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John C. Van Wagoner
Exxon Production Research Co.
Houston, Texas, U.S.A.
The Desert Member of the Blackhawk Formation and the Castlegate Sandstone in the Book Cliffs of Utah and Colorado were analyzed to develop a model of sequence stratigraphy and facies architecture in foreland basins. Stratal architecture of these rocks is controlled by two regionally extensive surfaces of incision and subaerial exposure interpreted as major sequence boundaries, and at least six surfaces of incision and subaerial exposure with more limited lateral extent, interpreted as minor sequence boundaries. Each sequence boundary is a regionally correlatable, single surface. Major sequence boundaries can be traced from marine into proximal alluvial plain environments. Sequence boundaries have different physical expressions depending upon where in the basin they are observed. Strata beneath sequence boundaries are unrelated to strata above, and time lines cannot be carried across these boundaries. Flooding surfaces, called parasequence or parasequence set boundaries, also exert an important control on facies architecture.
Based on parasequence stacking patterns, position of sequence boundaries, and facies associations, the Desert Member and Castlegate Sandstone have been divided into sequences with durations of 200,000 to 300,000 yr. There are an order of magnitude more sequences in this interval than are predicted by the global sea level chart of Haq et al. (1988). The major sequences have been divided into the following systems tracts or sequence sets: (1) Grassy highstand sequence set below the Desert sequence boundary, named for the Grassy Member of the Blackhawk Formation, (2) Desert lowstand, transgressive, and highstand systems tracts between the Desert and Castlegate sequence boundaries, (3) Castlegate lowstand sequence set resting on the Castlegate sequence boundary, and (4) Castlegate transgressive systems tract.
Parasequence stacking patterns suggest that the Grassy highstand sequence set and the Desert highstand systems tract were deposited during a relative rise in baselevel. The Grassy highstand sequence set is punctuated by minor relative falls in baselevel. Both of these stratal units contain thick,
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