In 2006, the base of the Sub-Boreal Baylei Zone, was named after its ammonite index species Pictonia baylei Salfeld, which was chosen as the base of the Kimmeridgian Stage by the Kimmeridgian Working Group of the Jurassic Subcommission. The Baylei Zone was first defined in the UK by Salfeld (1913), and Pictonia spp occur abundantly at many localities in the UK.
The Staffin Shale succession on the Isle of Skye (NW Scotland) is the most complete and richest in ammonites of all the UK successions which cover the Oxfordian/Kimmeridgian boundary interval. As well as yielding abundant ammonites belonging to the necessary Aulacostephanitid group, including Pictonia, of the Sub-Boreal Province, the succession in the Flodigarry section on Skye contains abundant Cardioceratids belonging to the Amoeboceras group characteristic of the Boreal Province.
The Flodigarry Shale Member of the Staffin Shale Formation, is developed as dark silty and shaley clays with bands of limestone nodules. The succession of dark shales is thus broken up by marker beds of limestone nodules (Morton & Hudson, 1995; Wright, 2001), with beds SS36 and SS40 being the most prominent.
Ammonite Biostratigraphy
Log of the Oxfordian/Kimmeridgian succession at Flodigarry close to the boundary, taken from Morton & Hudson (1995) and Matyja
et al. (2006).
Cymodoce Zone
SS45 Dark grey, very poorly fossiliferous clay with Rasenia cf. evoluta Salfeld and
Amoebites subkitcheni Spath, seen to 9 m
SS44 Distinctive bed of argillaceous sandstone forming a ridge, containing
Rasenia inconstans Spath. 0.15 m
SS43 Pale grey, poorly fossiliferous clay with R. inconstans. (3.2)
Baylei Zone: Normandiana Subzone
SS43 contd
Pictonia cf. normandiana Tornquist, Prorasenia hardyi Spath, Amoeboceras
bayi Birkelund and Callomon, A. cricki Salfeld (1.1) 4.3 m
SS42 Darker, silty clay with numerous P. hardyi, A. cricki and A. bayi preserved in
iridescent aragonite. 0.40 m
SS41 Pale grey, silty clay with crushed Pictonia normandiana, Prorasenia hardyi,
A. bayi, A. cricki, A. praebauhini (Salfeld), A. bauhini (Oppel), A. lineatum
(Quenstedt). 1.5 m
SS40 Band of large limestone nodules 0.15-0.45 m
SS39 Grey, silty clay with A. praebauhini, A. bauhini and A. lineatum. 1.8 m
:Densicostata Subzone
SS38 Black, shaley clay with abundant white, crushed ammonites: Pictonia
densicostata (Salfeld MS), A. praebauhini. A. bauhini. A. lineatum. 0.5 m
SS37 Series of grey, silty clays with Pictonia densicostata, Prorasenia bowerbanki
Spath, A. praebauhini. A. bauhini and A. lineatum. In the bottom metre
Pictonia is represented by P. flodigarryensis Matyja, Wierzbowski and Wright,
and Amoeboceras rosenkrantzi Spath and A. schulginae Mesezhnikov make
their last appearance. 4.3 m
SS36 Continuous band of large, lens-shaped limestone nodules 0.07 – 0.25 m
SS35 Medium to dark grey, slightly bituminous silty clays with Pictonia
flodigarryensis, Prorasenia bowerbanki, A. praebauhini, A.
aff. bauhini, A. rosenkrantzi and A. schulginae. (1.2)
Pseudocordata Zone: Evoluta subzone
SS35 contd
Ringsteadia evoluta Salfeld, Microbiplices/Prorasenia sp., A. rosenkrantzi (1.2)
:Pseudocordata Subzone
SS35 contd
Microbiplices microbiplex (Quenstedt), M. anglicus Arkell, A. marstonense,
A. rosenkrantzi. (4.0) 6.2 m
SS34 Impersistent layer of large, calcareous doggers 0.2m
Below SS34 come a further 30 m of hard, medium to dark grey, silty clays representing the lower 2 zones of the Sub-Boreal Upper Oxfordian. The fauna is however, dominated by Boreal Amoeboceras spp.
The boundary is defined on the first appearance of the genus Pictonia and specifically Pictonia flodigarryensis, 1.08 m below Bed SS36. The boundary at 1.08 m below bed SS36 also coincides with the traditional Boreal Oxfordian/Kimmeridgian boundary, that is marked by the incoming of the Amoeboceras bauhini group of ammonites. The last occurrence of the genus Ringsteadia indicating the latest Oxfordian, is at 1.24 m below Bed SS36. Correlation with the Sub-Mediterranean succession is gradually becoming clear, and it is already evident that the Sub-Mediterranean Planula Zone and Hauffianum Subzone, previously regarded as belonging to the Upper Oxfordian, are located in the Lower Kimmeridgian (Wierzbowski et al., 2016).
