SOME COMMENTS ON THE IMPACT ORIGIN OF THE MIOCENE PACIFIC NORTHWEST VOLCANICS HYPOTHESIS
FIELDS, Patrick F., Department of Plant Biology, Michigan State Univ, Plant Biology Building, East Lansing, MI 48824-1312, fieldspa@msu.edu.

The recent proposal that a southeastern OR meteorite impact was the causal agent for the Miocene lava flows in the Pacific NW (Alt, Hyndman, & others), & thus the trigger of: all the Columbia River Basalt flows, the origin of the mantle “hot spot” presently under the Yellowstone regions, & both the eastern and western Snake River Plains (SNRP) is here questioned. Problems with this scenario are: 1) The size of the proposed impact crater would occupy nearly all of southeastern OR, & yet there is no evidence for it. 2) The lack of laterites (supposedly created by the enhanced greenhouse effects of the impact) in the southwestern ID/eastern OR region, but present 400 km to the north (Latah flora region, northern ID); & the presence of warm temperate, but not tropical/subtropical fossil floras in both these regions. 3) The lack of any major west to east volcanic trends (i.e., successively younger calderas) through southwestern ID, but numerous northwest – southeast trending features. 4) The very diverse Succor Creek flora (representing a mixture of both successional & very stable vegetation) is the same age as & located right over the hot spot’s trajectory. It is not realistic to have formed at a time of such disturbance. This does not happen today & there is no precedence for it in the past. 5) The Cretaceous granitic northern Owyhee Mountains are located in the proposed pathway of this hot spot & yet fail to show Miocene volcanism along its north & eastern sides. But, Miocene volcanism is along the southern edge (Bruneau-Jarbidge caldera), where the traditional northeast-trending hot spot scenario predicts its presence. 6) A west to east hot spot migration scenario starting in eastern OR & ending in the present-day Yellowstone region requires several directional changes to explain the northwest-southeast trend of the western SNRP limb, but the eastern limb’s northeast-southwest trend. Alternatively, these data (in #1-6) appear to be consistent with a hot spot “migrating” in a northeast-trending straight line from northwestern NV, across eastern ID, to western WY creating the eastern SNRP limb only.

Cordilleran Section - 98th Annual Meeting (May 13–15, 2002)
Session No. 23
Impact Structures
CH2M Hill Alumni Center: Elle
4:00 PM-5:30 PM, Tuesday, May 14, 2002
 

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