Late Ordovician-early Silurian sedimentary cover sequence, Midland Valley Terrane, Northern Ireland

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Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.

M R Cooper and W I Mitchell

Introduction

Geological map of the Central Inlier (Corvanaghan Formation), Tyrone Igneous Complex and Pomeroy Inlier. (P947794)
Geological map of the Ordovician and Silurian rocks of the Pomeroy Inlier. (P947798)
Detailed geological map of the Ordovician and Silurian rocks in the Little River section at Pomeroy. (P947800)
Litho-, chrono-, and biostratigraphy of the Silurian Little River Group in the Pomeroy Inlier. (P947921)
Stratigraphy of the Lower Palaeozoic rocks at Pomeroy and Lisbellaw. (P947919)
Late Ordovician (mid-Caradoc) brachiopods from the Bardahessiagh Formation: 1 Bilobia etheridgei (x3) 2 Bimuria youngiana recta (x1.5) 3 Campylorthis discreta (x1.75) 4 Isophragma pseudoretroflexum (x1.5) 5 Rostricellula ambigua (x1.5) 6 Sowerbyites hibernicus (x1.5). (P948069)
Late Ordovician (early Ashgill) fossils from the Killey Bridge Formation: Brachiopods: 7 Schizophorella fallax fallax (x1.5) 8 Plaesiomys porcata lata (x2) 9 Skenidioides cf. greenhoughi (x4) 10 Dicoelosia simulata (x6) 11 Dalmanella testudinaria ripae (x3) 12 Rugosowerbyella ambigua (x3), 13 Sowerbyella thraivensis (x3), 14 Leptaena rugosa, 15 Chonetoidea radiatula (x6). Trilobite: 16 Tretaspis seticornis (x3 and x5.) (P948070)
Geological map of the Ordovician and Silurian rocks of the Pomeroy Inlier. (P947798)
Litho- and chronostratigraphy of the Ordovician Desertcreat Group in the Pomeroy Inlier. (P947920)
Litho-, chrono-, and biostratigraphy of the Silurian Little River Group in the Pomeroy Inlier. (P947921)
Early Silurian (Llandovery) graptolite zonal index species from Pomeroy (all x2) (after 17). (P947799)
Geological map of the Silurian rocks in the Lisbellaw Inlier (18). (P947801)

The Ordovician and Silurian rocks of the Pomeroy Inlier (P947794) and (P947798); (P947919) rest unconformably on the Tyrone Plutonic Group and on the granitoid intrusive complex at Craigbardahessiagh. Uplift and unroofing of the Central Inlier and Tyrone Igneous Complex had occurred by mid-Caradoc times, in the 5–15 million years after obduction and magmatism. Some 50 km to the southwest, is the fault-bounded inlier of Silurian rocks at Lisbellaw (P947919). Although composed of sedimentary rocks resembling those in the Southern Uplands-Down-Longford Terrane, the Lisbellaw rocks underwent a radically different tectono-thermal history being relatively uncleaved and with much lower organic carbon maturation levels.

Pomeroy Inlier

The Pomeroy succession (P947919) consists of over 400 m of late Ordovician rocks of the Desertcreat Group conformably overlain by about 600m of Silurian rocks of the Little River Group [1].

Desertcreat Group

The Bardahessiagh Formation (P947798), including the ‘Junction Beds’ (P947920), comprises a conglomerate at the base of the succession, overlain mostly by coarse-grained sandstone that passes up into fine-grained sandstone and grey mudstone that constitute the Junction Beds. Fossils consist mainly of large, thick-shelled brachiopods [1] such as Bilobia, Bimuria, Campylorthis, Isophragma, Rostricellula and Sowerbyites (P948069) Fossils 1–6 and rare trilobites.

The mid-Caradoc (Burrellian Stage) brachiopod and trilobite fauna in the Bardahessiagh Formation is of Laurentian aspect and has much in common with faunas found in rocks of similar age at Girvan in Scotland [2] and in North America.

The Killey Bridge Formation (P947798), (P947920) comprises grey siltstone and mudstone. Subordinate sandstone in the Killey Bridge stream [H 702 710] contains brachiopods including the typical Ashgill form Schizophorella (P948070) Fossil 7. Exposed in the Little River, about 210 m east of the Slate Quarry Bridge, silty mudstones contain abundant, well preserved fossils including numerous small brachiopods (P948070) Fossils 8-15, trilobites (P948070) Fossil 16, bivalves, gastropods, orthocone nautiloids, corals, bryozoa and the Ashgill zonal graptolite Dicellograptus anceps. Faunas in the formation are Ashgill (P947920). Compared to faunas in the Bardahessiagh Formation the Ashgill assemblage was more cosmopolitan and included many genera of brachiopods and trilobites that, in the Caradoc, were restricted to the margins of either Baltica or Avalonia and to separate Baltic or Anglo-Welsh faunules respectively.

The Tirnaskea Formation (P947920) is a relatively hard pyritous grey siltstone (P947798) , (P47920) with rare fossils including trilobites, ostracods and bryozoa [1]. The top of the formation and contact with the succeeding Little River Group represents the conformable Ordovician-Silurian systemic boundary and is exposed in the Crocknagargan stream section [H 724 734]. This section also exposes the middle and upper levels of the formation [3]. A fauna of small brachiopods of Rawtheyan age in the middle part of the formation includes Dedzetina, Sericoidea, Proboscisambon and Protozyga. The upper part contains Eostropheodonta and Dysprosorthis along with the trilobite Mucronaspis indicating a Hirnantian age. The faunas lived in deep, cold water in a dark, poorly oxygenated and nutrient-starved environment.

