The 23rd Annual David S. Snipes/Clemson Hydrogeology Symposium Field Trip Guidebook Remapping the Six Mile Quadrangle Tallulah Falls Biotite Gneiss exposed on Mile Creek Shoals, Six Mile, SC Field Trip Leaders: Victoria Sellers and Scott Brame March 24 and 26, 2015
Remapping the Six Mile Quadrangle Victoria Sellers and Scott E. Brame Environmental Engineering and Earth Sciences Clemson University Clemson, SC 29634 Abstract Griffin published the geology of the Six Mile Quadrangle in 1967. Griffin mapped hornblende gneiss, biotite gneiss, and mica schist in the quadrangle with minor quartzite (Figure 1). A metamorphic gradient was thought to exist between the biotite gneiss and the hornblende gneiss. Recent mapping efforts by Garihan (2005) of the Sunset Quadrangle yielded insight to the regional geology and necessitated improved geologic cohesion between the Sunset Quadrangle and the Six Mile Quadrangle. Particular focus has been on defining the Seneca Fault between the underlying Walhalla nappe and the overlying Six Mile Thrust Sheet, as the Seneca Fault was not named until 1974. Table Rock gneiss and Poor Mountain amphibolite of the kyanite-grade Walhalla nappe and Tallulah Falls gneiss and schist formations of the sillimanite-grade Six Mile Thrust Sheet have been outlined in the northern margin of the Six Mile Quadrangle bordering the southern boundary of the Sunset Quadrangle by Sellers and Brame. The northern portion of the Six Mile Quadrangle is dominated by hornblende gneisses and amphibolites with interspersed Table Rock gneiss. Table Rock gneiss is a fine-grained, primarily leucocratic gneiss with minor biotite and hornblende. The Poor Mountain amphibolite is comprised of hornblende, quartz, and alkali feldspar. Hanging wall formations are evident in the central portion of the map due to synclinal folding, with klippen surrounded by footwall formations in the east. The Tallulah Falls gneiss is composed of biotite and muscovite with porphyroblastic feldspar and quartz. The Tallulah Falls schist ranges from garnet-sillimanitemuscovite-schist to mica-amphibole-schist. Quartzite and pegmatite occur in both the hanging wall and footwall in the northern portion of the Six Mile Quadrangle. 1
Gradational metamorphic contact Area of significant biotite gneiss Synclinal folds Figure 1: Geology of the Six Mile Quadrangle, South Carolina by Villard Griffin. Griffin defined three units: hornblende gneiss, biotite gneiss, and muscovite schist. 2
Introduction-Griffin and Garihan s efforts The Six Mile Quadrangle contains a portion of the Inner Piedmont belt of the southern crystalline Appalachians. The Inner Piedmont is considered to be one of the largest areas of highgrade metamorphic rocks in the Appalachians (Griffin, 1971). The Inner Piedmont core consists of sillimanite-grade (upper-amphibolite facies) rocks that are thought to have been emplaced during a regional metamorphic event around 356 ma (Nelson, 1988). Recumbent and reclined folds of all scales occur within the area, with more recumbent folds occurring in northwestern South Carolina. Griffin s (1967) geologic map of the Six Mile Quadrangle was a vital initial step into understanding the lithological and structural features in the region. Griffin s lithologic description of Walhalla nappe rocks includes amphibolite or amphibolite gneiss with smaller amounts of granitoid gneiss. Griffin included a variety of rocks in the Six Mile Thrust Sheet, including: biotite gneiss, mica schist, granitoid gneiss, and amphibolite. He noted that the rock types are typically granoblastic and are moderate to coarse in grain size (1974). However, mapping efforts by Garihan (2005) have highlighted the need for clarification and improved mapping of the quadrangle. In Garihan s map, the southern extent of the Sunset Quadrangle has Middle Ordovician Table Rock gneiss (Trg, purple unit) and Poor Mountain amphibolite (Pma, green), with Tallulah Falls gneiss (TF, orange), and Tallulah Falls mica schist (Tfa, tan) and minor Quaternary alluvium (Qal, yellow). The leucocratic, biotite Table Rock gneiss dominates the area (Figure 1). The primary structural feature in Garihan s map is the appearance of Six Mile