Geology of the Dodgeville and Mineral Point Quadrangles Wisconsin GEOLOGICAL SURVEY BULLETIN 1123-D Prepared in cooperation with the Wisconsin Geological and Natural History Survey Geology of the Dodgeville and Mineral Point Quadrangles Wisconsin By JOHN W. ALLINGHAM GEOLOGICAL SURVEY BULLETIN 1123-D Prepared in cooperation with the Wisconsin Geological and Natural History Survey UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963 UNITED STATES DEPARTMENT OF THE INTERIOR STEW ART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington D.G., 20402 CONTENTS Page Abstract...-_--__-_________________-_______________--_-____-----__ 169 Introduction. _____________________________________________________ 171 Location and accessibility---____--_-___-__------_---_---_----__ 171. Fieldwork....___..___.___....____._____.....__.__._.._.._ 171 Acknowledgments ---____---_______________-__-----__---__--_-- 171 Previous work_________________ ______________________________ 173 Physiography-__.._._-____________.-__.________.___-__.._____-____ 173 Topography ___________________________________________________ 173 Drainage _____________________________________________________ 175 Stratigraphy----_.----_______-________________-___________________ 176 Cambrian system..____________________________________________ 177 Upper Cambrian series...__________________________________ 177 Ordovician system.____________________________________________ 178 Lower Ordovician series-_____-_-______-____-----_--__---.-_ 178 Prairie du Chien group_--__..---------------_-------_--- 178 Middle Ordovician series._________--______-____-__-___--___ 180 St. Peter sandstone.__------_____-_-____----____----_-_ 180 Platteville formation.._____--_____--__--__-____----__-- 182 Glenwood shale member_________.___________________ 184 Pecatonica dolomite member_:____-------__----____- 186 McGregor limestone member____-_------__---_--_-_- 186 Quimbys Mill member.. ___________________________ 188 Decorah formation.__________________________________ 190 Spechts Ferry shale member.___________._-____-_-._ 190 Guttenberg limestone member_--___-----------_---_- 193 Ion dolomite member._____________________________ 193 Galena dolomite.__.__-_-_--______--___-_----_-----.--_ 195 Cherty unit-..---..--------_.---------------_----- 195 Noncherty unit__________________________________ 196 Post-Galena deposits---.-------------.----------------- 197 Structure- ________________________________________________________ 199 Folds--_---------_.._--_-._-.-.._-----.-._--..-._..._.-..-..- 203 Fractures. _ ___________________________________________________ 204 Tectonics____________________________________________________ 209 Mineral deposits_______________________________ _____.____________ 210 Joint-controlled deposits__--_-__-______-_____-_-__-._-----_--- 211 Pitch-and-flat deposits._______._________'_____________-_-____--. 213 Structural and stratigraphic relations________-___---------_-- 213 Mineralogy _______________________________________________ 216 Rock alteration-_____._____________'___________________________ 219 Origin of the ores.-____________________________________________ 222 in IV CONTENTS Mineral deposits Continued - Page History and production. _______________________________________ 223 Early production._-__-_________-_^_______.________________ 223 Recent production.._______________________________________ 225 Potential production.______________________________________ 225 Guides to ore.________________________________________________ 226 Descriptions of mines._--__-____________-_--____-___-_-_-_ _____ 230 Nonmetallic deposits_____________________________________________ 233 Dolomite and limestone._-_-.___-__--___-___-_-_-___--_________ 233 Literature cited.___-_-___---___-___-__-___--_________-__-_________ 234 Index.______---__-___-________-__._..--_.-_.-_---_-----------_--- 241 PLATE '12. Geologic map of the Dodgeville quadrangle, Wis. 13. Geologic map of the Mineral Point quadrangle. 14. Plat showing trends of mineralized fracture zones and syn­ clinal and anticlinal axes in the Dodgeville and Mineral Point quadrangles. 15. Diagram showing the relation between residual, joint-con­ trolled, and pitch-and-flat deposits in the Mineral Point area. 16. Geologic map of the Richards mine, Mineral Point. 17. Geologic map of the Grayville mine, Mineral Point. 18. Geologic map of the Dodgeville No. 3 mine, Iowa County. Page FIGURE 25. Index map of Wisconsin-Illinois-Iowa zinc-lead district show­ ing the location of the Dodgeville and Mineral Point quad­ rangles. _ ________-______________-_--__--_--.«...------- 172 26. Diagrammatic section showing the undulating surface of the Shakopee dolomite.