Ian D. Somerville

Late Dinantian (Lower Carboniferous) platform carbonate stratigraphy of the Buttevant area North Co. Cork, Ireland

A thick sequence of late Dinantian (Asbian–Brigantian) carbonates crop out in the Buttevant area, North Co. Cork, Ireland. A mud-mound unit of early Asbian age (the Hazelwood Formation) is the oldest unit described in this work. This formation is partly laterally equivalent to, and is overlain by, over 500 m of bedded platform carbonates which belong to the Ballyclogh and Liscarroll Limestone Formations. Four new lithostratigraphic units are described within the platform carbonates: (i) the early Asbian Cecilstown Member and (ii) the late Asbian Dromdowney Member in the Ballyclogh Limestone Formation; (iii) the Brigantian Templemary Member and (iv) the Coolbane Member in the Liscarroll Limestone Formation. The Cecilstown Member consists of cherty packstones and wackestones that are inferred to have been deposited below fair-weather wavebase. This unit overlies and is laterally equivalent to the mud-mound build-up facies of the Hazelwood Formation. The Dromdowney Member is typified by cyclic-bedded kamaenid-rich limestones possessing shell bands, capped by palaeokarst surfaces, with alveolar textures below and shales above these surfaces. The carbonates of this unit were deposited at or just below fair-weather wavebase, the top of each cycle culminated in subaerial emergence. The Templemary Member consists of cyclic alternations of subtidal crinoidal limestones capped by subtidal lagoonal crinoid-poor, peloidal limestones possessing coral thickets. Intraclastic cherty packstones and wackestones characterize the Coolbane Member, which is inferred to have been deposited below fair-weather wavebase but above storm wavebase. The early Asbian Cecilstown Member has a relatively sparse micro- and macrofauna, typified by scattered Siphonodendron thickets, archaediscids at angulatus stage and common Vissariotaxis. Conversely, macro- and microfauna is abundant in the late Asbian Dromdowney Member. Typical late Asbian macrofossils include the coral Dibunophyllum bipartitum and the brachiopod Davidsonina septosa. The base of the late Asbian (Cf6γ Subzone) is recognized by the first appearance of the foraminifers Cribrostomum lecompteii, Koskinobigenerina and the alga Ungdarella. The Cf6γ Subzone can be subdivided into two biostratigraphic divisions, Cf6γ1 and Cf6γ2, that can be correlated throughout Ireland. Relatively common gigantoproductid brachiopods and the coral Lonsdaleia duplicata occur in the Brigantian units. The base of the Brigantian stage (Cf6δ Subzone) is marked by an increase in the abundance of stellate archaediscids, the presence of Saccamminopsis-rich horizons, Loeblichia paraammonoides, Howchinia bradyana and the rarity of Koninckopora species. Changes in facies at the Cecilstown/Dromdowney Member and the Ballyclogh/Liscarroll Formation boundaries coincide closely with the changes in fossil assemblages that correspond to the early/late Asbian and the Asbian/Brigantian boundaries. These facies changes are believed to reflect major changes in relative sea-level on the Irish platforms. The sea-level variations that are inferred to have caused the facies changes at lithostratigraphic boundaries also brought in the new taxa that define biostratigraphic boundaries. Moreover, many of the Dinantian stage boundaries that are defined biostratigraphically in Great Britain, Belgium and the Russian Platform also coincide with major facies boundaries caused by regressive and transgressive episodes. The integration of detailed biostratigraphic analyses with facies studies will lead to better stratigraphic correlations of Dinantian rocks in northwest Europe.

A revised correlation of Carboniferous rocks in the British Isles

The report revises and expands upon the 1976 and 1978 publications for the Dinantian and Silesian, respectively, combining them into a single account of British and Irish Carboniferous stratigraphy. The need to update the two Special Reports reflects the considerable advances in Carboniferous geology over the last 30 years. The report covers developments in international chronostratigraphy and incorporates wholesale reassessments of British lithostratigraphy. A huge volume of biostratigraphical information has been published over recent decades and the report summarizes the key information. Carboniferous rocks have long been of economic importance, but it is the search for hydrocarbons, in its infancy at the time of the previous reports, which has greatly increased our understanding of Carboniferous successions offshore and at depth, particularly in southern and eastern England.

