Lower Carboniferous Rocks Between the Curlew and Ox Mountains, Northwestern Ireland

Lower Carboniferous rocks between the Curlew and Ox Mountains, Northwestern Ireland

OWEN ARNOLD DIXON

CONTENTS i Introduction 7 I 2 Stratal succession 73 (A) General sequence 73 (B) Moy-Boyle Sandstones 73 (c) Dargan Limestone 74 (D) Oakport Limestone 75 (F.) Lisgorman Shale Group 76 (F) Bricklieve Limestone 78 (o) Roscunnish Shale 84 (H) Namurian rocks 84 3 Zonal stratigraphy . 85 (a) Fauna . . 85 (B) Zonal correlation 88 4 History of sedimentation 9o 5 Regional correlation. 95 6 References 98

SUMMARY Rocks in the Ballymote area, occupying one of sedimentary environments of a shallow shelf several broad downwarps of inherited cale- sea. The main episodes (some repeated) include donoid trend, provide a crucial link between the deposition of locally-derived conglomerates Vis6an successions north of the Highland and sandstones in a partly enclosed basin; the Boundary line (represented locally by the Ox accumulation of various thick, clear-water Mountains) and successions to the south, part limestones, partly in continuation with ad- of the extensive 'shelf' limestone of central jacent basins; and the influx of muddy detrital Ireland. The sequence, exceeding xo7o metres sediments from a more distant source. (35oo it) in thickness, ranges in age from early The rocks contain a succession of rich and to latest Vis~an (C~S1 to/2) and is succeeded, diverse benthonic faunas, predominantly of generally without interruption, by thick upper corals and brachiopods, but near the top these Carboniferous shales. The succession of differ- give way to several distinctive goniatite- ent rock types reflects changing controls in the lamellibranch faunas.

i. Introduction

THE LOWER CARBONIFEROUS rocks of the Ballymote map area underlie a shallow physiographic trough extending east-northeast from Swinford, Co. Mayo, to the upper Carboniferous shale highlands of Co. Leitrim. They are bounded to the north by metamorphic rocks of the Ox Mountains (Fig. I), and to the south by older Paleozoic rocks of the Curlew Mountains pericline (Charlesworth I96oA and 196oB).

Jlgeol. Soc. vol. x28, x972, pp. 7x-xoi, I2 figs. Printed in Northern Ireland.

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The area is generally of low relief and much of the bedrock is obscured by glacial deposits--conspicuous drumlins in the east, and moraine or fluvioglacial gravels in the west. Extensive tracts of bog and alluvial deposits effectively mask the underlying geology in the western and southwestern parts of the area, and around Templehouse and Cloonacleigha Loughs. Within the expanse of low- lying country are several prominent bedrock hills or scarps, notably in areas north and east of Tobercurry, northeast of Ballymote, in the Bricklieve Mountains and Highwood areas, north of Geevagh, and south of Manorhamilton. Griffiths (i836, et seq.) published some of the first accounts of Irish Carbon- iferous stratigraphy, and in his maps depicted the rocks of the Ballymote area in the regional setting of Irish geology. Other authors (Wynne I864, pl. 5; Hull I878, P- 34) used essentially the same subdivisions, and subsequently the Geo- logical Survey of Ireland adopted a slightly modified stratigraphic classification (Cruise i878; Symes & Kilroe I88o; Symes, Wilkinson & Kilroe I88I; Kilroe I885). Recent work in northwestern Ireland has augmented and partly modified this classification, as the faunal succession made possible more detailed and accurate subdivision and correlation. The area termed the 'Ballymote Syncline' (George & Oswald I957, fig. i; Caldwell x959, fig. I) is actually a composite downfold, involving two broad, essentially parallel synclines which plunge toward the east-northeast (Figs. xo and I~). It is one of a group of similar downfolds of caledonoid trend in northwest and north-central Ireland. The area is bisected by a major NE-SW wrench fault, the Killavil Fault, on which a sinistral displacement of as much as I6 kilometres is suggested by the local stratigraphy. Other faults include the Curlew Fault (along the northern margin of the Curlew pericline) with a large apparent downthrow to the north, and many small N-S to NW-SE faults, most with small

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normal displacements. These, and other structural features, are in accordance with the regional structural pattern in Carboniferous rocks in northwest Ireland (George and Oswald I957, p. I4o , pl. I6; Simpson I955, pp. 4o2-5, pl. I8; Caldwell i959, pp. i82-3, pl. 6; Dewey I963, fig. I). Credit for the geologic interpretation of the Bricklieve Mountains area is due to Caldwell and Charlesworth (i962) and for their detailed account of the limestone sequence and coral faunas there.

2. Stratal succession

(A) GENERAL SEQUENCE The lower Carboniferous rock sequence in the Ballymote area may be summarized conveniently as follows (see also Fig. I I): Zonal allocation Thickness (m) Macrofaunal Microfaunal Roscunnish Shale 12o P1-P2 ? Bricklieve Limestone 335 DI D1 Lisgorman Shale Group 275-520 $2-D1 $2 Oakport Limestone 170-27 ° $9 S/ Dargan Limestone 60 C2S1 S 1 Moy-Boyle Sandstones 3o-x 70 ?C2S1 ?C,~S1

The Moy and Boyle Sandstones rest everywhere with marked unconformity on rocks of older Palaeozoic or Precambrian age. The detrital sequence gives way upwards to a sequence of marine bioclastic, oolitic and micritic carbonates divided into a lower transitional formation, the Dargan Limestone, and an upper purely carbonate formation, the Oakport Limestone. The limestones are followed abruptly by a group of predominantly argillaceous rocks termed the Lisgorman Shale Group (the 'Calp' of many I9th century authors). The shale group grades upward into a sequence of bioclastic limestones, the Bricklieve Limestone, noted for an extreme abundance of chert, for an abundance of rich coral beds, and for the presence of many 'calcite mudstone reefs.' The limestones are succeeded abruptly by the Roscunnish Shale, a marine sequence of dark argillaceous limestone, shale, and sandstone which passes upward into marine and non-marine shales of upper Carboniferous age.

B) MOY-BOYLE SANDSTONES The basal detritalrocks everywhere show upward textural changes which may be related most reasonably to the gradual submergence of nearby land areas. In rock exposures between Coolaney and Cloonacool (along the headwaters of the River Moy), the Moy Sandstone contains alternating units of conglomerate (with mostly quartz pebbles), grey or green, coarse, pebbly subarkose and orthoquartzite (Folk 1968), and green or red finer orthoquartzite and siltstone.The coarser, immature sediments, predominant in the lower part of the formation, become subordinate to units of coarse-to-fine-grained,dolomitic, submature orthoquartzite higher in the formation. The allochems indicate a provenance in the

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metamorphic-granitic terrain of the adjacent Ox Mountains, in particular the rounded to angular pebbles and cobbles (up to 2o cm) of metaquartzite, pink and white vein quartz, and mica schist, and the chloritic or micaceous matrix of many of the rocks. Large scale foreset bedding corroborates this southeasterly sediment transport. From a maximum thickness of I4O metres in the west, the formation thins to between 30 and 6o metres east of Coolaney, along with a disappearance of the conglomeratic units. The sequence shows a transition from coarse, immature subarkose and orthoquartzite in the lower part, to fine, dolomitic, mature orthoquartzite and beds of impure, crinoidal biosparite in the top 9 to i2 metres. Thin lenses of quartz pebbles (up to 3 ° mm diameter) recur at intervals, and like the gross intercalations of pebble conglomerate and sandstone further west, may have been related to fluctuations in the supply of sediments. The Moy Sandstone east of Coolaney is regularly bedded, and although apparently lacking cross bedding, shows cross laminations within the sandstone beds. Local 'red-beds' occur near the base of the section in both areas--micaceous silt.stone and fine-grained, immature orthoquartzite with a dark, reddish-brown matrix rich in hematite. At the north end of Lough Key there are at least 17 ° metres of basal detrital rocks, the Boyle Sandstone (Caldwell 1959, pp. I65-7). The thickly bedded or massive lower beds consist of pale-grey, coarse, immature subarkose with scattered rounded pebbles of vein quartz, jasper, red and green shale, and greenish tuff (comparable to Old Red Sandstone and Lower Palaeozoic rock types in the Curlew Mountains inlier). The exposure is far from complete but the rocks appear to grade upwards into finer grained, flaggy, submature-to-mature orthoquartzite in the upper part of the formation. They become dolomitic near the top and grade within I-2 metres into the overlying bioclastic limestone. Large planar cross beds in the detrital rocks indicate local northerly sediment transport. A generally similar, but very poorly exposed sequence outcrops in the Carracastle area. Biosparite beds near the top of the Moy and Boyle Sandstones contain a fragmented and abraded benthonic invertebrate fauna of crinoids, brachiopods (chonetaceans, productaceans and spiriferids), and corals (zaphrentoids and tabulates, notably Michelinia sp.).

