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A. J. (Alfred John) Jukes-Browne.

The student's handbook of stratigraphical geology online

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Black limestone and chert \
Upper cherty zone /

The Fenestella limestone
Lower cherty zone
Shaly limestone .
.Lower limestone shale .



Feet.
. 80
. 1500

. 400

. 300

. 100

. 400

. 150

About 3000



The zonal distribution of fossils in the limestone of Clare county
has recently been ascertained by Mr. J. A. Douglas, 13 and proves to
be comparable with that in the Bristol sequence. He tabulates
the beds and zones as follows :



Survey Divisions.


Zones and Sub-zones.


Thickness.


Black Goniatite shales.
Upper Limestone. Bedded crinoidal
limestones, both light and dark
grey. Chert occurs in layers and
nodules, some beds of oolitic lime-
stone.


? = Pendleside Beds.
D 3 Cyathaxonia.
D 2 Lonsdaleia.
Dj Dibunophyllum 6.
S 2 Productus cora.
S x Prod, semireticu-
latus.


1900


Lower Unstratified Limestone. Mass-
ive grey and mottled limestones,
often dolomitic. Bryozoa abundant.


C. Syringothyris
cuspidata.


450
to
1000


Lower Stratified Limestone. Lime-
stone with cherts at top, dark-grey
or blue argillaceous limestones.


Z 2 Zaphrentis
konincki.


650


Lower Limestone shales. Black
shales and limestones, green and
yellow sandstones.


Z 2 Spirifer clathratus.
? Cleistopora zone.
Modiola phase.


150



274: STRATIGEAPHICAL GEOLOGY

The basal yellow sandstones contain Lamellibranchs only. The
succeeding shales have not yielded Cleistopora, but contain Spiri-
ferina octoplicata and Productus bassus. The other zones contain
assemblages which correspond closely with those of the South Wales
and Bristol areas. A noteworthy feature is the great development
of the Dibunophyllum and Cyathaxonia limestones, which have
been termed the Burren limestone, from the district of that name,
where they are magnificently exposed. It is thus described by
Jukes 14 :

" A range of hills, rather more than 1000 feet in height, sweeps
for about 20 miles along the south side of Galway Bay. They are
formed entirely of bare rock from the sea-level to the hill-tops, the
only soil being found in crevices of the rock, or in patches in
the hollows of the valleys. This rock is all limestone, in regular
beds, which dip gently to the south, at an angle of 1^ degrees only ;
and counting from the lowest bed that rises out on the sea-shore to
the uppermost, which caps the summit of the hills three or four
miles to the southward, there must be a thickness of at least 1600
to 1700 feet of solid limestone shown here."

In Limerick and Tipperary there is a similar succession, and
the limestone series attains a thickness of about 3500 feet, its
thickest member being the Fenestella limestone (1500 to 1900
feet), while the upper (Burren) limestone has decreased to only 240
feet. The same succession of limestones can be followed eastward
through Kilkenny and Carlow, and Fig. 93, across the Castlecomer
coalfield, illustrates this part of the area.

East of Thomastown the limestones overlap the lower shales so
as to rest directly on the granite of Carlow, and a little to the
northward, in County Dublin, shales of the calp facies overlap the
limestones, thus giving evidence of another shore-line.

This is confirmed by the sections on the coast of County Dublin
near Lough Shinny and Rush, which have recently been examined
by Dr. Matley 15 and Dr. Vaughan. The lowest beds there seen
are conglomerates and shales, indicating the close proximity of a
coast-line, and these are overlain by limestones belonging to the
Dibunophyllum zone. Still higher are limestones with Cyathax-
onia, overlain by limestones and black shales with Posidonomya
Becheri.

