Geology of Dupont State Forest and the Standingstone Mountain Quadrangle, SC and NC

The 25th Annual David S. Snipes/Clemson Hydrogeology Symposium Field Trip Guidebook Geology of Dupont State Forest and the Standingstone Mountain Quadrangle, SC and NC Triple Falls, Dupont State Forest, NC Field Trip Leaders: Jack Garihan and Scott Brame April 5 and 7, 2017 Overview of the Geology of Standingstone Mountain Quadrangle, South Carolina and North Carolina John M. Garihan, Earth and Environmental Sciences Department, Furman University, Greenville, SC 20613 Introduction Standingstone Mountain 7.5-minute quadrangle (SM) is situated within the rugged part of the western Inner Piedmont. Its southern portion lies across the North Carolina-South Carolina state line at the Eastern Continental Divide (~3000 foot elevation). The escarpment of the Blue Ridge Front winds its way across the southern margin of SM, with lines of impressive exfoliation domes (balds) formed on Table Rock gneiss. Balds that are developed on more feldspathic Henderson Gneiss are less common. The highest elevations in SM are 3600 - 3700 feet; local relief across this rugged landscape is 1400 - 2000 feet. Jones Gap State Park in South Carolina and the Dupont State Forest in North Carolina provide public access to the scenic landscape of the region via extensive hiking and horseback riding trails. Five Paleozoic fault-bound crystalline sheets occur in the Inner Piedmont in Standingstone Mountain quadrangle (Figure 1). Stacked structurally lowest to highest, northwest to southeast, they involve: 1) Henderson Gneiss, Sheet I; 2) Chauga River Formation (muscovite-biotite-feldspar- quartz gneiss and schist, with a siltstone protolith), Sheet II; 3) Henderson Gneiss, sheet III; 4) Table Rock gneiss of the Walhalla nappe, Sheet IV; and 5) Poor Mountain and Tallulah Falls Formations of the Six Mile thrust sheet, Sheet V. Ductile thrusts lie at the base of the Six Mile sheet (Sheet V), the Seneca fault (Garihan, 2001), and at the base of Sheet II, the Eastatoee fault (Garihan and Clendenin, 2007). Good exposures of both regional thrust faults will be seen on this field trip. The Slicking Gap normal fault lies between Sheets III and IV. Geology of the area was mapped at 1:24,000 scale by Garihan and Ranson (2007) and described by Garihan (2002). Sheet I: Henderson Gneiss In northwest SM, muscovite-biotite-two feldspar augen Henderson Gneiss underlies a belt 1- 2 km wide along the Crab Creek drainage (Figures 1, 2). Leucocratic, poorly foliated, granitoid gneiss and pegmatite are subordinate lithologies of sheet I. Typical Henderson gneiss is saprolitic to unweathered, fine- to medium-crystalline, and well foliated. It is moderately well layered compositionally (alternating more mafic and more felsic layers) on a scale of millimeters to tens of centimeters. Rounded to ovoid to lenticular microcline augen or porphyroclasts average 0.5 – 1 cm in long dimension; they may be sparsely distributed across an exposure. Henderson Gneiss is variably mylonitic. Many pink microcline augen in hand specimen have thin, gray margins of 1 myrmekite ± fine-crystalline, recrystallized microcline. Individual feldspar augen margins in places are asymmetric (winged) as result of ductile shearing. The base of Sheet I lies outside SM quadrangle, but is inferred to be bounded below by the easternmost fault in the regional Brevard fault zone (Hatcher, 2001). We will see typical Henderson augen Gneiss of Sheet I at the Dismal Creek stop. Figure 1. Generalized geologic map of Standingstone Mountain quadrangle, NC-SC. Sheets I-V are labeled; Sheet IV with stippled pattern is Table Rock gneiss. HGn- Henderson Gneiss, mbgn- Chauga River Formation, TRg- Table Rock gneiss, TRag- Table Rock augen gneiss, P- Poor Mountain Formation, T- Tallulah Falls Formation, sz- shear zone, diamond- sphalerite-chalcopyrite- galena prospect. Bar and ball symbol- Slicking Gap fault. A-A’, B-B’- cross sections shown in Garihan (2002). 2 Figure 2. Selected features of Standingstone Mountain quadrangle, NC-SC, with index to quadrangles. BF-Bridal Veil Falls, DSF- Dupont State Forest, CG-Camp Greenville, HF-High Falls, HQ-Holmes Educational Forest Headquarters, JGSP-Jones Gap State Park, MV-Mulligans View, SC-Symmes Chapel, TF-Triple Falls. 3 Sheet II: Chauga River Formation Ductile deformed muscovite-biotite-feldspar-quartz gneiss and schist of Sheet II underlies a narrow, arcuate zone (0.1-0.8 km wide) striking N55 ̊-75 ̊E. Sheet II is bound on the north edge by the Eastatoee thrust (present at the Dismal Creek stop) and on the south by an unnamed Paleozoic thrust that formed after the Eastatoee fault (cross section, Figure 3). It is conjectured that the unnamed Paleozoic thrust may sole into/merge with the deeper Eastatoee fault at depth. Chauga River Formation rocks form a prominent resistant ridge (200-500 feet relief) which we will see from the Pinnacle Peak overlook (Figure 4). Impressive waterfalls are produced on Chauga River rocks along north-flowing tributaries of west-flowing Crab Creek (Figure 2). SV SII SI SIII SIV Figure 3. Cross