Date of Award
Master of Science (MS)
Michael E. Nelson
The Early Cretaceous (Aptian/Albian) Cedar Mountain Formation of central Utah is composed of approximately 130 meters of an upper silty mudstone unit, the main body (unnamed shale member), and 15 meters of a basal conglomerate and sandstone, the Buckhorn Conglomerate Member. The main body lies unconformably below the Late Cretaceous (Cenomanian) Dakota Formation, while the Buckhorn Member unconformably overlies the Late Jurassic (Tithonian) Morrison Formation. The dominant lithologies of the Buckhorn Member are polymodal sorted well indurated cobble and boulder chert conglomerates and litharenites of varying maturity with chalcedony and calcite cement. In addition, anhydrite-rich (after gypsum) micritic mudstones are present in isolated deposits. The top of the Buckhorn displays a reduction in grain size and interfingers with mudstone of the overlying unit. Varying clast lithologies exist in the Buckhorn. Most abundant are colored cherts containing Late Paleozoic marine genera. Other clasts were derived from Early Mesozoic quartzites and conglomerates. Brown petrified wood pebbles are also common. All clasts are randomly distributed throughout the Buckhorn section. The thickness of the Buckhorn is highly variable within short distances ranging from zero to 32 meters which may be attributed to non-deposition, post-burial erosion, or recent erosion. Laterally, the Buckhorn terminates south of the Henry Mountains region, merges with the lower portion of the Burro Canyon Formation to the east, and merges with several coarse clastic toungues to the north. Westward, the Buckhorn may be correlative with the Indianola Group. The Sevier Belt had become a positive area by the middle of Early Cretaceous time. Paleocurrent data suggest that east to southeast-flowing streams deposited and reworked coarse clastic deposits in the depositional basin. Longitudinal and transverse bar forms are recognized in outcrops and indicate that the Buckhorn was deposited by braided streams. Incomplete fining upward sequences indicate a subsiding basin with pulsations of activity in the Sevier Belt. Periods of quiescence are noted by finer-grained sandstones and local lake deposits. Rejuvenation of upper flow regimes ripped up fragments of the mudstones and deposited them with the overlying conglomerates. The low quantity of mud in the depositional system of coarser clastics precludes debris flows as a mechanism of transport. Instead sheetfloods of low viscosity or hyperconcentration flows of higher viscosity moved the debris. Cross bedding, pebble imbrications, and sandstone channels preclude a non-fluvial mode of deposition. Instead, the Buckhorn was deposited by braided streams with highly variable flow regimes.
Copyright 1986 Steven J. Conley
Conley, Steven J., "Stratigraphy and Depositional Environment of the Buckhorn Conglomerate Member of the Cedar Mountain Formation (Lower Cretaceous), Central Utah" (1986). Master's Theses. 1984.