Southern Counties. When the Carboniferous rocks are traced to
the south-west through Cork and Kerry a still greater and much
more sudden change takes place in the constitution of the system.
This consists in the introduction of what appears to be a distinct
group of shales and slates between the Yellow Sandstone and the
Carboniferous limestone, and to which Sir R. Griffith gave the



THE CARBONIFEROUS SYSTEM 275

name of Carboniferous slate. This group of beds lias derived
special importance from its bearing on the Devonian question, and
the whole subject is so associated with the name of Jukes that it
would be idle to attempt a better exposition of the interest attach-
ing to the Carboniferous slate than is contained in Mr. Jukes's
own description quoted below. 16

" If we draw a parallel of latitude through the towns of Ken-
mare, Macroom, and Cork, the great development of Carboniferous
slate lies wholly south of that line. If we examine the neigh-
bourhood of the city of Cork itself, we find the [Upper] Old Red
Sandstone with plants in its upper beds, and a very short distance
above that we get solid Carboniferous limestone, with some black
shales or slates between the two, but not more than 200 or 300
feet in thickness. Passing southwards to the mouth of the harbour
of Monkstown or Queenstown, and then by Carrigaline and Cool-
more, these intermediate black slates or shales thicken to 2000 or
3000 feet, still having the Old Red [or Coomhola Beds] below and
the Carboniferous limestone above ; but going still farther south
t>y Ringabella to Kin sale, the dark -grey slates and grey grits
thicken rapidly to 5000 or 6000 feet, and are nowhere covered by
any part of the Carboniferous limestone, though they show here
and there highly calcareous bands."

On Whiddy Island at the head of Bantry Bay there are black
shales containing Posidonomya Becheri and P. membranaceus, the
characteristic species of the shales above the limestone, and it is
therefore highly probable that the Carboniferous slate is con-
temporaneous with the whole of the Carboniferous limestone, the
lower and upper parts of that formation passing laterally into
shales, just as its middle part does to the northward. We may
therefore regard the Carboniferous slate as representing the whole
of this limestone and its underlying shale, i.e. strata amounting to
a thickness of 2600 feet in the northern part of Ireland ; such, in
fact, was Jukes's opinion.

8. Devon and Cornwall

This area has been left till the last, because the equivalents of
the Avonian Series occurring therein are in many respects similar
to those of the south of Ireland and are very different from the
typical Avonian of the Bristol and Mendip area, in spite of the
close proximity of the two facies.

Devonshire. The delimitation of the Upper Devonian and
Lower Carboniferous rocks in Devonshire has not yet been worked
out. The junction-beds have not been specially described since



276 STRATIGRAPH1CAL GEOLOGY

Mr. Salter compared them with those of Pembrokeshire in 1863, 17
but his opinion was that the Pilton Beds included the equivalent
of the lower limestone shale. He says, " Nearer Barnstaple these
Pilton Beds begin to trough small patches of a barren softer slate
which is only seen well developed south of Pilton and occupying
the lower ground east and west of Barnstaple." In these soft slates
the prevalent fossils are Phillipsia seminifera, Spirifer bisulcatus,
8. laminosus, S. cuspidatus, Productus Martini, Orthis Michelini,
and other exclusively Carboniferous species.

Shales with a similar set of fossils occur at Fremington
west of Barnstaple, and are succeeded by the Coddon Hill Beds
consisting of shales and chert-beds ; the shales yield the Goiiiatites
characteristic of the lowest Pendleside Beds, i.e. Prolecanites com-
pressus, P. mixolobus, and Nomismoceras spirorbis, and the cherts
contain Radiolaria. They are overlain by black shales con-
taining Posidonomya Becheri, and black limestones with the same
fossil. 18

The combined thickness of all these beds is not very great,
apparently not more than 400 to 500 feet, unless a portion of the
Pilton Beds is included, and it is impossible to regard them as
representing the whole Avonian succession. From the fossils above
quoted it looks as if the base and the upper portion of the normal
sequence were present, and it is possible that the central beds are
cut out by faulting, for the boundary of the Pilton Beds at and
east of South Molton is believed to be a fault.

In South Devon and Cornwall nothing comparable with the
Pilton or with the Fremington Beds has yet been found. The
lowest Carboniferous Beds are either unconformable to or faulted
against the highest Devonian rocks ; but the cherts and limestones
of the Coddon Hill Group are well developed and appear to be
rather thicker than in North Devon. West of Dartmoor they are
associated with lava- flows, and the general succession according to
Mr. Reid is as follows : 19

Feet.
Upper lava of Brent Tor, etc. .... about 150

Beds of Radiolarian chert . . . ,, 70

Black shales and lenticular limestones . . ,, 250
Lower lavas .......,, 50

Hard black shale and chert-beds . . . ,, 150

I have elsewhere 20 suggested that the break which here seems to
exist between the Devonian and Carboniferous Systems may be a
result of the volcanic action indicated by the presence of the lavas
and by the intrusive mass of Brent Tor, which is supposed to have
been a volcanic vent. The sea-floor may have been slowly raised



THE CARBONIFEROUS SYSTEM



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278 STRATIGRAPHICAL GEOLOGY

or bulged upward by the rising mass of lava till several submarine
vents were established, and during such a movement little deposi-
tion would take place. The probability of this view is increased
by a comparison of the Cornish sequence with that of Brittany,
where a thick volcanic series is overlain by shales that are probably
of Visean and possibly of Yoredale age.



