Geology of Kennesaw Mountain, putting it together, Higgins et al, 2003
With the publication of the Geologic map of the Atlanta 30' x 60' quadrangle, Georgia in June of 2003, nearly everything changed in my understanding of the geology here. This map and accompanying text are the result of an amazing amount of field research that greatly increased our understanding of the rocks we live on. This map is a very small cropped section that includes Kennesaw Mountain and enough of its surroundings to give context to its regional geology. The descriptions below are from the map and its accompanying text booklet with my comments following.
Higgins, M.W., T.J. Crawford, R.L. Atkins & R.F. Crawford. 2003. Geologic map of the Atlanta 30' x 60' quadrangle, Georgia. Scientific Investigations Map 2602. U.S. Geologic Survey, Reston, VA.
Higgins, M.W., T.J. Crawford, R.L. Atkins & R.F. Crawford. 2003. Geologic map of the Atlanta 30' x 60' quadrangle, Georgia. Scientific Investigations Map 2602. U.S. Geologic Survey, Reston, VA.
Informal migmatite of Kennesaw Mountain at the summit of the mountain
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Informal migmatite of Kennesaw Mountain
Foreshadowing this map, mostly the same authors published a dramatic revision of geologic units in the Atlanta area, abandoning a large number of the names used or given by McConnell & Abrams and modifying most of the others. Their comments on Kennesaw Mountain's light-colored gneiss: "Hurst (1952) named the Kennesaw Mountain gneiss after Kennesaw Mountain, in the Marietta, Ga., 7.5-min quadrangle, for rocks he included in his migmatite category. McConnell and Abrams (1984) proposed formalizing the name Kennesaw Gneiss Member of their Laura Lake Mafic Complex for the migmatite. We here informally name the unit the migmatite of Kennesaw Mountain because we don't think it belongs with the Laura Lake Mafic Complex of McConnell and Abrams (1984). As recognized by Hurst (1952), the migmatite of Kennesaw Mountain is a massive, light-gray to whitish-gray biotite-quartz plagioclase gneiss that has the same general mineralogic composition as the nearby metatrondhjemite gneisses except that it contains abundant xenoliths of amphibolite that is lithologically identical to amphibolite in Ropes Creek Metabasalt. The migmatite is interpreted to be the result of intrusion of a trondhjemitic magma into amphibolite of the Ropes Creek Metabasalt, and is assigned an age of Late Proterozoic(?) to Early Ordovician(?)." Crawford, T.J., M.W. Higgins, R.F. Crawford, R.L. Atkins, J.H. Medlin & T.W. Stern. 1999. Revision of stratigraphic nomenclature in the Atlanta, Athens, and Cartersville 30' x 60' quadrangles, Georgia. Bulletin 130, Georgia Geologic Survey, Atlanta. |
OZkm Informal migmatite of Kennesaw Mountain (Middle Ordovician? to Late Proterozoic?)—Massive, light-gray to nearly white, medium-grained, potassium feldspar-poor, biotite-quartz-plagioclase gneiss identical to metatrondhjemite gneisses (OZmt) and Villa Rica Gneiss (OZv), but with abundant xenoliths of Ropes Creek Metabasalt (OZr).
Bands of dark Ropes Creek metabasalt in lighter metatrondhjemite gneiss on the Mountain Trail
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A 1.5 meter thick band of Ropes Creek metabaslt in metatrondhjemite gneiss on the Mountain road at the saddle
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Metatrondhjemite
The word metatrondhjemite begs the question of what the heck trondhjemite is!
"Trondhjemite is a light-coloured tonalite (leucotonalite) in which plagioclase is dominated by oligoclase. Trondhjemite is most common as part of Archean trondhjemite-tonalite-granodiorite suite of intrusions and are usually high-grade metamorphic rocks. They are also found as dykes in the sheeted dyke complexes of ophiolites." [Imperial College Rock Library]
Swiss-born mineralogist Victor Moritz Goldschmidt (1888-1947) studied the rocks of Støren Norway, just north Trondheim (modern spelling), in 1916 and gave them this name.
