Skip to Main Content
GEOREF RECORD

Reinterpretation of fractures at Swift Reservoir, Rocky Mountain thrust front, Montana; passage of a Jurassic forebulge?

E. M. Geraghty Ward and James W. Sears
Reinterpretation of fractures at Swift Reservoir, Rocky Mountain thrust front, Montana; passage of a Jurassic forebulge? (in Whence the mountains? Inquiries into the evolution of orogenic systems; a volume in honor of Raymond A. Price, James W. Sears (editor), Tekla A. Harms (editor) and Carol A. Evenchick (editor))
Special Paper - Geological Society of America (2007) 433: 197-210

Abstract

A sub-Middle Jurassic unconformity is exhumed at Swift Reservoir, in the Rocky Mountain fold-and-thrust belt of Montana. The unconformity separates late Mississippian Sun River Dolomite of the Madison Group (ca. 340 Ma) from the transgressive basal sandstone of the Middle Jurassic (Bajocian-Bathonian) Sawtooth Formation (ca. 170 Ma). North-northwest-trending, karst-widened fractures (grikes) filled with cherty and phosphatic sandstone and conglomerate of the basal Sawtooth Formation penetrate the Madison Group for 4 m below the unconformity. The fractures link into sandstone-filled cavities along bedding planes. Clam borings, filled with fine-grained Sawtooth sandstone, pepper the unconformity surface and some of the fracture walls. Sandstone-filled clam borings also perforate rounded clasts of Mississippian limestone that lie on the surface of the unconformity within basal Sawtooth conglomerate. After deposition of the overlying foreland basin clastic wedge, the grikes were stylolitized by layer-parallel shortening and then buckled over fault-propagation anticlinal crests in the Late Cretaceous-Paleocene fold-and-thrust belt. We propose that the grikes record uplift and erosion followed by subsidence as the Rocky Mountain foreland experienced elastic flexure in response to tectonic loading at the plate boundary farther to the west during the Middle Jurassic. The forebulge opened strike-parallel fractures in the Madison Group that were then karstified. The sandstone-filled karst system contributes secondary porosity and permeability to the upper Madison Group, which is a major petroleum reservoir in the region. The recognition of the fractures as pre-Middle Jurassic revises previous models that have related them to Cretaceous or Paleocene fracturing over the crests of fault-propagation folds in the fold-and-thrust belt, substantially changing our understanding of the hydrocarbon system.


ISSN: 0072-1077
EISSN: 2331-219X
Coden: GSAPAZ
Serial Title: Special Paper - Geological Society of America
Serial Volume: 433
Title: Reinterpretation of fractures at Swift Reservoir, Rocky Mountain thrust front, Montana; passage of a Jurassic forebulge?
Title: Whence the mountains? Inquiries into the evolution of orogenic systems; a volume in honor of Raymond A. Price
Author(s): Ward, E. M. GeraghtySears, James W.
Author(s): Sears, James W.editor
Author(s): Harms, Tekla A.editor
Author(s): Evenchick, Carol A.editor
Affiliation: University of Montana, Department of Geosciences, Missoula, MT, United States
Affiliation: University of Montana, Department of Geosciences, Missoula, MT, United States
Pages: 197-210
Published: 2007
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 46
Accession Number: 2008-088182
Categories: Structural geologyStratigraphy
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. strat. col., geol. sketch map
N48°04'60" - N48°13'60", W112°54'00" - W112°43'60"
Secondary Affiliation: Amherst College, USA, United StatesNatural Resources Canada, CAN, Canada
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200827
Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal