Title

Investigating Unconformities Through Detrital Zircon Geochronology: Initial Results from Precambrian, Cambrian, and Devonian Formations of NW Montana

Presentation Type

Poster

Abstract

Located predominantly in NW Montana, the intracratonic Belt basin was the depositional site of the Precambrian-aged Belt Supergroup. Paleozoic formations were deposited on top of the Belt Supergroup above geologic unconformities. An unconformity is a surface boundary between two geologic formations that represents a period during which no sediment is present. Unconformities may be indicative of significant tectonic activity. I have chosen to research the sedimentary provenance of formations that overlie geologic unconformities in the region of the Belt basin, namely, the Proterozoic Garnet Range, Cambrian Flathead, and Devonian Maywood formations. This research aims to assess the sources of mineral grains within the formations and then to make inferences about the tectonic settings in which the geologic formations were originally deposited. An accurate determination of provenance involves the radiometric dating of zircon, a detrital mineral that experiences U-Th-Pb isotope decay. Dateable zircon grains were acquired from each of the three target geologic formations from localities in NW Montana. I crushed, milled, and sieved rock samples from each locality, and isolated zircon from the milled material by using a Franz Magnetic Separator, followed by heavy-liquid separation. Selected zircon mineral grains were imaged using scanning electron microscopy (SEM), and dated at the Boise State University Isotope Geochronology Lab using Laser Ablation and Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). Initial results give peak ages of 1,724 million years for the Garnet Range formation, 1,768 and 1,825 million years for samples of the Flathead Formation from two different localities, and 1,886 million years for the Maywood Formation. These dates were then used to infer sedimentary provenance for each formation, and to evaluate the tectonic setting of the Belt basin region in Precambrian, Cambrian, and Devonian time.

Category

Physical Sciences

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Investigating Unconformities Through Detrital Zircon Geochronology: Initial Results from Precambrian, Cambrian, and Devonian Formations of NW Montana

Located predominantly in NW Montana, the intracratonic Belt basin was the depositional site of the Precambrian-aged Belt Supergroup. Paleozoic formations were deposited on top of the Belt Supergroup above geologic unconformities. An unconformity is a surface boundary between two geologic formations that represents a period during which no sediment is present. Unconformities may be indicative of significant tectonic activity. I have chosen to research the sedimentary provenance of formations that overlie geologic unconformities in the region of the Belt basin, namely, the Proterozoic Garnet Range, Cambrian Flathead, and Devonian Maywood formations. This research aims to assess the sources of mineral grains within the formations and then to make inferences about the tectonic settings in which the geologic formations were originally deposited. An accurate determination of provenance involves the radiometric dating of zircon, a detrital mineral that experiences U-Th-Pb isotope decay. Dateable zircon grains were acquired from each of the three target geologic formations from localities in NW Montana. I crushed, milled, and sieved rock samples from each locality, and isolated zircon from the milled material by using a Franz Magnetic Separator, followed by heavy-liquid separation. Selected zircon mineral grains were imaged using scanning electron microscopy (SEM), and dated at the Boise State University Isotope Geochronology Lab using Laser Ablation and Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). Initial results give peak ages of 1,724 million years for the Garnet Range formation, 1,768 and 1,825 million years for samples of the Flathead Formation from two different localities, and 1,886 million years for the Maywood Formation. These dates were then used to infer sedimentary provenance for each formation, and to evaluate the tectonic setting of the Belt basin region in Precambrian, Cambrian, and Devonian time.