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The Geological Society of America Field Guide 35 2014

Active features along a “passive” margin: The intriguing interplay between Silurian–Devonian stratigraphy, Alleghanian deformation, and Eocene magmatism of Highland and Bath Counties, Virginia

John T. Haynes* Elizabeth A. Johnson* Steven J. Whitmeyer* Department of Geology and Environmental Science, MSC 6903, 395 South High Street, James Madison University, Harrisonburg, Virginia 22807, USA

ABSTRACT

This two-day trip highlights new fi ndings from structural, stratigraphic, and pet- rologic research in the Valley and Ridge province of Highland, Bath, and Augusta Counties, Virginia, and Pendleton County, West Virginia. The structural emphasis on Days 1 and 2 will be at several scales, from the regional scale of folds and faults across the Valley and Ridge, to outcrop- and hand sample-scale structures. Stops will high- light deformation associated with previously unmapped faults and a second-order anticline in Silurian and Lower Devonian carbonate and siliciclastic strata, specifi cally the Silurian Tonoloway Limestone, the Silurian–Devonian Helderberg Group, and the Devonian Needmore Shale. The stops on Day 1 will also focus on facies changes in Silurian sandstones, the stratigraphy of the Keyser–Tonoloway formational con- tact, and new discoveries relevant to the depositional setting and regional facies of the McKenzie Formation in southern Highland County. The focus of the stops on Day 2 will be on the petrology and geochemistry of several exposures of the youngest known volcanic rocks (Eocene) in the eastern United States. Discussions will include the possible structural controls on emplacement of these igneous rocks, how these magmas and their xenoliths constrain the depth and temperature of the lower crust and mantle, and the tectonic environment that facilitated their emplacement.

*E-mails: [email protected], [email protected], [email protected].

Haynes, J.T., Johnson, E.A., and Whitmeyer, S.J., 2014, Active features along a “passive” margin: The intriguing interplay between Silurian–Devonian stratigra- phy, Alleghanian deformation, and Eocene magmatism of Highland and Bath Counties, Virginia, in Bailey, C.M., and Coiner, L.V., eds., Elevating Geoscience in the Southeastern United States: New Ideas about Old Terranes: Field Guides for the GSA Southeastern Section Meeting, Blacksburg, Virginia, 2014: Geological Society of America Field Guide 35, p. 1–40, doi:10.1130/2014.0035(01). For permission to copy, contact [email protected]. © 2014 The Geological Society of America. All rights reserved.

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2 Haynes et al.

INTRODUCTION in Rockingham, Augusta, Highland, and Pendleton Counties. These igneous rocks have been observed and studied by fi eld Recent and ongoing bedrock mapping in western Virginia geologists for more than 100 years (Darton, 1899; Garnar, and eastern West Virginia has clarifi ed several stratigraphic 1956; Kapnicky, 1956; Kettren, 1970; Johnson et al., 1971; and structural relationships in the Ordovician, Silurian, and Rader et al., 1986; Southworth et al., 1993; Tso et al., 2004; Tso Devonian sequence. This work has added to our understand- and Surber, 2006), but it was not until the 1960s that the fi rst ing of stratigraphic and sedimentologic relationships, and to paleomagnetic data implied these igneous rocks were Eocene our knowledge of local structures formed during late Paleozoic in age (Fullagar and Bottino, 1969). On Day 2 we will examine Alleghanian orogenesis. In addition, recent and ongoing pet- several exposures of these igneous rocks, and we will discuss rologic and geochemical analyses have furthered our under- textures and compositions of mafi c, intermediate, and felsic standing of igneous activity that occurred during the Eocene rocks, and recent geochemical data: in this region. Collectively, these research efforts have led us to contemplate some new ideas about the geologic history of this (1) The petrology and mineralogy of the mafi c, intermediate, region while simultaneously revisiting and refi ning old ideas and felsic rocks have been determined in appreciably more and models. The purpose of this fi eld trip and the selected stops detail from our recent studies. along its route (Fig. 1) is to showcase new fi ndings and interpre- (2) Geothermobarometers have been used to determine tem- tations, as well as to raise questions for lively discussion. perature and depth from mantle xenocrysts and lower crustal xenoliths. OVERVIEW Principal Structural Findings Sedimentary strata in the Appalachian orogen have a long and interesting history. They have been deposited, buried, lith- In addition to discussion throughout the fi eld trip of local ifi ed, deformed, eroded, buried again, intruded, uplifted, and and regional structures of interest, the major structural fi ndings eroded again. This fi eld trip will look at the effects of some and/or reinterpretations that will be highlighted on this fi eld trip of those processes in what we infer to be their chronological include: order, beginning with an overview of the sedimentology and stratigraphy, followed by igneous petrology and geochemistry, (1) Detailed geologic mapping and structural analyses along and concluding with deformation and structural relationships. Virginia State Highway 42, 9.5 km south of Millboro Springs in southeastern Bath County have confi rmed an Principal Stratigraphic Findings ~6-km-long, northeast-trending hill as the surface expres- sion of a ramp anticline that overlies a cryptic fault. The The three major stratigraphic fi ndings and/or reinterpreta- doubly plunging “breadloaf”-shaped fold is interpreted as a tions and their various components that will be highlighted on fault bend fold in the proximal hanging wall of a shallowly Day 1, as we examine several exposures of Lower Paleozoic southeast-dipping, northwest-directed thrust fault. strata are as follows: (2) Deformation features in the hinge region of the “breadloaf” anticline change along trend. Wedge faults with complex, (1) Facies changes and detailed lateral relationships in the Silu- small-scale folds are apparent in the southwest-plunging rian and Lower Devonian sandstones of this region are now hinge region of the anticline (Stop 3), but are less promi- better understood as a result of our work. nent to the northeast (Stop 4). (2) Quartz sandstones and quartzose oolitic grainstones that (3) Igneous dikes and plugs align with regional structural fab- occur in the McKenzie Formation of this area are identi- rics, suggesting that magma exploited faults and fractures fi ed as the easternmost exposures known thus far of the to ascend through the crust. unnamed middle sandstone member of the McKenzie For- mation and of the oolitic facies that comprise the upper SUMMARY OF STRATIGRAPHIC UNITS OF INTEREST beds of the Lockport Member of the McKenzie Formation in the subsurface to the west. Figures 2 and 3 show the various Ordovician, Silurian, and (3) The complicated and unconformable nature of the stratigraphic Devonian stratigraphic units and stratigraphic relationships that contact between the Tonoloway Limestone and the overlying are of interest at the stops on this trip. These units are summa- Keyser Limestone in this area is now better understood. rized below.

Principal Igneous Petrology and Geochemistry Findings Ordovician Carbonate Sequence Undivided

At least 150 dikes, plugs, sills, and diatremes are exposed (Beekmantown Formation undivided, Lurich Formation of in the Valley and Ridge province of Virginia and West Virginia Kay [1956], Blackford Formation of Butts [1940], Elway Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 3

Limestone of Cooper [1945], Five Oaks Limestone of Coo- The oldest exposed strata of interest in the fi eld trip area are per and Prouty [1943], New Market Limestone of Cooper a thick sequence of Ordovician carbonate rocks. These underlie and Cooper [1946], Lincolnshire Limestone of Cooper and the Shenandoah Valley, and they also crop out along the axes Prouty [1943], Big Valley Formation of Bick [1962], Benbolt of the several anticlinal valleys of the Valley and Ridge (Kay, Limestone of Cooper and Prouty [1943], McGlone Limestone 1956) including the Warm Springs, Bolar (Big), and Hightown of Kay [1956], McGraw Limestone of Kay [1956], and Neal- Valleys (Fig. 1). These are of interest because of several thermal mont Limestone of Kay [1944]) springs emanating from these carbonates in the Warm Springs

Figure 1. Generalized geology of the fi eld trip area showing stop locations and several major folds and faults (modifi ed from Diecchio, 1986). Inset map (upper left) shows the location of the fi eld trip area relative to Blacksburg. Downloaded from fieldguides.gsapubs.org on April 8, 2014

4 Haynes et al.

and Bolar Valleys (alternate Stop 4B) and because of the igne- ous intrusions in some of these carbonates in the Hightown Val- ley (Stops 8 and 9) and the Bolar Valley (Darton, 1899; Rader and Wilkes, 2001; Haynes and Diecchio, 2013). The lengthy and exceedingly complex history of nomen- clature that has been variously applied to these carbonate strata is hinted at by the various authors included in the above list of formation names. Each of these authors made an effort to subdivide this carbonate sequence, usually on the basis of fossil content. That approach to the naming of stratigraphic units is now forbidden by the North American Stratigraphic Code (North American Commission on Stratigraphic Nomen- clature, 1983, 2005), so there is a real need in this region to identify workable lithostratigraphic, rather than biostrati- graphic, differences that can be used to distinguish one unit from another in the fi eld. Based on our most recent mapping efforts in the fi eld trip area (Haynes and Diecchio, 2013), we recognize at the pres- ent time that the Nealmont Limestone with its interbedded bluish gray slabs of bioclastic wackestone and packstone con- trasts suffi ciently with the overlying Dolly Ridge Formation to make a useful and recognizable contact in the fi eld. The marked unconformity and associated overlying chert-rich breccias that elsewhere in this region are so prominent at the base of the Blackford Formation or New Market Limestone (Mussman and Read, 1986), which can be used as an unambiguous con- tact to separate the underlying Beekmantown or Knox Group dolomitic carbonate rocks from the overlying units that are pre- dominantly limestones, can likewise be a recognizable contact in many areas. In the Hightown Valley, Germany Valley, and the north end of the Big Valley, however, we have not yet iden- tifi ed that unconformity with any certainty. Although Wilkes (2011) mapped the Beekmantown separately from the overly- ing limestones, he noted nonetheless (p. 3) that “Contact of the Beekmantown with the overlying middle Ordovician limestone unit was not observed but is assumed to be unconformable…” Therefore, pending additional fi eldwork in the region that is needed to delineate this stratigraphic contact more conclu- sively, we have included the upper beds of Beekmantown with the overlying units as an undivided sequence of Ordovician car- bonate strata in central and northern Highland County (Haynes and Diecchio, 2013).

Ordovician Dolly Ridge Formation (80–130 m thick)

The type section of the Dolly Ridge Formation is at Dolly Ridge in Germany Valley, the northern extension of the High- town Valley in Pendleton County, West Virginia (Perry, 1972). These strata were initially mapped by Darton (1899) as part of the Shenandoah Limestone. Kay (1956) designated a new member (the Onego Member) of the Salona Formation for part Figure 2. Age and stratigraphic relations of units seen at the stops on of this stratigraphic interval, and Bick (1962) assigned these this fi eld trip, based in part on Denkler and Harris (1988a, 1988b), Har- strata to the Edinburg Formation. Perry (1971, 1972) noted that ris et al. (1994), and Leslie et al. (2011). in Pendleton County, these strata do not resemble the Salona Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 5

Formation or Edinburg Formation at their respective type sec- a thin outcrop of the Oswego Sandstone in northwestern High- tions, and he concluded that a new name was warranted. Subse- land County, but Bick (1962) did not recognize the Oswego. quent publications have used the name Dolly Ridge Formation Subsequently the Oswego has been recognized at several expo- (Rader and Wilkes, 2001; Haynes and Diecchio, 2013). sures in this region (Rader, 1984; Diecchio, 1985; Wilkes, 2011; Much of the Dolly Ridge Formation consists of thin beds Haynes and Diecchio, 2013). of dark gray to black lime mudstones, many of which weather Greenish-gray to brown beds of fi ne- to medium- to coarse- white, and interbedded dark greenish-gray shales, some of grained sublithic arenites up to 1 m thick, and cross-bedded in which are K-bentonite beds (altered tephras). The lower places, characterize the Oswego Sandstone. A common and 30–40 m has several cobbly weathering zones that produce distinctive character of these sandstones on a fresh surface is small nodules of white weathering, black lime mudstones the presence of limonite as small yellowish-orange specks. Thin that are common to abundant on some hillslopes underlain interbeds of green mudrock are also generally present. by this interval. Ordovician Juniata Formation (100–250 m thick) Ordovician Reedsville Shale (300–400 m thick) The Juniata Formation was named by Darton and Taff The type section of the Reedsville Shale is at Reeds- (1896), and a type section was designated by Clark (1897). ville in Miffl in County, Pennsylvania (Ulrich, 1911). Darton Darton (1899) mapped the Juniata Formation in this region, and (1899), Kay (1956), and Bick (1962) mapped these strata in this stratigraphic name has been in essentially continuous use Bath and Highland Counties as the Martinsburg Shale or the in this region ever since (Butts, 1940; Bick, 1962; Dennison, Martinsburg Formation. Because the Martinsburg Shale of the 1976; Diecchio, 1985; Rader and Wilkes, 2001; Wilkes, 2011; Shenandoah Valley area is a thick sequence of siliciclastic tur- Haynes and Diecchio, 2013). bidites with only a few tens of meters of carbonate strata at Interbedded yellowish-brown to reddish-brown sublithic its base, all deposited in open basin to basin margin settings arenites up to 1 m thick, some with cross-bedding, and olive- (McBride, 1962), whereas the strata of equivalent age west gray and red mudrocks are the major lithologies in the Juniata of the North Mountain front are mixed carbonate and silici- Formation. Skolithos (trace fossils of vertical burrows) occurs clastic sediments deposited in a storm-dominated shelf setting in some of the sandstone beds. In many sections, the top of the (Kreisa, 1981), Diecchio (1991) recommended that the name Juniata is characterized by several pink quartz to sublithic aren- Reedsville Shale be used for these strata on the west side of ite beds up to ~1.5 m thick that are transitional with the overly- the North Mountain front (marked by the North Mountain ing quartz arenites of the Tuscarora Formation. fault system along the west side of the Shenandoah Valley) and that the name Martinsburg Formation should be restricted Silurian Tuscarora Formation (15–25 m thick) in its usage to equivalent strata in exposures east of the North Mountain front. Reedsville Shale has been used to refer to The type section of the Tuscarora Formation is at Tusca- these strata throughout most of the fi eld trip area (Rader and rora Mountain in Pennsylvania. It was named by Darton and Wilkes, 2001; Haynes and Whitmeyer, 2010; Wilkes, 2011; Taff (1896), and soon thereafter Darton (1899) mapped these Haynes and Diecchio, 2013). It is the oldest formation shown strata in Bath and Highland Counties as the Tuscarora Quartz- in the regional cross section (Fig. 3), which includes specifi c ite. Woodward (1941) referred to these strata regionally as the stratigraphic information for the fi eld trip stops at Eagle Rock Tuscarora Sandstone, whereas Bick (1962) mapped them as (Stop 1), the Bullpasture River (Stop 5), and Bluegrass and the Clinch Sandstone. Subsequent publications have generally Forks of Waters (Stop 6). referred to these strata as the Tuscarora Formation (e.g., Bick, Thin greenish-gray to gray mudrock with thin interbeds of 1973; Rader and Wilkes, 2001). fi ne-grained sandstones, siltstones, and bioclastic limestones Thick to massively bedded white to grayish-white to comprise this interval. A prominent brachiopod-rich horizon pale-yellow to pale-pink silica-cemented supermature quartz known regionally as the Orthorhynchula zone occurs in the arenites, some with prominent cross beds, are the principal upper 3–4 m of the Reedsville, and is a useful marker bed for lithology of the Tuscarora. Because of its extreme durability, identifying the contact between the Reedsville Shale and the ledges of Tuscarora Sandstone are the dominant ridge-form- overlying Oswego Sandstone or Juniata Formation. ing stratigraphic unit of the central Appalachians. Thin beds of quartz-pebble conglomerate occur in the lower half of the Ordovician Oswego Sandstone (8–25 m thick) formation at many exposures, and at a few locations, notably along the west limb of the Wills Mountain anticline, a thin The Oswego Sandstone was named by Prosser (1888) for black shale is present in the middle of the unit. Trace fossils outcrops in Oswego County, New York, although a specifi c type including Skolithos and Arthrophycus (single to compound section was not specifi ed. These strata were mapped by Darton elongate burrows) can be found in some beds and on some (1899) as part of the Juniata Formation. Butts (1940) identifi ed bedding planes. Downloaded from fieldguides.gsapubs.org on April 8, 2014

6

Figure 3. Facies changes in Upper Ordovician and Silurian strata between selected exposures in and near the fi eld trip area, including Eagle Rock (Stop 1), the Bullpasture River Gorge (Stop 5), and Bluegrass and Forks of Waters (Stop 6). The lateral and vertical facies changes in the stratigraphic interval above the Tuscarora (TUSC) and below the Tonoloway will be a major part of the discussions at our stops on Day 1. Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 7

Silurian Rose Hill Formation (80–250 m thick) Silurian Keefer Formation (<1–8 m thick)

The type section of the Rose Hill Formation is at Rose The type section of the Keefer Formation is at Keefer Hill in the city of Cumberland, Allegany County, Maryland Mountain, a few kilometers northeast of Hancock in Washing- (Swartz, 1923). The earlier name for this stratigraphic unit was ton County, Maryland (Stose and Swartz, 1912). In Bath and the Cacapon Sandstone (Darton and Taff, 1896), and in Bath Highland Counties, this stratigraphic interval was mapped by and Highland Counties that name was used by Darton (1899). Darton (1899) as part of the Rockwood Formation. Woodward (1941) noted that strata previously mapped as the There is appreciable nomenclatural complexity to this Cacapon Sandstone in western Virginia should be mapped as stratigraphic interval, which in its restricted stratigraphic the Rose Hill Formation, and although Bick (1962) mapped sense refers to a white to grayish-white to pale-pink to red sil- these strata as the Cacapon Member of the Clinton Formation, ica-cemented quartz arenite, known informally in the region subsequent publications (e.g., Rader, 1984; Rader and Wil- as the “true” Keefer. In the Clifton Forge area southward, the kes, 2001) have nearly uniformly referred to this interval as name “Keefer” has been used to refer to what is more appro- the Rose Hill Formation. The name Cacapon sandstone is still priately referred to as “Eagle Rock sandstone.” Woodward widely used in this region, however, in reference to the dis- (1941) referred to these strata regionally as the Keefer Sand- tinctive dusky-red to dark-maroon hematite-cemented quartz stone; Bick (1962) mapped these strata as the Keefer Mem- and sublithic sandstones, some of which are ripple marked. ber of the Clinton Formation; Perry (1971) mapped these These are the most recognizable lithology in the Rose Hill strata in Germany Valley, Pendleton County, West Virginia, Formation and invariably are a major, and commonly the as the Miffl intown Formation and included the McKenzie major, component of colluvial and alluvial deposits that blan- Formation and the Williamsport Sandstone. Helfrich (1975, ket the dip slopes of ridges in this region that are underlain by 1980) mapped these strata at the Bluegrass section in north- the Tuscarora Formation. ern Highland County (Stop 6) as the lower hematitic mem- In addition to the distinctive dark-maroon to purplish ber of the Miffl intown Formation and the overlying Cosner hematitic sandstones, there are thin-bedded olive to gray Gap Member of the Miffl intown Formation, and stated that mudrocks interbedded with reddish shales and siltstones in the Cosner Gap Member is a limey equivalent of the Keefer the Rose Hill, some of which are sparsely to moderately fos- Sandstone. The Pennsylvania formation name “Miffl intown” siliferous with ostracodes, brachiopods, and trilobites. has not subsequently been used in this region, and Rader and The character of the Rose Hill Formation along the fi eld Wilkes (2001) mapped these strata as the Keefer Formation. trip route is relatively consistent, as we will see at Eagle Rock (Stop 1), the Bullpasture River (Stop 5), and Bluegrass (Stop 6).