In common parlance, trondhjemite (along with tonalite) is often called "white granite". It is an intrusive igneous rock that is often associated with ophiolites (a suite of crustal and oceanic mantle rocks found at ocean spreading centers).
The word metatrondhjemite begs the question of what the heck trondhjemite is!
"Trondhjemite is a light-coloured tonalite (leucotonalite) in which plagioclase is dominated by oligoclase. Trondhjemite is most common as part of Archean trondhjemite-tonalite-granodiorite suite of intrusions and are usually high-grade metamorphic rocks. They are also found as dykes in the sheeted dyke complexes of ophiolites." [Imperial College Rock Library]
Swiss-born mineralogist Victor Moritz Goldschmidt (1888-1947) studied the rocks of Støren Norway, just north Trondheim (modern spelling), in 1916 and gave them this name.
In common parlance, trondhjemite (along with tonalite) is often called "white granite". It is an intrusive igneous rock that is often associated with ophiolites (a suite of crustal and oceanic mantle rocks found at ocean spreading centers).
Albert Streckeisen's QAPF diagram for classification of plutonic rocks from the Wikimedia Commons with my revisions for this use.
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QAPF Diagram
To master all of the variations of what most people simply call "granite" requires an understanding of the mineralogy of the rock. The QAPF diagram greatly helps as it visually allows one to see the various minerals in the mix and the percentages of each that form the various rocks. The four points of the diamond represent Quartz, Alkali feldspar, Plagioclase and Feldspathoid. Each point represents 100% of that mineral. The A and P represent the feldspar family. The lines represent the somewhat arbitrary placing of boundaries on the various percent of minerals. The area within the quadrilaterals formed all have names for the kind of rock. Trondhjemite is a form of tonalite where the plagioclase is mostly in the form of oligoclase. From the QAPF diagram, where I've colored its section in gold, its feldspar is ≥90% plagioclase, ≤10% alkali feldspar; and 20 to 60% quartz. Tonalite/Trondhjemite were formerly included in the quartz diorite family and is still called that by some and used in older literature. To compare that to a contemporary definition of "granite" note the wide range that name encompasses on the diagram. Granite contains at least 20% quartz and a wide range of mixes of alkali and plagioclase feldspar. Rocks in the F end of the diagram are rare, the various forms of syenite with no quartz being the most common. |
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The summit of both Kennesaw and Little Kennesaw Mountain are capped with massive outcrops of highly weathered metatrondhjemite. Since they are completely exposed and surrounded by trails, they are walked on daily which keeps the flora of lichens limited. Most of the color comes from the chemical weathering of the rock. The yellows and browns come from the feldspar and the reds and oranges from the iron that occurs in every mineral crystal molecule in the rock.
If you look carefully, someone carved "Prepare to Meet Thy God". This graffiti is undated, but probably contemporaneous with the others. The swirls of lines, cracks and joints all are the result of the rock's migmatitic origin. The feldspars weather out faster than the quartz-rich minerals and form valleys, depressions, low spots. The very resistant quartz forms the ridges and high spots. While the rock here shows no evidence of being a dome like Arabia or Stone mountains, there is some exfoliation of the broader rounded portions of the rock. |
Informal migmatite of Kennesaw Mountain (metatrondhjemite gneiss) with post Civil War graffiti
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Crider Gneiss
OCcr Crider Gneiss (Middle Ordovician? to Late Proterozoic?)—Gray to nearly white, massive to slabby, medium- to coarse-grained, poorly to well foliated biotite muscovite- quartz-plagioclase gneiss that is locally contorted and generally weathers to a light-tan to dark-yellowish-tan soil containing corestones of gneiss. The gneiss commonly is found as residual boulders where the unit is deeply weathered.
OCcr Crider Gneiss (Middle Ordovician? to Late Proterozoic?)—Gray to nearly white, massive to slabby, medium- to coarse-grained, poorly to well foliated biotite muscovite- quartz-plagioclase gneiss that is locally contorted and generally weathers to a light-tan to dark-yellowish-tan soil containing corestones of gneiss. The gneiss commonly is found as residual boulders where the unit is deeply weathered.
Crider gneiss boulder used as trail edge material exposed long enough to show early weathering
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Crider gneiss boulder showing typical contorted banding consistent with migmatite in a fresh break from recent quarrying
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The diorite references of McConnell & Abrams becomes clear when I examine the Higgins et al map and find the Vulcan quarry is producing Crider gneiss. This detailed description comes form Crawford et al, 1999:
"The Crider Gneiss is here named for Crider Creek in the New Georgia, Ga., 7.5-min quadrangle ... The Crider Gneiss is a gray to nearly white, massive to slabby, medium- to coarse-grained, poorly to well-foliated biotite-muscovite-quartz-plagioclase gneiss that is locally contorted and generally weathers to a light-tan to dark-yellowish-tan soil containing corestones of gneiss. The presence of muscovite in excess of biotite in the Crider Gneiss is one of the features that distinguishes it from other biotite gneisses such as the Stonewall Gneiss. The Crider Gneiss is commonly found as residual boulders where the unit is deeply weathered. Crider Gneiss containing beds and lenses of finely laminated to massive calc-silicate rock containing diopside and locally diopside and garnets underlies a 5-km-long belt northwest of Villa Rica, Ga., in the Atlanta 30'X60' quadrangle (Higgins and others, in press); this calc-silicate-bearing gneiss is similar to some calc-silicate-bearing gneisses in the Wahoo Creek Formation (Higgins and Atkins, 1981; Higgins and others, in press). Map relations indicate that the Crider Gneiss, like the metatrondhjemites, intrudes the Ropes Creek Metabasalt."
The National Park Service is using small boulders and large gravel obtained from the Vulcan quarry in trail maintenance and reconstruction rather than harvesting and damaging local rock on thee mountain. These boulders are rarely encountered far from the trail and show little, if any, weathering (as in the right photo above). While similar to the metatrondhjemite, it is a different rock yet mapped as the same by Hurst and differentiated by McConnell & Abrams as the meta-quartz diorite member of their Laura Lake Mafic complex (now abandoned).
The protolith of Crider gneiss is probably very closely related, if not the same, as the metatrondhjemite gneisse of the Kennesaw migmatite.
"The Crider Gneiss is here named for Crider Creek in the New Georgia, Ga., 7.5-min quadrangle ... The Crider Gneiss is a gray to nearly white, massive to slabby, medium- to coarse-grained, poorly to well-foliated biotite-muscovite-quartz-plagioclase gneiss that is locally contorted and generally weathers to a light-tan to dark-yellowish-tan soil containing corestones of gneiss. The presence of muscovite in excess of biotite in the Crider Gneiss is one of the features that distinguishes it from other biotite gneisses such as the Stonewall Gneiss. The Crider Gneiss is commonly found as residual boulders where the unit is deeply weathered. Crider Gneiss containing beds and lenses of finely laminated to massive calc-silicate rock containing diopside and locally diopside and garnets underlies a 5-km-long belt northwest of Villa Rica, Ga., in the Atlanta 30'X60' quadrangle (Higgins and others, in press); this calc-silicate-bearing gneiss is similar to some calc-silicate-bearing gneisses in the Wahoo Creek Formation (Higgins and Atkins, 1981; Higgins and others, in press). Map relations indicate that the Crider Gneiss, like the metatrondhjemites, intrudes the Ropes Creek Metabasalt."
The National Park Service is using small boulders and large gravel obtained from the Vulcan quarry in trail maintenance and reconstruction rather than harvesting and damaging local rock on thee mountain. These boulders are rarely encountered far from the trail and show little, if any, weathering (as in the right photo above). While similar to the metatrondhjemite, it is a different rock yet mapped as the same by Hurst and differentiated by McConnell & Abrams as the meta-quartz diorite member of their Laura Lake Mafic complex (now abandoned).
The protolith of Crider gneiss is probably very closely related, if not the same, as the metatrondhjemite gneisse of the Kennesaw migmatite.
