REINVESTIGATION OF THE SEDIMENTARY PROCESSES, DEPOSITIONAL ENVIRONMENTS, AND PROVENANCE OF THE MESOPROTEROZOIC LAHOOD FORMATION IN JEFFERSON CANYON, MONTANA

The Belt Basin of North America records Mesoproterozoic syntectonic sedimentation, which is now preserved in southwest Montana and eastern Idaho. The Belt Basin’s genesis remains debated despite efforts for nearly a century. Our research focuses on the sedimentology, stratigraphic architecture, and provenance of the LaHood Fm. of the Belt Basin in the Whitehall-Jefferson Canyon area ~80 km west of Bozeman, Montana to determine its depositional environment and tectonic setting, namely rift or intracratonic basin. We document massive sandstones and normally graded bedding, matrix- and clast-supported polymictic pebble-boulder conglomerates, and less abundant massive and laminated siltstones. We interpret these facies to reflect deposition by high-density turbidity currents, subaqueous debris flows, and clastic suspension settling. The architecture of these deposits show alternating modes of deposition between laterally continuous sandstones that are cm to meter(s) thick with occasional normal grading and non-erosional contacts contrasted with bedding comprised of polymictic pebble-boulder conglomerates with erosional contacts, incising relationships, and channelized geometries. Sandstone petrographic and clast count analysis shows consistent arkosic composition among sampled locations, but there are subtle differences, namely the presence of limestone within the conglomeratic facies that are lacking in the sandstone beds. Detrital zircon (DZ) U-Pb geochronologic age populations are defined by a distribution of ages with minor peaks centered at 1.8 Ga and 2.5 Ga and major multimodal peaks between 3.2-3.6 Ga. Comparison of DZs from the LaHood Fm. exposures to the east, in the Bridger Range, show similar age distributions as samples from this study. However, DZs from the Highland Mountains ~40 km west are unique with unimodal age distribution at 1.8 Ga. Provenance for these samples indicate sourcing from Archean cratons as well as rocks resultant from the Big Sky Orogeny (ca. 1.8-1.71 Ga). Our findings point to a sub-aqueous depositional environment dominated by turbidity currents and debris flows within a point sourced fan complex. This is consistent with previous models of LaHood Fm. that interpreted deep-water facies, but differs in that we do not observe and interpret any alluvial fan facies.