HIGH-ARCTIC FAN DELTA RECORDING DEGLACIATION AND ENVIRONMENT DISEQUILIBRIUM

Abstract

Study of a Holocene fan delta in Adventfjorden, Spitsbergen, provides new insight into the nature of high-arctic coastal sedimentation and deglaciation dynamics. The fjord-side, gravelly Gilbert-type fan delta began to form at the local marine limit c. 10 ka BP, supplied seasonally with sediment by meltwater from a cirque glacier left behind by the retreating Late Weichselian ice sheet. Relative sea level had fallen by 63 m, and the fan delta reached a radius of c. 1 km by 6 ka BP, when the relic glacier eventually melted down and fluvial activity declined. A strong influence of marine processes is recorded by the fan-delta foreset facies, overlain by alluvium. Supplied with sediment by longshore drift, the fan-delta front continued to advance at a lower rate, while relative sea level fell further by 5 m and ceased to fall around 5·4 ka BP. The following transgression was countered by longshore sediment supply until 4·7 ka BP, when the delta-front beach aggraded and a spit platform began to climb onto the delta plain, recording a relative sea-level rise of 4 m. The subsequent regression was initially non-depositional, with the relative sea level falling by > 4 m in 200 years, outpacing fluvial supply, and the re-emerging fan delta being swept by longshore currents. A regressive beach began to form c. 4·3 ka BP, while relative sea level gradually reached its present-day position. The feeder braided stream was wandering across the delta plain during this time, but incised once the fan-delta shoreline began to retreat by wave erosion and turned into a receding modern escarpment. The stream has since been adjusting its profile by gradually eroding the pre-existing alluvium and distributing the coarse sediment supplied from catchment slopes by debrisflows and snow avalanches. Modern snowflows have also spread debris onto the abandoned fan surface. The erosional retreat of the fan delta has been accompanied by lateral shoreline accretion on both its sides. The study has important regional implications and demonstrates that Holocene fan deltas can provide a valuable record of the deglaciation history in high-arctic terrains, where glacial deposits are scarcely preserved on land.