Experimental exploration of the stratigraphy of fjords fed by glaciofluvial systems

Abstract

AbstractWhereas most Late Quaternary sedimentary systems experienced only sea-level rise, fjords record unique sequences because rapid uplift after the unloading of the Last Glacial Maximum (LGM) ice sheets outpaced global eustatic sea-level rise. This study aims to disentangle how rapid initial uplift and high variability of eustatic sea-level change affects fjord sedimentary records. Two numerical models are coupled, ICE-5G and SedFlux, and show that timing and duration of deglaciation and total uplift strongly affect fjord stratigraphy. The ICE-5G model predicts a number of distinct time intervals during which many fjords deglaciate, independent of latitude and short-term climate. Deglaciation of the entire fjord system takes significantly longer (c. 6 ka) for fjords that deglaciate early (17–15 ka BP) than for fjords deglaciating after 9 ka BP (c. 1 ka). Exponential uplift curves totalled c. 220–280 m, and have half-lives of 1–1.4 ka.High uplift rates consistently cause rapid progradation of the rivermouth over tens of kilometres. Thick packages of glaciomarine, and glaciofluvial sediments emerged above sea level and are subsequently incised. Sensitivity tests predict high frequency of submarine mass movements. Fjords that deglaciated early additionally show deposition to be strongly dominated by rapid sea-level rise; signs of drowning are pronounced and subsequent thick fine-grained sequences aggrade. We conclude that recently deglaciated fjords record solely deposition under falling sea-level and thus provide the best modern analogues of forced-regressive systems.