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85° 22' 30" W 85° 15' 0" W 85° 7' 30" W 85° 0' 0" W 84° 52' 30" W 84° 45' 0" W 84° 37' 30" W 84° 30' 0" W 84° 22' 30" W 84° 15' 0" W

85° 45' 0" W 85° 37' 30" W 85° 30' 0" W 85° 22' 30" W 85° 15' 0" W 85° 7' 30" W 85° 0' 0" W 84° 52' 30" W 84° 45' 0" W 84° 37' 30" W 84° 30' 0" W

33° 45' 0" N

33° 52' 30" N

34° 0' 0" N

33° 0' 0" N

33° 7' 30" N

33° 15' 0" N

GEOLOGIC MAP OF THE BREVARD ZONE FROM ABANDA, ALABAMA TO ATLANTA, GEORGIAby: Thomas J. Crawford and Randy L. Kath

Department of Geosciences, University of West Georgia, Carrollton, GA 30118

ABSTRACTDetailed geologic mapping at 1:24,000-scale from Abanda, Alabama, to north of Atlanta, Georgia (~148 km), detailedsurface mapping along four tunnel alignments in the metro-Atlanta area, and subsurface mapping of nearly 6 km of a5.5-meter diameter tunnel which trends along strike in the Brevard Zone have provided new insights into the nature andsignificance of this regional geologic feature. Exposures within this, and other tunnels excavated within the Brevard,show extreme lithologic heterogeneity and structural complexity that have not been previously documented within thiszone in metro-Atlanta. Although there are many definitions and interpretations to the meaning of the Brevard Zone, wedefine the Brevard in western Georgia as a relatively narrow zone of doubly plunging, northwest verging, overturnedfolds that have been overprinted by a pervasive shear fabric. Locally, the Brevard contains characteristic phyllonite andvarious types of mylonites and cataclastic rocks; however, these rocks are not always present.

In Alabama, the Brevard is defined as a terrane boundary between the Eastern Blue Ridge and Inner Piedmont/DadevilleComplex. This interpretation is not consistent with our detailed geologic mapping, in that continuous stratigraphic unitsare transected by Brevard shearing. For example, the Long Island Creek Gneiss (LICG) is north of the Brevard in themetro-Atlanta area. Further west along the Brevard trend, the LICG is within the Brevard and has been intensely shearedand silicified (flinty crush rock). Further southwest, near the Alabama state line, the LICG is south of the Brevard andhas a similar character as the exposures in the metro-Atlanta area.

The Brevard has been overthrust in the metro-Atlanta area by the Clairmont Formation (mélange). Rocks within theClairmont show no evidence of Brevard shearing and are interpreted to have been emplaced after Brevard shearingculminated. Both the Clairmont and the Brevard are intruded by the Ben Hill Granite. The Ben Hill cuts across buttonschist, phyllonite, and mylonitic rocks of the Brevard, and itself is unsheared. Preliminary age dates of mica growth inthe Brevard of Alabama by Poole (per com) suggest that the last shearing occurred ~317 Ma. This, coupled with an ageof the Ben Hill (~284 Ma), constrains emplacement of the Clairmont thrust sheet.

References-Etelman, D. J., and Kath, R.L., 2015, An Updated Geologic Map of Sweetwater Creek StatePark, Georgia, Geological Society of America Abstracts with Programs.Harden, H. M., Kath, R.L., Crawford, T.J., 2013, Geologic Map illustrating the

Tectonostratigraphy and Structural Geology of a part of the Brevard Zone in Georgia:Results from detailed geologic mapping in the Chattahoochee River NationalRecreation Area, Sandy Springs, 7.5-Minute Quadrangle: Geological Society ofAmerica, Abstracts with Programs, vol. 45, No. 7, p. 379.

Hatcher, R.D., Jr., and Bream, B.R., 2007, New Tectonic Map of the southern and centralAppalachians (v. 4.5WIP).

Higgins, M.H., 1968, Geologic Map of the Brevard Fault Zone near Atlanta, Georgia: USGSIMAP 511.

Kath, R.L., and Crawford, T.J., 2006, New Insight into the Brevard Zone in Georgia: Resultsform Detailed Surface and Subsurface Geologic Mapping of Three Large Tunnels inthe metro-Atlanta Area: Geological Society of America Abstracts with Programs, vol.38, pp. 79

Kath, R.L., and Crawford, T.J., 2001, Detailed Geologic Mapping along the ChattahoocheeTunnel, Cobb County, Georgia: in Kath, R.L., and Crawford, T.J., eds., Across theBrevard Zone: The Geology of the Chattahoochee Interceptor Tunnel, Cobb County,Georgia: Georgia Geological Society Guidebooks, vol. 22, No. 1, pp. 25-38.

