The sequence of ammonite correlated horizons in the British Sinemurian (Lower Jurassic)

Abstract

The meaning and usage of Horizons is discussed and then applied to the Sinemurian (Lower Jurassic) sequence within the Lower Lias Group of the British Isles. These rocks are of fundamental importance in Lower Jurassic stratigraphy and have yielded the type specimens of 15 of the 20 zonal and subzonal indices of the NorthWest European Standard for the Sinemurian Stage. By the recognition of Horizons a threefold increase in the resolution of the ammonite-correlated "time-scale" or Standard Zonation for the Sinemurian is possible (with 61 potentially correlatable hori zons identifiable within 17 established subzones). Comparisons and correlations with early and recent detailed subdivisions of the Sin em urian Stage are discussed. Resume. La signification et I'utilisation des horizons sont discutees et ensuite appliquees a la sequence Sinemurienne Ourassique inferieur) dans Ie groupe Lias inferieur des lies Britanniques. Ces roches sont d'une importance fondamentale dans la stratigraphie du Jurassique inferieur et en ont donne les specimens types de 15 a 20 indices zonaux et sous-zonaux du Standard Nord-Ouest Europeen pour l'etage Sinemurienne. Par la reconnaissance des horizons une echelle plus detaillee est possible et correlee avec les zones et sous-zones du Sinemurien. Les 61 horizons sont identifiables en 17 zones etablies. Les comparaisons et les correlations avec des anciennes et des recentes subdivisions detaillees de I'etage Sin emu rienne sont examinees.

From Oppel to Callomon (and beyond): building a high-resolution ammonite-based biochronology for the Jurassic System

“…). To help distinguish between Callomonian ‘faunal horizons’ and French ‘Horizons’ (which are effectively sub‐subzones), the term biohorizon is preferable as adopted by Page (, ) – although hemera would still be its time duration on a geochronological scale.…”

“…Application of a similar methodology subsequently led to the establishment of further biohorizonal schemes in the UK and biogeographically related areas for the Hettangian, Sinemurian, Lower Toarcian, Bathonian, Lower Callovian and across the Callovian-Oxfordian boundary and part of the Middle Oxfordian (Page 1992(Page , 2004(Page , 2005(Page , 2009 . This is not to imply, however, that other advances have not been made in the UK, as J.C.W.…”

“…But what are these observed ammonite 'events' and what is controlling them and facilitating their observation in the geological record? An attempt at answering this question considered the Upper Sinemurian biohorizonal sequence of Page (1992, as consolidated by Dommergues et al 1994 and recognized five main controlling factors (Page 1995;see Fig. 4 here but renumbered following Page 2009Page , 2010:…”

“…The most important aspect, however, is that biohorizonal schemes actually work for correlation purposes, suggesting that whatever are the controlling factors, many individual biohorizons could have a very wide geographical application. In the UK Sinemurian, for instance, the scheme of Page (1992) can be applied from northwest Scotland to southwest Englandand after further collaborative investigation and tweaking was also found to work very well across France to Switzerland as well (Dommergues et al 1994)effectively across the entire North West European ammonite Bioprovince (sensu Page 2008). Inevitably, however, some of the finest (bio-)stratigraphical detail might only be recognizable within parts of basins where more continuous sedimentation has provided a more complete record.…”

The palynology of the Pabay Shale Formation (Lower Jurassic) of SW Raasay, northern Scotland

“…The strata are Hettangian to Late Bajocian in age and the succession is approximately 600 m thick (Cope, Duff et al 1980;Cope, Getty et al 1980;Cox and Sumbler 2002;Simms et al 2004). It comprises six formations which are, in ascending stratigraphical order, the Breakish Formation, the Ardnish Formation, the Pabay Shale Formation, the Scalpay Sandstone Formation, the Portree Shale Formation, the Raasay Ironstone Formation and the Bearreraig Sandstone Formation (Lee 1920;Howarth 1956;Morton 1965;1976;Getty 1973;Page 1992;Morton and Hudson 1995;Hesselbo et al 1998;Morton 1999a, b;Cox and Sumbler 2002;Fig. 1).…”