The northeast Atlantic margin comprises underexplored sedimentary basins and poorly-characterised submarine basement highs (e.g., Porcupine High, Rockall Bank). Recent provenance work has suggested that these basement highs strongly influenced the supply of sediment to basins during the Mesozoic (Evans-Young, 2014, Tyrrell et al., 2007, 2010). Their composition, therefore, is a possible first-order control on the quality and distribution of potential reservoir sandstones in the adjacent Porcupine, Rockall and Slyne basins. The aim of this study is to address these issues by 1) developing a better understanding of the distribution, age and crustal affinity of the Porcupine High; and 2) to determine sedimentary dispersal pathways from the highs, and from surrounding areas, during the early Cretaceous. In order to achieve this, a novel, multi-proxy geochemical approach was implemented which combined petrography, heavy mineral analysis, in-situ isotopic analysis of individual sand components, and thermochronological techniques.
Work on newly sampled material from the northern Porcupine High has indicated the presence of an old and complex metamorphic terrane, dominantly comprising Archaean to Proterozoic gneisses. The samples, although collected by seabed dredging, are believed to be near in-situ and have a common uplift history as indicated by apatite fission track analysis. Pb isotopic analyses of K-feldspar and U-Pb zircon analyses from these rocks suggest that the basement here shares a close affinity with the Lewisian Complex of NW Scotland and its equivalents, with minor components that correspond with regional Grenvillian crust (e.g. Annagh Gneiss Complex, NW Mayo). This increasingly detailed view of Porcupine High geology is allowing sedimentary supply from these areas to be assessed for the first time.
Lower Cretaceous sands and sandstones have been sampled from wells in Erris and Porcupine basins and from the Goban Spur. Heavy mineral analysis highlight fluctuations in indexes of Garnet-Zircon (GZi), Rutile-Zircon (RuZi) and Zircon-Tourmaline (ZTi) data suggest changes in sand sourcing within the sampled stratigraphy in both the Erris and Porcupine basins. U-Pb zircon ages and Pb isotopes in K-feldspar suggest that older, Lewisian-type, sources dominate supply to the Erris Basin (likely from the adjacent northern Porcupine High), while Grenvillian-type and younger grains, dominate the sediment delivered to the Porcupine Basin. The presence of Grenvillian feldspar and zircon in both basins, paired with the absence of Lewisian-type detritus in the Porcupine Basin, suggests there is a significant drainage divide, buffering the delivery of sand southward from NW Ireland during the Early Cretaceous. In the Goban Spur, U-Pb zircon ages of ~650 Ma may indicate sourcing of detritus from the Flemish Cap to the south or an, as yet, unrecognised equivalent.
This project is funded by the Petroleum Infrastructure Programme (PIP).