Deposits and processes on the tide-influenced fjord-head delta in Dicksonfjorden, Svalbard

Abstract

This study investigates for the first time the tide-influenced fjord-head delta in Dicksonfjorden, a fjord in central Spitsbergen, Svalbard. In order to examine the deposits and processes, sediment cores, surface observations and bulk samples have been obtained. Sedimentary facies, log transects and composite logs are presented. Sediment cores are dated with the 210Pb dating method to provide an average sedimentation rate for the intertidal zone. High-resolution aerial images are used to produce a geomorphological map of the area. Landforms and surface features are identified, described and interpreted. Sedimentological trends of the delta are described and interpreted. An aerial photo from 1938 is used to describe and discuss the development of the delta.

Tidal flats, tidal channel, tidal bars, rill channels and cheniers are identified. 7 facies are identified which belong to tidal flat deposits or tidal bar deposits. The tidal flat deposits generally consist of muddy and sometimes sandy laminations, indicating mostly deposition from suspension during high tide. Deformational structures are common and together with circular depressions on the tidal flats they indicate shore ice influence. The tidal bar deposits generally consist of cross-laminated and structureless sand, indicating rapid deposition from a relatively strong heavy sediment-laden current. Heterolithic horizontal laminations, bedforms, reactivation surfaces and desiccation cracks together serve as a strong indicator of tidal influence. The wave-protected setting in which the delta is situated allows a microtidal range to be sufficient for influencing the deposits. Tidal flat deposits transition from mud flat to mixed sand-mud flats moving in a channel-ward direction, indicating a decrease of tidal current speed due to friction against the tidal flat. 210Pb profiles indicate non-steady deposition in the intertidal zone and an average sediment accumulation rate of 0,3 cm/year during the last century. Rill channels develop on tidal flats due to erosion by ebb tide and their distribution can therefor be used to determine the boundary between the intertidal and supratidal zone. Grain-sizes increasing towards the outskirts of the delta indicate a sediment input coming from the surrounding mountains, in addition to Dicksonelva. Because of this and the lack of sand flat deposits in the channel proximal areas of the tidal flats, the delta does not show a good correlation with the previously published facies model from Dalrymple (1992). The aerial photo from 1938 reveals that major tidal channels are located at approximately the same positions as today, indicating that the channel system is quite stable. This is probably linked to the cohesive properties of the muddy tidal flat deposits. Due to relative sea level fall since the last glaciation, the delta represents a forced regressive deposit. The forced regression causes the supratidal part of the delta to slowly increase. Fluvial downcutting and transport of older deposits out on the delta front and slope is likely to be occurring.