Missoula Floods provenance


Catastrophic outburst floods from glacial Lake Missoula had the potential to carry sediment with a unique geochemical signature, since Lake Missoula, and its major sediment sources, were spatially coincident with a zone of Proterozoic continental crust known as the Belt-Purcell Supegroup, and thus the sediments initially carried by an outburst flood should have had geochemical properties reflective of their distinctive source. Our measurements of argon and neodymium isotopes from fine-grained sediments in well-known Missoula Flood deposits confirm that outburst floods deposited geochemically distinctive sediments characterized by old 40K/ 40Ar* ages, and highly negative εNd values, and that the floods likely entered the Pacific Ocean with a sediment load that retained these distinctive characteristics, meaning that Missoula Floods sediment should be identifiable in marine sediments based on argon and neodymium geochemical analyses. Such analyses of sediment from MD02-2496, a core off Vancouver Island, indicate that fine-grained sediment from banded layers with an ~80 year cyclicity deposited between 19 and 15 ka has a geochemical signature that is similar to that from terrestrial Missoula Floods deposits.


The basemap is an interpolation based on K/Ar and 40Ar/39Ar data from USGS and NRCAN geochronology databases. This map is meant to show the expected K/Ar age of sediment sources. Superimposed on this basemap are the configuration and flow patterns of the Cordilleran Ice Sheet at the last glacial maximum, the location of glacial Lake Missoula, the drainage path of the Missoula Floods, turbidite channels of the Astoria Fan, and the shoreline at the last glacial maximum. Also shown are study locations, and isotope geochemistry of <63 μm sediments from terrestrial Missoula Floods and Lake Missoula deposits, and marine sediments that are possibly derived from the Missoula Floods.