ABSTRACT

Macrofracture analysis and dynamic analysis of calcite twin lamellae and quartz deformation lamellae in the Jurassic Swift and Rierdon formations as exposed on the northwestern end of the Dry Creek Ridge anticline illustrate a complicated stress history during folding. At least three and probably four separate principal stress orientations existed during evolution of the anticline, but only a maximum of three separate stress states can be demonstrated for any one location.

During the early stages of folding σ₁ was essentially parallel to bedding dip and σ₃ was parallel to bedding strike. At this time a dominant fracture set, F₁, formed and is identified by the classic relationship of shear and extension fractures. Evidence suggests that σ₂ was similar in magnitude to σ₃ and that these stresses may have changed position, perhaps more than once, during this early stage. Such a change would place σ₂ parallel to bedding strike and σ₃ normal to bedding. No fractures have been identified that correspond to this stress orientation.

Following this initial period of deformation a second fracture set formed when extension normal to local bedding strike became sufficiently large. This set is termed F₂ and is not particularly well developed. Principal stresses were now oriented such that σ₁ was parallel to bedding strike and σ₁ was parallel to bedding dip. No petrofabric evidence supports the existence of this stress state.

During this portion of anticlinal evolution both formations behaved as one mechanical unit. Both petrofabric and fracture evidence shows the position of this unit to be in the lower portions of a bent plate. At some later time the lower part of the Rierdon Formation became a separate mechanical layer and developed a stress state in agreement with that predicted for the upper fibers of a bent plate (σ₁ normal to bedding and σ₁ parallel to bedding dip). This inference is supported by dynamic analysis of calcite twin lamellae.