Field studies of joints: Insufficiencies and solutions, with examples from the Piceance Creek Basin, Colorado

Abstract

A standard approach for identifying joint sets (groups of mutually parallel joints) within an exposure of rock is to record the orientation of every available joint along a traverse of the exposure until 100 to 200 joints have been measured, and to assume that the most common strikes among the data represent valid sets. Sets are then correlated among scattered exposures on the basis of matching or similar strike; their geologic significance may be inferred in part from their orientation relative to nearby major structures. An ongoing study of joints in the Piceance Creek basin reveals some limitations of this method, and shows that misinterpretations of fracture history are likely to occur unless joint sets are identified and correlated by multiple attributes, not by strike alone. Difficulties in interpretation can arise from undue emphasis on joint strike where sets curve from one locality to another; jointing has recurred along similar trends at different times; multiple, coeval sets have identical strike but different dips and mode of fracture; dissimilar joint patterns exist in different beds within the same exposure; joints have been rotated from their original orientation; joints are unrelated to nearby major structures; and joints have formed with diverse orientationsmore » within the weathering zone. All of these difficulties can be forestalled by defining the relative age and style of the various joint sets, using simple field techniques.« less