Stratigraphic setting of fossil log sites in the Morrison Formation (Upper Jurassic) near Dinosaur National Monument, Uintah County, Utah, USA

Carole T Gee,Dale E Gray,Douglas A Sprinkel,Mary Beth Bennis

doi:10.31711/giw.v6i0.36

Abstract

The outcrop belt of the Upper Jurassic Morrison Formation in the northeastern Uinta Basin and southeastern flank of the Uinta Mountains is particularly rich in dinosaurian and non-dinosaurian faunas, as well as in fossil plants. The discovery of several well-preserved, relatively intact, fossil logs at several locations in Rainbow Draw and one location in Miners Draw, both near Dinosaur National Monument (Utah), has provided an opportunity to study the local paleobotany, stratigraphy, and sedimentology of the Morrison Formation in northeastern Utah. The Morrison Formation in northeastern Utah consists of four members. In ascending chronostratigraphic order, they are the Windy Hill, Tidwell, Salt Wash, and Brushy Basin Members. The lithology (including the presenceof glauconite grains) and fossil assemblage of the lower two members (Windy Hill and Tidwell) indicate a marine to marginal marine (coastal plain) depositional environment, whereas the lithology, fossil flora and faunaassemblage of the upper two members (Salt Wash and Brushy Basin) indicate a fluvial–lacustrine depositional environment. At least 10 fossil log sites in Rainbow Draw have been documented so far, and geologic mapping indicates that the logs and wood all occur in the same stratigraphic interval within the Salt Wash Member, approximately 17 to 27 m above the base of the member. The unit containing the logs and wood is about 11 m thick and consists of very fine to fine-grained sandstone and siltstone with indistinct bedding and no discernible sedimentary features.The logs are siliceous, some have a coaly exterior, and they range in exposed length from 0.5 to 11 m and reach diameters up to 1.1 m. In the Miners Draw area, a single siliceous log is documented in the upper part of the Salt Wash Member within a silty sandstone unit that is 4 m thick; its exposed length is about 6 m. Although the correlation of the Miners Draw log-bearing interval to the interval in Rainbow Draw is uncertain, both units are lithologically similar and both occur in the upper part of the Salt Wash Member. The logs have been identified as araucariaceous conifers that pertain to the same taxon originally described as Araucarioxylon hoodii Tidwell et Medlyn 1993 from Mt. Ellen in the Henry Mountains of southern Utah. Concurrent systematic work will prompt a nomenclatural transfer of this species to the genus Agathoxylon. Based on the abundance of large fossil logs and wood in the same stratigraphic interval in Rainbow Draw, wehypothesize that the area was covered by stands of moderately large trees of araucariaceous conifers. The sedimentological evidence suggests that the trees were not transported far from their original site of growth before they were deposited in a low-energy floodplain environment.

Highlights

The Upper Jurassic Morrison Formation is a spectacular and recognizable formation across the Colorado Plateau, and is distinctive because of its array of rainbow-colored mudstone, siltstone, and sandstone beds that typically form a “badlands” landscape
The purpose of this study is to describe the Morrison Formation strata, especially those related to the fossil logs, and to place the logs within a stratigraphic framework for the interpretation of the depositional environment of the log-bearing intervals at both locations
The fossil logs in both the Miners Draw and Rainbow Draw areas occur in the Salt Wash Member of the Morrison Formation

Summary

INTRODUCTION

The Upper Jurassic Morrison Formation is a spectacular and recognizable formation across the Colorado Plateau, and is distinctive because of its array of rainbow-colored mudstone, siltstone, and sandstone beds that typically form a “badlands” landscape. Sections of the Morrison Formation were measured at the Miners Draw and Rainbow Draw areas to describe beds and place the fossil log sites in stratigraphic and sedimentological context. At the Miners Draw area, several sections were measured to capture the entire Morrison Formation (Windy Hill, Tidwell, Salt Wash, and Brushy Basin Members) and the stratigraphic horizon of the fossil log site because distance between base and top of the Morrison is large and complicated by being exposed on the nose of a southwest-plunging anticline (figure 4). The measured section of the Salt Wash Member began at the eastern group of logs along a small drainage that contains the top of the Stump Formation to the top of a ridge where the fossil log-bearing interval and the overlying marker bed are exposed (figure 5A). Concurrent systematic work will prompt a nomenclatural transfer of this species to the genus Agathoxylon (Gee and others, 2019)

DISCUSSION

SUMMARY