Geology of Stone Mountain, Georgia

From: GEOLOGY AND GEOMORPHOLOGY OF STONE MOUNTAIN, GEORGIA
Southeastern Section, Geological Society of America Field Trip
Athens, GA
March 27, 1999

Led by:
James A. Whitney, Department of Geology, University of Georgia, Athens, GA
John Dennison, Department of Geology, University of North Carolina, Chapel Hill, NC
Pamela J. W. Gore, Department of Geology, Georgia Perimeter College, Clarkston GA

Directions to East Quarry: From the walkup trail, drive clockwise around the mountain, toward the carving. Pass the carving, cross "Old Route 78", and look for a parking lot on the left (near the grist mill). Park, cross the street, then cross the railroad tracks, and follow the path through the woods to the east quarry area.

Pamela J. W. Gore, Georgia Perimeter College

The East Quarry contains the freshest and least-weathered rocks exposed at Stone Mountain, as a result of quarrying that occurred there until the early 1970’s. As a result, the rocks in this area display petrologic and structural characteristics that are obscured by lichen and fungi in most other places. Quarrying began at Stone Mountain around the time of the Civil War. The East Quarry was operated by the Stone Mountain Granite Corporation from 1916 to 1934, and by the Works Progress Administration (WPA) from 1935 to 1940 (Herrmann, 1954). The quarry was leased by Arthur Kellogg from the Venable Brothers Estate, and operated after 1947. In 1950, this quarry produced 1200 tons of stone per week, including both curb stone and crushed rock, used in the Atlanta area (Herrmann, 1954).

Looking up at the mountain, you may see a white line which has been painted on it in the distance. This is the line above which park visitors are not allowed to climb.

The Stone Mountain granite is light gray and fine- to medium-grained, consisting of quartz, plagioclase, microcline, muscovite, and biotite (Size and Khairallah, 1989). The East Quarry contains excellent examples of flow structures, tourmaline pods (sometimes called "cat’s paws"), tourmaline aplites, zoned granite dikes, pegmatites, quartz veins, xenoliths, and autoliths. In addition, several examples of recent exfoliation are present.

Flow banding in the East Quarry

The granite in the East Quarry has primary flow structures which range from pronounced banding to weak flow foliation (Grant et al., 1980). The bands differ in grain size, foliation, and/or mineral proportion, and tend to show up best when the rock is wet (Size and Khairallah, 1989). In some areas, the flow banding is folded; folding varies from gently folded to tightly folded and ductily sheared (Size and Khairallah, 1989). Flow folds in this area suggest that the intrusion came from the east, although a flow fold on the south side of the mountain suggests intrusion from the south (Grant, 1986).

Tourmaline pods, commonly known as "cat's paws" are abundant in some areas of the East Quarry. The pods are several centimeters in diameter, and vary in shape from circular to elongated. They consist of a cluster of black, millimeter-scale, schorl-type tourmaline crystals, surrounded by a white halo or "bleached zone". Mineral zoning towards the center of a tourmaline pod indicates a decrease in plagioclase, muscovite, and biotite, and an increase in microcline and tourmaline (Size and Khairallah, 1989). A calculation of the amount of iron "missing" from the bleached halo is equal to the calculated amount of iron in the tourmaline in the center of the pod (Size and Khairallah, 1989). There is a mineral zoning towards the center of the tourmaline pods. Size and Khairallah (1989) noted a decrease in plagioclase, muscovite, and biotite, and an increase in microcline and tourmaline. The tourmaline pods are interpreted by (Size and Khairallah, 1989) as late-stage, post-magmatic zones formed during metasomatism by boron-rich fluids which stripped cations such as iron from biotite while migrating into small low-pressure pockets.

The tourmaline pods are scattered through the rock, but many are near pegmatite and aplite veins and dikes, which intrude the granite. Some of these intrusive structures contain large black tourmaline crystals. According to Whitney and Wenner (1980), the tourmaline probably nucleated in the vicinity of vapor bubbles left over from the terminal crystallization. The bleached zones are caused by the metasomatic removal of iron, magnesium and aluminum from the surrounding granite as the tourmaline grew (Whitney and Wenner, 1980). When critical elements (probably boron) were depleted from the surrounding rock, tourmaline crystallization stopped (Whitney and Wenner, 1980).