Silurian “Eagle Rock sandstone” (125 m thick)

The “Eagle Rock sandstone” was named by Lampiris (1975), and the not yet formalized type section is the series of cuts at Eagle Rock in Botetourt County (Stop 1, Fig. 4) where the James River has cut a steep-walled gorge that bisects a prominent ridge into two separate ridges, Rathole Mountain and Crawford Mountain. The thickness of the Silurian sand- stones at Eagle Rock is in marked contrast to the exposure at Fagg farther southeast (Fig. 3), where the entire Silurian is only a 1–3-m-thick bed of pebbly Tuscarora Sandstone that uncon- formably overlies bioclastic beds of the Martinsburg Formation and is unconformably overlain by the Rocky Gap Sandstone (Diecchio, 1985). The “Eagle Rock sandstone” is the modern name of this thick unit, which collectively includes the strati- Figure 4. Sketch of the exposure at Eagle Rock (Stop 1) that shows graphic equivalents of the Keefer, McKenzie, Williamsport, and formational boundaries, and how thick the Silurian “Eagle Rock sand- Wills Creek Formations that are separable into discrete strati- stone” is at this location relative to the other stratigraphic units present. graphic units at sections farther northwest (Figs. 2, 3), as we View is to the southwest. Omb—Martinsburg Formation; Os—Oswe- go Sandstone; Stu—Tuscarora Formation; Srh—Rose Hill Formation; will see in exposures at Crizer Gap (Stop 4), the gorge of the Ser—Eagle Rock sandstone. Not shown is the Tonoloway Limestone Bullpasture River (Stop 5), and in the vicinity of Bluegrass and at the northwest end of the exposure (modifi ed from fi gure 4 of Rader Forks of Waters (Stop 6). and Gathright, 1986). Downloaded from fieldguides.gsapubs.org on April 8, 2014

8 Haynes et al.

Haynes and Whitmeyer (2010) and Hazelwood et al. (2012) County, West Virginia (Reger, 1924). Darton (1899) mapped found that in southern Highland and northern Bath Counties this sandstone as part of the Lewistown Limestone. Butts (1940) (Stop 5) the Keefer is a mappable unit, but in west-central and included it as part of the Wills Creek Formation, which he defi ned northwestern Highland County, Wilkes (2011) and Haynes and as all of the strata between the McKenzie Limestone below and Diecchio (2013) found that the Keefer is too thin to map as a the Tonoloway Limestone above. Woodward (1941) fi rst identi- separate unit, and so it was included with the underlying Rose fi ed this sandstone as a separate formation in this region, which Hill Formation. In a petrographic investigation of the exposure he mapped as the Williamsport Sandstone. Although Bick (1962) at Bluegrass (Stop 6), Haynes et al. (2011) noted that little or mapped this sandstone as part of the Wills Creek Formation, no quartz arenite is present, and instead there is appreciable fer- subsequent publications have mapped these strata in Highland roan dolomite as well as ooids that are composed of hematite County as the Williamsport Sandstone (Helfrich, 1975; Diecchio and berthierine and/or chamosite. and Dennison, 1996; Wilkes, 2011), and our mapping efforts The character of the Keefer changes signifi cantly along have also shown that the Williamsport is a persistent and useful the route of this fi eld trip (Figs. 2, 3), from a massive, thick, marker bed in this area (Haynes and Whitmeyer, 2010; Hazel- and erosionally resistant quartz arenite that comprises the lower wood et al., 2012; Haynes and Diecchio, 2013). part of the “Eagle Rock sandstone” (Stop 1) to a thinner but The Williamsport Sandstone is a tough, erosion-resistant, still resistant ledge of quartz arenite (Stop 5) to thin ferruginous silica-cemented quartz arenite that weathers variously white to dolomites and mudrocks (Stop 6). tan to orange-brown to brown. Like the quartz arenites of the Tuscarora, the “Eagle Rock,” and the Keefer, the Williamsport Silurian McKenzie Formation (60–80 m thick) is generally very resistant and it makes prominent fl atirons on many of the dip slopes in this region, but because it is typi- The type section of the McKenzie Formation is at McK- cally medium bedded it commonly (but not always) breaks into enzie Station on the Baltimore and Ohio Railroad in Allegany smaller blocks than if it were more massively bedded, as the County, Maryland (Stose and Swartz, 1912). The unit was fi rst Tuscarora tends to be. It is common to fi nd some bedding planes mapped in Bath and Highland Counties as the lower part of with prominent ripple marks (Stop 5), a sedimentary structure the Lewistown Limestone (Darton, 1899). Woodward (1941) that seems to be far less common in the otherwise petrologically referred to this stratigraphic interval as the McKenzie Forma- similar quartz arenites of the Tuscarora, Keefer, and McKenzie tion. Bick (1962) mapped these strata in Bath and Highland Formations. In some intervals, isolated ostracodes to ostracode Counties as the “McKensie [sic] Limestone” of the Cayuga coquinas are present. On more weathered exposures, the ostra- Group, but the name “Cayuga Group” has since been aban- codes are now just shell moldic pores that are commonly lined doned as a lithostratigraphic term (http://ngmdb.usgs.gov/Geo- by orange limonite, but ostracode shells in unweathered sam- lex/NewRefsmry/sumry_937.html). Perry (1971) and Helfrich ples are commonly extensively pyritized, and it is the oxidation (1975, 1980) mapped these strata in Highland and Pendleton of this pyrite that contributes to the yellowish-brown limonite Counties as the McKenzie Member of the Miffl intown Forma- staining and patina that is common on the surface of ledges and tion, but as noted above that name has not subsequently been blocks of the Williamsport throughout this region. widely used, and these strata have since been mapped in this The Keefer of Lesure (1957, his Table 8, p. 39) includes area as the McKenzie Formation (Diecchio and Dennison, 52 ft (15.8 m) of “resistant…light-brown to grayish-orange” 1996; Rader and Wilkes, 2001; Haynes and Whitmeyer, 2010; sandstone as its uppermost bed, a description that is consis- Haynes and Diecchio, 2013). tent with typical Williamsport Sandstone of this region, further Where the McKenzie is recognized as a distinct strati- suggesting that the expanded Keefer, i.e., “Keefer” of previous graphic unit in the fi eld trip area (Stops 2–6) it is a heteroge- reports includes the Williamsport. neous sequence of thinly bedded and laminated dark-gray lime mudstones (Stop 4), oolitic and bioclastic (primarily ostracode) Silurian Wills Creek Formation (<1–70 m thick) grainstones, tan to green to blue-green to gray to black shales, and the aforementioned thick to massively bedded medium- to The type section of the Wills Creek Formation is at Wills coarse-grained, silica-cemented, yellowish-white quartz arenite Creek in Cumberland, Allegany County, Maryland (Uhler, beds that collectively comprise the middle sandstone member 1905). In this region, Darton (1899) included these strata as and are ~5 m thick in the Bullpasture River Gorge (Stop 5), and part of the Lewistown Limestone, and Butts (1940) mapped almost 10 m thick along Muddy Run to the southwest (White- them as the Wills Creek Formation, which he defi ned as all hurst, 1982). of the strata between the McKenzie Limestone below and the Tonoloway Limestone above (thus his defi nition would Silurian Williamsport Sandstone (9–10 m thick) include the Williamsport Sandstone). Woodward (1941) assigned these strata to the Wills Creek Limestone, and then The type section of the Williamsport Sandstone is on a Bick (1962) mapped them as the Wills Creek Formation, the branch of Patterson Creek, 1 km east of Williamsport in Grant name that has been used in this region by most authors since Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 9

(e.g., Helfrich, 1975; Diecchio and Dennison, 1996; Wilkes, mud cracked and algal laminated, and in a few beds thin intra- 2011; Haynes and Diecchio, 2013). clast grainstones and packstones are present. This member also Brown to green mudrocks are the principal Wills Creek contains 3–4 thin beds of calcite-cemented quartz arenites up lithology, with interbedded sandstone, sandy limestone, and to 4 m thick. At four exposures in this region (near Oak Flat on lime mudstone present in the Wills Creek as well. Ripple struc- U.S. 33 in Pendleton County, West Virginia; at the north end of tures, algal laminations, desiccation cracks, rip-up clasts, and Burnsville Cove along SR 609 in Bath County; just north of the molds of evaporite crystals are present in some of these beds. junction of Muddy Run Road and U.S. 220 in an unused quarry Fossils are not abundant, but include leperditian ostracodes, on the east side of U.S. 220 in Bath County; and in the bed of stromatolites, and brachiopods. the stream in Crizer Gap [Stop 4] south of Millboro Springs in The thickness of the Wills Creek Formation changes sig- Bath County), there is a vuggy bed, brecciated in places, and nifi cantly from south to north in the fi eld trip area (Fig. 2), from with rare “gypsum daisies” that are now pseudomorphed by < 2 m thick in the Bullpasture River Gorge (Stop 5) to ~70 m calcite. This bed, which is likely laterally persistent from this thick at the Bluegrass and Forks of Waters exposures (Stop 6). region westward, was originally an evaporite horizon of mostly gypsum or anhydrite that has now been modifi ed by diagenetic Silurian Tonoloway Limestone (50–180 m thick) processes and largely replaced by calcite, but with some of the original evaporite textures still present, which are an eastward The type section of the Tonoloway Limestone is at Tonolo- extension of the widespread evaporites of the Salina facies in way Ridge in Washington County, Maryland (Ulrich, 1911). the subsurface of the Appalachian basin farther west (Denni- This thick sequence of predominantly thin-bedded limestones son and Head, 1975; Smosna et al., 1977). The contact with was mapped by Darton (1899) as part of the Lewistown Lime- the overlying Keyser Limestone is sharp, and in places consists stone. Swartz (1930) mapped these strata as the Tonoloway of an intraclastic grainstone and packstone (“fl at-pebble con- Limestone of the Cayuga Group, whereas Butts (1940) and glomerate”) that varies in thickness from 0 to 1.4 m over short Woodward (1941) referred to these carbonates as the Tonolo- distances (Stop 5). way Limestone, the name that is used for this interval of strata in this region today (Bick, 1962; Perry, 1971; Helfrich, 1975; Silurian–Devonian Keyser Limestone of the Helderberg Diecchio and Dennison, 1996; Bell and Smosna, 1999; Wilkes, Group (15–70 m thick) 2011; Haynes and Diecchio, 2013). In this region, the Tonoloway Limestone consists of three The type section of the Keyser Limestone is at a quarry near unnamed members (Woodward, 1941; Perry, 1971; Bell and the town of Keyser in Mineral County, West Virginia (Ulrich, Smosna, 1999) that are laterally persistent across Pendleton, 1911). Darton (1899) included these strata as part of the Lew- Highland, and northern Bath Counties: istown Limestone, Swartz (1930) assigned them to the Keyser 1. The lower member is up to 60 m thick and it consists Limestone of the Helderberg Group, Woodward (1943) identi- primarily of thin-bedded and laminated gray to black lime mud- fi ed them as the Keyser Limestone of the Helderberg Group, stones, commonly peloidal and usually cut by many prominent and Bick (1962) mapped this unit as the Keyser Limestone and orthogonal fractures; in some of these limestones, there are considered it to be a separate formation below the Helderberg zones in which prominent pink to red to reddish-brown argil- Group. Most subsequent publications have mapped these strata laceous and dolomitic partings are present. This member also in Bath and Highland Counties as the Keyser Limestone of the contains two to three calcite- and quartz-cemented calcarena- Helderberg Group (Dorobek and Read, 1986). In this area, fi ve ceous quartz arenites in the area around the gorge of the Bull- named members have been identifi ed, and their stratigraphic pasture River (Stop 5) that are up to 3 m thick and have recently relations are complicated because of regional facies changes been found to have been mistaken for tongues of the Clifton and pinchouts. Forge Sandstone in this area for decades (White and Hess, 1982; Swezey et al., 2013). There are also a few thin beds of Byers Island Limestone Member of the Keyser Limestone of ostracode and gastropod packstones and grainstones, and thin the Helderberg Group (0–20 m thick) oolitic grainstones in this member. The type section of the Byers Island Limestone Member of 2. The middle member is up to 20 m thick and it consists the Keyser Limestone is a series of outcrops along the Susque- of thick to massively bedded bioclastic grainstones in which hanna River northeast of Selinsgrove in Snyder County, Penn- abundant crinoid fragments and lesser sponge, brachiopod, sylvania (Head, 1972). In the fi eld trip area it includes gray to coral, and bryozoan debris are most common, along with sparse pink bioclastic wackestones to grainstones and bryozoan and boundstones and coral-stromatoporoid framestones. stromatoporoid boundstones, and argillaceous limestone; crinoid 3. The upper member is up to 100 m thick and it consists of debris is abundant. We will see the Byers Island at Coronation thin-bedded and laminated gray lime mudstones that have some (Stop 3), where it is a massive grainstone to boundstone between to abundant and prominent orthogonal fractures that cut individ- the Tonoloway Limestone below and the Clifton Forge Sandstone ual beds; many of these lime mudstones are also peloidal, and above. The Byers Island is discontinuous over relatively short Downloaded from fieldguides.gsapubs.org on April 8, 2014

