U.S. DEPARTMENT OF THE INTERIOR Prepared in cooperation with the SCIENTIFIC INVESTIGATIONS MAP 2777 U.S. GEOLOGICAL SURVEY SOUTH DAKOTA SCHOOL OF MINES AND TECHNOLOGY FOUNDATION SHEET 2 OF 2 Pamphlet accompanies map

104°00' 103°30' 103°00' 104°00' 103°30' 103°00' ° ° EXPLANATION FOR MAPS F TO H 44 30' 44°30' EXPLANATION 44 30' 44°30' EXPLANATION Spearfish Geologic features 53 54 Tertiary igneous rocks (Tertiary and post-Tertiary Spearfish PHANEROZOIC ROCKS 90 1 90 sedimentary rocks not shown) Pringle fault 59 Tertiary igneous rocks (Tertiary and post-Tertiary Pre-Tertiary and Cretaceous (post-Inyan Kara sedimentary rocks not shown) Monocline—BHM, Black Hills monocline; FPM, Fanny Peak monocline 52 85 Group) rocks 85 Sturgis Sturgis Pre-Tertiary and Cretaceous (post-Inyan Kara A Proposed western limit of Early Proterozoic rocks in subsurface 55 Lower Cretaceous (Inyan Kara Group), Jurassic, Group) rocks 57 58 60 14 and Triassic rocks 14 Lower Cretaceous (Inyan Kara Group), Jurassic, B Northern extension (fault?) of Fanny Peak monocline and Triassic rocks Paleozoic rocks C Possible eastern limit of Early Proterozoic rocks in subsurface 50 Paleozoic rocks Precambrian rocks S Possible suture in subsurface separating different tectonic terranes 89 51 89 2 PRECAMBRIAN ROCKS of Sims (1995) 49 Contact St 3 G Harney Peak Granite (unit Xh) Geographic features—BL, Bear Lodge Mountains; BM, Bear Mountain; Fault—Dashed where approximately located G DT DT, Devils Tower 48 B Early Proterozoic rocks, undivided Anticline—Showing trace of axial surface and 1 St Towns and cities—B, Belle Fourche; C, Custer; E, Edgemont; HS, Hot direction of plunge. Dashed where G Mica schist and volcaniclastic rocks (units Xts, 2 Nc Springs; L, Lead; N, Nemo; Nc, Newcastle; P, Pringle; RC, Rapid approximately located Xtv) 47 City; Sp, Spearfish; St, Sturgis; S, Sundance; T, Tinton Syncline—Showing trace of axial surface and Metabasalt and metagabbro (units Xbo, Xby, 4 direction of plunge. Dashed where Xgb, Xgby, Xgbo, Xbd) 56 approximately located Late Archean granite and metamorphic rocks G 105° 104° 103° Monocline—Both axes shown. Short arrow indicates (units Wgr, Wos) ° 46 B 45 EXPLANATION 90 steep limb 90 5 Contact Xu Aeromagnetic field intensity, 85 85 in nanoteslas (nT)— Dome—Asymmetry indicated by length of arrow 3 Fault—Dashed where approximately located or underlain by 425 4 Contour interval 50 nT 7 inferred; dotted where concealed Wu Wu? x Minor dome—Related to possible conversion of 375 385 385 Fold of indeterminate age—May be F1 or F2. Wu 6 anhydrite to gypsum in underlying rocks 5 Showing generalized trace of axial surface B C 325 DT Form line—Shows schistosity in Precambrian rocks Overturned anticline 275 44 Inverted anticline Major Phanerozoic structure 6 BL Sp 225 1. Bear Butte dome 6 Overturned syncline S L St 175 2. Tilford dome 9 Stratigraphic younging unknown—Based on T 125 8 3. Anticline at Cottonwood Draw B G photogeology. Folds are isoclinal and typically 90 4. Big Elk anticline 90 Wyoming Wu 75 B overturned to northwest. Restricted to quartzite A 5. White Gate monocline G ? and sillimanite schist (unit Xqs) east and Province (Wu) C 25 10 6. Stagebarn monocline N 44 44 southeast of Custer 7. Piedmont anticline RC -25 43 Rapid City Rapid City Xu 8. Boxelder Creek anticline 7 F4 fold—Elongate dome related to emplacement of -75 9. Cement Plant anticlines 4 Harney Peak Granite. Dotted where concealed 44° underlain by 10. Rapid City monocline Wu -125 F3 fold—Showing generalized trace of axial surface 11. Victoria Creek fault B Nc C G -175 26 27 B F fold—Showing generalized trace of axial surface. Wu 11 12. Moon Meadows anticline 8 G 2 BM 45 13. Stratosphere Bowl fault 28 Queried where inferred Wu -225 12 14. Rockerville Gulch anticline ? Major antiform C -275 44°00' 44°00' 15. Murphy anticline 44°00' 44°00' -325 16. Hayward fault ? 