The North Sea Basin: mid-Miocene to Early Pleistocene
The lower limit of this chapter is within the Langhian (as explained in Chapter 12) corresponding to the base of the Hodde Formation and corresponding levels in onshore areas, except that, in the Netherlands and adjacent areas, the lowest Breda Formation, predating this level, is included. In the northern North Sea, this interval corresponds to the lower part of the Nordland Group, but owing to discrepancies in the de?nition of the base of the Nordland Group elsewhere, summarized below, the highest part of the Lark Formation is also included.
The base of the Calabrian has been chosen as the upper limit. Although this is within the Early Pleistocene, it corresponds approximately to a well-marked seismic reflector throughout much of the North Sea, and to a foraminiferid zonal boundary that is documented throughout the Basin. In addition, the earliest Pleistocene (Gelasian) was for a long period classi?ed as Late Pliocene, and has often generally been included in Tertiary rather than Quaternary studies in offshore areas.