Stratigraphy and provenance of mudstone in the Phosphoria Formation

Three chapters address different aspects of mudstone sedimentology and stratigraphy in the Middle Late Permian Phosphoria Formation.;In the first chapter, variations in the oxygen isotope composition and quantitative cathodoluminescence analysis of silt size quartz recorded changing provenance and sea level in the Meade Peak member of the Phosphoria Formation. The relative amount of authigenic quartz silt was estimated by combining oxygen isotopic geochemistry and computer-analyzed SEM cathodoluminescence imaging. Cathodoluminscence and isotopic analysis were consistent in their interpretation of authigenic vs. igneous/metamorphic quartz content of the clastic fraction. The relative amount of authigenic quartz varied more during the transgressive phase of sea level. A subsequent decrease in authigenic quartz was associated with continued sea level rise and submergence of the Phosphoria Basin during the deposition of the Meade Peak member.;Chapter 2 demonstrated how methods are compared for analyzing grain size of silt (specifically loess) including settling-technique and 2-D thin-section methods. Samples of loess taken from cores from locations in Iowa and Nebraska were used to determine the systematic relationship between the methods. The grain size characteristics determined from 2-D images of impregnated polished sections were correlated to those generated from settling grain size analyses using a Sedigraph 5100. The regression for mean grain size, (Ymean 3-D = 1.0931*Xmean 2-D - 0.5187; r2 = 0.91) was found to be similar to those determined by previous 2-D/3-D studies of well-sorted sandstones.;In chapter 3, the major facies of the Phosphoria Formation were placed within associations that helped identify their genetic relationships. A more complete interpretation was made of the interaction of authigenic and clastic processes in a sequence stratigraphic framework. Special attention was paid to the description and interpretation of the largely overlooked silty facies in the Meade Peak member. The incorporation of wind-transported silt into the geologic model has resulted in a more thorough look at sedimentologic processes and facies relationships in the Phosphoria Formation. The interpreted processes were then used to create a sequence stratigraphic classification of the Phosphoria Rock Complex recognizing three Type 1 sequences that make up the overall sequence of phosphatic mudstone and associated facies.