Structure Op the Rocky Mountains in Idaho and Montana1

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Structure Op the Rocky Mountains in Idaho and Montana1 BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 34, PP. 263-284 JUNE 30, 1923 STRUCTURE OP THE ROCKY MOUNTAINS IN IDAHO AND MONTANA1 BY GEORGE ROGERS MANSFIELD (Presented, before the Society December 28, 1922) CONTENTS Page Introduction................................................................................................................. 263 Fort Hail Indian Reservation................................................................................. 264 Area of seven quadrangles...................................................................................... 265 Teton basin.................................................................................................................. 266 Idaho and the northern Rockies............................................................................ 266 Thrust-faults of the northern Rockies................................................................. 266 Relative magnitude and importance of overthrusts........................................ 269 Relative ages of overthrusts.................................................................................. 269 Possible relationships of overthrusts.................................................................... 270 Deficiency of knowledge........................................................................................... 271 Overthrusting versus underthrusting.................................................................... 272 Restoration of structures in southeastern Idaho.............................................. 273 Isostatic features...'................................................................................................... 278 Hypotheses of mountain-building....................................................... .................... 280 Causes of mountain-building...................................................................................... 281 Tensional features in mountain structure........................................................... 282 Summary........................................................................................................................ 283 I ntroduction Most of the writer’s experience with Rocky Mountain structure has been gained in eastern Idaho in connection with the United States Geo­ logical Survey’s study of the western phosphate field, but some years ago he conducted a series of field parties from the Harvard University Sum­ mer School in southwestern Montana. He therefore feels some measure of acquaintance with the structural conditions in that State as well. The 1 Published by permission of the Director U. S. Geological Survey. This paper is one of a series composing a “Symposium on the structure and history of mountains and the causes of their development.” Manuscript received by the Secretary of the Society January 29, 1923. XVIII—Bull. Geol. Soc. Am., Vol. 34, 1922 (263) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/34/2/263/3414500/BUL34_2-0263.pdf by guest on 28 September 2021 264 G. R. MANSFIELD— STRUCTURE OF ROCKY MOUNTAINS remarks that follow are based chiefly on work in Idaho, but some mention is made of Montana and of other regions for comparison. Detailed or semi-detailed study has been made of three separate dis­ tricts in eastern Idaho, namely: (1) the Fort Hall Indian Reservation, (2) an area comprising 7 quadrangles lying about 15 miles east of the Fort Hall Indian Reservation and extending to the southeastern corner of the State, and (3) an area lying immediately west and south of the Teton basin. Since some of the structural features of these areas have already been described elsewhere, only brief reference need be made to them here, and the paper will be devoted chiefly to broader aspects of mountain structure suggested by the detailed studies. F o r t H a l l I n d i a n R e s e r v a t io n 2 The Fort Hall. Indian Reservation comprises an area of about 800 square miles in Bingham, Bannock, and Power counties and includes parts of several mountain ranges. The older sedimentary rocks, of Paleozoic and Mesozoic age, which make up these ranges are overlapped by Tertiary and Quaternary sediments and by igneous rocks as the ranges descend northward to the Snake River plain. The chief mountain-building activity in the reservation occurred after the deposition of the Jurassic beds, the latest Mesozoic rocks there ex­ posed, and before the deposition of the Tertiary rocks, which are believed to be of Pliocene age, but these last have been affected to a certain extent by subsequent movements. The major deformation was undoubtedly a part of the great Laramide Revolution. The Paleozoic and Mesozoic strata