Little River Group

The Silurian Little River Group [4] consists of dark grey graptolitic mudstones with rare thin sandstones (P947921). Although the graptolite faunas have been studied in detail (P947799) only an informal lithostratigraphy is available [4]. Lithological divisions are characterised in four main parts of the outcrop. The exposure of the Crocknagargan ‘beds’, representing the base of the Little River Group, in the Crocknagargan stream (op. cit.) comprises poorly graptolitic mudstone. In the Slate Quarry river [H 727 729], higher strata in this unit contain a more diverse graptolite fauna (P947921) and are succeeded by at least 100 m of the Slate Quarry ‘beds’ with 2 m of the Edenvale ‘beds’ at the south end of the exposure. The base of the Edenvale ‘beds’ is represented by the Dimorphograptus band [4], comprising 1m of calcareous mudstone with thin, bands of pale yellow bentonite. The most continuous exposure of this Group is in the Little River (P947798) where long sections in the Edenvale and Mullaghnabuoyah ‘beds’ contain diverse graptolite faunas ( (P947800), (P947921)). The Lime Hill ‘beds’ are exposed only in a stream section [H 694 739] at Lime Hill (P947798) which is the type locality for the graptolite species Stimulograptus (Graptolithus) sedgwickii and Lagarograptus (Graptolithus) tenuis [4]. Unusually for the Little River Group, these graptolitic mudstones also contain a sparse shelly fauna including bivalves and inarticulate and rhynchonnelid brachiopods. Unnamed ‘beds’ (P947921) at the top of the Little River Group are confined (P947798) to the stream at Aghafad [H 709 696]. Historically, because of their composition they were often included in the early Devonian Fintona Group but the presence of late Llandovery graptolites proves their early Silurian age.

Lisbellaw Inlier

The Lisbellaw Formation consists of 300m of grey and greenish grey fossiliferous mudstone and silty mudstone with some greywacke beds usually less than 5cm thick (P947801) . Near the middle of the succession, the 40 m thick Leambreslen Conglomerate Member [5] comprises conglomerate beds ranging from 10 cm to over 6 m thick. The conglomerate is exposed in the centre of Lisbellaw and in the Council quarry [H 295 411] west of the village, where 25 m of conglomerate is overlain by 25 cm of greywacke sandstone. Each bed has a sharp base with flute and groove marks and commonly grades up into coarse greywacke sandstone proving the succession is not inverted. The conglomerates are clast-supported with a medium- to coarse-grained matrix and only rarely show imbrication of pebbles. More than 90% of the well-rounded clasts consist of purplish and greenish grey quartzite up to 57 cm in diameter, with minor vein quartz, jasper and granite. Palaeocurrent data suggest that the sediments were derived from the north and northeast [6].

The oldest graptolite faunas occur [7] in mudstones in the stream east of Homeville [H 303 420] and belong to either the atavus Biozone or to the cyphus Biozone and include Normalograptus sp., Paraclimacograptus innotatus and Pribylograptus? sp. Thin mudstone beds in the conglomerate in the council quarry (P947801) contain a graptolite fauna of Monograptus triangulatus and Pseudorthograptus mutabilis of the early Aeronian triangulatus Biozone. Mudstones interbedded with greywacke in the by-pass quarry [H 301 412] (P947801) also contain a graptolite fauna of the triangulatus Biozone including M. triangulatus, Atavograptus cf. atavus, Coronograptus gregarius, Glyptograptus sinuatus, Metaclimacograptus cf. hughesi, Pribylograptus sp. and Raphidograptus toernquisti [7]. The youngest graptolite fauna occurs in mudstone in the slate quarry [H 297 417] and apart from trilobite remains contains Petalolithus folium and Pribylograptus leptotheca of the argenteus / leptotheca Biozone. The graptolite faunas range from the mid- to late Rhuddanian (atavus or cyphus biozones) to the mid-Aeronian leptotheca Biozone [8].

References

  1. 1.0 1.1 1.2 Mitchell, W I. 1977. The Ordovician Brachiopoda from Pomeroy, Co. Tyrone. Monograph of the Palaeontographical Society.
  2. Ingham, J K. 2000. Scotland: the Midland Valley Terrane-Girvan. In: Fortey, R A, Harper, D A T, Ingham, J K, Owen, A W, Parkes, M A, Rushton, A W A, and Woodcock, N H (eds.). A Revised Correlation of Ordovician Rocks in the British Isles, The Geological Society Special Report No. 24.
  3. Harper, D A T, Mitchell, W I, and Rong, Jia-Yu. 1994. New faunal data from the highest Ordovician rocks at Pomeroy, County Tyrone, Northern Ireland. Scottish Journal of Geology, 30, 187–90.
  4. 4.0 4.1 4.2 4.3 Fearnsides, W G, Elles, G L, and Smith, B. 1907. The Lower Palaeozoic rocks of Pomeroy. Proceedings of the Royal Irish Academy, 26 (B), 97–128.
  5. Geological Survey of Northern Ireland 1982. Enniskillen, Northern Ireland Sheet 45. Solid Geology. 1:50 000. (Southampton: Ordnance Survey for the Geological Survey of Northern Ireland).
  6. Simon, J B. 1986. Provenance and setting of some Ordovician and Silurian conglomerates in Northern Ireland. Scottish Journal of Geology, 22, 63–76.
  7. 7.0 7.1 Harper, J C, and Hartley, J J. 1938. The Silurian Inlier of Lisbellaw, County Fermanagh, with a note on the age of the Fintona beds. Proceedings of the Royal Irish Academy, 45B, 73–87.
  8. Williams, M, and Taylor, P. 2002. Graptolites from the Lisbellaw Inlier, Co. Fermanagh. British Geological Survey Internal Report, IR/02/089.