--> Fractures at the Surface in the San Juan Basin, New Mexico and Colorado: Implications for Petroleum, Groundwater, and Engineering Geology, by N. H. Whitehead, III; #90986 (1994).

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Abstract: Fractures at the Surface in the San Juan Basin, New Mexico and Colorado: Implications for Petroleum, Groundwater, and Engineering Geology

Neil H. Whitehead, III

Regional fractures are essentially restricted to brittle rocks. Most weakly to moderately indurated sandstones deform intergranularly and remain unfractured. Along the south basin flank, face cleats in Fruitland coals and one joint set in calcareous sandstones in underlying Pictured Cliffs and Cliff House Sandstones strike northerly. In Colorado, a northwest-striking joint set is prominent in rocks as young as the Eocene San Jose Formation.

Valley-parallel fractures form when gravity-induced stress causes rock to expand preferentially in a transverse direction until tensile strength is exceeded. Gravity-induced stress fractures: strike parallel to topographic contours; are vertical to almost vertical; have heights up to fifty feet in massive beds; have lengths as much as several hundred feet in massive beds; may be outlined by vegetation to show as photolinears; are restricted to moderately indurated sandstones; and stop at bedding planes. The number of fractures in outcrop progressively decreases away from the valley wall to zero in the interfluve area.

Regional fracture sets have a consistent orientation through a stratigraphic interval of several hundred feet, suggesting they may be extrapolated to Upper Cretaceous reservoir depths. Understanding that valley-parallel fractures are surface phenomena is important in photolinear analysis; in stability studies for highwalls, roadcuts, and dam foundations; and predicting groundwater yields or contaminant plumes in bedrock in a valley vs. an interfluve location.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994