CARBONIFEROUS SYSTEM, in geology, the whole of the great series of stratified rocks and associated volcanic rocks which occur above the Devonian and below the Permian sys terns. The period of time during which these rocks were accumu lated is known as the Carboniferous period, and forms the last and most important of the major Palaeozoic systems. During this period were deposited the thick coal-bearing strata, which are of such great economic importance in many countries of the North ern Hemisphere; and this system of rocks, first referred to as "Carboniferous" in England and Wales, has been widely recog nized by its fossils and by the similarity of its deposits in Europe, Asia, northern Africa and North America, and in small isolated areas in the Southern Hemisphere. Similar deposits of a slightly later date, usually referred to in England as Permo-Carboniferous, occur in India, the Urals, North America, South Africa and Aus tralasia. By American geologists these deposits are, however, now referred to the Permian.
As typically developed in England and Wales, the Carbonifer ous period opens with marine conditions, at first localized and confined to small areas, but later spreading and covering the greater part both of the north and south of the country. At the bottom of this sea or seas were laid down a varied series of sedi ments, mainly calcareous, but including a considerable amount of detrital matter in the north. This initial period, predominantly marine, is usually termed Lower Carboniferous, and includes but few workable coal-seams, the rocks being termed "Mountain Limestone" or "Shales with Limestone" according to their char acter. These marine conditions were succeeded by detrital de posits, consisting of sandstone, often feldspathic and more or less coarse in grain, with intercalated shales containing marine faunas. These rocks are generally referred to as the "Millstone Grit Series," and form the lower part of the Upper Carboniferous. Succeeding these beds occurs a third great group of rocks, the "Coal Measures," which consist of sandstones, shales and coal seams, with marls occurring in the highest portion of the sequence.
Thus as typically developed in the Midlands of England, there is a sequence easily divisible into three parts : Red and grey sandstones; marls and clays with occasional breccias; thin coals and (Upper limestones with Spirorbis; workable coals in the South Wales, Bristol, Somerset and Forest of Dean coalfields.
Coal Sandstones, marls, shales and the most Measures Middle important of the British coals.
Lower Flaggy hard sandstones (ganister), shales and thin coal-seams.
(Grits (coarse and fine), shales, thin coal-seams and Millstone occasional thin limestones. The shales contain bands Grit with marine fossils (goniatites and thin-shelled lamelli branches) .
A complex assemblage of beds, largely calcareous, but varying much according to locality. The uppermost Carbon- beds (Goredale series) present only in the north and in iferous Gower, exhibit a rhythmical succession of sandstones, Limestone limestones and shales. The lower beds are more Series dominantly calcareous, and the basement beds are seen only in the south. A distinct facies with goniatites occurs in Craven and in Devonshire and is closely allied to the Continental Culm.
The calcareous Lower Carboniferous rocks of England exhibit considerable changes of facies, accompanied by more or less dis tinct faunas. These may be grouped as : (r) Standard limestones and shales with fauna of brachiopods and corals.
(2) Knoll-reef limestones, more or less imbedded, with a rich fauna of brachiopods, lamellibranchs and occasionally goniatites.
(3) Thinly bedded dark argillaceous cherty limestones and shales with abundant crinoids, zaphrentid and cyathaxonid corals, and brachiopods such as Schizophoria resupinata.
(4) Calcareous shales with occasional thin limestone bands and bullions, containing a Posidonomya and goniatite fauna (Bowland shales).
(5) Cherty shales with radiolaria (Devonshire and West phalia).
(6) Algal, oolitic and dolomitic limestones, with bryozoa (la goon phase of Dixon).
In the north of England the standard limestone phase, whilst persisting later than in the south and midlands, became invaded by intercalations of detrital deposits (shales and sandstones), forming the Yoredale series. This series is splendidly developed in Wensleydale, Swaledale and Teesdale, and in its upper part, above the Main Limestone, becomes dominantly detrital in char acter, passing up into the overlying Millstone Grit, with a slight stratigraphical break at the junction. The three main strati graphical divisions represent fairly accurately the major phases of the Carboniferous system as developed in England, whilst leaving room for debate as to the position of the boundaries between these divisions. Taken as a whole, the system broadly shows us a complete cycle of deposits between the Old Red Sandstone and the upper red beds of the Coal Measures.
On commencing to trace the Carboniferous deposits away from England and Wales, it is found that the lithological character of the deposits affords but an uncertain guide, and recourse has to be had to fossils (whether of animals or plants), as a guide to relative age and position. Nevertheless, using these aids to cor relation it is found that considerable tracts of the earth's sur face were evidently connected either as seas or land areas, often surprisingly uniform in the character of their deposits. In western Europe there is no sharp line of demarcation between the Devo nian and the Carboniferous rocks ; neither can the fossil faunas and floras be clearly separated at any well-defined line ; this is true in Britain, Belgium, Russia and Westphalia. Again at the summit of the Carboniferous series, as in Russia, Bohemia, the Saar re gion and Texas, both the rocks and their fossil contents merge gradually into those of the succeeding Permian system.
American geologists elevate the Upper and Lower Carbonifer ous into distinct systems—the Pennsylvanian and Mississippian respectively. Further they group the Mississippian with the De vonian, and the Pennsylvanian with the Permian, restricting the term Carboniferous to the latter group. Having regard to the wide-spread unconformity which exists in North America be tween the Mississippian and Pennsylvanian, this is understand able, but no such wide-spread or prolonged break is known in In Europe the Lower Carboniferous (Dinantian or Avonian) is generally divided into two divisions, an older termed the Tour naisian, and a younger termed the Visean, and one or both of these divisions have been recognized also in Asia and north Africa. Similarly in North America the Lower Carboniferous (Mississippian) has been divided into two stages, an earlier termed the Waverlian, and a younger termed the Tennesseean.
In England and in Europe generally the Tournaisian seas were local and inhabited by varied faunas, rendering correlation fre quently difficult, as for instance in the correlation of the lower beds of Cumberland and Westmoreland with those of the Bristol and Belgian basins. The Visean sea, on the other hand, in Europe and especially in England and Scotland, occupied much wider areas than that of the Tournaisian period, and the fauna became more generally diffused. The separation of Visean from Tour naisian in western Europe is based almost entirely on faunal evi dence, there being few marked breaks in marine sedimentation.
In North America the conditions during the Lower Carbonifer ous period, whilst generally marine over large tracts of the pres ent United States, appear otherwise to have been largely the re verse of those prevalent in Europe. The Waverlian (Tournaisian) seas spread over great areas of the North American continent, estimated by Schuchert at its maximum at 26% of the present continental area. The Tennesseean (Visean) seas, on the other hand, appear to have been comparatively restricted in North America, with a maximum submergence of 12% of the continent. Coupled with this marked reversal of European conditions there were of course wide-spread physiographical changes in North America at the junction of the Waverlian and Tennesseean, as well as the more important break in the eastern part of the con tinent at the top of the Tennesseean.
Various attempts have been made to establish a uniform table of major subdivisions of the Carboniferous period, based on changes in the fossil fauna and flora. These have been locally suc cessful, but no one group of organisms was sufficiently wide spread for its component species to be of use as zonal indices throughout the period or in all areas. As a result, any complete zonal scheme is largely a patchwork of more or less overlapping divisions. The following main divisions are generally recognized throughout Europe :- (5) Stephanian (or Uralian) .
(4) Westphalian (or Moscovian).
(3) Lancastrian (or Namurian, or Lanarkian).
(r ) Tournaisian.