Abstract
Sedimentology and detrital geochronology of the Late Cretaceous Beaverhead Group, southwest Montana
Abstracts with programs - Geological Society of America, Vol.50(6)
Geological Society of America, 2018 annual meeting & exposition
11/2018
Abstract
The Beaverhead Group of southwest Montana is synorogenic; to the northeast of the study area it unconformably overlies and is deformed by Archean basement rocks of the Laramide Blacktail-Snowcrest uplift. To the west, it is deformed by and structurally juxtaposed against Paleozoic passive margin strata in the Sevier fold-thrust belt. The Beaverhead Group records coarse-grained, conglomeratic deposition in the northwestern foredeep of the Cordilleran foreland basin. Exposures of the Beaverhead Group are isolated, making it problematic to interpret depositional ages and relationships between exposures. The only age constraints for the Beaverhead were obtained from a limited sample of palynomorphs, which suggest Coniacian-Campanian ( approximately 89-72 Ma) depositional ages. Based on clast compositions, previous work suggests that Mississippian-Triassic limestone and Precambrian Belt Supergroup quartzite shed from thrust sheets to the west were the primary sediment sources. This study aims to better define the depositional age and provenance of the Beaverhead Group through the use of U-Pb dating on both detrital zircon and detrital apatite grains. Eight samples from a wide geographic and stratigraphic range within the Beaverhead were collected and analyzed using LA-ICPMS. Stratigraphic sections were measured where possible and detailed sedimentologic descriptions were recorded for each field site. The samples can be divided into three groups, where group 1 is quartzite conglomerate and interbedded sandstone; group 2 is limestone conglomerate and interbedded sandstone; and group 3 is distal, laterally equivalent sandstones. In our preliminary dataset, based on either the youngest single grain or youngest graphical peak age of the zircons the maximum depositional ages range from approximately 67.5-87.5 Ma (Coniacian-Maastrichtian). Zircon age distributions vary between samples; however, all groups have peaks between approximately 340-560 Ma, approximately 900-2100 Ma, and approximately 2350-3000 Ma, as well as a few single grains >3000 Ma. The primary difference is that the peaks in group 1 are relatively equal, whereas in group 2 there are large peaks at approximately 1440, 1720, 2490, and 2706 Ma. Group 3 differs in that it yields no Mesozoic zircons. Only four of the eight samples yielded detrital apatite, with a wide range of ages between approximately 70-560 Ma and approximately 1300-2000 Ma.
Details
- Title: Subtitle
- Sedimentology and detrital geochronology of the Late Cretaceous Beaverhead Group, southwest Montana
- Creators
- Kacey L Garber - University of IowaEmily S FinzelJustin A Rosenblume
- Resource Type
- Abstract
- Publication Details
- Abstracts with programs - Geological Society of America, Vol.50(6)
- Conference
- Geological Society of America, 2018 annual meeting & exposition
- Publisher
- Geological Society of America (GSA)
- ISSN
- 0016-7592
- Alternative title
- Geological Society of America, 2018 annual meeting & exposition
- Language
- English
- Date published
- 11/2018
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984229966802771
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