Abstract

Two distinctive, repetitive stratal patterns within members of the Upper Cretaceous Blackhawk Formation in Utah are recognized by comparison of (1) progradational distances of individual parasequences within each member and (2) the position of the updip termination of marine facies for each parasequence. The oldest parasequence within each member always progrades substantially farther than the younger parasequences within the same member, regardless of whether these parasequences are separated by sequence boundaries. The pattern of progradational distances results in an initial progradational to later aggradational stacking 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 establish the progradational distances. These stratal patterns are interpreted to define the path of relative change of sea level through time within the Cretaceous foreland basin and may provide information characterizing 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.

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