Episodic river flooding events revealed by palynological assemblages in Jurassic deposits of the Brent Group, North Sea

Spore and pollen (sporomorph) assemblages from Middle Jurassic marine deposits of the Brent Group in the northern North Sea are investigated to assess temporal and spatial variations in vegetation and depositional processes. Four wells were sampled for palynology from the Penguins Cluster and the Don North East fields through the Rannoch Formation shoreface succession. Hyperpycnite deposits occur throughout, but are concentrated within the lower part of the section. These are expressed by sand-prone beds displaying waxing and waning current motifs, normally graded muddy beds and structureless mudstones. Hyperpycnal/hypopycnal deposits resulting from episodic river flooding represent important sedimentary features as they may be preserved below fair weather wave base in more offshore settings and potentially be the only record of the former presence of a nearby river mouth. The hyperpycnites typically contain abundant Botryoccocus spp., Amorphous Organic Matter (AOM) and hinterland sporomorph taxa with relatively few marine components compared to associated marine shoreface facies. Variations in palynofacies assemblages and Botryococcus spp. abundances indicate frequent river mouth avulsion. Ordination of samples using non-metric multidimensional scaling (NMDS) indicates that shoreface samples of the sampled wells are relatively distinct, but hyperpycnite samples are highly similar regardless of their sampled well. This suggests that depositional processes and spore/pollen sources (i.e. catchment zones) were similar among hyperpycnite events across different wells. Abundant bisaccate pollen, Botryococcus spp. and AOM within interpreted hyperpycnites suggest sediment mixing along the fluvial drainage path during flooding events. The terrestrial signature of hyperpycnite sporomorph assemblages demonstrates that underflows remained coherent as they descended the shoreface profile with little turbulent mixing with ambient marine waters. Sporomorph assemblages display few large changes through time suggesting vegetation on the adjacent coastal plain was relatively static through the studied interval.