The Barr and Lower Ardmillan Series (Caradoc) of the Girvan District, South-West Ayrshire, with Descriptions of the Brachiopoda

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THE BARR AND LOWER ARDMILLAN SERIES (CARADOC) OF THE GIRVAN DISTRICT, SOUTH-WEST AYRSHIRE, WITH DESCRIPTIONS OF THE BRACHIOPODA. 1

CONTENTS Page Page I. INTRODUCTION 6 PART 2--PALAEONTOLOGY VIII. CARAOOC BRACHIOPODA 69 PART I--STRATIGRAPHY (a) Introduction 69 (b) Systematic description'of the Inarticulata 79 II. Tim SUCCESSION 6 Obolacea 79 Trimerellacea . 86 III. THE BARR AND LOWER ARDMILLAN SERIES 8 Acrotretacea 90 (a) The Lower Barr Series 8 Siphonotretacea 92 (i) The Stinchar Valley sector 8 Discinacea 94 (ii) The Brockloch-Dupin sector 13 Craniacea . 96 (iii) The Shalloch Hill sector 15 (c) Systematic description of the Articulata 97 (iv) The Tormitchell-Pinmacher sec- Orthacea 97 tor 17 Dalmanellacea. 132 (v) The Aldons ant'icline'sector 18 Clitambonitacea 150 (vi) The Craigneil sector. 21 Triplesiacea 151 Plectambonitacea 157 (b) The Upper Barr-Lower Ardmillan Series 23 ? Strophomenacea 194 (i) The Benan Conglomerate . 23 Strophomenacea 197 (ii) The Balclatchie Group 27 Orthotetacea 220 (iii) The Ardwell Group . 33 Syntrophiacea . 224 Rhynchonellacea 233 IV. THE TAPPINS GROUP . 41 Atrypacea 242 (a) The Traboyack division 41 Spiriferacea 250 (b) The Dalreoch division 45 Athyracea 254 (c) The Albany division . 45 IX. FossIL LOCALITIES 255 X. FAUNAL LISTS 256 V. THE CRAIGHEAD INLIER 48 X-[. ACKNOWLEDGMENTS. . 261 VI. CORRELATION 57 XII. INDEX OF BRACHIOPOD GENERA AND SPECIES FIGURED AND DESCRIBED 262 VII. CONDrrroNs OF DEPOSITION. 62 XIII. LIST OF REFERENCES . 266

SUMMARY A revision of the stratigraphy and the brachiopod faunas of the Caradoc rocks exposed in the Girvan district of south- west Ayrshire suggests that the current classification of the successions, both north and south of the Stinchar Valley, is misleading because it is based on the fallacy that the Caradoc comprises a small number of rock types which are repeated by isoclinal folding. This type of folding, however, is rare, and the recurrent impersistent conglomerates and limestones, for example, do not exclusively represent anticlinal cores but mainly rapid facies changes. The succession accordingly is very much thicker than was previously believed and as amended includes four additional conglomeratic members and one extra limestone horizon that have hitherto been mapped as the Benan or Balclatchie Conglomerate and the Stinchar Limestone respectively. It is also involved in a strong unconformity northwards from the Stinchar Valley that has eliminated all but the onshore correlatives of late Ardwell age in the Craighead area. Members of the Barr Series which are associated with the unconformity do not thin out gradually northwards but disappear rapidly along very steep gradients in certain parts of the district. This suggests that deposition took place in basins defined by step-faults and retained by continual slip below wave-base to accommodate the greywackes which make up much of the succession. The Tappins Group, south of the Stinchar Valley, consists of three thick sedimentary belts, one of which, the Albany division, is known from faunal evidence to be equivalent to the Stinchar Limestone and Benan Conglomerate. Another, the Traboyack division, is known to be mainly older than the Kirkland Conglomerate. It includes cherts and spilites that are probably Llandeilo in age. The third, the Dalreoch division, is possibly a lateral equivalent of the Albany facies. Rich shelly faunas have been collected from most of the succession up to, and including, the lower Ardwell Group and a systematic revision of the brachiopods has entailed the description of 180 species (27 of them new) and 11 new sub- species, belonging to 99 genera (5 of them new). The study provides a detailed correlation not only between various members of the Barr and Lower Ardmillan Series within the district but also between those Scottish rocks and the standard Ordovician sections of Alabama, Tennessee and West Virginia. 1 Read at the meeting of the Society held on 11 May 1960. The discussion is published in Proc. Geol. Soc. 1580,1960, 103-7. Downloaded from http://mem.lyellcollection.org/ by guest on October 2, 2021

