Cool-water Carbonate Contourites – Not Reefs – Examples from the Great Australian Bight and the North Sea Basin

Huuse, Mads¹, Finn Surlyk², Holger Lykke-Andersen³ (1) University of Aberdeen, Aberdeen, United Kingdom (2) University of Copenhagen, Copenhagen, Denmark (3) University of Aarhus, Aarhus, Denmark

Thick successions of cool-water carbonates occur in the Upper Cenozoic of the Great Australian Bight and in the Upper Cretaceous – Danian of the North Sea Basin, including on- and offshore Denmark. Traditionally these deposits have been considered largely pelagic or hemipelagic, locally resedimented along structural highs or with biogenic mounds, causing local bathymetric relief. However, data from recent offshore seismic investigations, seabed mapping and sampling suggest that contourites formed by long-lived bottom current systems are important elements in these deposits. In the Great Australian Bight, contourites are linked with palaeo-bathymetric variations at the shelf/slope break and the eastward flowing arm of the Leeuwin Current. The elongate contour-parallel contourite drifts form nucleation sites for bryozoan colonies and are thus best interpreted as ‘bryozoan-rich contourites' rather than ‘bryozoan reef mounds' as they have previously been described. In the Danish offshore chalk contourite drifts, moats and channels were formed along the Sorgenfrei-Tornquist Zone (STZ), which experienced significant tectonic inversion in the latest Cretaceous – Danian. The contourite system extends for more than 100 km along the STZ demonstrating how the bottom currents were focused along the inversion zone and exerted an important control on carbonate deposition in the area. These discoveries highlight that erosion, transport and redeposition of carbonate oozes by bottom currents are important processes in the formation of cool-water carbonates with immediate implications for basin reconstruction, stratigraphic architecture and reservoir properties of cool-water carbonate deposits.