- UW Department of Geology and Geophysics
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 J1 set and a secondary J2 set. Joints in J1 sets have two distinct orientations: a N19W average strike (47 stations) and a N32E average strike (6 stations). J2 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 J1 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 J1 joints are parallel to the last phase of local shortening and to Miocene mafic dikes, suggesting formation during post-Laramide extension. Subsequent NNW-striking J1 joints are parallel to local normal faults and may represent either regional Neogene extension or tectonic rebound from the main Laramide shortening. J2 cross joint sets in sandstones formed perpendicular to both bedding and J1 joints during uplift and erosion and thus probably do not extend far below the surface.
The NNW- and NNE-striking J1 joints may be open at depth and may act as preferential flow pathways for fluids. Nearer to the surface, systematic cross-joints (J2) increase fracture density, which may be important for shallow aquifers and coalbed methane plays.