The lithofacies and paleogeography of the Crazy Hollow Formation, central Utah
Norton, Kelly L.
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The Eocene fluviatile Crazy Hollow Formation of central Utah is characterized regionally by its lithic variety, and is differentiated stratigraphically from its enclosing lacustrine rock units. The Crazy Hollow is mineralogically and stratigraphically equivalent to the informal "Tawny beds" found along the western Gunnison Plateau, and represents the full fluvial sequence found above the Green River and below various pyroclastic formations. The 3 my (near Salina) to 11 my (in western Gunnison Plateau) interval of deposition is determined by dates from the enclosing formations. The very rapid vertical and horizontal lithic changes in the unit distinguish it from other fluvial sequences in central Utah. It is like them in having feldspathic sandstone, but it is conspicuously different in having salt-and-pepper sandstone (of quartz and both black and gray chert). The Crazy Hollow extends northwest into Juab Valley with patchy exposures on the Gunnison Plateau, northeast to the southern Cedar Hills, southeast along the western Wasatch Plateau to northern Fish Lake Plateau, and southwest to the eastern Valley Mountains. A section near Aurora is suggested as a sup- plementary reference section because of its lithic variety, completeness and accessibility. The distinction between the Green River and Crazy Hollow is not sharp and clean, but can be defined by a color change from pale gray and green to red, an increase in clastic material or, locally, evidence of an erosional surface. The contact is locally disconformable, but in many instances the boundary is gradational—even with interfingering units. The depositional environment of the Crazy Hollow changed regionally from mixed-load streams in the southeast to alluvial deposition in the northwest, close to the margin of the Lake Uinta basin. The source of most of the Crazy Hollow sediment was the Laramide uplifts to the east—notably the Uncompahgre Uplift. Local, more siliceous clastic materials were derived from the eroding Sevier Orogenic Belt, along the west margin of the area and minor amounts of sediment were derived from reworked underlying sediments.