LATE PALEOZOIC ZONES OF FAULTING AND CRYPTOVOLCANIC OR METEORITE IMPACT STRUCTURES 



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LAKE SUPERIOR FAULT ZONE 



The Tectonic Map of the United States shows a group of long and 

 subparallel faults extending from the Lake Superior region southwest- 

 ward into Wisconsin and Minnesota. The Keweenawan fault is probably 

 the best known. It runs lengthwise and approximately in the center of 

 the Keweenawan peninsula of Michigan and separates the copper-bearing 

 Keweenawan volcanic series from the Cambrian (?) sandstones. The fault 

 is clearly a thrust in one exposure near Houghton, but probably a fairly 

 high-angle one, with the Keweenawan series displaced southwestward 

 over the Cambrian (?) sandstone. 



North of the Keweenawan fault, the volcanic series is downfolded into 

 a broad syncline with dips on the southeast flank of about 30 degrees. 

 The beds rise and crop out on Isle Royal in Lake Superior. A fault which 

 cuts the north flank of the syncline has been postulated just north of Isle 

 Royal. This northern fault has been connected with the Douglas fault, 

 which runs almost east-west south of Superior, Wisconsin, and which, 

 according to Thwaites (1912, 1935) dips 38 to 60 degrees southward. He 

 believes the south block has been thrust northward 6 to 12 miles. The 

 Douglas fault swings southward after entering Minnesota, and there it 

 has been studied by geophysical means. Near Pine City, the fault is 

 believed to be nearly vertical, with the east side upthrown about 9000 

 feet (Welch, 1941). 



The great syncline between the oppositely dipping Keweenawan and 

 Douglas faults is thought by Thwaites to contain numerous minor folds 

 in Wisconsin and hence to be a synclinorium. The structure is discussed 

 in Chapter 4 and illustrated in Figs. 4.3 and 4.7. He also states that part 

 of the displacement could have occurred in late Keweenawan time, but 

 that part of it probably occurred later. The complementary relation of 

 the Keweenawan and Douglas faults suggests they are of the same age. 

 The syncline in the Keweenawan peninsula region appears to have sub- 

 sided partly at the time the volcanic flows and conglomerates were ac- 

 cumulating, according to Broderick (personal communication), but 

 considerable faulting undoubtedly occurred later. Thwaites ( 1943 ) agrees 

 in substance with this view. 



A fault along the north coast of Lake Superior has been surmised, 

 chiefly on physiographic evidence (Martin, 1908), but this is not sup- 

 ported by gravity surveys. 



Ten miles southeast of the Keweenawan fault in Michigan, two hills, 

 Limestone Mountain and Sherman Hill, are made up of a basal sandstone 

 and overlying dolomites and limestones. The sandstone, according to 

 Case and Robinson (1915) is either Cambrian or Lower Ordovician, and 

 the limestones and dolomites are Ordovician, Silurian, and Devonian. 

 According to Thwaites (1943) the sandstone is Upper Keweenawan, and 

 the dolomites and limestones are Trenton-Black River. The strata are 

 cut by small faults and, along the east side, exhibit dips up to 55 degrees. 

 A major fault may exist along the east margin, and the high dips may be 

 drag along the fault which would be approximately parallel with the 

 Keweenawan. The Ordovician beds in Limestone Mountain are 80 miles 

 from the nearest Ordovician outcrops; and the Devonian, if present, 150 

 miles from the nearest Devonian outcrops. 



Dating the faults in Limestone Mountain and Sherman Hill is difficult 

 because of lack of agreement on the age of the sandstones associated with 

 them (Cambrian or Precambrian ) , and the extensive swamp and drift 

 cover that prevents working out the geologic relations. Opinion seems 

 to favor an early episode of subsidence in which the Keweenawan basins 

 were formed, and a later episode of faulting in which the Paleozoic rocks 

 were affected. 



The disposition of an immense amount of material that came from 

 the truncation of thousands of feet of Keweenawan strata along the 

 Douglas and Keweenawan faults poses another problem. If most of the 

 movement were Precambrian, representative deposits possibly should 

 occur in the Cambrian, but the orogenic waste products do not seem 

 to make up any of the Paleozoic rocks nearby in Wisconsin or Michi- 

 gan. 



If all but a small part of the faulting were Precambrian and associated 

 with the downfaulting of a basin in which the Keweenawan series ac- 

 cumulated, and if the Keweenawan series is 1100 m.y. old as recounted 

 in Chapter 4, then during the next 500 m.y. before the basal Cambrian 

 sands were spread across the region, all relief could have disappeared. 



