Date of Award

Fall 8-10-2017

Degree Type

Report

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Benjamin J. Burger, PhD

Second Advisor

W. David Liddell, PhD

Third Advisor

Kenneth Carpenter, PhD

Abstract

This project examines a poorly studied sandstone ridge called Snake John Reef located 22 miles southeast of Vernal, in northeastern Utah. Previously this ridge was mapped as exposures of late Cretaceous, undifferentiated Mesaverde Group, and recently unidentified dinosaur fossils have been found along the ridge by the Utah Field House of Natural History State Park Museum. Stratigraphic sections, petrographic thin sections, and collection and study of fossils from Snake John Reef were undertaken to understand the stratigraphic relationship as well as to reconstruct the depositional environment of the dinosaur bearing units. Snake John Reef represents exposures of three late Cretaceous formations, the lower Sego Sandstone, middle Iles Formation, and upper Williams Fork Formation which can be diagnosed on differences in lithology. The units are capped by an unconformity with the Eocene Colton Formation. Fossil shark teeth (Scapanorhynchus, Cretolamna, and Squalicorax) are found in the lower Sego Sandstone, while dinosaur bones are located in the middle Iles Formation, and represent fragmentary but provisionally identified bones of ornithischian and tyrannosaurid dinosaurs. Fossil conifers (Geinitzia sp.) were also found in the Iles Formation, while fossil wood bored by Teredo (shipworms) is found in the upper Williams Fork Formation indicating close proximity to the ocean. This shows a marine to terrestrial transitional sequence, and an overall regression of the coastline. Petrographic study of the sandstone units indicate that they are best classified as calclithites composed of crystalline limestone with bituminous coal clasts. The absence of quartz grains indicate that the area represented a localized sediment starved coastal system, that may have been protected by barrier islands along a forested coastline. The presence of coal beds in the upper Williams Fork Formation indicate the presence of swamps higher in the section. Angularity of grains, abundance of poorly sorted fossil wood fragments, as well as sedimentary and paleontological evidence supports the interpretation that the coastline was prone to tropical storms that may have frequented the Western Interior Seaway during the Cretaceous Period. A major sequence boundary is found at the contact between the Sego Sandstone and Iles Formation representing a subaerial unconformity with an abundance of bioturbation 175.5 meters above the lower contact with the Mancos Shale. The Iles Formation represents a low stand system tract during a forced regression, with the upper Trout Creek Sandstone Member of the Iles Formation representing a short term transgressive system tract. In conclusion, the ridge along Snake John Reef presents a unique coastal depositional system during the final regression of the Western Interior Seaway that preserves dinosaur and plant fossils along a storm prone coastline during the Cretaceous.

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