Pb Isotopic Constraints on Mesozoic Paleodrainage from the Northwest European Margin: Implications for Reservoir Distribution

Tyrrell, Shane¹, Peter D.W. Haughton¹, J. Stephen Daly¹ (1) University College Dublin, Dublin, Ireland

Understanding the provenance of sedimentary rocks can place fundamental constraints on the scale and route taken by ancient drainage systems and, consequently, on the distribution of reservoir sandstones. However, many of the commonly applied provenance tools produce equivocal results due to non-unique source signals, inadequate characterization of potential source areas, post-depositional modification and/or averaging due to recycling and mixing. Some of these shortcomings can be overcome using a newly-developed technique based on the Pb isotopic composition of detrital K-feldspar grains. Rapid, in situ Pb isotopic analysis of single sand grains of K-feldspar by laser ablation ICPMS provides a provenance signal that has been shown to survive weathering, transport and diagenesis. Moreover, broad regional-scale variations in Pb isotopic composition in basement terranes mean that potential source areas can be readily characterized.

Mesozoic sedimentary basins on the northwest European margin record a complex history of active tectonism and thermal subsidence prior to and during the opening of the North Atlantic. The Pb isotopic K-feldspar provenance technique has been applied to prospective sandstones of Triassic, Jurassic and Cretaceous age from the Slyne and Porcupine basins and from the margins of the Rockall Basin, west of Ireland. In addition, basement Pb compositions have been compiled for circum-Atlantic basement terranes, and new onshore and offshore basement samples characterized. The data from the sandstones and potential source terranes challenge existing ideas concerning Pangean drainage during the onset of Triassic rifting, and demonstrate major drainage re-organisation later in the Mesozoic. These results have important implications for predicting the likely occurrence of similarly-aged reservoir sandstones over a wide region within the North Atlantic.