HR: 1330h
AN: V42B-0364    [PDF]
TI: Variations in Cross-Stratification within the Tuff Deposits at Sinker Butte, a Western Snake River Plain Hydrovolcano
AU: * Brand, B D
EM: georocks_1@hotmail.com
AF: Boise State University: Department of Geosciences, 1910 University Drive, Boise, ID 83725-1535 United States
AU: White, C
EM: CWHITE@boisestate.edu
AF: Boise State University: Department of Geosciences, 1910 University Drive, Boise, ID 83725-1535 United States
AB: Sinker Butte is the remnant of an emergent basaltic hydrovolcano that erupted about 1 Ma beneath the surface of Plio-Pleistocene Lake Idaho, near the southern margin of the Western Snake River Plain. The Snake River has eroded through the deposits, creating an unusual opportunity to study the entire eruptive sequence. One of the most interesting aspects of this volcano is the many different types of cross-stratification and dune bedding which can be found in the fragmental rocks. The first stage of this eruption created a series of subaqueous tuff deposits which formed a volcanic-sedimentary platform that grew towards the surface of the lake. The platform would have been growing at inconsistent rates around the vent causing the resulting deposits to vary around the platform. Cross-stratification in these deposits include channel scour and fill, dune bedding, trough cross bedding, and bouma-turbidite sequences. As the eruption progressed, this platform built above the water level, which represents the subaerial stage of the eruption. At this time the magma/water interaction was still significant, causing a pulsatory type of eruption creating many thin interbedded pyroclastic surge and airfall deposits. Near the vent large-scale regressive low-angle dunes (wavelength 1-2 m) are found interbedded with thin planar beds. The crests of regressive dunes migrate upstream, with the coarser fragments dropped on the upstream side of the crest. These are thought to form as a result of wet surges due to the presence of accretionary and/or armored lapilli as well as vesicles due to trapped volatiles in wet ash. Dunes in deposits further from the vent are generally thinly bedded and have gentler dip angles and shorter wavelengths. Other types of cross-stratification found at Sinker are wavy-planar beds, which are a mixture of airfall and surge, and deformed low angle dune beds (.5-1 m wavelength) containing abundant soft sediment deformation and sag structures. Found high in some sections are large-scale progressive dune-forms (1-2 m wavelength). These may be attributed to a late dry surge towards the end of eruption, just before the volcano isolated itself from all external water. The low angle cross-stratification and irregular truncations reflect the highly pulsatory nature of surges during deposition. The dune bed thicknesses and average wavelength appear to decrease away from the vent. This finer cross-stratification may be result of the same type of surges that formed the larger regressive dunes, but may represent a loss in momentum, density and turbulence as the surge travels further from its source.
DE: 5139 Transport properties
DE: 5480 Volcanism (8450)
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting