MAGNETIC TRACING OF LOST TIME IN CENOZOIC SEDIMENTS FROM THE NORTHERN ROCKY MOUNTAINS, USA

The early Miocene unconformity (EMU) occurred during a critical transition in the tectonic evolution of western North America, with profound impact on faunal biogeography and diversification. The Railroad Canyon section (RCS) in the northern Rocky Mountains provides the most complete geologic record of this period. There, our new magnetostratigraphic study in combination with published radiometrically-dated ash layers places the EMU at ~20.1 Ma with a duration of 0.1 – 1.0 Myr. The EMU is marked throughout SW Montana by a distinct change in color, an abrupt increase in magnetite concentration, and a sharp drop in carbonate abundance. Curiously, sedimentation rate in the RCS remains constant at 6.5 ± 0.1 cm/kyr through the unconformity, as do climate proxies. Alternative hypotheses to account for these observations consider (1) the exposure of magnetite-rich basement and/or (2) a change in drainage network. Both can be caused by a change in tectonic regime such as the onset of Basin and Range extension, the arrival of the Yellowstone plume, etc.