Ropes Creek metabasalt
OZr Ropes Creek Metabasalt (Middle Ordovician? to Late Proterozoic?)—Fine- to medium-grained, dark-green to greenish-black, ocher-weathering, massive to finely layered, locally laminated, locally pillowed, locally chloritic, commonly garnetiferous, locally magnetite-bearing, generally pyrite-bearing, generally epidotic, hornblende-plagioclase and plagioclase-hornblende amphibolites with insignificant amounts (generally less than a very small fraction of a percent) of fine- to medium-grained, generally amphibole-bearing granofels. The final weathering product of the amphibolites is a characteristic dark-red, clayey soil. Thinly layered, medium-grained, magnetite quartzite (OZmq), in units about 0.3 to 6 m thick, is common in and characteristic of the Ropes Creek Metabasalt
The Ropes Creek Metabasalt (OZr) consists of locally pillowed and generally garnet-bearing amphibolite and metalliferous quartzites. Closely associated with the Ropes Creek, which contains almost no metasedimentary components, are metatrondhjemites (OZv, OZmt) that have intruded the metabasaltic rocks. Amphibolite- and ultramafic-bearing pelitic units and the Stonewall Gneiss and Clarkston Formation are the pelitic parts of the allochthonous* oceanic assemblage. The metabasaltic rocks also are intruded by tonalitic orthogneisses of the Crider Gneiss that are chemically similar to the metatrondhjemites (Sanders, 1983, 1990). Amphibolite chemical compositions in the Ropes Creek Metabasalt indicate an oceanic origin (Stow and others, 1984; Higgins and others, 1988; Spell and Norrell, 1990; Sanders, 1990), as do its rare earth elements and other trace elements (Higgins and others, 1988; Spell and Norrell, 1990) and its isotopic compositions (Shaw and Wasserburg, 1984).
OZrs Spheroidally weathering amphibolite (Middle Ordovician? to Late Proterozoic?)—Dark-green, medium- to coarse-grained, salt-and-pepper textured, massive and flaggy, spheroidally weathering amphibolite. Holds up low ridges and knobs. Weathers to a dark-red soil with residual cobbles and boulders or flags.
Ophiolites
Of all the rocks here, I find the dark ones the most interesting. Granite and gneiss are not just common, they are abundant, easily the most expansive rock of the entire Piedmont. Scattered about are these outcroppings of very different dark rocks that are mafic in chemisty. With my growing understanding of their oceanic basalt origin, how they developed became less of an interest than of how they got here and metamorphosed into what they are today. It turns out how they formed is just as interesting! These rocks are ophiolites, a mélange of very curious oceanic crust rocks composed of mantle (the basalt), crust (graywacke) and ocean bottom sediments in that very specific bottom to top oreder.
"The most probable origin for these rocks is as oceanic lithosphere in a back-arc basin above a seaward (east) dipping subduction zone. The lack of IAT* signatures in metabasalts analyzed thus far from the easternmost area suggests this inferred back-arc basin opened sufficiently to separate spatially the spreading center from the subduction zone as in some modern backarc basins (for example, Mariana, Lau). The occurrence of granitic rocks with VAG signatures in the assemblage suggests that during convergence the back-arc basin lithosphere was thrust over the arc."
"The results of this study lead to the following basic conclusions:
1. Ophiolites do exist in the southern Appalachians, but their recognition has been hampered by poor exposures, deep weathering, and multiple metamorphic and deformational events.
2. The Ropes Creek assemblage probably represents the remnants of a disrupted, deformed, and metamorphosed ophiolite (or ophiolites).
3. Available evidence is compatible with the interpretation that rocks of the Ropes Creek assemblage represent a coherent lithologic suite contained within a single thrust sheet.
4. The most likely tectonic environment for formation of the Ropes Creek assemblage is in a back-arc basin above an eastward dipping subduction zone off the margin of North America.
5. The present structural, petrological, and chemical relationships suggest that during convergence back-arc basin lithosphere (Ropes Creek assemblage) was thrust over an associated arc, and volcanic arc granites were emplaced into it."
Spell, T.L. & G.T. Norrell. 1990. The Ropes Creek assemblage: petrology, geochemistry, and tectonic setting of an ophiolitic thrust sheet in the southern Appalachians. American Journal of Science, V.290 p.811.842.
*IAT = Island Arc Tholeiite, ocean basalt with higher silica and iron than most basalt; they are associated with mid-ocean spreading centers.
