Interpreting the Upper Carboniferous of the Dutch Cleaver Bank High

Abstract

The Dutch Cleaver Bank High is an area that has undergone a uniform structural and sedimentological history since the Lower Carboniferous. It is characterized by an absence of Upper Triassic and Jurassic sediments, and covers an area of some 8000 km 2 . The basement in this part of the Southern North Sea owes its origin to closure of the Tornquist Sea during the Acadian (late Caledonian) orogeny. NW–SE compressive structures were formed which were subsequently reactivated as normal faults during rifting in the late Devonian–Dinantian. Visean carbonates developed over most of the Dutch Cleaver Bank High but towards the Elbow Spit High, Yoredale facies sediments were deposited. After rifting ceased in the early Namurian, a thermal sag basin evolved which was infilled by marine and deltaic sediments of the Millstone Grit sequence. The Coal Measures sequence of Westphalian to early Stephanian age unconformably or disconformably overlies the Millstone Grit. The Coal Measures sequence consists of fluviatile and associated sediments and is at least 1200 m thick. It contains several regionally correlatable lithological units of remarkably constant thickness. Sedimentation appears to have been uninterrupted during the entire Westphalian in this area. This implies that subsidence was sufficiently rapid to counterbalance the tendency for erosion on the continental plain during periods of sea-level fall. The high rate of subsidence is thought to be due to flexure in the foreland of the Variscan mountain belt. The axis of the basin lies to the south of the Dutch Cleaver Bank High. The source area for the sediment was to the north. From base to top of the Coal Measures sequence there is a change in lithology from lacustro-deltaic to poorly drained continental sediments to well-drained continental sediments, indicative of overall terrestrial progradation. However, within this overall trend, higher frequency fluctuations in base level have had significant impact on lithological variations on a scale of 25–375 m. During periods of rapid base-level rise, thick packages of fluviatile sediment were deposited due to a high rate of accommodation on the continental plain. These fluviatile packages are equivalent to transgressive systems tracts (TST). Mapping reservoir intervals within the Coal Measures sequence is made possible by the uniformity of the strata over the entire Dutch Cleaver Bank High. The most important correlative unit is a TST of Westphalian A/B age which consists of a constant 375 m of poorly drained alluvial plain sediment. It represents a fluvial complex with dimensions greater than the Dutch Cleaver Bank High. The unit is highly reflective on seismic due to constructive interference of the seismic wavelet within fining-upwards parasequences of approximately 25 m thickness. A major rifting event occurred at the end of the Stephanian leading to extension, uplift and igneous activity. In places on the Dutch Cleaver Bank High more than 1000 m of the Coal Measures sequence were removed by erosion. The Elbow Spit High was formed at this time as a major fault-rotated tilt block, up-dip of the Dutch Cleaver Bank High. The Southern Permian Basin developed due to thermal sagging after rifting ceased. Three further inversion events have affected the area since the Permian.