JURASSIC JOINTS IN MISSISSIPPIAN CARBONATE, ROCKY MOUNTAIN THRUST FRONT, MONTANA: IMPLICATIONS FOR FRACTURED HYDROCARBON RESERVOIRS

A strike-parallel joint set in Mississipian carbonate along the Rocky Mountain thrust front of northwestern Montana contains Middle Jurassic neptunian dikes and therefore pre-dates the emplacement of the thrust sheets by at least 80 m.y. In a widely-accepted model, the joints have been interpreted as kinematically linked to extension over fault-propagation fold crests. Their pre-thrust age requires re-evaluation of the mechanics assumed in the development of regional fractured hydrocarbon reservoirs in Mississippian and Devonian carbonate. The Jurassic joints are well exposed on a dip-slope that reveals the sub-Middle Jurassic unconformity at Swift Reservoir, west of Dupuyer, Montana. Karstification widened the joints to a depth of ~2 meters below the Middle Jurassic erosional surface; some caves branched outward along bedding. Limestone-clast conglomerate with a fine, dark brown, cherty sandstone matrix filled the karsted joints. Borings made by a limestone-burrowing clam (Opertochasma?) are lined with fine sandstone on the unconformity and in the joint walls. Limestone clasts in locally preserved basal Jurassic conglomerate are also burrowed. Similar karsted joints with a depth of ~7 meters were identified in areas south of the Swift Reservoir in the Sun River Canyon area by Mudge (1972). The joints were strained by layer-parallel shortening prior to Laramide thrusting and exhibit ptygmatic folds and stylolites. On fold forelimbs, the joints were reactivated as small faults to accommodate flexural-slip folding. We suggest that the joints comprise a regional set that formed normal to basin-slope on the exposed carbonate platform, prior to inundation by the Middle Jurassic sea. Analogous karsted joints are documented in modern carbonate settings. The importance of the Jurassic joints as a structural anisotropy during later folding and thrusting, and their significance to fractured hydrocarbon reservoirs requires further analysis.