Sedimentary Architecture and Dynamic Stratigraphy of a Marine Ice-Contact System

Abstract

ABSTRACT The Mona moraine in southern Norway is a sedimentary ridge, 3.5 km long, formed at the mouth of a shelf trough during a late Younger Dryas readvance of the Scandinavian ice sheet. The moraine architecture, sedimentation processes, and record of ice-front behavior have been reconstructed in a high-resolution allostratigraphic framework on the basis of ground-penetrating radar profiles and sedimentologic analysis of outcrop sections. The shelf trough was draining ice from adjacent platforms, which resulted in ice-front protrusion with a moraine offset relative to the coeval swarm of small moraines formed in the shallow-water zone. The grounding-line system at Mona commenced its development as an ice-contact submarine fan, while the glacier front was thinning and grinding to a halt on a bedrock threshold. The ice flux declined, but fluctuated, and the subglacial sediment supply by meltwater prevailed. The ice-front stillstand allowed the submarine fan to aggrade to the sea level and turn into a Gilbert-type delta with a short (160 m) distributary plain. The delta foreset, 50-60 m thick, consists of gravelly massflow deposits, and its sandy turbiditic toe has been extended on the threshold's steep lee slope to a distance of at least 2 km. The delta was abandoned by rapid calving of the ice front, and the ice-contact slope was covered with marine mud and resedimentation products. A radiocarbon date from the base of this thin cover indicates ice retreat at around 10,100 yr BP. The moraine was surrounded by the sea and subject to reworking by waves, tides, and longshore currents. A thick package of regressive foreshore deposits with downstepping wave-cut terraces on the moraine slopes reflects gradual emergence due to regional glacioisostatic uplift. The study demonstrates that the facies architecture of marine moraines bears an important record of glacier dynamics and ice-front behavior.