C. CONTINENTAL REPRESENTATIVES

1. France and Belgium

Two very different facies of the Avonian Series are found in the
north of France ; quite as different as those of Gloucestershire and
Devonshire in England, the one being essentially calcareous and
the other argillaceous. The former is only found in the north-east
of France and in Belgium, the latter occurring in Brittany, Sarthe,
and the Loire valley, and round the borders of the Central Plateau.
It will be desirable to take the former first since the succession is
complete and comparable with that of the Bristol area.

Belgium and N.E. France. The Carboniferous rocks of this
district lie in a set of deep and narrow troughs which succeed one
another along a line extending from the Boulonnais on the west by
Bethune, Tournai, Mons, Namur, and Liege. French and Belgian
geologists use the term Dinantian for this part of the Carboniferous
System, but as defined by them its limits are not quite coextensive
with the Avonian either at the bottom or the top. They also
divide it into two stages, the Tournaisian and the Visean, names
taken from the towns of Tournai and Vise. The most complete
section of the series is that exposed in the valley of the Meuse by
Hastiere, Waulsort, Dinant, and Yvoir, and the succession is repeated
in the basin of Namur to the north, at the junction of the Meuse
and the Sambre. Vise lies much farther down the valley of the
Meuse to the north-east of Liege, and the whole of the Vise limestone
has been found to belong to the highest Avonian zone, that of
Dibunophyllum, and to be similar in character to that of Derbyshire ;
moreover, it is said to rest unconformably on the Devonian, so that
it is not a complete development of the so-called Visean stage.

The most recent descriptions of the Belgian Series are those by
Professor de Dordolot, Dr. G. Delepine, and Dr. Vaughan. The
following is a brief summary of the faunal sequence as determined
by Dr. Vaughan and Dr. Delepine in a joint visit to the Belgian
area. 21 The normal succession is :



THE CARBONIFEROUS SYSTEM



279



Visean,

about

1200 feet



Tournaisian,

about

600 feet



Dark shales with, locally, a Pendleside fauna.
Limestone with Productus longispinus and, locally, \ _

Dibunophylluwi and Lonsdaleia J

Pseudo-breccia with Productus undiferus =

Limestone with Lithostrotion and Seminula ficoides =
Shallow - water limestone with Productus 6 and ) _

Caninia bristolensis j

White oolite with Productus sublcevis and Gyatho- \ _

phyllum <(> j ~

Limestones of Paire and Yvoir with Spirifer \_

Konincki, Caninia patula, and Caninia cylindrica j =
Limestones and shales ("Gale- schistes") withS

Zaphrentis and Spirifer clathratus ( = S. tornacensis) I =
Shales with Spiriferina peracuta and Zaphrentis j

Vaughani J

Gale-grits and shales with JSumetria, Camarotcechia, \ _

and Ostracods ; the Upper Famennian of Belgian > =

authors



= Si



West of the Meuse in the Dinant area C l and C 2 take on a
" knoll " character and are termed Waulsortian ; the rocks are
massive mottled limestones with abundant Fenestellids. In these
beds characteristic fossils are : Spirifer princeps, S. pinguis, Productus
plicatilis, and Amplexus coralloides. East of the Meuse in the Dinant
area there is no Waulsortian phase ; the black marble of Dinant
("Marbre noir") belongs, probably, to C 2 . In the Namur basin
a large part of the. series (Z to C 2 ) is dolomiitised ("Grande
Dolomie "). At Vise only Dibunophyllum Beds occur, and these are
of the upper knoll type.

This calcareous facies of the Dinantian can be traced eastward
by Liege to Limbourg (Aix la Chapelle), but when it reaches the
Rhine the limestones are partly replaced by shales, and what we
know as the Pendleside fauna extends downward through a greater
thickness of beds. The succession near Dusseldorf is :



Shales with Posidonomya Becheri .

