Reply to Discussion on ‘Late Cenozoic geological evolution of the northern North Sea: development of a Miocene unconformity reshaped by large-scale Pleistocene sand intrusion’, Journal of the Geological Society , 170, 133–145

Abstract

Knowledge of small sand injection is far from new (Murchison 1827; Hurst et al . 2011) but that intrusive and extrusive bodies of sand can be more than 100 m thick and extend for kilometres is considered unlikely by Rundberg & Eidvin (2015), who discuss our paper (Loseth et al . 2013). Their discussion disagrees with our interpretation of the origin of these sand bodies and instead favours their previously proposed models (Eidvin & Rundberg 2001; Rundberg & Eidvin 2005). They claim that (1) the early Pleistocene sandstones are turbiditic sands deposited at the toe of the glaciomarine shelf clinoform foresets and not extrusive sands; (2) the Oligocene sandstones, located below mounds at the top Hordaland Group Unconformity, are in situ deposits representing turbiditic gravity sands shed from the Shetland Platform and not intrusive sands; and (3) the top Hordaland Group Unconformity in the northern North Sea was formed by submarine erosion and not during subaerial exposure. We have studied rock properties of Eocene to Pleistocene claystones and sandstones (Oygarden et al . 2015), carried out detailed seismic and well studies in the North Sea (Loseth et al . 2003, 2012, 2013), studied the physical processes of injection, extrusion and depletion of sand in analogue experiments (Rodrigues et al . 2009), and studied outcrops of intrusive sandstones through sand injection research consortiums (e.g. Hurst et al . 2006, 2011; Vigorito et al . 2008; Cartwright 2010; Scott et al . 2012). Our seismic and well observations above the Snorre and Visund fields were in conflict with existing geological models and therefore we proposed new models. This reply builds on knowledge of how the seismic expression of sandstone varies with the type of embedding claystone. Oygarden et al …