Stratigraphic interpretation of the Ordovician of the Appalachian Basin and implications for Taconian flexural modeling

Abstract

Models that reconcile the thicknesses of various Appalachian stratigraphic sequences in terms of subsidence associated with thrust loading are based on various assumptions about regional stratigraphy. Different assumptions, based on a different interpretation of Middle and Upper Ordovician stratigraphy, suggest that the geometry of lithospheric flexure due to Taconian thrust loading may have been different than predicted by existing models. Taconian flysch facies restricted to the eastern margin of the outcrop belt is the basis for identifying a thrust load‐induced foredeep. Coeval shallow marine facies were probably beyond the limit of thrust load influence. Stratigraphic thinning near the transition between the flysch and shallow marine facies is interpreted as a peripheral bulge and is located within the Valley and Ridge. Comparison is made with the Timor area north of Australia. Thrust load‐induced foredeeps and peripheral bulges predicted by this model are much narrower than predicted by previous models.

A Laurentian margin back-arc: The Ordovician Wedowee-Emuckfaw-Dahlonega basin

“…Additionally, isotopic evidence for the Deicke and Millbrig k-bentonites suggests they formed in a volcanic arc infl uenced by continental crust (Huff et al, 1992;Coakley and Gurnis, 1995;Kolata et al, 1996Kolata et al, , 1998Haynes et al, 2011;Samson et al, 1989). Although formation of the Blount basin in the southern Appalachians has been modeled as a typical foreland basin forming in advance of a developing thrust belt due to subduction of the Laurentian margin beneath an exotic island arc (Keller, 1977;Shanmugam and Walker, 1978;Shanmugam and Lash, 1982;Quinlan and Beaumont, 1984;Beaumont et al, 1988;Ettensohn, 2004;Diecchio, 1993;Finney et al, 1996), certain characteristics of the basin contrast with typical models of foreland basin evolution and with Ordovician clastic wedges in the central-northern Appalachians (e.g., Martinsburg). First, source rocks of the Blount basin consisted exclusively of Laurentian margin, shallow water sedimentary rocks and possibly basement igneous and metamorphic rocks (Kellberg and Grant, 1956;Cressler, 1970;Bayona and Thomas, 2003), but the sediments lack evidence or detrital contributions from volcanic or deep-water sedimentary sources (Mack, 1985), or Taconic-age detrital zircons (Merschat et al, 2010).…”

Stratigraphy, structure, and tectonics

“…2). West of Elkton, Virginia, these change into clastic sequences of the mid-to late Ordovician Taconic orogeny (Cooper and Cooper, 1946;Woodward, 1951;Diecchio, 1993), and in Fort Valley of the Massanutten synclinorium, Devonian black shales of the Acadian orogeny Biggs, 1975, 1976). West of Harrisonburg, Cambrian to Ordovician carbonates are again apparent in the hanging wall of the west-directed Little North Mountain thrust fault.…”

“…Oblique collisions of the Taconic terrane(s) left the present-day northern Virginia region in a protected reentrant that experienced only slight fl exure fold-ing (Thomas, 1977;Drake et al, 1989). This divided the Taconic foreland basin into a rapidly subsiding deep water eastern basin and a slowly subsiding shallower western basin (Diecchio, 1986(Diecchio, , 1993. These basins remained isolated from each other until deposition of the Oswego Formation in the late Ordovician.…”

“…Interpretive stratigraphic and tectonic cross sections for the Taconic sequences of Germany Valley, Little North Mountain (Brocks Gap), and the Massanutten Synclinorium. Top: cross section afterDiecchio (1993).…”

The Mid Atlantic Appalachian Orogen Traverse: a comparison of virtual and on-location field-based capstone experiences

“…Most of the visible, outcrop-scale deformation in the region resulted from the Alleghanian orogeny (Bartholomew and Whitaker, 2010;Whitmeyer et al, 2015), although the Blue Ridge geologic province preserves deformation and fabrics that derived from the Grenville orogenic cycle, along with younger Neo-Acadian high-strain zones (Bailey et al, 2006;Southworth et al, 2010). In contrast, stratigraphic data from the field trips provide evidence for earlier tectonic events, such as the Ordovician Taconic orogeny (e.g., Diecchio, 1993) and the Devonian Acadian Orogeny (e.g., McClung et al, 2013). Students use stratigraphic and structural field data that they collect on the field trips to draft a series of interpretive cross sections across the Blue Ridge and Valley and Ridge geologic provinces and then synthesize their data and interpretations in a report that describes the tectonic history of the region from the Mesoproterozoic Grenville orogeny through the Paleozoic assembly of Pangaea (Whitmeyer and Fichter, 2019).…”