Fourth-order nonmarine to marine sequences, middle Castlegate Formation, Book Cliffs, Utah

The lower part of the Campanian Castlegate Formation, one of the best known fluvial sheet sands of the Western Interior seaway, combines with an overl ying finer grained unit (middle member of the Castlegate Formation) to form an unconformity-bounded, third-order stratigraphic sequence (similar to 3 m.y. duration). This sequence contains a mappable muddy zone along the west ern Book Cliffs of Utah now mapped as contiguous with the open-marine parts of the succession (Buck Tongue, Sego Sandstone, Anchor Mine Tongue) to the east. The correlation within the middle Castlegate has been refined and th e nature of the link between tidally influenced fluvial strata in the west and marine strata in the east has been remapped, Five high-frequency strati graphic sequences (all within the upper part of the larger third-order sequ ence), forming a thickness to 160 m, have been mapped in the Price area sou theastward into the time-equivalent marine succession near Green River. Ind ividual sequences (probably <0.5 m.y. duration), reflecting transgressive t o regressive estuary infilling, have an internal architecture in which fluv ial and tidally influenced distributary-channel belts pass up into muddy ce ntral-basin sedimentary units, to bayhead-delta deposits, and in some place s up to more fluvial channel belts. The key to the mapping of individual se quences is recognition of the muddy, brackish-water interval, developed dur ing maximum transgression of the time-equivalent shoreline. The two most pr oximal marine-influenced sequences (2 and 3) are probably broadly time-equi valent with the Buck and Anchor Mine Tongues to the east. The analysis here , contrary to recent suggestions, shows that the marine sequences in the ea stern Book Cliffs can be traced far into the time-equivalent, western fluvi o-estuarine succession.