have been folded, locally with considerable intensity, and the folds, which have in general a northwesterly trend, have been inclined or even overturned northeastward. Transverse folding at right angles to the general trend is also present. Many of the folds are broken by thrust- faults and one overthrust-fault of considerable magnitude, the Putnam overthrust, has been recognized and described. Rock slices or cappings in the Portneuf Range, in the southeastern part of the reservation, which represent parts of an overthrust block now detached by erosion, are be­ lieved to be associated in some way with this great overthrust. Similar rock slices in the northern part of the Bannock Range, farther west, and in the unnamed range east of Bannock Valley, in the southwestern part of the reservation, may also be connected with this overthrust, but con­ 2 G. R. Mansfield: Geography, geology, and mineral resources of the Fort Hall Indian Reservation, Idaho, with a chapter on water resources by W. B. Heroy. U. S. Geol. Survey Bull. 713, 1920. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/34/2/263/3414500/BUL34_2-0263.pdf by guest on 28 September 2021 FORT HALL INDIAN RESERVATION 265 siderable areas which intervene have not yet been mapped, and in the mapped areas the older rocks and structures are largely concealed by younger sediments and by igneous rock. Tensional faulting of later date has also been recognized and described. These structural features are all illustrated in the geologic map and structure sections that accompany the report cited. A r e a o f s e v e n Q u a d r a n g l e s Seven quadrangles in the vicinity of the Wyoming border have been studied rather intensively. These include, the Montpelier 30-minute quadrangle at the south and six 15-minute quadrangles, namely, the Slug Creek, Crow Creek, Lanes Creek, Freedom, Henry, and Cranes Flat quadrangles. Together they occupy parts of Bingham, Bonneville, Cari­ bou, and Bear Lake counties and comprise an area of about 2,000 square miles. Preliminary accounts of some of the structural features of this area have already appeared in connection with official reports or in special papers,3 to which the reader is referred for available details. A compre­ hensive report covering the entire area has been prepared for publication as a professional paper of the United States Geological Survey. The area contains a number of mountain ranges, members of the Idaho-Wyoming chain, that have been complexly folded and faulted, and the folds show a pronounced curvature in trend from a direction a little east of north in the Montpelier quadrangle to northwest in the more northerly quadrangles. Some of the folds exceed 50 miles in length and 3 miles in breadth, the more important being synclinoria with relatively narrower intervening anticlines or anticlinoria. Usually they are unsym- metrical and inclined or even overturned eastward .or northeastward. The axes for long distances are nearly horizontal or slightly undulatory, due to the presence of relatively broad and low transverse folds, and the pitch is gentle, generally toward the north or northwest. In the last paper cited above the writer has given descriptions of the 3 H. S. Gale and R. W. Richards: Preliminary report on the phosphate deposits in southeastern Idaho and adjacent parts of Wyoming and Utah. U. S. Geol. Survey Bull. 430, 1910, pp. 457-535. R. W. Richards and G. R. Mansfield: Preliminary report on a portion of the Idaho phosphate reserve. U. S. Geol. Survey Bull. 470, 1911, pp. 371-451. R. W. Richards and G. R. Mansfield : Geology of the phosphate deposits northeast of Georgetown, Idaho. U. S. Geol. Survey Bull. 577, 1914. R. W. Richards and G. R. Mansfield: The Bannock overthrust, a major fault in southeastern Idaho and northeastern Utah. Jour. Geol., vol. 20, 1912, pp. 681-707. G. R. Mansfield: Igneous geology of southeastern Idaho. Bull. Geol. Soc. America, vol. 32, 1921, pp. 249-266. G. R, Mansfield: Types of Rocky Mountain structure in southeastern Idaho. Jour. Geol., vol. 29, 1921, pp. 444-468. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/34/2/263/3414500/BUL34_2-0263.pdf by guest on 28 September 2021 266 G. R. MANSFIELD----STRUCTURE OF ROCKY MOUNTAINS more important structural types of the area. These need not be repeated here. Suffice it to say that they include swallowtail folds, drag folds, fan folds, both upright and inverted, a transverse fold passing into an overthrust fault, an imbricated and folded overthrust fault, and horst and graben structure. T e t o n B a s i n The Teton Basin area, which lies about 40 miles north of the Freedom quadrangle, along the Wyoming border, comprises an area of approxi­ mately 300 square miles in Madison, Teton, and Bonneville counties. The eastern side of the basin is formed
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