The Geology of Rabun and Habersham Counties, Georgia
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THE GEOLOGY OF RABUN AND HABERSHAM COUNTIES, GEORGIA BY ROBERT D. HATCHER, JR. THE GEOLOGICAL SURVEY OF GEORGIA DEPARTMENT OF MINES, MINING AND GEOLOGY ' Jesse H. Auvil, Jr. State Geologist and Director ATLANTA 1971 For convenience in selecting our reports from your bookshelves, they will be color-keyed across the spine by subject as follows: Red Valley & Ridge mapping and structural geology Dk. Purple Piedmont & Blue Ridge mapping and struc- tural geology Maroon Coastal Plain mapping and stratigraphy Lt. Green Paleontology Lt. Blue Coastal Zone studies Dk. Green Geochemical and Geophysical studies Dk. Blue Hydrology Olive Economic geology Mining directory Yellow Environmental studies Engineering studies Dk. Orange Bibliographies and lists of publications Brown Petroleum and natural gas Black Field trip guidebooks. Colors have been selected at random, and will be aug mented as new subjects are published. THE GEOLOGY OF RABUN AND HABERSHAM COUNTIES, GEORGIA BY ROBERT D. HATCHER, JR. THE GEOLOGICAL SURVEY OF GEORGIA DEPARTMENT OF MINES, MINING AND GEOLOGY Jesse H. Auvil, Jr. State Geologist and Director ATLANTA 1971 BUIIITIN 83 Georgia Department· of Mines, Mining and Geology 19 Hunter Street, S.W. Atlanta, Georgia 30334 404/656-3214 Jesse H. Auvil, Jr. State Geologist and Director January 13, 1971 His Excellency, James E. Carter Governor of Georgia and Commissioner Ex-Officio State Division of Conservation Atlanta, Georgia Dear Governor Carter: I have the honor to submit herewith Georgia Geological Survey Bulletin 83, "The Geology of Rabun and Habersham Counties, Georgia," by Dr. Robert D. Hatcher, Jr. This report was supported under a contractual arrangement between Dr. Hatcher and the Georgia Department of Mines, Mining and Geology during the summer of 1970. This excellent report, a reconnaissance investigation of the bed rock of Rabun and Habersham Counties, is a I~ajor contribution to the geology of the region. It is particularily involved with the structure of the rocks in this area. This bulletin will be extremely useful in the preparation of the new geologic map of Georgia. Very respectfully yours, ks~:.::~: ~ Jt State Geologist and Director JHA:ldj Georgia Department of Mines, Mining and Geology 19 Hunter Street, S.W. Atlanta, Georgia 30334 404/656-3214 Jesse H. Auvil, Jr. State Geologist and Director January 13, 1971 His Excellency, ,James E. Carter Governor of Georgia and Commissioner Ex-Officio State Division of Conservation Atlanta, Georgia Dear Governor Carter: I have the honor to submit herewith Georgia Geological Survey Bulletin 83, "The Geology of Rabun and Habersham Counties, Georgia," by Dr. Robert D. Hatcher, Jr. This report was supported under a contractual arrangement between Dr. Hatcher and the Georgia Department of Mines, Mining and Geology during the summer of 1970. This excellent report, a reconnaissance investigation of the bed rock of Rabun and Habersham Counties, is a r:mjor contribution to the geology of the region. It is particularity involved with the structure of the rocks in this area. This bulletin will be extremely useful in the preparation of the new geologic map of Georgia. Very respectfully yours, t~~ 4--.:J, Jt State Geologist and Director JHA:ldj .CONTENTS j '"-> ·-' Page Abstract 1 fu t~6d ucti on --------------~--------------------------------------------------------- 2 .- Acknowledgements ------------------------------------~----------------------- 5 Rocks Units ___ .:_:_:·:_·_: __ ·_: _______ : _____ ~-------~-:--~-~~---~.::~~----"----~~-----::-----~· 5 ,, Introductl.on --------------------------------------------------~:-----------~---- · · 5 . Inner Piedmont Rocks: Rocks of Unknow~ Age .. ~-------·~c~-:~~~, -·' -· 8 · Gneiss-Schi~t.-A.ffi phibolite · (i pgsa) --~~~~------~~~------~---------- · 8 Gr~~itic Gneiss (i pgg) --~-----------------------~------~-----~-----C---:~~- .· -8 Age R€dations __ .::: ______________ : ______ ::~--------------------"-":"~-----~---- 8 . : Earlier Precambrian (?) RockS-Basement Rocks -------------"- Migmatitic ·Biotite- Gneiss (PCbgm) and Biotite Gneiss· (p c b g) ------------------------------------------------------------- --- 8 Granitic Gneiss (PCgg) ----------------------------------------------- 9 Later Pre cam brian Rocks ----------------------------------------------- 9 Hornblende Gneiss an<;l. Amphibolite (PChg) --------------- 9 Tallulah Falls Formation (PCt) ---------------------------------- 9 Graywacke-Schist-Amphibolite Member (PCtl) ----------- 11 Garnet-Aluminous Schist Member (PCtp) ------------------- 11 Graywacke-Schist Member (PCtg) ------------------------------- 12 Quartzite-Schist Member (PCtq) --~-------------------------------- 12 Cambrian-Precambrian (?) Rocks-Brevard Zone and Low Rank Belt ------------------------------------------------------- 12 Brevard Mylonite Zone (cz) ---------------------------------------- 12 Chauga River Formation -------------------------------------------- 15 Lower Brevard Phyllite Member (bp) ---------------------------- 15 Carbonate Member (ere) -------------------------------------------- 16 Upper Brevard Phyllite Member (bp) -;-------------------------- 16 Poor Mountain Formation -------------------------------------------- 16 Brevard-Poor Mountain Transitional Member (bpm) ---- 16 Later Paleozoic if:ntrusives (Post Metamorphic) ---------------- 16 Granitic"Rocks (PZg) --------------------------------------------------- 16 Ultramafic Rocks (PZum) -------------------------------------------- 17 Triassic (?) Rocks ------------------------------------------------------------ 17 Diabase (TRd) ------------------------------------------------------------ 17 Structural Data -------------------------------------------------------------------- 17 Mesoscopic Structural Elements ---------------------------------------- 21 Metamorphism --------------------------------------------------------------- 31 Stratigraphic and Structural Interpretation ------------------------ 3 3 Stratigraphic, Petrologic and Age Relationships ---------------- 33 Early Precambrian-Later Precambrian Nonconformity 40 Speculation Regarding the Structure of the Tallulah Falls Dome ---------------------------------------------------------------- 41 CONTENTS-continued Page Previous Interpreta ti ons ------------------------------------------------ 41 Nappe Hypo thesis -------------------------------------------------------- 41 Brevard Zone Structure ---------------------------------------------------- 42 Inner Piedmont Structure ------------------------------------------------ 43 Proposed Chronology of Events for Rabun and Habersham Co unties ---------------------------------------------------------------- 43 Summary of Major Conclusions -------------------------------------------- 44 References ---------------------------------------------------------------------------- 45 Figure ILLUSTRATIONS Page 1. Map of Rabun and Habersahm Counties, Georgia, showing the distribution of regional geologic pro vinces 3 2. View of the southeast side of Rabun Bald and the Blue 'Ridge from the Dahlonega Plateau in South Carolina 4 3A. Migmatitic Biotite Gneiss unit at Woodall Shoals on the Chattooga River (Georgia-South Carolina state line). Lenses of amphibolite are present in the gneiss here 6 3B. Closer view of the Migmatitic Biotite Gneiss at the same locality 7 4. Typical fresh exposure of amphibolite of the Horn blende Gneiss and Amphibolite unit on the north west side of the map area. Exposures located on U. S. 76 west of Clayton 10 5. Thick beds of quartzite of the Quartzite-Schist Member of the Tallulah Falls Formation exposed on U. S .. 441 south of Clayton near the Rabun- Habersham County line. 13 6. Upright graded bed of congolmetatic quartzite near the base of the Quartzite-Schist Member of the Tallulah Falls Formation. Note the extreme flat tening and elongation of pebbles. Located on U. S. 441 just east of Wiley 14 7. Exposure of a partially weathered surface of dunite in the dunite body on U. S. 76 west of Clayton 18 SA.· Triassic (?) diabase dike intruding amphibolite and hornblende gneiss on State Highway 197 near Lake Burton. Dike is about 15 feet thick 19 8B. Xenoliths of hornblende gneiss along the margin of a smaller diabase dike (about 10 feet thick) at the same locality as 8A 20 Figure 9. Contoured equal area projections of poles . to - folia:tion and compositional layerfng for structUr-al -· '5ubareas (mimbered with Romas numerals) in Rabun and Habersham Counties. Contours '(to upperright of each diagram) are in per cent per 1 . pet cent area. Number to the lower' ieft of each diagram is the number' of poles plotted'from ~hat subarea 10. ·synoptic diagram of 1737 poies to foliation· and compositional layering from the Blue Ridge of Rabun ai:id Habersham Counties, Contours 9%, 7%, 5%, 3%, 1%, and 0.1% per 1% area. - 22 11. -Point' diagram of 16 crystailizationlineations from the Blue Ridge of Rabun aiJ-d Haben:ham- Counties. 23 12. --Mineral streaking lineation exposed cin foliations - surfaces Unit 2- at Woodall Shoals on the- Chattooga - Rive':i: on the Georgia-South Carolina s~ate line. This is thought to be an a lineation related: to _movement within the Blue Ridge Thrust Sheet. Its strike is N 4-9° W and it plunges 22° southeast:· _ 24 ~ . ,-.· 13. Synoptic diagram of 84 fold axes from the- Blue Ridge of Rabun and Habersham Counties. Con tours 9%, 7%, 5%, 3%, and 1% per 1% area. 25 14. Flexural-slip fold in Migmatitic Biotite Gneiss at Woodall Shoals on the Chattooga River. 26 - 15A. Passive flow folds in