The Orustdalen Formation of Brøggerhalvøya, Svalbard: A fan delta complex of Dinantian/Namurian age

Abstract

The Orustdalen Formation of Broggerhalvaya is well exposed on two coastal sections and is 200–250 m thick. It consists dominantly of siliceous conglomerates and sandstones with subordinate shales and impure coal. One horizon low in the sequence yielded spores of uppermost Dinantian/lowermost Namurian age. Three sedimentary fades have been recognised. The fluvial channel facies (interbedded conglomerates and cross-stratified sandstones) are interpreted as braided stream deposits with flow directions to south and west. A shale facies, sometimes with drifted plant remains, is interpreted as overbank in origin. Highly quartzose, medium-to-coarse-grained, cross-stratified sandstones make up the reworked facies where palaeocurrents are bimodal or indicate movement to the NE. The Formation is thought to represent sediments shed from an early Carboniferous fault scarp eroding’a siliceous Lower Palaeozoic source terrain. The proximity of a nearby coastline led to the construction of fan deltas where fluvial deposits were reworked by waves and/or tides. Early diagenetic events include the local development of pyrite, kaolinite and calcite cements. Reddening’probably occurred during Middle Carboniferous times beneath an exposed land surface. Following burial, pressure solution and quartz cementation eliminated porosity at depths greater than 1000 m.