Abstract The Kaiparowits Basin, located mostly within Grand Staircase–Escalante National Monument, preserves an outstanding record of Late Cretaceous sedimentation in a foreland basin setting. Hosted in these rocks is one of the most continuous and complete records of this period’s ecosystems known from any one geographic area in the world. Recent work in the basin has emphasized macrovertebrate remains and documented many new sites of high scientific value. Recent stratigraphic studies have further refined our knowledge of the depositional systems and chronostratigraphic relationships. Provided is an overview of some of these recent advances, along with the necessary background to provide context .

The Kaiparowits Plateau comprises a 2-km-thick sequence of Upper Cretaceous rocks. The oldest Cretaceous unit in the sequence, the Dakota Formation, has Cenomanian palynomorphs in the middle nonmarine member and late Cenomanian molluscs in the overlying marine member. The Tropic Shale has been dated by marine molluscs as latest Cenomanian to middle Turonian. The Straight Cliffs Formation spans the middle Turonian through the Santonian and has been divided into four members. The Tibbet Canyon Member is of middle Turonian age, based on inoceramid bivalves, and the overlying terrestrial Smoky Hollow Member, though undated, is probably late Turonian in age. An unconformity of latest Turonian age separates the top of the Smoky Hollow Member from the overlying member. Molluscs in the marine facies of the John Henry Member indicate an early Coniacian through Santonian age. The overlying Drip Tank Member is undated. The Straight Cliffs Formation thickens northward across the Kaiparowits basin, indicating greater rates of subsidence in that direction. The overlying Wahweap Formation is considered to be of early Campanian age on the basis of its mammalian fauna. It is divided here into four informal members. The Kaiparowits Formation is middle to late Campanian in age, based on mammalian faunas and reevaluation of previously reported palynofloras. The Canaan Peak Formation has been dated as late Campanian based on palynomorphs, and if this date is valid, the folding and erosion of the underlying sequence occurred very rapidly near the end of the Campanian.

The Dakota Formation of south-central Utah was deposited in the foreland basin to the east of the Sevier orogenic belt. During the early Late Cretaceous, clastics shed from the west accumulated in valleys cut into Jurassic rocks along the western margin of the Western Interior Seaway. Braided stream deposits of horizontal and planar-tabular cross-bedded sand and gravel were the first sediments laid down on the erosional surface. Associated with a transgression of the seaway, anastomosed stream channel sands encased in fine-grained overbank sediments were the next to be deposited. Marine-influenced clastics overlie the fluvial sediments. Initially, these were lagoonal or estuarine beds of fine-grained material with a brackish fauna. The overlying lower shoreface sediments that are similar in appearance are differentiated by an open marine fauna and minor bedding characteristics. Tidal channel deposits of bimodally cross-bedded sandstone provide evidence for a barrier bar system. The final transgression that destroyed the topographically positive features of the barrier bar system is represented by a locally present ravinement deposit and the overlying open marine Tropic Shale. There are approximately equal percentages of spores, gymnosperm pollen, and angiosperm pollen in the Dakota Formation. Among angiosperm pollen, tricolpate forms are the principal types. There are very few coniferous pollen grains; the gymnosperm pollen content is almost entirely composed of the Araucariaceae and the Taxodiacaeae. A variety of fern and bryophyte spores was identified, plus an acritarch. The assemblage of pollen and spores suggests a warm temperate to subtropical climate. Palynomorphs, particularly the presence of psilate and reticulate tricolporate pollen, indicate the Dakota Formation is no older than Cenomanian in age. Just above the Dakota Formation in the Tropic Shale the Sciponoceras gracile ammonite zone indicates that the Dakota Formation is no younger than Cenomanian.

Lithology, carbonate and organic carbon geochemistry, and molluscan macrofauna were investigated in fine-grained clay-dominated strata spanning the Cenomanian-Turonian Stage boundary in the southern Colorado Plateau region. Analyses of these data show basinal and onshore to offshore changes in sedimentologic and molluscan assemblage trends through the upper Cenomanian Sciponoceras gracile and Neocardioceras juddii and lower Turonian Watinoceras coloradoense Biozones. Diverse infaunal/epifaunal bivalve assemblages are best developed in the Sciponoceras gracile Biozone and at the more clay-rich nearshore sections. Gastropod and infaunal bivalve abundance and diversity decrease in an offshore and up-section direction as carbonate and organic carbon percentages generally increase. The Neocardioceras juddii Biozone precedes the stage boundary and is characterized in Utah and Arizona by pulsed increases in organic carbon and detrital-feeding gastropods, and by a shift in infaunal bivalve dominance toward species adapted for soft-substrate conditions. These data suggest that pulses of increased organic detrital resources and decreased substrate firmness were associated with the stage boundary extinctions. The stage boundary is marked by a change to depauperate epifaunal assemblages at western sections. At all sections, the base of the Watinoceras coloradoense Biozone is reflected by an abrupt increase in carbonate and decrease in organic carbon percentages as well as by the immigration of Mytiloides bivalves. Periods of progressive recovery through this biozone were associated with carbonate-rich depositional periods.