The 24th Annual David S. Snipes/Clemson Hydrogeology Symposium Field Trip Guidebook Geology and Tectonic Framework of the Keowee- Toxaway Region, Northwestern South Carolina and Engineering Geology of the Bad Creek Pumped Storage Project, Northwestern South Carolina Malcolm F. Schaeffer HDR Engineering, Inc. 440 South Church Street Suite 900 Charlotte, NC 28202-2075 Field Trip Leaders: Malcolm Schaeffer and Scott Brame March 30, April 1, and April 28, 2016 GEOLOGY AND TECTONIC FRAMEWORK OF THE KEOWEE-TOXAWAY REGION, NORTHWESTERN SOUTH CAROLINA Malcolm F. Schaeffer, HDR Engineering, Inc., 440 South Church Street, Suite 900, Charlotte, North Carolina 28202-2075, [email protected] INTRODUCTION A significant amount of geologic work has been completed in the last fifteen years in the Keowee-Toxaway Region of northwestern South Carolina and vicinity (Garihan 2001, 2005, 2008; Garihan et al. 2005; Hatcher 2000, 2001a, 2001b; Hatcher and Lin 2001; Hatcher et al. 2001; Garihan and Ranson 2003; Merschat et al. 2003; Merschat and Hatcher 2007; Clendenin and Garihan 2004, 2007a, 2007b, 2008; Garihan and Clendenin 2007; Merschat et al. 2010) that builds on and extends the ground-breaking work by Villard Griffin (1967, 1969, 1971, 1973, 1974a, 1974b, 1975, 1993) and Bob Hatcher (1969, 1970, 1971, 1972, 1973, 1977, 1978a, 1978b, 1984; Hatcher et al. 1973; Hatcher and Acker 1984) and work by others in the area and region (Brown and Cazeau 1964; Cazeau 1967; Acker and Hatcher 1970; Lemmon 1973, 1981; Lemmon and Dunn 1973a, 1973b; Odom and Fullagar 1973; Roper and Justus 1973; Bond 1974; Horton 1974; Odom and Russell 1975; Clark et al. 1978; Fullagar et al. 1979; Harper and Fullagar 1981; Bobyarchick 1983, 1984; Edelman et al. 1987; Bobyarchick et al. 1988; Horton and McConnell 1991; Nelson et al. 1998)1. Hatcher (1993, 2002), Garihan et al. (2001), and Garihan and Ranson (2012), in field guides for the Carolina Geological Society, summarized the state of knowledge and advances in the understanding of the geology/tectonics of the region, in particular the Inner Piedmont at the time of those trips.2 Recent David S. Snipes/Clemson Hydrogeology Symposium field trips have examined the geology of the region in the light of the recent geologic work and interpretations (Goforth et al. 2012; Clendenin and Garihan 2013; Goretoy and Brame 2014; Sellers and Brame 2015). In the last twenty-five years major changes have occurred in geologic concepts and methods of analysis resulting in new theories of the tectonic development of the Appalachians. These geologic concepts and methods of analysis are continuously being updated as additional data are collected and different and sometimes contradictory interpretations are developed. A number of tectonic models of the southern and central Appalachians have been developed based on new data, including but not limited to state-of-the-art sensitive high-resolution zircon age-dating (SHRIMP), laser ablation inductively coupled plasma mass spectrometer (LAICP-MSI), new isotope data (Pb isotope and Sm-Nd), chemical analysis of metavolcanic and plutonic rocks, new detailed geologic mapping, an improved understanding of tectonic links between internal and external parts of the Appalachians, and modern geophysical data, primarily aeromagnetic and gravity (Hatcher et al. 2007) Recent models include those by Rankin et al. (1989), Horton et al (1989), Hatcher et al. (1990), Hibbard et al. (2002, 2006), Hatcher et al. (2007) and Hatcher (2010)3. This review seeks to summarize and synthesize the work in the Keowee-Toxaway Region in the context of a 1 This is by no stretch of the imagination all the work that has been done in the area; they are mainly the ones I am familiar with and have referred to in the past when working in and writing about the Bad Creek project and region. 2 A number of the Carolina Geological Society Annual Field Trip Guides are available on the Society’s website: http://carolinageologicalsociety.org/CGS/Guidebooks.html 3 Again, not an exhaustive list of models developed over the last twenty-five years. 