Sedimentary environments offshore norway — an overview

Abstract

The evolution of sedimentary environments in the Norwegian continental margin since the Early Carboniferous is directly linked with the evolution of the tectonic framework of the broader region of the northern North Atlantic. After the end of the continent-building Variscan and Uralian Orogenies in the Permo-Carboniferous, the tectono-magmatic history is that of a 200 million year period of general extension and rifting of the continent, ending with a final continental rupture and opening of the northern North Atlantic by seafloor spreading in Early Eocene times. The sedimentary environment of the Norwegian continental margin therefore is a record of an evolution from waning orogenic forelands and incipient rifts in an equatorial climate, through a stage of dominantly continental rifting while drifting from equatorial to temperate climates, to the present stage of a passive subsiding continental margin in a temperate to arctic climate. In earliest Carboniferous times the general lateral distribution of sedimentary environments reflects the existence of a northern ocean (the Boreal Ocean) and a southern ocean (the Proto-Tethys) separated by an area of emergent land with a central zone of continental rifting. In the south, the Carboniferous was generally a period of marginal marine, fluvial deltaic and alluvial deposits progressively filling up the central and southern North Sea. Purely continental alluvial, fluvial and lacustrine environments prevailed in the Norwegian Sea and East Greenland. In the Barents Sea, alluvial and fluvial deltaic environments were transgressed at an early stage by marine carbonates and evaporites. The Early Permian period of the North Sea, East Greenland and the Norwegian Sea was a time of continental environments including an early episode of widespread magmatism in the south. In the Barents Sea marine carbonate and evaporite environments prevailed. Middle Permian time was characterised by uplifts and large erosional breaks. Although most prominent in the southern and central areas, such an erosional hiatus is also recorded across much of the Barents Sea. At the end of the Permian period the sea transgressed the low-lying parts of the entire region — recorded by coarse clastics and evaporites in the south and central area and fine-grained clastics in the Barents Sea. Characteristic of the Triassic period were the numerous marine transgressions and regressions both in the north and south of the region. In the south, the evaporitic environment of the Permian continued but with an increased input of elastics. The northern North Sea, the Norwegian Sea and East Greenland were characterised by marine deposits in the Lower Triassic, followed by continental fluvial and alluvial systems interbedded with marine incursion cycles. A main feature of Triassic times was the shallowing of the Barents Sea by input of large volumes of clastic sediments. A relative sea-level rise, that started in latest Triassic times, caused the Lower and Middle Jurassic of the whole region to become uniformly dominated by shallow marine clastic shelf environments and approximately simultaneous delta oscillations. Early to Middle Jurassic domes and uplifts on regional and semi-regional scales caused a complex pattern of hinterlands, depo-centres and seaways. In the latest Middle Jurassic and through Late Jurassic times, a major sea-level rise considerably deepened the northern and southern seas and finally drowned the central area (between East Greenland and Norway). This caused the widespread accumulation of marine shale with intervals of very rich source rock. Following a period of marked oscillations of the sea level prior to the Aptian, the sea level continued to rise through the Cretaceous period and reached its peak in Late Cretaceous times. Lower Cretaceous deep-water shales and marls accumulated in the basins and rifts of the southern and central parts of the region, while shallow marine and coastal plain deposits dominate on the flanking platforms and in the vast platform of the Barents Sea. The facics pattern of the Lower Cretaceous continues unchanged into the Upper Cretaceous in the central province, while the high sea level gave rise to pelagic limestones in the southern. The central Barents Sea was transgressed with the development of a condensed Upper Cretaceous marine sequence of elastics and carbonate. The volcanism, tectonism and regional uplift preceding the earliest Tertiary continental break-up and subsequent seafloor spreading between Greenland and Norway, effectively ended the carbonate environments in the south, and the whole region became dominated by marine clastic deposits. In the Neogene the stratigraphy is a record of oscillating glaciations. The glaciations and regional uplifts caused deep erosion of the surrounding mainland areas and the Barents Sea shelf in the latest Neogene and the progradation of a huge sediment apron onto the margins of the Norwegian-Greenland Sea.