Abstract

Macrofracture analysis and dynamic analysis of calcite twin lamellae and quartz deformation lamellae detail the stress history during evolution of the Dry Creek Ridge anticline — a multilayered, multilithologic fold near Livingston, Montana. This study analyzes data from sandstone and limestone units in the Jurassic Rierdon, Swift, and Morrison Formations and in the Cretaceous Kootenai Formation. During the early stages of folding, σ1 was parallel to layering and was oriented normal to the anticlinal axis. A fracture system, F1, formed in all layers in response to this stress state. As predicted from mechanical bending theory, a neutral surface developed as folding proceeded. The location of this surface is indicated by petrofabric data and approximates the Kootenai-Morrison contact. In the lower portion of the anticline, σ1 remained parallel to bedding, although locally the stress rotated. In the upper portion, σ1 became reoriented normal to bedding. At this time (or later during fold growth), the lower units deforming in the compressional domain (σ1 parallel to layering) of a single mechanical plate subdivided into two separate plates. The stratigraphically lower of these units (Rierdon Formation) was then located in the extensional domain of a mechanical plate (σ1 normal to layering). During this stage of deformation, the upper portion of the fold (Morrison and Kootenai Formations) did not change behavior. The stress history of this anticline agrees in general with published theoretical models and explains the superposition of several fracture sets at one position in a fold.

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