Clinoform geometry, geomorphology, facies character and stratigraphic architecture of a sand-rich subaqueous delta: Jurassic Sognefjord Formation, offshore Norway

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The integration of core sedimentology, seismic stratigraphy and seismic geomorphology has enabled interpretation of delta-scale (i.e. tens of metres high) subaqueous clinoforms in the upper Jurassic Sognefjord Formation of the Troll Field. Mud-prone subaqueous deltas characterized by a compound clinoform morphology and sandy delta-scale subaqueous clinoforms are common in recent tide-influenced, wave-influenced and current-influenced settings, but ancient examples are virtually unknown. The data presented help to fully comprehend the criteria for the recognition of other ancient delta-scale subaqueous clinoforms, as well as refining the depositional model of the reservoir in the super-giant Troll hydrocarbon field. Two 10 to 60 m thick, overall coarsening-upward packages are distinguished in the lower Sognefjord Formation. Progressively higher energy, wave-dominated or current-dominated facies occur from the base to the top of each package. Each package corresponds to a set of seismically resolved, westerly dipping clinoforms, the bounding surfaces of which form the seismic ‘envelope’ of a clinoform set and the major marine flooding surfaces recognized in cores. The packages thicken westwards, until they reach a maximum where the clinoform ‘envelope’ rolls over to define a topset–foreset–toeset geometry. All clinoforms are consistently oriented sub-parallel to the edge of the Horda Platform (N005–N030). In the eastern half of the field, individual foresets are relatively gently dipping (1° to 6°) and bound thin (10 to 30 m) clinothems. Core data indicate that these proximal clinothems are dominated by fine-grained, hummocky cross-stratified sandstones. Towards the west, clinoforms gradually become steeper (5° to 14°) and bound thicker (15 to 60 m) clinothems that comprise medium-grained, cross-bedded sandstones. Topsets are consistently well-developed, except in the westernmost area. No seismic or sedimentological evidence of subaerial exposure is observed. Deposition created fully subaqueous, near-linear clinoforms that prograded westwards across the Horda Platform. Subaqueous clinoforms were probably fed by a river outlet in the north-east and sculpted by the action of currents sub-parallel to the clinoform strike.

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