A sedimentary provenance and detrital zircon study of the Bonneville flood: from Red Rock Pass, Idaho to Windust, Washington

Zachary John Coppa

This study focuses on two particularly noticeable continental North American landscapes that were influenced by Quaternary megafloods. These are the: Channeled Scabland of eastern Washington (Bretz et al., 1956), that were created from glacial Lake Missoula (Bretz et al., 1956; Waitt, 1985), and the Snake River valley in Idaho and Washington, which was modified by the pluvial Lake Bonneville Flood (Malde, 1963; Malde and O’Connor, 1968; O’Connor, 1993). These megafloods mobilized immense volumes of sediment that were transported from lithologically distinct source areas. Firstly, to relate known and postulated Bonneville Flood deposits to their transport and depositional histories and to develop a method for Bonneville Flood deposit identification downstream of Lewiston, Idaho. This study attempted to identify and evaluate the provenance of the Bonneville Flood samples primarily using heavy (accessory) mineral and field-based gravel point counts; then compared to the results of Glover (2008). Lastly, identify (any existing) relationships between the shapes, form, sizes, and ages of the sand-sized detrital zircon grains evaluated in Glover (2008). Systematic evaluation of the heavy mineral content of sand sized particles and lithologic content of gravel sized clasts in Bonneville Flood deposits provide are insufficient to identify or correlate these megaflood deposits; especially downstream from Lewiston, Idaho, where stratigraphic and sedimentary features that otherwise might facilitate recognition of this megaflood deposit become less distinctive. The mineral assemblage downstream of Lewiston, Idaho, contains two major populations (pyroxene and hornblende), which occur within the Snake River Plain rhyolite and basalt. The mineral assemblage upstream of Lewiston, Idaho, contains major populations (pyroxene and epidote), which occur in the Columbia River Basalt. The sharp transition of major mineral populations within these two composite assemblages is attributed to local sediment transport rather than to through-going transport. The morphology of detrital zircon grains reveal that there are no definitive trends relative to their form, shape, size, and age. This may be the result of unrepresentative sampling of the full set of grain sizes, the tendency for detrital zircon grains to be transported only relatively short distances from local source areas, and/or a higher concentration of grains older than 900 Ma.

A sedimentary provenance and detrital zircon study of the Bonneville flood: from Red Rock Pass, Idaho to Windust, Washington

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