__________________---_-_--__--_--- 179 27. Glenwood shale and Pecatonica dolomite members of the Platteville formation at the Mineral Point Branch------ 181 28. Plat showing areas of anomalous blocks of sandstone and thick­ ness of the St. Peter sandstone.___.__________-____--_-- 183 29. Section showing lithology of the Glenwood shale member in the Dodgeville and Mineral Point quadrangles-------------- 185 30. MeGregor limestone member of the Platteville formation, in City quarry, Mineral Point__----------_----_--__--___- 187 31. Quimbys Mill member of the Platteville formation, Spechts Ferry member and Guttenberg member of the Decorah formation, Mineral Point-. _-_______------------------- 189 32. Sections showing thickness and lithology of the Spechts Ferry shale member..--.--._-----------------_------------- 191 33. The effect of mineralizing solutions on the thicknesses of the Guttenberg, Spechts Ferry, and Quimbys Mill members at Mineral Point.__________----_._____-.--------------- 194 34. Section at the contact between Stewartville and Dubuque members, Johnson quarry.__________--__------_.------ 198 35. Major structures in the Dodgeville and Mineral Point quad­ rangles, Wis.------_----_--___-------.--------------- 201 CONTENTS V . Page FIGURE 36. Normal fault with displacement of 22 feet on Linden road near Mineral Point- __________________________________ 206 37. Diagrams showing strikes of joints in rocks of Ordovician age in the Dodgeville and Mineral Point quadrangles.__-. 207 TABLES TABLE 1. Thickness of strata of Cambrian age in the Dodgeville and Mineral Point areas.____________--___--_-_--___-_____ 178 2. Horizons and thickness of strata used for structural control in the Dodgeville and Mineral Point quadrangles.----.--- 202 3. Thickness of strata in surface exposure and in drill holes in the Dodgeville and Mineral Point areas._______.!---_____-___ 228 I * GEOLOGY OF PARTS OF THE UPPER MISSISSIPPI VALLEY ZINC-LEAD DISTRICT GEOLOGY OF THE DODGEVILLE AND MINERAL POINT QUADRANGLES, WISCONSIN By JOHN W. ALLINGHAM ABSTRACT The Dodgeville and Mineral Point quadrangles comprise 110 square miles in the Driftless Area of southwest Wisconsin. The Galena-Platteville cuesta, the major physiographic feature, dips gently to the south. Dolomite, limestone, and sandstone of Early and Middle Ordovician age are exposed and rocks of Early Ordovician and Late Cambrian age were penetrated in deep wells. The Prairie du Chien group of Early Ordovician age is about 250 feet thick, but only the upper 1X)0 feet is exposed. Relief on the Prairie du Chien surface exceeds 100 feet and is thought to be the result of differential compaction of limy muds over rigid reef structures and solution of the car­ bonate rock in the interdome areas. The St. Peter sandstone ranges in thickness from 45 to 210 feet, the result of compensating fill on the irregular Prairie du Chien surface. The St. Peter is a crossbedded friable sandstone and an important aquifer in the mining district. The Platteville formation is 53 to 65 feet thick and comformably overlies the St. Peter sandstone. This formation is an important source of dimension stone and crushed rock. It includes in ascending order the Glenwood, Pecatonica, McGregor, and Quimbys Mill members. The Glenwood shale member consists of 0.5 to 3 feet of green arenaceous metabentonitic shale and dolomite sand lenses. The Pecatonica dolomite member consists of 19 to 22i feet of massive yellowish-brown to gray dolomite. The McGregor limestone member, uniformly about 30 feet thick, consists of 2 submembers of variable thickness, a lower unit of wavy thin-bedded light-gray fossiliferous limestone, and an upper unit of mottled buff calcareous dolomite. The Quimbys Mill member, light- to dark- brown sugary-textured magnesian limestone, thickens southward, from 10 feet at Dodgeville to 13 feet near Calamine. The Decorah formation of Middle Ordovician age is about 35 feet thick and overlies the Platteville formation. This formation is the main host rock for zinc deposits. It includes in ascending order the Spechts Ferry, Guttenberg, and Ion members. The Spechts Ferry shale member consists of about 0.8 foot of green shale, shaly dolomite that contains phosphatic nodules, and a light- buff metabentonite. The Guttenberg limestone member consists of about 11 feet of thin-bedded nodular light-brown coquinoid limestone. The Ion dolomite 169 170 GEOLOGY OF UPPER MISSISSIPPI VALLEY ZINC-LEAD DISTRICT member is subdivided into blue beds of dark bluish-gray limestone and gray beds of greenish-gray argillaceous dolomitic limestone. Total thickness of the Ion member is 20 feet. The Galena dolomite of Middle Ordovician age has a total thickness of about 225 feet. In the mining district, two units