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Regional Relationships of the Beaucoup Formation in the Central Brooks Range, Alaska12.08 MB
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Bruxvoort DATA TABLEs_UPb_Hf1.59 MB
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Thesis
Regional relationships of the Beaucoup formation in the Central Brooks Range, Alaska
University of Iowa
Master of Science (MS), University of Iowa
Summer 2024
DOI: 10.25820/etd.007754
Abstract
Geologic mapping in the Philip Smith Mountains A1 NW and A2 NE 7.5’ quadrangles conducted under the auspices of the USGS EDMAP program was completed to evaluate the stratigraphic and structural history of Devonian–Mississippian sedimentary units assigned to the Endicott Mountains allochthon in the central Brooks Range. Primary units in the map area include the Skajit Limestone, Beaucoup Formation and Hunt Fork Shale. The Upper Devonian (Frasnian) Beaucoup Formation includes repeating sections of black shale, quartz arenite, carbonate and chert pebble conglomerate and discontinuous limestone. Contact relationships between the Beaucoup Formation and thick limestone layers previously assigned to the Skajit Limestone indicate that the Skajit Limestone is laterally equivalent to thinner limestone layers in the Beaucoup Formation. The Upper Devonian (Frasnian–Famennian) Hunt Fork Shale is dominated by black shale with minor thin (<5 cm) interbeds of quartz arenite. The Hunt Fork appears to depositionally overlie the Beaucoup Formation, although the contact is generally sheared. The Hunt Fork Shale is intruded by mafic dikes and sills of probable Devonian age. All units exhibit cleavage that is axial planar to outcrop-scale folds. Phyllitic shale and siltstone units have a slaty cleavage, whereas thicker sandstone, limestone and mafic intrusive units have a spaced cleavage. Folds in Beaucoup limestone vary from open upright to tight overturned with a north-vergent asymmetry. Cleavage-bedding relationships in clastic rocks throughout the area are consistent with north-vergent folding. Shallowly (<20º) south-dipping limbs are most common but separated by small domains characterized by moderately to steep (40-80º) south-dipping overturned bedding. Deformation is interpreted to reflect Mesozoic Brookian thrusting; there is no field evidence for earlier deformation. However, an older cleavage is observed at the thin section scale. The structurally highest units in the southeast corner of the map area include the Carboniferous Kayak Shale and Lisburne Group, which rest in thrust contact on the Hunt Fork Shale. Extensional deformation is recorded by a northeast-striking normal fault that offsets units and contractional structures across the map area. New U-Pb geochronologic and Hf isotopic data suggest that the Beaucoup Formation received detritus from the North Slope and Southwestern subterranes.
Details
- Title: Subtitle
- Regional relationships of the Beaucoup formation in the Central Brooks Range, Alaska
- Creators
- Samson E. Bruxvoort
- Contributors
- William C McClelland (Advisor)Justin Strauss (Committee Member)Jessica Meyer (Committee Member)
- Resource Type
- Thesis
- Degree Awarded
- Master of Science (MS), University of Iowa
- Degree in
- Geoscience
- Date degree season
- Summer 2024
- Publisher
- University of Iowa
- DOI
- 10.25820/etd.007754
- Number of pages
- viii, 44 pages
- Copyright
- Copyright 2024 Samson E. Bruxvoort
- Grant note
- Support for field work and analytical work was provided by National Science Foundation grant EAR1947071 and U.S. Geological Survey, National Cooperative Geologic Mapping Program Award No. G23AC00187-00 (EDMAP) awarded to McClelland. I received support from a Geological Society of America 2023 Graduate Student Research Grant. Additional support was provided by National Science Foundation grants awarded to Chris Connors (EAR1946680) and Justin Strauss (EAR1947074). The project was conducted in collaboration with David W. Houseknecht and Jared Gooley at the U.S. Geological Survey and Marwan A. Wartes at the Alaska Department of Natural Resources, Division of Geological & Geophysical Surveys. I thank Marwan Wartes and Jared Gooley for providing helicopter support through Maritime Helicopters for getting to and from the field area. U-Pb and Hf analyses were partially supported by NSF-EAR 2050246 awarded to the Arizona LaserChron Center. I also gratefully acknowledge Phil Kerr, Iowa Geological Survey, for his technical expertise and assistance in ArcGIS Pro. I thank Rick Langel, Iowa Geologic Survey, for teaching me the GEMS format and providing me resources to format the map data into a publishable format. (ii)
- Language
- English
- Date submitted
- 06/13/2024
- Description illustrations
- color illustrations, color maps
- Description bibliographic
- Includes bibliographical references (pages 24-33).
- Public Abstract (ETD)
Mapping the Philip Smith Mountains of the Central Brooks Range was done to understand the deformation history and structure of the Devonian Beaucoup Formation. The main rock types are the Skajit Limestone, Beaucoup Formation, and Hunt Shale. The Skajit is a large limestone reef complex. The Beaucoup Formation is made of black shale, quartz arenite, carbonate and chert pebble conglomerate, and limestone. The Hunt Fork Shale is mostly black shale with thin layers of quartz arenite and intruded by mafic dikes and sills. We found the thin limestone beds of the Beaucoup Formation to laterally extend into the Skajit reef complex leading us to include the previously mapped Skajit in the Beaucoup Formation. The Hunt Fork Shale overlies the Beaucoup Formation. The boundary of these units is normally arbitrary and sheared where shale overlies shale. All units contain folding, and cleavage caused by north-verging thrusting. There are also extensional normal faults striking northeast throughout the map area. New geochronologic data suggests that the Beaucoup Formation received material from the North Slope and Southwestern subterranes.
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984698352802771
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