Central Clare Group - Upper Carboniferous deltaic and coastal sediments

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; view the listed figures on Clare Basin Clastics page) can be widely traced across the area and the web site focuses mainly with the lowest cycle; the Tullig cyclothem (Fig. 18). 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.

Click on highlighted Deepwater Gallery for access to a complete index to this part of the section.

Summary - Key features of Central Clare Group

•Significant wave influence but possibly brackish water on account of high river discharge into a humid tropical basin remote from, or poorly connected with the ocean.
• Tectonically relatively quiescent, with the basin geometry largely inherited from an earlier stretching event.
• "Big picture’ is one of overall progradation and shallowing through time, such that deep basinal shales are overlain by turbidites and then slope deposits and finally the shelf/delta deposits.
• cycle frequency is probably tied to glacioeustatic changes in relative sea level and sediment flux occurred throughout deposition.

Useful References