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Research Article| February 01, 1985 Revised interpretation of the age of allochthonous rocks of the Uncompahgre Formation, Needle Mountains, Colorado B. J. TEWKSBURY B. J. TEWKSBURY 1Department of Geology, Hamilton College, Clinton, New York 13323 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1985) 96 (2): 224–232. https://doi.org/10.1130/0016-7606(1985)96<224:RIOTAO>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation B. J. TEWKSBURY; Revised interpretation of the age of allochthonous rocks of the Uncompahgre Formation, Needle Mountains, Colorado. GSA Bulletin 1985;; 96 (2): 224–232. doi: https://doi.org/10.1130/0016-7606(1985)96<224:RIOTAO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The middle Proterozoic Uncompahgre Formation consists of a deformed and mildly metamorphosed sequence of interlayered quartzites and metapelites. Previous workers advanced the notion that the Uncompahgre Formation represents an isolated remnant of a sequence of supracrustal sediments deposited on deeply eroded crystalline basement sometime after 1680 Ma and deformed and metamorphosed before 1430 Ma. These conclusions followed from previous interpretations that the contact between Uncompahgre lithologies and adjacent metamorphic units is an unconformity truncating dikes ranging in age from 1640 to 1680 Ma. A considerable body of evidence suggests, however, that the Uncompahgre Formation is, in fact, allochthonous and that age interpretations must be revised.Significant displacements between Uncompahgre units and adjacent metamorphic sequences are suggested by an increase in accumulated strain toward the contact zone, coupled with a 3- to 25-m-thick zone of mylonites, protomylonites, and phyllonites in the contact zone itself. The geometry of the contact and the presence of faults of similar attitude within the Uncompahgre itself indicate an allochthonous sheet of Uncompahgre units emplaced along a sole thrust of variable dip, with imbricate internal north-dipping thrusts and a northern thrust contact of similar attitude. Folding within the Uncompahgre Formation was intimately involved with southward emplacement and shortening of the allochthon.Because the Uncompahgre Formation is allochthonous, time of deposition of the protolith cannot be bracketed by the ages of surrounding crystalline basement. Deformation in the Uncompahgre Formation must, however, postdate 1680 Ma and predate intrusion of the Eolus batholith at ∼1430 Ma. The Uncompahgre Formation may well be older than previously presumed and likely has equivalents in northern New Mexico. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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