section across part of Standingstone Mountain quadrangle, coinciding with the northwest half of the A-A’ cross section line shown on Figure 1. Sheets are labeled SI, SII, SIII, SIV, and SV. Rock units: Henderson Gneiss- forest green, Chauga River Formation- red, Table Rock gneiss- pink, Six Mile thrust sheet rocks- light green. Eastatoee fault separates SI and SII. Paleozoic thrust (post-Eastatoee fault age) separates SII and SIII. Slicking Gap normal fault separates SIII and SIV. Seneca fault separates SIV and SV. Cross section modified from Garihan and Ranson, 2007. The Chauga River Formation is a uniformly hard, dark, fine-crystalline, poorly to moderately layered muscovite-biotite-feldspar-quartz gneiss. Its variable texture suggests an original metasiltstone protolith; it is locally ‘pebbly’ in appearance in coarser layers. At the Dismal Creek exposures, thin light gray, siltier layers outline isoclinal folds with axial planar schistosity. Locally, shape-modified K-feldspar porphyroclasts (generally < 0.5-1 cm in long dimension, parallel to foliation) are rounded or strongly tapered by ductile deformation. Shear sense indicators are consistent with west- or northwest-directed transport. Coarser muscovite ductile fish (‘buttons’) with finer biotite inclusions are visible on schistosity surfaces, particularly at exposures along NC 1257 at Shoal Creek (Figure 2, which we will not visit). 4 Figure 4. View to the northwest from Pinnacle Peak. Slopes on left side underlain by Table Rock gneiss. Chauga River Formation ridge is indicated by arrows. Valleys on either side of the ridge are underlain by Henderson Gneiss. Sheet III: Henderson Gneiss Henderson Gneiss of Sheet III underlies a zone 0.7-3 km wide (Figure 1). The lithologies are the same as the Henderson Gneiss of thrust sheet I. Foliation attitudes within Sheet III (N32 ̊E, 13 ̊SE; n=60) are essentially the same as foliation attitudes within Sheet I (N37 ̊E, 19 ̊SE; n=34). Impressive, well-jointed exposures of Henderson Gneiss occur within Dupont State Forest at Triple Falls (which we will visit) and High Falls along the Little River (Figure 2, labeled TF and HF). The foliation attitudes in Henderson Gneiss are truncated by a fault at the top of Sheet III. The nature of this contact between Henderson Gneiss (below) and Table Rock gneiss (above) is equivocal because the southeast-dipping contact is not exposed. Nonetheless, mutual intrusive relationships are absent. The contact is interpreted to be the northeastern continuation of the Slicking Gap normal fault found 5 in Table Rock quadrangle (Garihan and Ranson, 2003) because older orthogneisses occur in the footwall and younger orthogneisses in the hanging wall. This normal fault is shown with the bar and ball symbol on Figure 1. In a few places the Slicking Gap normal fault is offset by small faults of north-northwest strike (local offsets of 0.5-1 km). Sheet IV: Table Rock gneiss Sheet IV (Figure 2) contains resistant (quartzose) Table Rock Plutonic Suite biotite gneiss and, particularly in southern SM, lesser biotite-feldspar augen gneiss (labeled TRag on Figure 1). The latter augen gneiss, which is not Henderson Gneiss, is middle Ordovician, U-Pb dated at 450 Ma (Ranson and others, 1999). Locally interlayered amphibolite and minor schist are present in Sheet IV. Metagabbro and ultramafic schists, found commonly in adjacent Table Rock quadrangle to the southwest, are notably absent along strike in SM. At the overlook ridge (Blue Moon at Pinnacle Peak community, 'The Pinnacle' on Figure 5) we will see rounded, exfoliated exposures and landscape blocks of typical Table Rock gneiss, some with chevron and isoclinal folds. From the overlook to the northwest we can see the narrow resistant ridge of Sheet II underlain by Chauga River Formation rocks (Figure 4). Sheet V (Six Mile) Poor Mountain and Tallulah Falls Formations Rocks of the Poor Mountain and Tallulah Falls formations lie at the higher elevations of SM (2200-3600 feet). They comprise discontinuous klippen of the Six Mile thrust sheet, with the Seneca fault at its base. We will take a short walking traverse across the margin of a klippe of Six Mile rocks, tracing the Seneca fault (Figure 5). The irregular regional trace of the Seneca fault in central and south SM is due to a combination of factors: 1) a shallow regional southeastward dip; 2) the rugged relief in the region, prompted by Cenozoic uplift and deep erosion; and 3) the complex geometry of the fault surface. The latter is produced by several phases of folding younger than and hence deforming the Seneca fault surface. Post-Seneca, southwest-divergent, overturned antiformal folding of the Seneca surface has produced several windows exposing footwall Table Rock gneiss at structural culmination positions. That is, at these locations the Seneca thrust has been arched up and subsequently eroded through, exposing underlying footwall rocks at the surface. In Figure 1, the culminations are shown as dotted ovals and circles surrounded by hanging wall Six Mile rocks.

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