Waterloo Bay, Larne, Northern Ireland: A potential Global Stratotype Section and Point (GSSP) for the base of the Jurassic System
Michael J. Simms and Andrew J. Jeram
Since the pioneering work of Portlock (1843) and Tate (1864), late Triassic and early Jurassic strata of Northern Ireland have remained neglected. Only now is it becoming apparent that the early Jurassic succession in Northern Ireland is exceptionally well-developed, if only very patchily exposed. One site in particular offers significant potential for defining the base of the Jurassic System.
At Waterloo Bay, on the east coast of Northern Ireland near the ferry port of Larne, the foreshore exposes an almost uninterrupted succession from the upper part of the Mercia Mudstone Group (Triassic, Norian) through to the Bucklandi Zone of the Lias Group (Jurassic, Sinemurian).
PDF files of the 2 posters (poster 1; poster 2) displayed at the Jurassic Congress are available to download (these files are at A3 size and hence the ammonites on poster 2 are illustrated at a 50% reduction compared with the original A1 posters).
View, from the adjacent cliff top, of part of the foreshore exposed in Waterloo Bay at low tide. Strata dip to the north-west and the section seen here extends from the Cotham Member (Rhaetian, Penarth Group) on the far right through the Langport Member and basal Lias Group to the basal beds of the Johnstoni Subzone of the Planorbis Zone (Hettangian) on the extreme left.
The Triassic-Jurassic boundary beds (upper Penarth Group to basal Lias Group) at Waterloo Bay are proving to be superior in many respects to correlative strata at St. Audrie’s Bay, in south-west Britain, a site which, for decades, has been cited as a candidate Global Stratotype Section and Point (GSSP) for the base of the Jurassic System.
What has the Larne section to offer as a potential GSSP?
The Triassic-Jurassic boundary succession at Larne is significantly thicker, and experienced more continuous deposition, than correlative strata at St Audrie’s Bay or at any other site in NW Europe. In particular, the depth of the Larne Basin ensured that there was significant accommodation space for sediment during lowstand episodes, commonly represented by condensed sequences or even hiatuses at other sites.
The greatly expanded succession in the Larne Basin, compared with correlative strata elsewhere in the UK and beyond, offers the potential for refinement of the isotope curve of Hesselbo et al. (2002) to a significantly higher resolution.
Potential for global correlation
Two orders of sedimentary cycles, largely unaffected by diagenetic limestones, can be recognised at this site. Analysis, combining sequence stratigraphy and cyclostratigraphy, indicates that these cycles reflect eustatic fluctuations (see website for more detailed discussion). As such they have significant potential for global correlation. A major seismite unit and associated desiccation-cracked bed in the Cotham Member of the Penarth Group (taken as the base of the measured section above) provides a datum that has already been correlated across the UK (Simms 2003, Geology, 31, 557-560; click here for more information on this part of the succession).
A more detailed, albeit preliminary, discussion of the nature and significance of this sedimentary cyclicity can be downloaded by clicking here.
Diverse macro- and microfauna
The fossil macro- and microfauna (ammonites, bivalves, gastropods, echinoids, crinoids, trace-fossils, vertebrates, ostracods, foraminifera) are abundant and diverse. Data for the nearby Larne borehole are already published (Ivimey-Cook 1975, Bull. Geol. Surv. G.B., 50, 51-69; Warrington and Harland 1975, Bull. Geol. Surv. G.B., 50, 37-50). The diversity and preservation of the earliest Jurassic ammonite fauna in the Larne Basin is superior to that at St. Audrie’s Bay or at any other UK site.
Small shale block from the basal Planorbis Subzone (lower part of Bed 25) containing typical Lias Group macrofauna; Diademopsis tomesi (echinoid plates), upper left; Modiolus cf. minimus (bivalve), lower left; Psiloceras ? tilmanni (ammonite), right. Width of block about 6 cm.
The basal Jurassic ammonite succession in north-east Ireland is exceptionally well-developed and represented by various species of Psiloceras, Neophyllites and Caloceras. Although material is commonly crushed, well-preserved specimens of most species has been recovered from carbonate nodules or as pyrite casts. Preservation of these specimens is superior to that of any in situ occurrences in the UK.
Large Psiloceras erugatum (diameter: 68mm) from Bed 24, showing exceptionally long development of ribbing. This is the earliest ammonite recovered from the section at Waterloo Bay.
Pyrtitic internal cast of Neophyllites antecedens (diameter: 32mm) from near the top of Bed 26.
Ex-situ specimen of Caloceras johnstoni (diameter: 78mm), probably from a localised early diagenetic carbonate lens in Bed 35 or 36.
Possible limitations of the Waterloo Bay site
The Triassic-Jurassic boundary section at Waterloo Bay has many advantages, listed above, over other sites in the UK but it does have some potential limitations.
1. The relatively steep dip and limited extent of the foreshore means that the actual area of outcrop of the critical interval at this site is perhaps smaller than at some other localities.
2. Palaeogene igneous activity has affected a significant area of the foreshore and adjacent cliff. Strata have been thermally metamorphosed to hornfels in places. Potentially this might affect attempts to establish a chemostratigraphy or magnetostratigraphy through the succession at Waterloo Bay.
However, other less complete sections in the Penarth Group and basal Lias Group in north-east Ireland can, with confidence, be correlated lithostratigraphically with the succession at Waterloo Bay. Data from these sites could supplement that from Waterloo Bay.
There are also several sites within a few km of Larne where, although no exposure currently exists, shallow trenching could potentially expose a temporary section through this part of the succession.
Cliff section through the upper part of the Penarth Group at Cloghan Point, near Whitehead, about 10km south of Waterloo Bay. Individual lithostratigraphic units at this site can confidently be correlated with the succession at Waterloo Bay and have the advantage of not having been thermally metamorphosed.
This active mudflow, at Minnis North on the Antrim coast, has been the source of many well-preserved basal Jurassic ammonites, particularly Psiloceras erugatum, Psiloceras plicatulum, Caloceras johnstoni, Caloceras intermedium and Caloceras multicostatum. Alas the strata are all highly disturbed but many distinctive lithologies found in the debris here can be matched with specific beds at Waterloo Bay some 10 km to the ESE.