4. The FAUNAL SUCCESSION in the CARBONIFEROUS LIMESTONE (UPPER
AVONIAN) of the MIDLAND AREA (NORTH DERBYSHIRE and NORTH
STAFFORDSHIRE). By THOMAS FRANKLIN SIBLY, B.Sc., F.G.S.
(Read December 4th, 1907.)

[PLATE I-FOSSILS.]

CONTENTS.

I. Introduction

II. The Typical Sequence of the Midland Area

III. The Faunal Succession

(1) Dibunophyllum 0-Subzone.

(2) Lonsdalia-Subzone.

Variation of the Faunal Facies of the Lonsdalia-

Subzone.

(3) Cyathaxonia-Subzone.

IV. Description of certain Sections of the Lonsdalia-Subzone:

Variation of the Lithological Facies

V. Description of certain Sections of the Cyathaxonia-Subzone:

Relation of the Cyathaxonia-Subzone to the Pendleside

Series

VI. Local Unconformity between the Carboniferous Limestone

and the Pendleside Series

VII. Comparison of the Faunal Succession in the Midland Area

with that in other Areas

VIII. Summary of Conclusions.....

IX. Description of certain Corals and Brachiopods from the

Midland Area

I. INTRODUCTION.

THE area of Avonian rocks with which this paper deals, and which
I term the Midland Area, includes the large, irregularly-shaped
periclinal mass of Carboniferous Limestone, forming the southern
termination of the Pennine anticline, south of the Peak, and com-
prises also a few small inliers adjacent to this main outcrop. (See
map, fig. 1, p. 35.) The main part of this area is included in
Derbyshire, but a small, south-western portion lies in Staffordshire.
In the Geological-Survey Memoir on North Derbyshire, the
Carboniferous-Limestone succession of this area, as shown by the
extensive section between Buxton and Monsal Dale, is briefly
described1; details of a few other sections are given; and the
nature of the junction between the Carboniferous Limestone and
the overlying shales, as seen at various points, is discussed. An
account of the elementary tectonics of the area, and of the general
features of the Carboniferous Limestone, is contained in a paper by
Mr. H. H. Arnold-Bemrose, entitled 'A Sketch of the Geology of
the Lower Carboniferous Rocks of Derbyshire,' published in the

Geology of the Carboniferous Limestone, &c., of North Derbyshire'

Mem. Geol. Surv. 2nd ed. (1887) pp. 18-21 & pl. ii.

2 Ibid. pp. 21-24.

3 Ibid. pp. 26-33.

Proceedings of the Geologists' Association, vol. xvi (1899-1900) p. 165. Details of a few sections in the area are given in papers by Mr. Arnold-Bemrose and by Dr. Wheelton Hind, published in

the Quarterly Journal of the Geological Society and elsewhere: I refer to these in the course of the present paper.

The tectonic structure of this area has not yet been adequately investigated. However, the detailed mapping of the igneous rocks

associated with the Carboniferous Limestone, by Mr. ArnoldBemrose, whose results are embodied in a recent paper,' has laid the foundations for such an investigation; and, moreover, the structure of the south-eastern portion of the area is now being worked out by Mr. C. B. Wedd, of H.M. Geological Survey, in the course of the re-survey of the East Derbyshire coalfield.

No systematic attempt to zone the Carboniferous Limestone of the Midland area was made until Mr. C. B. Wedd commenced his examination of the Matlock district, an investigation which is now approaching completion. In the Summary of Progress of the Geological Survey' for 1904, pp. 8 & 9, Mr. Wedd gave a preliminary account of the faunal sequence in the Matlock district, and in the same publication for 1905, p. 14, he stated that

'The corals and brachiopods show that the bulk of the limestone of the district, at least down to the second toadstone, belongs to the Upper DibunophyllumZone (D) of Dr. Vaughan's classification.'

My own work in this area, carried out in 1906 & 1907, during the tenure of an 1851-Exhibition Science-Research Scholarship, has had for its object the investigation of the faunal succession in the Carboniferous Limestone throughout the area. The account here presented deals mainly, therefore, with the palæontological features of that formation; but a few important tectonic features which came under my notice are described, and I have also attempted a brief description of those striking lithological changes which take place, between different parts of the area, in certain portions of the sequence.

