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Abstract

John C. Lorenz, 2013, Fractures, hydraulic injections, and strain accommodation in the Tensleep Formation at Flat Top Anticline, Carbon County, Wyoming, in C Knight. and J. Cuzella, eds., Application of structural methods to Rocky Mountain hydrocarbon exploration and development: AAPG Studies in Geology 65, p. 157174.

DOI:10.1306/13381695St653572

Copyright © 2013 by The American Association of Petroleum Geologists.

Fractures, Hydraulic Injections, and Strain Accommodation in the Tensleep Formation at Flat Top Anticline, Carbon County, Wyoming

John C. Lorenz

FractureStudies LLC, 99 Rainbow Rd, Edgewood, New Mexico, 87015, U.S.A. (e-mail: [email protected])

ABSTRACT

Several types of meter-scale structures accommodated strain during folding of the Tensleep sandstones at Flat Top Anticline, a compound fold overlying an east-northeast-west-southwest striking Laramide thrust fault in southeastern Wyoming. The suite of structures includes (1) syn-depositional hydraulic injection fractures that were reactivated in shear and extension during folding, (2) early-formed hinge-oblique extension fractures, (3) later-formed hinge-parallel extension fractures concentrated on the crest and forelimb, (4) scattered small shear planes oriented both parallel and oblique to the large-scale eolian cross-bed foresets, (5) larger-scale bedding-parallel shear between sedimentary units on the steeper forelimbs, (6) faults, and (7) rare scattered deformation bands. The hinge-oblique extension fractures strike parallel to the direction thrusting, which was not normal to the basement fault. Most of these structures formed due to extension of the strata parallel and oblique to the anticlinal hinge as folding developed over the thrust fault. The degree and type of extension fracturing vary by structural position: hinge-oblique fractures dominate the unfolded backlimb, whereas both hinge-oblique and hinge-parallel fractures developed on the forelimb and anticlinal crest. Paradoxically, extension fracturing is minimal where folding is most acute at the westernmost Pine Butte substructure, where small faults, bedding-parallel shear, and reactivation of the preexisting, well-developed suite of hydraulic injectites accommodated most of the strain. Many of these structures record more than one structural event.

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