STRATIGRAPHIC ASSOCIATION AND DEFORMATION SEQUENCE OF THE UNCOMPAGHRE GROUP AND VALLECITO CONGLOMERATE, SOUTHEASTERN NEEDLE MOUNTAINS, COLORADO

Paleoproterozoic metasedimentary units in the Needle Mountains include the Uncompaghre Group (~ 2.5 km of quartzite and metapelite) and the Vallecito Conglomerate (~ 3 km of meta-quartz-pebble conglomerate). New geologic mapping and structural analysis from the southeastern Needle Mountains places the units in a new stratigraphic framework and correlates their deformation histories. In the eastern region of its exposure, Vallecito Conglomerate grades up and is transitional into quartzite and pelite identical to that of the Uncompaghre Group. The two units have a similar deformation history that consists of early layer parallel shear and mesoscopic folding (D1), followed by development of upright macroscopic folds and conjugate crenulation cleavages within the Uncompaghre Group (D2) and, late-stage, long wavelength folding that is synchronous with the growth of andalusite prophyroblasts (D3). Andalusite growth is presumably associated with emplacement of the ~1.43 Ga Eolus Grantie. Macroscopic D2 folds, similar in style to those in the Uncompaghre Group, are not well expressed within the siliclastic facies of the Vallecito Conglomerate, apparently due to contrasting rheologic properties of the two units. Shear zones with shallowly plunging stretching lineations parallel to L1 place the Irving Formation (volcanogenic basement) structurally above the Vallecito and are interpreted as D1 thrusts. Local top-to-the south and top-to-the west shearing of the Uncompaghre Group and Vallecito over the basement respectively records late stage reactivation of D1 shear zones. Recognition of this polyphase deformation sequence effectively solves one of the longstanding issues regarding the apparent orthogonal trends of macroscopic structures within the Uncompaghre Group and Vallecito Conglomerate. Furthermore, stratigraphic correlation of these units improves our understanding of regional syntectonic quartzite deposition during the Paleoproterozoic.