In many parts of the world the Carboniferous deposits preserve the record of a major transgression of the seas onto land areas, and the subsequent retreat of the seas to give continental or near-continental deposits in the upper part of the system. Palaeomagnetic evidence indicates that much of the United States, Europe and parts of Asia lay in the equatorial belt (Fig. h) and there is evidence of hot climates, including evaporite deposits, in these regions.
Although the early Carboniferous transgression was general, it was not a continuous one; its progress was interrupted with pauses and temporary withdrawals of the sea. Except in areas in which subsidence was greater than elsewhere (and such areas are commonly referred to as basins) the hot, arid climate and shallow seas often gave rise to limestone deposition. With a few local exceptions the lands invaded by the sea were apparently not of high relief and land detritus generally occurs only near coastlines.
There were wide expanses of shallow or very shallow seas in which many different types of benthic communities flourished, each specially adapted to the prevailing local ecological conditions. Two factors appear to have been paramount in controlling the composition of these communities: depth and salinity. The nature of the enclosing sediment, be it mudstone or limestone, is also important but seems to have been of lesser significance. Sand in the Carboniferous was often too rapidly deposited for extensive faunal communities to develop.
The depth of water involved in most cases was probably rather small and abyssal deposits are very rarely seen. In the lower part of the Carboniferous a series of communities, linked with gross lithology was outlined by Ramsbottom (1973, 1974) (see Table V). Each type of limestone or mudstone contained its characteristic species of bottom-attached benthic fossils; these were dominantly brachiopods and corals, the mollusca being much less important. Swimming animals such as goniatites, fish and active bivalves are much rarer in the shallow water calcareous coastal deposits, but they were often the only types which flourished in the subsiding basinal areas in which mudstone was being deposited and in which the sea bottom may well have been de-oxygenated;
this prevented colonization by a bottom-dwelling fauna. The reasons for the probable de-oxygenation of the sea floor in basins are still a matter for debate.
At times seas of extremely shallow depth developed over wide areas. These conditions produced intense evaporation which gave rise to varying degrees of hypersalinity, and the fauna and floras which flourished contained only those euryhaline species which could tolerate such specialized conditions. Here stromatolitic algae became dominant, and grew on sheet or mound-like masses. It is
Fig. h. The world during the Carboniferous. Positions of continents modified after Briden et al. J974; land areas after Hodso and Ramsbottom, 1973.
probable that the amount of relief on the stromatolite growths is directly proportional to the closeness of the shoreline and the disturbance of the water, for those with most relief appear to have grown furthest from the shorelines.
When hypersaline conditions occur in any locality, a faunal diversity gradient develops between those areas where salinity is normal and where it is extreme. In the Lower Carboniferous beds of the Northumberland Trough, in northern England, normal open sea conditions became more saline towards the east. There is moreover a definite order of disappearance of different fossil groups, reflecting their varying tolerance of hypersalinity (Table VI).
A similar type of faunal diversity gradient occurs when conditions become less saline than normal; these conditions are best studied in some of the Upper Carboniferous marine bands (Calver, 1968). In this case the freshwater end of the gradient contains 'non-marine' bivalves of a type comparable with living freshwater types of today.
A third type of diversity gradient reflects the transition from the diverse communities of the shelf to the fewer species of the subsiding basinal areas. Such gradients can be inferred from data (R. B. Wilson, 1974) from the Lower Carboniferous Macgregor Marine Beds of south-west Scotland, and also from the increase in diversity towards the old shorelines in the Namurian beds of South Wales.
Foraminifera Rugose corals
Spiriferoids, chonetids, most productoids
Normal Marine Limestones
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