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Department of Geological Sciences and Institute for Geophysics, The University of Texas at Austin, 8701 North Mopac Expressway, Austin, Texas 78759-8397, USA
Department of Petroleum Geosciences, University Brunei Darussalem, Brunei Darussalem
The Celtic Sea, offshore southern Ireland, is underlain by several discrete rift basins, which are part of a larger set of such basins that developed marginal to the North Atlantic within the tectonic framework of Pangaean break-up and episodic opening of the North Atlantic Ocean. This paper documents the development of depositional systems within a rift basin setting by analysis of the sequence stratigraphic evolution of the North Celtic Sea Basin, the largest basin in the area, with particular emphasis on the influence of pre-existing structural fabric on that evolution. The Caledonian and Variscan orogenies each imprinted distinct structural grains on this area, and the intersection pattern of their respective tectonic trends provided the structural framework which influenced subsequent Triassic, Late Jurassic and Early Cretaceous phases of rifting. Detailed analysis of an extensive well and seismic database indicates that the mode of reactivation of pre-existing lines of structural weakness depended on the orientation of the principal extensional stress direction relative to these lineaments. Triassic and Late Jurassic northwest-southeast orientated extensional stress reactivated Caledonian lineaments as half-graben boundary faults and Variscan lineaments as accommodation zones and lateral transfer faults which offset the basin margin faults. In contrast, during the Early Cretaceous, extensional stress was orientated approximately north-south, oblique to the Caledonian trend. This resulted in transtensional pull-apart basin geometries dominated by reactivation of Variscan structures. The intrinsic relationship between plate tectonic setting, pre-existing structural fabric and the resulting structural style provides insight into the influence of tectonism on depositional system development in an evolving rift basin. During Late Jurassic and Early Cretaceous rifting, depositional system development was predominantly controlled by tectonism and partly conforms to empirically derived tectono-stratigraphic models of sequence development in non-marine rifts. Depositional environments changed from fluvial-dominant to lacustrine-dominant as each rifting episode evolved through its early to active rift stages. During post-rift subsidence, marine transgressions dominated depositional system development, overriding the influence of local tectonism. Spatially, pre-existing structural fabric and the style of structural deformation are the dominant influences on sediment input and drainage systems within the evolving rift basin. Recognition of the intrinsic structural fabric of a rift basin is one of the key factors in understanding and predicting stratigraphic distributions within this type of basin.
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