Geomagnetic polarity
The secondary marker for the boundary is a change in geomagnetic polarity close to the boundary. The base of reverse magnetozone F3r is located between 1.28 and 1.48 m (the two sampling points straddling the polarity boundary) below the base of bed SS36, so the ammonite marker and the upwards change to reverse polarity are a minimum of ~20 cm from each other (maximum of 40 cm). Assuming that the base of F3r correlates to the base of marine magnetic anomaly M26r (Przybylski et al., 2010), and using the marine magnetic anomaly age scale, this equates to an age difference of no more than ~30 kyrs. Therefore, the boundary of magnetozones F3n–F3r provides a fairly precise secondary marker for the base of the Kimmeridgian in the Flodigarry section and so provides an additional correlation marker into other marine and non-marine strata (Wierzbowski et al., 2016).
Other stratigraphic tools
Rhenium–Osmium (187Re–188Os) dating of black shale samples at Flodigarry from 1.20– 1.40 m below the base of bed SS36, yield an age of 154.1 (± 2.2) Ma (Selby, 2007). This level corresponds close to the base of the Kimmeridgian Stage (Pseudocordata-Baylei zone boundary placed at the base of the Pictonia flodigarriensis horizon of the Subboreal subdivisions) but closer to the magnetozone F3n–F3r boundary.
Analysis of the carbon isotope composition of both marine carbonates and terrestrial organic matter shows the presence of a gradual fall in δ13C values in the Upper Oxfordian and the lowermost Kimmeridgian. Like other sections across the boundary, the most negative δ13C from values occur in the early part of the Densicostata Subzone seen from well preserved belemnite rostra in the Flodigarry section (Nunn et al., 2009).
Visiting the site:
The site is within Trotternish Ridge Site of Special Scientific Interest. It lies on the foreshore just below high tide mark, and is freely accessible. Field guides to the area are available to the Staffin Coast, Raasay and to Skye.
Consent to collect rock material: This must be obtained from Scottish Natural Heritage, Bridge Road, Portree, Skye, IV51 9ER. However, permission must first be obtained from the Owner, SGRPID (Scottish Government and Payments Directorate), Estates Office, Portree, Isle of Skye, IV51 9DH, Tel. +44 1478 612 516.
(John Wright & MWH)
Bibliography
Matyja, B.A., Wierzbowski, A. and Wright, J.K. 2006. The Sub-Boreal/Boreal ammonite succession at the Oxfordian/Kimmeridgian boundary at Flodigarry, Staffin Bay (Isle of Skye), Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 96, 387-405.
Morton. N. and Hudson, J.D. 1995. Field Guide to the Jurassic of the Isles of Raasay and Skye, Inner Hebrides, N.W. Scotland. In Taylor, P.D. (ed.) Field Geology of the British Jurassic. Geological Society of London, 267-278.
Nunn E.V., Price G.D., Hart M. B., Page K. N., Leng M.L. 2009. Isotopic signals from Callovian–Kimmeridgian (Middle–Upper Jurassic) belemnites and bulk organic carbon, Staffin Bay, Isle of Skye, Scotland. Journal of the Geological Society of London, 166, 633–641.
Przybylski P.A., Ogg J.G., Wierzbowski A., Coe A.L., Hounslow M.W., Wright J.K., Atrops F., Settles E., 2010. Magnetostratigraphic correlation of the Oxfordian-Kimmeridgian boundary. Earth and Planetary Science Letters, 262, 267–288.
Salfeld, H. 1913. Certain Upper Jurassic Strata of England. Quarterly Journal of the Geological Society of London, 69, 423-432.
Selby D. 2007. Direct Rhenium–Osmium age of the Oxfordian–Kimmeridgian boundary, Staffin Bay, Isle of Skye, U.K., and the Late Jurassic time scale. Norwegian Journal of Geology, 87, 291–299.
Wierzbowski, A., Atrops, F., Grabowski, J., Hounslow, M.W., Matyja, B.A., Oloriz, F., Page, K.N., Parent, H., Rogov, M.A., Schweigert, G, Wierzbowski, H and Wright, J.K. 2016. Towards a consistent Oxfordian-Kimmeridgian global boundary: current state of knowledge. Volumina Jurassica, XIV, 14–49.
Wierzbowski. A., Coe, A.L., Hounslow, M.W, Matyja, B.A., Ogg, J.G., Page, K.N., Wierzbowski, H. & Wright, J.K. 2006. A potential stratotype for the Oxfordian/Kimmeridgian boundary: Staffin Bay, Isle of Skye, UK. Volumina Jurassica, IV, 17-33.
Wright, J.K. 2001. Staffin. In: British Upper Jurassic Stratigraphy (Oxfordian to Kimmeridgian), Geological Conservation Review Series No. 21, (J.K. Wright and B.M. Cox), Joint Nature Conservation Committee, Peterborough, pp. 201-210.