CENOZOIC TECTONIC SEQUENCES AND BASIN EVOLUTION IN WESTERN MONTANA

Cenozoic sedimentary and volcanic basin-fill in western Montana can be packaged into sequences related to regional tectonic events. We use interpreted seismic, borehole, and gravity data, regional mapping, and detailed age constraints on Cenozoic deposits, employing both fossil vertebrates and isotopic ages, to delineate sequences and to generate a more data-integrated paradigm for Cenozoic basin evolution. The “Eocene unconformity” or Rocky Mountain erosion surface underlies the oldest sequence. This sequence, known as Cenozoic Sequence 1/Challis Sequence, ranges in age from ~50 to 44 Ma. It preserves a record of orogenic collapse and basin development related to metamorphic core complex formation in the Cordilleran hinterland and listric normal faulting in the fold and thrust foreland, as well as abundant arc-magmatism. The overlying sequence, Cenozoic Sequence 2/lower Kittitas Sequence, spanning a time duration of ~38 to 30 Ma, documents a tectonic interval of extensional collapse that occurred throughout the entire Cordilleran orogen. Significantly, this sequence is bounded by the ~30 Ma unconformity which may be related to eustatic and/or climatic changes rather than to tectonic events. The overlying ~27 to 21 Ma Cenozoic Sequence 3/upper Kittitas Sequence is bounded at its top by the Hemingfordian unconformity, which represents a hiatus from ~20 to 17 Ma. The Heminfordian unconformity is largely isochronous across the western U.S., and marks the change from orogenic collapse to backarc spreading of the Basin and Range event, the next major Cenozoic episode of crustal extension and magmatism. Its overlying sequence, Cenozoic Sequence 4/lower Walpapi Sequence, ranges in age from ~16 to 4 Ma. This sequence records the development of the present topographic basins which includes reactivated Paleogene and newly defined basins. Some of these newly delineated basins contain only portions of the older, Paleogene basins whereas others extend geographically beyond the original Paleogene basins. The youngest sequence, Cenozoic Sequence 5/upper Walpapi Sequence ranges in age from ~ 2 Ma to the present. This sequence correlates to the tectonic pulse documented in the Yellowstone/Teton area and to the initiation of the Yellowstone Plateau volcanic field at ~ 2 Ma.