Asking the pikas politely…
The Hermosa formation lies within a sedimentary basin situated in southwestern Colorado, northwestern New Mexico, northeastern Arizona, and southeastern Utah. Spencer described and defined the Hermosa formation in the Animas Canyon north of Durango, Colorado, in 1900, and subsequently Pennsylvanian rocks of similar stratigraphic position and faunal characteristics near Moab, Utah, were correlated with the Animas Canyon sediments. Pennsylvanian sediments exposed in the San Juan River Canyon of southeastern Utah were described in 1909, but were given a tentative, separate nomenclature. Later, the sediments in southeastern Utah were correlated with those of the Hermosa formation of the Animas Valley in Colorado, and the name Hermosa was applied to these sediments in Utah. The Rico formation has generally been accepted as a unit separate from, and above, the Hermosa, although some authors designate the Rico formation as a provisional unit only. The absence of distinct paleontological and lithologic features makes the validity of the Rico division questionable. Its stratigraphic position cannot be established on a regional basis. The Rico division is omitted and sediments usually assigned to this unit are included within the Hermosa formation. The Pennsylvanian evaporites, exposed by fault contact in the salt anticlines of western Colorado and eastern Utah, have been designated previously as the Paradox formation, separate from, and below, the Hermosa formation. Evidence from exposures of the Hermosa formation in the Raplee anticline in southeastern Utah and from well logs indicates that the Paradox evaporites and shales lie within the Hermosa formation. The Hermosa formation of this thesis includes, in descending order, the Upper Hermosa, the Paradox member, and the Lower Hermosa. Above the Hermosa are red, arkosic, and conglomeratic sandstones and shales of the Cutler formation; beneath the Hermosa are cherty, conglomeratic red shales of the Molas formation. Exposures of the Hermosa formation at Ouray, Colorado, and at Fisher Valley near Moab, Utah, as well as logs of wells along the northeastern rim of the Hermosa Basin, indicate that the Cutler formation interfingers with the Hermosa formation. Cutler-like sediments were deposited continuously along the steep southwestern flank of the Uncompahgre Uplift, San Juan Mountains, and the Ft. Defiance Uplift during Pennsylvanian and Permian time. Thin section study of exposed rocks of the Hermosa formation indicates that the sandstones are generally arkosic with oligoclase as the most common detrital fragment. Oligoclase and quartz are common detrital constituents in the limestones. The presence of this unstable sodic plagioclase as well as extreme angularity of involved grains indicates rapid deposition with a minimum amount of abrasion of detrital fragments. Lithofacies maps based upon information obtained from measured sections and well logs delineate lateral variations in Hermosa lithology. Interpretation of lithofacies and isopach maps indicates that the basin in which the Hermosa formation was deposited was deepest, or had the greatest amount of subsidence, adjacent to the Uncompahgre Uplift. The southwest flank of this basin sloped gently from the uplands of the Ft. Defiance area into the deep parts of the basin. Uplift of the Uncompahgre-San Juan area and the Ft. Defiance area occurred following deposition of the Leadville limestone. These highlands provided source areas for sediments of the Hermosa basin. The lithology of the Hermosa formation is a reflection of the recurrent uplift of the surrounding highlands and fluctuating shorelines within the basin. Deposition of the evaporites and shales of the Paradox member occurred in the interior of the basin while alternating marine limestones and continental sandstones and shales were deposited around the periphery of the basin. The Paradox evaporites are correlated with similar evaporites of Pennsylvanian age deposited near Eagle, Colorado. Increasing crustal movement toward the end of Hermosa time may have involved salt flowage within the Paradox member which in turn produced structural features now obscured by overlying sediments. This increasing uplift of the highland areas brought Paradox deposition to a close and the alternating sandstones and limestones of the Upper Hermosa spread across the Hermosa Basin. Eventually uplift was so great that the Hermosa Basin was filled completely with subsequently eroded material. Coarse, Cutler-like sediments that had been deposited continuously along the flanks of surrounding highlands since the end of deposition of the Leadville limestone now spread across the Hermosa Basin. Hermosa time ended with the filling of the basin and the elimination of marine conditions. The lithofacies map indicates areas that should be favorable for the accumulation of petroleum by outlining the lithologic associations that are producing now, and also by outlining those areas of similar lithologic associations that have not yet been tested. Reefing present in the San Juan Canyon probably extends northwestward parallel to the isolith lines of the lithofacies map, and thus provides zones in which subsurface Hermosa reefing might be expected. Many favorable stratigraphic and structural features remain to be explored by drilling.
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