(C) DARGAN LIMESTONE The Dargan Limestone typically comprises three members--a lower dolomitic, bioclastic limestone, a middle oolitic limestone, and an upper argillaceous limestone--with a total thickness of about 6o metres. The lower limestone consists of unevenly bedded, dark-grey, sandy biosparite with thin shaly and micaceous partings. The bioclastic material is mainly a poorly sorted mixture of coarse-to- fine crinoidal debris in a sparry calcite or dolomite cement. Lenticles of quartz sand recur sporadically throughout the section. Other exposures of similar rocks occur above the Boyle Sandstone north of Lough Key and in the Carracastle area. The lower limestone, about 17 metres thick at Lough Dargan, thins eastward and

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westward and apparently passes laterally into the topmost Moy Sandstone in the west. The lower limestone contains the Moy Sandstone fauna augmented by caniniid corals, orthid and strophomenid brachiopods, euomphalid gastropods, fenestellid bryozoans, and calcareous algae. The larger fossils show ample evidence of attrition--rounded crinoid fragments, abraded coral epithecae, and much fragmented shelly material--although a few bedding planes are strewn with fairly well preserved articulated strophomenid shells and other fossils. The Dargan oolite, exposed only on the north flank of the downfold, attains a maximum thickness of I I metres in the east (Dromahair) and thins slightly to the west. The well-stratified and thickly bedded (locally cross-bedded) oosparite contains, in addition to the well-sorted and well-washed oolitic sediment, substantial amounts of fine crinoid and shell debris, and detrital quartz and feldspar grains. Various oolith nuclei (detrital sand grains, bioclastic material, calcareous algal material, oolith fragments) reflect the preponderance of different clastic sediments in the contemporary environments along the Ox anticline--mainly bioclastic sands east of Collooney, but mainly detrital sands to the west. The upper and lower boundaries of the oolite are usually abrupt. The upper limestones also display important facies changes. East of Collooney the oolite is overlain by as much as 37 metres of dark-grey to black, argillaceous rocksmirregularly bedded, commonly nodular, argillaceous biomicrite with black shale partings, and local black chert nodules. Crinoidal debris is ubiquitous, but coarse bioclastic material is much less common than in the underlying limestones. The rocks contain a distinctive and locally rich fauna of gastropods (including euomphalids, bellerophontids, straparollids, naticopsids, and many small loxonematids and turritellids) and pelecypods (notably Conocardium spp.). Sponge spicules form felted layers at several horizons. A few nautiloids, brachiopods, trilobites (Phillipsia sp.) and rare corals make up the remainder of this rather specialized marine fauna. The fossils tend to show little evidence of attrition, the delicate spines of brachiopods, the calices of crinoids, and some fragile, openly- coiled mollusc shells being preserved intact. Much of the larger crinoidal material is slightly disarticulated, and this is in keeping with the turbid appearance of the sediments in this section--possibly a product of organic burrowing. Farther west, the upper limestones contain much more broken and abraded fossil material, lenticles of detrital sand and rounded quartz pebbles, and a sparry calcite cement. All these imply conditions of considerably greater water agitation and reworking than in the areas further east where contemporary muddy sediments were accumulating. The fauna also differs in being dominantly of crinoids, corals (many large caniniids, zaphrentoids, and tabulates, notably Michelinia spp.), a few gastropods, and common foraminiferans in the higher beds. Lime- stones similar faunally and lithologically occur near the Curlew Fault southwest of Gorteen.

(D) OAKPORT LIMESTONE The Oakport Limestone (defined by Caldwell 1959, pp. I67-9), consisting of fairly pure, shallow water, carbonate rocks, attains a maximum thickness of nearly

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270 metres in the Ballintogher-Ballygawley area and thins westward to about 17 ° metres near Cloonacool. The sequence throughout displays alternations of several different well-bedded, regularly and often closely jointed limestones (individual lithologic units a few centimetres to one or two metres thick). Thickly bedded biosparite, made up largely of well-washed, strongly abraded, crinoidal debris, is common and grades locally into biosparite with a more varied content of abraded and rounded fossils. Splintery, grey-brown micrite is also common and frequently contains an abundance of calcareous algae. Certain gradational textures suggest that much of the dense micrite was derived from algal material through penecontemporaneous disintegration and/or subsequent diagenesis. Thin beds of oosparite recur at several levels, the majority of ooliths having nucleated about tiny fossils (commonly foraminiferans) or fossil fragments. In general, biosparite predominates in the upper Oakport, while micrite and oosparite are more common in the lower half. Well preserved fossils are rather sparse throughout the sequence, but clisiophylloid and syringoporoid corals, and large linoproductid brachiopods are fairly characteristic. Smooth athyridaceans form several distinctive shell beds at horizons between 15 and 23 metres above the base (compare the Composita Limestones--Caldwell 1959, p. 168). Other brachiopods and corals occur sporadically, along with euomphalid gastropods, fenestellid bryozoans, and calcareous algae (Koninckopora sp. and codiaceans). Foraminiferans and ostracods are abundant in many of the rocks. In thin sections, many of the rocks show features strongly suggestive of penecontemporaneous reworking by burrowing organisms. These include a general mixture of coarse bioclastic material of all sizes; a random orientation of large elongate particles in much of the biosparite; a random orientation in the micrite of ostracod carapaces and other small fossils (which otherwise should lie approximately parallel to the bedding); and commonly a lack of internal bedding lamination in the micrite. Local lenses of sand and small quartz pebbles, and a few thin argillaceous micrite and calcareous shale beds, occur in the lower Oakport Limestone in the western part of the syncline. They are evidence of a continued influx of detrital sediment from the contemporary Ox Mountains. Secondary dolomitization is particularly common in the coarse biosparite, and has led to the progressive destruction of primary sedimentary textures. Locally these rocks have been altered to brown, saccharoidal dolomite (notably in areas adjacent to the Dromahair Fault). Dark-grey, rounded nodules of chert occur very locally in the topmost beds of the Oakport Limestone.

(E) LISGORMAN SHALE GROUP The Lisgorman Shale Group is best exposed in a few stream sections between the former Lisgorman Station and O'Donnell's Rock, where the section incorporates a lower shale, about 185 metres thick, a middle sandstone, about 60 metres thick, and an upper shale, about 275 metres thick. These correspond respectively to the Bundoran Shale, Mullaghmore Sandstone, and Benbulben Shale of the Sligo syncline (Oswald 1955, PP. 172-5). The sandstone thins to the southwest and

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disappears within a few miles of O'Donnell's Rock, and elsewhere in the map-area the two shale formations are not readily differentiated. The group as a whole thins southwestward to about 275 metres near Knocknashee. In the O'Donnell's Rock area, the Bundoran Shale consists of dark-grey, micaceous, calcareous shale grading into platy, argillaceous micrite, with subordinate interbeds of hard, pyritic micrite, argillaceous biomicrite, thin layers of calcareous siltstone, and a few dark, non-calcareous shales. Traced to the southwest, the shale becomes subordinate to micritic limestone. Chert nodules occur very rarely in the lowermost limestones. Fossiliferous beds are rather widely spaced and thick parts of the Bundoran Shale and equivalent beds contain little fossil material. Fossils most common are crinoidal fragments, brachiopods (large pustulose productaceans, spiriferids, orthids, and strophomenids), fenestellid bryozoans, and sponge spicules. Other brachiopods (spinous athyridaceans, chonetaceans, and terebratulids), trilobites (Gri~thides sp.), and conulariids are less common elements of the fauna, and corals are rare (except for some large caniniids, phaceloid lithostrotiontids and zaphrentoids which occur locally in the uppermost beds). Ostracods are abundant in much of the shale. Traces of burrowers are restricted mostly to the fossiliferous beds, and the majority of sparsely fossiliferous rocks show a lack of organic disturbance. Most have well-defined bedding laminations (locally cross laminations in the siltstone) and preferred orientations of the few elongate fossil particles parallel to the bedding. The Bundoran Shale is succeeded in the northeast by the Mullaghmore Sandstone, a sequence of interbedded orthoquartzite, siltstone, shale, and sandy micrite. In the lower 5 ° metres, orthoquartzite beds up to a metre in thickness alternate with comparable thicknesses of shale and siltstone; in the remaining upper part, beds of dark, impure micrite alternate with shale, and the coarser detrital rocks are much reduced. The grey, calcareous, submature orthoquartzite is well stratified (locally cross-bedded) and contains thin argillaceous laminations. Well-sorted, fine-to-medium-grained quartz sands are mixed with varied amounts of bioclastic material, and locally the elastic particles have thin oolithic skins. The rusty-brown, highly micaceous siltstone is commonly deeply weathered and friable. It forms, together with well-bedded, dark, calcareous shale, recessive beds between the resistant orthoquartzites. Dark micrite near the top of the sequence contains much comminuted bioclastic material and some sand, silt, and terrigenous mud as impurities. Fossils in the Mullaghmore beds include a few brachiopods, corals, foraminiferans, and fenestellid bryozoans in the micrite, and scattered ostracods and fenestellid bryozoans in the shale. Worm burrows, especially abundant in the siltstone and some of the other detrital rocks, probably account for much dis- ruption of internal layering in the rocks. In the O'Donnell's Rock area, the Mullaghmore beds change laterally from thick, clean, cross-bedded orthoquartzite (at the map margin to the northeast-- Fig. 12.) through calcareous orthoquartzite, siltstone, and shale, to dark shale and argillaceous micrite (within Io kilometres to the southwest). In the northeastern part of the area, the lower part of the Benbulben Shale

