ASH FALL TUFF MARKER BEDS WITHIN CENOZOIC BASIN-FILL OF SOUTHWESTERN MONTANA

Ash fall tuffs are significant marker beds within Cenozoic continental basin-fill of southwestern Montana. The tuffs provide important intra- and intervalley stratigraphic markers because they can be followed through strata that have rapid lateral facies changes, complex stratal geometry, limited outcrop continuity, and that are often unfossiliferous. Some ash fall tuffs are associated with calcic paleosols at regional sequence boundaries. Radioisotopic age dates of the tuffs that are associated with sequence boundaries are of great consequence because the use of unconformity bounded units has proven to be a better stratigraphic approach than the use of lithostratigraphic units for regional studies of continental strata. The radioisotopic ages of tuffs also place additional constraints on Tertiary vertebrate fossil assemblages and their exact placement with respect to North American Land Mammal Ages. The ash fall tuff marker beds with radioisotopic age dates are from the Upper Ruby, Blacktail, Gallatin, Melrose-Divide, Rocker-Ramsey, Jefferson, and Toston-Townsend valleys, and range in age from mid Tertiary to Quaternary. The Tertiary tuffs currently identified as regional marker beds in southwestern Montana are 28.3, 21.7, 18.3, 15.0, 10.4, 9.5, and 3.7 Ma in age. Quaternary volcanic ashes are the Glacier Peak ash at circa 11,125 RCYBP, and the Mazama ash at 6,500 RCYBP. Huckleberry Ridge ash flow tuff at 2.1 Ma in age is also a marker bed in basin-fill within valleys closely situated to Yellowstone National Park. These same ash fall tuff marker beds are tentatively identified in many other southwestern Montana areas, including the Deer Lodge, Beaverhead, Madison, Three Forks, Paradise, and Big Hole valleys.