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Several regional or local unconformities occur in the latest Jurassic-Early Cretaceous sequences of the North Sea and adjacent areas. Each may have been identified locally as the "late Cimmerian unconformity," a supposed major break at the base of the Valhall Formation (or Rodby Formation where the Valhall is locally absent). Although a major hiatus (or a condensed sequence) may occur at basin margins or above structural highs, over most of the North Sea the base of the Valhall Formation is isochronous, and conformable with underlying sediments. It is detected on seismic reflection profiles because it represents a widespread facies change marking the late Ryazanian transgression.
Most of the unconformities and associated sedimentary and/or biologic events are of eustatic origin and, even in the tectonically active areas of the North Sea, the effects of eustatic sea level changes were never completely masked by local tectonics. Thus, in the modeling of individual oil fields, the possibility of sedimentary breaks occurring can be predicted in part by reference to regional or eustatic events.
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