The Triassic — early Jurassic succession in the northern North Sea: megasequence stratigraphy and intra-Triassic tectonics
Authors: Steel R. , Ryseth A. 
Published: April 2008

The occurrence of Triassic to early Jurassic mudstone and sandstone sequences in the northern North Sea basin, and their possible significance in terms of basinal subsidence pattern is reviewed. The latest phase (Scythian) in a period of crustal stretching is illustrated from the Horda Platform where block rotation has been accompanied by infilling (> 2000 m) of alluvial deposits in a series of N-S oriented, asymmetric sub-basins.

The ‘post-rift’ succession is subdivided into three megasequences which, in their broad areal development over most of the northern North Sea basin, contrast with the rift-infill sequences. The late Scythian-Ladinian megasequence (PR1) documents an early phase of widespread, fine-grained, floodbasin deposition which was interrupted by prograding, sandy sheetflood lobes (Teist Fm). The latter were derived from marginal alluvial fan bajadas along the Norwegian hinterland and from certain segments of the East Shetland uplands. This phase of infilling culminated with basinwide, sand deposition (Lomvi Fm) from amalgamated fans and braided river systems. Although classified as ‘post-rift’, megasequence PR1 does show some ‘growth’ along the older rift lineaments.

The Carnian-early Rhaetian megasequence (PR2) records retrogradation of the earlier sandy systems towards the basin margins (lower Lunde Fm) followed by renewed establishment of extensive lacustrine and coastal floodbasin conditions (middle Lunde Fm.). Low-lying tracts within the northerly floodbasins allowed influx of brackish lagoonal waters from a sea to the north. The sequence culminates with repeated progradation of extensive sandy alluvial plains (lower and middle parts of upper Lunde Fm).

The early Rhaetian-Sinemurian megasequence (PR3) also records retrogradation of the earlier sandy fluvial systems (middle to upper parts of upper Lunde Formation), followed by the development of extensive, fine-grained floodbasins and occasional brackish-marine lagoons (uppermost Lunde Fm.) into which isolated sandy streams and fan deltas prograded from both East Shetland and Norwegian margins. Subseqeunt progradation of streams and fan systems eventually allowed the development of northward, through-flowing, axial river systems (Statfjord Fm). There are clear signs of more active intra-basinal tectonics during this third stage in the basin’s post-rift history, with latest Triassic-earliest Jurassic depocentres tending broadly to occupy the sites of the future Viking and Sogn Grabens.

In the central North Sea the three-fold megasequence development cannot easily be recognized, though broadly time-equivalent units can be identified. The earliest phase was dominated by inland floodbasin deposition (Smith Bank Fm.) but this was replaced towards the Norwegian hinterland by, at first, lacustrine/lagoonal and inland sabkha deposition, and later, by sandy alluvial fan development (Skagerrak Fm.). The second phase was dominated by sandy alluvial deposition in the northern and eastern reaches but became muddy floodbasin-dominated farther west. The latest phase is only fragmentarily preserved and therefore poorly recognized because of major mid-Jurassic erosion over much of the central North Sea. Where preserved it tends to be sandy, especially in the Norwegian sector.

The pattern of initial retreat of sandy fluvial systems followed by extensive, fine-grained floodbasin development, subsequent progradation of sandy alluvium and eventual development of axial, through-flowing river systems appears to have been characteristic of each of the 3 post-rift intervals (particularly of the first and third, but also, albeit more crudely, of the second) and is interpreted in terms of an increasing then decreasing rate of relative base-level rise. This, in turn, is argued to have been caused largely by increasing then decreasing basinal subsidence rates.

Intra-Triassic tectonics are reflected in (a) early Triassic rifting, (b) three distinct phases of post-rift subsidence and (c) clear signs of differential fault movement and sediment growth along certain intra-basinal lineaments, particularly in the third post-rift interval.