Stable Isotope Evidence for Diagenesis of the Ordovician Courtown and Tramore Limestones, South-Eastern Ireland
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Key.qxd 21/10/05 19:59 Page 25 STABLE ISOTOPE EVIDENCE FOR DIAGENESIS OF THE ORDOVICIAN COURTOWN AND TRAMORE LIMESTONES, SOUTH-EASTERN IRELAND MARCUS M. KEY, Jr., PATRICK N. WYSE JACKSON, WILLIAM P. PATTERSON and M. DUSTIN MOORE (Received 7 October 2004. Accepted 13 April 2005.) Abstract Use of Lower Palaeozoic carbonate fossils for palaeothermometry as derived from stable isotope geochemistry is increasing. With all these studies, there is the concern of how pristine the skeletal carbonate is in relation to diagenesis. In this study, we compared the stable isotopes of calcitic brachiopod and bryozoan allochems with secondary cements to determine if the skeletal isotopes were reset during diagene- sis. The study was conducted on the Courtown and Tramore Limestone Formations of the Ordovician (Llanvirn–Caradoc) Duncannon Group, which outcrops in south- eastern Ireland. These formations consist of calcareous mudrocks and argillaceous limestones. Bryozoans and brachiopods are common allochems in both formations. The Duncannon sediments were cleaved, folded and faulted by compression asso- ciated with the Caledonian and Hercynian orogenies. Bulk rock samples were slabbed and thick-sectioned. Fifty three ≥20µg samples of carbonate from bry- ozoans, brachiopods and matrix (cements and veins) were collected with 1µm spa- tial precision using a computer-driven micromilling device to minimise mixing of allochems and matrix. Results indicate ␦13C values of –8.8‰ to ϩ0.3‰ VPDB and ␦18C values of –17.8‰ to –11.5‰ VPDB. In the Courtown samples, the brachio- pod and bryozoan ␦18C values were consistently more similar to each other than either was to the matrix, but this was not true for the Tramore samples. This suggests different alteration histories (more likely) or a different source of carbonate for the Courtown matrix (i.e. sparry cement) as opposed to that of the Tramore matrix (i.e. abraded skeletal material) (the less likely explanation). Palaeotemperature calcula- tions from the oxygen isotopes (78°C to 128°C) indicate exposure to isotopically light groundwater or meteoric fluids following the previous and hotter Caledonian and Hercynian orogenies. All three phases of alteration would have overprinted any original palaeoenvironmental signal that might once have existed. Introduction is a well-established approach for geoscientists. Originally this work was done on relatively The use of carbonate fossils for palaeothermom- young fossils (such as those from the Cenozoic etry as derived from stable isotope geochemistry era) with obviously unaltered original skeletal Irish Journal of Earth Sciences 23 (2005), 25–38. © Royal Irish Academy 25 Key.qxd 21/10/05 19:59 Page 26 26 Irish Journal of Earth Sciences (2005) mineralogy (e.g. aragonite). Because of the interbedded blue-gray argillaceous nodular effectiveness of this tool, its use has been limestone beds and contain a rich shelly fauna. expanded (e.g. Stanton et al. 2002) to include Bryozoans and brachiopods are common older fossils in which diagenetic alteration of the allochems in both formations. The two units sit skeletal mineralogies is more difficult to deter- unconformably on sediments of the Lower mine (for example, those with low-to-high Mg Ordovician Ribband Group (Brück et al. 1979; calcite). With all these studies, there is the con- Carlisle 1979; Tietzsch-Tyler and Sleeman cern about (1) how pristine the skeletal carbon- 1994b; 1995b; Sleeman and McConnell 1995b; ate is in relation to diagenesis and (2) whether or Harper and Parkes 2000), which has been shown not diagenetic cements were included in the to be Arenig in age on the basis of graptolites sample of skeletal carbonate. (Skevington 1970; Rushton 1996) and does not In this study, we compare the light-stable extend into the Llanvirn as once thought isotopes of low-Mg calcite brachiopod and bry- (Brenchley et al. 1977). The age of various ozoan allochems from the Ordovician Duncan- formations in the Duncannon Group have been non Group with their surrounding matrix to constrained using trilobites, brachiopods, grap- determine if the skeletal isotopes were reset dur- tolites and microfossils (Harper and Parkes ing diagenesis. Our first hypothesis was that if 2000), and here we follow the recent revised the brachiopod and bryozoan isotope values correlation for the Ordovician (Fortey et al. were consistently more similar to each other 1995; 2000). than either was to the matrix and/or cements, then there was minimal diagenetic alteration of Courtown Limestone Formation the geochemistry of the allochems, and thus The Courtown Limestone Formation, which their skeletal carbonate would be amenable for crops out at various