Stratigraphy of Silurian Sandstones in Western Virginia from Eagle Rock to Bluegrass John Haynes1 1James Madison Univ. Rick Diecchio 2 2 George Mason Univ. Steve Whitmeyer1 45th45th AnnualAnnual VirginiaVirginia GeologicalGeological FieldField Conference,Conference, SeptemberSeptember 25-26,25-26, 20152015 Figure A. Facies changes in Upper Ordovician and Silurian strata between selected exposures along and near the eld trip route, including Eagle Rock (Stop 1), Chestnut Ridge (Stop 3) and Williamsville (Stop 4), in and near the Bullpasture River Gorge, and Bluegrass/Forks of Waters (Stop 5). See back cover for stop locations. The thinning and stratigraphic splitting of the “Eagle Rock Sandstone” into separate sandstones of the Keefer, McKenzie, and Williamsport formations, and the possible equivalence of the calcareous upper part of the “Eagle Rock Sandstone” with similar sandstones in the Tonoloway, will be the primary focus of this trip. Front Cover. Deformation in the Keefer facies of the lower “Eagle Rock Sandstone” at Eagle Rock (Stop 1). The prominent pinnacle is one of the resistant quartz arenites in the “Eagle Rock Sandstone” that denes the summit of Crawford Mountain. View is to the southwest looking across US 220 at the southwest end of the bridge over the James River near the town of Eagle Rock. 45th Annual Virginia Geological Field Conference September 25 – 26, 2015 Natural Bridge Hotel, Natural Bridge, Virginia STRATIGRAPHY OF SILURIAN SANDSTONES IN WESTERN VIRGINIA FROM EAGLE ROCK TO BLUEGRASS John Haynes Department of Geology and Environmental Science James Madison University Richard Diecchio Department of Atmospheric, Oceanic & Earth Sciences George Mason University Steven Whitmeyer Department of Geology and Environmental Science James Madison University Figure B. Age and generalized stratigraphic relations of Upper Ordovician, Silurian, and Lower Devonian strata at and near the stops on the field trip. Uncertainties in upper and lower contacts of the “Eagle Rock Sandstone” and in the extent of the Ordovician – Silurian boundary are indicated by question marks. Based on field work by the authors, and the work of Denkler and Harris (1988a, 1988b), Harris et al. (1994), and Cohen et al. (2013). 2 INTRODUCTION This year’s field trip will examine bedrock exposures of Silurian strata in the western Valley and Ridge with a principal focus being the dramatic facies changes that occur in a SSE to a NNW direction of this region (Fig. A [inside front cover]), especially in the stratigraphic interval above the siliciclastics of the Rose Hill Formation and below the carbonates and thin sandstones of the Tonoloway Limestone (Fig. E [inside back cover]). We will examine Silurian strata in exposures at 5 stops in Botetourt, Alleghany, Bath, and Highland Counties (Fig. F, [trip route on back cover]). New findings based on our recent and ongoing bedrock mapping in Highland County (Haynes and Whitmeyer, 2010; Hazelwood et al., 2012; Haynes and Diecchio, 2013) will be a principal part of today’s field trip. Stratigraphic highlights will include (A) the presentation of evidence that supports suggested new correlations of the upper several meters of the “Eagle Rock Sandstone” at Eagle Rock (Stop 1) with the Wills Creek Formation as well as the lower Tonoloway Limestone (Stop 3); (B) recognition that the quartz sandstones in the middle of the McKenzie Formation exposed at Williamsville in the Bullpasture River Gorge (Stop 4) are the middle sandstone member of the McKenzie, which extends the known exposures of this sandstone; (C) recognition that the quartzose oolitic grainstones in the lower McKenzie Formation exposed in the Bullpasture River Gorge (Stop 4) are the easternmost known exposures of the oolitic facies that comprises the upper beds of the Lockport Member of the McKenzie Formation in the subsurface of western West Virginia; (D) a reinterpretation of prior stratigraphic findings and correlations in and near the Bullpasture River Gorge (Stops 3 and 4); and (E) confirmation that of the Keefer, McKenzie, and Williamsport equivalents which collectively comprise the sandstones in the ~140 m thick “Eagle Rock Sandstone” at Stop 1, only the Williamsport Sandstone persists as a quartz arenite sandstone to the exposures in northernmost Highland County at Stop 5 near Forks of Water and Bluegrass where, as we will see, it is only ~8 m thick. Structurally, we will see deformation at several scales, from the regional scale of folds and faults across the Valley and Ridge, to outcrop- and hand sample-scale structures (front cover). The route of the field trip will transect (and parallel) kilometers-scale folds, including the Rich Patch, Warm Springs, Bolar, and Hightown (Wills Mountain) anticlines. Individual stops will highlight fault-related deformation, such as outcrop-scale folds and faults at Eagle Rock that are related to the Pulaski Fault, and other smaller-scale structures. BACKGROUND AND GEOLOGIC SETTING In the Mid-Atlantic region, the Appalachian Mountains are divided into the Blue Ridge, and Valley and Ridge, and Appalachian Plateaus physiographic