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Style and mechanics of liquefaction-related deformation, lower Absaroka Volcanic Supergroup (Eocene), Wyoming

By
Paul L. Decker
Paul L. Decker
Department of Geological Sciences, University of Wisconsin–Madison, Madison, Wisconsin 53706
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Published:
January 01, 1990

Stratigraphically equivalent sections within the Aycross and lower Wapiti Formations (middle Eocene) in the east-central Absaroka Range of northwest Wyoming locally are intensely deformed in a wide variety of structural styles. Deformation zones are primarily restricted to a well-defined, largely volcaniclastic interval within these two units of the lower Absaroka Volcanic Supergroup (AVS). Unlike the Mesozoic and Paleozoic formations that constitute the major displaced masses of the Heart Mountain, Reef Creek, and South Fork detachment faults in the northern Absaroka Range, the pre-volcanic strata (lower Eocene and older units) in the east-central Absaroka Range appear to have acted as basement during deformation. The rootless character of deformation, and the lack of involvement of younger middle to upper Eocene AVS units, prompted previous workers to propose middle Eocene detachment faulting as the principal process of deformation in the east-central Absaroka Range. However, no source area can be identified for the deformed intervals previously considered "allochthonous," nor has any persistent detachment horizon been discovered. In addition, the structures present represent an exceptionally broad range of structural styles whose geometries imply deformation prior to pervasive lithification, and whose attitudes suggest a nearly random kinematic pattern. Furthermore, the recognition of large-scale liquefaction phenomena in close association with most other deformational features throughout the study area suggests that the volcanic-volcaniclastic pile was more profoundly affected by in situ disturbances than by extensive detachment faulting in this area. Apparent lateral transport within local domains is generally limited, although well-developed sheath folds imply locally high shear strains ( ≥10). Where subhorizontal mass transport is documented, deformation probably began in response to local instabilities generated by liquefaction at depth. Folding may have proceeded in places at creep rates for decades or centuries after initial liquefaction.

Stratigraphic relations within the AVS in the east-central Absaroka Range are complex, but reveal a pattern of south-southeastward migration of intrusive/extrusive activity throughout middle to late Eocene time. The distal taper of the volcanic-volcaniclastic apron is revealed by southeastward thinning of the Wapiti/Aycross interval as a whole, and individual informally recognized volcanic (Twuf unit) and dominantly epiclastic (Twmb unit) members within this interval. I propose that the deformed basal AVS units in the study area may be the relatively distal stratigraphic equivalents of the Cathedral Cliffs Formation—the basal unit of the AVS in its more proximal northerly reaches. This tentative correlation suggests a genetic link between the spatially and stylistically distinct phenomena of liquefaction-related deformation and Heart Mountain faulting, both of which may stem from a common external high-energy stimulus, probably large-magnitude seismic activity.

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GSA Special Papers

Style and mechanics of liquefaction-related deformation lower Absaroka Volcanic Supergroup (Eocene), Wyoming

Paul L. Decker
Paul L. Decker
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Geological Society of America
Volume
240
ISBN print:
9780813722405
Publication date:
January 01, 1990

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