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Characteristics of progradational shoreface units have been described from several sandstones in the Cretaceous Western Interior seaway. Little work has been done to document the characteristics of shoreface deposits in a transgressive setting. Outcrop studies of the Sego Sandstone (Campanian), in Rio Blanco County, Colorado, indicate that an initial episode of progradation was followed by a stagnation of the system in which minor transgressions occurred. Transgression of the foundering coast is evidenced by landward movement and storm breaching of a thinned shoreface unit and by the aggradation of a thick associated tidal flat and lagoonal complex.
Three lithofacies are recognized in the shoreface deposits from 80 measured sections; each is composed of very fine-grained, laminated sandstone. (1) Stacked sequenced, 1 to 1.5 m (3.3 to 5 ft) thick, of interbedded sandstone and rippled sandy mudstone. The sandstone beds have 0.5 to 1 m (1.6 to 3.3. ft) thick, imbricated wedge-planar laminations that thin rapidly to the northwest into broad low-angle trough and antiform bedding before pinching out into the mudstone as rippled and burrowed beds. (2) Sandstone with low-angle trough cross-stratification and imbricated wedge-planar sets, but no associated mudstone. This facies ranges up to 5 m (16 ft) thick, but is most commonly 2 to 3 m (6 to 10 ft) thick. (3) Sandstone in superposed accretionary sets ranging from 0.5 to 3 m (1.6 to 10 t) thick and 8 to 30 m (26 to 100 ft) wide.
Facies 1, which always forms the base of a sequence, is a stacked series of washover fans that thinned rapidly beyond the confined shoreface breach and spread landward into adjacent lagoonal lows. Facies 2 is a stabilized shoreface and is interrupted along strike by facies 3, the longshore-derived fill of storm channels. The stacking and lateral occurrence of filled channels indicates that active zones of weakness existed, and that net landward movement of the shoreface occurred as result of storm washovers.
A thick (80 m, 260 ft) sequence of tidal and paludal sediments is found above the shoreface. Facies include: (1) ripple-stratified, bioturbated humates, and very fine-grained sandstone, interbedded with thin (1 to 30 cm, 0.4 to 12 in.) continuous coals. (2) Thin-bedded fine-grained sandstone and mudstone with flaser bedding and starved current ripples. (3) Very fine-grained, laminated sandstone with low-angle through cross-stratification and accretionary sets; nearly identical to facies 2 and 3 of the shoreface but thinner (1 m, 3.3 ft). (4) Small to large scale, stacked accretionary sets of fine-grained sandstone ranging to 10 m (33 ft) thick (individual sets 0.1 to 3 m, 4 in. to 10 ft thick), containing rippled foresets and abundant Ophiomorpha. Mud clast conglomerates are found in ome of the smaller channel forms.
Study of 30 measured sections indicates that lagoonal deposits (facies 1) become sandier upward and are overlain by sand tidal flats and mixed tidal flats (facies 2). A system of meandering tidal creeks eroded into the contemporaneous tidal flat and lagoon facies. These creeks coalesced to form a central tidal inlet (facies 4). A subsequent transgression reworked shoreface sandstones (facies 3). Influx of sediment by longshore currents reinitiated progradation of the system. Meander-belt and floodplain facies were deposited over the tidal flat-lagoon complex during this regressive phase.
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