The structure and origin of the Waulsortian (Lower Carboniferous) “reefs” of west-central Eire

Abstract

The structures into which the Irish Waulsortian limestones are organized have been regarded as reefs. They are re-interpreted as carbonate mudbanks. Their growth mechanism has been deduced from a study of bank morphology and depositional structures. The banks grew from Upper Tournaisian to Lower Visean times, occupying an offshore position on a shallow water shelf. They formed a bank complex covering thousands of square miles, and many smaller masses scattered in the lagoon on its northern and eastern sides. Southwards the Waulsortian Complex was bounded by the ‘Culm’ mud belt. Individual banks were detected and their internal structure mapped by studying the spatial arrangement of small-scale bedding features. The sparry masses ( Stromatacis or ‘reef tufa’) proved the most useful of these because their shape and orientation were found to depend directly upon the depositional attitude of the bank bed containing them. Form lines, constructed from measurements on bedding features, were used to delineate banks incompletely exposed. When compared with the size of the Complex the banks were not large. At any one time they may neither have risen much more than 50 ft. above the sea floor nor exceeded a few hundred yards in diameter. They are constructed from irregular, thin, lenticular limestone bodies here termed ‘bank beds’. These, which are sometimes difficult to detect, apparently represent growth increments not erosional remnants. Flat-lying beds characterize the earliest stage of bank growth. Later, the depositional slope gradually increased. In the final ‘climax form’ depositional dips up to 50° are known. Bank geometry was controlled by the size, shape and arrangement of the bank beds (affected by several factors) and the relative rates of bank and off-bank sedimentation. Most banks conform to a basic ‘knoll’ growth pattern. ‘Sheet’ forms, probably highly modified knolls, are rare. Single, isolated knoll-form banks have not been seen: the existence of one bank always seems to have promoted the formation of others. Four examples illustrate stages in the aggregation of banks to form a complex. These, taken from localities in Counties Longford, Galway, Tipperary and Limerick, show successively less intercalation of lagoonal limestone and shale until finally all the banks overlap one another directly. Lithological variation in the Waulsortian limestones can be expressed in terms of their five main components: (i) calcite mudstone, (ii) coarsely crystalline calcite mosaics (including Stromatactis and ‘ reef tufa ’), (iii) in situ fenestellid Bryozoa, (iv) crinoidal, shelly and bryozoan debris, and (v) entire fossils other than Bryozoa. Except at bank margins no simple pattern of lithological changes has been recognized. Fenestellids are often common, acting as baffles trapping fine sediment. However, they did not constitute a rigid framework and cannot be regarded as the sole agents of bank growth. Depositional structures in the calcite mudstones provide the key to an understanding of bank genesis. Stromatactis spars, which elsewhere have attracted most attention, are less important because their form depends directly upon the depositional sequence in the mudstones. In any one sample several distinct mudstone generations are present. Most were deposited before any spar formed. Their present distribution mainly results from internal sedimentation. The earliest mud generation ( M 1) forms discrete patches or loose ‘ flocculent ’ masses often occupying less than half the total volume of mud present. It is generally surrounded by later muds ( M 2 et seq .) and spars. The arrangement of the generations and the structures within them suggest that (i) M 1 behaved as lumps of sediment (compacted but not lithified) while later muds were finely particulate, (ii) both M 1 and M 2 arrived in their present positions together by downward movement in a gently collapsing system, and (iii) loose packing of M 1 and M 2 left cavities roofed by mechanical ‘bridges’. Similar ‘bridging’ and cavities can be produced experimentally. The collapse features can be explained by decay of the organisms (perhaps plants or sponges) around and within which the mudstones of the bank accumulated. M 1 may then represent mud trapped between the organisms and M 2 that trapped or produced within them. A local origin for the mud is favoured. Cavities remaining after collapse were filled by geopetal muds trickling from higher parts of the bed, and by precipitated spars (thus producing Stromatactis ). In some instances, at least, the spars were formed before deposition of the next bed. They could thus have provided an inorganic skeleton supporting the bank until the mud lithified. It is concluded that the morphology of the banks, their steep depositional slopes, and the presence, bulk and arrangement of the calcite mudstones all point to the baffling activity of organisms (not preserved) as the mechanism of bank growth. Details of the physical and chemical environment around the banks cannot be surely deduced from evidence available at present. The sedimentary structures give few direct clues because of extensive internal resedimentation. Further, the value of any of the mechanical structures as indicators of the state of the water around the banks is doubtful. Even if the banks grew in agitated water the presence of baffles could inhibit formation of mechanical depositional and erosional features otherwise associated with these conditions. Absence of such features may thus be misleading.