Limestones with a Visean fauna ....

Shales with Spirifer clathratus ( = S. tornacensis) .



Feet.

1000

300

1500



Brittany and Basse Loire. The Avonian Series of Brittany
commences with a volcanic group which is believed to have a thick-
ness of more than 1500 feet in the west, consisting of conglomerates
and felspathic tuffs with thick masses of andesite ; but these beds
only occur on the northern side of the basins of Chateaulin and
Laval. The conglomerates rest unconformably on older rocks from
Devonian to Cambrian, and as they contain no marine fossils their
zonal age is unknown.

Above them in the Chateaulin basin is a great thickness of
unfossiliferous black shales with layers of felspathic sandstone,



280 STRATIGRAPHICAL GEOLOGY

probably 3000 feet thick, and a few fossils (Phillipsia and Productus}
nave been found in lenticles of limestone near the top. In the
basin of Laval there is a more massive black limestone containing
Productus giganteus and other Visean species, overlain by shales,
coal-seams, and sandstones with 'plant remains (Bornia transitionis,
etc.), and in the centre of the basin is a band of red and green
limestone.

Only the plant -bearing beds pass into the basin of Ancenis and
the Basse Loire district, but these are more than 3000 feet thick,
consisting of shales and sandstones with many beds of anthracitic
coal. The plants are those mentioned on p. 245.

From the above account it will be seen that the Tournaisian
does not appear to be represented, except perhaps by conglomerates
and lavas. The greater part of the series belongs to the Visean, and
the place of the Yoredale Beds is taken by a group of lagunic and
estuarine deposits.

2. Germany

East of the Rhine in Germany a very different facies prevails,
which has long been known as that of the Culm. This consists of
a thick series of shales and sandstones ; limestones only occurring as
lenticular beds or as layers of calcareous nodules. The series is
generally divided into a lower and an upper group, the lower
being essentially marine, while the higher seldom contains anything
but plant remains. In Westphalia, Hesse, and Nassau the Lower
Culm has at the base chert-beds and siliceous limestones about 200
feet thick, containing Prolecanites compressus, Pronorites cyclolobus^
and Orthoceras striolatum ; the overlying beds are dark thin-bedded
limestones containing Glyphioceras sphcericum and G. crenistria,
with Productus giganteus and Ghonetes papillionaceus. Above this
are shales abounding in Posidonomya Becheri, and the total thick-
ness is about 1400 feet. The Upper Culm consists of coarser sand-
stones, often pebbly, and contains the characteristic plants Knorria
imbricata, Bornia transitionis, Lepidodendron veltheimianum.

This Culm facies is found also in the Harz Mountains, Tlmringia,
the Fichtelgebirge, and over a large area in Saxony. It likewise
occupies parts of Silesia and Moravia, where it is of great thickness
and has a somewhat different facies. According to Stur, the
Silesian succession is as follows :

Shales with plant remains.

Sandstones and shales with Posidonomya Becheri, Glyphioceras sphoeri-

cum, etc. , and other beds with plants.
Sandstones, shales, and conglomerates with both marine and terrestrial

fossils.



THE CARBONIFEROUS SYSTEM 281

Carboniferous deposits of the Culm type are also found in the
south-west of Germany, in the Black Forest and in the Vosges
district, with which the tracts in Central France were probably
continuous.



3. South of France, Pyrenees, and Spain

When we pass into Southern France we enter a region where
still another facies prevails. As a type of this the succession found
in the Montagne Noire, near Corbieres, may be taken. As described
by M. Bergeron this is :

Grey limestone with Productus giganteus.
Sandstones with plants (Lepidodendron veltheimianum).
Shales with Spirifer tornacensis and Posidonomya Becheri.
Radiolarian chert-beds with phosphatic nodules (Glyphioceras diadema),
succeeding Upper Devonian.