29 9 Overturned anticline ? 16 17. Hayward anticline 16 P -375 18. Lower Spring Creek syncline G 10 Major synform Hill ? St 14 19. Charles Draw anticline Hill -425 City 13 Overturned syncline ? 20. Coolidge Creek anticline 21 City 11 G S HS Xu Early Proterozoic rocks 21. Spokane fault St Minor antiform or synform—Shown only in 24 22. Anticline at L7 Draw representative areas to indicate deformation Wu Late Archean rocks E 23. Red Hill anticline St 25 pattern Xu C 15 24. Fairburn anticline K 20 St F fold—Showing generalized trace of axial surface. underlain by Xu 244 244 S 1 25. Dow Ridge anticline Queried where inferred Wu underlain by Wu 0102030 KILOMETERS 19 23 26. Martin Valley anticline Wyoming 18 22 Anticline Wu 16 27. Buffalo Gap fault South Dakota 051015 MILES 18 12 28. Buffalo Gap monocline Overturned anticline 43° 17 29. Cold Brook anticline 16A 17 16A Syncline x 19 30. Dome at Loring Siding S 31. Hawkright Creek anticline sS Overturned syncline MAP F. AEROMAGNETIC MAP OF THE 32. Barker anticline 36 36 42 33. Pass Creek anticline Major Precambrian structure BLACK HILLS REGION, SOUTH DAKOTA AND WYOMING 16 16 34. Pass Creek fault 16 1. Poorman anticline Custer 20 sS 21 35. Hells Canyon anticline 2. Lead anticline Custer S 36. Shenk Canyon anticline 3. Benchmark fault 16A 13 16A ° ° ° 37. Pleasant Valley fault St 4. Pactola Lake fault 105 104 103 S 45° 22 38. Fourmile fault 5. Nemo fault EXPLANATION 41 6. Estes fault 39. Jewel Cave fault Complete Bouguer gravity field, 79 40. Teepee Canyon anticline 15 79 7. Pactola Dam fault 40 in milligals (mGals)— 39 8. Silver City fault -62.5 16 41. Antelope Ridge anticline 16 Contour interval 5 mGals 42 9. Rockerville syncline Wu . Dead Horse Flat anticline Wu? -67.5 10 Xu 43. Crooks Tower anticline . Boulder Hill synform B x 44. Deerfield syncline 11. Keystone fault B -72.5 x 23 DT 12. Harney Peak dome 385 45. Axis of central Black Hills dome 385 C -77.5 13. Stockade Lake fault x 38 46. Hellsgate syncline BL Sp -82.5 x 47. Little Spearfish Creek dome 14. Pringle fault x 24 48. Fanny Peak monocline 15. Western Star syncline x S L St -87.5 x 25 49. Edge of Tinton dome 16. Atlantic Hill anticline x 37 xx 50. Tollgate dome 17. Grand Junction fault T -92.5 x x 36 x 51. Lead dome 18. St. Elmo fault Wu -97.5 x x x x x 52. Citadel Rock dome 19. Bear Mountain dome C -102.5 x 53. Crow Peak dome 20. Thompson Draw synform N x x 21. Union Hill anticline 35 x x 54. Spearfish monoclines RC -107.5 30 55. Green Mountain dome 22. Burnt Fork fault BHM -112.5 33 x xx 56. Brownsville dome 23. Humbolt Mountain fault Xu x 34 xx 44° x x 26 57. Whitewood Peak dome 24. Bishop Mountain syncline -117.5 27 14 xx 58. Crook Mountain dome 25. Summit Peak anticline Nc x -122.5 x xxx 59. Elkhorn Mountain dome 26. Slate Prairie syncline BM x 29 28 27. Rochford antiform Wu -127.5 32 60. Whitewood anticline 28. Lowden Mountain synform C 31 C -132.5 29. Empire fault 43°30' 43°30' 43°30' 43°30' ° ° ° ° ° ° -137.5 104 00' 103 30' 103 00' 104 00' 103 30' 103 00' B Metamorphic isograd—First appearance of index ? G P -142.5 0 10 20 KILOMETERS 0 10 20 KILOMETERS mineral noted on side of isograd. B, biotite; G, garnet; K, kyanite; S, sillimanite; sS, second -147.5 ? sillimanite; , staurolite. Dotted through St HS outcrops of Harney Peak Granite Xu Early Proterozoic rocks 0 5 10 MILES 0 5 10 MILES C Late Archean rocks Line indicating outer limit of small pegmatite E Wu bodies related to Harney Peak Granite

