Alluvial and sequential architecture of the lower and middle Castlegate Formation, Book Cliffs, east-central Utah

The Castlegate Formation consists of lower and middle members that show varying architecture of fluvial sandbodies. The lower Castlegate is an amalgamation of channel belts and the middle Castlegate has more isolated sandbodies encased in muddy deposits. The lower and middle Castlegate members combine to form an unconformity-bounded, third-order stratigraphic sequence (∼4 million years). Within these units, a three-tiered hierarchy is established. The three tiers within the Castlegate are (1) channel-bar system (<10 m thick), (2) channel belt (10--15 m thick), and (3) high-frequency sequences (>15 m thick). An idealized channel-bar system consists of a lower, massive, thalweg unit overlain by bar and occasional overbank deposits. Channel-bar systems combine to form channel belts that define avulsion events. Channel belts are stacked to form high-frequency sequences.; The lower Castlegate member was deposited by low- to moderate-sinuosity rivers during low rates of base level rise. Channel-bar development is poor, with the remnants of multiple channel-bar systems forming the basal parts of channel-belts. The preservation of a bar overlying the composite remnants of multiple channel-bar systems suggests avulsion of the active channel belt. Alternations of thalweg-dominated zones and associated overlying zones of preserved bar deposits suggest high-frequency sequences (fourth-order). Three such successions are defined within the lower Castlegate member. The middle Castlegate member was deposited by a higher-sinuosity meandering system during higher rates of base level rise. Channel-bar systems are well-preserved and nested within larger channel belt bodies encased in overbank deposits. The overbank deposits locally contain brackish-water trace fossils. Such a succession of sandy channel-belt and overlying brackish, muddy deposits can delineate high-frequency sequences. Within the fluvial channel belts of the middle Castlegate member, a wedge of estuarine deposits consisting of central basin muds and bayhead deltas overlies a transgressive surface. These estuarine and fluvial deposits define up to five high-frequency sequences within the middle Castlegate succession. Correlation of these sequences with time-equivalent, shallow marine successions suggests the estuarine deposits are the landward expression of both the Buck Tongue and Anchor Mine Tongue transgressions. Thus, nonmarine and shallow marine successions can be linked within a sequence stratigraphic framework.