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GEOREF RECORD

A new conceptual fold-fracture model including prefolding joints, based on the Emigrant Gap Anticline, Wyoming

Stephan Bergbauer and David D. Pollard
A new conceptual fold-fracture model including prefolding joints, based on the Emigrant Gap Anticline, Wyoming
Geological Society of America Bulletin (April 2004) 116 (3-4): 294-307

Abstract

Symmetric relations between synfolding fractures and fold geometry have been used to explain the formation of fractures in folded sedimentary rocks. Based on field observations at the Emigrant Gap, a Laramide anticline located near Casper, Wyoming, we propose that the orientation of synfolding fractures was influenced dominantly by two tectonic joint sets, J1 and J2, which developed prior to folding. Rotations appropriate for unfolding the strata reorient most fractures such that they strike sub-parallel to one or the other of the two joint sets and oblique to the fold hinge line. On the fold limbs, these prefolding joints apparently propagated along strike and infilled during folding to form a denser network of joints bedding perpendicularly with spacing approximately 1/5 to 1/10 that of the same joint set in nearby unfolded outcrops of the same formation. With proximity to the hinge line of the anticline, where bending of strata and surface curvature are elevated, the style of deformation is different. Whereas most J2-parallel fractures still are joints, fractures in the J1 orientation include sheared prefolding joints and bedding-inclined shear fractures. Apparent normal offsets of bedding by these shear fractures are an order of magnitude larger than offsets observed across sheared joints on the fold limbs. The proposed model of fracture-fold relationships differs from existing conceptual models because it postulates that folding-related fracture orientations are controlled by the orientations of prefolding fractures. For the Emigrant Gap anticline, these fractures are not oriented symmetrically with respect to the fold hinge line.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 116
Serial Issue: 3-4
Title: A new conceptual fold-fracture model including prefolding joints, based on the Emigrant Gap Anticline, Wyoming
Affiliation: Stanford University, Department of Geological and Environmental Sciences, Stanford, CA, United States
Pages: 294-307
Published: 200404
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 71
Accession Number: 2004-028816
Categories: Structural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. block diags., sects., geol. sketch map
N42°47'60" - N43°00'00", W106°49'60" - W106°34'60"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200409
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