PROVENANCE OF THE BELT-PURCELL SUPERGROUP OF NW MONTANA, USA AND SW ALBERTA, CANADA: IMPLICATIONS FOR THE EARLY MESOPROTEROZOIC PALEOGEOGRAPHY OF LAURENTIA

The Mesoproterozoic Belt-Purcell basin contains a ~5 km thick siliciclastic and carbonate succession spectacularly exposed along the USA-Canada border in the Glacier, Waterton Lakes, and Castle national and provincial parks. The depositional chronology and influence of Laurentian and non-Laurentian sources on the Belt-Purcell Supergroup remain unresolved, necessitating a comprehensive reevaluation. To address these issues, we acquired laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) detrital zircon (DZ) U-Pb geochronologic results from the Lewis thrust salient (N=27 samples, n=2,558 analyses), calculated their maximum depositional age (MDA) constraints, and qualitatively and quantitatively determined sediment provenance relationships to Laurentian igneous and metamorphic sources (references=148, N=20 samples, n=12,159 analyses) by employing multidimensional scaling (MDS) and an inverse Monte-Carlo model (DZmix). Our findings yield a basin age of ca. 1490-1375 Ma, a depositional duration of ca. 115 Myr, and an accumulation rate of ~170 m/Myr. At the base of the succession, the Waterton to lower Helena formations, which have a ca. 1490-1415 Ma MDA, are dominated by Paleoproterozoic (1920-1770 Ma) and Archean (3000-2500 Ma) grains, suggesting sediment derivation from the Canadian Shield. A range of zircon populations from the early Mesoproterozoic (1500-1375 Ma), Paleoproterozoic (1920-1600 Ma), and Archean (2700-2500 Ma) are observed in the upper Helena to Sheppard formations that deposited ca. 1415 Ma. The Gateway to Rooseville formations, with depositional ages ca. 1415-1375 Ma, contain almost entirely late Paleoproterozoic (1800-1600 Ma) zircons, consistent with present-day southern Laurentian sources. We interpret the provenance shift from northeastern to southern terranes to indicate a continental-scale reorganization of the North American drainage system at ca. 1415 Ma, with establishment of a Mesoproterozoic trans-continental fluvial system, likely influenced by the ca. 1480-1350 Ma Picuris orogeny (southwestern USA) associated with the addition of juvenile crust along southern Laurentia. These findings have implications for Precambrian paleogeography and paleotectonic studies during the Columbia-Rodinia supercontinent transition.