanding of Laramide events on the Colorado Plateau and those events specifically associated with the subsequent gradual emergence of an integrated, modern Colorado River system are hampered by a scarcity of paleontologically well-dated sediments or volcanic rocks of Paleogene age. The lower Eocene lacustrine Duff Brown Tank Locality (L4371) contains an assemblage of gastropods, charophytes, and ostracodes that constrain the stratigraphic record within the erosionally truncated arkosic Rim gravel on the east side of Long Point, on the Coconino Plateau, Arizona.

The Duff Brown Tank molluscan faunule of the Music Mountain Formation is interesting to interpret in the absence of similar fossils nearby. These fossils are effectively unique in Arizona. Correlation to potentially equivalent faunules in Utah and Colorado poses interesting study questions: 1) early (historical) publications typically constrain poorly the type stratigraphic and geographic occurrence of continental molluscan taxa; and 2) later, even relatively modern molluscan research, may not represent well the species concepts of Meek, White, and others developed for Western Interior taxa. Subsequent research has promoted broad morphological variability for species diagnoses.

The Duff Brown Tank fossils are comparable to fossils that have previously been reported elsewhere but are 1) poorly known (e.g., type lost, not reidentified); 2) well known, but misidentified; or 3) not well preserved to be compared, identified, and correlated with confidence. Even with these issues, the Duff Brown Tank faunule composition is sufficiently robust as to indicate a likely age for the enclosing strata.

The viviparid, pleurocerid, hydrobioid, physid, planorbid, and ellobiid taxa of the Duff Brown Tank assemblage indicate a biostratigraphic correlation with lower Eocene strata of the main body of the Wasatch Formation, the Niland, Luman, Tipton Shale, and Laney Tongue/Members of the Green River Formation (southwestern Wyoming); Flagstaff Member/Formation (Uinta Basin and elsewhere, Utah), and San Jose Formation (Sandoval County, New Mexico). A result of this study indicates a required taxonomic revision of species of at least these taxa in the Green River and Flagstaff Formations.