The base of the Carboniferous Limestone is not visible in the Midland area, and the whole of the series exposed constitutes a greatly-expanded development of the uppermost zone of the typical Avonian succession of the South-Western Province, namely, the Dibunophyllum-Zone. In this Midland development three subzones can be distinguished: these are as follows, in descending order :

D3 = Subzone of Cyathaxonia rushiana.
Subzone of Lonsdalia floriformis.
Subzone of Dibunophyllum 0.

D2

D1

=

=

The scheme of the present paper is as follows. The section along the line of the Midland Railway, between Longstone and Buxton, being the most extensive in the area, is selected as typical, and is described in some detail. The general faunal succession in the Midland area is then described, and the fauna of each subzone analysed in detail. In this connexion, the marked variation of faunal facies exhibited by the Lonsdalia-subzone is discussed. Certain important sections of the Lonsdalia-subzone are next

1

2

The Toadstones of Derbyshire: their Field-Relations & Petrography Quart. Journ. Geol. Soc. vol. Ixiii (1907) p. 241.

Throughout this paper attention is confined entirely to the corals and brachiopods, in interpreting the faunal succession. These two groups alone are sufficiently abundant to be reliable in zoning.

described briefly, and the changes of lithological facies in that subzone discussed. The Cyathaxonia-subzone is then traced, in its varying development, throughout certain parts of the area, and the relation of this highest subdivision of the Avonian to the overlying Pendleside Shales is exemplified. A local unconformity between the Carboniferous Limestone and the Pendleside Shales is described in a separate section. After a comparison of the succession in the Midlands with that in other areas, and a summary of conclusions, the paper concludes with the description of certain corals and brachiopods.

II. THE TYPICAL SEQUENCE OF THE MIDLAND AREA.

Section along the course of the Midland Railway between Longstone and Buxton.

This is by far the most extensive sequence exposed in the area. The cuttings on the railway, together with numerous quarries and other exposures adjoining the line, afford a nearly-continuous section, which extends over more than 8 miles, in a roughly east-and-west direction, through the dales. The succession is described and illustrated in the Geological-Survey Memoir,' pp. 18-21, fig. 2 & pl. ii. At the eastern end of the section a cutting west of Longstone Station shows the uppermost beds of the Carboniferous Limestone, dipping eastwards to pass conformably under the Pendleside Shales. Westwards from this point, as far as Pig-Tor Tunnel, a distance of about 6 miles, successively lower parts of the Carboniferous Limestone are seen. Throughout this distance the beds have a very gentle easterly dip, interrupted here and there by slight undulations. The base of the Carboniferous Limestone is not seen, for at Pig-Tor Tunnel the anticlinal axis is reached, and the westerly dip which there sets in persists to the end of the section. At the western end, on the railway near Buxton, a considerable thickness of the upper beds of the Carboniferous Limestone has been faulted out, and the junction of the lower beds with the Pendleside Shales is obscured.

In a section so extensive as this one, and with slightly-inclined and undulating beds, estimations of vertical thickness are necessarily open to considerable error. In the accompanying table (p. 38) my own reading of the sequence is given, together with that of the Geological-Survey Memoir. The difference between the two estimates of total thickness is comparatively small, and lies within the probable error of determination. With reference to these two estimates, we may conclude that the total thickness of Carboniferous Limestone exposed in this section, exclusive of the intercalated toadstones, approximates to 1500 feet.

In one or two important points, my own reading of the section differs from that which is set forth in the Geological-Survey Memoir. Geology of the Carboniferous Limestone, &c., of North Derbyshire' 2nd ed. 1887.

THE TYPICAL CARBONIFEROUS-LIMESTONE SEQUENCE OF THE MIDLAND AREA.

Section along the course of the Midland Railway between Longstone and Buxton.

Reading of the same section given in the Geological-
Survey Memoir.1

1 'The Carboniferous Limestone, &c., of North Derbyshire' 2nd ed. (1887) p. 18.