Doctoral Dissertations

Date of Award

12-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Steven G. Driese

Committee Members

Kenneth R. Walken

Abstract

A detailed sedimentologic and stratigraphic examination of the Juniata Formation, Tuscarora Sandstone, and Rose Hill Formation (Upper Ordovician to Lower Silurian) revealed new insights on the process sedimentology and regional stratigraphic architecture of these units, exposed within Alleghanian thrust sheets of the Appalachian Valley and Ridge in southwestern Virginia. The examined stratigraphic units are part of the Martinsburg-Shawangunk clastic wedge, which represents the fill of a peripheral foredeep formed during the Taconic orogeny in the central Appalachians. The stratigraphic units succeed the Martinsburg Formation and mirror the secular development of the Taconic orogen. The regional "Tuscarora unconformity" bisects the Martinsburg-Shawangunk clastic wedge. It separates a regionally extensive and sedimentologically distinct "upper" Tuscarora Sandstone from various subjacent stratigraphic units, including the regionally restricted “lower” Tuscarora Sandstone. Isostatic basin rebound is favored over glacio-eustasy as the causative mechanism for the "Tuscarora unconformity." Following completed terrane accretion (Chopawamsic island-arc and accretionary complex) and cessation of thrusting, erosional and tectonic (extensional collapse?) unloading of the orogen caused isostatic uplift of both the orogen and adjacent foredeep. This led to development of a regionally extensive erosional surface within the foredeep. Vestiges of tectonic thirming of the orogen are preserved in the extensional successor basins (Arvonia and Quantico basins) within the Virginia Piedmont Creation of accommodation space for the upper part of the Martinsburg-Shawangunk clastic wedge can be linked to another phase of load-induced subsidence within the foreland area. This renewed phase of tectonic deepening of the foredeep might be related to accretion of a second terrane (Goochland microcontinent?), outboard of the earlier accreted terrane, and to associated thrusting. The Juniata Formation and "lower" Tuscarora Sandstone beneath the "Tuscarora unconformity" represent storm-dominated deposits of a shallow-marine foredeep. Identified shallow-marine hydrodynamic zones range from the middle shoreface to the offshore marine. The "lower" Tuscarora Sandstone is interpreted as a wedge-shaped sediment body, which constitutes the lithostratigraphic equivalent of the upper Juniata Formation in more proximal outcrop belts. The thick section of shallow-marine deposits of the Juniata Formation and "lower" Tuscarora Sandstone implies a delicate balance between creation of accommodation space (subsidence predominantly through tectonic loading, supported by sediment compaction and loading) and sediment supply. The sandier-upward trend from the upper Martinsburg Formation through the "lower" Tuscarora Sandstone probably reflects a combination of increase in relief of the orogen and change in source-rock lithologies. A cyclic organization is not readily apparent within the Juniata Formation and "lower" Tuscarora Sandstone. The "upper" Tuscarora Sandstone, situated above the "Tuscarora unconformity," represents predominantly storm-dominated deposits of the upper shoreface to the offshore transition zone. The "upper" Tuscarora Sandstone constitutes a continuous transgressive facies succession, which is bound at the base by a sequence boundary ("Tuscarora unconformity") and at the top by a (maximum) marine-flooding surface, along which the storm-dominated deposits of the Rose Hill Formation commence. The "upper" Tuscarora Sandstone, furthermore, forms a transgressive systems tract composed of a single retrogradational parasequence (except at the most distal outcrop locality). Development and preservation of this transgressive facies succession is believed to be related to an episode of load-induced flexural subsidence within the Taconic foredeep. Gradual and continuous subsidence created accommodation space at a rate which outpaced the rate of ample sediment supply. On a regional scale (central Virginia to northern Alabama/Georgia), the uppermost Ordovician to Lowct Silurian stratigraphic units above the Upper Ordovician red-bed succession in the central and southern Appalachians can be subdivided into the distinctly different White Sandstone Lithosome' (Tuscarora Sandstone, Poor Valley Ridge Sandstone Member of the Clinch Sandstone, and some beds/bedsets within the Rockwood Formation ) and 'Heterolithic Lithosome* (Rose Hill Formation, Rockwood Formation, and Hagan Shale Member of the Clinch Sandstone). In southwestern Virginia, the ‘White Sandstone Lithosome' dominates, whereas in extreme southwestern Virginia and eastern Tennessee, the 'Heterolithic Lithosome' dominates. The latter commences with the ‘basal transgressive sandstone' and encapsulates the more exotic 'White Sandstone lithosome’. The ‘White Sandstone Lithosome’, contrary to ruling opinion, is not a simple, homogeneous and uniform quartzarenite blanket, but is divisible into separate components. Each component possesses its distinct tie to different developmental stages of the Taconic orogen in the central Appalachians and to eustatic sea-level changes. Detritus for the "lower" Tuscarora Sandstone was shed from the growing Taconic orogen and was deposited within the foredeep during thrust-load induced foredeep subsidence. The glacio-eustatic sea-level events during the Hirnantian (latest Ordovician) are thought to be reflected in the Whirlpool Sandstone (subsurface of southern Ohio and parts of West Virginia), whose stratigraphic position and glado-eustatic origin corresponds to that of the ‘basal transgressive sandstone' (extreme southwestern Virginia to northern Alabama/Georgia). Detritus for the Whirlpool Sandstone was derived from erosion of "lower" Tuscarora Sandstone during glado-eustatic sea-level drop. Flexural isostatic uplift/rebound converted part of the Taconic foredeep from an active depositional site to an active erosional site. In the area of uplift (denudation area), evidence for the Hirnantian glado-eustatic sea-level events was cut out, and the produced erosional detritus (derived predominantly from the "lower" Tuscarora Sandstone) , was redistributed radially away from the denudation area into parts of the foredeep not affected by rebound, and into areas at the fringe of the foredeep and beyond (parking area and fill area). The ‘White Sandstone Lithosome’ in extreme southwestern Virginia and eastern Tennessee (Poor Valley Ridge Sandstone and bedsets within the Rockwood Formation) is interpreted to be part of this redistributed material, which interrupted the normal, mud-dominated sedimentation in that area. The "Clinton" sandstone in the subsurface of southern Ohio is regarded as a lithostratigraphic equivalent. The quartz-rich detritus was reworked and distributed within the fill area by the ambient storm system. The isolated bedsets of the ‘White Sandstone Lithosome' within the Rockwood Formation, demarcating the farthest spread of this lithosome, might represent forced regressions. Renewed compressional tectonism and active foredeep subsidence converted the foredeep back to a depositional site, thereby gradually shutting off supply of white quartz sand to peripheral areas. Transgressive reworking of erosional detritus covering the denudation area, together with new sediment contribution from the Taconlc orogen during this new phase of thrusting, provided the detritus for the "upper" Tuscarora Sandstone. Deposition of the ‘White Sandstone Lithosome' within the active foredeep was terminated by rapid marine flooding during the ? early Telychian eustatic sea-level rise.

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