Browsing by Author "Harris, Charles William"

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    A sedimentological and structural analysis of the Proterozoic Uncompahgre Group, Needle Mountains, Colorado
    Harris, Charles William (Virginia Polytechnic Institute and State University, 1987)
    Siliciclastic sediments of the Proterozoic Uncompahgre Group can be subdivided into stratigraphic units of quartzite (Q) and pelite (P); these units include a basal, fining- and thinning-upward retrogradational sequence (Q1-P1) that records the transition from an alluvial to a shallow-marine setting. Overlying the basal sequence are three thickening- and coarsening-upward progradational sequences (P2-Q2, P3-Q3 and P4-Q4) that were influenced by tide-, storm- and wave-processes. The progradational units are subdivided into the following facies associations in a vertical sequence. Outer-to inner-shelf mudstones, Bouma sequence beds and storm beds of association A are succeeded by inner-shelf to shoreface cross-stratified sandstones of association B. Conglomerates and cross-bedded sandstones of upper association B represent alluvial braid-delta deposits. Tidal cross-bedded facies of the inner shelf/shoreface (association C) gradationally overlie association B. Interbedded within the tidal facies in upper association C are single pebble layers or <1 m-thick conglomerate beds and trough cross-bedded pebbly sandstones. Single pebble layers could be due to storm winnowing whereas conglomerates and pebbly sandstones may record shoaling to an alluvial/ shoreface setting. A temporally separated storm/alluvial and tidal shelf model best explains the origin and lateral distribution of facies in the progradational sequences. The presence of smaller progradational increments in the mudstone dominated units (P3) and the recurrence of facies associations in the thick quartzite/conglomerate units (Q2, Q3, Q4) suggests that external cyclic factors controlled sedimentation. A composite relative sea level curve integrating glacio-eustatic oscillations and long-term subsidence may account for the evolution of the thick progradational sequences of the Uncompahgre Group. Sedimentary rocks of the Uncompahgre Group have been subjected to polyphase deformation and greenschist facies metamorphism. Phase 1 structures (localized to the West Needle Mountains) include bedding-parallel deformation zones, F₁ folds and an S₁ cleavage. Phase 2 coaxial deformation resulted in the development of upright, macroscopic F₂ folds and an axial-planar crenulation cleavage, S₂. In addition basement-cover contacts were folded. Phase 3 conjugate shearing generated strike-parallel offset in stratigraphic units, a macroscopic F₃ fold, and an S₃ crenulation cleavage. In addition, oblique-slip, reverse faults were activated along basement-cover contacts. The Uncompahgre Group unconformably overlies and is inferred to be parautochthonous upon ca. 1750 Ma gneissic basement that was subjected to polyphase deformation (DB) and amphibolite facies metamorphism. Basement was intruded by ca. 1690 Ma granitoids. Deformation of gneissic and plutonic basement together with cover (DBC) postdates deposition of the Uncompahgre Group. The structural evolution of the Uncompahgre Group records the transition from a ductile, north-directed, fold-thrust belt to the formation of a basement involved “megamullion" structure which was subjected to conjugate strike-slip faulting to accommodate further shortening. DBC deformation may be analogous to the deep foreland suprastructure of an orogenic belt that developed from ca. 1690 to 1600 Ma in the southwestern U.S.A ..
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    An unconformity in the Carolina slate belt of central North Carolina: new evidence for the areal extent of the ca. 600 Ma Virgilina deformation
    Harris, Charles William (Virginia Polytechnic Institute and State University, 1982)
    Detailed mapping in the Ramseur, N.C. 7 1/2' quadrangle has shown that lithostratigraphic units of the Virgilina sequence, units 11 and 111 (Glover and Sinha, 1973) in the Roxboro-Durham, N. C. area can be extended into central North Carolina. The volcanic stratigraphy is composed of the Hyco, Aaron and Uwharrie Formations, all of which have been subjected to greenschist facies metamorphism. The oldest map unit, the Hyco Formation, consists of intermediate(?) lava flows, pyroclastic and volcaniclastic rocks. Deposition of these units was in a subaqueous environment although some units are indicative of transient subaerial conditions. The Aaron Formation is a volcanic epiclastic sequence composed of conglomerate, pebbly and feldspathic arenite with intercalated siltstone, argillite and vitric tuff. The arrangement of sedimentation packages in the Aaron Formation are analogous to those of a coarse grained retrogradational submarine fan sequence. In the western part of the map area the Uwharrie Formation unconformably overlies the Hyco Formation. The Uwharrie consists of a bimodal (felsic-mafic) sequence of lava flows, pyroclastic and volcaniclastic rocks, all of which were deposited in a subaqueous environment. Structural data indicates that the older units of the Hyco and Aaron Formations were folded (F₁) and faulted during the Virgilina deformation (D₁). The entire volcanic sequence of Hyco, Aaron and Uwharrie Formations was subsequently folded (F₂) and metamorphosed during the Taconic deformation (D₂). Associated with the Taconic event is the development of a pervasive spaced anastomosing cleavage (S₂) in the volcanic lithologies. Previous regional correlations preferred by Wright and Seiders (1980) are thought to be incorrect. It is proposed in this paper, as first suggested by Glover (1974), that an angular unconformity separates the older volcanic strata of the Virgilina sequence from the younger units of the central N.C. sequence. The presence of an unconformity is indicated by 1) the truncation of lithologies comprising the Hyco Formation at the contact between the Hyco and Uwharrie Formations, 2) the deviation of macroscopic fold trends from the Hyco and Aaron Formations to the adjacent Uwharrie Formation and 3) the intrusion of felsic dikes equivalent to those comprising the Uwharrie Formation, which crosscut the older units and structures of the Virgilina sequence. The Virgilina deformation is probably correlative in time with the Late Precambrian Monian, Cadomian and Pan-African orogenies which effect similar age volcanic terranes like the older Carolina slate belt. In this study it is proposed that the name Virgilina deformation should have precedence over the term Avalonian event, because of the relation of the former 'to compressional tectonics versus the extensional tectonism prevalent in the latter. The Virgilina deformation may be attributed to active plate margin tectonics associated with a former volcanic arc.