10 Haynes et al. distances, as we will see from Coronation (Stop 3) to Crizer Jersey Shore Limestone Member of the Keyser Limestone of Gap (Stop 4), a distance of just under 3.5 km. the Helderberg Group (8–45 m thick) The type section of the Jersey Shore Limestone Mem- Clifton Forge Sandstone Member of the Keyser Limestone of ber of the Keyser Limestone is near the community of Jer- the Helderberg Group (0–25 m thick) sey Shore in Lycoming County, Pennsylvania (Head, 1972). The type section of the Clifton Forge Sandstone Mem- In the fi eld trip area, the Jersey Shore Member is lithologi- ber is at Clifton Forge in Alleghany County, Virginia (Swartz, cally heterogeneous and it includes gray to blue-gray to pink 1930). North of the type section, the Clifton Forge Sandstone bioclastic packstones and grainstones with abundant crinoid, Member splits into two sandy beds (Woodward, 1943). The brachiopod, and bryozoan debris; shaly lime mudstones with lower of these sandy beds merges in south-central Highland sparse black chert nodules and lenses and more abundant County with the Big Mountain Shale Member of the Keyser siliceous beds that weather similar to chert but are not yet Limestone, whereas the upper of these sandy beds extends fully silicifi ed (we refer to these horizons in the fi eld as the north into Highland County and presumably pinches out “Keyser faux chert”); nodular bedded bioclastic wackestones (Dorobek and Read, 1986). and packstones; and, in many areas, two prominent beds of The Clifton Forge is a calcarenaceous quartz arenite, with stromatoporoid-coral boundstones, rudstones, grainstones, crinoid fragments as the primary non-quartz framework grains and packstones (Dorobek and Read, 1986). A bed of nodular but with some distinctive pink corals and/or stromatoporoids as limestone above the Clifton Forge Sandstone at Crizer Gap well. Cross bedding is prominent in some horizons. At Corona- (Stop 4) is a common Jersey Shore lithology. tion (Stop 3) and Crizer Gap (Stop 4) it is ~20 m thick, and in and near the Bullpasture River (Stop 5) where much of the Clif- LaVale Limestone Member of the Keyser Limestone of the ton Forge is actually a quartzose limestone, it is ~10 m thick. Helderberg Group (<1–9 m thick) From southern Highland County northward the Clifton Forge The type section of the LaVale Limestone Member of the Sandstone Member changes facies laterally into green calcare- Keyser Limestone is at the Corriganville Quarry near the town ous shale that is identifi ed as the Big Mountain Shale Member of La Vale in Allegany County, Maryland (Head, 1972). The of the Keyser Limestone (Swartz, 1930). LaVale Member is a laminated blue-gray to gray sandy lime- Deike (1960) considered the prominent and erosionally stone to calcite-cemented calcarenaceous sandstone, which can resistant sandstone beds that form the fl oor and ceiling of be confused with the Healing Springs Sandstone in this region Breathing Cave in northern Bath County (near Stop 5) to be because the quartz sand laminae, stringers, lenses, and thin beds the same two sandstone beds that Woodward (1943) identi- are commonly wavy to anastomosing, although the sandy beds fi ed as tongues of the Clifton Forge Sandstone Member, and in the Healing Springs are generally thicker and more laterally he informally named these beds the “lower Breathing sand- persistent than those in the LaVale. stone” and the “upper Breathing sandstone.” White and Hess (1982) applied the name lower Clifton Forge Sandstone to Devonian New Creek Limestone of the Helderberg Group the lower bed and the name upper Clifton Forge Sandstone to (<1–10 m thick) the upper bed. Our mapping efforts in this area (Walker et al., 2010; Haynes and Whitmeyer, 2010; Hazelwood et al., 2012; The type section of the New Creek Limestone is a quarry Swezey et al., 2013) indicate that the “upper Breathing sand- on the north side of U.S. Route 50, ~0.8 km south of the town stone” and “lower Breathing sandstone” of Deike (1960) are of New Creek in Mineral County, West Virginia (Bowen, 1967). instead correlative with two sandstones in the lower member of Darton (1899) mapped these strata as part of the Lewistown the Tonoloway Limestone, and thus they cannot be tongues of Limestone. Subsequently Swartz (1930) identifi ed these strata the Clifton Forge Sandstone Member of the Keyser Limestone. as the Coeymans Limestone of the Helderberg Group, Butts (1940) identifi ed them as the Coeymans Limestone Member of Big Mountain Shale Member of the Keyser Limestone of the the Helderberg Limestone, Woodward (1943) identifi ed them Helderberg Group (0–10 m thick) as the Coeymans Formation of the Helderberg Group, and Bick The type section of the Big Mountain Shale Member of (1962) mapped these strata as the Coeymans Limestone of the the Keyser Limestone is at Big Mountain, ~2.5 km west of the Helderberg Group. Bowen (1967) demonstrated that these strata community of Upper Tract in Pendleton County, West Virginia. cannot be traced continuously from Virginia through interven- It was named by Swartz (1930), who identifi ed these strata ing areas to the type section of the Coeymans Limestone in as a separate, mappable unit within the Keyser Limestone in New York, and so use of the name Coeymans Limestone has Highland County that are characterized by olive-gray shales been discontinued in areas south of central Pennsylvania, and and some thin beds of sandstone and sandy limestone. Swartz the name New Creek Limestone is used instead. (1930), Woodward (1943), and Dorobek and Read (1986) rec- The New Creek Limestone is thick to massively bedded light- ognized the facies relationship that these shales have with the gray to pink crinoidal grainstone, and in some exposures the cri- Clifton Forge Sandstone Member to the south. noid columnals can be noticeably pink and up to 3 cm in diameter. 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Active features along a “passive” margin, Highland and Bath Counties, Virginia 11

Devonian Corriganville Limestone of the Helderberg nomenclatural history, and almost equally complex strati- Group (0–8 m thick) graphic relationships. Darton (1899) mapped them as part of the Lewistown Limestone, and both Swartz (1930) and Butts The type section of the Corriganville Limestone is at a rail- (1940) identifi ed these strata as the Becraft Limestone of the road cut 0.5 km southeast of the town of Corriganville in Alle- Helderberg Group. Woodward (1943) abandoned the name gany County, Maryland (Head, 1972). Darton (1899) mapped Licking Creek, and instead used the names Port Jervis Lime- these strata as part of the Lewistown Limestone, Swartz (1930) stone and Port Ewen Chert, which he stated were equivalent identifi ed these strata as the New Scotland Limestone of the in part to the Becraft Limestone and Shriver Chert of previous Helderberg Group, and Woodward (1943) and Bick (1962) like- reports. Lesure (1957) revived the name Licking Creek Lime- wise used the name New Scotland Limestone of the Helderberg stone for the distinctive sequence of limestones that contain Group. Head (1972) demonstrated that these strata cannot be common to abundant nodules and lenses of black chert and traced continuously from Virginia through intervening areas to which are stratigraphically below the Oriskany Sandstone. the type section of the New Scotland Limestone in New York, Bick (1962) subsequently mapped these strata in Bath and and so the name New Scotland Limestone has been discontin- Highland Counties as the Licking Creek Limestone of the ued in areas south of central Pennsylvania, and the name Cor- Helderberg Group. riganville Limestone is used instead. Head (1974) redefi ned the Licking Creek Limestone so The Corriganville Limestone is recognizable by its com- as to comprise all limestone and cherty limestone above the mon and distinctive light-gray chert nodules and lenses, and Corriganville Limestone and below the Oriskany Sandstone, common large curved brachiopod shell fragments that can be and he defi ned two members of the Licking Creek (below). up to 6 cm in length. From southern Highland County south- In this region, these cherty and sandy limestones are pres- ward the Corriganville becomes less cherty and more quartzose ently mapped as the Licking Creek Limestone of the Helder- as it changes facies into the Healing Springs Sandstone (Swartz, berg Group. 1930; Perry, 1971; Dorobek and Read, 1986). In the vicinity of Burnsville Cove, the Licking Creek Limestone is an ~25–60-m-thick unit of gray cherty lime- Devonian Healing Springs Sandstone Member of the stone and argillaceous limestone, overlain by gray limestone Corriganville Limestone of the Helderberg Group and sandy limestone. In Bath and Highland Counties, these The type section of the Healing Springs Sandstone Member strata were mapped by Darton (1899) as part of the Lewis- is at Healing Springs in Bath County, Virginia. Swartz (1930) town Limestone. fi rst identifi ed these strata as a separate unit, which he named the Healing Springs Sandstone Member of the New Scotland Devonian Cherry Run Member of the Licking Creek Limestone of the Helderberg Group. Woodward (1943) and Limestone of the Helderberg Group (5–30 m thick) Bick (1962) mapped these strata as the Healing Springs Sand- The type section of the Cherry Run Member of the Lick- stone Member of the New Scotland Limestone of the Helder- ing Creek Limestone is near Cherry Run in Washington berg Group, but because Head (1972) indicated that the name County, Maryland (Head, 1974). The Cherry Run Member Corriganville Limestone should be used instead of New Scot- comprises the lower and middle Licking Creek Limestone and land Limestone in Virginia, these strata are more appropriately is characterized by lime mudstones to bioclastic wackestones mapped as the Healing Springs Sandstone Member of the Cor- and some packstones with minor argillaceous and shaly lime riganville Limestone of the Helderberg Group. The Healing mudstones, all of which have common to abundant nodules, Springs changes facies laterally northward from northern Bath lenses, beds, and irregular masses of tough black chert. The County into the cherty Corriganville Limestone (Dorobek and Cherry Run Member is equivalent to the Port Ewen Chert of Read, 1986). Woodward (1943). The Healing Springs Sandstone is a calcarenaceous quartz At Porters Cave (Stop 2), the Cherry Run Member is not arenite to sandy limestone in which bioclastic debris is com- present along Virginia State Highway 42 because it has been cut mon. It is distinctive for its wavy laminae and thin beds of out by the fault that places the Little Cove Member of the Lick- quartz grains that typically stand 1–2 mm in relief above the ing Creek on the Needmore Shale. But, just a few tens of meters enclosing limestone. to the east from the exposures along Highway 42 are superb subterranean exposures of the Licking Creek in the passages Devonian Licking Creek Limestone of the Helderberg of Porters Cave. Exploration of that cave shows that the fault Group (10–40 m thick) seen along the highway is not exposed in the cave. Furthermore, a complete composite section of the Little Cove Member and The type section of the Licking Creek Limestone is at a several meters of the underlying cherty Cherry Run Member bluff on the south side of Licking Creek ~1.6 kilometers east of are present in Porters Cave, thus suggesting that the shallowly Warren Point in Franklin County, Pennsylvania (Swartz, 1939). dipping fault seen along the highway must dip more steeply This sequence of cherty and sandy limestones has a complex approaching the cave to the east. Downloaded from fieldguides.gsapubs.org on April 8, 2014

12 Haynes et al.

Devonian Little Cove Member of the Licking Creek stone as the Monterey Sandstone, which he stated was approxi- Limestone of the Helderberg Group (8–12 m thick) mately equivalent to the Oriskany Sandstone. Kindle (1911) The type section of the Little Cove Member of the Licking identifi ed these strata in Bath County as the Oriskany Sand- Creek Limestone is near Franklin County, Pennsylvania (Head, stone, but shortly thereafter Schuchert et al. (1913) mapped 1974). The Little Cove Member, which is equivalent to the Port equivalent strata in West Virginia and Maryland as the Ridge- Jervis Limestone of Woodward (1943), can be distinguished ley Sandstone Member of the Oriskany Formation. Schuchert from the underlying Cherry Run Member by the commonly et al. (1913) divided the Oriskany into a lower member named abrupt disappearance vertically of the black chert nodules and the Shriver Chert Member of the Oriskany Formation, and an lenses that are so prevalent in the underlying limestones and are upper member named the Ridgeley Sandstone Member of the the distinguishing lithology of the Cherry Run. The Little Cove Oriskany Formation. They designated a type section for the Member is a thick-bedded quartzose bioclastic wackestone to Ridgeley Sandstone Member at the town of Ridgely [spelling packstone that commonly weathers massively, to such an extent later changed to “Ridgeley”] in Mineral County, West Virginia. in some places that bedding can be diffi cult to identify, as is the Swartz (1930) later identifi ed these strata as the Ridgeley Sand- case along much of the exposed length of the massive bluffs at stone of the Oriskany Group, but Butts (1940) used the name Stop 2. Oriskany Sandstone for these strata, stating that the Oriskany At Porters Cave (Stop 2, along the road) the Little Cove Sandstone “corresponds exactly with the Ridgely Sandstone” Member forms the hanging wall of a low(?)-angle thrust fault (Butts, 1940, p. 291); Butts also indicated that the Oriskany over the younger Needmore Shale. At Coronation (Stop 3), Sandstone is the same as the Monterey Sandstone of Darton the Little Cove and Cherry Run Members are characterized by (1899). Bick (1962) mapped these strata in Bath and High- appreciable small scale folds and contractional features by vir- land Counties as the Ridgeley Sandstone, but Avary and Den- tue of their being in the footwall immediately beneath a fault nison (1980) assigned these strata in Highland County to the that has placed the Byers Island and Clifton Forge Members Oriskany Sandstone. In subsequent publications, however, the of the Keyser above the Licking Creek Limestone. Quartz sand name Ridgeley Sandstone has persisted for these strata in Bath grains in samples of the Little Cove Member from the footwall and Highland Counties (e.g., Rader and Wilkes, 2001), but the beneath this fault have been pervasively fractured. existing rules of stratigraphic nomenclature (North American There is an interesting and unusual stratigraphic story asso- Commission on Stratigraphic Nomenclature, 1983, 2005) dic- ciated with Head’s (1974) naming of this member. In his dis- tate that that name Oriskany Sandstone has priority and thus is sertation, Head (1969) divided the Licking Creek into a lower the name that should be given to these strata throughout this Cherry Run Member and an upper Warren Point Member, and region. Therefore, this is the name we are using in our mapping the name “Warren Point Member” was used by Whitehurst efforts across this region (Haynes and Whitmeyer, 2010; Hazel- (1982) in his detailed study of the Muddy Run area in Bath wood et al., 2012; Haynes and Diecchio, 2013). County, and it was also shown in the stratigraphic column for The Oriskany is a calcareous to calcarenaceous quartz are- the Burnsville area of Bath and Highland Counties by White nite, with some pebbly zones, and one of its principal charac- and Hess (1982, p. 69), just west of the Bullpasture River Gorge teristics throughout the central Appalachian region is the abun- (Stop 5). The stratigraphic column of White and Hess (1982) has dance in some beds of prominent biomoldic pores formed by even been used as recently as 2010 (Chess et al, 2010). Since dissolution of large spiriferid brachiopod shells. 1925, however, the name “Warren Point” has been used for a Pennsylvanian sandstone in the southern Appalachians from Devonian Needmore Shale (6–45 m thick) Virginia to Georgia (Culbertson, 1963). So according to the rules of the North American Stratigraphic Code (North Ameri- The type section of the Needmore Shale is located between can Commission on Stratigraphic Nomenclature, 1983, 2005), the towns of Needmore and Warfordsburg in southern Fulton its use by Head (1969) was invalid. As a result, Head revis- County, Pennsylvania (Willard, 1939). Darton (1899) mapped ited this stratigraphic matter, and resolved it when he applied these shales as part of the Romney Shale, but Butts (1940) the name Little Cove Member (Head, 1974) to the upper and stated that the name Romney Shale should be abandoned, and mostly chert-free limestones of the Licking Creek Limestone, he mapped these strata at Bullpasture Mountain in Highland a name that is in use at the present time (e.g., Wilkes, 2011). County as the Onondaga Formation. Woodward (1943) assigned these strata to the Needmore Shale of the Onondaga Group, and Devonian Oriskany Sandstone (3–25 m thick) Bick (1962) described these strata in Bath and Highland Coun- ties as predominantly shale, which he mapped as the Onondaga The type section of the Oriskany Sandstone is at Oriskany Formation. Avary and Dennison (1980) used the name Need- Falls in New York (Vanuxem, 1839). This stratigraphic unit, more Shale for these strata, and that name is now widely used like some of the other formations we will see, has had a fairly in this region. complex nomenclatural history since it was fi rst named almost The Needmore Shale is a sequence of greenish-gray to gray 180 years ago. Darton (1899) mapped this distinctive sand- to black fi ssile to blocky weathering mudrock, with some beds Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 13 of gray lime mudstone that commonly have bioclastic debris details that this thick sequence of Lower Silurian quartz arenites weathering in relief. Intact Phacops sp. trilobites are not infre- preserves may ultimately support a basin rebound hypothesis quently found in weathered exposures of the blocky weather- for the depositional environment of these sandstones (Driese ing mudrocks of the Needmore; discovery of a Phacops in the et al., 1991; Dorsch et al., 1994; Dorsch and Driese, 1995), as Needmore Shale of the footwall at Porters Cave (Stop 2) helped well as the hypothesis that the Taconic Orogeny persisted into to confi rm that those shales are most likely the Needmore. the Silurian (Ettensohn and Brett, 2002). They may also help At many exposures, the lowermost few meters of the Need- improve our understanding of the Late Ordovician glaciation more Shale are notably darker than the overlying shales, and (Hambrey, 1985). Dennison (1961) named these black mudrocks the Beaver Dam At Eagle Rock (Stop 1), near the eastern margin of the black shale subfacies of the Needmore Shale. Hasson and Denni- depositional basin, an amalgamated 125-m-thick sequence of son (1988) later mapped these strata in Bath and Highland Coun- Silurian quartz arenites comprises the “Eagle Rock sandstone” ties as the Beaverdam Shale Member of the Needmore Shale. (Fig. 3), the modern name for Woodward’s stratigraphic “tan- gle.” It is worth noting that in the Massanutten Synclinorium Devonian Millboro Shale (70–400? m thick) to the northeast of the fi eld trip area (Fig. 1), another thick Silurian sandstone, the Massanutten Sandstone, is present. Its The type section of the Millboro Shale is at Millboro stratigraphic relations have been understood for some time, as Springs in Bath County, Virginia, along the route of this fi eld it is recognized as the equivalent of the Tuscarora, Rose Hill, trip (Cooper, 1939; Butts, 1940; Hasson and Dennison, 1988). and Keefer Formations in the fi eld trip area (Roberts and Kite, Darton (1899) mapped these black fi ssile mudrocks as part of 1978). That contrasts with the diffi culties geologists have had the Romney Shale. Butts (1940) stated that the name Romney in working out the regional stratigraphic relations of the “Eagle Shale should be abandoned, and he mapped the upper portion of Rock sandstone” over the decades. the Romney Shale in Bath and Highland Counties as a separate A few of the many fundamental questions about these Silu- formation he named the Millboro Shale after a name that had rian sandstones that can be asked as we examine and discuss been used by Cooper (1939). Farther north in Pennsylvania and some of the changes that are evident in a southeast to northwest New York, these strata are mapped collectively as the Marcellus traverse across part of the depositional basin include: (1) What Shale, Mahantango Formation, Harrell Shale, and (or) Hamil- might the subtle petrographic details of these quartz arenites ton Formation, but Butts (1940) had diffi culty differentiating tell us about the provenance of all the sand that was transported these units in Virginia, so he proposed the name Millboro Shale to the Eagle Rock depocenter? (2) What tectonic or eustatic for these strata in Virginia. Woodward (1943) used the names event(s) accompanied or preceded the erosion, transport, and Millboro Shale and Harrell Shale for these strata and he also deposition of this quantity of quartz sand? (3) Are any/all of noted that Butts had proposed the name Millboro Shale instead these sandstones fi rst-cycle quartz arenites (Johnsson et al., for the entire sequence of shales. The name Millboro is now 1988)? (4) How does the composition of the silt and clay frac- widely used in this region (Bick, 1962; Avary and Dennison, tion in the mudrocks vary across the region (Taylor and McLen- 1980; Rader and Wilkes, 2001). nan, 1985). And, (5) how and why did accommodation space The Millboro Shale is a sequence primarily of black vary so signifi cantly from Fagg to Eagle Rock (Fig. 3)? mudrock (shale) that is fi ssile and typically weathers to thin Our recent and ongoing mapping efforts in this area platy chips. Calcareous concretions are locally present, as are (Haynes and Whitmeyer, 2010; Walker et al., 2010; Hazelwood occasional silt- to sand-rich layers, some of which display et al., 2012; Haynes and Diecchio, 2013; Haynes et al., 2010, convolute bedding. Trimble Knob (Stop 7) and several other 2011) have led to the development of a working stratigraphic igneous intrusions in Highland and Pendleton Counties are model that evolves as we continue to map and trace out these emplaced in the Millboro Shale. several Silurian (and Lower Devonian) sandstones across this region (Fig. 3), from the area of Woodward’s “tangle” north- STRATIGRAPHY AND SEDIMENTOLOGY ward. In Bath County and southern Highland County, the indi- vidual sandstones become progressively more stratigraphically Facies Changes in the Silurian Sandstones distinct and “untangled” as the overall volume of mudrocks and carbonates in the section increases relative to quartz arenites. Local and regional stratigraphic relationships among the This change in lithologic ratios effectively divides and sepa- various Silurian sandstones are complex, especially those in rates what at Eagle Rock is the massive and undifferentiated the Keefer–McKenzie–Williamsport–Wills Creek interval, “Eagle Rock sandstone” (Stop 1, Fig. 3) into the readily dif- and correlations have been puzzled over by geologists in this ferentiated Keefer, McKenzie, and Williamsport quartz arenites region for decades, even before Woodward (1941, p. 103, 169) that comprise discrete stratigraphic horizons at other exposures aptly referred to exposures in the Eagle Rock and Clifton Forge such as Crizer Gap (Stop 4). area (Stop 1) as a stratigraphic “tangle of Silurian sandstones.” Each of these sandstones makes obvious ledges in the gorge From a tectonic perspective, the stratigraphic and sedimentologic of the Bullpasture River (Stop 5), but when our mapping in this Downloaded from fieldguides.gsapubs.org on April 8, 2014