Irish Dinantian biostratigraphy: practical applications

Abstract For the last two decades there have been considerable advances in the dating and correlation of marine Dinantian carbonate sequences in Ireland, as elsewhere in Europe, primarily through the increased precision made possible by the use of microfossils, such as conodonts, foraminifers and miospores, together with macrofossils such as rugose corals. The advance was accelerated by the availability of boreholes up to 2km deep drilled by mineral exploration companies in Ireland who routinely use biostratigraphy. Detailed biostratigraphic biozonations are now established and are used for geological mapping and borehole zonation. Data are assessed and attempts are made to highlight some of the practical problems encountered in locating Dinantian stage boundaries, recognizing biozones in Ireland, and suggesting correlations with other biozonation schemes in Europe. Although there are many difficulties in recognizing each of the stage boundaries in Ireland, two in particular present major problems — the Courceyan/Chadian and Holkerian/Asbian boundaries. From the work of Conil, Groessens and co-workers in Belgium, conodont and foraminiferal biozonation has been applied to the British and Irish Dinantian stages. Unfortunately, the bases of both the Chadian and Asbian stages rarely contain the zonal taxa. Also, at both stratigraphic levels, there are difficulties in comparing basinal and platform faunas. New biostratigraphic data in Ireland have permitted the recognition of two new intervals within the late Asbian Cf6γ Subzone, referred to informally as Cf6γ1 and Cf6γ2. Until the Dinantian stages in Britain and Ireland are redefined biostratigraphically in the existing stratotype sections, or new stratotype sections are defined with faunal criteria, it is becoming more expedient and practical to recognize and define biozones in Ireland which can be identified and correlated with other Dinantian sections in continental Europe. This paper presents correlations with Belgian and Russian biostratigraphic schemes.

Permo-Carboniferous carbonate platforms and reefs

The results are presented from an integrated sedimentologic, structural, stratigraphic, and diagenetic study of syndepositional faults and fractures that cut the Permian Capitan reef and equivalent platform strata within three outcrop windows in Slaughter Canyon, Guadalupe Mountains, New Mexico. The studied faults are dip slip, have displacement of up to 30 m parallel the shelf margin, and are vertically and laterally segmented. The faults dip steeply shelfward and basinward, and grew incrementally during deposition of the Yates and Tansill Formations. Steep polycyclic paleocavern systems developed along faults and fractures. These can have cumulative vertical relief of more than 270 m, extend at least 1.2 km along strike, and are typically less than 10 m wide but can attain a width of 90 m. The paleocaverns can extend more than 110 m below the top of the Capitan reef. The paleocaverns are filled mainly with sediments deposited during Capitan progradation and aggradation. Seven Permian lithologies are distinguished: (1) limestones and limestone breccias, (2) microspar-lithified breccias, (3) carbonate-rich breccias, (4) reworked and remnant breccias, (5) beige dolomitic siltstone–sandstone and associated breccias, (6) pink dolomitic siltstone–sandstone and associated breccias, and (7) spar-cemented breccias. The fills vary vertically and along strike within the paleocaverns but have a clear organization and stratigraphy. The integration of the stratigraphy of the paleocaverns with the structural and sequence stratigraphic framework developed in back-reef strata provides evidence for incremental fault growth and multiple episodes of dissolution, brecciation, collapse, deposition, cementation, and dolomitization within the paleocaverns during Capitan times. Solution-modified syndepositional faults extend at least 33 km along strike from Slaughter Canyon and are considered to be an integral component of the Capitan platform. The fault-zone paleocaverns contain a unique internal record of events and processes that have no counterpart in the shelf succession. Their study provides new insights into the internal heterogeneity and diagenesis of the Capitan platform. The results have important implications for the Capitan platform, and more generally for the heterogeneity of syndepositional fracture-controlled karst systems formed in carbonate platforms with steep unstable margins, subject to compaction-induced tilting and/ or developed in active tectonic settings. Permo-Carboniferous Carbonate Platforms and Reefs SEPM Special Publication No. 78 and AAPG Memoir 83, Copyright

Late Asbian to Brigantian (Mississippian) foraminifera from southeast Ireland: comparison with northern England assemblages

Foraminiferal assemblages from platform carbonates in the Carlow district (SE Ireland) are analysed. This platform contains a near-continuous succession of Upper Asbian to lower Upper Brigantian strata. Detailed sampling of several quarry and borehole sections allows characterization of foraminiferal assemblages throughout the succession. Assemblages typifying the Late Asbian, Early Brigantian and Late Brigantian are described, with the most common genera and species, as well as the guides for the recognition of these substages. In addition, three successive faunal events are recognized within the Early Brigantian. A comparison with northern England foraminiferal assemblages from the Asbian and Brigantian stratotype sections shows a great similarity in the recorded taxa. Furthermore, these taxa have closely comparable stratigraphical ranges, demonstrating the biostratigraphic utility of these foraminifera throughout Ireland and Britain. Taxa proposed here as guides for the basal Brigantian are potentially an alternative to the previously published taxa (which are either unrecorded, or recorded at higher stratigraphic levels in the Brigantian).

