Upper Carboniferous Slope to Basin Floor Sediments, Western Ireland: Outcrop Analogues for Passive Margin Turbidite Plays

Martinsen, O.J., Norsk Hydro Research Center, N-5020 Bergen, Norway, R.G. Walker, Calgary, Canada, Trond Lien, Norsk Hydro Research Center, N-5020 Bergen, Norway

Abstract

The Upper Carboniferous deep water deposits of the Shannon Basin, western Ireland show world-class exposures which have been visited by many hundreds of geologists over the last 20 years. The succession is particularly well suited as an analogue for sand- rich and mud-rich turbidite plays in passive margin basins around the world, because of its tectonic context, the type and scale of turbidite sandstone bodies, and the overall character of the stratigraphic succession.

The Shannon Basin is an east northeast-west southwest elongate basin which originated from extensional collapse of the Iapetus Suture in latest Devonian and Early Carboniferous time. The tectonic context and basin fill compare with productive passive margin situations, such as the North Sea Basin and the Cenozoic of the highly prospective eastern North Atlantic margin. Several aspects compare with the Cenozoic Gulf of Mexico Basin and the Neogene of the West African margin.

The deep water rocks of the Shannon Basin consist of an 1150 m thick, shallowing upward succession of shales, mudstones, and turbidite sandstones. The sandstones mainly belong to the 380 m thick, axially supplied basin floor successi on of the Ross Formation, which is analogous to many sand-rich submarine fan play types in the Paleogene of the North Sea and the Cretaceous of the Vøring and Møre Basins. There are several reservoir-type elements of the Ross Formation: (1) randomly organized tabular bed packages, up to 10 m thick, in the lower part of the formation; (2) packages of shallow, sinuous channel sandstones, up to 20 m thick, in the middle and upper parts of the formation; and (3) sandier, and locally thickening upward packages, mostly less than 5 m thick and capped by giant flutes, which represent marginal deposits of the sinuous channels. Reservoir elements (2) and (3) occur in a predictive succession which has important implications for the understanding of similar subsurface successions.

In contrast, the overlying 550 m thick Gull Island Formation is a mud-rich turbidite system and can be divided into two parts:

1. The lower 350 m, representing proximal basin floor deposits, are composed of three laterally variable facies associations: (a) axially transported turbidite sandstones in packages up to 50 m thick; (b) up to 50 m thick slump and slide sheets; and (c) a few meters thick hemipelagic draping mudstones and shales. The main reservoir type sandstones are found in this part of the Gull Island Formation and include channels, overbank lobe and splay deposits, sandstones trapped in hangingwall areas of slides in front of growth faults, and channel sandstones bounded by mud diapirs. The interaction between reservoir type sandstones and slumped deposits is complex.
2. The upper 350 m represent the prograding basin slope which advanced laterally across the axially supplied basin floor deposits, and contains no reservoir type sandstones.

The stratigraphy and sedimentology of the Ross and Gull Island formations form an important analogue for turbidite systems in rifted basins. The change from a very sand-rich turbidite system, having abundant but architecturally complex reservoir elements, to a mud-rich system, having sparse widely spaced reservoir elements, is particularly important to understand for predictive purposes in both frontier and mature basins. The most likely explanation in the present case is simply the overall progradation of the muddy basin slope over the sandy basin floor. Nevertheless, in the subsurface, such transitions are difficult to define and understand, even on high quality 3-D seismic. Valuable, detailed understanding can be gained from extensiv e outcrop sections. Even if the scale of outcrops seldomly matches the scale of subsurface systems, crucial concepts can be extracted which lead to significant breakthoughs in understanding.