ENIGMATIC PROTEROZOIC LAHOOD FORMATION, MONTANA: REVISITED

The Lahood Formation (Montana) has traditionally been interpreted as a narrow wedge of arkosic sediment deposited along the southern margin of the Belt basin and constituting the basal member of the Mesoproterozoic Belt Supergroup. New observations and analyses of samples from several classic outcrops reveal important mineralogical variations, including: 1) local Archean sources to the south are mostly tonalite-trondhjemite gneisses, yet the Lahood has abundant k-spar not found in these rocks; 2) hornblende is abundant in the local basement, but is mostly absent in the Lahood, with the exception of its type-section in Jefferson River Canyon (JRC); and 3) detrital muscovite is common in all Lahood outcrops, except at JRC, but rare in currently exposed basement. These observations challenge conventional wisdom regarding the provenance and depositional setting of the Lahood Formation. Furthermore, zircon ages demonstrate that the Lahood (and Neihart Quartzite) are only post-1.7 Ga, opening the possibility that these units might not be equivalent to the lower Belt, but instead might be slightly older pre-Belt rocks. Regardless of age, the provenance of the Lahood was likely more complicated than a simple south-to-north transport system from an eroding Archean highland into adjacent fault-bounded basins, a model that seems to apply only to the JRC section. Other sediment sources to the ENE are necessary, such as the Great Falls Tectonic Zone, formed during the Medicine Hat-Wyoming collision at 1.86-1.79 Ga. Furthermore, rapid changes in the nature of bedding, sedimentary structures, thickness and lithology suggest a more complicated depositional model for the Lahood in toto than a single massive arkosic clastic wedge. If the Lahood Formation is older than the lower Belt, it may have been deposited during the post-collisional extension phase of the Wyoming-Medicine Hat collision (Great Falls orogeny). Alternatively, if the Lahood is equivalent to the lower Belt, we suggest several localized basins in a “syn-rift shoulder” tectonic setting adjacent to the Helena embayment. In either scenario, multiple small, probably asymmetric, extensional basins are suggested by rapid variations in Lahood rocks from the northern Bridger Range westward, including a rapid decrease in 3.5 Ga detrital zircons from west (JRC) to east.