Download Guidebook as .pdf (29.1 Mb) - Carolina Geological Society

Allen J. Dennis and Others
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 (“shearsense”,
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)
STOP 12: MAFIC METAVOLCANIC ROCKS
EXPOSED ALONG DUTCHMANS CREEK
LOCATION
The outcrops are located on Dutchmans Creek. Access
is via S-42-511 (a/k/a S-44-68), and entrance can be gained
at the Bridge over the Creek or at gates 0.1 or 1.0 miles from
the intersection with SC 56. (Fig. 21, Cross Anchor quadrangle)
DESCRIPTION
: A good section of mafic metavolcanic rocks is exposed
in this reach of Dutchman Creek. These rocks are thought to
be typical, if very well exposed, examples of the mafic
metavolcanic rocks that are representative of the Carolina
terrane in this area. They are comparable to the McClure
Creek section discussed above at Stop 5, even to the extent
of being intruded by a foliated 579±4 Ma granodiorite (Dennis
and Wright, 1993, 1995). The metabasalts here are well
foliated, and many contain a strong lineation. These rocks
are cut by leucocratic granitic to pegmatite dikes that are
clearly unfoliated and undeformed, and cut across the metamorphic
fabric (Fig. 22; Dennis, 1995). At this location we
are less than 3 km from the surface trace of the central Piedmont
suture to the west, and less than 1 km from felsic intrusive
rocks associated with the Buffalo gabbro to the east and
southeast.
Foliation dips in this area are moderately steep, in the
range of 45-55° to the south or southeast. Mineral lineations
plunge 35° to 45° within a few degrees of 180. This is comparable
to lineations discussed above in the stop 10 description,
and the regional pattern of lineations in this segment of
the central Piedmont suture (Dennis, 1995) that are subparal-
18