gb 1978.book - Carolina Geological Society
gb 1978.book - Carolina Geological Society gb 1978.book - Carolina Geological Society
CAROLINA GEOLOGICAL SOCIETY Guidebook for 1978 Annual Meeting Pages 21-24 STRUCTURAL CHRONOLOGY OF THE LAKE MURRAY SPILLWAY, CENTRAL SOUTH CAROLINA Martha Carr Department of Geology, University of North Carolina, Chapel Hill, North Carolina 27514 (present address: Chevron Resources, 8026 Vantage, Suite 100A, San Antonio, Texas 78230) INTRODUCTION The Lake Murray spillway is a spectacular exposure of polyphase-deformed rocks near the northeast terminus of the Kiokee belt, central South Carolina. Although the geology of the spillway has been studied by many geologists (Heron and Johnson, 1958; Kiff, 1963; Tewhey, 1968, 1977), detailed structural analysis has not been attempted. The purpose of this article is to summarize the structural chronology of the spillway as deduced from the analysis of mesoscopic deformation features. Over 2000 structural measurements were collected with a Brunton compass throughout the length of the spillway. LITHOLOGIES The main rock units exposed in the spillway can be broadly divided into two gneiss units and two schist units (See STOP 9--Secor and Snoke, this volume). The contacts between these units are sharp and straight and have all appearances of being conformable. The gneiss varies in composition from granodiorite to quartz monzonite. It is an even-grained, relatively homogeneous, light gray rock with a well-developed gneissosity. Large porphyroblasts (up to 5 or 6 mm in length) of white to pink microcline are conspicuous. A decrease in grain size and a corresponding increase in the development of mylonitic texture is apparent from west to east across the exposures. The schist unit is considerably less monotonous than the gneiss/ Silver-gray, garnetiferous quartz-muscovite schist is interlayered with quartzo-feldspathic gneiss and green to black amphibolite. Garnet porphyroblasts in the schist reach 2 cm in diameter and impart a knobby appearance on weathered surfaces. Aplite, pegmatite, and mafic dikes intrude the above units. The mafic dikes are metamorphosed to biotite- and amphibole-rich schists. Lamprophyre and diabase dikes are late aspects of the igneous history of the area and do not display evidence of penetrative deformation and metamorphism. Textural, compositional, isotopic, and mineralogical data indicate that the gneiss units were originally igneous rocks. A sedimentary origin is assumed for the compositionally-layered schist units. Therefore, prior to the main phase deformation, it is inferred that a granitic pluton (i.e., the protolith of the gneiss) intruded a heterogeneous sequence of sedimentary rocks. The mechanism of emplacement of the pluton is uncertain, but perhaps a diapiric intrusion of semicrystalline magma is a viable hypothesis (see Tewhey, 1977). STRUCTURAL FEATURES Initial analysis, employing several domains based on lithology and geography, revealed that the structural chronology for both the gneiss and schist units was the same. Therefore, the spillway is considered a single, homogeneous domain with respect to the described deformational events. All planar and linear features measurable within the spillway are on the mesoscopic