Field Reports:
Western Snake River Plain volcanic field, Idaho
Role of phreatomagmatism in the evolution of the Mio/Pliocne Western Snake River Plain volcanic field, Idaho: pitfalls in application of volcanological data for paleogeographical reconstruction.
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Fig. 1 Thick accumulation of predominantly phreatomagmatic pyroclastic rocks in the Snake River canyon at Sinker Butte |
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Fig. 2 Typical base surge and phreatomagmatic fall beds close to Con Shea Butte along the SnakeRiver canyon. |
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Fig. 3 Intra-vent peperite in an eroded small phreatomagmatic volcano of 71 Gulch Volcano in the Western Snake River Plain. |
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Fig. 4 Pillow lava pile associated with shallow subaqueous eruption in the Western Snake River Plain |
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Fig. 5 Lava delta cross section in the Snake River canyon wall. |
The western Snake River Plain volcanic field in southwest Idaho is considered to be an extensive Mio/Pliocene volcanic field that formed voluminous lava shields, pahoehoe lava fields, scoria cones, and great variety of phreatomagmatic volcanoes. In many ways the Snake River Plain volcanic fields are considered as a special type of volcanic fields, where broad, large volume shield volcanoes form a complex network. In spite of the generally poorly exposed volcanic features in Idaho due to grass cover, along the Snake River, deep canyon walls expose thick accumulation of pyroclastic rocks (Fig. 1), many of them indicating widespread and probably long lasting phreatomagmatic eruptive events through the volcanic history of the region. A newly initiated research project in cooperation between Massey and Boise State University researchers reports significant new findings to demonstrate that the general view of the Western Snake River Plain volcanism as a continuum between deep subaqueous to subaerial eruptions may need significant revision. Previously, the location of many phreatomagmatic volcanic eruptive sites were considered to be the locations of deep subaqueous volcanoes erupted in the lake floor of a large intra-continental lake, called Lake Idaho. The identification of pyroclastic deposits suggestive for magma – water interaction has exclusively been used to draw a paleogegraphical map to locate the former deep sub-basins of the Lake Idaho in spite of the lack of independent supporting stratigraphical data from the extension of lacustrine beds.
Recent field based volcanological observations gave evidences of widespread presence of phreatomagmatic pyroclastic rocks (Fig. 2), however most of them exhibit textural features characteristic for sub-aerial depositional environment. In addition intrusive and extrusive magmatic bodies with textural features associated with peperite (Fig. 3) formation have also been identified. Most of the peperite however can be interpreted to be preserved in intra-crater/conduit or vent settings and therefore cannot be used for as widespread paleoenvironmental indicators to demonstrate magma and surface water (e.g. lake) non-explosive interaction. It seems that the common presence of pillow lava (Fig. 4) and associated hyaloclastite breccia deposits (Fig. 5) may also represent lava deltas formed in relatively shallow (few tens of metres) lacustrine basins.
Initial pyroclastic facies analyses are seemingly confirm that the formation of pyroclastic deposits/rocks inferred to deposited and formed in deep sublacustrine environment don’t need such environment for their formation. This recent finding opens up many questions in regard of the Mio/Pliocene evolution of SW Idaho, the timing of the volcanism and its association with the evolution of the Lake Idaho and/or other large intra-continental lacustrine systems in the region. In addition, these re-evaluations of the volcanism in SW Idaho have some general implication for the usage of phreatomagmatic pyroclastic rocks for paleoenvironmental reconstruction. The potential cooperation between Massey and Boise State University through research projects mostly carried out by MSc and PhD students will definitely bring some new exciting results in the near future.
Selected References
- Axen, G.J., Taylor, W.J. and Bartley, J.M., 1993. Space-Time Patterns and Tectonic Controls of Tertiary Extension and Magmatism in the Great-Basin of the Western United-States. Geological Society of America Bulletin, 105(1): 56-76.
- Brand, B., 2004. Basalt emergent volcanoes and maars, Sinker Butte - Snake River Canyon, Idaho. In: K.M. Haller and S.H. Wood (Editors), Geological field trips in southern Idaho, eastern Oregon, and northern Nevada. Boise State University, Boise, Idaho, pp. 106-116.
- Cummings, M.L., Evans, J.G., Ferns, M.L. and Lees, K.R., 2000. Stratigraphic and structural evolution of the middle Miocene synvolcanic Oregon-Idaho graben. Geological Society of America Bulletin, 112(5): 668-682.
- Godchaux, M. and Bonnichsen, B., 2002. Syneruptive magma-water and posteruptive lava-water interactions in the Western Snake River Plain, Idaho, during the past 12 million years. In: B. Bonnichsen, C.M. White and M. McCurry (Editors), Tectonic and magmatic evolution of the Snake River Plain Volcanic Province. Idaho Geological Survey, University of Idaho, Moscow, Idaho, pp. 387-435.
- Godchaux, M.M., Bonnichsen, B. and Jenks, M.D., 1992. Types of phreatomagmatic volcanoes in the Western Snake River Plain, Idaho, USA. Journal of Volcanology and Geothermal Research, 52(1-3): 1-25.
- Greeley, R., 1982. The Snake River Plain, Idaho: Representative of a new category of volcanism. Journal of Geophysical Research, 87(B4): 2705-2712.
- Hamilton, W.H. and Myers, W.B., 1962. Menan Buttes, cones of glassy basalt tuff in the Snake River Plain, Idaho. United States Geological Survey Professional Paper, 450-E: 114-118.
- Hughes, S.S., Wetmore, P.H. and Casper, J.L., 2002. Evolution of Quaternary tholeiitic basalt eruptive centers on the Eastern Snake River Plain, Idaho. In: B. Bonnichsen, C.M. White and M. McCurry (Editors), Tectonic and magmatic evolution of the Snake River Plain Volcanic Province. Idaho Geological Survey, University of Idaho, Moscow, Idaho, pp. 363-387.
- Ore, H.T., Reid, T.V. and Link, P.K., 1996. Pre-Bonneville-level, catastrophic overflow of Plio-Pleistocene Lake Bonneville, south of Rockland, Idaho. Northwest Geology, 26: 1-15.
- Womer, M.B., Greely, R. and King, J.S., 1980. The geology of Split Butte - A maar of the South-Central Snake River Plain, Idaho. Bulletin of Volcanology, 43(3): 453-471.
- Wood, S.H. and Clemens, D.M., 2004. Tectonic and magmatic evolution of the Snake River Plain volcanic province. In: B. Bonnichsen, M. McCurry and C. White (Editors), Tectonic and magmatic evolution of the Snake River Plain volcanic province. Idaho Geological Survey, Moscow, pp. 69-103.
Karoly Nemeth
INR, Volcanic Risk Solutions,
Massey University,
Palmerston North, New Zealand
Email k.nemeth@massey.ac.nz