Fig. 1

The Upper Carboniferous sections of the Co. Clare coast expose part of the fill of the Clare Basin (also known also as the Carboniferous Shannon Basin (Martinsen et al., 2000 and 2003; Pyles in Press))(Figs 2, & 3). The axis of this basin coincides with the Shannon estuary where it intersects the coasts of southern Co Clare and the northern Co Kerry. Prolonged subsidence in this area of western Ireland started in the early Carboniferous period. This subsidence was initiated by regional crustal stretching in the Lower Carboniferous that enabled the initial accumulation of a thick succession of limestones that are particularly well exposed in the Burren area of north Co. Clare. The character of limestone associated with this early phase of subsidence supports the contention that western Clare was the site of a platform-to-basin transition (Fig. 4). The area to the north marked a persistent shallow water platform but in the vicinity of the Shannon Estuary, the so-called Shannon Trough, the platform sloped southwards into a deeper water setting of what was an enclosed structural basin. By the end of the Lower Carboniferous, Kerry and probably South Clare were part of this deep water setting with limestone turbidites already accumulating on the basin floor. The succeeding Upper Carboniferous of the Clare Basin can thus be thought of as a successor basin that inherited much of its geometry from this earlier phase of subsidence and coincident bathymetry.

Fig. 4

Fig. 2

Fig. 3

Fig. 9

Fig. 10

Shannon Group - Ross Sandstone Formation
The overlying Ross Sandstone Formation is thickest in the area of south Clare (c. 350m) and thins towards the north, east and south. It is primarily composed of sandstones (around 65% at Loop Head (Fig 12); Chapin et al. 1994), with subordinate interbedded shales and slumped horizons of mixed lithology. The sand is mostly fine to very fine grained, in dm to m-scale beds, many of which have a sheet geometry. These tend not to show systematic vertical trends, although laterally discontinuous channelised units (Figs. 13 & 14) and interbedded slumped and disturbed horizons are more common towards the top of the formation (Fig. 15). As indicated the base of the formation is seen at Ballybunnion, on the south side of the Shannon Estuary (Figs. 2, & 10), and it is marked by a gradual increase in the thickness and abundance of sandstone sheets sitting in black shales. A number of laterally extensive 'marine bands' are seen within the formation. The sandstone beds are interpreted as a turbidites dispersed from an unseen source lying to the southwest, and blanketing the deepest part of the basin floor as a fan or sheet system (Fig. 8, & 11). A submarine high in north Clare prevented all but the youngest of the flows from depositing sand here (Figs. 3, & 7). The Ross Sandstone Formation is transitional with the overlying Gull Island Formation, with the boundary placed at the widespread "Reticuloceras paucicrenulatum" marine band (Fig 3).

Shannon Group - Gull Island Formation
The Gull Island Formation reaches 550 m in the Loop Head region, but thins to c. 140 m in North Clare. The lower part of the formation contains siltstones interbedded with bundles of sheet sandstones (turbidites), whereas the upper section is primarily siltstones (Fig. 16). The main feature of this formation, however, is the extensive soft sediment deformation (growth faults and slump features; Fig. 17) which occurs within in it. It is interpreted as a fine grained, unstable (delta?) slope which prograded eastwards into the basin (Fig. 6).

Central Clare Group
The delta systems of the Central Clare Group transitionally overlie the Gull Island progradational slope system. The group has a maximum thickness of 900 m and can be broken down into five major cycles (or cyclothems) separated by prominent marine bands. Only the lower three cycles (Tullig (Fig. 18 & 19), Kilkee (Fig. 20) and Doonlicky) can be widely traced across the area and the web site focuses mainly with the lowest cycle; the Tullig cyclothem (Fig. 08). Each cyclothem broadly coarsens upwards and has been classically interpreted as a prograding delta system (Fig. 18), perhaps analogous to the modern Mississippi Delta); the deltas were fine grained, river-dominated with moderate wave-reworking of the delta front mouth bars (Fig. 19); the delta front slopes showing varying degrees of instability and this may partly relate to their position (with relatively stable shoal water deltas sitting on the shelf and unstable shelf margin deltas where rivers fed sediment to the shelf-slope break).

Each cyclothem typically commences with a silty succession reflecting the build out of unstable deep water delta fronts. Mouth bar sandstones are embedded in the top of these progradations; these can be up to 15 m thick and 2-3 km across. These are erosively cut by sandstone-dominated fluvial channels (Fig. 19). In the Tullig cyclothem, the sandstones are up to 35 m thick and are referred to as the Tullig Sandstone. Original interpretations of the cyclothems stressed autocyclic mechanisms, with the erosively based sandstones reflecting the advance of the distributive feeder channels of the delta top over the delta front with its mouth bars. However, discrepancies in the grain size of the sediment in the ‘channels’ and the delta front, the huge scale of the channel sandbodies (>20 km wide) and evidence for significant erosion at their base indicate that these may be palaeovalleys rather than distributive channels (Fig. 18). The stratigraphy may thus be responding to external forcing by changes in sea level, with falls in sea level driving shelf incision and the cutting of incised valleys. This is important for understanding the turbidites down dip; the evidence from the deltas suggests that there were times when much of the sediment was held in shallow water shoal deltas and estuaries, and other times when either deltas migrated to the shelf edge or the shelf itself was incised and rivers fed sediment directly to the top of the slope. These latter two scenarios would have favored transfer of sediment into deeper water.