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Rhuddanian GSSP, Dob's Linn, Moffat, Scotland

The Rhuddanian Stage is the lowest stage of the Silurian System and is named after Cefn-Rhuddan Farm, Llandovery, Wales.

Lithostratigraphic units: The Moffat Shale Group covers the boundary interval. Below the boundary is the Hartfell Shale Formation (48 m thick), consisting largely of pale grey mudstone with subordinate black shales and several interbedded bentonites. Above this is the 43 m-thick Birkhill Shale, which consist predominantly of black graptolitic shale with subordinate grey mudstones and meta-bentonites.

Locality:  Dob’s Linn is near the A708 between Moffat and Selkirk at Birkhill, just past the Grey Mare’s tail (waterfall) beauty spot. The GSSP section is just north of the Linn Branch Stream (national Grid Ref: NT 1962 1584). Earthwise has a useful excursion guide. 

Lithology and boundary definition

The Moffat Shale Group at Dob’s Linn is exposed in a series of faulted inliers formed by imbricate thrusts. The Moffat Shale was deposited as hemipelagites and distal microturbidite deposited on the eastern continental margin of Laurentia during the closure of the Iapetus Ocean.

The Upper Hartfell Shale comprises principally organic-poor (∼0.1–0.4% total organic carbon) grey shale, with no preserved graptolites or sedimentary structure with minor amounts of disseminated pyrite. This unit is interbedded with six, ∼5 to 20 cm thick bands of organic-rich (∼1–2% TOC) black to dark laminated grey shale. These bands contain abundant graptolites and disseminated pyrite.

The Lower Birkhill Shale Formation is black, organic-rich (0.63 to 1.69% total organic carbon), laminated, and graptolite-rich. Disseminated pyrite is abundant throughout, as regular sub mm layers parallel to lamination. Both units are interbedded with 1 cm to ∼5 cm bentonite horizons. Poorly developed low angle cleavage occurs through the section and peak metamorphism occurred at ∼340 °C during the mid Silurian (428–422 Ma), coinciding with the Scandian orogeny.

The base of the Rhuddanian Stage is defined at 1.6m above the base of the Birkhill Shale Formation, coinciding with the first appearance of the graptolite Akidograptus ascensus, defining the base of the A. ascensus Biozone at the first occurrence of graptolites Akidograptus ascensus and Parakidograptus praematurus.

Graptolite and chitinozoan biostratigraphy

Originally in the GSSP section (Williams, 1983, 1988) the GSSP point was regarded as coincident with the local base of the Parakidograptus acuminatus Biozone, marked by the first appearances of P. acuminatus s.l. and Akidograptus ascensus. However, since then in other regions it is clear that A. ascensus first appears below P. acuminatus, and the base of the Silurian is now regarded as coincident with the first appearance of A. ascensus in the section.

Resampling and systematic revision (Rong et al., 2008) have shown that A. ascensus first appears at the level of the GSSP at Dob's Linn, and that P. acuminatus s.s. first occurs 1.5 m higher in the section (Melchin & Williams, 2000), the lower occurrences of previous accounts now being referable to P. praematurus.. Therefore, the graptolite zonation at Dob's Linn includes an older A. ascensus Zone and a younger P. acuminatus Zone, like in other regions. The GSSP point, while not changed in its position, is therefore now regarded as the first appearance of A. ascensus, defining the base of the A. ascensus Biozone in the section.

Chitinozoans from the section do allow additional detail to be added and provide additional means for correlation from the GSSP section. The Armoricochitina reticulifera Biozone has been recognised, allowing correlation with the Newport Sands section of the Cardigan area in Wales (which has good graptolite control). Across the Ordovician–Silurian boundary, a fauna dominated by Cyathochitina and Ancyrochitina spp. and in ascending order attributed to the Ancyrochitina ellisbayensis, Ancyrochitina laevaensis and Belonechitina ?postrobusta biozones, allow correlation with several other sections in Laurentia and Baltica, and the global chitinozoan biozonation for the Silurian System (Verniers & Vandenbroucke, 2012).

Conodonts, brachiopods, trilobites

Conodonts are best represented in the late Ordovician, with Amorphognathus superbus (in Pleurograptus linearis graptolite zone), A. ordovicicus (in the Dicellograptus complanatus Zone and Anceps Bands A-E). In the overlying Normalograptus persculptus Zone, specimens of Amorphognathus sp. and Scabbardella sp. cf. S. altipes occur. A poor single element assigned tentatively to Oulodus? kentuckyensis (now assigned to genus Rexroadus); was found in the P. acuminatus Zone (Barnes & Williams, 1988), which in Anticosta Island (Québec, Canada) ranges through the Rhuddanian into the Aeronian( Zhang & Barnes, 2002). The conodont colour alteration index is 5-7, commensurate with peak metamorphism. In addition, inarticulate brachiopods are known from the late Ordovician D. complanatus and D. anceps zones (Williams and Lockley, 1983; Williams, 1988), and trilobites from the extraordinarius ‘Band’ (Lespe’rance, 1988).

