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Constraints on the Emplacement Age of the Heart Mountain Slide, Northwestern Wyoming

David H. Malone, John P. Craddock, Mark H. Anders and Andrew Wulff
The Journal of Geology
Vol. 122, No. 6 (November 2014), pp. 671-685
DOI: 10.1086/678279
Stable URL: http://www.jstor.org/stable/10.1086/678279
Page Count: 15
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Constraints on the Emplacement Age of the Heart Mountain Slide, Northwestern Wyoming
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Abstract

AbstractThe Heart Mountain slide is the largest terrestrial landslide deposit as yet recognized on Earth. The slide covers an area of at least 3400 km2, and the upper-plate rocks include 2–4 km of Paleozoic carbonate and Eocene volcanic rocks thrust out over 45 km of Eocene landscape. The precise age and duration of sliding is critical to emplacement models as well the slide’s effect on regional Eocene river systems. To address the timing issues, we sampled zircons from the basal fluidized layer 2 km from the slide’s breakaway fault (Silver Gate, MT) and 40-km downslope, nearer the slide’s toe (White Mountain, WY). Within this basal layer, we have identified mineral content and features consistent with a partially solidified magma. We interpret these observations to be consistent with the slide catastrophically dismembering an active magma body that mixed with the basal fault layer. The results yield remarkably similar U/Pb zircon crystallization ages at the proximal and distal locations: 48.78 ± 0.51 Ma at Silver Gate (n = 48) and 48.88 ± 0.22 Ma at White Mountain (n = 22). These zircon ages from the basal layer are tightly bracketed using various radiometric ages of Eocene Absaroka volcanic units involved in the movement phase of the slide and those deposited after emplacement, including detrital U/Pb zircon ages from the dissected Crandall Conglomerate river system. Our interpretation of the data is that the slide was catastrophically emplaced at 48.87 ± 0.20 Ma.

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