Hauling field gear up Gothic Mountain…
Abstract The Robinson Member is one of eight carbonate members of the 6,000-ft (1,824 m), predominantly sandstone, Minturn Formation (Desmoinesian) in the study area west of the Gore Range in central Colorado. The Robinson Member consists of three limestone units (lower, middle, and upper) interstratified with marine and nonmarine sandstone, siltstone, and conglomerate. Total thickness of the Robinson Member ranges from 200 to 400 ft (60–120 m). Several fusulinid biozones can be recognized in the Robinson Member. They are parallel with and equivalent to the lower, middle, and upper limestones. Fusulina pristina and Fusulina curta do not appear to range above the lower Robinson. Fusulina rockymontana and Wedekindellina coloradoensis are present throughout the Robinson. Fusulina truncatulina has been found only in the upper Robinson and Fusulina plattensis is limited to the upper Robinson and overlying Elk Ridge Member. The areal distribution and the thickness of four time-transgressive facies which make up each of the Robinson limestone units imply that the late Paleozoic Eagle basin was an asymmetric depositional basin with a relatively deep marginal trough along the east side. The basin was bounded on the east by the Ancestral Front Range highland and on the southwest by an Ancestral Uncompahgre highland. From east to west the limestone units contain (1) an oolite facies composed of superficial oolites, pseudo-oolites, and oncolites in microspar or micrite; it is interpreted to be a shallow-water, high-energy deposit on the east edge of the marginal trough; (2) a tubular Foraminifera facies composed mainly of tubular Foraminifera micrite; (3) a phylloid algae facies of biomicrite containing abundant Archaeolithophyllum, Eugonophyllum, fusulinids, and Komia; and interpreted to be a deeper water deposit near the middle of the trough; (4) a stromatolite facies of stromatolites, laminated micrite, and vuggy ostracod-bearing intramicrudite, which indicates quiet-water to supratidal deposition near the western margin of the trough. Small Solenopora bioherms are present locally in this facies. The stromatolite facies grades westward into and is interbedded with siltstones and evaporites of the Eagle Valley Evaporite near the center of the Eagle basin. The overall patterns of the limestone facies and sandstone beds indicate intervals of rapid nondepositional marine transgression, followed by relatively slow regression in which a series of limestone facies was deposited. A typical sequence of lithofacies in the Robinson limestones from bottom to top is: argillaceous micrite, phylloid algae biomicrite, tubular Foraminifera biomicrite, and oosparite and/or oomicrite. Bioherms as thick as 80 ft (24 m) are in the phylloid algae and stromatolite facies. Nonphylloid algae appear to be the major components of these bioherms. However, recrystallization and dolomitization make recognition of the alga types difficult. Diagenesis of the limestones involves recrystallization, silicification, dolomitization, and intrastratal solution.
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