Sedimentary facies and reconstruction of a transgressive coastal plain with coal formation, Paleocene, Spitsbergen, Arctic Norway

Analysis of coal-bearing strata in the Paleocene Firkanten Formation in the Central Tertiary Basin of Spitsbergen (Svalbard) has shown that deposition was in a coastal plain setting. Interpretation of numerous cores indicates that deposition occurred in coastal plain mires, lagoons, tidal inlets, barrier bars and shoreface environments. These form a series of shallowing-up successions metres to tens of metres thick. Correlation between boreholes over distances in excess of 100 km proved challenging with no regional marker horizons. However, by constructing paleogeographic maps as part of the process of correlating, and by using analogous modern coastal areas as indicators of facies belt widths, a correlation scheme that provides patterns of facies distribution that are consistent in three dimensions has been created. The resulting stratigraphic framework comprises parasequences grouped into parasequence sets that show an overall retrogradational pattern representing a stepwise transgression over a basin floor that was subsiding due to flexural subsidence. Absence of incision or other evidence for relative sea-level fall within the whole succession suggests that flexural subsidence was sufficient to keep pace with eustatic sea-level falls. The basal unconformity surface is overlain by a poorly sorted deposit of reworked regolith that forms a strongly diachronous unit, occurring at the base of successive parasequences. Individual parasequences formed in response to relative sea-level rises that are considered likely to be eustatic in origin and resulted in flooding of the coastal plain. Centimetre-scale analysis of the coal maceral distribution through the thickness of the lowermost major coal seam indicates that there were also higher-frequency fluctuations of relative sea level, possibly at Milankovitch-scale, during the deposition of individual parasequences. This new facies model and interpretation of the subsidence history of the basin provides a framework for understanding the distribution of sediments within a coal-bearing coastal plain succession.