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/1/71/4884440/gsjgs.128.1.0071.pdf by guest on 03 October 2021 78 O. A. Dixon consists of dark-grey calcareous shale with subordinate dark, platy, argillaceous micrite, and a few thick biomicrite beds. The lithology changes gradually upward to medium-grey micrite, with only a few thin beds of dark, platy, argillaceous micrite and calcareous shale near the top. In addition there is a gradual reduction from northeast to southwest in the amount of shale in the upper half of the shale group. The Benbulben Shale is distinctive for an abundance and great variety of fossils at many horizons (although there are sparsely fossiliferous exposures more similar to much of the Bundoran Shale). The fauna typically includes many large caniniid and zaphrentoid corals, fragmentary phaceloid lithostrotiontids, strophomenids, spinous athyridaceans, spiriferids, and a few clisiophylloid corals, large pustulids and other productaceans, rhynchonellids, orthids, chonetaceans, and terebratulids. Some of the shale is packed with crinoidal debris (with locally preserved crinoid calices). Many beds contain a matted fabric of fenestellid, rhabdomesid, stenoporid, and acanthocladiid bryozoans. Ostracods are abundant in much of the shale, and foraminiferans in the micrite and shale in the upper part of the formation. Sponge spicules occur throughout and locally the rocks contain masses of felted spicules. Other less common fossils include small lamellibranchs (pectinids), euomphalid gastropods, trilobite pygidia (Phillipsia sp.), and rare goniatites. Worm casts are particularly common in much of the shale and in the shaly margins of the thicker micrite beds. The preservation of many of the fossils is excellent, and they show little evidence of attrition. The rarity of textures resulting from active water transport of sediments and the turbid appearance in thin section of many of the fossiliferous rocks strongly suggest both the absence of any significant sea bottom transportation and the post-depositional reworking of the sediments by benthonic organisms. The Lisgorman Shale Group-Bricklieve Limestone boundary is diachronous, which partly explains the marked thickening of the shale group in the northeast. At O'Donnell's Rock, shale-with-biomicrite in the top 3 ° metres of the shale group is packed with fragmentary Lithostrotion pauciradiale, and is correlated with the pauciradiale beds in the Bricklieve Limestone to the southwest (see Bricklieve Limestone below).

F) BRICKLIEVE LIMESTONE

The Bricklieve Limestone is the best exposed of all the rock formations, and in several parts of the downfold forms hills or mountains with steep or precipitous scarp faces, as in the Bricklieve Mountains, the Highwood area, or the Carrane Hill area. In these areas of excellent exposure the sequence can be subdivided into groups of beds (informally named Groups I to 6 as in Fig. 2) separated by characteristic lithostrotiontid-rich beds, or where the coral beds are impersistent, subdivided simply as parts of a vertically varying but horizontally persistent lithologic sequence. The general lithologic sequence, and variations in the different areas of exposure are illustrated in Fig. 2. Shaly material persists in minor amount in the basal beds, but disappears within a short distance upwards. Much of the Bricklieve

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succession consists of grey bioclastic limestone which varies from the lithostrotiontid beds, crammed with coral material, through coral- and brachiopod-rich biosparite and crinoidal biosparite, to fine-grained biosparite and biomicrite. The outstanding faunal character is the profusion of lithostrotiontids, which occur in far greater abundance and variety than any other group of corals in the sequence. Of particular importance are the concentrations of the phaceloid species (Lithostrotion pauciradiale and L. martini) in two groups of beds in the Lower Brick- lieve Limestone; the abundance of cerioid forms in the limestones between them; and the recurrence of beds packed with L. junceum in the Upper Bricklieve Lime- stone. Giganteid caniniids (mainly Caninia spp.) are especially abundant in the basal beds of the sequence, and recur sporadically above. The coral fauna also includes other caniniids, clisiophylloids, a few zaphrentoids, and tabulate corals. Brachiopods occur in abundance and diversity throughout, the productaceans and spiriferids being the dominant forms, although all the main groups are represented. Fenestellids and other bryozoans, calcareous algae, foraminiferans, and ostracods are all fairly common. The abrasion and breakage of many of the larger bioclastic components indicates a considerable degree of bottom turbulence, although sorting of the sediments tends to be poor, and there is normally a mixture of different types of fossils and fragment sizes. Depositional textures or alignment of particles is obscure and suggests post-depositional organic disturbance. Some of the crinoidal biosparite beds show better sorting and a few of them are oolitic. Beds of pale, grey-brown, splintery micrite are common but subordinate in amount.

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FIG. 3- Sketch map showing the locations of the detailed maps (Figs. 4, 5 and 6), the published Bricklieve Mountains map (CaldweU and Charlesworth I962, fig. 5) and the vertical cross sections (Fig. xo). The Bricklieve Limestone is stippled; pre- Carboniferous rocks are ruled obliquely.

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Chert in a great variety of forms (nodules, lenticles, beds, and ragged interconnecting masses) occurs in abundance throughout much of the succession, and culminates in Group 6 in massive, cliff-forming beds composed of as much as 70 per cent chert. It tends to be sparse in the crinoidal biosparite and micrite beds. Chert nodules of distinctive shapes occurring in successive groups of beds have proved reliable for local correlation when used in conjunction with the sequence of coral beds and the general lithologic sequence. Many of the corals (and to a lesser extent, the brachiopods) in the Bricklieve Limestone are preferentially but incompletely silicified. Details of the lithologic and faunal sequence (in particular the lithostrotiontid- rich beds) in the Bricklieve Mountains have been described by Caldwell & Charlesworth (I962). Of the sections elsewhere in the syncline, some are closely comparable (Figs. 4 & 5) differing only in details, while others differ quite significantly. The important variations in thickness and development of the lithostrotiontid beds are summarized in Fig. 2. The pauciradiale and martini beds are

.. -,,, GEOLOGICAL MAP J ...... , ' ~ ~-- HOUSE---- 0 900 METRES _ _------"~---'~ ~ ------of the HORIZONTALAND VERTICAL _ "_~_------_--z(--_ '--~__-3OO.--~. SCALE . ~--_--..l-__--__ ------~'~---~'~..~. ~~

,o woo0 .... COUNTY SLIGO . ~T_------Z_-- .... _ ._-~'------~Z~_ ~ ---.-,..,;;.~ l

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~roup UPPER BRICKLIEV1 '~Jgroup, LIMESTONE-" ~group r--_~_--_~Z-Z- ~E~ LOWERBRICKLIEVE AGH LIMESTONE(undivided) A drumtin

/ (KILMACTRANNY]

FIo. 4- Geological map of the Highwood area, Co. Sligo. Contours are in feet above mean sea level (after the Ordnance Survey, Dublin).

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persistent widely. The junceum beds are poorly defined outside the Bricklieve Mountains and are indistinguishable (apart from their position in the lithologic sequence) from other coral-rich beds developed in the Upper Bricklieve Limestone. Coarse crinoidal biosparite units show the most lateral variation of any of the Bricklieve rock types. In the Lower Bricklieve Limestone (Group ~) they are most important in the Tobercurry and Carrane Hill areas, where they are interbedded with the pauciradiale beds. Cross-bedded, crinoidal biosparite is conspicuous in Group 4 in the Bricklieve Mountains (Caldwell & Charlesworth I962 , pp. 367-9), but markedly reduced in the Carrane Hill and Carrickbanagher-Ballymote areas. Worthy of comment is a similarity in details of lithology and coral bed development between the Carrane Hill area and the Carrickbanagher-Ballymote area. The similarity is much closer than between the Bricklieve Mountains and the Carrickbanagher-Ballymote area which are much closer geographically. A qualitative comparison of the available exposures suggests that there may have

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:~"~ ~i ,,, : . z ' ~L/thostrotion ~ martini bedsLu~LI -r ~group 2 :> L./x~c/rod/o/e~ F-~-~doterlte p~' ~;-~-r----~24~ ~ beds ~1 • I dyke _ F----I'reef ' ~ ~nomuri, =L_.~ time.stone ~ ~ rocks ~gro~p I ~ I~ROSCU, ~ t~ SHALE

GEOLOGICAL MAP of the -~" ~ " ~-7~--- ..... ~~4.~ CARRANE HILL AREA ~11 ~:- • ~--C---o 3000 Feet " COUNTY SLIGO -7'![cLOONEEN ~--Z~. 6 ~O METREs t ~. ~ " . , ------,;" GEEVAGH

FXO. 5- Geological map of the Carrane Hill area, Co. Sligo.

