The Kregnes moraine in Gauldalen, west‐central Norway: anatomy of a Younger Dryas proglacial delta in a palaeofjord basin*

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The Kregnes “moraine” ridge in Gauldalen, a north‐trending valley south of Trondheim, is a Gilbert‐type delta formed at a Younger Dryas glacier terminus. The gravelly delta consists of a north‐dipping foreset, 150 m thick, comprised of turbidites, debrisflow beds and debrisfall deposits. The bottomset consists of turbiditic sand and mud layers. The topset, 2–3 m thick, is a braided‐river alluvium with local beach deposits, matching the marine limit of 175 m a.s.l. The fjord‐wide delta front had an extent of 3 km and prograded over a distance of 1.5 km, in probably less than 100 years, with the delta toe climbing by 50 m against the basin's rapidly aggrading muddy floor. The delta advanced through the alternating episodes of its toe aggradation and progradation, related to the increases and decreases of the delta‐slope gradient. Slope steepening led to intense sediment sloughing by chutes and occasional large‐scale failures. The fjord's wave fetch was low and the wave base no deeper than 1.5‐2 m, but strong storm waves occasionally reworked the delta front to a depth of 6 m. Glacitectonic deformation was limited to the system's upfjord end. Allostratigraphic analysis suggests that the proglacial system commenced its development as an ice‐contact submarine fan that was deformed, quickly aggraded to the sea surface and turned into an ice‐contact delta, which further evolved into the large glaciofluvial delta. The Kregnes ridge represents an episode of the ice‐front re‐advance due to climatic deterioration and is tentatively correlated with the Hoklingen substage.