With my botanical wanderings on the mountain looking for calciphiles (plants that require large amounts of calcium) on the mountain, I became fascinated by this geologic control on botany and began an exploration of nearly all the mafic rocks in the Piedmont and Blue Ridge of Georgia. Some of those will be another story.
The Ropes Creek Metabasalt (OZr) consists of locally pillowed and generally garnet-bearing amphibolite and metalliferous quartzites. Closely associated with the Ropes Creek, which contains almost no metasedimentary components, are metatrondhjemites (OZv, OZmt) that have intruded the metabasaltic rocks. Amphibolite- and ultramafic-bearing pelitic units and the Stonewall Gneiss and Clarkston Formation are the pelitic parts of the allochthonous* oceanic assemblage. The metabasaltic rocks also are intruded by tonalitic orthogneisses of the Crider Gneiss that are chemically similar to the metatrondhjemites (Sanders, 1983, 1990). Amphibolite chemical compositions in the Ropes Creek Metabasalt indicate an oceanic origin (Stow and others, 1984; Higgins and others, 1988; Spell and Norrell, 1990; Sanders, 1990), as do its rare earth elements and other trace elements (Higgins and others, 1988; Spell and Norrell, 1990) and its isotopic compositions (Shaw and Wasserburg, 1984).
OZrs Spheroidally weathering amphibolite (Middle Ordovician? to Late Proterozoic?)—Dark-green, medium- to coarse-grained, salt-and-pepper textured, massive and flaggy, spheroidally weathering amphibolite. Holds up low ridges and knobs. Weathers to a dark-red soil with residual cobbles and boulders or flags.
Ophiolites
Of all the rocks here, I find the dark ones the most interesting. Granite and gneiss are not just common, they are abundant, easily the most expansive rock of the entire Piedmont. Scattered about are these outcroppings of very different dark rocks that are mafic in chemisty. With my growing understanding of their oceanic basalt origin, how they developed became less of an interest than of how they got here and metamorphosed into what they are today. It turns out how they formed is just as interesting! These rocks are ophiolites, a mélange of very curious oceanic crust rocks composed of mantle (the basalt), crust (graywacke) and ocean bottom sediments in that very specific bottom to top oreder.
"The most probable origin for these rocks is as oceanic lithosphere in a back-arc basin above a seaward (east) dipping subduction zone. The lack of IAT* signatures in metabasalts analyzed thus far from the easternmost area suggests this inferred back-arc basin opened sufficiently to separate spatially the spreading center from the subduction zone as in some modern backarc basins (for example, Mariana, Lau). The occurrence of granitic rocks with VAG signatures in the assemblage suggests that during convergence the back-arc basin lithosphere was thrust over the arc."
"The results of this study lead to the following basic conclusions:
1. Ophiolites do exist in the southern Appalachians, but their recognition has been hampered by poor exposures, deep weathering, and multiple metamorphic and deformational events.
2. The Ropes Creek assemblage probably represents the remnants of a disrupted, deformed, and metamorphosed ophiolite (or ophiolites).
3. Available evidence is compatible with the interpretation that rocks of the Ropes Creek assemblage represent a coherent lithologic suite contained within a single thrust sheet.
4. The most likely tectonic environment for formation of the Ropes Creek assemblage is in a back-arc basin above an eastward dipping subduction zone off the margin of North America.
5. The present structural, petrological, and chemical relationships suggest that during convergence back-arc basin lithosphere (Ropes Creek assemblage) was thrust over an associated arc, and volcanic arc granites were emplaced into it."
Spell, T.L. & G.T. Norrell. 1990. The Ropes Creek assemblage: petrology, geochemistry, and tectonic setting of an ophiolitic thrust sheet in the southern Appalachians. American Journal of Science, V.290 p.811.842.
*IAT = Island Arc Tholeiite, ocean basalt with higher silica and iron than most basalt; they are associated with mid-ocean spreading centers.
With my botanical wanderings on the mountain looking for calciphiles (plants that require large amounts of calcium) on the mountain, I became fascinated by this geologic control on botany and began an exploration of nearly all the mafic rocks in the Piedmont and Blue Ridge of Georgia. Some of those will be another story.