The Temporal and Geometrical Evolution of a Middle Carboniferous Extensional Basin in the Eastern Campine Basin ( <scp>NE</scp> Belgium)

ABSTRACT Major thickness changes of the Namurian interval (Serpukovian and early Bashkirian) recorded in wells across several basins in northwestern Europe provide indications for rifting, but little is known about the dynamics of this extensional phase. On the basis of new biostratigraphic analyses, well‐log correlations, seismic interpretations, and geological models for the Campine Basin (northeastern Belgium), this study is the first to provide detailed constraints on the timing and geometry of this middle Carboniferous tectonic phase. The karstification of the lower Carboniferous carbonates, which is key for geothermal and gas storage applications in the region, was strongly influenced by the middle Carboniferous tectonics, highlighting the importance of a thorough understanding. During the early to late Visean, the Campine Basin was characterised by carbonate platform development under relative tectonic quiescence. The latest Visean was a time of major shifts in tectonics throughout Western Europe and marked the start of differential subsidence of the eastern Campine Basin. In this part of the basin, a several hundred meters thick sediment succession was deposited, with a progressive increase in siliciclastics at the expense of carbonates. In the western Campine Basin, in contrast, the latest Visean and earliest Namurian represented a major depositional hiatus. As differential subsidence continued from the Visean into the Namurian up to the earliest Westphalian A (late Bashkirian), the contemporaneous depositional sequences, mainly delta‐systems, were consistently developed thicker in the eastern Campine Basin compared to the western Campine Basin. The regional thickness model of the Namurian, on the basis mainly of seismic data, indicates that the differentiation between the western and eastern Campine Basin took place along a narrow basin margin comprised of tilted blocks and faults with normal vertical throws having predominantly NNE–SSW orientations. This fault direction could have resulted from an ESE–WNW extensional direction that likely originated from N‐S maximum horizontal compression by the northwards migrating Variscan thrust front further south. The extensional phase ended in early Westphalian A and marked the start of a thermal subsidence phase that likely continued up to the late Westphalian B when compressional foreland dynamics gradually started dominating the region. This transition from extension to thermal subsidence coincided roughly with a regional change in sedimentary deposition from delta progradation towards lower delta plain deposition.