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.

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