scale. Structures range in size from minute crenulations, only a few millimeters in length, to folds having amplitudes and wavelengths of 5 to 6 meters. Four, possibly five, deformational phases are evident from comparison of the geometric relationships between individual structures. The mesoscopic structural elements as deciphered from the spillway are summarized in Tables 1 and 2. D 1 phase 1 The effect of the D 1 deformational event is manifested as the development of foliation in the gneiss and schist unites and in the early mafic dike (metamorphosed to biotite schist) in the gneiss. The gneissosity (S 1 ) is defined by the segregation of light and dark minerals in thin layers, only a few millimeters wide. Quartz, microcline, muscovite, and minor sodium plagioclase compose the light bands; whereas biotite flakes compose the dark bands. The schistosity (S 1 ) in the muscovite schist is parallel to compositional layering in the schist and is defined by the parallel alignment of muscovite flakes. The schistosity in the biotite schist is the result of the alignment of biotite plates. F 1 folds were not observed in the gneiss or biotite schist (metadike), but scarce “refolded” folds were found in the schist unit. However, the S 1 foliation in all units is interpreted to have developed during an intense isoclinal folding event (F 1 ). Subsequent to D 1 , a mafic dike (altered to amphibole schist) intruded the gneiss. This dike along with many pegmatite intrusions appear to be pre-D 2 , for they are locally deformed by second generation structures. 1.Editor’s note: Secor and Snoke (this volume) believe that the dominant synmetamorphic penetrative foliation in the Lake Murray gneiss is regional S 2 ; therefore, the numerical sequence used by Carr should be considered in a local sense rather than a regional. 21
Martha Carr TABLE 1. SUMMARY OF MESOSCOPIC STRUCTURES, LAKE MURRAY SPILLWAY DEFORMATIONAL PHASE FOLDS SURFACES FOLDED PLANAR STRUCTURES LINEAR STRUC- TURES ATTITUDE F 1 , isoclinal D 1 D 2 D 3 D 4 D 5 S 0 , bedding; and original igneous structures in the gneiss S 1 , dominant metamorphic foliation --- overturned folds to SE, with present NE axial trend F 2 , open to isoclinal, some attenuated limbs, intrafolial, rootless folds, chevrons locally F 3 , open folds and crinkle folds S 0 and S 1 , foliation S 0 , S 1 , and S 2 S 0 , S 1 , S 2 and S 3 (?) S 2 , secondary foliation subparallel to S 1 , locally dominant axial surface to F 2 folds L 2 , intersection lineation between S 1 and S 2 overturned folds to SE, moderate to steeply plunging, NE axial trends S 3 , axial planar to F 3 open folds, crinkle folds L 3 , crenulation lineation subhorizontal axial surface dipping slightly SE S 4 , local slip or fracture cleavage L 4 , crenulation lineation -- -- subvertical axial surface, E-W trend S 0 , S 1 , S 2 , S 3 (?), S 4 (?) S 5 , axial surfaces of elongate wraps L 5 , elongate linear ridges subvertical axial surface, NE trend TABLE 2. ORIENTATION OF STRUCTURAL ELEMENTS, LAKE MURRAY SPILLWAY 1 DEFORMATIONAL EVENTS AND RELATED STRUCTURES WESTERN GNEISS SCHIST (both units) EASTERN GNEISS BIOTITE SCHIST AMPHIBOLE SCHIST APLITE DIKES PEGMATITE DIKES F 1 -- 2 ~65°, N43E -- -- -- -- -- D 1 presently S 1 N12W, 52NE N48E, 68SE