Geologic Mapping Supported by- University of West Georgia, Department of Geosciences, Center for Water Resources Geological Society of America, GeoCorps America #2013018 Petrologic Solutions, Inc., Douglasville, GA.

DESCRIPTION OF MAP UNITS

Zoned Feldspar Gneiss-epidote-muscovite-biotite-quartz-feldspar gneiss, fine- to medium-grained,with disseminated very coarse zoned feldspar crystals; very feldspathicoverall; deep weathering is characteristic. Locally contains discontinuouspods/lenses of white mylointe

OZzf

Clairmont Melange (Contorted Unit)-sphene-epidote-muscovite-biotite-quartz-feldspar gneiss, medium-grained,schistose in part; interlayered with sphene-epidote-muscovite-quartz-feldspar-biotite schist, medium- to coarse-grained; garnets may be present,but are small and scarce. Hornblende gneiss/amphibolite lenses and layers(commonly boudinaged) are common. Contains, in many places, lensesand discontinuous layers of granite on a scale of feet and ten's of feet.Vein quartz is common. Pegmatitic layers and coarse pegmatites up to 60inches thick are abundant and characteristic; shear foliation in thegneiss/schist wraps around the coarse pegmatites, which are not sheared;this rock mass is extremely contorted; foliations are quite variable overshort distances, and are generally low-angle and undulatory.

OZcm

Phyllonite, Button Schist, Mylonite, and Mylonitic Biotite Gneiss-phyllonite, button schist, mylonite, and mylonitic biotite gneiss; allinterlayered on a scale of inches, feet, and 10's of feet. The phylloniteconsists of fine recrystallized muscovite along schistosity surfaces,formed by dislocation (shearing) metamorphism. The mylonitic buttonschist is composed primarily of fine sericite, muscovite, quartz, andfeldspar; with medium- to coarse-grained muscovite forming distinctive“eyes”; there is a well-developed shear foliation. The mylonite iscomposed of sericite, quartz, and feldspar, extremely fine-grained, with apoorly developed foliation. The mylonitic biotite gneiss is composedprimarily of biotite, quartz, and feldspar, very fine-grained; with awell-developed shear foliation.

OZbs

Brevard Zone Mylonite (white)-biotite-muscovite-quartz-feldspar mylonite, very fine-grained andfeldspar-rich. This rock unit includes mylonitized granite, composed ofmuscovite, quartz, and feldspar; much of the feldspar is pink andcoarse-grained. Shearing was pervasive and produced a well-developedshear foliation. Weathering is generally moderate to deep and producesa light colored; white, cream, buff, and pink soil.

OZmw

Brevard Zone Mylonite (black)-biotite-quartz-feldspar mylonite, with quartz “eyes” and quartz-richlayers; very fine-grained, contains pods and lenses of quartz-tourmaline(?), fine-crystalline. The quartz-rich layers stand out in saproliteexposures. A shear foliation is well-developed and generally verycontorted; jointing is well-developed. Weathering is generally shallowand produces a dark brown to red soil.

OZmb

Clairmont Melange Gneiss-epidote-biotite-quartz-feldspar gneiss, medium-grained, schistose in part;locally contains hornblende gneiss/amphibolite lenses and layers.

OZcg

Brevard Zone Mylonite (black) and Button Schist-biotite-quartz-feldspar mylonite, with quartz “eyes” and quartz-richlayers; interlayered with mylonitic button schist which is composedprimarily of fine sericite, muscovite, quartz, and feldspar; with medium-to coarse-grained muscovite forming distinctive “buttons” due to S-Cmylonitic fabric.

OZmbs

Schist, Amphibolite, and Ultramafic-garnet-kyanite-feldspar-muscovite-biotite-quartz schist, fine- tocoarse-grained; very quartzose, in part. Garnet-kyanite-muscovite-biotite-quartz-feldspar gneiss, fine- to coarse-grained,schistose in part; quartzite; medium-grained and clean in part; also, fine-to coarse-grained and micaceous and/or feldspathic.Amphibolite/hornblende gneiss, fine- to medium-grained. Ultramafic-coarse-grained pyroxenes and amphibole; massive; apparently as smallovoid bodies several 10's of feet to a few 100 feet in their longestdimension.

Quartzite-fine-grained, moderately- to well-foliated, locally micaceous, locallycontains fine-grained black opaques.

OZsau

OZq

Granitic-Gneiss-biotite-quartz-feldspar gneiss, very feldspathic; quartz and feldspar aremedium- to coarse-grained; biotite is fine- to medium-grained. Muscoviteis present where this gneiss is sheared. Shear foliation is commonlydeveloped.

