The Tonian-Cryogenian System boundary is to be defined at a GSSP (Global Boundary Stratigraphic Section and Point) beneath the first evidence of widespread glaciation. A candidate lies within the Dalradian Supergroup of Scotland and Ireland, which is least deformed and metamorphosed in Argyll, western Scotland. We present new stratigraphic profiles and interpretations from the Isle of Islay and the Garvellach Islands, update the chemostratigraphy of the Appin Group Tonian carbonates underlying the thick (ca. 1 km) glacigenic Port Askaig Formation (PAF) and demonstrate an environmental transition at the contact.The Appin Group forms a regionally extensive, >4 km-thick, succession of limestones, shales and sandstones deposited on a marine shelf. On Islay, the upper part of the lithostratigraphy has been clarified by measuring and correlating two sections containing distinctive stratigraphic levels including molar tooth structure, oolite, stromatolitic dolomite and intraclastic microbial mounds. Significantly deeper erosion at the unconformity at the base of the overlying PAF is demonstrated in the southern section. Carbonate facies show a gradual decline in δ 13 C VPDB from +5 to +2‰ upwards.In NE Garbh Eileach (Garvellach Islands), a continuously exposed section of Appin Group carbonates, 70 m thick, here designated the Garbh Eileach Formation (GEF), lies conformably beneath the PAF. The GEF and the GEF-PAF boundary relationships are re-described with new sedimentological logs, petrological and stable isotope data. Interstratified limestone and dolomicrosparite with δ 13 C of -4 to -7 ‰ (a feature named the Garvellach anomaly, replacing the term Islay anomaly) are overlain by dolomite in which the isotope signature becomes weakly positive (up to +1 ‰) upwards. Shallow subtidal conditions become peritidal upwards, with evidence of wave and storm activity. Gypsum pseudomorphs and subaerial exposure surfaces are common near the top of the GEF. The basal diamictite (D1) of the PAF is rich in carbonate clasts similar to slightly deeper-water parts of the underlying succession. D1 is typically several metres thick with interstratified sandstone and conglomerate, but dies out laterally. Scattered siliciclastic coarse sandstone to pebble conglomerate with dropstones associated with soft-sediment deformation is interbedded with carbonate below and above D1. Dolomite beds with derived intraclasts and gypsum pseudomorphs are found above D1 (or equivalent position, where D1 is absent).Published and new Sr isotope studies, including successive leach data, demonstrate primary Tonian 87 Sr/ 86 Sr values of 0.7066-0.7069 on Islay, decreasing to 0.7064-0.7066 in the younger GEF limestones on the Garvellachs, with 1700-2700 ppm Sr. Other typically Tonian characteristics of the carbonates are the Sr-rich nature of limestones, molar tooth structure, and dolomitized peritidal facies with evidence of aridity. Seabed surveys suggesting uniformly-dipping strata and shallow borehole core material illustrate the potential for extending the...

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The Mesozoic history of a number of Atlantic borderland sedimentary basins can be related to the early opening history of the southern North Atlantic Ocean. Regional tectonic controls such as plate motion vectors and the pre-existing tectonic grain had an important role in basin development and are expressed as local tectonostratigraphic events. The evolving palaeogeographies for the region are demonstrated in a series of maps based on computer-generated plate reconstructions. The Porcupine Basin, centrally located in the study area, lay close to the intersection of three plate boundaries that separated Eurasia from North America and controlled opening of the Bay of Biscay. The south Porcupine Basin, where there is relatively poor data control, is considered in the context of broader platetectonic controls, which were also responsible for the development of contiguous and better understood basins during Mesozoic time. This approach provides new insight into the structural evolution and likely facies development in the south Porcupine Basin, allowing broad inferences for petroleum prospectivity to be made. Initial Permo-Triassic fault-controlled extension led to continental deposition, which, if associated with aeolian and/or fluvial reservoir rocks, will mostly be too deep to be prospective. Thermal subsidence during Early Jurassic time was associated with flooding and fine-grained clastic deposition with anticipated moderate source rock potential. Regional uplift of the northern Porcupine area during Mid-Jurassic time forced shorelines and shelves southwards and the south Porcupine Basin could contain good reservoir quality sandstones and possible waxy deltaic-type source rocks of this age. In Late Jurassic time, major crustal extension took place with potential for reservoir and source rocks in locally expanded footwall successions. Further extensional faulting occurred in earliest Cretaceous (Neocomian) time with further synrift plays possible at this level. Growth of the Porcupine Median Volcanic Ridge is attributed to Barremian-Aptian time and related to continuing extension associated with a northwesterly arm of a triple junction positioned to the south of the Porcupine area. Strong subsidence of the basin centre during this time will have a significant impact on source rock maturation and flank trap geometries in the south Porcupine Basin.

The exploratory drilling of 200 wildcat wells along the NE Atlantic margin has yielded 30 finds with total discovered resources of c. 4.1×109 barrels of oil equivalent (BOE). Exploration has been highly concentrated in specific regions. Only 32 of 144 quadrants have been drilled, with only one prolific province discovered – the Faroe–Shetland Basin, where 23 finds have resources totalling c. 3.7×109 BOE. Along the margin, the pattern of discoveries can best be assessed in terms of petroleum systems. The Faroe–Shetland finds belong to an Upper Jurassic petroleum system. On the east flank of the Rockall Basin, the Benbecula gas and the Dooish condensate/gas discoveries have proven the existence of a petroleum system of unknown source – probably Upper Jurassic. The Corrib gas field in the Slyne Basin is evidence of a Carboniferous petroleum system. The three finds in the northern Porcupine Basin are from Upper Jurassic source rocks; in the south, the Dunquin well (44/23-1) suggests the presence of a petroleum system there, but of unknown source. This pattern of petroleum systems can be explained by considering the distribution of Jurassic source rocks related to the break-up of Pangaea and marine inundations of the resulting basins. The prolific synrift marine Upper Jurassic source rock (of the Northern North Sea) was not developed throughout the pre-Atlantic Ocean break-up basin system west of Britain and Ireland. Instead, lacustrine–fluvio-deltaic–marginal marine shales of predominantly Late Jurassic age are the main source rocks and have generated oils throughout the region. The structural position, in particular relating to the subsequent Early Cretaceous hyperextension adjacent to the continental margin, is critical in determining where this Upper Jurassic petroleum system will be most effective.

The Atlantic margin region of northwest Europe includes, on its inboard side, five basins which have been explored since the 1970s and contain proven, Jurassic-sourced petroleum systems: Halten