The correlation between joint orientation and transport direction in the Sawtooth Salient, Northern Montana.

Open Access
Author:
Cannon, David L
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 26, 2008
Committee Members:
  • Terry Engelder, Thesis Advisor
Keywords:
  • Swift Reservoir
  • stress field
  • fracturing
  • transport direction
  • joint
  • Sawtooth
  • Teton Anticline
  • Sun River
Abstract:
The Sawtooth Range in Montana is a Sevier orogenic event. Recent dating determined the main phase of thrusting to have occurred between 74-59Ma. The Northern Disturbed Belt is a section of the Sevier Orogenic Belt that contains the Sawtooth Range with its four major structural provinces; Flathead Range Complex, Sun River Valley, Sawtooth Range Complex, and the High Plains Complex. Situated in northern Montana, the 250 km Northern Disturbed Belt thrust front has a common transport of ENE. At Sawtooth Range, the 100 km Sawtooth Salient has three general transport directions; E, ENE, and NE. Seismic evidence, well control, and structural measurement suggests that the orocline bend is a manifestation of thin-skinned Sevier tectonics interacting with basement topography. Jointing in the Sawtooth Range and High Plains Complex help determine paleostress trajectories. Andersonian stress regimes for thrust faulting favors extension fracturing on a horizontal plane during thrust faulting. At Sawtooth Salient, vertical jointing prevails. A stress regime that produces vertical extensional fractures and maintains a horizontal maximum stress is strike-slip faulting. In order to facilitate vertical extension fracturing in a thrust faulting regime, there must be strike parallel stretching to counteract the less horizontal stress and convert it to the least compressive stress (ó3). During orocline development, significant strike parallel stretching occurs as an accommodation mechanism. In response to this transport with accommodating strike parallel stretching, joints will form parallel to localized transport. All locations contain a major set of ENE joints. Northern joint measurements contain both ENE and NE joints. Southern joint measurements contain both ENE and E joints. Abutment relationships in the field suggest that the ENE joint set was first to form and then both NE and the E set formed afterwards. Additional field evidence of the ENE joint set occurs at Teton Anticline and Lesser Teton Anticline. Previously, jointing at Teton Anticline has been wholly attributed to folding stresses. Current field evidence shows that the ENE joint cuts through both Teton Anticline and Lesser Teton Anticline. Convincing evidence occurs at the nose of Lesser Teton Anticline. One hypothesis for jointing at an anticlinal nose is strike parallel stretching, creating an array of joints that follow dip direction. However, at Lesser Teton Anticline the ENE joint set dominates regardless of structural position. This evidence suggests the ENE joint set formed before the development of Teton Anticline and Lesser Teton Anticline. Finally, a switch of stress regimes from thrust faulting to strike-slip faulting occurred. First introduced as bedding parallel strike slip faults, large conjugate wrench faults occur at Teton Anticline and thrust sheets toward the hinterland. Current measurement shows slip sub-normal to bedding, suggesting wrench faulting after bedding rotation has occurred in the thrust sheets and Teton Anticline.