Date of Award
Master of Science
Mervin Jerome Bartholomew
Roy Van Arsdale
The Little Water syncline is a complex structural feature within the Tendoy thrust sheet which formed during two temporally overlapping deformational episodes during the Late Cretaceous. The NNW-trending Four Eyes Canyon thrust and the structurally lower Tendoy thrust are associated with NE-SW-shortening related to the Cordilleran (Sevier-style) fold-thrust belt. The Tendoy thrust forms the structural front of the Cordilleran fold-thrust belt in southwestern Montana and its detached, NNW-trending overturned limb (of the Little Water syncline) was over-ridden by the Four Eyes Canyon thrust. Earlier research primarily used stratigraphic evidence coupled with geometric interpretations to suggest that: 1) clasts found in the syn-tectonic (Cretaceous) Beaverhead Formation were derived from the Four Eyes Canyon thrust; 2) Laramide-style deformation occurred before emplacement of the Tendoy thrust sheet; and 3) that the Tendoy thrust was then emplaced over the syn-tectonic Beaverhead Formation. The NE-trending (approximately 30o), overturned northern flank (at nearly right angles to the detached overturned limb) of the Little Water syncline has long been argued to be a structural feature associated with NW-SE-shortening (~120o) generally related to the Laramide Snowcrest thrust system. In this study of the Little Water syncline, I have used layer-parallel-shortening (LPS) strain-indicators (fossils and pellets) to obtain the initial direction of shortening preserved in: 1) the NNW-trending, detached, vertical-to-overturned, western limb; 2) the steeply-dipping-to-overturned, northwestern limb; and 3) the moderately W-dipping, upright, N-trending eastern limb to determine the initial deformation (Sevier or Laramide) that affected the syncline. When bedding is unfolded, LPS strain: at 11 of 14 sites is consistent with NE-SW-shortening (221o) associated with the Sevier orogeny; at 2 sites is consistent with N-S-shortening; and at 1 site is consistent with E-W-shortening. No sites have LPS strain consistent with NW-SE-shortening. Hence Sevier LPS strain occurred before Laramide deformation. The 3 sites (sites 1, 2, and 5) where LPS strain lies outside of Sevier-shortening (221o +20o), but not within the range of Laramide shortening (~110o-290o) likely represent Sevier LPS strain affected by local factors not accounted for in the retro-deformational sequence used to restore bedding to horizontal in this study.
dissertation or thesis originally submitted to the local University of Memphis Electronic Theses & dissertation (ETD) Repository.
Judy, Kera Ann, "Layer Parallel Shortening: A Mechanism Used to Determine the Sequence of Deformation of the Little Water Syncline in the Tendoy Mountains, Southwestern Montana." (2012). Electronic Theses and Dissertations. 516.