Correlating palynofacies assemblages with sequence stratigraphy in Upper Cretaceous (Campanian) sedimentary rocks of the Book Cliffs, east-central Utah

摘要

Some of the most important factors that govern the preservation of depositional sequences in passive margins are the rate of sediment supply, the type of sediment, subsidence history, sea-level changes, and climate. These factors also control the nature of components, such as organic matter, palynomorphs, and other fossils incorporated within the sediments. Because detrital organic matter (palynodebris) behave like sedimentary particles, their distribution in sediments can be used to infer palynofacies, and consequently depositional conditions. This study correlates palynofacies assemblages with sequence stratigraphic interpretations of Upper Cretaceous (Campanian) sedimentary rocks in the Book Cliffs, east-central Utah, namely the Kenilworth, Grassy, and Desert Members of the Blackhawk Formation, Mancos Shale, Castlegate Sandstone, and Buck Tongue of the Mancos Shale. Sedimentological criteria have been used to identify the depositional framework at 8 localities, and 12 palynodebris types have been identified in the sediments: sporomorphs, marine palynomorphs, fresh-water algae, fungal remains, amorphous organic matter, resins, cuticles, parenchyma, degraded bundles, wood, black debris, and degraded debris. The samples and palynodebris were analyzed statistically using average linkage cluster analysis, principal components analysis, and detrended correspondence analysis. Cluster analysis yielded seven palynofacies assemblages, which have been integrated with sedimentological criteria, and correlated with sequence stratigraphic interpretations. Although all the samples are dominated by woody debris, the palynofacies assemblages are defined primarily by marine palynomorphs, degraded bundles, black debris, and sporomorphs, and to a lesser extent by cuticles and resins. Highstand systems tracts and lowstand systems tracts are each represented by major palynofacies assemblages, indicating that there is some correlation with sequence stratigraphy. Two other minor assemblages are indicative of the highstand systems tract. In addition, three assemblages, which are dominated by siltstone and shale samples interbedded with sandstones, are richer in black debris; this is probably due to the effect of oxidation in the depositional environment and, possibly, microbial respiration in the interbedded sandstones. Marine environments are usually characterized by an abundance of marine palynomorphs and palynodebris over sporomorphs and other nonmarine palynodebris, and this characteristic has been used to recognize initial marine flooding events and maximum flooding intervals (condensed sections) in several palynological studies. In this study, however, one possible initial marine flooding event was not easily detected palynologically. In situations such as this, there is a need to integrate palynology with sedimentological and other criteria in order to establish valid regional sequence stratigraphic interpretations.