Structure and Stratigraphy of the Southwest Marias Pass Area, Flathead County, Montana

Abstract

Research Article| February 01, 1963 Structure and Stratigraphy of the Southwest Marias Pass Area, Flathead County, Montana MILTON O CHILDERS MILTON O CHILDERS 2364 Logan Way, Salt Lake City, Utah Search for other works by this author on: GSW Google Scholar GSA Bulletin (1963) 74 (2): 141–164. https://doi.org/10.1130/0016-7606(1963)74[141:SASOTS]2.0.CO;2 Article history received: 24 Apr 1961 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation MILTON O CHILDERS; Structure and Stratigraphy of the Southwest Marias Pass Area, Flathead County, Montana. GSA Bulletin 1963;; 74 (2): 141–164. doi: https://doi.org/10.1130/0016-7606(1963)74[141:SASOTS]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The southwest Marias Pass area includes parts of the Lewis and Clark, Sawtooth, and Lewis ranges in the northern Rocky Mountains of Montana.The western two-thirds of the map area is underlain by a Precambrian sequence, the Belt Supergroup, consisting of argillites, quartzites, and silty microcrystalline carbonate rocks more than 14,000 feet thick. The Belt has been subdivided into seven conformable formations in addition to an unnamed sequence of beds at the top.About 1300 feet of Mississippian carbonate rocks occur in the southeast quarter. These rocks are delimited above and below by thrust faults.Jurassic and Lower Cretaceous shales, mudstones, and sandstones of unknown thickness crop out in the eastern part of the map area.The trace of the Lewis thrust fault extends across the area from north to south separating the Belt strata from the Paleozoic and Mesozoic rocks. The Lewis fault surface cuts upward through the thrust sheet as it is traced southeastward through the map area, and the deformation of the autochthonous Mesozoic and Paleozoic rocks increases southeastward. There is no tear fault cutting the Lewis thrust sheet in the Marias Pass area. At the south end of the Clarke Range salient, erosion has resulted in apparent accentuation of an original shift in the trend of intersection between the fault surface and the beds of the thrust sheet.The Mesozoic and Paleozoic strata are imbricated by southwest-dipping thrust faults which appear to be genetically related to the Lewis thrust. This zone of imbricate structure plunges at a low angle, N. 50°–85° W. and is the northwest end of the Sawtooth Range structural salient.The folds within the Lewis thrust sheet and the imbricate structures in the Mesozoic rocks east of the north end of the Sawtooth Range salient (beyond the map area) are parallel and plunge at low angles, S. 20°–30° E. These relationships suggest an early stage of regionally concordant deformation before the Sawtooth Range salient was fully developed.After movement on the Lewis fault ceased (late Eocene or early Oligocene), normal faulting took place. The largest of the normal faults dip southwest and offset the earlier thrust faults. The easternmost normal fault has a measured stratigraphic separation of 14,000 feet. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.