14 Haynes et al. region began, we found that identifying which sandstone ledge sandstones in the lower member of the Tonoloway Limestone was which is no simple task. This fundamental stratigraphic (Stops 4 and 5) that for almost 50 years were mistakenly identi- issue evidently eluded previous geologists who worked in this fi ed in the Bullpasture River Gorge and nearby areas as upper region as well (e.g., Bick, 1962), perhaps because no strati- and lower tongues of the Clifton Forge Sandstone (White and graphic column for the gorge and nearby areas that is based on Hess, 1982; Walker et al., 2010; Swezey et al., 2013). Other one or more measured sections has ever been published, a situ- calcarenaceous sandstones we will see include the “real” Clif- ation that we have rectifi ed (Fig. 3). One major advance in our ton Forge Sandstone Member of the Keyser Limestone (Stops understanding of the Bullpasture Gorge is the identifi cation of 3, 4, and 5), the Healing Springs Sandstone Member (Stop 3 the McKenzie sandstone, discussed in more detail below. and alternate Stop 4A), the sandy limestones of the Little Cove At Stops 1, 5, and 6, we will also discuss an intriguing Member of the Licking Creek Limestone (Stops 2, 3, and 4), stratigraphic aspect of the Keefer Formation, specifi cally what and the Oriskany Sandstone (Stops 2, 3, and 5 and alternate is true Keefer Sandstone versus “Keefer Sandstone” versus Stop 4A). “Eagle Rock sandstone.” Woodward (1936) may have been the fi rst to refer to the thick sequence of sandstones in the Roanoke Quartz Sandstones and Quartzose Oolitic Grainstones in area and vicinity (including Eagle Rock) as Keefer, and sev- the McKenzie Formation eral authors since then (e.g., Lesure, 1957; Rader, 1967, 1984; Patchen, 1974; Dennison et al., 1992) have used the expanded A second stratigraphic fi nding that has come from our map- Keefer or “Keefer” as a convenient name and useful mapping ping in this area is that we now recognize that the unnamed mid- unit, but one that is stratigraphically inappropriate vis-à-vis the dle sandstone member of the McKenzie Formation (Travis, 1962; North American Stratigraphic Code (North American Commis- Patchen, 1974) extends ~25 km northward of the exposures along sion on Stratigraphic Nomenclature, 1983, 2005), because the Muddy Run (Fig. 3), which is the previously identifi ed northern restricted Keefer is a well-defi ned stratigraphic unit. limit of that sandstone (Whitehurst, 1982; Diecchio, 1986; Diec- We favor discontinuing the use of “Keefer Sandstone” in chio and Dennison, 1996). It is a thick bed in the Bullpasture the central Appalachians and substituting instead the “Eagle River Gorge (Stop 5), and it thins northward from there (Fig. 3) Rock Sandstone” of Lampiris (1975) as an acceptable alterna- toward McDowell and Monterey, but it can be recognized as a tive name (and one that is far less stratigraphically confusing) thin bed on Jack Mountain west of McDowell. for this markedly thickened sequence of Silurian sandstones at In addition, our mapping has led us to conclude that the exposures in this area where its use would be appropriate. This exposures of oolitic limestones in the gorge of the Bullpasture would be analogous to the way that the Massanutten Sandstone River and along nearby Tower Hill, and which form prominent is used farther north. “Eagle Rock” as a stratigraphic name is ledges that total ~8 m in thickness along State Road 678 (Stop available, because the term Eagle Rock tuff in Idaho, named 5), are quite likely an older and earlier eastern equivalent of the by Stearns (1936), has been abandoned (Stearns and Isotoff, oolitic facies that in the subsurface farther west comprises the 1956). For this to happen, though, the name “Eagle Rock Sand- upper part of the Lockport Member of the McKenzie (Patchen stone” will need to be formally proposed as a stratigraphic unit and Smosna, 1975; Smosna and Patchen 1978; Smosna, 1984). in accordance with the North American Stratigraphic Code And, because these oolitic limestones in the McKenzie Forma- (North American Commission on Stratigraphic Nomenclature, tion along the Bath–Highland County line are stratigraphically 1983, 2005). below the unnamed middle sandstone member of the McKen- Initial study of the petrographic characteristics of these zie, in contrast to the oolitic carbonates in the Lockport Mem- sandstones has helped us begin to identify criteria that are use- ber farther west that overlie the middle sandstone, we conclude ful in helping to distinguish one from another on the basis of that these oolitic limestones represent an earlier development of more than just stratigraphic position. Petrographic study has the sandy oolitic lithofacies in the basin. also helped with our efforts to identify these sandstones at three Thin sections of these oolitic grainstones and related bio- additional and previously unmeasured exposures in Highland clastic grainstones from the Bullpasture River Gorge and Lower County: Lower Gap, Trimble, and McDowell (Fig. 3). Gap indicate that dolomitization has not been as pervasive in In addition to these sandstones, we will see and discuss this area as in many of the cores described by Smosna (1984), some of the facies changes that occur in other Silurian and but the cementation history of these carbonates is complex Lower Devonian sandstones of this area, several of which are nonetheless, with a later diagenetic ferroan baroque dolomite calcarenaceous quartz arenites (“calcarenaceous” refers to a (Spötl and Pitman, 1998) that effectively reduced any remain- siliciclastic sediment in which 10%–50% of the total framework ing interparticle pore spaces in these sediments. grains are carbonate allochems, e.g., peloids or bioclasts; Pet- Our discovery that the middle sandstone and the oolitic tijohn et al., 1972, p. 190; Riley et al., 1997, p. 437) in contrast grainstones in the McKenzie Formation extend into central and to the predominantly quartz arenites of the Tuscarora, Keefer, southern Highland County and northern Bath County could be and Williamsport (in which quartz framework grains comprise of some signifi cance in the search for hydrocarbons in the Silu- ≥95% of the total framework grains). These include unnamed rian of this region. The presence of baroque dolomite in these Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 15 and other Silurian and Devonian strata of this region (Dorobek, relatively short distances in the area of this fi eld trip, as shown 1987; Haynes et al., 2010, 2011) is evidence that these strata on the detailed stratigraphic column that is applicable to Stops have been in the oil window (Spötl and Pitman, 1998). There 2, 3, and 4 (Figs. 5, 6). This abrupt thinning and discontinuous is a long history of natural gas production in the central Appa- nature of the Byers Island Member in this area, which we will lachian basin from the Lockport oolite facies (Patchen and see at Coronation and Crizer Gap (Stops 3 and 4), has appar- Smosna, 1975) and more recently from the Williamsport Sand- ently not been recognized previously. At Coronation (Stop 3), stone (Patchen, 1974); Patchen (1974) noted that thicknesses the Byers Island is at least 18 m thick, and it separates the older of the Williamsport Sandstone were inversely proportional to Tonoloway Limestone (below) from the younger Clifton Forge the thickness of the underlying McKenzie Formation. Given Sandstone (above), but just less than 3.5 km to the northeast, at the proximity of these McKenzie exposures in the Bullpas- Crizer Gap (Stop 4), the Byers Island is absent, and the upper ture River Gorge to the scattered gas production in neighbor- beds of the Tonoloway Limestone are overlain instead by the ing Pocahontas County (Limerick et al., 2005), further study Clifton Forge Sandstone. of these units may be warranted at some future time for their Our mapping (Haynes and Whitmeyer, 2010; Hazelwood potential to assist in identifi cation of drilling prospects in the et al., 2012) has shown that a similar stratigraphic relationship Silurian of this area. also occurs in southernmost Highland County, where the Clif- Stratigraphically, this sequence of oolitic limestones and ton Forge unconformably overlies the Tonoloway in and around the overlying unnamed middle sandstone member have turned the gorge of the Bullpasture River (Stop 5), but that in cen- out to be useful marker beds. This has been especially help- tral and northern Highland County (vicinity of Stop 6 although ful to our ongoing mapping efforts in central and northern not exposed at our stop there) the Byers Island unconform- Highland County, an area where the Keefer sandstone thins to ably overlies the Tonoloway. In northern Highland County and such an extent that it is no longer mappable, and the sequence northward into adjacent Pendleton County, the Keyser stratig- becomes an otherwise shale-dominated stratigraphic interval raphy is further complicated by the facies change in the middle from the middle Rose Hill Formation up section to the Wil- Keyser, as the Clifton Forge Sandstone passes laterally into the liamsport Sandstone. Big Mountain Shale Member. The nature of the Tonoloway–Keyser contact has not been Unconformable Nature of the Tonoloway–Keyser Contact discussed in any appreciable detail in previous studies. Wood- ward (1941, 1943) observed fl at-pebble conglomerates discon- In the central Appalachians, the oldest of fi ve named mem- tinuously at the contact in some of his many measured sections, bers of the Keyser Limestone is the Byers Island Member of and Dorobek and Read (1986) noted that the lowest beds of Head (1972, 1974). Our mapping efforts have shown that the the Keyser represent a deepening of the water column and thickness of the Byers Island varies greatly and abruptly over development of more open marine conditions compared to the

Figure 5. Stratigraphic column for units in the exposures by Porters Cave (Stop 2), Coronation (Stop 3), and Crizer Gap (Stop 4). Downloaded from fieldguides.gsapubs.org on April 8, 2014

16 Figure 6. Elevated cross section showing our interpretation of the geology in the vicinity of Stops 2 and 3. Note that the top of the cross section represents the ground surface, and theof the cross section represents ground surface, our interpretation of the geology in vicinity Stops 2 and 3. Note that top cross section showing Figure 6. Elevated (mostly the Clifton Forge Formation Limestone; SDk—Keyser Sandstone, Stw—Tonoloway Swp—Williamsport Formation; Smc—McKenzie geology. colored units represent subsurface Landscape Needmore Shales undivided. Dmn—Millboro and Sandstone, Licking Creek Limestone, and Healing Springs Sandstone undivided; Sandstone Member); Dolh—Oriskany Group; Dm—Millboro and Needmore Shales; (1965) in the vicinity of Stop 3 (Dhl—Helderberg the cross-section interpretation of Kozak The inset shows imagery is from Google Earth. interpretation. is missing from Kozak’s at thrust fault Limestone). Note that the west-directed ramp-to-fl Group; Sk—Keyser Sandstone; Scl—Clinton Group; Scy—Cayuga Do—Oriskany Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 17 restricted conditions that prevailed during deposition of much Geochronology of the Eocene (and Late Jurassic) of the underlying Tonoloway (Smosna et al., 1977), and that Igneous Rocks were described in more detail by Bell and Smosna (1999). The Eocene volcanic swarm (Fig. 7) was originally PETROLOGY AND GEOCHEMISTRY assumed by initial investigators to be a part of Central Atlan- tic magmatic province volcanism (May, 1971; Ragland et al., Overview 1983; Philpotts and Martello, 1986; McHone et al., 1987; Rag- land, 1991; Marzoli et al., 1999; McHone, 2000, 2002). The The Eocene volcanic rocks of western Virginia and West Vir- Central Atlantic magmatic province volcanic activity occurred ginia are the expression of the youngest-known magmatic event at ca. 190–200 Ma along the East Coast of the United States, in the eastern United States. Why these Eocene magmas erupted during the breakup of Pangea (Nomade et al., 2007; Marzoli et is an especially intriguing question, because then, as today, the al., 1999). It was not until the 1960s that the fi rst paleomagnetic eastern United States was a passive margin, far from the Mid- data implied these igneous rocks were Eocene in age (Elvers et Atlantic Ridge. The answer to this question must involve the long al., 1967; Fullagar and Bottino, 1969). Limited K-Ar and Ar-Ar and complicated tectonic history of the Appalachian region (i.e., dates (Core et al., 1974; Wampler and Dooley, 1975; Ressetar Hatcher, 1989). The Eocene dikes as well as the ca.150 Ma Late and Martin, 1980; Southworth et al., 1993) confi rmed the young Jurassic dikes in the area (Southworth et al., 1993) also contain age of these igneous rocks with reported ages ranging from 48 crustal and mantle xenoliths that help constrain the deeper stra- to 35 Ma. The most recent Ar-Ar dates constrain Eocene erup- tigraphy of the lower crust and mantle. These volcanic rocks hold tion ages to 47–48 Ma (Bulas and Johnson, 2012). important keys to the interpretation of structural and seismic data The region of Eocene volcanic rocks overlaps with an area in the region and our understanding of the long-term, continued (Fig. 7) that includes ca. 150 Ma Late Jurassic dikes which evolution of the rift-to-drift transition for eastern North America, postdate the opening of Pangea by 50 Ma (Zartman et al., 1967; as well as for rift margins worldwide. Johnson et al., 1971; Southworth et al., 1993). Although only

Figure 7. Igneous rocks of Jurassic and Eocene age in west central Virginia and eastern West Virginia. (A) Location of the Virginia–West Vir- ginia Eocene and Late Jurassic volcanic swarm (bold black lines within B) relative to the ca. 200 Ma igneous rocks (bold black lines) of the Central Atlantic magmatic province (CAMP). Also shown are the proposed hotspot track (line from middle left to top right) and areas of lower lithospheric anomalies (gray boxes) from Chu et al. (2013). Central Atlantic magmatic province locations from Ragland et al. (1983), McHone et al. (1987), Ragland (1991), and McHone (2000). State boundaries are shown for reference. (B) Expanded view of the Eocene volcanics and Late Jurassic dikes showing fi eld trip stop locations and additional locations where lower crustal xenoliths have been found. 5S—5 Springs; AF—Arey Farm; VQ—Vulcan Quarry; numbers correspond to fi eld trip stops. The Eocene volcanics lie in clusters along strike with faults (black curves) and major Alleghanian fold structures (as refl ected by topography). In contrast, the Late Jurassic dikes strike perpendicular to regional strike, but parallel to the local Central Atlantic magmatic province dikes. Locations from Garnar (1956), Southworth et al. (1993), Helsley et al. (2013), and Rossi et al. (2013). Bold line is the Virginia–West Virginia state line. Topographic basemap from Ryan et al. (2009) created using GeoMapApp (www.geomapapp.org). Downloaded from fieldguides.gsapubs.org on April 8, 2014

