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The Pennsylvanian/Permian Hermosa Formation in southwestern Colorado is composed of arkosic sandstone, marine limestone, and shale that formed at a time when the region consisted of open-marine carbonate shelves and coastal plains. During the time of deposition, there were many tectonically active highlands in the region, including the Ancestral Rocky Mountains, San Luis Highlands, and the Uncompahgre, Emery, Zuni-Defiance-Kaibab Uplifts. Previous studies have attributed the source of the main sediment supply of the Hermosa Formation to the Uncompahgre branch of the Ancestral Rocky Mountains with smaller amounts of elastics from the Defiance-Zuni uplifts (Wengerd and Matheny, 1958; Wengerd and Strickland, 1954; Baars and Stevenson, 1981). None of these studies, however, specifically investigated source areas and provided no evidence for their claim of provenance. Because the area around the Hermosa Formation consisted of many dynamic regional uplifts, it is possible that there was more than one sediment source area. By analyzing the sediment from the Hermosa Formation and determining the provenance, it will provide a better understanding of the paleogeographic evolution of this region. This provenance study utilized Gazzi-Dickinson point counts in conjunction with optical mineralogy techniques to better constrain the source location. The results indicated that sediment was derived from a continental block provenance, contained moderately mature grains, and had a plagioclase compositional range near the pure albite endmember. Based on these findings, the Hermosa samples most likely were derived primarily from the Ancestral Rocky Mountains, substantiating the claims of provenance from previous studies in the area.
Pourtabib, Kristina P., "Determining the Sediment Source Area Through Petrographic Analysis of the Hermosa Formation, Silverton, Southwestern CO" (2013). Undergraduate Honors Theses. 54.