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Style and mechanics of liquefaction-related deformation, lower Absaroka volcanic supergroup (Eocene), Wyoming

Paul L. Decker
Style and mechanics of liquefaction-related deformation, lower Absaroka volcanic supergroup (Eocene), Wyoming
Special Paper - Geological Society of America (1990) 240: 71 pp.


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 (< or = > or =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.

ISSN: 0072-1077
EISSN: 2331-219X
Serial Title: Special Paper - Geological Society of America
Serial Volume: 240
Title: Style and mechanics of liquefaction-related deformation, lower Absaroka volcanic supergroup (Eocene), Wyoming
Author(s): Decker, Paul L.
Affiliation: Univ. Wis.-Madison, Dep. Geol. Sci., Madison, WI, United States
Published: 1990
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
ISBN: 0-8137-2240-3
ISBN: 978-0-81372-240-5
Number of pages: 71
References: 149
Accession Number: 1990-051612
Categories: Structural geology
Document Type: Serial Map
Bibliographic Level: Monographic
Illustration Description: illus. incl. 6 tables, sects., block diags., geol. sketch maps
Map Scale: 1:20,000
Map Type: geologic maps
N41°00'00" - N45°00'00", W111°04'60" - W104°04'60"
N43°30'00" - N45°00'00", W110°15'00" - W109°00'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2021, American Geosciences Institute.
Update Code: 1990
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