localities close to the town use in palaeothermometry. Our second hypothe- of Courtown, Co. Wexford (Fig. 1D), comprises sis was that if all the ␦18O values were isotopi- a 50m-thick sequence of white to blue fossil- cally light, then there had been some iferous limestones, calcareous sandstones and post-depositional alteration confounding any siltstones, with a basal conglomerate developed original palaeoenvironmental signal in the in places (Gardiner 1974). Biogenic reworking allochems that might once have existed. in the limestones has resulted in some fragmen- tation of the allochems, particularly the crinoids Stratigraphy of the Duncannon Group (Crimes and Crossley 1968). A moderately rich brachiopod–trilobite fauna has been described This study was conducted on bryozoans and bra- from the Courtown Limestone Formation chiopods collected from the Ordovician Cour- (Brenchley and Treagus 1970; Mitchell et al. town and Tramore Limestone Formations of the 1972; Parkes 1994). It has been deformed, Duncannon Group, which crop out in south- which has produced a well-developed cleavage eastern Ireland (Fig. 1). The Duncannon Group in the formation. (Gardiner 1974), which occurs in a band trend- The age of the Courtown Limestone Forma- ing north-east to south-west from its southern tion has been debated by various authors. On extremity near Dungarvan, Co. Waterford, and palaeontological and stratigraphical evidence, north-eastwards to south County Wicklow, is Carlisle (1979) argues that the formation ranged part of the Leinster Terrane. It comprises a Mid- from Middle Llandeilo (a stage name that is now dle to Upper Ordovician suite (Upper Llanvirn not in use) to the Costonian Stage of the to Caradoc) of limestones, black shales and Caradoc, whereas Brück et al. (1979) give a basaltic, andesitic and rhyolitic arc volcanics Llandeilo age. The brachiopods (Mitchell et al. (Owen and Parkes 2000), of which the laterally 1972) suggest a Caradoc age, possibly extend- equivalent Courtown Limestone Formation and ing down into the Llandeilian age. However, in the Tramore Limestone Formation locally form a review of the geology of the region, Tietzsch- basal units. These both consist of gray calcare- Tyler and Sleeman (1995a; 1995b) give an older ous mudstones, siltstones and sandstones with age range of Late Llanvirn to Middle Llandeilo Key.qxd 21/10/05 19:59 Page 27 Key—Diagenesis of Ordovician limestones 27 BG Sil RG N Leinster Granite IRELAND UP RG B Courtown D A RG BG Sil RG UP Sil Precambrian Waterford RG with UP UP RF TG RG UP UP C Dungarvan Tramore B 0 20 km TRAMORE * COURTOWN * Courtown * Newtown Cove Ballinatray Bridge Harbour C 0 1.5 km D 0 1 km U. PALAEOZOIC DUNCANNON Campile Formation Ballinatray Formation Campile Formation GROUP DUNCANNON Courtown Limestone Fmn. Carrighalia Formation GROUP RIBBAND GP. Undifferentiated Tramore Limestone Fmn. RIBBAND GP. Tramore Shale Fmn. Felsic volcanics CAMBRIAN Fault Fault * Sample locality * Sample locality Fig. 1—Geological map of southeastern Ireland indicating position of the Duncannon Group (diagonally shaded) as well as Courtown and Tramore sampling localities. (A) Map of Ireland showing general area of interest; (B) modified from Owen and Parkes (2000, Fig. 1). (C) modified from Tietzsch-Tyler and Sleeman (1994a); (D) modified from Tietzsch-Tyler and Sleeman (1995a). Abbreviations: BG = Bray Group; RF = Ross Formation (probably part of the Duncannon Group and late Caradoc in age); RG = Ribband Group; Sil = Silurian; TG = Tagoat Group; UP = Upper Palaeozoic. Key.qxd 21/10/05 19:59 Page 28 28 Irish Journal of Earth Sciences (2005) for the formation. Here we are following the Dunabrattin Head (Sleeman and McConnell findings of Williams (1972), Carlisle (1979) and 1995b). Parkes (1994) and the recent correlation chart The Tramore Limestone Formation has been (Harper and Parkes 2000) that shows the forma- assigned an age range from the Middle tion to be exclusively Caradoc in age (base: Llandeilo to the Costonian stage of the Caradoc Aurelucian stage, Velfreyan substage, gracilis on the basis of shelly faunas (Williams et al. biozone; top: Aurelucian stage, Costonian sub- 1972a; Brenchley et al. 1977; Carlisle 1979; stage, gracilis biozone). It lies unconformably Parkes 1994; Sleeman and McConnell 1995b). on the Riverchapel Formation, the youngest part Conodonts suggest the E. lindstroemi subzone of of the Ribband Group, which has been dated as the P. serra biozone (Bergström and Orchard Lower Arenig on the basis of graptolites (Crimes 1985). In a recent revision of Ordovician and Crossley 1968; Skevington 1970; Harper chronostratigraphy, the age of the Tramore and Parkes 2000). Succeeding the Courtown Limestone Formation was given as ranging from Limestone Formation is the Ballinatray Forma- the Llandeilian stage of the Llanvirn series to the tion; shales at