The structural and stratigraphic evolution of the Celtic Sea Basins, offshore Ireland




Rowell, Philip, 1952-

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The Celtic Sea area, offshore southern Ireland, is underlain by several discrete rift basins, part of a larger set of such basins which developed marginal to the North Atlantic within the tectonic framework of Pangea break-up and episodic opening of the North Atlantic Ocean. This study investigates tectonic processes of rift basin formation by analysis of the structural and stratigraphic evolution of the Celtic Sea basins, with particular emphasis on the influence of preexisting crustal structure on that evolution. The Paleozoic Caledonian and Variscan orogenies each imprinted distinct structural grains on this area, and the intersecting pattern of these tectonic lineaments provided the structural framework for the subsequent Triassic, Late Jurassic and Early Cretaceous rifting phases. Detailed study of the largest basin in the area, the North Celtic Sea Basin, using an extensive well and seismic database, indicates that the mode of reactivation of these preexisting lines of structural weakness depended on the orientation of the principal extensional stress direction relative to these lineaments. The Triassic and Late Jurassic northwest-southeast oriented extensional stress reactivated Caledonian lineaments as half-graben boundary faults and Variscan lineaments as transfer zones. On the other hand, the Early Cretaceous extensional stress was oriented north-south, oblique to the Caledonian trend, resulting in transtensional pull-apart basin geometries dominated by Variscan lineament reactivation. Subsequent north-south compressional stresses related to the Alpine orogeny reactivated both Caledonian and Variscan lineaments in a conjugate shear pattern of strike-slip deformation. This structural evolution model demonstrates the intrinsic relationship between plate tectonic setting, preexisting crustal structure and the resulting structural style. It also provides insight into the variable influence of tectonism on depositional system development. During Late Jurassic and Early Cretaceous rifting, depositional sequences were predominantly controlled by tectonics and partly conformed to empirically derived tectono-stratigraphic models of sequence development in nonmarine rifts. Depositional environments changed from fluvial to lacustrine as each rifting episode evolved through its early to active rift stages. In addition, axial transport of sediment, accentuated by regional tectonics and climatic changes, was a dominant control on facies distribution during Early Cretaceous rifting. During periods of tectonic quiescence, changes in base level were more eustatically driven and sedimentation was less influenced by preexisting crustal structure. The identification and discrimination of these controls on depositional system development in evolving rift basins aids in determining potential stratigraphic distributions within these basins