Wu FPM Xu 0102030 KILOMETERS

MAP B. PHANEROZOIC STRUCTURES OF THE CENTRAL BLACK HILLS MAP C. PRECAMBRIAN STRUCTURES OF THE CENTRAL BLACK HILLS Wyoming

South Dakota 051015 MILES 43° MAP G. COMPLETE BOUGUER GRAVITY MAP OF THE

104°00' 103°30' 103°00' 104°00' 103°30' 103°00' BLACK HILLS REGION, SOUTH DAKOTA AND WYOMING 44°30' 44°30' 44°30' 44°30' EXPLANATION Spearfish Spearfish EXPLANATION 90 PRECAMBRIAN ROCKS 90 105° 104° 103° PHANEROZOIC ROCKS Harney Peak Granite (unit Xh) 45° EXPLANATION

85 Tertiary igneous rocks 85 Muscovite schist (units Xts and Xtv) Xu Early Proterozoic rocks Sturgis Sturgis Xu PRECAMBRIAN ROCKS Metabasalt and metagabbro (units Xgb, Wyoming underlain by Wu Late Archean rocks 14 14 Xgby, Xgbo, Xbo, Xby, and Xbd) Province (Wu) Wu Wu? Harney Peak Granite (unit Xh) Crystalline rocks in the All other stratified metamorphic rocks B central Black Hills Muscovite schist (units Xts and Xtv) DT B Archean granite and iron-formation C 89 Metabasalt and metagabbro (units Xgb, 89 Xgby, Xgbo, Xbo, Xby, and Xbd) BL Sp All other stratified metamorphic rocks Contact S T L St All Archean rocks Fault—Dashed where approximately located; dotted where concealed Wu A C Contact—Dashed where interpreted N contacts of units Xts, Xtv, and Xby Complete Bouguer gravity field, in RC shown beneath Phanerozoic rocks in -62.5 milligals (mGal)—Contour interval BHM ? Xu ? Rochford and Bear Mountain areas -65.0 2.5 mGal 44° -67.5 underlain by 90 Fault—Dashed where approximately 90 -70.0 Nc Wu located; dotted where concealed -72.5 BM 85 85 Wu -85.0 -77.5 C C Aeromagnetic field intensity, in -80.0 385 385 425 nanoteslas (nT)—Contour interval -82.5 ? 400 -85.0 25 nT P 375 -87.5 350 -90.0 Xu ? 325 -92.5 S HS 300 -95.0 underlain by -97.5 Wu 275 C 250 -100.0 E 90 225 90 -102.5 -105.0 Xu 200 Wu 0102030 KILOMETERS 175 -107.5 underlain by 44 44 FPM 150 -110.0 Wyoming

South Dakota Wu Rapid City 125 Rapid City -112.5 051015 MILES 100 -115.0 43° -117.5 75 -120.0 50 -122.5 25 MAP H. SUMMARY MAP SHOWING GEOLOGIC -125.0 0 -127.5 -25 STRUCTURES INFERRED FROM GEOPHYSICAL DATA, -130.0 ° ° -50 ° ° 44 00' 44 00' 44 00' 44 00' -132.5 ? -75 BLACK HILLS REGION, SOUTH DAKOTA AND WYOMING -135.0 -100 16 -137.5 16 -125 -140.0 Hill -150 Hill -142.5 City -175 City -145.0 -200 -147.5 -225 -250 -275 244 -300 244 -325 -350 -375 -400 -425 16A 16A

36 36

16 16 Custer Custer ENLARGEMENT

16A 16A

79 79 16 16

385 385

43°30' 43°30' 43°30' 43°30' 104°00' 103°30' 103°00' 104°00' 103°30' 103°00' 0 10 20 KILOMETERS 0 10 20 KILOMETERS

0 5 10 MILES 0 5 10 MILES

MAP D. AEROMAGNETIC MAP OF THE CENTRAL BLACK HILLS MAP E. COMPLETE BOUGUER GRAVITY MAP OF THE CENTRAL BLACK HILLS

A A′ METERS METERS U.S. Highway 85 Spearfish Creek E. Spearfish Creek Pillar Peak

