BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 48, PP. 631-666, 4 PLS., 1 FIG. MAY I. 1937 STRUCTURE OF THE UINTA MOUNTAINS BY J. DONALD FORRESTER CONTENTS Page Introduction........................................................................................................................ 632 Acknowledgments............................................................................................................... 633 Topographic features.......................................................................................................... 633 Geosyncline of deposition.................................................................................................. 634 Relation of Uinta geosynclinal sediments to Rocky Mountain sediments........ 634 Trend of Uinta Mountain trough relative to the Rocky Mountain geosyncline and the Wasatch Mountain trough..................................................................... 636 Iithology of the region...................................................................................................... 637 Structural features.............................................................................................................. 642 Folding......................................................................................................................... 642 Faulting........................................................................................................................ 644 South Flank Fault.............................................................................................. 644 Brush Creek Zone............................................................................................... 645 Yampa Fault..................................................................................................... 645 Uinta or Crest Fault.......................................................................................... 645 North Flank Fault.............................................................................................. 646 Faulting adjacent to the Uintas....................................................................... 647 Jointing........................................................................................................................ 648 Diastrophism....................................................................................................................... 648 General statement...................................................................................................... 648 Deformation of Uinta and Wasatch geosynclines during the Laramide Revolu­ tion............................................................................................................................ 649 Folding of the Wasatch and other northward-trending ranges...................649 Folding of the Uinta Mountains...................................................................... 650 General symmetry of the Uinta Fold.............................................................. 650 Minor folds adjacent to the Uinta Range...................................................... 651 Detailed consideration of Uinta Mountain structure............................................ 652 Western portion.................................................................................................. 652 Eastern portion................................................................................................... 652 Faulting........................................................................................................................ 653 Time of folding............................................................................................................ 654 Post-folding erosional history................................................................................... 655 Vertical uplift and large-scale faulting.................................................................... 656 General statement.............................................................................................. 656 Extent and intensity of uplift........................................................................... 657 Uinta Mountain area.......................................................................................... 657 Wasatch Mountain area.................................................................................... 658 Coalville-Weber River and Daniels Canyon-Strawberry areas................... 659 Axial Basin region.............................................................................................. 659 Rock Springs area............................................................................................... 659 Time of vertical uplift........................................................................................ 659 Theory as to cause of vertical uplift................................................................ 660 (631) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/48/5/631/3415499/BUL48_5-0631.pdf by guest on 26 September 2021 632 J . D. FORRESTER— STRUCTURE OF THE UINTA MOUNTAINS Page Post-vertical uplift erosional history................................ 661 Continental uplift in late Pliocene or early Pleistocene, 663 General Plateau area.................................................. 663 Time of continental uplift......................................... 663 Bibliography................................................................................ 664 ILLUSTRATIONS Figure Page 1. Isopach map..................................................................................... 635 Plate Facing page 1. Index map of the Uinta Mountains.............................................. 632 2. Composite geologic columnar sections of the Uinta Mountains 636 3. Areal geology and structure sections of the Uinta Mountains. 640 4. Regional structural relationships in the Uinta Mountains........ 644 INTRODUCTION Geologists have long been interested in the Uinta Mountains, partly because of the unusual erosional features, but chiefly, perhaps, because of the orientation with respect to most mountain ranges of the Cordilleran region. These mountains, which form the largest east-west-trending range in the western hemisphere, are in northeastern Utah and northwestern Colorado, just south of the Utah-Wyoming and Colorado-Wyoming boun­ dary. They extend as a physiographic unit from Kamas, Utah, on the west, to Cross or Junction Mountain, Colorado, on the east, a distance of approximately 160 miles. The width of the range, averaging about 45 miles, varies considerably, and, because of certain structural features, is somewhat greater toward the eastern end. As a structural element, the Uinta Range is a prominent feature of the Rocky Mountain system and extends into the north-south-trending Wa­ satch Range and possibly even into the Oquirrh Range, still farther west. Eastward, its anticlinal axis extends through the Axial Basin uplift into the Grand Hogback, which has a north-south trend. The Uinta Mountains have been the subject of study and discussion since the time of the earliest exploratory surveys of the West. Indeed, some of the classics of geological literature deal, to a greater or lesser extent, with this interesting feature of the central Cordilleran region. Of these earlier papers, with the exception of the Fortieth Parallel reports of the King Survey, none has covered the entire range as a structural unit. The primary purpose of this paper is to consider, from field evidence, the Uinta Range as a structural unit, in an attempt to find a satisfactory explanation of the origin and peculiar orientation of this range with respect to neighboring Rocky Mountain ranges. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/48/5/631/3415499/BUL48_5-0631.pdf by guest on 26 September 2021 BULL. GEOL. SOC. AM., VOL. 48 POEBESTEE, PL. 1 INDEX MAP OF THE UINTA MOUNTAINS Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/48/5/631/3415499/BUL48_5-0631.pdf by guest on 26 September 2021 INTRODUCTION 633 The greater parts of the field seasons of 1933 and 1934 were spent in studying that portion of the range west of the Utah-Colorado boundary. The area eastward into Colorado was covered mostly by reconnaissance, in an attempt to study as much of the structure as possible in the time available. Only a reconnaissance study of the eastern portion seemed warranted, as a part of the eastern end of the range had already been ade­ quately mapped. The base maps used consisted of United States Geological Survey topo­ graphic sheets, Land Office plats, and Forest Service maps. The survey­ ing methods employed were chiefly Brunton triangulation and Brunton pace traverse. Because of the size of the area, it was found impossible to show, on the scale of the published map, many details mapped in the field. ACKNOWLEDGMENTS This study was made possible through the Eleanor Tatum Long Scholar­ ship in Structural Geology at Cornell University. To Dr. C. M. Nevin, of Cornell University, belongs much of the credit for the presentation of this paper. The author also wishes to express his appreciation to Tom Lyon and Clem Pollock, of the International Smelting Company; Dr. Hyrum Schneider, of the University of Utah; Dr. A. J. Eardley, of the University of Michigan; Mr. R. E. Redden, of Hoytsville, Utah; and J. Leroy Kay and F. W. Kohler, of the Carnegie Museum, Pittsburgh, Pennsylvania. Residents of the area always showed interest