Sedimentological and geomorphological study of a postglacial fjord-side landslide and fan-delta complex at Russenes, inner Balsfjord, Troms county

Abstract

A comprehensive study of the Russenes postglacial fjord-side fan delta and landslide complex has been undertaken in inner Balsfjord, Troms county, northern Norway. High-resolution bathymetric data, digital terrain models, aerial photography, ground penetrating radar profiling, radiocarbon dating and field mapping have been used to determine the geometry, structure, lithofacies and evolution of the Russenes fjord-side fan delta. A 1:10 000 scale Quaternary Geological Map of the Russenes fjord-side fan delta and associated deglacial landforms has been produced during this study. Previously unrecognised deglacial ice-contact landforms have been mapped above the Russenes fjord-side fan delta. These landforms provide new insights into the deglaciation history of Balsfjord because they indicate that there was a halt in the retreat of the Balsfjord glacier at Russenes. Radiocarbon dating of Balanus shells located within these deglacial ice-contact landforms provides a minimum age for the glacier standstill of 10 884 ± 96 cal. yr BP (1σ).

Detailed field investigation of a distinctive rock-avalanche complex located immediately above the Russenes fjord-side fan delta suggests that it was deposited by multiple rock-avalanche events. The composition of the boulders in the lower section indicates that they have been sourced from outside the catchment area, transported on the Balsfjord glacier and subsequently, deposited along with the ice-contact landforms. The composition of the rock-avalanche boulders in the upper section of the rock-avalanche complex indicates another rock-avalanche event was triggered on the mountains located directly above the Russenes fjord-side fan delta.

Investigation of the present-day morphology and structure of the fjord-side fan delta indicates there are two distinct depositional environments within the postglacial fan delta: (1) a subaerial alluvial fan; and (2) a subaqueous Gilbert-type delta. Within both environments sediment transport occurs during episodic high-energy flood events. The subaerial alluvial fan contains two perennial river channels that migrate over the fan via channel back-filling and avulsion processes. Additionally, the subaqueous component of the fan delta can be further divided into three depositional environments: (1) a flatly lying intertidal delta platform; (2) a steeply dipping subaqueous delta slope; and (3) a deep, subaqueous, gently dipping prodelta region. Investigation of the deltaic subsurface reveals the delta has a Gilbert-type tripartite structure composed of bottomset, foreset and topset units and the relationship between the subsurface structure and the present-day fan delta morphology has been discussed. The surficial morphology and subsurface structure of the postglacial fjord-side fan delta has been used to determine the progradational and aggradational history of the fan delta. Finally, the response of the Russenes fjord-side fan delta to variations in climate, sea level and sediment supply over the Holocene has been investigated.