INFLUENCE OF MECHANICAL STRATIGRAPHY ON JOINT PATTERNS AT SHEEP MOUNTAIN ANTICLINE

Joint patterns associated with folding can be influenced by both fault-tip stresses and the mechanical characteristics of the rock stratigraphy. The mechanical stratigraphy of a rock sequence describes the thickness and material properties of each layer along with the nature of the layer contacts. While stresses associated with fault- tips contribute to all fault-cored folds, not all folds record these stresses in the joint patterns along the overlying fold. Differences in mechanical stratigraphy could account for the differing expression of jointing on otherwise similar folds. For example, stratigraphic sequences of thick, stiff carbonate reef sequences may more closely resemble deformation in an elastic half-space whereas interbedded strata may deform as individual isolated beds. We explore this hypothesis with joint observations and models of Sheep Mountain Anticline, WY. Joint data was collected around the northern fold termination of Sheep Mountain Anticline within a sequence characterized by interbedded sandstones, limestones, and shale. The joint patterns are compared to joint patterns simulated using two end-member mechanical models: a plate-bending model, which examines only flexural stresses, and a Boundary Element Method (BEM) model where a fold is formed due to slip along an underlying fault. Because the plate-bending model assumes zero friction along the top and bottom of the plate, this model simulates deformation of an isolated layer. In contrast, the BEM model simulates deformation of a thick layer and includes fault-tip stresses. The joint patterns observed at Sheep Mountain Anticline most closely resemble the plate-bending model in all layers observed. This indicates that these layers are deforming in relative isolation without significant expression of the fault-tip stress field. We suspect that the distributed strain within shale layers between the fractured sandstone beds acts to dissipate flexural strain and mechanically isolate the sandstone layers. This is supported by the observation that the closest match to the plate-bending model was a bed bounded by thick shale sequences. It follows that the fault-tip stresses may not be expressed within the mapped strata because thick shale sequences above or at the fault tip may attenuate the fault-tip stresses at Sheep Mountain Anticline.