Forest Fires, Climate, and Sea-Level Changes in a Coastal Plain-Shallow Marine Succession (Early-Middle Oxfordian Jakobsstigen Formation, North-East Greenland)

Abstract

ABSTRACT The Lower-Middle Oxfordian Jakobsstigen Formation of North-East Greenland was deposited in a semi-enclosed marine embayment along the western margin of the rifted seaway between Greenland and Norway. It consists of rhythmically interbedded coastal-plain and shallow marine deposits, reflecting repeated alternation of two markedly different environments. The formation is 128 m thick and forms an overall coarsening-upward unit. The basic motif is a high-order sequence, 0.1-10 m thick, consisting of a thin, laterally extensive carbonaceous mudstone overlain by coarsening-upward offshore-transition-zone and shoreface heteroliths and sandstones. The mudstone was deposited during early base-level rise in a shallow, flat-bottomed lake or lagoon on a flat, low-lying coastal plain. The plain was subsequently flooded during maximum rise in sea level, followed by shoreface progradation during late rise, stillstand, and early fall. The boundary between coastal-plain and shallow-marine deposits represents the maximum flooding surface, separating the transgressive and highstand systems tracts. The organic material of the carbonaceous coastal-plain mudstones is dominated by inertinite macerals resulting from wildfires. In the shallow-marine sediments the organic material contains a high proportion of huminite and a significantly lower proportion of inertinite macerals, indicating wetter conditions and a lower frequency of wildfires in the hinterland during times of shoreface deposition. The different climatic regimes may reflect high-order sea-level fluctuations that exerted an important influence on the areal extent of the marine environments in large areas of the rifted seaway between Greenland and Norway due to the flat, low-gradient basin floor and overall shallow-water conditions. Periods of late rise and highstand were characterized by a humid climate, possibly caused by higher precipitation in the nearby source areas related to evaporation over the extensive sea. During lowstands only the topographically deeper parts of the rift complex remained submerged and a drier continental climate prevailed. The carbonaceous mudstones were deposited in the transitional period from the drier to the more humid climate corresponding to early sea-level rise. This was the time of large seasonal variations with optimum conditions for wildfires in the vegetated hinterland. This time interval was also characterized by onset of base-level rise in the low-lying coastal plain, where the wildfire-derived inertinite was trapped in shallow lakes and lagoons.