18 Haynes et al.

three of the Late Jurassic dikes have been dated using K-Ar tures (637–984 °C; Helsley et al., 2013) extend into the ultra- and Ar-Ar geochronology (Zartman et al., 1967; Johnson et al., high temperature (UHT) fi eld of metamorphism. This is only 1971; Southworth et al., 1993), dikes with similar-trace element the second reported occurrence of UHT rocks in the eastern geochemistry and orientation are assumed to be Late Jurassic in United States (Ague et al., 2013). The depth (pressure) of origin age (e.g., Furman and Gittings, 2003) and are common within of the xenoliths is not as well constrained, but a pseudosection Augusta and adjacent Rockingham and Pendleton Counties from Ague et al. (2013) implies that maximum pressure was at (Fig. 7). least 7.25 kbar and no more than 14.5 kbar. The gneiss xenoliths record Grenvillian detrital U-Pb zircon ages (Rossi et al., 2013) Whole-Rock Geochronology and Petrology with no evidence for zircon rim growth during subsequent oro- genic events. In contrast, the anorthosite and gabbro xenoliths Whole-rock geochemical data for the Eocene volcanics are have igneous U-Pb ages of ca. 150 Ma. compiled in Southworth et al. (1993) and additional data are Two studies (Sacco et al., 2011; Jones et al., 2012) have reported in Tso et al. (2004) and Furman and Gittings (2003). The used olivine-melt, clinopyroxene-only, and clinopyroxene-melt Eocene volcanics form a bimodal alkaline series ranging from geothermobarometers to determine temperature and depth from picrobasalts and basanites to trachydacites and rhyolites. The mantle xenocrysts. At Mole Hill, the xenocrysts record a tem- mafi c volcanics have LREE (light rare earth element)-enriched perature of 1230 °C and a pressure of 13 kbar, corresponding to REE (rare earth element) patterns with steep negative slopes simi- ~39 km depth (Sacco et al., 2011). These conditions fall close lar to those of oceanic-island basalts (OIBs), and the felsic rocks to the solidus within the spinel peridotite stability fi eld, con- have even more highly enriched LREE (Southworth et al., 1993; sistent with the assemblage of olivine, high Al-augite, and spi- Furman and Gittings, 2003) than the mafi c magmas. Extremely nel xenocrysts found within the basalt. The recorded depth is limited Sr, Nd, and Pb isotopic data from Mole Hill (Furman and close to the 40 km depth of the Moho (Benoit and Long, 2009). Gittings, 2003) and a composite Sr isotope analysis of felsic rocks Clinopyroxene megacrysts from a Highland County picrobasalt from Highland County (Fullagar and Bottino, 1969) indicate little record a pressure similar to that at Mole Hill (P = 14.1 ± 1.8 interaction of the Eocene magmas with the crust. kbar), but a signifi cantly higher temperature of 1371 °C (Jones The mafi c rocks are generally aphanitic to porphyritic et al., 2012). Preliminary calculations using olivine melt inclu- and contain microcrystalline plagioclase and olivine or clino- sion compositions from Mole Hill estimate the depth of melting pyroxene within the groundmass. Phenocrysts of olivine and to be 60–70 km at that location (O’Reilly et al., 2012), which clinopyroxene are common within the mafi c magmas. Vesicles would likely lie within the garnet peridotite stability fi eld. Fur- and/or amygdules fi lled with carbonates, sulfi des, and zeolites ther geothermobarometric work is needed to completely inter- are often present (Kearns, 1993). The intermediate and felsic pret these preliminary results. rocks typically have a devitrifi ed glassy groundmass, some- times including microcrystals of plagioclase, with phenocrysts STRUCTURAL GEOLOGY of plagioclase, alkali feldspar, amphibole, and biotite. Some of the felsic dikes have glassy chilled margins. Alleghanian Deformation

Geochemistry of Xenoliths and Xenocrysts This fi eld trip visits excellent exposures that characterize outcrop- to mountain-scale fold and fault structures commonly Geochemical analyses of lower crustal xenoliths and man- associated with thin-skinned tectonics. The region is commonly tle xenocrysts that occur within the Eocene and Late Jurassic viewed as a type example of a foreland fold-and-thrust belt volcanic rocks can be used to understand the vertical cross sec- (Rodgers, 1970), as it represents the foreland of the late Paleo- tion of the lower crust and mantle underneath western Virginia zoic collision of Gondwana with eastern Laurentia. The region and eastern West Virginia. highlighted by this fi eld trip is dominated by a series of large- Lower crustal xenoliths of paragneiss and anorthosite- scale anticlines and synclines, bounded by the west-directed gabbro are included within several dikes in the area (Fig. 7B). Pulaski and North Mountain fault systems to the east, and dis- The gneisses contain a granulite-facies mineral assemblage of sipating at the Allegheny structural front to the west (Fig. 1). quartz + alkali feldspar + garnet + rutile + sillimanite + zircon From west to east the major fold structures include the Wills ± apatite ± graphite with secondary chlorite, ilmenite, mona- Mountain anticline, Warm Springs–Bolar anticline (Kulander zite, sulfi des, and barite along grain boundaries and fractures and Dean, 1986; Rader and Wilkes, 2001), and Rough Mountain (Helsley et al., 2013). Metamorphic temperatures were deter- syncline. These and other large-scale fold structures are inter- mined using Zr-in-rutile thermometry (Tomkins et al., 2007) on preted to overlie a series of duplexes of Cambrian to Ordovician rutile inclusions within garnets that record maximum tempera- clastic and carbonate rocks (Kulander and Dean, 1986; Mitra, tures, as well as rutile grains within the matrix of the xenolith 1986). Second-order deformation features occur as parasitic that record resetting during infi ltration of hydrous fl uids from folds on the large-scale structures, and as fault bend folds or the magma into the xenolith during ascent. Calculated tempera- ramp anticlines (e.g., the “breadloaf”-shaped anticline of Stops Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 19

2–4; Fig. 6) potentially produced by subsurface blind and/or to the surface, especially considering our fi ndings which indi- cryptic thrusts (Wilson and Shumaker, 1992; Dunne, 1996). cate that magma ascent rates were rapid (Stempniewicz and The geometry and kinematics of thin-skinned fold-and- Johnson, 2011). For example, even though Mole Hill (Stop thrust structures are typically facilitated by mechanically weak, 13) is exposed within carbonates of the Ordovician Beekman- shale-dominated lithologies (e.g., Millboro, Needmore, Bral- town Formation, xenoliths of a sandstone unit beneath Mole lier Formations) that alternate with more competent and brittle Hill were brought to the surface during eruption. Petrographic sandstone-dominated units (e.g., Clifton Forge Sandstone, Oris- analyses (texture, grain size, and composition) indicate that the kany and Tuscarora Formations). Low-angle thrusts, such as the Silurian Tuscarora Formation is the most likely stratigraphic one that underlies the anticline of Stops 2, 3, and 4 (Fig. 6), typ- unit from which the sandstone xenoliths were derived (Johnson ically propagate through weak shales, though many ramp sec- et al., 2013). We hypothesize that the Eocene Mole Hill magma tions probably cut through more competent lithologies at depth exploited splays of the North Mountain fault system (exposed (e.g., Kropp et al., 2013). Outcrop- and larger-scale folds com- ~8 km to the west) to ascend to the surface. monly exhibit contractional wedge faults in the hinge regions (Perry, 1978; Fichter et al., 2010) as a mechanical accommoda- Eocene Tectonics tion by brittle lithologies to the reduction of space precipitated by folding. The resulting increased thickness of hinge regions Several hypotheses have been proposed to explain the from structural interfi ngering can produce elevated hills that are Eocene magmatism on a plate tectonic scale. Southworth et al. resistant to weathering (Fig. 6), in contrast to the more typi- (1993) proposed that a shift in worldwide plate tectonic motion cal inverted topography of kilometer-scale breached anticlines at ca. 50 Ma may have resulted in a change in the stress fi eld (e.g., the Germany Valley region of the Wills Mountain anti- in the North American plate, reactivating fault and fracture sys- cline; Fichter et al., 2010). tems in the deep and shallow crust and inducing magmatism. In general, differential weathering in the southeastern Valley The 38th parallel fracture zone (Dennison and Johnson, 1971) and Ridge province has resulted in northeast-trending mountain has recently been extended and reinterpreted as a hotspot track ridges that are underlain by resistant Silurian and Devonian sand- produced by linear regions of lower lithospheric thinning and stones, and valleys that are typically underlain by weak Devonian asthenospheric upwelling (Chu et al., 2013). This proposed shales (e.g., Enomoto et al., 2012) or Ordovician carbonates (e.g., hotspot track crosses directly through the Virginia–West Vir- Diecchio, 1985, 1986). Smaller-scale, fault-related folds can add ginia Eocene volcanic swarm (Fig. 7A), although Chu et al. intriguing complexities to regional topographic patterns. How- (2013) places the hot spot in this area at ca. 65 Ma rather than ever, the lack of exposed deformational features across much of at 50 Ma. the Valley and Ridge province can lead to underestimates of the Other hypotheses combine mantle dynamics with plate complexity of deformation in this region. On this fi eld trip, we tectonics. Asthenospheric upwelling and melting could have are fortunate to have several key outcrops that we can use as a occurred underneath Virginia as a response to the Farallon plate proxy for the style and intensity of deformation that likely per- sinking into the deep mantle below the eastern United States vade much of the Appalachian foreland. (Moucha et al., 2008; Forte et al., 2010). Edge-driven convec- tion of the asthenosphere at the edge of the continental litho- Structural Controls on Eocene Magmatism sphere also could have caused upwelling and melting to occur (King and Anderson, 1998). Helium isotope data obtained from Eocene volcanic rocks are generally exposed within Ordo- waters at Warm Springs (R/RA > 1) record a mantle component vician, Silurian, and Devonian limestones and shales (Darton, (Baedke and Silvis, 2009), implying that at least some fractures 1899; Wilkes, 2011), and the intrusions mostly (but not always) in the shallow crust are connected in some way to the mantle seem to have avoided intrusive pathways through the stronger below, perhaps by deeper fractures in the lower crust. sandstone units. The shales and limestones are generally more These scenarios might be able to explain upwelling at a complexly deformed than the sandstones, and there is usually distance from the continental margin that corresponds to west- evidence for many parasitic folds within larger-scale fold struc- ern Virginia, but they are less able to explain the unique loca- tures (Tso and Surber, 2006). Exposures of Eocene volcanic tion of young volcanic rocks along the edge of eastern North rocks are generally aligned along or around fold axes and noses America. Preexisting structural and thermal conditions caused of plunging folds in the region (Darton, 1899; Wilkes, 2011). by prior orogenic and rifting events might help to explain why This contrasts with Late Jurassic dikes, which generally strike magmatism occurred only in the western Valley and Ridge dur- northwest (Southworth et al., 1993). The strike orientations of ing the Eocene. For example, Wagner et al. (2012) interpret their individual bodies, where determined, have been shown to align seismic data to show a remnant subducted slab and evidence with jointing within the sedimentary unit(s) into which they for crustal delamination below the Piedmont and Blue Ridge intrude (Tso and Surber, 2006). of North Carolina. Regional variations in the lower crustal and It is reasonable to hypothesize that the Eocene dikes and lithospheric mantle structure could have produced magmatism plugs exploited preexisting fracture and fault systems to ascend in western Virginia. At present, existing hypotheses for these Downloaded from fieldguides.gsapubs.org on April 8, 2014

20 Haynes et al. enigmatic intrusions are poorly constrained and await future graph, are folded and crosscut by several faults (McGuire, 1970; investigations to improve our understanding of Eocene tectonic Kattenhorn and McConnell, 1994; McConnell et al., 1997). conditions in the eastern United States region. Hanging wall anticlines and footwall synclines are manifest as both fault propagation folds and fault bend folds. Fault-bounded ROAD LOG AND STOP DESCRIPTIONS horse blocks are apparent in the hanging wall of the main thrust surface, the footwall of which also exhibits secondary thrust Our fi eld trip will travel on Day 1 from Blacksburg, Vir- splays (McConnell et al., 1997). This locality is incorporated ginia, to Monterey in Highland County, Virginia, via U.S. 460, within the Eagle Rock allochthon (a thrust-bounded klippe; I-81, U.S. 220, I-64, Virginia State Highways 42 and 39, Vir- Bartholomew, 1987) produced by west-directed thrusting of ginia State Road 678, and U.S. 250 and U.S. 220. We will spend the Pulaski fault system. Similar geometries of thrust bounded the night in Monterey. On Day 2 we will travel from Monterey horses and interrelated folds are in evidence along the North to Blacksburg via U.S. 220, Virginia State Roads 637, 640, and Mountain fault system (Orndorff, 2012), and perhaps apparent 614, West Virginia State Road 21, U.S. 33, I-81, and U.S. 460. in part as the doubly plunging fault bend anticline that we will The road log begins at Christiansburg, just east of Blacks- examine at Stops 2, 3, and 4. burg, where U.S. 460 intersects I-81. The coordinates for lati- At this exposure, look for trace fossils in the “Eagle Rock tude and longitude for all stops are given in the WGS 84 system. sandstone,” look for criteria that distinguish the “Eagle Rock” from the Rose Hill and the Tuscarora, and for the more subtle Day 1 differences in texture and composition that occur in the almost 130-m-thick sequence of quartz sandstone that comprises the “Eagle Rock Sandstone” and that might be useful for help- Mileage Directions ing understand Woodward’s “tangle” of Silurian sandstones (Cumulative) in this region. 0.0 Intersection of U.S. 460 and I-81 (Mile Marker Mileage Directions 118 on I-81). Proceed north on I-81. (Cumulative) 32 (32) Exit 150B at Daleville and junction with U.S. 220. Exit from I-81 and take U.S. 220 north. 0.0 (52.7) Continue north from Eagle Rock on U.S. 220. 20.7 (52.7) Eagle Rock, and the long exposure along the 22.5 (75.2) Junction with I-64. Take the entrance ramp onto southwest side of U.S. 220 at the bridge over the I-64 East (right). James River. 1.4 (76.6) Exit 29 on I-64 and the junction with Virginia State Highway 42. Leave I-64 and drive on State Stop 1. Eagle Rock Highway 42 heading north (left). 37.64125° N, 79.80735° W 10.2 (86.8) Arrive at the pullout on the west side of Highway At this exposure (Fig. 4), the prominent ridge-forming 42 across from the trail to Porters Cave. This is a sandstone is the Silurian “Eagle Rock sandstone” of Lampiris relatively busy road, so cross it cautiously. (1975). It is underlain by the Rose Hill Formation and overlain by the Tonoloway Limestone. Gathright and Rader (1981) and Stop 2. Porters Cave Rader and Gathright (1984) correlated the middle red zone of 37.91743° N, 79.72533° W the “Eagle Rock sandstone” with the Bloomsburg Formation, At this exposure along the eastern side of the broad Cow- which in the Massanutten Synclinorium to the northeast is at pasture River Valley, a low-angle, west-directed thrust fault has the approximate stratigraphic level of the McKenzie Formation emplaced chert-free limestones of the Little Cove Member of and Williamsport Sandstone (Figs. 2, 3). Because the Tusca- the Devonian Licking Creek Limestone over the younger Devo- rora Formation underlies the Rose Hill Formation here at Eagle nian Needmore Shale (Figs. 5, 6, 8A), yet there is a signifi - Rock, the “Eagle Rock sandstone” cannot be completely correl- cant spring that emerges at the base of the embankment, appar- ative with the thick Massanutten Sandstone of that area, which ently from beneath the shale, suggesting that there are karstic includes the Tuscarora and Rose Hill equivalents. Furthermore, limestones present, at this lower elevation and lower structural there is no nearby exposure of the Bloomsburg along strike position. The stratigraphic sequence of shale-limestone-shale toward the Massanutten Synclinorium, and there is no Blooms- at this location was fi rst noted by Holsinger (1961), who aptly burg in the western Valley and Ridge, but rather the sequence of described it thusly: McKenzie, Williamsport, and Wills Creek formations. The structural complexity of Eagle Rock has been well documented in other fi eld guides (Gathright and Rader, 1981; Limestone outcrop just west of the entrance to Porter’s Cave. Note the well defi ned contact between the limestone (upper bed) and the shale Rader and Gathright, 1986; Spencer et al., 1989), and thus we (lower bed). The shale layer here is about 15 ft thick while the lime- will provide only a few highlights. Steeply inclined to over- stone layer is about 50 ft. Overlying the limestone is another layer of turned sandstones and shales, as outlined in the preceding para- shale averaging from 20 to 25 ft thick. (Holsinger, 1961, p. 67) Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 21