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been a sinistral displacement on the Killavil Fault in the order of x6 kilometres. More detailed analysis of the Bricklieve Limestone facies may refine this estimate. The succession in the O'Donnell's Rock area (Figs. 2 and 6) is unique in its greatly reduced thickness, due in part to the diachronous rise of the top of the Lisgorman Shale Group, and in part to a possible disconformity at the top of the limestones. A conspicuous accumulation of L. pauciradiale in the top 30 metres of the Lisgorman Shale Group is taken to be the easternmost exposure of the pauciradiale beds. The sequence between the base of the Bricklieve Limestone and the martini beds contains a high proportion of crinoidal biosparite, much of which is altered to brown saccharoidal dolomite. The martini beds are well developed and are overlain by no more than 45 metres of biosparite and biomicrite (the Upper Bricklieve Limestone). As attitudes are conformable in the succeeding shales and sandstones, a non-sequence of beds or a disconformity may be held to account for the abbreviated succession. In several areas the Lower Bricklieve Limestone contains 'calcite mudstone reefs' in knoll-like forms, in coalescing lenticular forms, and in long continuous

FIo. 6. Geological map of the O'Donnell's Rock area, Co. Leitrim.

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sheets (Fig. 12). The central portions of the larger 'reefs' typically consist of massive, indistinctly bedded, pale-grey micrite, locally rich in fragmentary bryozoans, with pockets of small brachiopods and lenses of crinoidal material. Microscopic features compare closely with those described by Lees (i964, pp. 5o8-I8) in the Waulsortian reef limestones of west-central Ireland. In the smaller 'reefs' and at the margins or bases of larger ones, the rocks are typically thickly bedded, varying from light-grey intramicrite to very coarse, crinoidal biosparite. The distinctive bryozoan-brachiopod fauna includes abundant fenestellids (and a few rhabdomesids), many rhynchonellids, small productaceans, and spiriferids, and a few terebratulids, smooth athyridaceans, and plicochonetids. Lamellibranchs (pectinids and others), trilobite pygidia, foraminiferans, ostracods, and rare phaceloid lithostrotiontids are subordinate elements of the fossil fauna.

(G) ROSCUNNISH SHALE The most continuous (but far from complete) exposures of the Roscunnish Shale (defined by Caldwell x959, pp. 178-8o) are to be found in two streams just over two kilometres south of Carrane Hill. The sequence of rock types is outlined in Fig. 7. The pale micritic rocks at the top of the Bricklieve Limestone give way abruptly to dark, sparsely fossiliferous, argillaceous rocks. The fauna includes a few zaphrentoid and tabulate corals, spiriferid, productacean, and chonetacean brachiopods, scattered crinoidal material, and rare goniatite fragments. Muddy calcareous sediments with a varied goniatite-lamellibranch fauna succeed the nearly-barren basal beds, and the sequence is completed by non-calcareous, ferruginous shales closely comparable to the overlying Namurian shales. About 3 metres of fine-grained, cross-bedded, grey orthoquartzite and micaceous siltstone are exposed on the southeast slope of the O'Donnell's Rock escarpment, between 15 and 3 ° metres above the Bricklieve Limestone. Coarser orthoquartzite on the opposite side of the Killavil Fault may be of the same age although its relationship to other rocks in the vicinity is rendered uncertain by local faulting. Thinly bedded siltstone and impure, fine orthoquartzite is exposed in the Roscunnish Shale about I. 5 kilometres south of Belhavel Lough. How- ever, no equivalent sandstone is exposed in the Carrane Hill area.

(H) NAMURIAN ROCKS The boundary between the Roscunnish Shale and overlying shales of Namurian age has only a faunal basis. The non-calcareous shale near the top of the former continues upward into the Namurian with little lithologic variation (apart from a few carbonaceous beds, and varied concentrations of clay-ironstone nodules) for nearly 15° metres to the base of a prominent sandstone escarpment at the summit of Carrane Hill. Shales and coal seams appear above the sandstone in the Geevagh Coal Field to the south-east. The shales are readily divisible stratigraphically using the contained succession of distinctive goniatite-lamellibranch faunas. Black, ferruginous shale occurs in an abandoned shaft in the Highwood area about 0. 7 kilometres north-northwest of Kilmactranny (Fig. 4). Some is highly

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carbonaceous, but all is apparently unfossiliferous. Farther north, near the centre of the Highwood graben, other non-calcareous shales contain a Namurian goniatite fauna.

3. Zonal stratigraphy

A) FAUNA Throughout most of the succession, corals and brachiopods are the dominant elements of a fauna which includes representatives of many invertebrate phyla. The stratigraphic distribution of these two groups of invertebrates is shown in the

}hard, black, non-calcareous shale K- Eumorphoceras pseudocoronula KozakhoceraS sp Ela Posidomo corrugata AF~ROX }hard, black, non-caLcareous shale P2 30 METRES ','

[dark, calcareous shale & r ~ platy, argillaceous micrite ~ }calcareous shale & platy ~ argillaceous micrite; minor ] thicker, pyritic micrites ::j J -Canayella membranacea horizontali$ hard, black, ferruginous shale

dark, platy, argillaceous micrfte dark, platy, argillaceous micrite & minor thicker, pyritic micrites H-Sudeheeras sp.

G - Goniatites granosus P2 dark, caLcareous shale & Pld platy, argillaceous micrite dar k, calcareous shale, hard pyritic m~cnte & platy, argillaceous micnte dark, Laminated micrites, platy, grey intramicrite, minor argillaceous micrite & cal.c, shale shale & laminated micrite dark.calcareous shale & clork grey, argillaceous micrite Laminated m~crite (goLgot) t I dark, calcareous shale & /~ platy, argillaceous micrite e.) pole. laminated micrites __~ I F - Posidoma becheri ,,]dark, calcareous shale, platy. I Hibermcoceras sp. .~l argillaceous micrite & J dark, Laminated micrite hard, dark grey micrites & minor B-zo.phrentoids, crinoid debris platy, argillaceous micrite A-fragmented Michelinta sp. grey, Laminated micnte & ?p~ th~n shales w :x~ie micrite & intramicrite; D dark, calcareous shale & scattered chert nodules platy, argillaceous micrite r E - Hibernicoceras rudls Posidonia sp. ,ight grey, fine-grained { biospQrite; dark chert I D - PosJdonia becheri, worm trails ,,>, nodules ~ {dark micrite C -Hibernicoceras ?hibernicus, ?, Light grey, fine-grained t4. carraunense var. tumida, biospo.rite; 50-?0% chert Thrincoceras sp., orthocones, gostropods. ,®a3 i Posidonia sp. brochiopodso zophrentoids in irregular, interconnecting framework dark, calcareous shale & platy • argiLLaceous micrite

FZO. 7- Columnar section representing the Roscunnish Shale exposed near Carrane Hill.

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faunal lists below under the following headings: I. Dargan Limestone (and minor limestones in the Moy Sandstone) 2. Oakport Limestone 3. Bundoran Shale (lower Lisgorman Shale Group) 4. Mullaghmore Sandstone 5. Benbulben Shale (upper Lisgorman Shale Group) 6. 'Reef' limestones 7. Lower Bricklieve Limestone 8. Upper Bricklieve Limestone I234567 8

Chaetetes sp.. __ X Michelinia ?favosa (Goldfuss) . X M. megastomata (Phillips) x M. tenuisepta (Phillips) . X Syringopora geniculata Phillips . xx Amplexus coralloides J. Sowerby X AUotropiophyllum sp. • . XX Amplexizaphrentis enniskilleni (Edwards & Haime) XXX Cyathaxonia cornu Michelin . . X zaphrentoids XXXXX XX Caninia cornucopiae Michelin X X Caninophyllum archiaci (Edwarc~ & Haimei X X C. sp. X X XX Rylstonia sp. X X Caninia benburbensis Lewis XX XX C. cylindrica (Seouler) XXXX C. el. cylindrica X X ClisiophyUum sp. X X clisiophylloids X X Dibunophyllum bipartitum "(McCoy) XX .Koninckophyllum sp. . . X X XX Palaeosmilia murchisoni Edwards & I--Ialme X X P. sp. X Lithostrotion aj~ne (Fleming) . X XXXX L. aranea (McCoy) XX Lithostrotion of. decipiens (McCoy) (see Caldwell & Charlesworth I962, P. 379) x L. junceum (Fleming) X L. martini Edwards & Haime X XX XX L. pauciradiale (McCoy). XX L. el. pauciradiale (see Caldweil & Charlesworth ,962, p. 377) xx L. aft. portlocki Bronn (see Caldwell & Charlesworth x962, p. 378) x L. scoticum Hill x phaceloid lithostrotiontids xxxxx XX Rhipidomella michelini (Leveille) x xx x Schizophoria resupinata (J. Sowerby) XXX Leptaena analoga (Phillips) xx x ScheUwieneUa ?aspis Smyth XX ScheUwieneUa sp. . X x Schuchertella ?fascifera (Tornquist) xxx X S. radialis (Phillips) x X Delepinea carinata (Garwood) . x D. sp. x x

