A sedimentological study of the deltaic De Geerdalen Formation in Fulmardalen and of fluvial deposits in the Snadd Formation on the Finnmark Platform

Abstract

In the Late Paleozoic, Svalbard and the Barents Sea were part of a large, shallow embayment located at the north-western corner of the supercontinent Pangea. During the Triassic this embayment was gradually filled with erosional products from the UralianMountains located in south-east. A large delta systemevolved depositing the upper Triassic De Geerdalen and Snadd formations, on Svalbard and in the Barents Sea, respectively. This study investigates the sedimentology of the two formations, through fieldwork in Fulmardalen on Spitsbergen, and through investigations of a 3D seismic reflection cube and a sediment core from the Finnmark Platform in the south-western Barents Sea. The study aims to complement the understanding of the evolution of the delta systemthat evolved in the region during the Triassic.

Six sections were measured from different mountains in Fulmardalen. Facies analyses of the outcrops indicate an overall shallowing upwards depositional environment, with open marine shelf and prodelta deposits in the lower part, shallow marine and delta front deposits in the middle part and delta plain deposits in the upper part. This is concordant with previous studies of the formation. Laterally extensive and upwards coarsening sandstone unitswith sedimentary structures typically associated with wave- and tidal activity, characterize the shallow marine and delta front deposits in Fulmardalen. These units have been interpreted as barrier bars, forming as a result of low accommodation space allowing for basinal processes to rework the sediment.

The seismic investigation of the equivalent Snadd Formation on the Finnmark Platform reveals a much more fluvial dominated depositional environment compared to Fulmardalen. A range of fluvial seismic geomorphological features have been interpreted, such as point-bar deposits from large meandering rivers, braided river morphologies with mid-channel-bars and smaller ribbon shaped channel bodies. The sediment core, retrieved from a large point-bar complex, shows characteristics typical of fluvial deposits. The channels show a general WNW-ESE orientation, reflecting the overall progradation of the delta towards west and north-west. Width-to-thickness plots of selected channels compare well to plots of mainly meandering rivers and distributaries from ancient and modern fluvial systems. Moreover, channel bodies in the lower part appear larger than channels further up in the formation. This trend is thought to be related to tectonic changes in the hinterland.