ၧ U.S. Highway 385 Whitewood Creek P m MDpe Poorman anticline 2,000 MDpe MDpe Tp Oww Tr 2,000 0 5 CENTIMETERS Tp ၤ Dome Mountain Tr Tr Oၤd O d Tu Boulder Creek

Td BEND IN Xby Tr Tt Tt SECTION Oww Td Oၤd Mp Whitewood anticline

ၤ Oww Xs U.S. Highway 14A O d ? ? ? ? Tql Interstate 90 1,500 ? Xqg Xtv Tql 1,500

Xqg Bear Butte dome Xu Xu ? ? Pၧm ၧ U.S. Highway 79 Xbo ? Xu Pmo ၣ P m Pmo E. Spring Creek Xqg Xtv? ? ? Ps Kl Jmu Large sieve-like microcline metacrysts (MM) indicate potassium metasomatism of metagraywacke near bodies ? ? Xtv? Xqg? Xu ၣ Jsg Jmu Km Kf Jmu ? Kl Kf Ks Km Kf Km of Harney Peak Granite. The metacrysts develop in quartz-feldspar-biotite (QFB) relict graywacke and are re- 1,000 ? Xs ? MDpe Kb Ks Kl Jsg ၣPs Jsg Kl Ks Kb KcKn Kp 1,000 Xif Xh? Xu Xu placed by sillimanite (S) nodules (var. fibrolite) and microcline perthite (MP) plus quartz (Q) “granitic” segrega- ? ? ? ?? tions. A 1-cm oval mass of coarse-grained muscovite (M) probably represents an earlier metamorphic mineral Xst Oၤd Oww ? Kg ? ? ?? (andalusite?). Sample from about 7 km northeast of Custer above the high-sillimanite isograd. 500 ? ? Xu 500 ? ? MDpe ? ? ? Tr Oၤd ? ? SEA LEVEL Xif ? SEA LEVEL ? Oww ? ? Oww MDpe Oၤd -500 ? ? ? ? -500 Xu ? ?

Xbs1 Xbo ? -1,000 ?? ? -1,000 Quaternary units not shown. VERTICAL EXAGGERATION X2 Unit Knc too thin to show

B B' METERS METERS

2,000

anticline 2,000 State Highway 44 Norris Peak anticline anticline Boxelder Creek Nemo Road Cement Plant Cement Plant Xqs Oၤd State Highway 79 1,500 Tw Xmt MDpe Tw 1,500 Interstate 90 Boxelder Creek ၧ U.S. Highway 16 ? P m Kf Xeq Pmo Jmu Kl Ks Km ၣPs Jsg Kb Kg 1,000 Kc Kn Kp Oၤd ? ? 1,000 ? MDpe Pၧm Xu

500 ၤ ? O d 500

?

Xu SEA LEVEL ? ? MDpe SEA LEVEL

ၤ -500 O d -500 ၤ Xu O d

Quaternary units not shown. VERTICAL EXAGGERATION X2 Units Knc and Tw too thin to show

Bear Mountain dome Xgw 3 ? Gra ? nd C ? Jun C' Xbs2 ct Xts ion METERS Xsic f au METERS Xds Xgbo lt MDpe Oၤd MDpe Xbo Xds 2,000 Deerfield Lake Xgbo Wos Xsic

Xs BEND IN SECTION N. Fork Castle Creek N. Fork

Slate Prairie syncline 2,000 Xtv Xs Xs Xgw Xts ? 3 Xbs ? Xbs 2 Xsic 2 ? ? 1,500 ? ? ? ? ? 1,500