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I2345678 Megachonetes cf. zimmermanni (Paeckelmann) x M. sp. x x x Pliocochonetes crassistrius (McCoy) x P. tricornis (Semenew) . x P. sp. X x Rugosochonetes spp. xx Tornquistia sp. x Antiquatonia hindi (Muir-Wood) xx A. insculpta (Muir-Wood) X A. sp. (antiquatus-type) X A. sp. xxxx Argentiproductus margaritaceus (Phillips) xxx Avoniayoungiana (Davidson) . X X XX Dictyoclostus semireticulatus (Martin) X D. ?pinguis (Muir-Wood) X dictyoclostids X XXXXXX Echinoconchus elegans (McCoy) i XX E. punctatus (J. Sowerby) X KX E. defensus (I. Thomas) X E. sp. . . XX XK Eomarginif era ?frechi Paeckelmann X E. sp. X Fluctuaria undata (Defiance) X Gigantoproductus giganteus (J. Sowerby) X G. gigantoides (Paeckelmann) . XX G. sarytscheffi (Paeckelmann) . X Krotovia spinulosa (J. Sowerby) XX K. sp. X X L inoproductus sp. ( " corrugato-hemisphaericus"-type ) XX linoproductids XXXXX Overto. s mbriata iJ. c. S;werb;) X X Plicatifera plicatilis (J. de C. Sowerby) XX X Productus ?concinnusJ. Sowerby X Pugilis scoticus (J. Sowerby) XX pugilids X X XX Pustula pustulosa (Phillips) XXX X P. pyxidiformis (de Koninck) . X X P. sp. X XXX Stipulina deshayesiana (de'Koninck) X Camarotoeehia spp. X XXX Pugnax pugnus (Martin) X P. sp. X Stenoscisma sp. . X Aainoconchus ?expansus (Phillips) X A. lamellosus (Leveille) . X A. obtusus (McCoy) X A. sp. X X Brachythyris integricosta (Phillit~s) X B. pinguis (J. Sowerby) X Cleiothyridina cf. glabristria (Phillips) XX C. el. royssii (Leveille) . . X XXX XX Composita aft. ambigua (J. Sowerby) X X XX G. ficoides (Vaughan) X Martinia sp. X

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x2345678 Phricodothyris sp. . x xx Spirifer princeps McCoy . xx S. spp. XXXXXXXX Spiriferellina insculpta (Phillipsi X S. octoplicata (J. Sowerby) X X Syringothyris sp. X Tylothyris laminosa (McCoy) • X X X Dielasma hastata (J. de C. Sowerby) • x x xxx

The cephalopod-lamellibranch fauna in the lower (PI) and upper (P~) Roscunnish Shale is as follows (See Fig. 7)"

Goniatites granosus (Portlock) . x G. robustus Moore & Hodson . x G. striatus (J. Sowerby) and related forms x Hibernicoceras carraunense var. tumida Moore & Hodson x H. ?hibernicus Moore & Hodson x H. rudis Moore & Hodson x Sudeticeras turneri Moore & Hodson x S. sp.. x x Thrincoceras sp. . x orthoconic nautiloids x x Caneyella membranacea subsp, horizontalis Yates x Obliquipecten costatus Yates x Posidonia becheri Bronn . x P. corrugata (R. Etheridge) x Pseudamu~ium sp. . x Pterinopecten sp. x

B) ZONAL CORRELATION A precise application of the classic Avonian zonation to the Irish Lower Carbon- iferous Series has proven impracticable. Faunal studies have revealed the geographic limitations of characteristic faunal assemblages, apparent in Ireland in the extension of the stratigraphic ranges of certain zone fossils, and the insignifi- cance or absence of others. Certain fossils are useful for approximate dating and correlation but more detailed faunal studies are necessary before precision can be attained• The earliest Vis6an fossils in the sequence are Lithostrotion, which occurs in beds a few metres above the Dargan oolite, and Linoproductus, which appears near the top of the Dargan Limestone. The basal limestones also contain the alga Koninck- opora, elsewhere restricted to the Vis6an (Wood 1943, pp. 217-18; Johnson & Konishi 1956, p. 48). The presence of Linoproductus "corrugato-hemisphaericus" in the topmost beds of the Dargan Limestone, and widely in the Oakport Limestone, and of Composita ficoides in the lower part of the latter, suggests correlation with the $2 subzone of the Bristol standard. Probably most of the Dargan Limestone is of C~S1 age and the Moy and Boyle Sandstones may be of the same age as their restricted faunas are quite similar.

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Ostracods from the Bundoran Shale and Mullaghmore Sandstone, in particular Seminolites sp., Youngiella sp., Glyptopleura sp. (G. costata group), and Cavellina cf. longula (Coryell.), compare with S~ Scremerston Coal Group faunas of North- umberland (E. Robinson, personal communication). The foraminiferal fauna (including Endothyranopsis sp.) in the sandstone points to F.Z. 6 (S~ subzone). The presence, in the upper part of the Lisgorman Shale Group, of goniatites referable to Beyrichoceras and the Muensteroceras micronotum group, indicates that these beds are of Beyrichoceran age (top S~ or D1). The Bricklieve Limestone contains a fauna of undoubted Dibunophyllidan age (D1), as Dibunophyllum appears in limestones below the pauciradiale beds and recurs throughout the formation. Other fossils present which typify the D 1 subzone elsewhere in the British Isles, are Gigantoproductus, giganteid caniniids (mainly Caninia benburbensis), and the great variety of lithostrotiontids, including L.junceum and many cerioid forms. Among the foraminiferans, Tetrataxis spp. (forms with high acute spires), Archaediscus aft. moeUeri Rauser-Cernoussova, and Saccam- minopsis are common Dibunophyllidan forms. The microfossil Draffania (Cummings I957) , which appears in the topmost $2 and the D Zone in Scotland, occurs throughout the Bricklieve Limestone. (It also occurs at O'Donnell's Rock in the top 3 ° metres of the Lisgorman Shale Group). Zone fossils representing most of the Bollandian subzones are present in the Roscunnish Shale. Stream sections near Carrane Hill (Fig. 7) provide a near- complete exposure of the P2 and P1d subzones. The former is defined at the base by the first appearance of Goniatites granosus, and at the top by the first appearance of Namurian fossils. CaneyeUa membranacea horizontalis, a subspecies dominant in late P~ time, and extending into the early Namurian (Yates x962, p. 399), is abundant near the top of the Roscunnish Shale. The 3 ° metres of section exposed beneath the lowest G. granosus beds contain abundant Posidonia becheri and belong to the P1 subzone. Near the base of this exposure, several beds containing species of Hibernicoceras can be compared with a series of closely spaced goniatite beds at the base of the Pad subzone at Dough Mountain, Co. Leitrim (Moore & Hodson i958 ) where the subzone is a comparable 3 ° metres thick. South of Ballymote, the Roscunnish Shale adjacent to the Killavil Fault contains, in addition to Posidonia becheri, the Pac goniatites Sudeticeras turneri and Goniatites robustus. G. striatus, an index fossil of the Plb subzone, which is about 15 metres thick at Dough Mountain (Hodson & Moore I959, p. 389), occurs in the Roscunnish Shale on the southeast slope of O'Donnell's Rock, less than 3o metres above the Bricklieve Limestone. Forms of Goniatites referable to the striatus-group occur in isolated shale exposures south-southwest of Belhavel Lough, and north-northeast of Ballyfarnan. The lowermost shales exposed near Carrane Hill (Fig. 7) possibly represent at least part of the basal Bollandian (Pla), but the absence of zonal indicators leaves the age in doubt. The shales immediately succeeding the Roscunnish Shale near Carrane Hill contain Eumorphoceras pseudocoronula Bisat and Kazakhoceras sp., indicating a basal Ela age (Yates i962 , p. 367). Shales near the centre of the Highwood shale graben