Pegmatites, tourmaline aplites, zoned granite dikes, and quartz veins are present in the East Quarry. Pegmatites occur mainly as meter-sized pods, and some contain large feldspar and/or tourmaline crystals. Tourmaline aplites are elongated but discontinuous, and resemble the tourmaline pods with the bleached halo around the tourmaline. Tourmaline crystals within these aplites are up to about a centimeter in diameter.

Zoned granite dikes are lighter-colored than the rest of the granite tend to be narrow (about 20 cm) but continuous. They have symmetrical zoning or layering parallel to the sides, and some have a lighter-colored, coarser-grained margin. Some of these dikes are also cut by tourmaline aplites.

Several types of xenoliths are present in the Stone Mountain granite, including biotite, muscovite-biotite, garnet-mica schists, biotite-plagioclase gneiss, granite gneiss, and less commonly, tourmaline mica-schist with centimeter-sized tourmaline crystals (Grant et al., 1980). Near the base of the walk-up trail, most of the xenoliths are lens-shaped mica schist and garnet-mica schist fragments, ranging up to a few tens of centimeters in length. In the East Quarry, however, larger biotite gneiss xenoliths are present. These biotite gneiss xenoliths are blocky, elongated, or irregular in shape, ranging up to about 1 m in length. At least some of the xenoliths are surrounded by bleached halos resembling those around the tourmaline pods. Size and Khairallah (1989) referred to these halos as reaction rims. Some of the xenoliths are also cut by tiny white quartz or aplite veins, resembling the material in the halo.

The xenoliths are irregularly distributed locally, but tend to be more abundant at the eastern and western ends of the mountain (Grant et al., 1980). Grant et al. (1980) studied the orientation of the schistosity and gneissic banding in the xenoliths, and determined that these structures are generally parallel to the flow banding and lineations, approximately N65^oW. Granite autoliths are present locally and contain flow banding.

Evidence of recent exfoliation can be found in the East Quarry area. There are several places in which a piece of rock has recently exfoliated. The pictures below illustrate how the rock has exfoliated and broken up over a period of about six years.

Directions to Weathered Granite and Diabase Dike at "Old Route 78" near Stone Mountain lake: This stop is between the carving and the grist mill. From the East Quarry, return across the railroad tracks either the way you came, or along the gravel road. Return to cars, and drive toward the carving to intersection with "Old Route 78". Turn right and park in open unpaved area. Do not block park access road leading up into the woods. You will be examining outcrops at the parking area and down the abandoned road toward the lake.

Pamela J. W. Gore, Georgia Perimeter College

At this stop, you can observe two things: (1) a weathering profile on the Stone Mountain granite in the old roadcuts on either side of the abandoned road, and (2) a Jurassic diabase dike.

The Stone Mountain granite at this location has weathered to saprolite. The saprolite is white with a bulk density of about 1.5, and is strongly kaolinized (Grant, 1986). The kaolinite is derived from the weathering of the feldspars. None of the original oligoclase feldspar remains, although there is still some microcline (Grant, 1986). Quartz and muscovite are basically unaltered by weathering, and the biotite in the granite has weathered to produce a small amount of iron oxide. Halloysite and gibbsite are also present as weathering products (Grant et al., 1980). The light color of the saprolite and the overlying pale brown soil reflects the low iron content of the granite. Careful examination of the saprolite reveals the original structure of the granite. For example, tourmaline pods or "cat's paws" are present in the saprolite here.

Walk toward the lake, and observe the color of the soil and the weathered rocks in the old roadcut. At some point, the color changes from white to brownish orange. Where the soil color changes, stop and look carefully at the rock along the road and in the old roadcuts on the left as you face the lake. The outcrops on the right side of the road are covered by kudzu (a legume vine introduced to Georgia from Japan to control erosion).