OZgg

Rottenwood Creek Mixed Unit-garnet (small, scarce)- biotite-muscovite-quartz-feldspar gneiss, fine- tocoarse-grained; interlayered with garnet-feldspar-biotite-muscovite-quartzschist, medium- to coarse-grained; all gradations from well-developedgneiss to well-developed schist (gneiss, schistose gneiss, gneissic schist,and schist); thin zones contain abundant garnets.

OZsg

Biotite Gneiss-garnet (small, minor)-muscovite-biotite-quartz-feldspar gneiss, fine- tomedium-grained, schistose in part; interlayered with garnet (small,minor)-biotite-feldspar-quartz-muscovite schist, medium- tocoarse-grained; some garnet-rich zones, all layereded with concordant anddiscordant pegmatite pods, lenses, and layers up to 10 feet thick; foliationwraps around pegmatite pods/lenses.

OZbg

Garnet Schist-kyanite-garnet-biotite-quartz-muscovite schist, coarse-grained, sheared,with abundant pegmatite pods and lenses; structurally overlain andunderlain by quartzite (OZq), fine- to coarse-grained in layers 1-to8-inches thick. The quartzite structurally overlying the schist appears tobe several 10's of feet thick; while the quartzite structurally underlying theschist appears to be less than 10 feet thick.

OZgs

Migmatitic Gneiss-muscovite-biotite-quartz-feldspar-gneiss, with scattered small garnets,fine- to coarse-grained; concordant pegmatites and pegmatitic zones arecommon; quartz veins are small and scattered.

OZmg

Muscovite Schist-garnet-biotite-quartz-muscovite schist, coarse-grained, with tourmalineand thickly disseminated fine black opaques; sheared, with a buttontexture.

OZsh

Amphibolite-amphibole/hornblende gneiss, thinly laminated, fine- to medium-grainedhornblende and plagioclase; and chlorite-actinolite schist, veryfine-grained; joints are close-spaced and abundant.

OZa

Pappasito Schist-garnet-biotite-muscovite-quartz-schist, coarse-grained, with thicklydisseminated fine black opaques; muscovite is generally coarser-grainedthan biotite; sheared, with a button texture. Joints are poorly developedand scarce; shallow-weathering.

OZps

Mixed Schist/Gneiss-garnet-muscovite-biotite-quartz-feldspar gneiss, fine- to medium-grained,and garnet (common to abundant) -feldspar-quartz-muscovite-biotiteschist, medium- to coarse-grained, interlayered; and all extensivelypegmatized with coarse-grained muscovite-quartz-feldspar. Micas in theschist are often coarse-grained and in part oriented across thecompositional layering creating a “tough” rock which resists breaking.This rock unit is very non-uniform. It weathers unevenly, and often innon-planar directions. Where compositional layering and foliations arewell-developed, differential weathering produces a slabby outcrop. Thethickness of weathering residuum varies from thin to thick over shortdistances; and thick residuum often contains large boulder masses ofrelatively fresh rock. Small, apparently ovoid bodies ofbiotite-quartz-feldspar granite (OZg), medium- to coarse-grained,equigranular, massive, without foliation or with a poorly developedfoliation, are scattered in this unit.

OZmu

Orange Gneiss-biotite-quartz-feldspar gneiss, fine- to medium-grained, schistose in part;biotite-hornblende-feldspar gneiss, fine- to medium-grained; andamphibolite, fine- to medium-grained; all are interlayered or intermixed.Together, an in approximately equal parts, these three lithologies make upgreater than 90 percent of this rock unit. The rock weathers to ayellow-brown, pink, or brownish gray saprolite/residuum. The remainingpart of this lithologic unit consists of small medium- to coarse-grainedmafic and ultramafic bodies, apparently ovoid and several feet to a fewhundred feet in their longest dimension, and muscovite-quartz-feldsparpegmatites as much as 8 to 12 feet thick. This rock unit weathers deeply toa soft, feldspathic residuum, except for the coarse-grained mafic andultramafic bodies and the pegmatites. The coarse-grained mafics yield ahard, tough saprolite with well-developed box work; and pegmatites arepresent in residuum as resistant boulders.

OZog

Palisades Quartzite-fine-grained, equigranular, weakly-foliated, massive, cherty, vitreous.OZpq

Gneissic Granite-muscovite-biotite-quartz-feldspar gneissic granite, feldspathic; quartz andfeldspar are medium- to coarse-grained; biotite is fine- tomedium-grained. Shear foliation is commonly developed. Locallycontains schist xenoliths.

OZgg

Long Island Creek Gneiss-sphene-epidote-biotite-quartz-feldspar gneiss, medium- to coarse-grained;very felsic; yields light-colored soil; foliation is moderatelywell-developed.

