REPETITIVE STRATAL PATTERNS IN A FORELAND BASIN SANDSTONE AND THEIR POSSIBLE TECTONIC SIGNIFICANCE

Two distinctive, repetitive stratal patterns within members of the Upp er Cretaceous Blackhawk Formation in Utah are recognized by comparison of (1) progradational distances of individual parasequences within ea ch member and (2) the position of the updip termination of marine faci es for each parasequence. The oldest parasequence within each member a lways progrades substantially farther than the younger parasequences w ithin the same member, regardless of whether these parasequences are s eparated by sequence boundaries. The pattern of progradational distanc es results in an initial progradational to later aggradational stackin g pattern within each member. This pattern is observed in all members of the Blackhawk Formation where there is sufficient outcrop exposure to trace the nonmarine-to-marine transition, which is used to establis h the progradational distances. These stratal patterns are interpreted to define the path of relative change of sea level through time withi n the Cretaceous foreland basin and may provide information characteri zing thrust-sheet emplacement. Flooding events bounding parasequences may represent periods of punctuated thrusting, with the main flooding surface (member boundaries) representing the period of greatest thrust -sheet movement.