Holsinger’s (1961) Figure 3 shows this contact, which tion, we welcome discussion and speculation on the hydrogeo- we now interpret as a fault, as a normal stratigraphic contact. logic relationship of the (presumably) karst spring at the base Although Holsinger did not attempt to name or correlate either of the embankment to the structure and stratigraphy at this stop. of the two shale horizons regionally as to their stratigraphic Mileage Directions position, he did conclude that Porters Cave is formed in the (Cumulative) New Scotland Limestone of the Helderberg (Holsinger, 1961, p. 66), which is the Corriganville Limestone of current regional 0.0 (86.8) Continue northward on Virginia State High- stratigraphic usage. way 42. By contrast, our fi eldwork here indicates that (1) the Heal- 0.7 (87.5) Pull off along the right (south) side after the ing Springs Sandstone, not the facies equivalent cherty Cor- sharp bend to the east. The sharp, blind curves riganville Limestone, is present in this area and in fact forms in both directions make this a particularly the ceiling of nearby Black Oak Cave; and (2) Porters Cave is treacherous road to traverse, so as at Stop 2, developed in the Licking Creek Limestone (both the Little Cove cross the road cautiously. and Cherry Run Members, Fig. 5), rather than the Corriganville Limestone. Between here and Highland County to the north Stop 3. Coronation (Stops 5 and 6), the Healing Springs changes facies into the 37.92466° N, 79.72002° W cherty Corriganville Limestone, and this is one of the several At this exposure, previously documented by Montgomery signifi cant facies changes that occur in the Silurian–Devonian (1967) and by Rader and Gathright (1984) as a stop in their sequence of this region (Figs. 3, 8, 9, 10). road log, a complexly deformed sequence of strata that includes This km-long outcrop is at the south end of a southwest- Tonoloway limestones, Byers Island and Clifton Forge Mem- plunging anticline that is located between regionally extensive bers of the Keyser Formation, and Licking Creek limestones, synclines in which Rough Mountain (to the east) and Beard is present in the core of a broad anticline. The upper 6–8 m Mountain (to the west) are the most prominent features. The of thinly bedded, laminated and algal-laminated, and in places exposures are astride the axis (nose) of the anticline. The fault, mud-cracked lime mudstones of the Silurian Tonoloway Lime- which crops out along the east side of Virginia State High- stone are the oldest strata exposed. In the old roadside quarry way 42 at this locality, exhibits cm-scale deformation bands at the east end of this exposure, the approximate contact of the and drag structures indicating top-to-the-west movement. The uppermost thin beds of Tonoloway lime mudstones with the geometry of hanging wall rocks defi nes an anticline with a overlying massive bryozoan and crinoidal grainstones and in wavelength of several tens of meters, plunging gently to the places boundstones of the Byers Island Limestone Member southwest. of the Keyser Formation can be found with little diffi culty. Several trips into Porters Cave, 50 m east of the road, The massive Byers Island is the principal limestone in the old have thus far failed to locate the fault plane in the lower cave quarry, and it is overlain by cross-bedded calcarenaceous sand- passages. Instead, a good composite section of the Licking stones of the Clifton Forge Sandstone Member of the Keyser Creek Limestone can be seen in the cave, with the ceiling of (Fig. 11B), with the contact being well exposed at the east end many of the cave passages in the upper levels being either the of the quarry. basal Oriskany Sandstone (exposed in the south wall of the Deformation features become more apparent to the west, sinkhole in which the cave’s only known entrance occurs) or with abundant small-scale folding apparent in the Tonoloway the uppermost sandy limestones of the Little Cove Member limestones adjacent to a shallowly east-dipping fault that trun- of the Licking Creek. A complete section of the Little Cove cates the unit. The fault continues upward to the west, where it Member is present in the cave, and it is underlain by a nearly underlies a wedge of lighter gray Byers Island limestone. These complete section of the underlying Cherry Run Member of limestones are, in turn, emplaced over darker-gray deformed the Licking Creek, with its distinctive and relatively abundant cherty and sandy limestones of the Licking Creek Limestone lenses and nodules of black chert. These stratigraphic rela- along a second subhorizontal fault. Pervasively fractured quartz tions, in combination with local structural features (i.e., Little sand grains with numerous fracture lamellae (Fig. 11A) like Cove Member faulted on Needmore Shale along the high- those discussed at length by Perry (1971) in Pendleton County, way), suggest that the shallowly dipping fault plane apparent West Virginia, are adjacent to the fault plane. Near the western along Highway 42 steepens to the east, perhaps in a fl at-to- end of the outcrop, the overlying Clifton Forge sandstone dips ramp geometry (Fig. 6). westward, defi ning the western part of the broad anticlinal struc- At this exposure, look for bedding in the massive bluffs of ture, but exhibiting none of the internal deformation seen in the the Little Cove Member, for evidence of movement and slippage underlying limestone units. The western part of this exposure along the fault plane, for fossils including Phacops (Fig. 8A) in is a steeply dipping succession of Healing Springs Sandstone, the Needmore Shale beneath the fault plane, and, at the south- Licking Creek Limestone, Oriskany Sandstone (Fig. 11C), and ern end of the exposure, look at the thin Oriskany Sandstone a meter or two of Needmore Shale, all in normal stratigraphic and the overlying Needmore Shale in normal sequence. In addi- sequence (Figs. 5, 6). Overall, the geometry of interfi ngered, Downloaded from fieldguides.gsapubs.org on April 8, 2014

22 Haynes et al. thrust-bounded wedges of limestone overlain by gently folded Mileage Directions but otherwise undeformed sandstone beds suggests that a series (Cumulative) of contraction faults accommodated reduced space in the hinge 0.0 (87.5) Continue northward along Virginia State High- of a broad anticline. This anticline likely overlies a northeastward way 42. continuation of the west-directed thrust apparent at Stop 2. 2.2 (89.7) At the T-intersection with VA 632 (Crizer Gap At this exposure, look for the lithologic criteria that indi- Road), which comes from the west (left), note cate that the lowest limestones in this section are Tonoloway the exposed ledges of cherty limestones of limestones, look for bryozoans and other fossils in the Byers the Cherry Run Member of the Licking Creek Island, compare and contrast the Clifton Forge and Heal- Limestone along the east (right) side of the road. ing Springs and Oriskany Sandstones, and look at the contact Turn west (left) onto VA 632. between the cherty Cherry Run Member of the Licking Creek 0.6 (90.3) Drive to the pullout on the west side of the and the overlying Little Cove Member. Questions to consider at bridge over the creek. this exposure: Why is the Oriskany relatively thin in this area (generally less than 4 m thick)? What caused this anticline to Stop 4. Crizer Gap develop between the two wide and regionally extensive syn- 37.95041° N, 79.69866° W clines to the east and west. And, why is there apparently so little (Weather permitting, this will be our lunch stop.) Keyser and evidently no New Creek between the Clifton Forge At this location, a creek has cut an erosional window and the Healing Springs? through a broad anticline defi ned by Oriskany Sandstone and Coronation, the name of this location, is also a curiosity. There is no such town now shown on maps of this region, and there are no road signs directing travelers in this area to it either. Coronation is shown on the 1928 Geologic Map of Virginia and Figure 8. Outcrops of Silurian and Devonian strata in and around the the accompanying topographic map, and Woodward (1941, fi eld stops by Porters Cave (Stop 2), Coronation (Stop 3), Crizer Gap 1943) referred to this exposure as the Coronation section, but (Stop 4), and Windy Cove Gap (alternate Stop 4A). (A) Fault contact since then the town seems to have disappeared. The apparent between quartzose chert-free limestones of the Little Cove Member of the Devonian Licking Creek Limestone and the underlying but younger fate of Coronation seems to be connected with a tornado: Devonian Needmore Shale along State Highway 42 by Porters Cave (Stop 2). Inset (lower right) shows part of a Phacops sp. found in the Needmore below the fault plane. The Licking Creek is overlain here by May 2, 1929, Virginia’s “Deadliest Tornado Outbreak”: It has been a normal stratigraphic sequence of the Oriskany Sandstone, Needmore said that tornadoes do not occur in mountainous areas. This is false. It Shale, and Millboro Shale, each of which is exposed on the hillside was a warm May day with a cold front moving in from the west. The above this outcrop and above the nearby sinkhole entrance to the cave. fi rst tornado hit Rye Cove in Scott County in extreme southwest Vir- Photo by P. Lucas. (B) Wedge fault in algal laminated limestones of the ginia... . In Bath and Alleghany Counties lies Cowpasture Valley. This Silurian Upper McKenzie Formation in the axis of the anticlinal arch in valley is at an elevation of 1500 feet and lies between two ridges that Crizer Gap (Stop 4). Photo by J. Haynes. (C) Ledges of the Silurian Wil- rise 1000 feet above the valley. A tornado struck around 6 p.m. Prop- liamsport Sandstone on the west limb of the anticline exposed in Crizer erty losses in Coronation and Sitlington were great. At least 10 people Gap (Stop 4); these are underlain by the upper several meters of the were injured, but none were killed. An eyewitness watched the tornado McKenzie Formation, and overlain by a complete exposure of the Silu- form near his home. He described everything within 250–800 yards rian Tonoloway Limestone. Photo by J. Haynes. (D) Arrow points to the of the tornado’s path being destroyed. The postmaster at Covington base of a thick, calcarenaceous, and in places cross-bedded (inset, upper followed the storm 17 miles. He watched it take out 150 apple trees, left) sandstone in the undivided Tonoloway Limestone at Crizer Gap lift the roof off a house, and sweep away a barn. In the barn, a woman (Stop 4). We are tentatively correlating this sandstone with a Tonolo- was milking a cow. She was found some distance from where the barn way sandstone in Highland and northern Bath Counties (Stop 5, Fig. had stood, under its fl oor. One edge of the barn fl oor was resting on 10F), which in that region separates the lower and middle members of a stone wall and she, miraculously, was not injured, nor were the six the Tonoloway, and is known informally as the “upper Breathing Cave cows that had been in the barn. Poultry houses were swept away and sandstone” (Swezey et al., 2013). Photos by J. Haynes. (E) Contact be- chickens were found dead and almost featherless. (Historical Virginia tween the lowest exposed bed of the Clifton Forge Sandstone Member Tornadoes [NOAA]; www.erh.noaa.gov/lwx/Historic_Events/va-tors of the Keyser Formation (arrow #1), which is cross-bedded (inset, upper .html [accessed 10 January 2014]) right), and the uppermost exposed thin-bedded lime mudstones of the Tonoloway Limestone (arrow # 2) in the streambed at Crizer Gap (Stop 4). The Byers Island Member of the Keyser, which 3.4 kilometers away at Coronation (Stop 3) separates the Clifton Forge and the Tonoloway The town of Sitlington mentioned in the above quote like- and is ~15 m thick, is absent here. Photos by J. Haynes. (F) Exposure wise no longer seems to exist. So it is possible these two com- of the Devonian Healing Springs Sandstone along State Highway 39 munities were damaged so signifi cantly by this 1929 tornado in Windy Cove Gap (alternate Stop 4A), which in this area typically exhibits these distinctive wavy laminae and thin beds of calcarenaceous that they were either not resettled, or the inhabitants eventually sandstone, which weather in relief and contrast with the surrounding moved away. sandy limestone. Photo by J. Haynes. Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 23

Figure 8. Downloaded from fieldguides.gsapubs.org on April 8, 2014

24 Haynes et al.

Licking Creek Limestone. An evidently unfaulted sequence is Above the Clifton Forge are just a few meters of nodular Keyser present from the Licking Creek Limestone down section to the limestones that are overlain by another sandstone, which is in the upper McKenzie Formation, which is exposed along the road right stratigraphic position to be the Healing Springs Sandstone in the anticlinal axis (note the wedge fault in the McKenzie based on the section at Windy Cove Gap to the north (Fig. 8F). here, Fig. 8B). The massive rock arch (Fig. 8C) is formed by The New Creek Limestone that elsewhere occurs between the the resistant quartz arenite ledges of the ~15-m-thick William- Keyser and the Healing Springs or the cherty Corriganville Lime- sport Sandstone, which overlies the McKenzie. The William- stone, is very thin or absent. The Healing Springs Sandstone is sport–Tonoloway contact on the west limb is covered at the overlain by the cherty limestones of the Cherry Run Member of pull-off, but it is exposed in the creek bed on the east limb. The the Licking Creek, and above that is the sandy chert-free Little Wills Creek Formation, which elsewhere in the region sepa- Cove Member. The Oriskany is either very thin here or com- rates the Williamsport Sandstone and the Tonoloway Limestone pletely absent, as no exposures can be found in the creek bed (e.g., Stop 6), is absent here. The Tonoloway is recognizable below the fi rst exposures of Needmore and/or Millboro Shale. by its thinly bedded, laminated limestones, a thick sandstone In the interval of the Tonoloway Limestone to Helderberg (Fig. 8D) that we tentatively correlate with a Tonoloway sand- Group part of the sequence at this exposure (i.e., above the Wil- stone in the Burnsville area of Bath and Highland Counties, and liamsport Sandstone and below the Millboro Shale), it is useful by a distinct vuggy bed that is likely related to deposition and to determine the number of discrete sandstone-dominated beds subsequent dissolution of evaporites. that are present in the section. In addition, consider how the Of particular interest is the Tonoloway–Keyser contact, McKenzie Formation and Williamsport Sandstone here differ which is exposed just downstream from the pullout (Fig. 8E). lithologically from the equivalent interval that we saw at Stop The lowest bed of the Clifton Forge Sandstone is just a meter 1 where the “Eagle Rock sandstone” occupied much of this or two up section from the uppermost exposed thin Tonoloway same stratigraphic interval. In addition, note the difference in limestones, and the Byers Island Member is either very thin and small-scale structures here (a single wedge fault in the McKen- not exposed, or it is absent, which is also what is seen in the zie Formation, Fig. 8B) versus the intricate wedge faulting and vicinity of the Bullpasture River Gorge (Stop 5, Figs. 10E, 10G). complexity of structures we saw at Stop 3 (Figs. 6, 8B).

Figure 9. Aerial view of the ledges of Silurian Rose Hill, Keefer, McKenzie, and Williamsport sandstones that form rapids in the gorge of the Bullpasture River, and as identifi ed in the stratigraphic column shown in Figure 2. Images from Google Earth. (A) West limb of the Bullpasture Mountain anticline at the mouth of White Oak Draft. (B) East limb of the Bullpasture Mountain anticline just upriver from Williamsville (Stop 5). Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 25

Figure 10. Outcrops of Rose Hill, Keefer, McKenzie, Williamsport, Tonoloway, and Clifton Forge sandstones (Silurian) in and near the gorge of the Bullpasture River (Stop 5). (A) Nearly horizontal ledges of the Caca- pon sandstone facies of the Rose Hill Forma- tion in the Bullpasture River along the axis of the Bullpasture Mountain anticline. Photo by J. Haynes. (B) The massive ledge of Keefer sandstone on the east limb of the Bullpasture Mountain anticline on the north bank of the Bullpasture River. Photo by J. Haynes. (C) The massive ledge of McKenzie sandstone that forms Beaver Dam Falls in the Bullpas- ture River above Williamsville. Photo by J. Haynes. (D) The Williamsport Sandstone along State Road 678 in the Bullpasture River Gorge. Inset shows bedding planes with nu- merous ripple marks, which are a common sedimentary structure on many bedding sur- faces of the Williamsport in this region. Pho- tos by J. Haynes. (E) Flat-pebble conglomer- ate at the base of the Clifton Forge Sandstone where it unconformably overlies the upper- most lime mudstones of the Tonoloway Lime- stone in the Water Sinks Subway Cave west of the Bullpasture River Gorge (Stop 5). Photo by P. Lucas. (F) Cross-bedded calcarenaceous quartz arenite that separates the lower and middle members of the Tonoloway Limestone along State Road 614 near Burnsville south- west of the Bullpasture River Gorge (Stop 5), and which we correlate with the sandstone in the Tonoloway Limestone at Crizer Gap (Fig. 8D). Photo by J. Haynes. (G) Cross-bedded Clifton Forge Sandstone in the main stream passage of Aqua Cave at the west end of the Bullpasture River Gorge. This is near the northern extent of the Clifton Forge and its transition into the Big Mountain Shale, and most beds within the Clifton Forge in this vi- cinity are sandy limestones rather than sand- stones (Stop 5). Photo by J. Haynes. Downloaded from fieldguides.gsapubs.org on April 8, 2014