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(Fig. 4) contain a fauna including E. bisulcatum subsp, ferrimontanum Yates, Kazakhoceras scaliger (Schmidt), Cravenoceras cf. gairense Currie (see Yates I962, p. 388), Anthracoceras sp., and Posidonia corrugata (R. Etheridge). These, together with Ghaenocardiolafootii (Baily) and Pseudamusium sp., are typical of the E. bisulcatum fauna (high E2a) present about I 15 metres above the base of the Namurian and 3 ° metres below the sandstone escarpment on Slieve Anierin (Yates I962, pp. 368, 37I-2). 4- History of sedimentation Visdan sedimentation in northwest Ireland began with a major transgression and the accumulation of fluvial and marginal marine detrital sediments in discrete basins of deposition. The Ballymote area was separated from a region to the north by a ridge of metamorphic and granitic rocks, the Ox Mountains, and there is ample evidence of derivation of the Moy Sandstone from this variably salient land area (Fig. 8). In exposures in the River Moy headwaters, the repeated, poorly graded conglomerates mark the intermittent, perhaps torrential, spread of land- eroded debris as fans or deltas into the adjacent sea. During intervening periods of marine reworking, finer sands were dispersed widely in the shallow sea and now alternate with the conglomerates. Further east, a thinner and more completely graded detrital sequence slowly accumulated along the contemporary Ox ridge (possibly represented only by a chain of islands--Fig. 8). The Curlew pericline remained emergent briefly after the Vis6an transgression. The Boyle Sandstone which accumulated along its flanks now incorporates rock fragments of local origin as well as finer material (strained quartz and fresh detrital feldspar) of more distant provenance, possibly from the Ox Mountains terrain (Caldwell I959, p. I67). With the gradual submergence of the remaining land areas, detrital sedimentation diminished and was gradually superceded by carbonate sedimentation. The carbonate-forming environments, only intermittently established during upper Moy Sandstone deposition, became widespread and persistent during Dargan Limestone time, and generally were occupied by a characteristic benthonic fauna of crinoids, brachiopods and corals. Most of the bioclastic and oolitic limestones formed in shallow, persistently agitated waters, and a large proportion of the contained fossil material is broken or abraded. However, locally accelerated subsidence east of Collooney in later Dargan Limestone time, apparently produced deeper, calmer bottom conditions in which fine-grained limy detrital mud and a characteristically well preserved gastropod-lamellibranch-sponge fauna collected. The presence of fine detrital sediment in the east, and sand lenses in equivalent upper Dargan beds west of Collooney, indicate the persistence of a land area in the western Ox Mountains throughout this period. Indeed minor evidence for the existence of this land area occurs as high as the middle of the Oakport Limestone. This first phase in the sedimentary history ended with the widespread establishment of carbonate environments largely free of detrital sediment, and over much of the area this change is represented approximately by the Dargan-Oakport formational boundary. The Oakport Limestone, with its repeated alternation of

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beds of oolite, biosparite, and algal micrite, represents throughout its entire thickness a succession of shallow, often strongly agitated, highly oxygenated, marine environments. This is remarkable not only for the delicate balance maintained between sedimentation and subsidence during this interval, but also for the geographic extent of the environments, as equivalent beds in the Carrick syncline (Caldwell ~959, P. ~68) are closely comparable. Exposures in the Sligo

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FIG. 8. Facies patterns during early Moy Sandstone deposition. The size and configuration of land areas is diagrammatic. The rose diagrams indicate the directions of sediment transport, and the number of foreset bedding measurements made. The effect of a 16 km sinistral displacement on the Killavil Fault has been removed. The present position of the fault, the present outcrop distribution of the Moy Sandstone and equivalent basal beds (black), and the present distribution of pre-Carboniferous outcrops (diagonal ruling) are shown on the inset map.

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Syncline do not permit direct comparison but probably the Ox ridge was largely if not entirely submerged by the end of Oakport sedimentation. Shales succeeding the Oakport Limestone represent the abrupt influx of muddy sediments into these carbonate shelf environments, and the beginning of a pro- longed period of detrital sedimentation during which the Lisgorman Shale Group accumulated. The effect on the benthonic faunas was striking. Many of the lower shales of the Group apparently accumulated in excessively mud-choked environments, virtually barren of invertebrate life, although the isolated fossil-rich muddy limestones and calcareous shales are evidence of brief, less turbid conditions. The culmination of this detrital sedimentary phase was achieved with a influx of sand (the Mullaghmore Sandstone) in the O'Donnell's Rock area, the edge of an extensive deltaic sand facies (Fig. 9) which spread generally from the north (George I958, fig. I3). The distribution of facies in the Lisgorman Shale Group supports the suggestion (ibid., pp. 271-2) that the Ox anticline, although not emergent, formed a persistently positive area which restricted the southward dispersal of deltaic sands, and separated them from the dominantly calcareous beds to the south. Excepting the sands deposited in the O'Donnell's Rock area, the only detrital sediments which crossed the Highland Boundary Line in the Ballymote region, were muds. The transition from a shale-dominated sequence in the northeast, near the limit of deltaic sandstones, to a limestone-with-shale sequence in the southwest, approaching the limit of dispersal of the suspended detrital muds, is part of a much larger gradient in the detrital 'Calp' sequence of the region. Through much of the Upper Lisgorman Shale Group, the fossil faunas represent benthonic faunas which flourished in response to the gradual waning of detrital sedimentation, and the return of the less restrictive conditions of open marine carbonate sedimentation. The fourth phase in the history of sedimentation began with the re-establishment of shallow, mud-free, carbonate-forming environments in which large solitary and compound corals and robust brachiopods abounded. The alternation of coral biostromes (with in situ and overturned coralla), variably bioclastic shelly limestones, crinoidal sands and oolites, and thin algal micrites mark minor fluctuations in depth and water agitation (compare in part with Pareyn i959, P. 353). The similarity of rock successions in adjacent areas indicates the existence of a very extensive shelf sea in northwestern Ireland. Several features in the Bricklieve Limestone support the suggestion (George 1953, p. 72; Caldwell 1959, p. 173) that the Ox and Curlew anticlines maintained a positive topographic expression throughout this period. The distribution of micrite 'reefs' shows a striking relationship to these two basement structures as well as to the Killavil Fault. The southward thickening of the coarse crinoidal limestones in the Bricklieve Mountains (Caldwell & Charlesworth 1962, p. 369) suggests that their source may have been in crinoid 'meadows' on shoal areas along the Curlew anticline. Similarly, the richly crinoidal carbonate succession near O'Donnell's Rock, and the discontinuity indicated at the top, may be evidence of shoaling related to contemporaneous tectonic movements of either the Ox anticline or the Killavil Fault. At variance with the reconstruction by

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FIG. 9" Facies patterns during mid-Lisgorman Shale Group deposition. Base map reconstructed as in Figure 8. Heavy dot and line patterns represent the positions of present outcrops of the mid-Lisgorman Shale Group and equivalent beds; lighter patterns are assumed extensions of the facies. Arrows in the Carrowmoran Sandstone indicate two important sediment transport directions (after Hubbard I966A, fig. xo).

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George (i958 , fig. i3) , this study suggests that the persistent positive area of the Ox anticline was narrow, as the Lower Carboniferous sections in the Ballymote and Carrick downfolds are not significantly different in thickness (Figs. I o & 1 I). The fifth and final phase in this sedimentary history began with another major influx of argillaceous sediments, and again this produced environments initially barren of marine organisms. Part of this impoverishment in the fossil record may be attributed to restricted evaporitic conditions (West et al. I968 ) proposed for equivalent beds near Drumshanbo (Co. Leitrim). However, the specialized fauna of goniatites and lamellibranchs which eventually became established suggests, perhaps, that the evaporitic environments were discontinuous laterally as well as vertically. In the Roscunnish Shale near Drumshanbo (Caldwell I959, p. x79 ) and O'Donnell's Rock, the occurrence of a coarse sandstone which thins and disappears southwestward, suggests that the detrital sequence again was derived from an actively eroding land mass to the north and east.

5. Regional correlation Following the Carboniferous marine transgression in northwestern Ireland, similar conditions of sedimentation were established rapidly in the Ballymote and Carrick-on-Shannon areas, and the Visdan sequence can be compared closely in lithology and faunal content. The fact that as close a correlation cannot be made with the Sligo syncline north of the Highland Boundary line, probably indicates the effect which the emergent Ox Mountains had on sedimentation during the early Visdan. The Moy Sandstone and Dargan Limestone have approximate equivalents (Fig. I I) in the Boyle Sandstone and Kilbryan Limestone in the Carrick syncline (Caldwell I959, pp. I65-7). The mapped boundaries between the sandstone and limestone formations likely do not correlate precisely in the two areas due to the varied local persistence of clastic sedimentation. The Oakport Limestone com- pares quite closely in the two areas (ibid., pp. I67-9) , and together with the underlying formations probably is equivalent to the Ballyshannon Limestone and basal beds of the Sligo syncline (Oswald I955, pp. I68-72), although excluding certain "Ballyshannon" coastal exposures in Sligo which since have been proven younger (Hubbard & Sheridan i965; Hubbard I966B ). The Ballymote area provides intermediate exposures which show the relationship between the markedly different 'Calp' sequences in the Carrick and Sligo synclines. The Ballymore Beds and Croghan Limestone in the former (Caldwell i959, pp. x69-7o ) compare in thickness and in proportion of shales with the Lisgorman Shale Group west of Ballymote. The shales increase in thickness and proportion northeastwards until, at O'Donnell's Rock, with the appearance of a sandstone in the middle of the shale group, the section can be compared to the 'Calp' sequence in the Sligo syncline (Fig. I I). From this thin tapering edge, the Mullaghmore Sandstone thickens in the Sligo syncline (Oswald I955, pp. I73-4) and is thought to be equivalent to the Carrowmoran Sandstone in western Sligo (Hubbard I966A). The shales immediately above the Mullaghmore Sandstone at O'Donnell's Rock probably are successively replaced to the north and east by

