12 Mesozoic Paleontology, Sedimentology, and Geochronology of the Rocky Mountains and Colorado Plateau

Tuesday, 12 May 2009
1:00 PM - 5:00 PM

EARLY TRIASSIC CONODONTS IN THE PAHVANT RANGE OF CENTRAL UTAH

Paper number: 12-3
Program: Theme Sessions
Time: 1:40 PM – 2:00 PM

Authors

Abstract

The Pahvant Range lies near the junction of three major geological provinces in the western USA: the Basin and Range, the Rocky Mountains, and the Colorado Plateau. The complex geological milieu of central Utah provides both challenges and opportunities. Among these is the elucidation of the precise age relationships of the strata found in the Pahvant Range. The Thaynes Group (formerly called Thaynes Formation in Utah) is a marine unit deposited in the Sonoma Foreland Basin during the Early Triassic. Rocks of this group are now exposed in various locations in Utah and adjacent states. In the Pahvant Range, the Thaynes Group is represented by numerous, thin limestone beds separated by fissile shale intervals. The group rests disconformably upon the Permian Kaibab Formation and is overlain by the Upper Triassic Chinle Group.

In an effort to more precisely constrain the age of the Thaynes Group in the Pahvant Range, we conducted a detailed sampling at two measured sections. Macrofossils were collected as well as bulk rock samples for microfossil analysis. Ammonoids were abundant, especially in the upper limestone beds, whereas palynomorphs seem to be lacking in the shale intervals. The Early Triassic age of the Thaynes Group has been independently confirmed by analyses of the ammonoid fauna (reported in another presentation at this meeting). A meager collection of poorly preserved conodonts was recovered from the limestone beds. From this sparse conodont assemblage, we have tentatively identified Neospathodus waageni, which indicates a Smithian age for at least some of the Thaynes Group in the Pahvant Range. This age determination is corroborated by studies of the ammonoid assemblages.

Lower Triassic conodont biostratigraphy in North America has apparently been in a state of flux for decades, and it remains problematic for several reasons, perhaps most importantly because of trouble in defining and reliably identifying zonal boundaries. We hope our ongoing research in the Pahvant Range of central Utah may ultimately help resolve some of these issues.