PALEOCLIMATE AND FACIES DEVELOPMENT ACROSS THE MID-TERTIARY UNCONFORMITY IN THE MADISON INTERMONTANE BASIN, SOUTHWESTERN MONTANA

Facies of the Oligocene Dunbar (T_rd) and Miocene Madison Valley Formations (T_mva) flank the mid-Tertiary unconformity and reflect changing depositional settings associated with post-Laramide extensional tectonics. Below the unconformity, the T_rd is characterized by relatively thin (cm to dcm scale) interbeds of diatomite and vitric tuff. Wave-ripple bedding and faunal remains document lacustrine settings that were episodically inundated with ash. Thin immature paleosols containing calcareous rootlets and pedoturbation fabric developed during intervening periods of emergence. Paleosols overlain by epiclastic tuff with an erosional base, unidirectional ripple cross-stratification and a wave-rippled, diatom-rich cap indicates subaerial runoff events and playa filling. Rapid deposition commonly resulted in locally abundant deformed and disrupted bedding. Directly above the unconformity, lateral pinching and discontinuity of basal T_mva muddy sand- to gravel-rich fluvial bodies reflect initial deposition in relatively narrow paleovalley tracts with up to 20 m relief along the unconformity surface. Subsequent alluvial plain buildup and gradient decrease is indicated by the occurrence of a widespread, tabular, upward-fining sequence (~5 m thick) of m-scale gravity-flow deposits distinguished by gravel-filled basal scour pockets, massive bedding, faint laminations, mud-supported pebbles/cobbles and rare inverse grading. A widespread tabular unit (6-11 m thick) of laterally and vertically stacked channel bodies containing conglomerate to medium-grained sandstone, abundant large-scale trough cross-stratification and well-defined lateral accretion surfaces overlies the mud-flow sequence and documents the encroachment of a wide, northeastward-axial, sinuous-channel system (with channel depths ~4-5 m) in a widened paleovalley tract. All T_mva facies are slightly to heavily pedogenically overprinted. Calcareous and carbonaceous rootlets, tubules, granular/blocky peds and calcium rich zones with sparse calcrete nodules signify compound soil development in an arid to semi-arid climate.