? ? ? Wgr ? ? Xeq ? ? ? ? 1,000 ? Xeq ? 1,000 ? Wgr Xif ? ? Xsic ? Wgr Xgbo ? ? ? ? ? ? 500 Xbo ? 500 ? ? Xby ? ? 0 5 10 CENTIMETERS ? ? ? SEA LEVEL ? ? ? Aerial view of Harney Peak on skyline (elev. 2,207 m) showing numerous large sill-like exposures of Harney Peak Granite dipping to Spodumene-quartz core of Etta pegmatite mine near Keystone. Partial cast of a large elon- ? Wos SEA LEVEL ? the left. View is mainly of southern (foreground) and western part of dome. gate spodumene crystal is visible sloping away from man's feet. Its original length was in ex- Wos ? ? ? cess of 8 m. Crystals and casts of other spodumene crystals are also visible. Etta pegmatite ? Bedded pyrrhotite-arsenopyrite gold ore from the Homestake Formation, Homestake Mine, Lead, S. Dak. ? ? ? Ghost beds of pyrrhotite inclusions preserved in arsenopyrite crystals indicate extensive areas of pyrrhotite is a pipelike body that was prospected for tin in late 1800s and produced spodumene and -500 -500 ? ? mobilization in fold nose and right limb. Similar pyrrhotite ghosts indicate renewed growth of arsenopyr- other minerals during first half of 20th century. The Etta is one of hundreds of zoned peg- ? ? ? matites peripheral to the Harney Peak Granite. Photograph taken in 1975. ? ? ? ite following pyrrhotite mobilization into discordant “veins” in left limb of fold. Discordant quartz and car- ? ? Xh? bonate (lower left), to left of recrystallized chert in fold core, also indicate mobilization but probably at a -1,000 ? -1,000 ? ? somewhat later time. Xbs ? 2 Xts ? ? ? ?

? ? ? ?

? ? ? ? ? Quaternary units not shown VERTICAL EXAGGERATION X2

D fault D' pegmatite

METERS Outer limit of METERS syncline Grand Junction State Highway 87 Lightning Creek French Creek U.S. Highway 16 Harney Peak dome St. Elmo fault(?) anticline Western Star State Highway 89 Iron Creek Xby Atlantic Hill Xts 2000 Xgby U.S. Highway 16A 2000 fault zone MDpe Oၤd Xts Xgw Xh Xbo Jewel Cave Xgwd U.S. Highway 16 Xgwp Xh Xts Xgw 1 Xbs1

Xgwd 2 Silver City fault

ၧ State Highway 40 Boulder Hill synform Keystone fault ၧ P m Xgw2 Xgw1 Xeq Battle Creek Pၧm P m Xh Murphy anticline Pmo ၤ ? ? Xif ၣ Pmo O d ? Xgw2 Xh Xif 1500 ၣPs Ps Xh ? ? Xgw Xqc 1500 Xqc Tw State Highway 79 ? 2 Xbs2 Xif Oၤd ? ? ? Xh ? Xgw MDpe Tw Rapid Creek ? 2 ၧ Jmu Rapid Creek ? P mၣPs Pmo ၣ Jsg ? Ps Xh ? ? Xu Kl Kf Tw ? ? ? Xqc Ks 1000 Xbo ? Wu? Km Kb Kg Kc 1000 ? ?

ၤ ? ? Kp ? Kn

O d ? ? Xu ? ? ? Xh ?

Xbs ? Xgw Xu 500 2 ? 1 ? 500 ? ? Xbs Xqc 2 ? ? Xh Xu SEA LEVEL ? Xgw1 SEA LEVEL ? ? Xu ? ? ?

? Xu Xqc ? ? ?

? Xu ?

-500 Wu ? -500

? ? ? Xh? Wu? ? ? ? Xu -1,000 Xu? -1,000 ? ?

? -1,500 -1,500

-2,000 -2,000 Quaternary units not shown. VERTICAL EXAGGERATION X2 Unit Tw too thin to show

MAPS SHOWING GEOLOGY, STRUCTURE, AND GEOPHYSICS OF THE CENTRAL BLACK HILLS, SOUTH DAKOTA By Jack A. Redden1 and Ed DeWitt2

Including contributions by Any use of trade names is for descriptive purposes only 1South Dakota School of Mines and Technology and does not imply endorsement by the U.S. Government 3 2 4 2 4 2 2U.S. Geological Survey James Berry , C. Gilbert Bowles , W.A. Braddock , C. Mark Cattermole , J.C. Harkeson , Robert Kucks , For sale by U.S. Geological Survey Information Services 3 Box 25286, Federal Center, Denver, CO 80225 Homestake Mining Company, Lead, S. Dak. 1-888-ASK-USGS 4 Alvis L. Lisenbee1, James J. Norton2, Zell Peterman2, J.C. Ratté2, and R.E. Zartman5 Deceased ArcInfo coverages and a PDF for this map are available at 5 Printed on recycled paper Max Planck Institute for Chemistry, Mainz, Germany 2008 http://pubs.usgs.gov