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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/1/71/4884440/gsjgs.128.1.0071.pdf by guest on 03 October 2021 72 O. A. Dixon 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 " t VISEAN LIMESTONES • ." ~ & BASAL BEDS ~i'~i PRE- CARBONIFEROUS PALAEOZOIC ROCKS BAY~,I~ ~{.'. ~~ ~~..""SL GO "" ":::'::: "" " " "":'."~IL.""" "" .'~~'"::'T~:!:v":"" """ " "'"" '~ . J-~:/..: o " '; ....... ~ M, ::!~.:.:::.'.~..'.': i ~_ x~l o. ~. Geological sketch map of northwestern Ireland. The limits of the Ballymote map area (Fig. x2) are indicated. 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 73 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 sum- marized 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/1/71/4884440/gsjgs.128.1.0071.pdf by guest on 03 October 2021 74 O. A. Dixon 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

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