An example of a highly bioturbated, storm-influenced shoreface deposit: Upper Jurassic Ula Formation, Norwegian North Sea



Integrated ichnological and sedimentological analyses of core samples from the Upper Jurassic Ula Formation in the Norwegian Central Graben were undertaken to quantify the influence of storm waves on sedimentation. Two main facies associations (offshore and shoreface) that form a progradational coarsening upward succession are recognizable within the cores. The offshore deposits are characterized by massive to finely laminated mudstones and fine-grained sandstones, within a moderately to highly bioturbated complex. The trace fossil assemblage is dominated by deposit-feeding structures (for example, Planolites, Phycosiphon and Rosselia) and constitutes an expression of the proximal Zoophycos to distal Cruziana ichnofacies. The absence of grazing behaviours and dominance of deposit-feeding ichnofossils is a reflection of the increased wave energies present (i.e. storm-generated currents) within an offshore setting. The shoreface succession is represented by highly bioturbated fine-grained to medium-grained sandstones, with intervals of planar and trough cross-bedding, thin pebble lags and bivalve-rich shell layers. The ichnofossil assemblage, forming part of the Skolithos ichnofacies, is dominated by higher energy Ophiomorpha nodosa ichnofossils and lower energy Ophiomorpha irregulaire and Siphonichnus ichnofossils. The presence of sporadic wave-generated sedimentary structures and variability in ichnofossil diversity and abundance attests to the influence of storm-generated currents during deposition. As a whole, the Ula Formation strongly reflects the influence of storm deposits on sediment deposition; consequently, storm-influenced shoreface most accurately describes these depositional environments.