Casella, Clarence J.
M.S. (Master of Science)
Department of Geology
Precambrian rocks of the West Fork Lake Quadrangle are suites of metamorphic and igneous rocks. The metamorphic rocks are principally gneisses of the amphibolite grade which includes mafic (amphibole), felsic, and quartzo-felspathic gneisses. This sequence was last affected by a major thermal event 1.8 billion years ago. Small bodies of garnet and biotite gneiss and small lenses of marble and calc-silicate rock are associated with the gneissic complex described above. Igneous rocks consist of a small mafic pluton and granites. The mafic rocks are bodies of periodotite enclosed in a large mass of olivine gabbro. Ophitic amphibolite surrounds the olivine gabbro. No radiometric dating has been done on this pluton. Granites of the West Fork Lake Quadrangle correlate with the Boulder Creek Granite of the Front Range and are 1.7 billion years old. Post-Precambrian igneous rocks are porphyry dikes which cut Precambrian and Cretaceous age rocks. Early and Middle Paleozoic sedimentary rocks are absent; the oldest Paleozoic rock is the Forelle Limestone which is 10-15 feet thick. Triassic rocks are red mudstone and siltstone, red and white sandstone and local limestone. The Chugwater and Jelm Formations and Chinle equivalent rocks suggest marginal marine environments and are 600 feet thick. Jurasic age rocks are pink, frosted sandstone, mud and siltstone and minor limestone lenses. Montmori1 loni t i c clays are present. Environments appear to be mainly non-marine. The Jurassic Formations of the Entrada, Sundance and Morrison reach a thickness of 500 feet. Cretaceous rocks are marginal marine and marine sandstone, shale and calcarenite. Maximum thickness is 850 feet. Tertiary rock consists of one basal, conglomeratic alluvial facies member of the Browns Park Formation. Ash beds in the upper members of this formation have been dated (near the West Fork Lake Quadrangle) at 11 million years by K-Ar methods. Precambrian structures in the study area are relict features of earlier deformations. Well-defined structural elements can be seen in the field and are a result of Laramide activity. The prominent features are north-south trending thrust faults which dip east. Synclines of Mesozoic rock are located at the face of the faults. Dip-slip faulting occurred during the folding of these synclines. Evidences of at least two major glaciations are seen in types of till and glacial -morphological features. Other surficial deposits, mass movement slumps, alluvial benches, and colluvium are present.
Graff, Paul, "The areal geology of the West Fork Lake Quadrangle, northern Colorado" (1973). Graduate Research Theses & Dissertations. 5315.
viii, 59 pages
Northern Illinois University
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