HR: 1340h
AN: T53A-0478    [Abstracts]
TI: Drainage Evolution in Response to the Migration of the Yellowstone Hotspot
AU: * Whitchurch, A
EM: amy.whitchurch@imperial.ac.uk
AF: Department of Earth Science and Engineering, Imperial College London, South Kensington, London, SW7 2AZ United Kingdom
AU: Gupta, S
AF: Department of Earth Science and Engineering, Imperial College London, South Kensington, London, SW7 2AZ United Kingdom
AU: Dawers, N
AF: Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA 70118 United States
AU: Densmore, A
AF: Department of Earth Sciences, ETH Zurich, Zurich, CH-8092 Switzerland
AB: The topography of the Yellowstone region in northwestern USA, preserves a record of the interaction of a hotspot with continental crust. The track of the hotspot has been defined by the time-transgressive migration of caldera forming volcanic centres; a distance of 700 km north-eastward to the present Yellowstone volcanic field since 16 Ma (Pierce and Morgan, 1992). The passage of such a long wavelength deformation field associated with large-scale mantle processes is likely to have exerted a significant effect on the spatial and temporal organisation of river drainage systems, the sedimentological consequences of which form the focus of this investigation. Fritz and Sears (1993) hypothesised that the deformation field associated with passage of the Yellowstone hotspot was responsible for a complete Neogene drainage reversal in southwestern Montana. In order to test this hypothesis, sedimentary deposits preserving a record of potential fluvial disruption in a Mio-Pliocene half-graben in southwestern Montana were analysed in order to investigate changes in river system geometry and dispersal patterns. Integration of a comprehensive sedimentary facies study with detailed palaeocurrent analysis combined with chronostratigraphic constraints from dated ash horizons across the region, enable reconstruction of drainage evolution and palaeogeography. The sedimentary record reveals basin-fill evolution from alluvial fan-dominated deposition during the mid-Miocene, to externally sourced, axial-flow dominated deposition in small isolated streams, during the mid- to late- Miocene. Palaeocurrent data shows clear northeasterly drainage throughout the mid-Miocene to early Pliocene, with no evidence to support the hypothesis of drainage reversal. During the late Miocene to early Pliocene, a distinct change in the axial-fluvial depositional system is recorded by the abrupt transition from pebble-grade, isolated stream channel deposits, to broad, sheet-form, cobble-boulder deposition. Reconstruction of past hotspot positions (based on caldera locations and patterns of faulting) indicate that topographic doming associated with the hotspot would have impacted south-western Montana at $\sim$ 6 Ma, a time coincident with this gravel progradation. We propose that the change in sedimentary style was a consequence of source area uplift and erosion associated with hotspot-related deformation.
DE: 9350 North America
DE: 8109 Continental tectonics--extensional (0905)
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting