Stratigraphic framework, structural evolution and tectonic implications of the eastern Blue Ridge sequence in the central Appalachians near Warrenton, Virginia
MetadataShow full item record
The metasediments in the Warrenton area are the Lynchburg and Fauquier Groups, the latter of which is raised herein from a formation to a group. The lowest unit, the Bunker Hill Formation, is dominated by coarse- to very coarse-grained feldspathic sandstone, typically trough and planar cross-bedded, with minor fine-grained sandstone and siltstone or granule conglomerate. The overlying Monumental Mills Formation is dominated by thin bedded fine sandstone and siltstone, with minor laminated mudstone towards the top. The next overlying unit, Ball Mountain Formation, is composed of thick beds of medium to coarse sandstone and rare conglomerate, interbedded in black schists and dark laminated mudstones. The facies observed and correlation with previous work farther south suggests that these three formations record a continuous transgressive alluvial to deep water rift sequence. The Ball Mountain passes northwards to the lower Swains Mountain Formation, which is dominated by massive sandstone, and farther north to the upper Swains Mountain and Carter Run Formation, a continuous fining upwards succession from medium sandstone to laminated mudstone. From south to north the clastic sequence shows an overall thinning, and at the time of eruption of the Catoctin volcanics probably was shoaling as well.
The overlying Catoctin shows a basal volcanic breccia unit covered by a thick succession of lavas. Medium grained, parallel bedded feldspathic sandstone occurs as lenses along a regionally continuous belt in the upper part of the formation. Numerous metamorphosed mafic intrusives were observed within the clastic sequence, and evidence for Late Proterozoic faulting was documented.
The whole assemblage of lithofacies is interepreted to represent a continuous rift sequence, and the presently exposed section was probably oblique to the trend of the original rift. The rift stratigraphy can be traced in all the Warrenton area. The whole sequence is characterized by one low grade progressive metamorphism. Deformation is minimal and took place in two stages: Development of pervasive foliation (S 1) in varying degrees throughout the sequence (event: Dl). Subsequent folding and faulting and development of a localized discrete cleavage (S2) (event: D2). The observed structural features can be correlated with features described by other workers in adjacent areas. The map-scale structure is dominated by a change in attitude of the strata from east-dipping next to the basement, to open folded and gently dipping farther east. Considering that the whole Blue Ridge is an allochthonus thrust sheet, this change probably corresponds to the eastern subsurface edge of the basement block, on which the rift sediments were deposited, above a gently dipping decolement.
Some workers have proposed tectonic models of the central Appalachians in which accreted terrane boundaries, are present along the eastern Blue Ridge. The continuous rift stratigraphy and the tectonic evolution proposed herein do not support such models.
- Masters Theses