Thrust Sheet units Tallulah Falls gneiss and Tallulah Falls mica schist in the central part of the map (Figure 3). The western extent of these units is considered to be the Seneca Fault, separating the underlying Walhalla nappe from the overlying Six Mile Thrust Sheet. A major inconsistency between the mapping efforts is the later discovery of the Seneca Fault. The Six Mile thrust sheet was initially thought to have rested on the underlying Walhalla Nappe as tectonic slide (Nelson, 1988). As the Seneca Fault separating the nappes was not named and described until 1991 by Horton and McConnell, Griffin s map lacks thrust faults, fensters, and klippen that are apparent in the Sunset Quadrangle mapped by Garihan (Figure 2). 3
Seneca Fault Table Rock gneiss Tallulah Falls schist Poor Mountain amphibolite Tallulah Falls gneiss Figure 2. The boundary between the Sunset Quadrangle and the Six Mile Quadrangle with units outlined. 4
Recent Work Sellers and Brame began delineating the northern portion of the Six Mile Quadrangle in July 2014. A goal of the mapping effort was to differentiate the biotite gneisses of the quadrangle. From Garihan s map and Sellers and Brame s mapping efforts, the Table Rock gneiss contains biotite (5-10%) and is leucocratic, whereas the Tallulah Falls gneiss is darker, and contains both biotite and muscovite in larger quantities (30-80%) (Table 1). This has helped make distinctions in the gneisses that Griffin did not surmise, making the hanging wall and footwall easier to delineate. Griffin defined significant amounts of hornblende gneiss in the western portion of the Six Mile Quadrangle at a lower metamorphic grade that he separated from the biotite gneiss, muscovite biotite schist, and quartzites found in the central and eastern portions of the quadrangle (1967). The mica schist that Griffin mapped in the Six Mile Quadrangle is also more complex according to Garihan and Sellers and Brame. Variations include mica amphibole, garnet-sillimanite-mica-, garnet-mica, and muscovite schist. The updated map (Figure 4) by Sellers and Brame shows significant discontinuity of the hanging wall units in the northern part of the quadrangle. Structural data indicate predominately northeast striking foliation with southeast dip, with minor northwest dips. Figure 3. Cross sectional view of the structures mapped by Garihan (2005). 5
Table 1: Dominant lithologic characteristics of major rock types found in the Six Mile Quadrangle. Representative Lithology Table Rock gneiss: leucocratic, biotite (5-10%) orthogneiss with minimal foliation. Poor Mountain Amphibolite: unit ranging from hornblende gneisses with <50% hornblende to amphibolites (see picture left) with >50% hornblende. Other minerals include plagioclase (10-50%) and quartz (10-40%). Tallulah Falls schist: variable unit with dominant minerals muscovite, biotite, sillimanite, and garnet with minor quartz and hornblende. Commonly forms crenulation with axial planar minerals aligning to create lineations. Tallulah Falls gneiss: paragneiss containing muscovite and biotite with plagioclase and quartz. Plagioclase and quartz often form porphyroblasts (0.1-2cm) in deformation-related areas. 6
Seneca Fault Tallulah Falls schist Tallulah Falls gneiss Poor Mountain amphibolite Figure 3. Preliminary interpretation of lithology and associated structures by Sellers and Brame showing edge matching with Sunset Quadrangle geology. References J.M. Garihan., 2005, Geologic Map of the Sunset 7.5-minute quadrangle, Pickens County, South Carolina Griffin, V.S., 1974, Analysis of the Piedmont in northwest South Carolina: Geological Society of America Bulletin, v. 85, p. 1123-1138. Griffin, V.S., 1971, The Inner Piedmont belt of the southern crystalline Appalachians: Geological Society of America Bulletin, v. 82, p. 1885-1898. Nelson, A.E., 1988, Stacked crystalline thrust sheets and episodes of regional metamorphism in northeastern Georgia and northwestern South Carolina; a reinterpretation: USGS, Report 1822. 7
Field Trip Stops 8
