A glaciogenic sequence from the Early Pleistocene of the Central North Sea

Abstract

A depositional sequence bounded by upper and lower erosion surfaces covering a 35‐km × 20‐km area within the northern embayment of the Early Pleistocene Central North Sea is dated close to the Jaramillo Normal sub‐chron at approximately 1.0–1.1 Ma. The basal surface cuts shallow marine clays of the upper Nordland Group and is characterized by furrow incisions, mega‐scale glacial lineation textures and bedforms that are all orientated NNW–SSE. The sequence comprises a suite of distinctive bedforms, including a chaotic deposit up to 60 m thick, a series of moulded and lineated depositional bodies and some more complex bedforms, with lithologies shown by petrophysical data to be dominated by overconsolidated clays and thin sand layers. This sequence is considered to represent deposition from the advance and subsequent decay of a marine‐terminating ice‐sheet. The upper surface of the sequence coincides with the so‐called ‘Crenulate Reflection’ and is characterized by a fluvial channel system, representing final retreat of the ice‐sheet, which was followed by shallow marine conditions and deposition of the Aberdeen Ground Formation. It is considered that the Aberdeen Ground Formation comprises at least two main sequences spanning the Early to Middle Pleistocene.

The early Quaternary North Sea Basin

“…Sedimentation rates for the Quaternary were calculated from sediment thickness maps between the four seismic stratigraphical packages (2.58 -2.35, 2.35 -1.94, 1.94 -1.1 and 1.1 Ma to present Nielsen et al (2008); Buckley (2012Buckley ( , 2017; Ottesen et al (2014) and Thöle et al (2014). mbsf, metres below seafloor; n.a., not analysed.…”

“…For comparison, the basal Quaternary as defined at Josephine-1 by Knudsen & Asbjörnsdóttir (1991) and correlated to the 'crenulate reflector' by Buckley (2012Buckley ( , 2017 using the first occurrence of benthic foraminifera species Cibicides grossus, was also mapped across the dataset. C. grossus was identified as a marker species for the North Sea by King (1983), who calibrated C. grossus and its related species Elphidiella hannai to the Gauss-Matuyama Reversal, and hence the 2.58 Ma boundary.…”

A petrophysical approach to the investigation of shallow marine geology

“…A synthetic seismogram was constructed by Vardy et al . () for comparison of piston core logs with chirp seismic data in a Norwegian fjord, and Buckley () used a synthetic well‐tie analysis in the investigation of a glaciogenic sequence in the Central North Sea. The use of seismic inversion techniques for informing shallow geological site investigations has been considered by Dai, Snyder and Dutta (); Hamilton et al .…”

“…(); and briefly by Digby (). More recently, Vardy () inverted single‐channel seismic data calibrated against multi‐sensor core‐log data to determine physical properties of a shallow geological unit, and Buckley () has described a North Sea glaciogenic sequence from 3D seismic data inverted with a model‐based deterministic inversion algorithm. Rose et al .…”

“…An exploration 3D seismic volume and associated near‐, middle‐, and far‐angle stacks from an area of the central North Sea were employed in the investigation of potential shallow gas accumulations at the level of 20” casing shoes of planned wells in the area. Seismic responses at the level of the so‐called “Crenulate” reflection (Holmes ; Buckley ; Rose et al . ) display several characteristics indicative of shallow gas, including reverse‐polarity (with respect to seabed) amplitude anomalies (Figure a), as well as velocity and frequency responses.…”