Carbon isotopes and radiometric dating

Carbon isotope fluctuations from the late Ordovician (Hirnantian) through into the Rhuddanian provide a very clear global signature for environmental change across this boundary, with the most negative carbon excursion close to the base Rhuddanian (Underwood et al., 1997). The extraordinarius and early persculptus graptolite biozones of the Upper Ordovician coincide with the glaciation of southern Gondwana. The end of this glacial interval is marked by the deposition of black anoxic shales, beginning in the persculptus Biozone, and the overlying decline in d13C globally occurred as the climate returned to greenhouse conditions.

Multi-grain U– Pb zircon geochronology of bentonites ∼4.5 m below and ∼6 m above the base Rhuddanian has yield dates of 445.7±2.4 Ma and 438.7±2 Ma, respectively (Tucker et al., 1990). Mathematical modelling of these dates gives an age of 443±1.5 Ma for the Ordovician/Silurian boundary.

Sources: text and figures taken directly from Rong et al. (2008), Verniers, & Vandenbroucke (2012) & Finlay et al. (2010).



Barnes, C.R. & Williams, S.H. (1988). Conodonts from the Ordovician-Silurian boundary stratotype, Dob's Linn, Scotland. In: Cocks, L.R.M. & Rickards, R.B. (eds). A global analysis of the Ordovician-Silurian boundary, Bulletin of the British Museum (Natural History), 43, 31-40.

Finlay, A. J., Selby, D. & Gröcke, D. R. (2010). Tracking the Hirnantian glaciation using Os isotopes. Earth and Planetary Science Letters, 293, 339-348.

Lespe’rance, P. J. (1988). Trilobites. In: Cocks, L.R.M. & Rickards, R.B. (eds) A Global Analysis of the Ordovician–Silurian boundary. Bulletin of the British Museum (Natural History), Geology Series, 43, 359–376.

Melchin, M.J. & Williams, S.H. (2000). A restudy of the Akidograptine graptolites from Dob's Linn and a proposed redefined zonation of the Silurian stratotype. . In: Cockle P., Wilson G.A., Brock G.A., Engerbretsen, M.J. & Simpson A. (eds.), Palaeontology Down-Under 2000. Geological Society of Australia, Abstracts 61, pp. 63.

Rong, J., Melchin, M., Williams, S. H., Koren, T. N. & Verniers, J. (2008). Report of the restudy of the defined global stratotype of the base of the Silurian System. Episodes, 31, 315-318.

Tucker, R.D., Krogh, T.E., Ross Jr., R.J. & Williams, S.H. (1990). Time-scale calibration by high-precision U–Pb zircon dating of interstratified volcanic ashes in the Ordovician and Lower Silurian stratotypes of Britain. Earth Planet. Sci. Lett. 100, 51–58.

Underwood, C. J., Crowley, S. F., Marshall, J. D. & Brenchley, P. J. (1997). High-Resolution carbon isotope stratigraphy of the basal Silurian Stratotype (Dob's Linn, Scotland) and its global correlation. Journal of the Geological Society, 154, 709-718.

Verniers, J. & Vandenbroucke, T. R. (2006). Chitinozoan biostratigraphy in the Dob's Linn Ordovician-Silurian GSSP, Southern Uplands, Scotland. GFF, 128, 195-202.

Williams, S.H. (1983). The Ordovician-Silurian boundary graptolite fauna of Dob's Linn, southern Scotland. Palaeontology, 26, 605-639.

Williams, S H. (1988). Dob's Linn:-the Ordovician-Silurian boundary stratotype. In Cocks, L.R.M., and Rickards, R.B. (eds), A global analysis of the Ordovician-Silurian boundary. Bulletin of the British Museum (Natural History), 43, 17-30.

Williams, S. H. & Lockley, M. G. (1983). Ordovician inarticulate brachiopods from graptolitic shales at Dob's Linn, Scotland; their morphology and significance. Journal of Paleontology, 57, 391-400.

Zhang, S. & Barnes, C. R. (2002). A new Llandovery (early Silurian) conodont biozonation and conodonts from the Becscie, Merrimack, and Gun River formations, Anticosti Island, Québec. Memoir, Journal of Paleontology, 57 (Supplement to Vol. 76), 1-46.