The field trip begins in the Sunset Quadrangle mapped by Garihan. You will be seeing an east to west transect of the footwall, hanging wall, then footwall mapping units. See Figure 3 for structural clarification. Stop 1. Massengill Memorial Rd. Looking towards the south, there is a saprolite exposure displaying interfoliated amphibolite and mica amphibole schist. Garihan (2005) interpreted this transition as a visible exposure of the Seneca Fault. The upper part of the exposure (Figure 5) reveals a nearly flat lying weathered biotite gneiss belonging to the Tallulah Falls Formation (hanging wall) thrust over weathered footwall rocks dipping to the east. Garihan has mapped this as a klippe of Tallulah Falls Formation (Six Mile Trust sheet) surrounded by Walhalla Nappe rocks. Figure 4. Inclined foliation of footwall intersecting overlying subhorizontal foliation. Orientation of underlying foliation: N32W/36NE. Exposed machete is approximately 1ft in length. Yellow line denotes thrust fault associated with Seneca fault. 9
Stop 2. Intersection of Windmont & Skyland Rds. On the north side of the road in the ditch and embankment is an exposure of Tallulah Falls mica amphibole schist associated with the hanging wall. Stop 3. AR Lewis Elementary School North and south ditch exposures of porphyroblastic Tallulah Falls biotite gneiss. Stop 4. Martin Grove Church Outcrop to the south of the roads contains a large exposure of leucocratic Table Rock gneiss. Stop 5. Little Eastatoee Rd. The Nine Times ultramafic body is found in this location. This ultramafic body is a greenishgold colored schist and contains the minerals clinoamphibole and chlorite (Gober, 2005). Stop 6. Lunch Crowe Creek Boat Access overlooking Lake Keowee. After lunch, we will go to the Six Mile Quadrangle and perform an east to west transect of hanging and footwall formations. 10
Stop 8. Concord Church Rd. Vertically oriented mica schist with locally occurring crenulated cleavage (Figure 6). Amphibolite pods are emplaced in the schist body (Figure 7). Figure 6. Crenulated schist boulder with axial planar cleavage forming lineations. Yellow line denotes crenulation. 11
Figure 7. Amphibolite pods emplaced in red colored schist. Stop 9. Brandy Ln. Outcrop of locally crenulated, mica-garnet-sillimanite-schist on north side of road. The red colored schist is rich with garnets that have been differentially weathered to iron oxides. A photomicrograph (Figure 8) of the schist reveals that the interior of the garnets are preserved while the rim and fractures have been replaced. 12
Iron oxide Garnet Sillimanite Figure 8. Photomicrograph showing garnet with iron oxide halo. Stop 10. Brandy Ln. Outcrop of porphyroblastic hornblende gneiss on south side of road with feldspar and quartz porphyroblasts ranging from 0.1-1cm (Figure 9). Lithologic change from the last stop is interpreted as crossing the Seneca Fault from the hanging wall to footwall. Figure 9. Hornblende gneiss with porphyroblastic texture. 13
Stop 11. Mountain View Church Small outcrop of garnet-mica-schist on west side of road opposite church parking lot. Beware of vicious dog protecting sheep in adjacent field (Figure 10).. Figure 10. Victoria taming the beast. Stop 12. Ferguson Rd. Outcrop of leucocratic Table Rock Gneiss on the south side of road (Figure 11). The drastic change in rock type from last stop is interpreted to be the Seneca Fault. Figure 11. Outcrop of Table Rock gneiss. 14
Stop 13. Highway 133 (optional if time permitting) Outcrop of green-colored schist of unknown lithology with adjacent amphibolite and localized folding. Garihan (personal communication, 2015) stated that color is potentially related to shearing. Stop 14. Mile Creek Shoals Creek exposures of Tallulah Falls biotite gneiss (Figure 12). Figure 12. Tallulah Falls gneiss exposed along Mile Creek. 15
Acknowledgement This work and field trip would not have been possible without the tremendous contribution of Jack Garihan. His ongoing quest to map geologic quads (14 and counting) in the upstate is unrivaled. While that is a remarkable legacy, he will be remembered mainly by these authors through his willingness to mentor (Figure 13) and translate complex geologic concepts into ordinary language. Jack has left us much richer both scientifically and personally. Figure 13. Jack and Victoria conducting an in-field mapping session. 16