Geometry, Origin and Significance of Late Syn-Rift Canyon Systems

McAndrew, Andrew E.¹, Christopher A.-L. Jackson¹, Tom Dreyer², Mike Charnock³ (1) Imperial College, London, United Kingdom (2) Norsk Hydro Research Centre, Bergen, Norway (3) Norsk Hydro ASA, Bergen, Norway

Erosional unconformities are commonly identified in Upper Jurassic syn-rift stratigraphy of the northern North Sea. These unconformities are typically localised on and related to the erosion of structural highs such as uplifted fault-block footwall crests. Conversely, corresponding hanging-wall lows often lack widespread or long duration unconformities, as subsidence rates are typically greater than even the most rapid base-level falls. Along the eastern flank of the North Viking Graben, it is however possible to identify at least one regionally extensive unconformity within the Upper Jurassic syn-rift, which is not restricted to structural highs and spans a significant length of time.

Over the majority of the area the unconformity is seismically expressed as an angular discordance with pronounced truncation of underlying, regionally-tilted and faulted strata. Superimposed on top of this widespread erosion are more deeply-incised, canyon-like features, up to 4 km wide, 200 m deep (after compaction) and whose length can be mapped for more than 10 km. Biostratigraphic zonation of marine fauna from wells penetrating the units above and beneath the UJUNC suggests erosion occurred in Late Kimmeridgian to Early Volgian times.

The working hypothesis is for erosion to have occurred in sub-marine conditions, with a probable change in slope gradient required to facilitate such deep incision. This gradient change probably occurred through a reduction in ratio between sediment supplied and the rate of accommodation creation towards the centre of the basin axis due to syn-rift faulting or post-rift thermal subsidence.