Devonian depositional environments and processes of Spitsbergen (DEPOS)

Late Silurian (?) to early Devonian sediments, forming a part of the Old Red Sandstone (ORS) are exposed in a fault-bounded, continental basin located in central north Spitsbergen. The terrestrial deposits attain a cumulative maximum thickness of about 8000m and cover an area of around 9000km2. They were deposited under arid to semi-arid climatic conditions in a near-equatorial position at the northern margin of the ORS landmass, which arose from the plate collision of the continents Baltica and Laurentia during the Caledonian Orogeny.

To reconstruct the sedimentary processes, depositional environments and the spatial and temporal development of the basin, microfacies studies are combined with geochemical analyses (carbonate content, stable isotopes, TOC) and the interpretation of sedimentary structures, lithologies, fossil and trace fossil associations. Fieldwork includes the mapping of key areas, establishment of type sections and the collection of fossils and rock samples for further investigations.

Investigations concentrate on the Wood Bay Formation and the conformably overlying Grey Hoek Formation, which show completely different facies, marked by an overall colour change from red to grey, clearly recognizable in the field.

Participants

Project contact: Dierk Blomeier

The Dicksonfjorden Member

The Dicksonfjorden Member of the Wood Bay Formation is seen as the typical red-bed unit of the ORS in Spitsbergen and forms a large-scale, fining-upward succession. The siliciclastic red beds are arranged in 1) generally coarser grained, fining-upward sequences composed of intraformational conglomerates grading up into sandstones, sandy siltstones or silty mudstones, and 2) fine-grained facies associations consisting of thick, massive, silty mudstones with thin sandstone or siltstone interbeds and local horizons of nodular calcretes. The fining-upward sequences were mainly deposited in the channels and adjacent overbank areas (levees, crevasse splays) of rivers and smaller, probably ephemeral distributaries. The fine-grained facies associations represent the accumulated, suspended load of flood waters, which recurrently inundated broad alluvial plains during high water stands. Carbonate glaebules point to the local development of paleosols (vertisols, calcisols) during emersion.

The Verdalen Member

At the top of the Wood Bay Formation, multicoloured, calcareous horizons, up to several meters in thickness are repeatedly intercalated in the red beds, characterizing the Verdalen Member. The sediments consist of partly sandy, silty, calcareous mudstones, marls and limestones. These can be arranged in three successive facies units, forming regressive, shallowing-upward sequences. The sediments are interpreted as the depositional products of perennial, low-relief, low-energy watercourses, which repeatedly formed in low-lying areas of the alluvial plain. The water bodies were probably connected to the open ocean and thus may represent restricted lagoons marked by brackish-water conditions, indicating repeated marine ingressions before the final turnover to a marginal-marine environment at the boundary to the Grey Hoek Formation.

The Skamdalen Member

The lowest member of the Grey Hoek Formation, the Skamdalen Member mainly consists of successive coarsening-upward sequences, formed of dark-gray, silty claystones, gradually passing into poorly bedded, partly sandy, massive, calcareous mudstones to siltstones and massive calcisiltites to calcarenites at the top. The facies marks a basic environmental change into a coastal depositional system, which corresponds to the back-barrier area of an interdistributary bay or estuary. Here, the successive coarsening-upward sequences were deposited on broad tidal flats, which were influenced by both continental and marine processes. They reflect a shift from a protected mudflat environment with quiet-water conditions into a mixed- to sand-flat environment with more open, higher energy conditions. This process may reflect seasonal variations in climate connected with increasing freshwater discharge and sediment input from the hinterland.

Combining all information enables us to reconstruct a detailed picture of the overall palaeogeographic development of the continental basin. During the deposition of the Wood Bay Formation, the basin developed from a steep-sloped, high-energy, braided river-dominated molasse trough into a level, low-energy alluvial plain marked by meandering streams. The gradually proceeding geomorphologic maturity can be connected with general decreasing sedimentation rates and the local development of palaeosols. As a result of a continuous rise in relative sea level, the entire depositional area was gradually submerged. Broad lagoons, probably connected to the open sea during high stands, developed repeatedly in topographic depressions of the lowland. With the sedimentation of the Grey Hoek Formation the entire area was permanently submerged and sedimentation occurred in the tidal flats of an interdistributary bay or estuary at the interface of continental and marine settings.

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