374 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



geophysical prospecting, is somewhat west of the geographical center. 



The folds of the Rig Horn Rasin, according to Fanshawe ( 1947 ) , are 

 due to an interplay of two forces. The Precambrian basement was faulted 

 as it adjusted to Laramide mountain building on both sides, and the 

 Paleozoic and Mesozoic strata flexed over the fault scarps. Also, as the 

 sides of the basins were upturned, the upper beds of the basin were 

 crowded and buckles developed. 



Map, Fig. 22.4, shows the Rig Horn Rasin to have come into existence 

 in Montana time, and Van Houten (1952) notes that Precambrian rock 

 had been exposed in places in the surrounding ranges by late Paleocene 

 time (Fig. 22.5). Sandstone, mudstone, and coal beds accumulated to a 

 thickness of 7000 feet just east of the Reartooth front during Paleocene 

 time. 



The early Eocene Willwood formation overlies older beds unconform- 

 ably at the margin of the basin, and this time is taken as one important 

 deformation of the surrounding uplifts. As previously noted, the detached 

 blocks of the Heart Mountain thrust (?) rest on the Willwood. See Fig. 

 22.6. The Willwood is spread widely over the Rig Horn basin. 



Middle Eocene time saw the accumulation of the Tatman formation, 

 which is almost free of volcanic debris except at Lysite Mountain at the 

 south end of the basin. West of the southern half of the Rig Horn basin 

 the Tatman is overlain by more than 1000 feet of volcanic debris of the 

 Absaroka Range. Remnants of the volcanics are noted elsewhere, and it 

 is postulated by Van Houten (1952) and Love (1956a,b) that sedimenta- 

 tion continued after Tatman time. 



Ry late Eocene time the Rig Horn basin had sunk relative to the uplifts 

 on either side about 17,000 feet. About 9000 feet of the depression had 

 been filled. Yet all the while, Mackin (1947) and Van Houten (1952) 

 contend, the climate had not been changed, and the orogenic debris 

 accumulated in a warm, humid lowland near sea level. In middle Ceno- 

 zoic time gradual regional uplift occurred. Pediments were widely cut in 

 the uplands and the lowlands were broadly alluviated, producing an 

 extensive graded surface. Ry late Cenozoic time further regional uplift and 

 increased aridity initiated the present cycle of erosion, and the graded 

 surface was widely dissected. 



Intrusive Rocks 



A belt of Laramide intrusions extends across the Rlack Hills about at 

 the north end of the exposed Precambrian core. Most have been con- 

 sidered laccoliths or modified laccoliths, such as Ragged top laccolith 

 (Fig. 24.11), Rear Rutte, Deadman Mountain, Cook Mountain, White- 

 wood Peak, Rlack Ruttes, and Devils Tower (Robinson, 1956). Within 

 the Precambrian basement complex the intrusions are chiefly sills and 

 dikes, and by charting the base of the Cambrian sandstone Noble et al. 

 (1949) have shown that the sedimentary rocks overlying the Precambrian 

 have been domed in two places notably, and believe that intrusive stocks 

 are the cause. Some of the so-called laccoliths are believed to be stocks. 



BLACK HILLS AND POWDER RIVER BASIN 



General Characteristics of Black Hills 



The Rlack Hills rise island-like several thousand feet above the sur- 

 rounding Great Plains in western South Dakota and northeastern Wyo- 

 ming. They are the easternmost of the outer ranges of the Rockies, and 

 in point of Laramide structure involving the sedimentary rocks, perhaps 

 the simplest. Their ridges, peaks, and valleys are the erosional remains of 

 a broad dome, some 120 miles long and 60 miles wide. A Precambrian 

 core of crystalline rocks trends nearly north-south and is flanked by up- 

 turned and truncated Paleozoic and Mesozoic strata. The broad anticline 

 trends and pitches northwestward beyond the crystalline area. The east 

 flank is fairly steep, with dips up to 45 degrees and more; the broad top 

 is fairly flat; and the west flank is fairly gentle, with dips of a few to 20 

 degrees. Four geomorphic units are distinct, namely, the central Pre- 

 cambrian core of fairly rugged mountains, a plateau area in the west cen- 

 tral part that is formed of Paleozoic limestones not yet stripped from the 

 Precambrian rocks, a remarkably continuous strike valley around the 

 Hills eroded in Jurassic and Triassic strata between the inner Paleozoic 

 formations and the outer Cretaceous sandstones (Fig. 24.11), and a bold, 

 inward-facing hogback held up by the Fall River and Lakota Cretaceous 

 sandstones. The strike valley is known as the Red Valley from the red 



