Sequence stratigraphy in a marine moraine at the head of Hardangerfjorden, western Norway: evidence for a high-frequency relative sea-level cycle

Abstract

The depositional architecture and stratigraphy of the Hæreid moraine, a sedimentary ridge ca. 1.5 km long and up to 113 m in height at the head of Hardangerfjorden, West Norway, have been reconstructed in a high-resolution sequence stratigraphic framework. Information from ground-penetrating radar (GPR) profiles, outcrop sections and borehole data have been used in the analysis. The basal, forced regressive deposits comprise relicts of a Gilbert-type delta that commenced its development at the front of a grounded glacier. On the seaward (distal) side of the ridge, there is a major channel-fill/mouth-bar complex covering large parts of the delta's former frontal slope and prodelta zone. The complex occurs in the seaward lengthening of a fluvially incised channel and is interpreted as a lowstand to early transgressive succession of fluvial and fluvial-estuarine facies. The channel-fill/mouth-bar complex and the dissected delta top have been planed off through transgressive ravinement and the delta top is overlain by an upvalley-thinning sedimentary wedge, 3–8 m thick and with a general landward onlapping and propagating internal bedform geometry. This wedge is thought to be composed mainly of transgressive, marine (estuary–mouth barrier) facies. On the landward (proximal) side of the ridge, the transgressive deposits include a retrograding landward-dipping foreset interpreted as relicts of a flood-tidal delta overlain by possible washover fan deposits. These deposits are again conformably overlain by laminated clay, interpreted as the remains of a highstand mud drape. The depositional sequence delineated in this study, up to 20 m thick and comprising deposits of a lowstand-wedge, a transgressive and a highstand systems tract, developed in response to a relative sea level (RSL) rise of 12–15 m. This RSL rise, which interrupted the general RSL fall following the deglaciation, is correlated with the Younger Dryas transgression, a regionally recorded sea-level event. The highstand mud drape was deposited as the rising RSL ultimately led to drowning of the moraine. The highstand and transgressive deposits are erosively truncated by a thin gravelly lag, deposited during final emergence in the early Holocene.