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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 bound-aries, 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 bound-ary, 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, amalgamated lower-shoreface sandstones with sharp, slightly erosional lower boundaries. These boundaries are interpreted to be caused by storm processes and are unrelated to sea level falls or forced regressions (Posamentier et al., 1992).

Lowstand strata in both the Desert and Castlegate sequences are braid-ed-stream deposits forming fluvial sheet sandstones contained within incised valleys at least 50 mi wide. Sheet sandstones are regionally wedge- shaped and thin and fine basin ward. These fluvial lowstand deposits form megafans that prograded to the east off of the Sevier thrust belt. In both Desert and Castlegate lowstand systems tracts, fluvial systems do not connect to the sea, but die out into a broad, swampy complex of shallow lakes. Lowstand fluvial sandstones have no coeval siliciclastic lowstand beaches or deltas in the study area. Instead, the lowstand shoreline is marked by oolites in the Castlegate. Distal ends of these mega fans behave similarly to terminal fans described in literature (Friend, 1978). Channelized sediment at the distal end of these terminal fans downlaps onto the sequence boundaries.

The incised valleys within which the mega fans are contained are interpreted to form and fill in response to relative falls in baselevel and upstream migration of knickpoints through highstand alluvial channels. In the Castlegate lowstand sequence set, interpreted intersection of upstream- migrating knickpoints with alluvial fans in front of the Sevier thrust belt result in a significant increase in discharge causing deeper incision of the valley and development of the observed wedge-shaped, basinward-thin-ning sandstone body. This occurs independently of sea level change.

Sequence boundaries and systems tracts within the Desert and Castlegate can be continuously traced in both outcrop and subsurface, moving updip from the marine to totally nonmarine parts of the system. These relationships are used to develop an integrated model for nonmarine sequence stratigraphy applicable to the subsurface elsewhere. Nonmarine sequences fine upward. The lowstand systems tract is commonly a multi-story, high net/gross sandstone with excellent lateral continuity filling the updip end of an incised valley. The transgressive systems tract is composed of thinner, single-story sandstones, commonly deposited by point bars and associated overbank and crevasse-splay strata, forming a moderate to low net/gross interval. Trace-fossil assemblages typical of brackish water commonly occur in this systems tract. The highstand systems tract is dominated by muddy levee, lake margin, and crevasse-splay deposits punctuated by thin, singlestory channels, forming a low net/gross interval.

Major sequence boundaries split the Desert Member and Castlegate Sandstone. For this reason, the more downdip strata, called Desert and Castlegate using formation names, are temporally and physically different from updip strata with the same names. This difference between sequence stratigraphy and lithostratigraphy reflects alternative ways of thinking about the stratigraphic and paleogeographic evolution of these units, with signifi-cant practical implications for basin exploration and production of hydrocarbons.

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