Stop 11: Slice of tightly folded mylonite of central Piedmont suture along Hackers Creek
LOCATION: The outcrops are located on Hackers Creek. Park at the gate just past the S-42-115 bridge over Hacker Branch, 0.9 mile from SC 56. S-42-115 is 4.0 miles north of the Enoree River Bridge on SC 56. Walk northeast approximately 500 m to the first outcrops on the creek. (Fig. 19, Cross Anchor Quadrangle)
DESCRIPTION: This spectacular series of outcrops was first recognized and described by Willis (1984, p. 49) and later by Dennis (1991). Quartzo-feldspathic and darker grey garnetiferous mylonites are complexly interleaved and cut by folded pegmatites at this location. Willis (1984) interpreted these rocks as prograde mylonites, recrystallized and equilibrated at sillimanite zone conditions. They are not annealed because the quartz in them does not make equant, polygonal grains, but instead the quartz forms large, elongate grains with little or no subgrain development. Both quartzofeldspathic and darker grey rocks contain garnet, and the garnets in the darker grey rocks are up to 3 cm in diameter. In the darker grey rocks, the mode of occurrence of the garnets is unusual. The garnets are often observed in the middle of highly altered aggregates of plagioclase +/- K-feldspar (Fig. 20). The feldspars are recognized on the basis of twinning. The feldspars appear to have high relief because of the extensive development of sillimanite +/- white mica along cleavage planes. The feldspars are typically the same size or slightly larger than the garnets they enclose. Some garnets appear to show two phases of growth with irregular fragments of garnet contained within larger well-formed crystals. Garnets are fractured and filled with retrograde chlorite; chlorite also rims grains. Mafic minerals, primarily micas, are much less common away from the garnets. Perhaps the darker grey rocks are paragneisses, it is difficult to say on the basis of their mineralogy or texture.
The orientation of mylonitic foliation here ranges from 125 80 (S, right hand rule) 100 45, 304 60, 320 45; with southwesterly dips common to the southern end of the outcrop and northeasterly dips common to the northern exposures. These are not orientations one typically associates with the central Piedmont suture, and they speak to the folding of the fault (where it has not been reactivated by Alleghanian strike slip motion, e.g. Kings Mountain shear zone) along the 50 km length of the central Piedmont suture between Spartanburg and Clinton in general (Dennis, 1991), and the fact that this is a fault-bounded slice in particular. At the northernmost exposures of the mylonite, the creek takes a hard left (northwest) bend, and highly fractured (brecciated?) rock is observed. Dennis (1991) interpreted that most of the mylonitic rocks along this segment of the central Piedmont suture had been removed by excision at higher structural levels. The generally non-cylindrical folding of mylonitic fabric is observed at outcrop and greater scales. Because these rocks are fault-bounded and folded at several scales it is difficult to make a convincing argument about the original orientation and/or asymmetry of fabric elements. The complex folding and refolding of mylonitic layering at this location defies conventional kinematic analysis ("shear-sense", e.g., Simpson and Schmid, 1983). This aspect of these rocks is frustrating for workers studying the history of ductile motion along the central Piedmont suture.
Mineral separates have been prepared from darker grey gneiss (monazite) and folded pegmatites (zircon) from this site, and U-Pb analyses are forthcoming from the laboratories of J.E. Wright, Rice University.
Nearby is the site of a significant revolutionary war battle. This outcrop is about a mile west-southwest of the Blackstock battlefield, where action took place 20 November 1780. The much better known Battle of Kings Mountain took place six weeks prior on 7 October 1780. The following account is Lumpkin's (1981, p. 268) summary of the battle (which is also treated in an entire chapter in his book):
"Banastre Tarleton, pursuing the retreating Thomas Sumter [retreating from Fishdam Ford on the Broad River], pushed forward with his calvary and mounted infantry, leaving his slower infantry and artillery to follow at their best speed. Sumter meanwhile had determined to make a stand at the farm of William Blackstock overlooking the Tyger River. Tarleton with an inferior force [270] frontally attacked [900-1,000] strongly posted Americans and was beaten back with heavy casualties [92 killed, 100 wounded]. The Americans lost only 3 killed and 4 wounded but among the latter was Thomas Sumter. Colonel John Twiggs of Georgia assumed command of the Americans and retreated that night with his little army across the Tyger leaving the field to Tarleton, who claimed victory."
"The battle of Cowpens, where Tarleton was to experience his greatest defeat, occurred less than two months later on 17 January 1781. The importance of the battle of Blackstock's therefore lay in the fact that the dreaded "Bloody" Tarleton had been fought and checked by American militia, a fact that was to influence markedly the future course of the war in the South. Thomas Sumter survived his serious wound and took the field again in a few months." (p. 115)
Fig. 20. Pavement surface showing large garnets enclosed in K-feldspar +/- plagioclase within the darker grey mylonite of Hackers Creek at Stop 11. Hammer handle is 66 cm.