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sandstones, and the Benbulben Shale at O'Donnell's Rock is considerably thicker than the Benbulben Shale in the Sligo syncline (Oswald I955, p. I75 ). The Bricklieve Limestone in the Bricklieve Mountains bears close resemblance to the Cavetown Limestone in the Carrick syncline (Caldwell & Charlesworth I962, pp. 37I-2). The coral fauna of the Bricklieve Limestone suggests it is equivalent in age to the entire Glencar-Dartry sequence in the Sligo syncline. The diachronism at the top of the shale group east of Ballymote continues to the north, where shales occur (diminishing upwards) through much of the Glencar Limestone (Oswald 1955, PP. 175-6). The Upper Limestone in the Clogher-Slieve Beagh area (Padget I952 , pp. 67-7 I) probably is equivalent in age to part of the upper Lisgorman Shale Group and most of the Bricklieve Limestone. However it is succeeded by sandstones and shales of Cracoean age (B2), and this suggests that the top beds of the Bricklieve Limestone pass eastwards into detrital rocks. The Roscunnish Shale correlates with the Bollandian Roscunnish Shale near Slieve Anierin (Caldwell I959, pp. 178-8o). In a sedimentological study of part of the sequence, West & others (i968) applied the name Aghagrania Formation to the basal 71 metres of the type section of the Roscunnish Shale. The original formation name has been retained herein to facilitate mapping of isolated unfossiliferous outcrops which cannot be assigned with certainty to the Aghagrania Formation or to the overlying sequence of shales and carbonate rocks ("Carraun Formation"--ibid., fig. 2). The Roscunnish Shale probably correlates in part with the Glenade Beds in the Sligo syncline (Oswald I955, PP. I79-8o). The transgression of the Glenade Beds over the underlying limestones, identified north and northeast of Manorhamilton (ibid., p. I8o; discussion, p. 185), strengthens the possibility of a disconformity between the Bricklieve Limestone and Roscunnish Shale at O'Donnell's Rock. Correlation of the Sligo and Carrick successions with more distant areas is discussed in George (i958 , pp. 266-272, 276 ) and Caldwell (I959, p. I8o-I8I), to which the reader is referred for a broader analysis of Lower Carboniferous palaeogeography in northwestern Ireland. After the main paper was submitted, a further note on the Microfossils from the area, prepared by B. Mamet of the Department of Geology, University of Montreal, Canada, was lodged in the Library of the Geological Society, Burlington House, London. Although Mamet reported no major discrepancy between the micro- and macrofaunal zonations, he considered that the microfossils indicate an Sx rather than an $2 age for the Oakport Limestone. He also commented on the uncertainty of assigning the Lower Bricklieve Limestone to the D1 subzone, based either on the microfossils or on Dibunophyllum, the earliest elements of which appear below the D zone of the standard Bristol succession.

ACKNOWLEDGEMENTS. This research was carried out during the tenure of a Post-graduate Scholar- ship from the National Research Council of Canada, and forms part of a Ph.D. thesis submitted to the University of Glasgow. The author is indebted to Professor T. Neville George for supervision of the research and critical reading of the manuscript, and to the following persons for assistance in identifying particular elements of the fossil fauna; Dr K. A. G. Shiells (late of The University, Glasgow) ; Dr E. Robinson and Dr J. W. MacKay (University College, London) ; and Mr R. W. L. Oldroyd (Robertson Research Company, Colwyn Bay). 7--(12 pp.)

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6. References CALDWXLL, W. G. E. I959. The Lower Carboniferous rocks of the Carrick-on-Shannon syncline. q. Jl geol. Soc. Lond. ix5, i63-87. CALDWRL~, W. G. E. & CHARLESWORTH, H. A. K. I962. Vis6an coral reefs in the Brieklieve Mountains of Ireland. Proc. Geol. Ass. 73, 359--82. CHARLESWORTH, H. A. K. 196oA. The Lower Palaeozoic inlier of the Curlew Mountains anticline. Proc. R. Ir. Acad. 6xB, 37-5o. 196oB. The Old Red Sandstone of the Curlew Mountains inlier. Proc. R. Ir. Acad. 6xB, 51--8. CRuisE, R.J. 1878. Explanatory memoir to accompany sheets 66and 67 of the maps of the Geological Survey of Ireland. Dublin. CuMxuxNos, R. H. I957. A problematic new microfossil from the Scottish Lower Carboniferous. Micropaleontology 3, 4o7-9. 196 x. The foraminiferal zones of the Carboniferous sequence of the Archerbeck borehole, Canonbie, Dumfriesshire. Bull. geol. Surv. Gt Br. 18, Io7-28. DEWEY, J. F. I963. The Lower Palaeozoic stratigraphy of central Murrisk, County Mayo, Ireland, and the evolution of the south Mayo trough, q. Jl geol. Soc. Lond. xx9, 313-44 • FOLK, R. L. 1968. Petrology of sedimentary rocks. Hemphill's, Austin, Texas. 17o p. GEOROE, T. N. I953. The Lower Carboniferous rocks of North-Western Ireland. Adv. Sci. xo, 65-73. I958. Lower Carboniferous palaeogeography of the British Isles. Proc. Yorks. geol. Soc. 3~r, 227-318. & OSWALD, D. H. I957. The Carboniferous rocks of the Donegal syncline, q. Jlgeol. Soc. Lond. xx3, I37-79. GRIFFITH, R. I836. On the geological map of Ireland. Rep. Brit. Ass. 5 [for I835], 56--8. 1838. On the leading features of the geology of Ireland, etc. Rep. Br. Ass. Adv. Sci. 6 [for 1837], 88-9o. 1857. Notes explanatory of the subdivisions of the Carboniferous System, etc. J. geol. Soc. Dublin 7, 267-77. HoDson, F. & MOORE, E. W. J. i959. Goniatites striatus and related forms from the Visfian of Ireland. Palaeontology, Lond. T, 384_96. HUBBARD, J. A. E. B. 1966A. Facies patterns in the Carrowmoran Sandstone (Vis~an) of Western Co. Sligo, Ireland. Proc. Geol. Ass. 77, 233-54. 1966B. Population studies in the Ballyshannon Limestone, Ballina Limestone, and Rinn Point Beds (Vis6an) of N.W. Ireland. Palaeontology, 9, 252-69. HUBBARD, W. F. & SHERIDAN, D. J. R. i965. The Lower Carboniferous stratigraphy of some coastal exposures in Co. Sligo, Ireland. Scient. Proc. R. Dubl. Soc. A, 2, 189-95. HULL, E. I878. The physical geology and geography of Ireland. London (Edward Stanford). JOHNSON, J. H. & KomsHi, K. 1956. Studies of Mississippian algae. Quarterly, Colorado School of ~lines 5 x, I -- 13 I. KILROE, J. R. 1885. Explanatory memoir to accompany sheet 55 of the maps of the Geological Survey of Ireland. Dublin. LEES, A. 1964. The structure and origin of the Waulsortian (Lower Carboniferous) 'reefs' of west-central Eire. Phil. Trans. R. Soc. B, 247 , 483-53 I. MooRE, E. W. J. & HODSON, F. 1958. Goniatites from the Upper Vis~an shales of County Leitrim, Eire. Lpool. Manchr. geol. d. 2, 86-Io 5. OSWALD, D. H. 1955. The Carboniferous rocks between the Ox Mountains and Donegal Bay. q. dl geol. Soc. Lond. xxl, 167-83. PADOE'r, P. 1952. The geology of the Clogher-Slieve Beagh area, County Tyrone. Scient. Proc. R. Dubl. Soc. 26, 63-83. 1953. The stratigraphy of Cuilcagh, Ireland. Geol. Mag. 9o, I7-26. PAREYN, C. I959. Les r~cifs carbonif~res du Grand Erg occidental. Bull. Soc. gdol. Fr. (7) x, 347-63. SIMPSON, I. M. I955. The Lower Carboniferous stratigraphy of the Omagh syncline, Northern Ireland. q. dl geol. Soc. Lond. xxo, 39I-4o7 .