N53E, 71SE N48E, 58SE -- -- -- D 2 S 2 N28E, 70SE N42E, 62SE N55E, 73SE N20E, 54SE N42E, 70SE N32E, 47SE N39E, 50SE F 2 52°, N78E 65°, N46E 60°, N54E -- -- -- -- L 2 49°, N51E 26°, N58E 31°, N46E -- -- -- -- D 3 S 3 N55E, 21SE N65E, 24SE -- N42E, 26SE F 3 -- -- -- -- -- -- -- L 3 05°, N65E 26°, S72W -- 21°, N66E -- -- -- S 4 N73E, 72SE N88W, 74SE N88W, 70SW D 4 L 4 -- 65°, S50E -- -- -- -- -- S 5 -- -- -- -- -- -- -- D 5 L 5 -- 68°, N50E -- -- -- -- -- 1 -- Values reflect statistical maxima or most representative orientation. 2 -- The symbol “--” indicates that either no structure was present or it was not measurable. D 2 phase The second phase of deformation was by far the strongest and most pervasive. All lithologies show evidence of this deformation with the exception of the late lamprophyre and diabase dikes. Structural features associated with D 2 include F 2 folds, S 2 foliation (parallel to axial surfaces of F 2 folds) and L 2 lineations, Folding associated with D 2 is quite consistent with respect to orientation, but quite variable with respect to style and scale. When plotted on a stereonet, F 2 fold axes cluster closely; the minor scatter is easily accounted for by effects of subsequent milder deformation episodes. The folds are consistently overturned to the southeast with moderately to steeply plunging axes. F 2 folds are widespread in the gneiss and schist units as well as the biotite schist metadike. Aplite and pegmatite dikes have locally also been deformed by F 3 structures. Folding is commonly similar in style, some open, but 22
- Page 2 and 3: CAROLINA GEOLOGICAL SOCIETY FIELD T
- Page 4 and 5: CONTENTS CAROLINA GEOLOGICAL SOCIET
- Page 7 and 8: CAROLINA GEOLOGICAL SOCIETY Guidebo
- Page 9 and 10: STRUCTURE OF WEST FLANK OF THE RALE
- Page 11 and 12: CAROLINA GEOLOGICAL SOCIETY Guidebo
- Page 13 and 14: GEOLOGIC RESEARCH IN THE RALEIGH BE
- Page 15 and 16: Van Price, Jr. and R. Bruce Ferguso
- Page 17 and 18: Van Price, Jr. and R. Bruce Ferguso
- Page 19 and 20: CAROLINA GEOLOGICAL SOCIETY Guidebo
- Page 21 and 22: MAGNETOMETER ANOMALIES IN THE HAILE
- Page 23: MAGNETOMETER ANOMALIES IN THE HAILE
- Page 27 and 28: Martha Carr Columbia, South Carolin
- Page 29 and 30: Alexander W. Ritchie and Wallace C.
- Page 31 and 32: CAROLINA GEOLOGICAL SOCIETY Guidebo
- Page 33 and 34: PETROLOGY OF THE GREENWOOD PLUTON p
- Page 35 and 36: Harmon D. Maher Figure 1. Geologic
- Page 37 and 38: Harmon D. Maher Augusta fault zone
- Page 39 and 40: David C. Prowell Figure 2. Contour
- Page 41 and 42: David C. Prowell cates a horizontal
- Page 43 and 44: Stephen A. Kish Figure 1. Distribut
- Page 45 and 46: DONALD T. SECOR, JR., AND ARTHUR W.
- Page 47 and 48: DONALD T. SECOR, JR., AND ARTHUR W.
- Page 49 and 50: DONALD T. SECOR, JR., AND ARTHUR W.
- Page 51 and 52: DONALD T. SECOR, JR., AND ARTHUR W.
- Page 53 and 54: DONALD T. SECOR, JR., AND ARTHUR W.
- Page 55 and 56: DONALD T. SECOR, JR., AND ARTHUR W.
- Page 57 and 58: DONALD T. SECOR, JR., AND ARTHUR W.
- Page 59 and 60: DONALD T. SECOR, JR., AND ARTHUR W.
- Page 61 and 62: DONALD T. SECOR, JR., AND ARTHUR W.
- Page 63 and 64: DONALD T. SECOR, JR., AND ARTHUR W.
- Page 65 and 66: CAROLINA GEOLOGICAL SOCIETY Guidebo
- Page 67 and 68: FIELD TRIP ITINERARY Figure 17 - A.