A similar succession is found at intervals all along the chain of
the Pyrenees, with the addition at the base of compact limestones
(the " marbres griottes "), variegated with red and brown, which
contain Glyphioceras crenistria, Pronorites cyclolobus, and Prolecanites
Henslowi. Still farther west in Asturia Professor Barrois has
described the following sequence : 22

Feet.
Shales and limestones of Lena, with Amplexus coralloides, Lons-

daleia floriformis, and Fusulina ...... 240

The Canon limestones with few fossils, Poteriocrinus crassus,

Prod, aculeatus, Spirifer striatus ...... 700

''Marbres griottes" with the Goniatites above mentioned . . 60

It is curious to find limestones with what we regard as
Pendleside species of Goniatitidce here conformably succeeding the
Upper Devonian ; but there is 110 doubt about the sequence, and
we may regard these forms as southern species which did not find
conditions suitable to them in the north-west of Europe until
a late date in Dinantian time.



4. Russia

In this region the Lower Carboniferous Series is a combination
of the Culm and the Dinantian limestone facies. The beds lie for
the most part horizontally and are exposed in three large areas,
that of Donetz in the south, the Moscow basin in the centre, and
on the slopes of the Ural Mountains to the east. Ignoring local
variations the general sequence in the Ural and Moscow areas is :



282 STKATIGRAPHICAL GEOLOGY

3. Limestones with Prod, giganteus.

2. Sandstones, shales, and coal-seams ; some of the beds yield Prod.

giganteus.
1. Sandstones and shales with lenticular limestones containing Prod.

mesolobus.

These beds are from 5000 to 6000 feet thick and appear to
represent both the Tournaisian and Visean stages. In the Donetz
area, according to Tschernyschew, nearly the whole series consists
of limestones.



D. CONDITIONS OF DEPOSITION

The geographical conditions which prevailed in the British area
during Avonian time are fully discussed in my Building of the
British Isles (3rd ed., 1911), but it will be useful here to extend
our view to those parts of the European region which have been
briefly mentioned in the preceding pages.

The complete absence of any trace of Carboniferous strata in
Norway, Sweden, Lapland, and Finland seems to indicate the
existence of land over a large part of Northern Europe during this
period. This inference is confirmed by the character of the deposits
found in Northumberland, Scotland, and the north-east of Ireland ;
these being mainly detrital deposits carried down by rivers and
such as would be accumulated in a large bay or gulf. Hence we
conclude that the Scandinavian land extended westward across the
North Sea and across the north of Scotland into the North Atlantic
region.

Again on the eastern side we find a broad band of Carboniferous
strata in Russia extending from the Valdai Hills, south-east of St.
Petersburg, northwards by the south end of Lake Onega. In this
band the lowest beds are terrigenous deposits similar to those
of Scotland, and here again, therefore, we are on the borders of
continental land.

With regard to the southern border of the Scandinavian land
there is less certainty, because all evidences of it are buried and
concealed beneath the Neozoic deposits of the Germanic region.
Probably, however, the coast-line passed across the southern part
of the North Sea and across Denmark into the Baltic basin, whence
it must have curved north-eastward through the Baltic provinces of
Russia. Here it may be pointed out that the absence of Avonian
deposits in Poland is no proof that this country was land at this
time. The Carboniferous deposits may have been largely removed
from that area before Permian time, and seeing the great thickness
which they have in Silesia and as far east as Cracow, it would be



THE CARBONIFEROUS SYSTEM 283

surprising if they had not extended for some distance to the north
of these places.

Passing now to the Atlantic region there is reason to think that
a large part of this was also occupied by land, and that a part of
its coast-line passed through the extreme north-west of Ireland.
Again, the'great thickness of shale in the south-west of Ireland and
the north-west of France indicates the neighbourhood of land and
the debouchures of large rivers which carried much sediment
derived from such land. We may therefore picture a continent
which almost encircled the British area, lying not only to the
north but also to the west and south-west of it; and indeed it
seems probable that a promontory of this land actually occupied
parts of Cornwall, Brittany, and the intervening channel area
during the Tournaisian epoch.

Another tract of land which was probably connected with the
Atlantic continent seems to have stretched across Central France,
for in the basin of the Loire the Dinantian ( = Avonian) deposits
are entirely of terrigenous origin, and are probably all of Visean
and Namurian age, while to the southward in La Vendee even
these are absent, and small tracts of Westphalian Coal-measures rest
directly on Archaean rocks. We have seen also that when Dinantian
Beds come in again to the southward in Asturia, the Pyrenees, and
the Montagne Noire they are of quite a different facies, hence the
land above indicated seems to have separated a southern from a
northern Carboniferous Sea.

From Central France this land may have extended across the



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