Upper Carboniferous and Lower Permian tectonostratigraphy on the southern margin of the Central North Sea

Abstract

Hydrocarbon exploration in UK licence blocks 31/26 and 39/2 on the southern margin of the Central North Sea in 1996 and 1997 proved the presence of a Palaeozoic sedimentary succession, previously undescribed in the area. Biostratigraphic data from the sediments and radiometric dating of interbedded basaltic lavas indicate that the succession spans the Upper Carboniferous (Westphalian B) to Lower Permian (Asselian) interval; this has been informally split into three sedimentary units. The ‘Lower Flora Sandstone’ is of Westphalian B–C age. This clastic interval is overlain by the first of two main lavas, which is in turn overlain by the Westphalian C–D to Stephanian ‘Upper Flora Sandstone’. The Flora Sandstones are interpreted to be lithostratigraphic equivalents to the Schooner Formation of the Southern North Sea. The Flora Sandstones are separated from the ‘Grensen Formation’ by an angular unconformity, possibly related to Variscan (Asturic) inversion. The Grensen Formation is in turn overlain by a second basaltic lava flow. Both basaltic units are assigned to the Inge Volcanics Formation, Rotliegend Group. Petrographic data indicate that the Grensen Formation also forms part of the Rotliegend Group. A major extensional event of Permian age post-dates all of these strata. Extensional faults are planed by a regional unconformity (Saalian unconformity) that is overstepped by Permian to Cretaceous strata.