107-10 Sediment Sources of the Basal Cambrian Flathead Sandstone reveal insights into Precambrian-Cambrian Paleogeography of western Laurentia

Session: Sedimentary Geology Division/SEPM Student Research Poster Competition: Dynamics of Stratigraphy and Sedimentation

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Abstract:

The Middle Cambrian Flathead Sandstone (~540-520 Ma) is the first Paleozoic sedimentary unit overlying the Great Unconformity in southwest Montana, lying directly on Precambrian basement. The Flathead Sandstone records the early transgression of epeiric seas across Laurentia following the breakup of Rodinia. Previous work interpreted the Flathead Sandstone as being locally derived from underlying basement and from recycling of the Belt Supergroup during the development of the Great Unconformity. To test this hypothesis and better understand sediment sources and paleogeography during early to middle Cambrian time, we present new detrital zircon U-Pb geochronology from samples of the basal conglomeratic sandstone of the Flathead Sandstone across southwest Montana.

Results reveal a variable Archean component (>2.5 Ga) and a Proterozoic component (~1.78 Ga). The Archean age component contains age peaks present throughout the Wyoming Province, with peaks of 2.5 Ga, 2.6-2.7 Ga, 2.9 Ga, and 3.2 Ga, with an assortment of 3.5-3.9 Ga zircons. The highly diverse Archean age populations from sample-to-sample present additional challenges to the existing interpretations of paleogeography and sediment transport in western Laurentia. While some Flathead samples indicate local recycling, comparison of Flathead Sandstone age spectra to Precambrian sources across northwestern Laurentia indicate regional-scale transport of sediment from Archean basement exposed in the north and north-central Wyoming Province, rather than being limited to localized erosion and recycling, with predominantly north-northwesterly sediment routing. The Proterozoic age signature in the Ruby Range suggests sediment input from underlying magmatic rocks or metamorphic rocks. In other samples, possible sources include the Great Falls Tectonic Zone to the north, or the Cheyenne Belt along the southern margin of the Wyoming Province. Comparison of Flathead Sandstone age spectra to Belt Supergroup datasets have largely ruled out significant input from the Belt Basin.

Ongoing and future work will clarify the provenance of Proterozoic-aged zircon grains through Lu-Hf analysis, and further sampling of strata higher in the Flathead section will clarify previous studies’ interpretation of the timing of uplift of the Transcontinental Arch and its effect on sediment provenance throughout western Laurentia. In addition to sampling, field investigation of faulting and more detailed provenance analysis in the Ruby Range may uncover new insights into potential structural inheritance of Precambrian structures reactivated during Sevier-Laramide orogenesis.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6952

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