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Microfossils from Cretaceous coal-bearing strata can be used to establish key stratigraphic surfaces that mark marine flooding events with intermediate-frequency (fourth-order) and high-frequency (fifth-order) periodicities. We document several examples of this cyclicity from the transgressive and regressive facies at the land-sea transition of the Greenhorn Marine Cycle on the Colorado Plateau. Estuarine strata from the upper Cenomanian Dakota and middle Turonian Straight Cliffs Formations yield four primary fossil assemblages: Assemblage A, the lagoonal assemblage, comprising a rich agglutinated foraminiferal population of Trochammina and Verneuilinoides and brackish ostracodes and molluscs in a skeletal shell accumulation; Assemblage B, the proximal estuarine assemblage, comprising the brackish ostracode Fossocytheridea, charophytes, and smooth admetopsid gastropods within bituminous coal zones giving rise to distal estuary with the addition of brackish gastropods and sparse agglutinated foraminifera in sandy marlstones; Assemblage C, the open-bay (distal estuarine) assemblage, comprising the ostracodes Fossocytheridea posterovata, Cytheromorpha, Looneyella, and Cytheropteron, the foraminifera Trochammina and Ammobaculites, and ornate brackish molluscs in calcareous shelly mudstones; and Assemblage D, the marsh, comprising an exclusive population of the foraminifera Trochammina, Miliammina, and Ammobaculites in rooted lignites. Intermediate flooding surfaces are marked by normal marine taxa that are superimposed on the background of a primary marginal marine assemblage. In general, intermediate flooding events approximate lithologic and biostratigraphic boundaries and record basin-wide paleoenvironmental changes with the advancing Greenhorn Sea. We correlate coal zones from the coast to maxima in calcium carbonate and planktic foraminifera in the offshore. The intermediate cycles approximate ammonite biostratigraphic zones and therefore maintain periodicities within the 100-400 kyr bandwidth. The onshore-offshore correlations suggest that a regional and perhaps global sea-level mechanism controlled the stratigraphic position of the coal zones. Superimposed on the intermediate cycles are higher-frequency cycles that represent short-lived flooding events. As many as six high-frequency cycles constitute an intermediate cycle, and therefore periodicities fall within an approximate 10-25 kyr range. The general asymmetry of the packages suggests that a combination of oceanographic, climatic, and autogenic processes influenced the high-frequency stratal architecture. Overall, the primary mechanism controlling the stratigraphic position of the coals was tectono-eustasy. Compactional processes and/or climate modulations contributed to the observed internal coalzone cyclicity that we interpret as a secondary coal-forming process.

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 KaiparowitsFormation is middle to late Campanian in age, based on mammalian faunas and réévaluation 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.

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