- Page 69 and 70: FIELD TRIP ITINERARY apparently was
- Page 71 and 72: FIELD TRIP ITINERARY formed during
- Page 73 and 74: FIELD TRIP ITINERARY dike (Fig. 20-
Martha Carr<br />
TABLE 1. SUMMARY OF MESOSCOPIC STRUCTURES, LAKE MURRAY SPILLWAY<br />
DEFORMATIONAL<br />
PHASE<br />
FOLDS<br />
SURFACES FOLDED<br />
PLANAR<br />
STRUCTURES<br />
LINEAR STRUC-<br />
TURES<br />
ATTITUDE<br />
F 1 , isoclinal<br />
D 1 D 2 D 3 D 4 D 5<br />
S 0 , bedding; and original<br />
igneous structures<br />
in the gneiss<br />
S 1 , dominant metamorphic<br />
foliation<br />
---<br />
overturned folds to SE,<br />
with present NE axial<br />
trend<br />
F 2 , open to isoclinal,<br />
some attenuated limbs,<br />
intrafolial, rootless folds,<br />
chevrons locally<br />
F 3 , open folds<br />
and crinkle folds<br />
S 0 and S 1 , foliation S 0 , S 1 , and S 2<br />
S 0 , S 1 , S 2 and S 3<br />
(?)<br />
S 2 , secondary foliation<br />
subparallel to S 1 , locally<br />
dominant axial surface to<br />
F 2 folds<br />
L 2 , intersection lineation<br />
between S 1 and S 2<br />
overturned folds to SE,<br />
moderate to steeply<br />
plunging, NE axial trends<br />
S 3 , axial planar to<br />
F 3 open folds,<br />
crinkle folds<br />
L 3 , crenulation<br />
lineation<br />
subhorizontal<br />
axial surface dipping<br />
slightly SE<br />
S 4 , local slip or<br />
fracture cleavage<br />
L 4 , crenulation lineation<br />
-- --<br />
subvertical axial surface,<br />
E-W trend<br />
S 0 , S 1 , S 2 , S 3 (?), S 4<br />
(?)<br />
S 5 , axial surfaces of<br />
elongate wraps<br />
L 5 , elongate linear<br />
ridges<br />
subvertical axial surface,<br />
NE trend<br />
TABLE 2. ORIENTATION OF STRUCTURAL ELEMENTS, LAKE MURRAY SPILLWAY 1<br />
DEFORMATIONAL<br />
EVENTS AND<br />
RELATED<br />
STRUCTURES<br />
WESTERN<br />
GNEISS<br />
SCHIST<br />
(both units)<br />
EASTERN<br />
GNEISS<br />
BIOTITE<br />
SCHIST<br />
AMPHIBOLE<br />
SCHIST<br />
APLITE DIKES<br />
PEGMATITE<br />
DIKES<br />
F 1 --<br />
2 ~65°, N43E<br />
-- -- -- -- --<br />
D 1<br />
presently<br />
S 1 N12W, 52NE N48E, 68SE N53E, 71SE N48E, 58SE -- -- --<br />
D 2 S 2 N28E, 70SE N42E, 62SE N55E, 73SE N20E, 54SE N42E, 70SE N32E, 47SE N39E, 50SE<br />
F 2 52°, N78E 65°, N46E 60°, N54E -- -- -- --<br />
L 2 49°, N51E 26°, N58E 31°, N46E -- -- -- --<br />
D 3 S 3 N55E, 21SE N65E, 24SE -- N42E, 26SE<br />
F 3 -- -- -- -- -- -- --<br />
L 3 05°, N65E 26°, S72W -- 21°, N66E -- -- --<br />
S 4 N73E, 72SE N88W, 74SE N88W, 70SW<br />
D 4<br />
L 4 -- 65°, S50E -- -- -- -- --<br />
S 5 -- -- -- -- -- -- --<br />
D 5<br />
L 5 -- 68°, N50E -- -- -- -- --<br />
1 -- Values reflect statistical maxima or most representative orientation.<br />
2 -- The symbol “--” indicates that either no structure was present or it was not measurable.<br />
D 2 phase<br />
The second phase of deformation was by far the strongest<br />
and most pervasive. All lithologies show evidence of<br />
this deformation with the exception of the late lamprophyre<br />
and diabase dikes. Structural features associated with D 2<br />
include F 2 folds, S 2 foliation (parallel to axial surfaces of F 2<br />
folds) and L 2 lineations,<br />
Folding associated with D 2 is quite consistent with<br />
respect to orientation, but quite variable with respect to style<br />
and scale. When plotted on a stereonet, F 2 fold axes cluster<br />
closely; the minor scatter is easily accounted for by effects of<br />
subsequent milder deformation episodes. The folds are consistently<br />
overturned to the southeast with moderately to<br />
steeply plunging axes. F 2 folds are widespread in the gneiss<br />
and schist units as well as the biotite schist metadike. Aplite<br />
and pegmatite dikes have locally also been deformed by F 3<br />
structures.<br />
Folding is commonly similar in style, some open, but<br />
22
Change language