The brownish-orange color of the soil and saprolite indicates that the rock in this area is not Stone Mountain granite. Examination of the rock on freshly-broken surfaces reveals a fine-grained dark gray to black igneous rock identified as a diabase dike intruding the granite. The diabase, closely resembling basalt, is rich in iron and magnesium. Through weathering of the diabase, abundant iron oxides are produced, which color the soil.

This dike is one of many Jurassic diabase (or dolerite) dikes which intrude Piedmont rocks throughout the eastern United States from northern Florida (in the subsurface) and Alabama to New England, and into Canada. The dikes are associated with Jurassic tholeiitic lava flows farther north (Manspeizer, 1988). Radiometric ages of these dikes in eastern North America range from 185 to 195 million years, although some are slightly younger, about 175 million years (McHone, 1988).

The dikes in the southern states tend to be low-titanium olivine and quartz-normative tholeiite (McHone, 1988). In the southeastern US, these dikes predominantly strike NW-SE and N-S, with a mode about N35W from Alabama through South Carolina (McHone, 1988). In the mid-Atlantic to New England states, the dikes strike N-S or NE-SW (McHone, 1988, and Manspeizer, 1988).

The presence of these diabase dikes is related to the breakup of the supercontinent Panega during the Early Mesozoic. Diabase dikes of similar chemistry, age, and orientation in western Africa, indicate that these dikes were formed parallel to and roughly adjacent to the final rifts that separated the continents and opened the Atlantic Ocean (McHone, 1988). The nearest rift basin with Mesozoic sedimentary rocks is buried beneath the coastal plain, southeast of Macon, GA, about 150 miles away (Chowns and Williams, 1983).

Chowns, T. M. and Williams, C. T., 1983, Pre-Cretaceous rocks beneath the Georgia coastal plain - regional implications, in Studies related to the Charleston, South Carolina, earthquake of 1886 - Tectonics and Seismicity, U. S. Geological Survey Professional Paper 1313, p. L1-L42.

Grant, Willard H.,1986, Structural and petrologic features of the Stone Mountain granite pluton, Georgia: Geological Society 9foAmerica Centennial Field Guide, Southeastern Section, site 65, p. 285-290.

Grant, Willard H., Size, William B., and O'Connor, Bruce J., 1980, Petrology and Structure of the Stone Mountain Granite and Mount Arabia Migmatite, Lithonia, Georgia, Field Trip No. 3, in Excursions in Southeastern Geology, Volume 1, Robert W. Frey (ed.), Geological Society of America Annual Meeting, Atlanta, GA, p. 41-57,

Herrmann, Leo Anthony, 1954, Geology of the Stone Mountain – Lithonia District, Georgia: Georgia Department of Mines, Mining and Geology, The Geological Survey, Bulletin No. 61, 139 p.

Manspeizer, Warren, 1988, Triassic-Jurassic rifting and opening of the Atlantic: An Overview: in Triassic-Jurassic Rifting, Continental Breakup and the Origin of the Atlantic Ocean and Passive Margins, Part B, W. Manspeizer (ed.), Developments in Geotectonics 22, Elsevier, The Netherlands, p. 41-79.

McHone, J. Gregory, 1988, Tectonic and paleostress patterns of Mesozoic intrusions in eastern North America: in Triassic-Jurassic Rifting, Continental Breakup and the Origin of the Atlantic Ocean and Passive Margins, Part B, W. Manspeizer (ed.), Developments in Geotectonics 22, Elsevier, The Netherlands, p. 607-620.

Size, William B. and Khairallah, Nayla, 1989, Geology of the Stone Mountain Granite and Mount Arabia Migmatite, Georgia: in Excursions in Georgia Geology, W. J. Fritz (ed.), Georgia Geological Society Guidebooks, v. 9, no. 1, p. 149-177.

Whitney, James A. and Wenner, David B., 1980, Petrology and structural setting of post-metamorphic granites of Georgia, Field Trip No. 18, in Excursions in Southeastern Geology, Volume 2, Robert W. Frey (ed.), Geological Society of America Annual Meeting, Atlanta, GA, p. 351-378.

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