Electronic Theses and Dissertations

Identifier

626

Kera Ann Judy

Date

2012

Document Type

Thesis

Degree Name

Master of Science

Major

Earth Sciences

Concentration

Geology

Committee Chair

Mervin Jerome Bartholomew

Committee Member

Roy Van Arsdale

Committee Member

Randall Cox

Abstract

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.

Comments

Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to the local University of Memphis Electronic Theses & dissertation (ETD) Repository.

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