Deposition in the Northwest Porcupine Basin During the Cretaceous

Abstract

The paper details the sedimentary infill of the northwest part of the Porcupine Basin, offshore west Ireland, during the Cretaceous. This is a critical area for understanding the sediment input and incremental infilling of the basin, particularly during the early Cretaceous. Late Jurassic rifting and early Berriasian erosion produced a northward narrowing valley on the Base Cretaceous unconformity surface, which provided an entry point for sediment into the basin and also constrained the lateral extent of initial sedimentation. Following Berriasian marine transgression, the first deposits in the Porcupine Basin were turbidites located in localised depressions on the unconformity surface (named here informally as Basal Deposits 1–4). As the unconformity surface became smoother, the succeeding layers were more widespread (Sequences 1–3, the top being of probable Barremian age). Several features, interpreted as mounded fans sourced from the basin flanks, are developed in the Valanginian-Barremian section. Where drilled, the Berriasian to Barremian section is predominantly fine-grained. The rifted marginal horsts were covered in late Barremian time, and Aptian and later sediments onlapped the flanks of the Porcupine High to the west. The ‘Near Base Aptian’ is a well-defined prograding surface and the ‘Near Top Aptian’ horizon is a marked unconformity. The Aptian section is comprised predominantly of deep-water mudrocks and siltstones, although a distinctive, possibly shallower, erosive Intra-Aptian event was also mapped. A renewed sediment influx, comprising of sand prone deltaics, occurred at the northern margin of the basin during the Albian. Sand content decreases southwards into the basin. Sedimentation in the late Cretaceous was dominated by the basinwide deposition of the Chalk Group (Cenomanian-Selandian) deposited in a stable thermal subsidence setting. At the northern basin margin Upper Cretaceous deposits are thin and lithologically variable. The well-defined ‘Top Chalk’ reflector, which shows mounded features, is overlain unconformably by Paleocene clastics.