26 Haynes et al.

Mileage Directions Like Blowing Cave, the thermal springs here were also (Cumulative) mentioned by Thomas Jefferson in his 1785 publication Notes 0.0 (90.3) Turn around and drive back along VA 632. on the State of Virginia. These springs are some of the several 0.3 (90.6) At the junction with State Highway 42 turn thermal springs that occur along the length of the Warm Springs north (left) and continue to Millboro Springs. anticline, which topographically forms an anticlinal valley as 5.5 (96.1) Millboro Springs, and the junction of State it is fl oored by the soluble Ordovician carbonates from the Highway 42 and State Highway 39. Turn west Beekmantown Formation stratigraphically up section through (left) on State Highway 39. the Nealmont Formation. At ~36 °C (96 °F) the springs here at 0.8 (96.9) T-intersection of State Highway 39 and VA 678. Warm Springs are among the warmest in the valley, exceeded only by the thermal stream in Mud Pot in Alleghany County Directions to Alternate Stop 4A and Alternate Stop 4B that was measured at 37 °C (99 °F) by James Madison Uni- (These are options if the weather limits our time at Crizer Gap.) versity student Meghan Moss in January 2013, and the several thermal springs at Hot Springs, the hottest of which is “The Mileage Directions Boiler” at 41 °C (106 °F). (Cumulative) The source of heat that warms these waters has been a 0.0 (96.9) T-intersection of State Highway 39 and VA 678. source of speculation for decades, with one hypothesis (Denni- 0.3 (97.2) Continue west on State Highway 39 to pullout son and Johnson, 1971) being heat that is radiated by the cool- on north (right) in front of old quarry. ing of the magma chamber(s) associated with the igneous intru- sions we will see on Day 2. The other prevailing hypothesis Alternate Stop 4A. Windy Cove Gap and Blowing Cave (Perry et al., 1979; Severini and Huntley, 1983) is that deeply 38.00477° N, 79.63801° W circulating waters enter the Ordovician sequence at the Browns At this location, the upper Helderberg Group limestones Mountain anticline (Fig. 1) in West Virginia, at elevations are exposed both in the quarry and along the roadcut, as well as in the passages of Blowing Cave (a cave that was described in some detail by Thomas Jefferson in his 1785 publication Notes on the State of Virginia including a comment about the airfl ow Figure 11. Photomicrographs of selected features in units seen on the fi eld trip. All samples have been stained with the standard Dickson at the cave entrance from which the cave takes its name), the method. (A) Pervasive fractures in monocrystalline quartz grains that entrances to which are in the quarry. The oldest unit exposed is persist through the grains but not into the adjacent carbonate cements the very thin New Creek Limestone with its large pink crinoids, and framework grains. Little Cove Member of the Licking Creek which is overlain by the Healing Springs Sandstone and its dis- Limestone, plane-polarized light (PPL), footwall of fault at Corona- tinctive anastomosing sandy laminae and beds (Fig. 8F). Above tion (Stop 3). (B) Syntaxial sparry calcite overgrowths on echinoderm fragments that contrast with quartz overgrowths on adjacent mono- the Healing Springs Sandstone is the Licking Creek Limestone crystalline quartz grains; note the euhedral crystal faces outlined by with the lower cherty Cherry Run Member and the upper sandy, the calcite cement that encloses them on several of the quartz grains. and in places fossiliferous, Little Cove Member. Above this is Microstylolites are outlined by the opaque sediment including pyrite the Oriskany Sandstone, and there are a few scattered hillslope and organic matter concentrated along them. Clifton Forge Sandstone, exposures of the Needmore Shale. PPL, hanging wall of the fault at Coronation (Stop 3). (C) Spiriferid fragments exhibiting typical fabrics of these brachiopod shells, in- If we visit this exposure, things to consider in comparison cluding prismatic and foliated textures, and accompanying grains of with the prior four stops are the increased thickness of the Oris- monocrystalline quartz grains several of which are partly cemented by kany here and the presence of a prominent bed of collophane ferroan baroque dolomite. Oriskany Sandstone, cross-polarized light grains and pebbles in it, the very gradational nature of the Lick- (XPL), west end of the outcrop at Coronation (Stop 3). (D) Curvilinear ing Creek–Oriskany contact, the change in the quartz-calcite biomoldic pores formed as ostracode shells originally deposited with the quartz sand were dissolved at a later time. The opaque mineral ratio in the Healing Springs such that there is much more lime- in these moldic pore spaces is mostly limonite derived from weather- stone in the Healing Springs here, and the extreme thinness (10s ing (oxidation) of pyritized ostracode shells. Williamsport Sandstone, of cm) of the New Creek Limestone here. PPL, from the Bullpasture River Gorge (Stop 5). (E) Ooids of cham- osite and berthierine and a few grains of monocrystalline quartz silt Mileage Directions and fi ne sand, all cemented by ferroan dolomite. Keefer ironstone, (Cumulative) PPL, along State Road 642 between Forks of Waters and Bluegrass (Stop 6). (F) Framework grains of monocrystalline quartz and a couple 0.0 (97.2) Continue west on State Highway 39. of tourmalines, all cemented by hematite. The few grain moldic pore 11.9 (109.1) Warm Springs, and the junction with U.S. 220. spaces may have formed from destruction of less stable framework Drive across U.S. 220 and park in the gravel grains that are present in these sandstones including collophane and parking area. mudrock SRFs (sedimentary rock fragments). Even though the quartz grains are far more abundant than the hematite, these sandstones are a dark maroon because the hematite is a strong pigment. Cacapon sand- Alternate Stop 4B. The Jefferson Pools at Warm Springs stone lithofacies of the Rose Hill Formation, PPL, from slope of Jack 38.05417° N, 79.78051° W Mountain near Monterey, Highland County. Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 27

Figure 11. Downloaded from fieldguides.gsapubs.org on April 8, 2014

28 Haynes et al.

~260 m higher than the fl oor of the Warm Springs Valley, and why these sandstones have no value as an iron ore—a question are then confi ned to the Ordovician stratigraphic interval and asked not infrequently by students and others—and the answer on their journey toward the various thermal resurgences in the is very straightforward: The ratio of hematite to quartz is uneco- Warm Springs Valley, are heated by the geothermal gradient. nomically low. Along VA 678 we will also examine the dark- gray quartzose oolitic grainstones of the lower McKenzie that Mileage Directions overlie the Keefer and are 6–8 m thick (Figs. 12C, 12D). These (Cumulative) form prominent ledges with small solution openings along the (Continuation of planned fi eld trip stops) road, and just a few km south of the road atop Round Hill, there 0.0 (96.9) T-intersection of State Highway 39 and VA 678. are additional small caves developed in these oolitic McKenzie Turn north (right) on VA 678. limestones. The presence of this oolitic grainstone lithofacies 16.8 (113.7) Continue on VA 678 northward to Williamsville here suggests that it may likewise be present in the subsurface and just beyond, to the wide pullout on the north to the west between this outcrop and the core described by (right) just at the lower end of the Bullpasture Smosna (1984) from an area where active hydrocarbon produc- River Gorge. tion in the Silurian occurs. At these exposures, compare and contrast the stratigraphic Stop 5. Bullpasture River Gorge sequence with what we saw at Eagle Rock (Stop 1) and Crizer 38.19913° N, 79.57322° W Gap (Stop 4), and look for the lithologic criteria (some of them At the roadcuts and riverbed exposures we will visit here being quite subtle) that along with stratigraphic position might on the east limb of the Bullpasture Mountain anticline, we will be used to identify and differentiate these sandstones from each see all of the strata that at Eagle Rock (Stop 1) in Botetourt other. It is also worthwhile to look at the thin-bedded shales County were part of the massive “Eagle Rock sandstone” as which occur at the Rose Hill–Keefer contact and which may well as the underlying Rose Hill Formation (Fig. 10A). Here represent a feather edge of the Rochester Shale. at the Bath–Highland County line, there are now three distinct Mileage Directions quartz arenites above the Rose Hill (Figs. 2, 3, 9, 10B, 10C, (Cumulative) 10D), and their aggregate thickness is far less than the “Eagle Rock sandstone” at Stop 1. We will fi rst examine the youngest 0.0 (113.7) Continue north along VA 678. of the three, the Williamsport Sandstone, along VA 678. At this 14.0 (127.7) McDowell, and the junction with U.S. 250. Turn exposure there is not a massive ledge as at Crizer Gap (Stop west (left) onto U.S. 250. 4), but instead there are medium-bedded sandstones, many 9.6 (137.3) Monterey, and the junction with U.S. 220. Turn with ripple-marked bedding planes (Fig. 10D) and some with north (right) onto U.S. 220. ostracode molds (Fig. 11D) that are variably fi lled with limonite 6.5 (143.8) Forks of Waters, and the junction with VA 642. derived from oxidation of pyrite that commonly has replaced Turn west (left) onto VA 642. the ostracode shells in the McKenzie and Williamsport of this 0.7 (144.5) Drive to the pullout at the Keefer Formation. region (Fig. 12B). There is appreciably more mudrock in the Williamsport here than at Crizer Gap (Stop 4) or in the “Eagle Stop 6. Bluegrass and Forks of Waters Rock sandstone” at Eagle Rock (Stop 1). 38.48811° N, 79.52744° W The second sandstone we will see is the unnamed middle At this long series of roadcuts (Fig. 13) on the east limb of sandstone member of the McKenzie (Figs. 2, 3, 9, 10C, 12A) the Wills Mountain anticline, we will see a long section of the that we have identifi ed in this region, thus pushing its known Silurian from the Rose Hill up into the Tonoloway, with very geographic extent well into Highland County several tens of km good although not continuous exposures. Our focus will be on north of its prior known northernmost extent, at Muddy Run the hematitic ferroan dolomite of the Keefer at the west end of in central Bath County (Fig. 3). Here the sandstone is a silica- the exposures, the shaly McKenzie, the quartz arenites of the cemented quartz arenite that forms the prominent ledges at Bea- Williamsport, the thick Wills Creek Formation, and the lime ver Dam Falls in the Bullpasture River as well as one of the mudstones of the lower Tonoloway. The Keefer here consists prominent ledges along VA 678. of some quartz grains, but they are cemented by ferroan dolo- The third sandstone we will see is the Keefer Sandstone, mite (Fig. 11E), not silica. In addition, ooids of chamosite and/ which likewise forms a prominent ledge in the riverbed (Fig. 9) or berthierine (Fig. 11E) and hematite are present, making this and along VA 678. It is also a silica-cemented quartz arenite, exposure a thin but distinctly unambiguous occurrence of the and it is underlain by hematite-cemented maroon quartz are- Clinton iron ore lithofacies (Brett et al., 1998) that elsewhere nites of the Cacapon lithofacies of the Rose Hill Formation in the Appalachians, e.g., upstate New York and Birmingham, (Figs. 9, 10A). The Cacapon sandstones are colored thusly Alabama, has been mined for many decades. from the strong pigmenting action of the hematite, even though The McKenzie at this stop has no sandstone, only shales and the hematite is just a coating on the quartz framework grains shaly limestones, which are not well exposed. The uppermost as seen in thin section (Fig. 11F). The thin section also shows McKenzie that is exposed immediately beneath the lowest quartz Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 29

Figure 12. Photomicrographs of the middle sandstone and oolitic and bioclastic grainstones in the McKenzie Formation from Highland County. All samples have been stained with the standard Dickson method. (A) Bimodal texture of quartz framework grains, with the larger grains being more rounded than the smaller grains. Middle sandstone, plane-polarized light (PPL), Jack Mountain west of McDowell, Highland County. (B) Moderately to extensively pyritized ostracode shells, typical of these bioclastic grainstones. Note the presence of late stage ferroan dolomite cements that reduced much of the remaining interparticle pore spaces that had initially been reduced by non-ferroan calcite. Limestones in the lower McKenzie, PPL, from Lower Gap, Highland County. (C) Quartzose oolitic grainstone in which many ooids have radial textures that prob- ably indicate elevated salinity. A few ooids are superfi cial, some have cores of echinoderm fragments, and others have cores of siltstone SRFs (sedimentary rock fragments) or quartz silt grains. Limestones in the lower McKenzie, PPL, from the gorge of the Bullpasture River along State Road 678 (Stop 5). (D) Quartzose oolitic grainstone showing a variety of cements that reduced interparticle pores during diagenesis. Cementa- tion by carbonate minerals began with an initial non-ferroan and probably meniscus fi brous cement (pink colored) that formed thin coatings on many of the ooids. It was followed by ferroan calcite (lilac colored) that appreciably reduced much of the remaining interparticle porosity, and the fi nal cement is baroque ferroan dolomite (patchy blue color). Many of the quartz grains have some quartz overgrowths on them. Compare this photomicrograph and Figure 12C above with the photomicrographs in fi gures 6C and 8D of Smosna (1984, p. 33, 45), which show texturally similar ooids in the Lockport oolite complex of West Virginia. Although these ooids in the McKenzie of Highland County are probably older than the Lockport oolites, they were evidently deposited in a very similar facies as the oolitic grainstones of the Lockport farther west, implying a westward transgression of the oolite complex with time. Limestones in the lower McKenzie, PPL, gorge of the Bullpasture River along State Road 678 (Stop 5). Downloaded from fieldguides.gsapubs.org on April 8, 2014

30 Haynes et al.

Figure 13. Sketch of the exposure along VA 642 in northern Highland County between Forks of Waters and Bluegrass (Stop 6), showing for- mational boundaries of the stratigraphic units and selected lithologic features in certain beds. Compare the thickness (or thinness) of the Wil- liamsport Sandstone here with the “Eagle Rock sandstone” at Eagle Rock (Stop 1). The Williamsport and thinner sandstones in the Wills Creek Formation are the only quartz arenites at this location in the “Eagle Rock sandstone” stratigraphic interval, as the Keefer and McKenzie have changed facies into other lithologies (modifi ed from fi gure 10 of Diecchio and Dennison, 1996).

arenite of the Williamsport is a green mudrock with abundant domal LLH (laterally linked hemispheroidal) stromatolites, white speckles that give this bed a texture consistent with a gutter casts, ripple marks, lenses of ostracode debris, and pos- paleosol, possibly one in a semi-arid environment in which cali- sibly some hummocky cross-stratifi cation. Collectively, these che or even some evaporite minerals formed small clumps and structures suggest shifting depositional environments from a nodules in the soil. storm-dominated shelf to supratidal fl ats with stromatolitic The Williamsport forms a low hogback in this vicinity microbial mats. The Wills Creek–Tonoloway contact is read- by virtue of the sandstone’s erosional resistance, but the total ily identifi ed by the change from a sequence of shaly weath- thickness of quartz arenites is less than 10 m, vastly dimin- ering strata to a sequence of thin- to medium-bedded bluish- ished from the “Eagle Rock sandstone” at Eagle Rock (Stop gray lime mudstones. 1) and the Williamsport at Crizer Gap (Stop 4). Above the Mileage Directions Williamsport here is the ~60-m-thick Wills Creek Formation, (Cumulative) which we have not seen at other stops because it is thin to absent in areas south of here, and the thinning seems to occur 0.0 (144.5) Turn around and drive east on VA 642. quite abruptly (Fig. 3). It is a heterogeneous unit that consists 7.2 (151.7) Return to Monterey via VA 642 and U.S. 220. of ostracode grainstones and laminated lime mudstones, and 0.2 (151.9) Monterey, and the junction with U.S. 250. Turn siliciclastic mudrocks and thin calcarenaceous quartz arenites west (right) on U.S. 250 and park in front of the with common ostracode shell fragments as framework grains. Highland Inn (68 West Main Street, Monterey, Look for sedimentary structures in the Wills Creek including Virginia). Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 31

END OF DAY 1 ROAD LOG. We propose that the exposed rocks at Trimble Knob rep- resent the root zone of a diatreme (Fig. 15) produced by an Day 2 explosive eruption (Guzmán and Johnson, 2012). In the root zone or base of a diatreme, feeder dikes that channel the magma upward intersect with brecciated blowout zones in the country Mileage Directions rock and with the main cone-shaped structure of the diatreme. (Cumulative) The presence of carbonate in the rock implies an abundance of

0.0 Depart Highland Inn on U.S. 250 heading west. CO2 within the magma, either from a primary source or from

0.1 (0.1) Junction with VA 636 (Spruce Street). Turn assimilation of carbon from regional limestone layers. The H2O south (left) on VA 636. concentration in the Trimble Knob magma is currently uncon- 0.6 (0.7) Park at base of hill on private property. strained, but magma-groundwater interactions were likely responsible for the phreatic eruption that produced the diatreme Stop 7. Trimble Knob Diatreme (Lorenz and Kurszlaukis, 2007). 38.4048° N, 79.5881° W Mileage Directions Trimble Knob is on private property. Request permission (Cumulative) from landowner for access. Trimble Knob is located near the hinge of the Mon- 0.0 (0.7) Turn around and drive north on VA 636. terey syncline, just east of the contact between the Devonian 0.6 (1.3) Monterey, and junction with U.S. 250. Turn Millboro and Needmore Shales and the Devonian Oriskany west (left) on U.S. 250. Formation (Wilkes, 2011). The limbs of the syncline are 4.7 (6.0) Junction with VA 637, Maple Sugar Road. Turn expressed as NE–SW-trending ridges and are easily viewed north (right) on VA 637. from the top of Trimble Knob. Another basaltic plug, Sound- 0.3 (6.3) Park in pullout on the east (right) side of the ing Knob, is visible to the SE within the Bolar anticline. A road just before outcrop. felsic dike containing biotite and feldspar phenocrysts within a white groundmass was recently re-exposed during expan- Stop 8. Hightown Dike sion of the medical center near the base of Trimble Knob. This 38.4295° N, 79.6167° W intimate spatial juxtaposition of felsic and mafi c magmas is a At this location in the Hightown and Blue Grass Valleys common pattern for the Eocene volcanics of Highland County along the Wills Mountain anticline, a cross section through an and strongly suggests a cogenetic relationship between these aphanitic basaltic dike within the Ordovician Beekmantown bimodal magma compositions. Formation is observed in outcrop. The dike is visible to the west Trimble Knob was previously called Pyramid Hill due to within a creek bed, and also extends up the hillside to the east of its angular shape (Darton, 1899; Rader et al., 1986). A tran- the outcrop. The dike has an orientation of N53°W that is nearly sect from west to east reveals a texturally heterogeneous cross perpendicular to the orientation of the Wills Mountain anticline section that likely infl uenced the resulting geomorphology of fold hinge (approximately N30°E) and clearly cross-cuts the the hill. On the western side of the peak of the hill is exposed prevalent bedding orientation. This is in contrast to the sills and aphanitic-porphyritic basalt containing augite and olivine phe- dikes oriented roughly parallel to the fold hinges and bedding nocrysts in a devitrifi ed matrix containing microphenocrysts of that we will observe nearby, including within the Hightown plagioclase. The basalt forms poor- to moderately developed Quarry (Stop 9). Southworth et al. (1993) proposed that dikes columnar joints similar to those seen at Mole Hill (Stop 13). On such as this one oriented perpendicular to the regional strike the east side, exposed just below the summit of Trimble Knob, exploited deep fracture systems within the lower crust during is a ridge of more massive basalt with sheet joints that transects ascent. While such deep fracture systems are likely to exist, we the hillside. Below this dike-like feature on the east side, the hypothesize that formation of a doubly plunging fold, such as texture of the basalt becomes progressively brecciated as one the Wills Mountain anticline, would produce fractures perpen- travels down the eastern slope away from the summit. This dia- dicular to the fold hinge, and this dike could have exploited one treme breccia contains rounded to sub-rounded xenoliths of the of these fractures within the shallow crust. Millboro and/or Needmore Shale, the Oriskany Sandstone, and Miarolitic cavities >2 cm across and smaller amygdules an unidentifi ed limestone possibly originating from the Helder- containing calcite and zeolites (Johnson et al., 1971; Rader berg Group or the Tonoloway Limestone. The clasts become et al., 1986) are observed within the dike. Small ~1 cm xeno- more abundant and their maximum size (up to 0.5 m) increases liths of limestone are also entrained within the basalt. The dike with distance from the summit. Carbonate cement occurs exhibits complex columnar jointing patterns, including a set of within the basaltic groundmass surrounding basaltic scoria and subhorizontal, curving columns that extend across the central the crustal xenoliths, and in thin section, carbonate inclusions portion of the outcrop (Fig. 16A). It has been suggested (John- can be seen within some augite crystals (Fig. 14A). son et al., 1971; Rader et al., 1986) that this dike changes to a Downloaded from fieldguides.gsapubs.org on April 8, 2014