provinces. Today’s field trip is entirely within the Valley and Ridge that, with its distinctive northeast-trending linear topography of parallel ridges and valleys, is a classic fold-and-thrust belt. In the area along the field trip route, the Valley and Ridge is underlain by a thick sequence of Cambrian to Mississippian sedimentary rocks. These Lower and Middle Paleozoic sedimentary strata now exposed in the Valley and Ridge province of western Virginia have had a long and interesting history of deposition, burial, lithification, deformation, exhumation and erosion, a second round of burial, intrusion, and uplift, and ongoing erosional sculpting, which collectively have produced the landscape we see today. The ridges are held up by mechanically and chemically resistant quartz sandstones, and the valleys are underlain by mechanically weak mudrocks and/or by chemically weak carbonate rocks. With its overview of the regional sedimentology and stratigraphy, and of the regional deformational and structural relationships, today’s field trip will allow participants to have a look at the effects of several of these processes. The earliest geologic mapping in the area of the field trip route includes the work of Darton (1892, 1899), Schmitz (1896), and Butts (1933). From that time to the present, many regionally focused geologic studies (both stratigraphic and structural, as well as geologic mapping) that are of relevance to this field trip have been carried out in the area of today’s trip, including Butts (1940), Woodward (1941, 1943), Lesure (1957), Deike (1960), Folk (1960), Hunter (1960), Bick (1962, 1973), Travis (1962), Appalachian Geological Society (1970), McGuire (1970), Patchen (1974), Lampiris (1975), Patchen and 3 Smosna (1975), Smosna et al. (1977), Smosna and Patchen (1978), White and Hess (1982), Whitehurst (1982), Smosna (1984), Kulander and Dean (1986), Rader and Gathright (1986), Bartholomew (1987), Denkler and Harris (1988a), Diecchio and Dennison (1996), Brett et al. (1998), Bell and Smosna (1999), Haynes and Whitmeyer (2010), Hazelwood et al. (2012), Haynes and Diecchio (2013), Haynes et al. (2014), Martin et al. (2014), Swezey and Haynes (2015), and Swezey et al. (2015). Of the stratigraphic studies on the Silurian of this region, several deserve additional mention. Dennison (1970) made note of two important relationships related to Silurian stratigraphy in the outcrop belt of Virginia and West Virginia. First, southwest of Roanoke, the strata are incomplete, being cut-out by one or more unconformities. Second, toward the west and north (including the area covered by this field guide), where the strata are more complete, the quartz arenite facies are dominant at the southeast basin margin, but become tongues to the north and west with some eventually pinching. The name “Eagle Rock Sandstone” was introduced informally by Lampiris (1975) to describe the quartZ arenite unit that occurs between the Rose Hill Formation and the Tonoloway Limestone. The “Eagle Rock Sandstone” is thickest at Eagle Rock, near the southeast basin margin, and thinner toward the southwest, northwest and northeast. It thus thins into the basin and along the basin axis. Lampiris also recogniZed that the “Eagle Rock Sandstone” splits and thins into tongues toward the west and northwest. These sandstone tongues are the emphasis of this field guide. It should be mentioned that the surface exposures of distal or basinal facies in the Valley and Ridge, including those which we will see on this trip, are not the limit of these facies. Basin center is in West Virginia, where most Silurian strata occur in the subsurface, and the Silurian strata continue to change facies westward into the basin center, where they contain more shale, limestone and dolomite than anything we will see on this trip. These relationships are illustrated by Woodward (1941), Knight (1969), and Horvath et al. (1970) in the form of stratigraphic cross-sections. STRATIGRAPHIC UNITS OF INTEREST Figure B shows the various Upper Ordovician, Silurian, and Lower Devonian stratigraphic units and stratigraphic relationships in this region, many of which will be of interest at the stops on this trip as indicated at the top of each column. The stratigraphic units are summarized below. Ordovician Reedsville/Martinsburg Shale (300–400 m thick) The type section of the Reedsville Shale is at Reedsville in Mifflin County, Pennsylvania (Ulrich, 1911), and the type section of the Martinsburg Shale is at Martinsburg in Berkeley County, West Virginia (Geiger and Keith, 1891). Darton (1899) and Bick (1962) mapped these strata in Bath and Highland Counties as the Martinsburg Shale or the Martinsburg Formation. In the Shenandoah Valley, area the Martinsburg Shale is a thick sequence of siliciclastic turbidites with only a few tens of meters of black laminated argillaceous limestone at its base, all deposited in open basin to basin margin settings. In contrast, the strata of equivalent age west of the North Mountain front are mixed carbonate and siliciclastic sediments deposited in a storm-dominated shelf setting.