Stratigraphy and Palynology of the Upper Lewis Shale, Picture Cliffs Sandstone, and the Lower Fruitland Formation (Upper Cretaceous) near Durango, Colorado

Abstract

The stratigraphy and palynology of Upper Cretaceous units (upper Lewis Shale, Pictured Cliffs Sandstone, and lower Fruitland Formation) were investigated near Durango, Colorado, on the northern rim of the San Juan basin. This investigation: (1) confirms an interdeltaic depositional setting for these formations in the study area, (2) provides new biostratigraphic data for the Upper Cretaceous rocks on the northern rim of the San Juan basin, and (3) indicates a mostly freshwater origin for the Carbonero coal bed at the base of the Fruitland Formation. The upper Lewis Shale was deposited in lower offshore to upper offshore marine environments. The Pictured Cliffs Sandstone records a major regression on the western margin of the Late Cretaceous sea. The northeastward regression was not one steady event but a series of superimposed transgressive pulses. The depositional environments identified in the Pictured Cliffs Sandstone are upper offshore, shoreface, foreshore, and backshore. The lower part of the Fruitland Formation represents a coastal swamp deposit which developed just landward of the Pictured Cliffs Sandstones. The Carbonero coal bed lies at the base of the Fruitland Formation. At one location, a small tidal channel developed and inundated the local swamp environment. Ten Late Cretaceous palynomorph guide fossils were identified in the Lewis Shale, Pictured Cliffs Sandstone, and Fruitland Formation. The palynomorphs of the Lewis Shale and Pictured Cliffs Sandstone are a marine assemblage. The Fruitland Formation contains a terrestrial assemblage of pollen and spores. Trudopollis meekeri, Kuylisporites scutatus, and cf. Myrtaceoipollenites are a late Campanian Stage palynomorph assemblage in the Fruitland Formation. These findings refute the recent interpretation of a late Maestrichtian age for the Fruitland Formation. The palynological results obtained in this research can be used for correlation and mapping of interbasin and intrabasin sedimentary facies.