32 Haynes et al.

Figure 14. Photomicrographs of igneous rocks in Highland and Rock- ingham Counties. (A) An Al-augite phenocryst (Cpx) containing unusual carbonate inclusions (Cc) within the Trimble Knob diatreme facies (Stop 7). Cross-polarized light (XPL). (B) Phenocrysts of hornblende (Hbl), biotite (Bt), and plagioclase (Pl) surrounded by a trachytic groundmass of plagioclase and glass, in a sample from the large tra- chydacite dike exposed near the top of the Hightown Quarry (Stop 9). The biotite phenocryst has been shattered into two pieces. The horn- blende phenocryst lacks a dehydration rim, indicating rapid ascent of the magma from an intermediate storage system. XPL, from Ruther- ford and Hill (1993). (C) Sandstone xenolith (Ss), olivine xenocryst (Ol), and Al-augite crystal (Cpx) within the Mole Hill basalt (Stop 13). XPL. A reaction rim surrounds the sandstone xenolith. The olivine xe- nocryst is >1 cm, has a core composition of ~Fo90, contains Cr-rich spinel inclusions, and exhibits kink bands with undulatory extinction. The sandstone xenoliths at Mole Hill are believed to originate from the Tuscarora Formation (Johnson et al., 2013).

more andesitic composition on its eastern end, but given the Along the way to the quarry we will observe low-lying hills adjacent exposure of bimodal compositions elsewhere in the to the west of Blue Grass Valley Road; these hills contain mul- Blue Grass Valley, and the lack of outcrop between the basaltic tiple felsic and mafi c dikes and plugs intruded into the Ordo- and andesitic segments (Rader et al., 1986), we prefer to inter- vician Beekmantown Formation. These dikes, sills, and plugs pret any nearby felsic rocks as separate intrusions. align roughly parallel to the regional strike, and are interpreted to intrude an overturned anticline (Wilkes, 2011). Mileage Directions Within the abandoned quarry, at least three felsic sills and (Cumulative) a basaltic dike intrude the carbonate rocks of the Beekmantown 0.0 (6.3) Turn around and drive south on VA 637. Formation. Bedding is near-vertical and dipping to the NW on 0.3 (6.6) Junction with U.S. 250. Turn west (right) on the west side of the quarry, but beds dip more shallowly to the U.S. 250. SE near the top of the quarry (Wilkes, 2011). A thin basaltic 0.8 (7.4) Junction with VA 640, Blue Grass Valley Road. dike cross-cuts the Beekmantown bedding planes, and is off- Turn north (right) on VA 640. set just above the debris slope. Two felsic dikes run across the 2.2 (9.6) Park in wide pullout on the east (right) side of quarry wall. the road in front of quarry. Exposed on the north end of the quarry is a highly brec- ciated felsic sill and a small 15–20-cm-wide felsic sill, which Stop 9. Hightown Quarry intrude parallel to the near-vertical bedding of the Beekman- 38.4554° N, 79.6075° W town. Fingers of obsidian extend from the brecciated sill, and Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 33 both sills exhibit glassy chill margins at contacts with the coun- ing Devonian Brallier Formation. The brecciated contact zone try rock (Fig. 16B). between the igneous rock and shale is exposed in places along The felsic dikes at this location are trachydacites contain- on the south side of the intrusion and at the top of the small hill. ing phenocrysts of alkali and plagioclase feldspar as well as The trachybasalt has massive, blocky jointing, and is Eocene in biotite and amphibole (Fig. 14). The amphibole phenocrysts age. The rock might be more properly called a gabbro, as it is show little evidence of dehydration reaction rims with the composed of interlocking plagioclase and clinopyroxene crys- magma, suggesting rapid ascent during the last stage of erup- tals. Some augite crystals exhibit carbonate inclusions in thin tion from at least 7 km depth (Rutherford and Hill, 1993; Bulas section, similar to those found at Trimble Knob. and Johnson, 2012). Mileage Directions Mileage Directions (Cumulative) (Cumulative) 0.0 (44.0) Continue north on WV 21 (Sugar Grove Road). 0.0 (9.6) Turn around and drive south on VA 640. 1.1 (45.1) Park near outcrop; only limited turnout space is 2.2 (11.8) Junction with U.S. 250. Turn east (left). available. 5.7 (17.5) Stop in Monterey for restrooms. Continue east on U.S. 250. Stop 12. “Mica Pyroxenite” Basanite Dike Containing 13.9 (31.4) Junction with VA 614, Cowpasture River Road. Crustal Xenoliths near Sugar Grove Turn north (left) on VA 614. 38.4869° N, 79.3249° W 8.5 (39.9) Virginia/West Virginia state line. VA 614 The dike at this location cross-cuts the sandstone and shale becomes WV 21, Sugar Grove Road. Continue of the Devonian Brallier Formation (McDowell et al., 2004) north on WV 21. and has blocky jointing. It has been age dated at 149 Ma using 1.7 (41.6) Park on left side of road just south of the basalt Ar-Ar geochronology (Southworth et al., 1993) and is therefore outcrop. 100 m.y. older than the other igneous rocks observed on this trip. This dike is similar in age to other Late Jurassic dikes exposed in Stop 10. Steeply Dipping Sill or Dike near Turkey Farm Augusta County, Virginia, to the southeast of this location (John- 38.4479° N, 79.3563° W son et al., 1971; Southworth et al., 1993). The origin of these Late At this location, a basaltic sill or dike intrudes the Devo- Jurassic dikes is also puzzling, because they postdate the 200 Ma nian Millboro Shale (Fig. 16C) on the western limb of a com- Central Atlantic magmatic province (McHone, 2000) magma- plex antiformal structure (McDowell et al., 2004). The Millboro tism associated with the rifting of Pangea by 50 m.y. Shale at this location dips 44° SE, toward the road, and it con- tains 60–120° jointing. The igneous body is intruded parallel to bedding as a sill in the cross-sectional exposure, but it dips below the shale to the south, and it also disappears beneath the surface exposure at the top of the hill, thus giving it character- istics of a dike as well. The intrusion exhibits jointing perpen- dicular to the contact with the shale. Miarolitic cavities and amygdules within the basalt are aligned in bands that are roughly parallel to the edges of the intrusion. These cavities contain calcite, aragonite, barite, pyrite, and many zeolites including notronite, analcites, thomp- sonite, mesolite, harmotome, and chabazite (Kearns, 1993). Mileage Directions (Cumulative) 0.0 (41.6) Continue north on WV 21 (Sugar Grove Road). 2.4 (44.0) Park in pullout across from church.

Stop 11. Trachybasalt Plug at Wilfong (St. Michael) Church Figure 15. Schematic diagram with vertical exaggeration of a diatreme (Optional) cone and maar (inset) and the root zone of the diatreme exposed at 38.4743° N, 79.3323° W Trimble Knob (Stop 7). The root zone consists of a feeder dike (black), and diatreme (gray) within country rock (white). Diatreme and feeder A trachybasalt containing clinopyroxene, plagioclase, and dike structure modifi ed from Lorenz and Kurszlaukis (2007). Interpre- olivine is visible at this location. It is intruded near the con- tation of Trimble Knob outcrop from data collected in Guzmán and tact of the Devonian Millboro/Needmore Shale and the overly- Johnson (2012). Downloaded from fieldguides.gsapubs.org on April 8, 2014

34 Haynes et al.

Figure 16. Exposures of igneous rocks in Highland and Pendleton Counties. (A) Subhorizontal curved columnar jointing in the Hightown ba- saltic dike (Stop 8). Rock hammer ~40 cm long is at the center of the image. (B) Contact between a trachydacite sill and the limestone of the Beekmantown Formation in the Hightown Quarry at Stop 9. A chilled margin of black obsidian is present near the contact. (C) Steeply dipping basaltic sill (top) in contact with the Millboro Shale (bottom) near Sugar Grove at Stop 12. A rock hammer ~40 cm long lies on the cleavage surface of the shale at mid- to lower-left of the image. (D) Angular granitic xenolith (light color, center-right of image) within a mica pyroxenite dike just south of Sugar Grove at Stop 12.

In addition to its perplexing age, the mineralogy and appear- Igneous and metamorphic lower crustal xenoliths have ance of this igneous rock is unusual. Garnar (1956) referred to this been found at multiple sites in the region, including the Vul- rock as a “mica pyroxenite” due to its mineralogy, but it would can Quarry in adjacent Augusta County, Virginia; just north of likely be classifi ed as a hydrous basanite using the modern Inter- Staunton; within a dike in Five Springs Cave along Bullpasture national Union of Geological Sciences classifi cation. The rock Mountain south-central Highland County; near Centerville in contains abundant microphenocrysts of mica and plagioclase as northern Augusta County; and at this location (Helsley et al., well as larger mica, almost colorless pyroxene (Garnar, 1956), 2013; Rossi et al., 2013). and amphibole phenocrysts. The cleavage planes of the mica and Mileage Directions plagioclase give the rock a sparkling appearance in hand sample. (Cumulative) Small (>0.5 cm) vesicles and amygdules are common. Also within the dike are large (>1 cm) xenocrysts of black 0.0 (45.1) Continue north on WV 21 (Sugar Grove Road). augite, and felsic lower crustal xenoliths that range from <1 cm 12.5 (57.6) Brandywine, and the junction with U.S. 33. Turn up to several cm in width (Fig. 16D). The xenoliths are either east (right) on U.S. 33. igneous granitoids or granulite-facies gneiss, and they contain 24.4 (82.0) Junction with VA 734 (Bank Church Road). feldspar and quartz and are granular in appearance. Turn south (right) on VA 734. Downloaded from fieldguides.gsapubs.org on April 8, 2014

Active features along a “passive” margin, Highland and Bath Counties, Virginia 35

0.3 (82.3) Junction with VA 733 (Dale Enterprise Road). 0.0 (87.0) Turn right onto South High Street from Memo- Turn east (left) on VA 733. rial Hall parking lot. 0.2 (82.5) Park in front of trail. 0.4 (87.4) Turn left onto Port Republic Road. 0.9 (88.3) Turn right to merge onto I-81 south, and drive Stop 13. Mole Hill Volcanic Neck back to Blacksburg. 38.4481° N, 78.9528° W This is private property. A signed waiver fee from each END OF DAY 2 ROAD LOG. fi eld trip participant is required for access to be granted by the landowner. Mole Hill is an elliptical volcanic neck exposed within the ACKNOWLEDGMENTS Ordovician Beekmantown Formation. Gravity and magnetic data (Elvers et al., 1967) indicate that below the surface the JTH acknowledges support from the U.S. Geological Survey, intrusion dips to the N-NW at 60–65°. The igneous rocks are National Cooperative Geologic Mapping Program (EDMAP) exposed on the top third of the hill in blocky, irregular columns. under USGS award numbers G09AC00122 (to JTH and SJW), The rock at Mole Hill is basaltic in composition (Furman G11AC20278 (to JTH and SJW), and G12AC20312 (to JTH and Gittings, 2003; O’Reilly et al., 2012) and is aphanitic with and Richard J. Diecchio of George Mason University) for a groundmass of plagioclase and olivine crystals. Sandstone bedrock mapping in Highland and Bath Counties. JTH thanks xenoliths and xenocrysts of olivine, Al-augite, and hercynite Philip C. Lucas, past president of the Virginia Speleological spinel can all be found within the fi nely crystalline basalt (Fur- Survey, for many days in the fi eld and especially for pointing man and Gittings, 2003; Beard, 2010; Johnson et al., 2013). out the Needmore Shale along State Highway 42 by Porters Figure 14C shows a sandstone xenolith surrounded by a reac- Cave, for introducing us to the Crizer Gap exposure, and for tion rim within the basalt. The sandstone xenoliths most likely much help with landowner relations that have greatly aided originated from the Silurian Tuscarora Formation (Johnson et this effort. JTH thanks Richard A. Lambert, president of the al., 2013), which is thought to extend under Mole Hill within Virginia Speleological Survey, also for help with landowner and beneath the Alleghenian North Mountain fault system, as relations and for many days in the fi eld in support of this horse blocks such as we saw at Eagle Rock (Stop 1) within the research. EAJ gratefully acknowledges support from Jeffress fault zone as well as more continuous beds beneath the fault Research Grant J-984 and NSF EAR-1249438. The authors plane(s). Like other igneous intrusions in this region, the Mole would like to thank James Madison University undergradu- Hill basalt is postulated to have exploited previously existing ate students M. Bulas, S. Cole, D. Guzman, K. Hazelwood, fracture and fault systems to ascend to the surface. J. Helsley, D. Jones, Z. Kiracofe, T. Kropp, K. McConahy, S. Large (>1 cm) olivine xenocrysts have typical mantle com- O’Reilly, N. Rossi, B. Sacco, T. Stempniewicz, and S. Walker positions (~Fo90) in their cores and rims, with compositions for help with observations and interpretations presented in ranging to Fo78. Kink bands and undulatory extinction associated this fi eld guide. Reviews by C. Bailey and C. Bentley greatly with deformation are commonly observed in the olivine xeno- improved this manuscript. crysts, and Cr-rich spinel inclusions consistent with a mantle ori- gin are likewise found inside some olivine crystals (Beard, 2010; REFERENCES CITED his fi gure 2-8). High-Al augite mega-xenocrysts have composi- Ague, J.J., Eckert, J.O., Jr., Chu, X., Baxter, E.F., and Chamberlain, C.P., 2013, tions consistent with originating from a metasomatized clinopy- Discovery of ultrahigh-temperature metamorphism in the Acadian orogen, roxenite mantle source (Wilshire and Shervais, 1975; Irving, Connecticut, USA: Geology, v. 41, p. 271–274, doi:10.1130/G33752.1. 1980; Beard, 2010). Olivine and augite compositions were used Avary, K.L., and Dennison, J.M., 1980, Back Creek Siltstone Member of Devo- nian Brallier Formation in Virginia and West Virginia: Southeastern Geol- to calculate a depth of origin of ~13 kbar, or 39 km (Sacco et ogy, v. 21, p. 121–153. al., 2011). This is consistent with a preliminary Moho depth of Baedke, S.J., and Silvis, N.V., 2009, Determining the source of heat impacting 40 km determined from the TEENA (Test Experiment for East- thermal springs in the Warm Springs Valley of Bath and Alleghany Coun- ties, Virginia: Geological Society of America Abstracts with Programs, ern North America) seismic array (Benoit and Long, 2009). v. 41, no. 7, p. 448. Bartholomew, M.J., 1987, Structural evolution of the Pulaski thrust system, Mileage Directions southwestern Virginia: Geological Society of America Bulletin, v. 99, (Cumulative) p. 491–510, doi:10.1130/0016-7606(1987)99<491:SEOTPT>2.0.CO;2. Beard, J.S., 2010, Reconnaissance mineralogy of the Eocene Mole Hill dia- 0.0 (82.5) Continue east on VA 733 (Dale Enterprise treme, Rockingham County, Virginia: Jeffersoniana, v. 25, p. 1–16. Road). Bell, S.C., and Smosna, R., 1999, Regional facies analysis and carbonate ramp development in the Tonoloway Limestone (U. Silurian; Central Appala- 0.7 (83.2) Junction with U.S. 33. Turn east (right) onto chians): Southeastern Geology, v. 38, p. 259–278. U.S.-33 East. Benoit, M.H., and Long, M.D., 2009, The TEENA experiment: A pilot project 3.6 (86.8) Turn right onto S Willow Street. to study the structure and dynamics of the eastern US continental margin: American Geophysical Union, Fall Meeting, abstract #U53A-0053. 0.2 (87.0) Turn left into James Madison University Memo- Bick, K.F., 1962, Geology of the Williamsville Quadrangle: Virginia Division rial Hall parking lot for restrooms. of Mineral Resources Report of Investigations 2, 40 p. Downloaded from fieldguides.gsapubs.org on April 8, 2014

36 Haynes et al.

Bick, K.F., 1973, Complexities of overthrust faults in central Virginia: Ameri- West Virginia: Geological Society of America Bulletin, v. 82, p. 501–508, can Journal of Science: Cooper Volume 273-A, p. 343–352. doi:10.1130/0016-7606(1971)82[501:TIAAPN]2.0.CO;2. Bowen, Z.P., 1967, Brachiopoda of the Keyser Limestone (Silurian–Devonian) Dennison, J.M., Bombach, R.K., Dorobek, S.L., Filer, J.K., and Shell, J.A., of Maryland and Adjacent Areas: Geological Society of America Memoir 1992, Silurian and Devonian unconformities in southwestern Virginia, in 102, 103 p. 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