1 Southern and Central Appalachians tectonic framework model developed by Hatcher et al. (2007) that incorporates the advances made by geologic investigations and increased knowledge of the region over the last twenty-five years. GEOLOGY OF THE KEOWEE-TOXAWAY REGION Introduction The crystalline rocks of the southern Appalachians occur in northeast-trending parallel geologic terranes. The Keowee-Toxaway Region is within the Tugaloo terrane that includes rocks of the eastern Blue Ridge northwest of the Brevard zone as well as the rocks of the western Inner Piedmont southeast of the Brevard zone (Figure 1). The Tugaloo terrane maintains the same stratigraphic sequence (Tallulah Falls-Ashe Formation) across the Brevard zone (Figure 2; Hatcher et al. 2007) and contains detrital zircons of Laurentian provenance (Bream et al. 2004; Bream 2003, in Hatcher et al. 2007). The late Cambrian to early Ordovician Chauga River Formation and middle Ordovician Poor Mountain Formation sedimentary and volcanic rocks overlie the Tallulah Falls-Ashe sequence and are intruded by various granitoid rocks (Henderson Gneiss, Table Rock suite, and others) in the Inner Piedmont south of the Brevard zone (Hatcher 2002; Bream 2003; in Hatcher et al. 2007). A compiled geologic map of the Keowee-Toxaway Region is presented in Figures 3a, 3b, and 3c. Blue Ridge - Introduction The Blue Ridge is a mountainous zone that extends from southern Pennsylvania to central Alabama and varies in width from less than 24 km to about 100 km. Its greatest width is in the Tennessee/Carolinas/North Georgia segment. It is a complex crystalline terrane consisting of Precambrian gneissic basement core structurally overlain by a vast thickness of metasedimentary and metavolcanic rocks of Precambrian to lower Paleozoic age (Hatcher 1978a, 1978b). Numerous igneous bodies of mafic to felsic composition intrude into the basement core and the overlying metasedimentary and metavolcanic sequence. The structure of the Blue Ridge is controlled by major thrust faults, associated complex polyphase folding, and later brittle faulting (Hatcher 1978a). The southern Blue Ridge is divided into three belts: 1) a western belt of imbricate thrust sheets involving upper Precambrian and lower Paleozoic rock and some basement rocks, 2) a central belt containing most of the basement rocks exposed in the Blue Ridge terrane along with higher grade upper Precambrian and possible lower Paleozoic metasedimentary rocks, and 3) an eastern belt of high grade early Paleozoic metasedimentary and metavolcanic rocks (Hatcher 1978a, 1978b; Hatcher et al. 2007). The eastern belt of the southern Blue Ridge comprises those portions of the Tugaloo terrane that occur northwest of the Brevard zone (Figure 1). Blue Ridge – Keowee-Toxaway Region The principal rock units of the western Tugaloo terrane (eastern Blue Ridge belt) within the Keowee-Toxaway Region are the Tallulah Falls Formation (Cambrian) and the underlying Toxaway Gneiss (Hatcher 1977; Figure 2) which underlie the upper portion of Lake Jocassee northwest of the Rosman fault and Brevard zone (Figure 3c). Rocks of the Tallulah Falls 2 Keowee-Toxaway Region Figure 1: Tectonostratigraphic terrane map of the southern and central Appalachians (from Hatcher et al. 2007) and approximate location of the Keowee-Toxaway Region (Figures 3a, 3b, and 3c). 3 Formation consists of metagraywacke, pelitic schist, mafic volcanic rocks, quartzite and are migmatitic. They are intruded by Paleozoic granitoid rocks and overlie 1,000 to 1,200 million years ago (Ma) Grenville basement in the Toxaway Dome (Hatcher 1977) and Paleozoic oceanic crust (Figure 2; Hatcher 2002). The formation consists of four members: 1) the quartzite-schist member, 2) the lower graywacke-schist-amphibolite member, 3) the garnet-aluminous schist member, and 4) the upper graywacke-schist member (Hatcher 1977). The lower member contains quartzite with interlayered schist. The lower graywacke-schist-amphibolite member contains metagraywacke (quartz-biotite-plagioclase-muscovite gneiss), amphibolite, muscovite schist, and biotite schist. Layers of granitic gneiss and pegmatites also occur in the lower member. Overlying this member is the garnet-aluminous schist member. It consists of muscovite-garnet-kyanite schist with interlayered amphibolite, muscovite schist, metagraywacke, granitic gneiss, and pegmatites. It is generally easily recognizable by abundant garnet and kyanite. The upper graywacke-schist member contains metagraywacke, muscovite schist, muscovite-biotite schist, and minor amounts of amphibolite, granitic gneiss, and pegmatites. In some areas surrounding the Toxaway Dome, the lower members are absent, suggesting either non-deposition or faulting out of the lower members (Hatcher 1977; Merschat et al. 2003; Schaeffer 2007; Clendenin and Garihan 2007a). Mylonitic fabrics, up to several thousands meters thick, in the Talluluh Falls rocks northwest of the Brevard zone are attributed to deformation along the zone (Clendenin and Garihan 2008, Cattanach et al. 2012). Although recent mapping (Merschat et al. 2003; Clendenin and Garihan 2007a; Cattanach et al. 2012) of the Tallulah Falls Formation in the area does not distinguish the members described by Hatcher (1977), they describe similar lithologic sequences. Figure 2: Stratigraphic relationships in the Tugaloo terrane (from Hatcher 2002). The Toxaway Gneiss, part of the Precambrian basement of the eastern Blue Ridge,