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Origin of Cretaceous to Holocene fractures in the northern San Juan Basin, Colorado and New Mexico

Jason C. Ruf and Eric A. Erslev
Origin of Cretaceous to Holocene fractures in the northern San Juan Basin, Colorado and New Mexico
Rocky Mountain Geology (July 2005) 40 (1): 91-114


The timing and origins of minor faults and extensional fractures (joints) in the San Juan Basin of the Rocky Mountain foreland have generated intense debate. Hypotheses include nearly syn-sedimentary fracturing guided by pre-existing Precambrian weaknesses, syn-Laramide fracturing, and overburden-release jointing during Holocene erosion. Furthermore, the Laramide deformation that formed the San Juan Basin is also poorly resolved, with hypotheses including single-stage, unidirectional shortening and multi-stage, multidirectional shortening. The stress and fracture history of the basin is critical for predicting subsurface fracture trends that can control hydrocarbon and aquifer permeability and production. In order to test fracture and tectonic hypotheses, minor fault (n = 842) and joint (n = 2619) data were collected from 76 outcrops of Jurassic through Paleogene rocks in the northern San Juan Basin. Multiple conjugate sets of minor faults with consistent crosscutting relationships indicate multi-directional shortening with four episodes of deformation: (1) N56E Laramide shortening, (2) N55W shortening, (3) N17E post-Laramide shortening, and (4) N59E post-Laramide extension. Most joint localities show two regional joint sets, a dominant J (sub 1) set and a secondary J (sub 2) set. Joints in J (sub 1) sets have two distinct orientations: a N19W average strike (47 stations) and a N32E average strike (6 stations). J (sub 2) cross-joint sets have a N72E average strike and were probably generated at shallow depths. Subsurface information for the Dakota Sandstone from a well in the basin indicates two sets of N14W- and N18E-striking fractures, similar to J (sub 1) surface joint sets. NE-striking coal cleats are oblique to the main NNE-trending Laramide shortening and perpendicular to the later NW-trending shortening. They may have formed either during earlier shortening related to the Sevier fold-and-thrust belt or during tectonic rebound from the last Laramide shortening. NNE-striking J (sub 1) joints are parallel to the last phase of local shortening and to Miocene mafic dikes, suggesting formation during post-Laramide extension. Subsequent NNW-striking J (sub 1) joints are parallel to local normal faults and may represent either regional Neogene extension or tectonic rebound from the main Laramide shortening. J (sub 2) cross joint sets in sandstones formed perpendicular to both bedding and J (sub 1) joints during uplift and erosion and thus probably do not extend far below the surface. The NNW- and NNE-striking J (sub 1) joints may be open at depth and may act as preferential flow pathways for fluids. Nearer to the surface, systematic cross-joints (J (sub 2) ) increase fracture density, which may be important for shallow aquifers and coalbed methane plays.

ISSN: 1555-7332
EISSN: 1555-7340
Serial Title: Rocky Mountain Geology
Serial Volume: 40
Serial Issue: 1
Title: Origin of Cretaceous to Holocene fractures in the northern San Juan Basin, Colorado and New Mexico
Affiliation: Colorado State University, Department of Geosciences, Fort Collins, CO, United States
Pages: 91-114
Published: 200507
Text Language: English
Publisher: University of Wyoming, Dept. of Geology and Geophysics, Laramie, WY, United States
References: 81
Accession Number: 2005-066692
Categories: Structural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, geol. sketch maps
N38°00'00" - N46°00'00", W110°00'00" - W104°00'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, University of Wyoming
Update Code: 200540
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