Mid-Dinantian Waulsortian buildups in the Dublin Basin, Ireland

Abstract The sedimentary history and biostratigraphic setting of Waulsortian carbonate buildups of the Feltrim Limestone Formation (late Courceyan to early Chadian) within the Dublin Basin are described. There is no unique precursor or successor facies to this formation, and the massive Waulsortian banks are composed predominantly of peloidal, skeletal wackestones and lime mudstones with packstones near the tops of banks. These banks form tabular bodies of moderate relief and are interbedded with thin shales and argillaceous crinoidal limestones of inter-bank facies. In the southwest of the basin inter-bank facies are rare and the bank facies have abundant stromatactis cavities, and uniquely at Roselawn a fauna of rugose corals. All buildups in the Dublin Basin have Phase C and/or D component assemblages of Lees and Miller (1985) and are interpreted as accumulating in moderately shallow-water depths, near or within the photic zone. Isopachs for the Feltrim Limestone Formation show a NE-SW-trending axial depocentre where the Waulsortian facies is in excess of 400 m thickness. Deposition appears to have taken place on this “double-sided” ramp, in a manner similar to the model of Lees (1982) for Belgium and southern Britain. Soft-sediment deformation such as large-scale slumping, shale-injections and water-escape structures, not previously recorded from these rocks is widespread. The upper surface of the Feltrim Limestone Formation is fissured and displays a prominent erosion surface. Termination of Waulsortian facies deposition and influx of terrigenous sediment was caused by rapid uplift, attributed to Chadian tectonism. However, eustatic sea-level fall cannot be ruled out as a partial cause of the demise of the Waulsortian.

Controls on ramp, platform and basinal sedimentation in the Dinantian of the Dublin Basin and Shannon Trough, Ireland

Abstract In the Dublin Basin a Courceyan ramp phase of sedimentation was followed in the Chadian by tectonic break-up of the basin into distinct shallow-water platforms, on which production of carbonate sediments continued in considerable volume, and ‘deep’ basinal areas in which it ceased. Progradation of the platforms across these basinal areas was limited, and mainly confined to the dip-slope of hanging wall blocks; progradation across fault scarps was rare. In the Shannon Trough basement-fault control was evident in the distribution and migration patterns of volcanic centres in the Chadian to Arundian, but despite this, ramp sedimentation occurred throughout the Dinantian, evolving into a purely constructional large platform by late Dinantian time. There was no break-up of the basin as in the case of the Dublin Basin. The reason for the contrasting behaviour of the two basins is related to the rate of upwards movement of extensional faults relative to sedimentation rates. In the Dublin Basin these faults penetrated to the palaeosurface to form scarps by the late Chadian, and this topography survived into the Brigantian. In the Shannon Trough these faults failed to surface, but deep basement structures controlled the distribution of Dinantian volcanic centres which lie on a series of ENE-trending lineaments. These lineaments, which parallel the axis of the Shannon Trough, almost certainly mark the traces of active down-to-basin faults that controlled its half-graben structure. The basement rocks of the two basins are clearly of a different nature; the Dublin Basin is floored by basement of a much more heterogeneous nature than the Shannon Trough, the former lying south and the latter north of the putative Iapetus Suture line.

New Foraminifers in the Visean/Serpukhovian Boundary Interval of the Lower Limestone Formation, Midland Valley, Scotland

