Abstract 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 paleoenvironmen-tal 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 coal-zone cyclicity that we interpret as a secondary coal-forming process.

Abstract Bate (1978, p. 254) commented of the Great Estuarine Group succession from the Inner Hebrides, Scotland (Fig. 1 ) that it ‘is so imperfectly known with respect to its ostracod fauna that it is one area in urgent need of investigation’. This was rectified by Wakefield (1991, 1994) in which the largest freshwater and brackish water ostracod fauna from the British Bathonian was described, particularly with respect to the number of darwinulid and lim-nocytherid species. However, the similarity with freshwater and brackish water ostracod assemblages recorded in the English Midlands, although generically high, was specifically low. Studies recording freshwater and brackish water ostracods from the English Midlands, and therefore of com-parative interest, are those of Bate (1965, 1967) , Ware (1978) , Stephens (1980) , Ware & Whatley (1980) , Ware & Windle (1981) , Jacovides (1982) , Timberlake (1982), Barrington (1986) , and Stride (1994) . Figure 2 illustrates the stratigraphical coverage of these studies and their overlap with the Great Estuarine Group succession. The development of such brackish water ostracod faunas during the Bathonian was discussed in Whatley (1990). Lithostratigraphical correlation of Bathonian strata from the English Midlands highlighting successions from which freshwater and brackish-water ostracods have been analysed. The lithostratigraphy of the Great Estuarine Group, Inner Hebrides, Scotland, is also shown to enable an approximate correlation with the English successions. English stratigraphy after Bradshaw (1978) , Torrens (1980b) , Boneham & Wyatt (1993) ,