Master's Theses

Department

Geosciences

Degree Name

Master of Science (MS)

Abstract

The Aptian to Albian Cedar Mountain Formation of the western San Rafael Swell in east-central Utah contains two members, the Buckhorn Conglomerate and an unnamed upper main member. The Buckhorn Conglomerate disconformably overlies the Late Jurassic Morrison Formation and is an indurated, discontinuous braided stream deposit composed mainly of coarse clast-supported conglomerate that often grades upward into a matrix-supported conglomerate and coarse sandstone. The Cedar Mountain main member rests conformably upon the Buckhorn Conglomerate and may be paraconformable with the upper Morrison Formation where the Buckhorn is absent. The main member is largely unconsolidated floodplain silts and clays that generally were pedogenically altered to semi-arid alfisols in the lower part of the member and were less altered, to inceptisols, in the upper part. Inceptisols and fossiliferous deposits suggest shorter hiatuses in the upper main member. Channel and overbank siltstones and sandstones in the main member indicate a sluggish high-sinuosity fluvial regime. The Cenomanian Dakota Sandstone disconformably overlies the main member of the Cedar Mountain Formation. The Dakota Sandstone varies by locality from thin shoreline-sandstone facies to thicker channel-dominated delta depocenters. The delta depocenters contain some interdistributary deposits. Recent studies indicate that widespread Lower Cretaceous gravels, including the Buckhorn, were generally distributed eastward from the Sevier Belt hinterland across the Western Interior. Sometime during deposition of the Buckhorn Conglomerate, initial thrusting in the Sevier Belt loaded the hinterland, which resulted in eventual isostatic lowering of the uplift, and deposition shifted closer to the thrust belt. This aggradational trend began with the appearance of matrix-supported conglomerate in the Buckhorn and continued throughout deposition of the Cedar Mountain main member. The Dakota Sandstone exhibits fluvial characteristics in the lower parts of the thicker deposits, but an increase in marine influence by encroachment of the Mancos Sea is demonstrated in the upper parts of the thicker sections and in the thinner deposits by the appearance of marine fossils and marine sedimentary features. The Dakota represents transgressively overprinted delta depocenters connected by shoreline deposits. Twenty - three measured sections of the Cedar Mountain-Dakota Sandstone sequence demonstrate its variable thickness in the western San Rafael Swell. The variation in thickness suggests secondary depositional control was exerted by paleotopography that formed during the hiatus between the Morrison and Cedar Mountain formations.

Advisor

Michael E. Nelson

Date of Award

Summer 1990

Document Type

Thesis - campus only access

Rights

© 1990 David A. Hunter

Comments

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