OZli

Granite (undifferentiated)-biotite-quartz-feldspar granite, medium- to coarse-grained, equigranular,massive, without foliation or with a poorly developed foliation.

OZg

Ben Hill Granite (Carboniferous)-generally massive, poorly foliated to unfoliated, light-gray, porphyritic,muscovite-biotite-quartz plagioclase-microcline granite. Commonly containszoned microcline phenocrysts as long as 5 cm. Occasionally formspavement-style outcrops; weathers to a tan-yellow saprolite with microclinephenocrysts and quartz grains, where more deeply weathered forms a light-redsoil.

Cbh

Palmetto Granite (Carboniferous)-massive, poorly foliated to unfoliated, light-gray, porphyritic,muscovite-biotite-quartz plagioclase-microcline granite. Commonly containszoned microcline phenocrysts as long as 5 cm. Tends to form pavement-styleoutcrops and large pedestal-boulder outcrops; weathers to a tan-yellow saprolitewith microcline phenocrysts and quartz grains, where more deeply weatheredforms a light-red soil. Lithologically identical to the Ben Hill Granite (Cbh).

Cp

Granites and Gneissic Granites

Austell Gneiss (Early Silurian)-medium- to coarse-grained, light- to medium-gray,±muscovite-biotite-quartz-feldspar gneissic granite with megacrysts ofmicrocline (orthoclase?). Locally contains accessory garnet, ilmenite, titanite,tourmaline, apatite and zircon. The microcline megacrysts are typically 1- to 2-mm in length, but range up to 4 mm in length. Generally forms pavement-styleoutcrops and weathers to a yellowish, sandy soil. Near the margins of the body,shearing forms distinct feldspar augens.

Sa

Brevard Zone Lithologies

Dadeville Complex Lithologies

Sand Hill Granite (Silurian?)-medium- to coarse-grained, light- to medium-gray, ±muscovite- biotite-quartz-feldspar gneissic granite with megacrysts of microcline. Themicrocline megacrysts are typically 1- to 2- mm in length, but range upto 4 mm in length. Generally forms pavement-style outcrops andweathers to a yellowish, sandy soil.

Ssh

Camp Odum Granite (Silurian?)-medium- to coarse-grained, light- to medium-gray,±muscovite-biotite-quartz-feldspar gneissic granite withmegacrysts of microcline; locally contains accessorygarnet. Generally forms pavement-style outcrops andweathers to a yellowish, sandy soil; highly shearedthroughout.

Sco

Alteration Zone Rocks-characterized by chlorite, abundant large garnets, coarse hornblende, andpyrite.

OZca

Graphitic Schist-grapite-quartz-biotite-muscovite schist, graphitic quartzite, andinterlayered non-graphitic quartz-muscovite schist; locally garnetiferous.

OZgr

Muscovite Schist with garnets-garnet-biotite-quartz-muscovite schist, quartzose schist andmeta-graywacke, with occasional interlayered feldspar-hornblendegneiss/amphibolite. Locally grades into porphyroblastic garnet schist(OZgs).

OZsg

EXPLANATION OF MAP SYMBOLS

Lithologic unit contact- Approximate location

Fault (high angle)- Approximate location

Fault (thrust)- Approximate location; teeth onupper plate.

36

f.a.

Trend and Plunge of small-scale fold axes

Strike and Dip of Foliation65

56 Map station location with associated number

36

c

Trend and Plunge of crenulations (c), quartz rods (r),minerals (m), intersections (lin), and fold generations(fi).

Fault (strike/slip)- Approximate location

Strike and Dip of Exfoliation23

Overturned, northwest verging, antiform

Overturned, northwest verging, synform

Biotite Gneiss-garnet (small, minor)-muscovite-biotite-quartz-feldspar gneiss, fine- tomedium-grained, schistose in part; interlayered with garnet (small,minor)-biotite-feldspar-quartz-muscovite schist, medium- tocoarse-grained; some garnet-rich zones, all layereded with concordant anddiscordant pegmatite pods, lenses, and layers up to 10 feet thick; foliationwraps around pegmatite pods/lenses.

OZbg

Amphibolite-amphibole/hornblende gneiss, thinly laminated, fine- to medium-grainedhornblende and plagioclase; and chlorite-actinolite schist, veryfine-grained; joints are close-spaced and abundant.

OZa

Orange Gneiss-biotite-quartz-feldspar gneiss, fine- to medium-grained;biotite-hornblende-feldspar gneiss, fine- to medium-grained; and amphibolite,fine- to medium-grained; all are interlayered or intermixed. The rock weathersto a yellow-brown, pink, or brownish gray saprolite/residuum. The remainingpart of this lithologic unit consists of small medium- to coarse-grained maficand ultramafic bodies.

OZog