I. INTRODUCTION The early history of geological research into the stratigraphy and the faunas of the Ordovician rocks exposed in the Girvan area is so admirably reviewed in the publications of Lapworth (1882) and Peach &Horne (1899) that it would be presumptuous to reiterate the sequence of inquiry which led to their classic interpretations of the area. Indeed, the contributions of these three geologists are so fundamental that it is more relevant to touch upon inconsistencies which arose in the course of their work and which, because some of them have become part of geological tradition, have necessitated the present revision. Yet the issues involved in tracing the origin of contradictions which exist today are complicated by the fact that since 1882 the fossil faunas have received far more attention than the stratigraphy; and monograph by monograph palaeontological opinion has diverged from stratigraphical interpretation to a state of irreconcilability. In such circumstances it would be confusing to comment on more recent publications in strict historical sequence. Instead it seems more appropriate to deal with their implications in two distinct sections, one as a preface to the proposed stratigraphical revision and the other as introductory to the systematic description of the Caradoe brachiopods.

PART 1--STRATIGRAPHY II. THE SUCCESSION The two outstanding achievements of Lapworth's work were the demonstrations, first, that previous stratigraphical arrangements included tectonicaUy duplicated and even triplicated parts of a general succession which, contrary to general belief, could be correlated with those of structurally less complicated areas; and secondly, that this assorted succession, estimated by him to be almost 3000 ft. thick, was equivalent to about 100 ft. of graptolitic shales exposed in the Central Belt of the Southern Uplands. These relationships are well summarized by Peach & Home (1899, pp. 82-3), who accepted Lapworth's succession with no more than minor revisions of formational thicknesses and the transference of the Balclatchie Mudstones, Grits and Conglomerates from the top of the Barr Series to the base of the Ardmillan Series. In effect, all the difficulties experienced by earlier geologists were shown to be due to an inadequate appreciation of the importance of fossil faunas and facies changes, not only in determining successions but also in resolving the structure of the region. Yet, paradoxically, Lapworth paid scant attention to either faunal or lithological changes when preparing his interpretation of the Caradoc sequences, mainly because he believed the Benan Conglomerate to be a reliable datum horizon (1882, pp. 551,659). By using the formation in this capacity he showed that three belts of limestones, exposed around Benan Hill, were not impersistent lenses within a thick conglomerate but simply one limestone member underlying the Benan Conglomerate and brought to the surface by folding. With regard to this particular section he was correct, but unfortunately he then concluded that all Caradoc limestones were of the same age. In fact, there are three distinct members, namely the Auchensoul, Stinchar and Craighead limestones, in ascending order. The first was noted by Lapworth (p. 561) to be differently coloured from the Stinchar Limestone but he attributed this difference to faulting and ignored its occurrence below and not above the Valcourea confinis Flags. The last is isolated from the main exposures of the Stinchar Limestone, and although there were some stratigraphical reasons for comparing the entire sequence at Craighead with the Lower Barr Series, the lithological likenesses are not so close as he maintained and the faunas are quite different. This error in correlation, which was repeated by Peach &Horne and not even queried until 1930 (Ulrich, p. 84), had serious repercussions in all later palaeontological studies. Lapworth's failure to recognize the individuality of impersistent members of similar lithology is found also in his arrangement of the non-calcareous Caradoc sequences, but in respect of these sediments his attitude is more understandable for, according to his experience in the Southern Uplands, lenticular out-