Magnetostratigraphy and Source Characterisation Across the Early Miocene Unconformity, Northern Rocky Mountains, <scp>USA</scp>

ABSTRACT The early Miocene unconformity (EMU) formed during a transition in the tectonic regime of western North America that coincided with faunal diversification. The Railroad Canyon section in the northern Rocky Mountains provides a complete geologic record around this event. Our new magnetostratigraphic study in combination with published U‐Pb ages from intercalated ash places the end of the EMU at ~20.1 Ma with a duration of up to 1.5 Myr. The EMU is marked by an abrupt change in rock colour, an increase in magnetite concentration, and a decrease in calcite abundance. Average sedimentation rates are similar above and below the EMU, as are climate proxies; therefore, the changes in mineralogy likely reflect a reorganisation in sediment source. Drainage network reorganisation seemingly contributed to this, possibly related to a change in tectonic regime, such as the onset of Basin and Range extension and/or arrival of the Yellowstone plume.