AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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The Slyne and Erris Basins: exploring varied structural style along strike in narrow rift-basins

Abstract

The Slyne and Erris Basins belong to a chain of basins extending from the Norwegian Atlantic Margin in the north to the Porcupine Basin in the south. They are narrow, elongate and interconnected sub-basins orientated NE-SW (Erris) and NNE-SSW (Slyne) that began forming in the later Permian and Early Triassic. Basin fill consists of Permian, Triassic, Jurassic, Cretaceous and Cenozoic strata with key reservoir intervals in the Triassic, Lower- and Middle Jurassic. The Slyne Basin is divided into northern, central and southern sub-basins. The northern sub-basin is a large graben with major NE-SW trending basin-bounding faults to the east and west, with considerable influence from salt-layers. The Corrib gas field and nearby structures are heavily influenced by these, forming forced anticlinal folds over Jurassic normal faults in the pre-salt. The central sub-basin is a steeply dipping half-graben with a major fault along its western margin and significant detachment along Permian salt producing many distinct horst blocks. Permian salt also intrudes diapirically up the basin-bounding fault, creating forced folds in the Triassic and Jurassic hanging-wall stratigraphy. The southern Slyne sub-basin is a large graben cut by major, long-lived faults that also divide it from the Northern Porcupine Basin. The whole Slyne Basin is cut by a significant composite Base-Cretaceous Unconformity (BCU) and Base-Cenozoic Unconformity (BCzU), with the BCU removing the Upper Jurassic in the central and southern sub-basins and the BCzU removing the Cretaceous in the same areas. Evidence for E-W trending strike slip faults and reverse reactivation of normal-faults in the Cenozoic can be seen throughout the Slyne Basin and has significant implications for the integrity of hydrocarbon traps. The Erris Basin can be divided into northern and southern, elongate grabens, bounded to the NW by the Erris Ridge and to the SE by the Irish Mainland Shelf which underwent normal extension during the Triassic and Jurassic. The northern sub-basin is locally affected by a significant Base-Cretaceous Unconformity (BCU) which removes most of the Jurassic section down to the Hettangian Broadford Beds Formation, while thicker Jurassic stratigraphy is preserved in the deepest parts of the sub-basin, bounding the crystalline Erris Ridge. The southern sub-basin preserves a thicker Jurassic sequence but has experienced prolonged uplift along its western flank during the Cretaceous and Cenozoic, with a significant angular unconformity placing the Base-Cenozoic against Middle and Lower Jurassic sediments. Like the Slyne Basin, there is evidence of strike-slip faulting and reactivation of faults during the Cenozoic in the Erris Basin. The variety of structural styles within individual sub-basins produces a range of potential petroleum plays and trapping structures, most of which remain untested. The change in structural style is linked to the nature of the transfer zones separating the sub-basins, likely inherited from the deeper structures that separate the Caledonian terranes. Understanding these zones will inform the structural history of both the Slyne-Erris area and other basins along the Irish Atlantic Margin. ICRAG is funded in part by a research grant from Science Foundation Ireland (SFI) under Grant Number 13/RC/2092 and is co-funded under the European Regional Development Fund and by PIPCO RSG and its member companies.