DETRITAL ZIRCON U/PB AND LU/HF ANALYSES OF THE BUFFALO HUMP FORMATION AND UNDERLYING DEER TRAIL GROUP: INSIGHTS INTO THE PROTEROZOIC TECTONOSTRATIGRAPHIC RECORD OF WESTERN LAURENTIA

Significant uncertainties regarding the Proterozoic tectonic history of the western margin of Laurentia hampers our understanding of the timing, geometry, and evolution of the supercontinents Nuna and Rodinia. Although the occurrence of a complete Wilson cycle along the western margin of Laurentia during the Nuna to Rodinia transition has been suggested, substantiating this proposed tectonic event requires a far better understanding of the sedimentary, metamorphic, and structural record of the region.

The Deer Trail Group and the overlying Buffalo Hump Formation constitute a ~3km thick siliciclastic and dolomitic sequence that are typically thought to be the western most exposures of the (1.47–1.38 Ga) Mesoproterozoic Belt Supergroup. Early work suggested a possible exotic western provenance for some of the units. Here we present new detrital zircon data that revise these proposed correlations. The detrital zircon spectra from the predominantly argillaceous and dolomitic Deer Trail Group (Togo Fm.) has a maximum depositional age (MDA) of ~1365 Ma, but consist mostly of a ~1.6 to 1.8 Ga population with εHf values of -9 to +7, suggesting it is slightly younger than, but shares similar sources with, the upper Belt Supergroup equivalent strata in the Lemhi sub-basin. Up-section, detrital zircon spectra from sandstones of the Buffalo Hump Formation have an MDA of ~1.0 Ga and a dominant Stenian (1.2–1.0 Ga) population that shows juvenile Lu-Hf isotopic values (εHf of +2 to +12). Several tectonic models propose the existence of Stenian-age, Rodinia-forming orogenic belts along the western margin of Laurentia, offering a proximal, non-Laurentian source for this detritus at ca. 1.0 Ga. However, nearby (<780 Ma) Windermere Supergroup strata deposited during Rodinia breakup also show similar detrital zircon populations that are interpreted to have been sourced from distal Laurentian Grenville provinces.

This new data corroborates Box et al.'s (2019; GSA Cordillera abstract) that the Buffalo Hump Formation likely unconformably overlies the Deer Trail Group, and both are younger than Belt Supergroup. Further study, including paleocurrent analysis, is required to evaluate their potential for constraining the late Proterozoic tectonic evolution of western Laurentia and its proposed conjugate margins within Nuna and Rodinia.