Masters Theses

Date of Award

8-1992

Degree Type

Thesis

Degree Name

Master of Science

Major

Geography

Major Professor

Steven G. Driese

Committee Members

Robert D. Hatcher, Jr., Harry Y. McSween

Abstract

Crustal extension and initiation of rifting of Laurentia during the Late Proterozoic resulted in formation of a northeast-trending system of discontinuous to continuous, half-graben basins situated cratonward of the Iapetus Ocean spreading ridge. Thick accumulations of sandstone, siltstone, bimodal volcanic rocks, conglomerate, diamictite, and minor limestone were deposited largely in response to rifting and relief formation on the basin margins.

The Grandfather Mountain Formation contains five stratigraphically and compositionally distinct conglomerate/diamictite units and one pebbly sandstone unit which cap coarsening-upward, basin-fill sequences. The progradational sequences average 1300 m thick and are composed of a succession of volcanic flows (basalt/rhyolite) and/or siltstone, succeeded by fine- to coarse-grained feldspatholithic sandstone, succeeded by pebbly sandstone and conglomerate. Major rifting events (or clusters of events) occurred during deposition of volcanic rocks and fine-grained lacustrine or marine, and fluvial sediment near the basin margin fault After a time lag, alluvial fans and fan-deltas prograded basinward from the margin over the fine-grained sediment. Smaller-scale coarsening-upward sequences (few to 10's m) are attributed to avulsion and lobe progradation due to inherent fan/fan-delta/subaqueous slope processes and to progradation following localized faulting events.

Southwest-fining along strike of three of the five conglomerate units suggests: 1) derivation from the northeast, possibly from an accomodation zone and from the Mount Rogers Formation, or 2) more extensive, coarser-grained, southeastward progradation in the northern half of the basin. The Grandfather Mountain and Mount Rogers basins may have developed as an asymmetric, alternating, half-graben pair and at various times were joined or separated by an accommodation zone.

The polymictic conglomerate of the Grandfather Mountain Formation is dominated by felsite and basalt clasts and contains lesser amounts of crystalline basement and sedimentary clasts. Two compositional sequences (upper and lower) are present within the conglomerate and are delineated by the presence or absence of perthite phenocrysts in felsite clasts. The lower sequence is dominated by porphyritic quartz-perthite felsite clasts and details an unroofing sequence: felsite -> sandstone and siltstone -> crystalline basement. In contrast, the upper sequence is dominated by felsite clasts containing only quartz phenocrysts (in the Banner Elk conglomerate) and basalt clasts (in the Broadstone Lodge diamictite).

Certain conglomerate clasts are most reliably matched to nonconformably underlying Grenvillian Blowing Rock Gneiss and the intraformational Montezuma basalt Felsite clasts may be derived from either Grandfather Mountain Formation or Mount Rogers Formation rhyolite. Other clasts were derived from other, as yet unidentified, source terranes that have been eroded away or are not exposed.

Four facies associations are composed of thirteen descriptive facies. Lateral and vertical changes in facies and facies associations of the conglomerate units of the Grandfather Mountain Formation indicate that coarse-grained alluvial fans, fan-deltas/subaqueous slopes, and braidplains prograded from the basin margins displacing finer-grained braidplain and marine or lake deposits back toward the basin center. Subaqueous (marine or lake?) slope and large-scale subaqueous channel deposits are more significant basin fill environments in the Grandfather Mountain Formation than previously thought. Their presence is particularly indicative of high relief due to basin-margin faulting.

Differing clast composition and grain size between conglomerate units as well as interpreted hydrodynamics produce heterogeneous longitudinal bar sequences, braidplain and fan styles. The heterogeneous styles are due to heterogeneous fluvial processes and the complex interplay between proximal and distal environments such as at the alluvial fan-to-braidplain transition. Evidence in support of a glacial or proglacial origin for deposits in the upper part of the Grandfather Mountain Formation is either absent or ambiguous at best.

Methods used in this study, if applied to other ancient rift sequences, especially those exposed in the Appalachian Blue Ridge, will further delineate rifting episodes, rift shoulder and basin paleogeography, and provide insight into subsurface stratigraphic patterns within rift basins along modem passive margins.

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