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SYMES, R. G. & KILROE,J, R. x880. Explanatory memoir to accompany sheet 54 and the southwestportion of 4~ of the maps of the Geological Survey of Ireland. Dublin. , WILKINSON,S. B., & KILROE,J. R. x88x. Explanatory memoir to accompany sheet 65 of the maps of the Geological Survey of Ireland. Dublin. WEST, I. M., BRaNDON, A., & SMITH, M. 1968. A tidal flat evaporitic facies in the Vis6an of Ireland. J. sedim. Petrol. 38, IO79-93. WOOD, A. I943. The algal nature of the genus Koninckopora Lee; Its occurrence in Canada and Western Europe. Q. Jl geol. Soc. Lond. 98, 2o5-2 I. WYNN~, A. B. I864. On the geology of parts of Sligo, etc. J. geol. Soc. Dublin xo [for I862-64] , 33-41 • YATES, P.J. i962. The palaeontology of the Namurian rocks of Slieve Anierin, Co. Leitrim, Eire. Palaeontology. 5, 355-443. Received I I September 1969; revised manuscript received 2o November i 97o; read 3o June x97 x. O. A. Dixon, Department of Geology, University of Ottawa, Ottawa 2, Canada

DISCUSSION Dr H. G. READING queried the author's interpretation of the Moy Sandstone as entirely marine. The unimodal palaeocurrent directions pointing towards the basin centre, the feldspathic nature of the grits and the local derivation of the clasts would all seem to point towards a fluvial rather than a marine origin. Was there any positive evidence for a marine origin, such as polymodal palaeocur- rents, wave ripples, tidal currents, well sorted compositionally mature beach or bar sandstones ? The calcareous nature of the cement was no reason for postulat- ing a marine origin because many fluvial sandstones have a calcite cement. Could this sequence of sandstones represent environmental changes from fluvial below into shallow marine above ? If this was found to be so, it would suggest a southerly source area of much wider extent than the relatively small islands postulated by Dr Dixon.

In a written reply, the AUTHOR said, any impression conveyed that the Moy Sandstone had a history which was entirely marine was quite unintentional. Indeed the account of the history of sedimentation mentioned the accumulation of both fluvial and marginal marine detrital sediments during the Vis6an transgression, although evidently it failed to give due emphasis to the initial fluvial aspects of the sequence. The isolated exposures along the headwaters of the River Moy show a pronounced change from dominantly cross-bedded, immature, coarse-grained, arkosic rocks in the lower part to planar bedded, near-mature, finer grained rocks in the upper part. This is in keeping with a sequence of environmental changes from fluvial below to shallow marine above, as envisaged by Dr Reading. The marine nature of the uppermost beds is apparent from contained fragmental fossil material, notably crinoidal debris. The southerly source area could have been substantially larger only at a stage of the transgression earlier than is represented by present surface exposures. The outcrop distribution of the Boyle Sandstone which fringes much of the Curlew pericline, effectively limits the possible extent of a contemporary source area to an elongate island or islands not appreciably larger than already suggested.

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The similarity of the sedimentary successions on either side of the pericline achieved early in the sedimentary history, also tends to suggest a land mass limited in areal extent and topographic expression, and hence emergent only briefly during the Visfian transgression'.

Dr M. E. PHILCOX commented that just east of the south-east corner of Dr Dixon's map the extension of the Ballinafad-Lough Meelagh reef belt lies for about ten miles apparently in direct contact with older rocks, including the Boyle Sandstone, this contact being the Curlews fault. Between Keadew and Drumshanbo there are several exposures of a breccia at the base of the reef, which include sandstone pebbles. The coincidence of the breccia with the Curlews fault suggests at first that this is a fault-breccia, particularly since west of Keadew, where the reef is separated from the fault by sub-reef beds, at the one locality where the base of the reef is visible, it is gradational down into darker limestones with thin shaly interbeds. (Stream section 1.6 km. ( I mile) south-west of Ballinafad.) The composition of the breccia, however, strongly suggests a sedimentary origin, although later movement, either faulting or slumping, has modified it slightly. The breccia includes rounded pebbles of sandstone and angular clasts of calcilutite, some containing isolated grains of sub-angular quartz. The loosely packed clasts, which reach up to 15 cm. (6") across, are cemented in a matrix of calcilutite, some of it with quartz grains, geopetal laminated calcilutite and cavity- filling fibrous calcite; the matrix itself is partly brecciated. It is evident that sandstone pebbles and quartz grains were being supplied at the time that carbonate mud was being deposited, lithified and reworked. Elsewhere in Ireland laminated calcilutite and fibrous calcite occur as contemporaneous cavity-fillings in shallow- water limestones, such as the Waulsortian reefs. It is at least possible that this breccia was likewise deposited in a shallow marine environment. Reworking prior to final deposition could have resulted from gravitational movement down a fault-scarp. If this interpretation of the breccia is correct, it has important consequences for the palaeogeography. The most likely source of the sandstone pebbles and quartz grains seems to be Boyle Sandstone to the south. If so, the reef was probably sited directly on an island or shoal consisting of Curlews 'basement'. If the reef belongs to the lower part of the Bricklieve Limestone as interpreted by Dr Dixon, some 6IO m. (2000 ft.) of section between the reef and the Boyle Sandstone (Dixon, Fig. I I) has been cut out, either through very rapid thinning southward against the Curlews island, or through pre-reef faulting. In either case the Ballymote and Carrick synclines would have been distinct sedimentary basins in mid-Vis~an times, as Dr Dixon infers (Fig. 8) for the early Vis~an. No evidence for thinning of sub-reef units against the Curlews island is reported by Dr Dixon, but exposures are in any case poor. Sandstone pebbles have not been recorded from units between Boyle Sandstone and the reef in this area, which might have come from an emergent Curlews island during this interval. It seems more likely that the Curlews fault was active immediately before reef deposition and provided a linear topographic feature on which the reef developed. The startling figure of 6IO m. (2000 ft.) of stratigraphic displacement

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/1/71/4884440/gsjgs.128.1.0071.pdf by guest on 03 October 2021 Lower Carboniferous rocks between the Curlew and Ox Mountains I01

depends on the source of sandstone pebbles and on the correlation of the reef. In conclusion, Dr Philcox asked the author to comment on the possibility that the Drumshanbo-Ballinafad reef belt might correlate with Caldwell's Ballymore reef horizon, some 15o-3oo metres lower in the section and a possible alternative source for the sandstone pebbles. (Permission to publish the above remarks was granted by Gortdrum Mines Limited, Exploration, whose geologists, notably T. N. McKillen, discovered the breccia localities.)

In a written reply to Dr Philcox the AUTHOR said, 'The simplest stratigraphic interpretation of the succession north of the Curlew Fault would place the Bal- linafad-Lough Meelagh reefs in the Lower Bricklieve Limestone. To place the reefs at a lower level (comparable to the Ballymore reef) would require either that they be faulted at the top against the Bricklieve Limestone, or that they be part of a greatly condensed sequence. I found no evidence in the area to suggest either alternative. From gravity readings, Thirlaway (1951, Dublin Inst. Adv. Studies, Sch. Cosmic Phys. Geophys., Mem. 2, pt. 2, p. 17-I8 ) concluded that as much as 2700 feet (~--~825 m) of Carboniferous rocks lie immediately adjacent to the Old Red Sandstone near Ballinafad, and this suggests no appreciable thinning of this part of the succession toward the Curlew Fault. There is substantial evidence that the stratigraphic displacement on the Curlew Fault is large, but the proportions of displacement attributable to syn-depositional and post-depositional movements are open to interpretation. The limited exposures in the area immediately north of the fault contribute little to an understanding of the sub-reef sedimentary history. However, in the Carrick Syncline the un- interrupted rock succession between the Woodbrook Fault and the Curlew pericline apparently shows no definite evidence of subaerially exposed bedrock along the pericline during post-Boyle Sandstone deposition. The stratigraphic successions on either side of the pericline (as much as the limited exposures to the north can be used for comparison) show sufficient similarity to suggest some marine connection across the pericline during the early Visdan. In the Bricklieve Limestone discussed in the paper, a few sedimentary features were interpreted as indications of syn-depositional shoaling along the Curlew pericline later in the Visdan. I have not seen a breccia such as that reported beneath the reefs by Dr Philcox. The breccia is the strongest suggestion yet of syn-depositional movements on the Curlew Fault. The nearest possible source for the sandstone pebbles is the Boyle Sandstone, although this poses a considerable problem in reinterpreting the local paleogeography. The Mullaghmore Sandstone, although it occurs at a more appropriate stratigraphic level much closer to that of the reefs, might be considered a source only if it extends in the subsurface much farther south in Co. Leitrim than is apparent from surface exposures. Derivation of pebbles from a sandstone of Mullaghmore age would require later Vis6an uplift of the Curlew pericline, and this perhaps is more in accordance with the existing evidence in the adjacent sedimentary succession. Clearly this is highly speculative, and the problem must be pursued with further field studies and detailed petrographic work on the detrital sediments.' 7a---(4 pp.)

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