Transgressive development of coal-bearing coastal plain to shallow marine setting in a flexural compressional basin, Paleocene, Svalbard, Arctic Norway

Abstract

The most extensive Paleogene succession on Svalbard, in the Arctic north of Norway, is found in the Central Tertiary Basin of Spitsbergen. It consists of a clastic basin fill of mudstone, sandstone, coal and rare conglomerate beds. A coastal plain to shallow marine setting is suggested for the Late Paleocene Firkanten Formation, the lowermost unit of the Paleogene succession. This is the first comprehensive facies model, sequence stratigraphic analysis, and paleogeographic reconstruction of the Firkanten Formation, based on new borehole cores and field data. The facies analysis reveals that sedimentation occurred in a flat relief coastal plain environment with tidal, wave, and storm influence but only minor fluvial sediment input. Previous interpretations have described the Todalen Member, the lower part of the Firkanten Formation as delta plain deposits. The detailed sequence stratigraphic analysis and paleogeographic reconstruction show that the Firkanten Formation consists of parasequences combined into parasequence sets bounded by major flooding surfaces. The succession is dominated by aggradation in a step-wise transgressive setting. The general tectonic subsidence was at all times greater than any eustatic sea level fall since there are no relative sea level falls detected in the succession. The basin was formed as a depression in front of the West Spitsbergen Fold and Thrust Belt. Thick sections of coastal plain deposits of coal, carbonaceous shale, and other fine grained clastic sediments were deposited on the coastal plain, in mires and swamps that graded into tidally influenced lagoons. The coastal plain was protected from wave reworking by sandy barrier bars but was flooded during periods of increased relative sea level rise probably from eustatic sea level rise. The foreshore and shoreface deposits are characterised by fine grained sandstone and a few pebbly beds, making up the Endalen Member, the upper part of the Firkanten Formation. The foreshore was characterised by sandy barrier bars of long shore transported fine grained sandstone. The foreshore and shoreface show a high degree of wave and storm influence. Alluvial fan deltas built out from the thrust front, transporting coarse grained material to the basin. The base of the Paleocene succession is made up by the unconformity to the Lower Cretaceous Carolinefjellet Formation, representing the lower sequence boundary characterised by poorly sorted sediment of re-deposited weathered material and vegetation. Large, newly discovered footprints of the Pantodont ‘Titanoides’ from the Todalen Member coal layers are the earliest evidence of a large mammal on Svalbard and the northernmost discovery from the Paleocene. The traces are named Thulitheripus svalbardii Ign nov. isp. nov. Large Paleocene Pantodonts are previously only known from North America and their presence on Svalbard, confirms the postulated DeGeer route for migration of mammals in the Late Paleocene to Eocene. The Central Tertiary Basin is interpreted as being of flexural origin, formed as a result of crustal shortening in West Spitsbergen due to convergence with Greenland related to the opening of the Northern Atlantic in the early Paleogene. The Late Paleozoic clasts in conglomerate beds provide evidence that there was uplift and erosion of at least 2000 m of rocks in the West Spitsbergen Fold and Thrust Belt, directly adjacent to the western margin of the basin. The sand came from Mesozoic strata uplifted to the north and northwest of the basin. The deformation zone is relatively narrow and the strata are folded to vertical on the western side of the basin. The Central Tertiary Basin shows very little deformation. It is suggested that the most important factor creating the Central Tertiary Basin was compressional folding and not extension or foreland basin flexural loading as has been postulated previously, in accordance with initial continental and shallow marine basin deposits. The compressional folding model suggests that the orogeny did not necessarily create an extensively elevated mountain belt. The footprints suggest that there was no obstruction for migrating Pantodonts such as a seaway or mountain range between Svalbard and Greenland/Ellesmere Island in the Late Paleocene.