Pockmarks with Radiating Ripples: Fluid Expulsion In a Storm-Influenced Early Cretaceous Delta, Svalbard, Norway

Sedimentary features consisting of remarkably circular pockmarks with straight-crested ripples radiating from their centers occur on bedding surfaces of medium-grained quartz arenites in the deltaic Lower Cretaceous Helvetiafjellet Formation of Spitsbergen (Svalbard), Norway. The pockmarks are 0.15 - 5.1 m in diameter, and their depths vary proportionally to their diameter from 0.03 m to 0.55 m. The distribution of the pockmarks on the bedding surfaces appears random. The pockmarks are interpreted to have formed under estuarine conditions during a relative sea-level rise that submerged an earlier deltaic lobe. Models are presented for both the generation of the pockmarks as well as their fining-upward fill and development of radiating bedforms. The initial depressions are attributed to erosion by fluids expelled from the subsurface. The radiating ripples were evidently generated subaqueously by circular flow at the base of fixed vertical vortex instabilities. A preferred interpretation is presented in which pockmarked bottom topography interacted with storm surges to produce narrow, steep boundary shear-stress gradients along which vortex instabilities developed. Abundant hummocky cross-stratification provides additional evidence for storm influence on the stratigraphic sequence, and storms are thought to have augmented an otherwise micro-tidal circulation.