Abstract The biostratigraphy of the upper part of the Mississippian Lower Limestone Formation in the Midland Valley, Scotland is revised using foraminiferal assemblages. This formation was previously assigned exclusively to the upper part of the Brigantian Substage (upper Cf6δ, P2 or latest Visean), whereas the succeeding Limestone Coal Formation (barren of foraminifers and conodonts) was assigned to the Pendleian Substage (lower Cf7, E1 or early Serpukhovian). The foraminifers, in particular those recorded from the Second Hosie and Top Hosie limestones and their lateral equivalents (Anvil and MacDonald limestones), are comparable to Serpukhovian assemblages from the Ukraine and Russia. Consequently, the Visean/Serpukhovian stage boundary is repositioned at the base of the Second Hosie Limestone. This assignment of the Second Hosie Limestone to the Pendleian is reinforced by the first occurrence of the ammonoid Emstites (Cravenoceras), although it occurs 1 m below the Top Hosie Limestone. Pendleian foraminiferal assemblages from northern England, its geographically closest equivalent, are similar but not well enough known for a detailed comparison. A few similarities are found between the foraminiferal assemblages from the Midland Valley and those from southwest Spain and North African basins. Faunas from the Second and Top Hosie limestones and their lateral equivalents allow us to propose an assemblage zone, valid for regional correlations within the British Isles, and the faunas can be potentially used as Serpukhovian markers for western Paleotethyan basins. Two new genera and species are described, Praeplectostaffella anvilensis n. gen. n. sp. and Praeostaffellina macdonaldensis n. gen. n. sp., and two new species, Tubispirodiscus hosiensis n. sp. and Euxinita pendleiensis n. sp.

Potential foraminiferal markers for the Visean–Serpukhovian and Serpukhovian–Bashkirian boundaries—a case-study from Central Morocco

Abstract The Carboniferous succession in Adarouch (Central Morocco, north of the Atlas Transform Fault) contains thick carbonate beds including upper Visean, Serpukhovian and basal Bashkirian rocks. Foraminifers enable precise recognition of the Visean/Serpukhovian (V/S), early/late Serpukhovian (eS/lS) and Serpukhovian/Bashkirian (S/B) boundaries. Insolentitheca horrida, Loeblichia ukrainica, “Millerella” spp. and Endostaffella? sp. 2 are regarded as regionally useful indices to the V/S boundary, whereas Eostaffellina spp., Eostaffella pseudostruvei and some evolved species of Archaediscus exhibit greater reliability for worldwide correlation of this level. Similarly, the eS/lS boundary is marked locally by Brenckleina rugosa, Eosigmoilina sp., and Monotaxinoides spp. and globally by Loeblichia minima, Bradyina cribrostomata, Plectostaffella spp., Eostaffellina “protvae” and “Turrispiroides”, and the S/B boundary is marked locally by Globivalulina bulloides and globally by Seminovella elegantula, and Novella?. Occurrences of these taxa in Morocco allow correlations with the Moscow Basin, the Urals, the Donetz Basin and North America. The Moroccan assemblages share few taxa in common with Saharan basins south of the Atlas Transform Fault. Correlations with western European basins are difficult because of the paucity in the latter of foraminiferal-bearing carbonate strata.

The Carboniferous Archerbeck Borehole, near Canonbie (Dumfriesshire, southern Scotland): biostratigraphic revision of the late Asbian to early Pendleian succession using foraminiferans and regional correlations

The upper half of the Archerbeck Borehole contains a continuous Mississippian succession from the late Asbian (late Visean) to the Pendleian (early Serpukhovian), with numerous limestone horizons. The borehole sequence lies within the Solway Basin (western end of the Northumberland Trough) and bridges the successions between the Midland Valley of Scotland and the Pennines (northern England). The rich foraminiferal and algal assemblages are compared to those described by previous authors, and genera as well as species are updated as far as is possible. In addition, some other overlooked or underestimated taxa have been identified and illustrated, which improve notably the biostratigraphic resolution of the Archerbeck Borehole succession. Within the abundant foraminiferal assemblages can be highlighted the richness and diversity of representatives of the family Archaediscidae. In general, the succession developed in the Archerbeck Borehole is closely comparable with that in the Alston Block of northern England. The Asbian/Brigantian transition mimics that observed in the Janny Wood boundary stratotype section, with clear late Asbian horizons (lower part of the Archerbeck Beds), transitional beds (middle and upper parts of the Archerbeck Beds), and Brigantian limestones (from the Cornet Limestone upwards). The base of the early Brigantian is placed at the base of the Cornet Limestone (equivalent to the Lower Peghorn Limestone in the Alston Block). The base of the late Brigantian is placed at the base of the Gastropod Limestone (equivalent to the Scar Limestone Member in northern England), and the base of the Pendleian is repositioned at the base of the Under Limestone (equivalent to the Four Fathom Limestone Member in northern England). Throughout, the borehole, protista and microfloral elements are abundant, which allows the recognition of Assemblages 4 to 10, previously recognised in northern England and the Midland Valley of Scotland. Furthermore, other local assemblages are recognised and related to palaeoecological controls, although they do not represent any improvement in biostratigraphical resolution.