Bellevue

Also within

Antrim County

Climate exposure

This is about exposure to a nationwide pressure this habitat type carries — not a verdict on this site's own management.

Geology & landscape

Geological Feature: Karst · Mining / Industrial: Quarry · Geological Period: Cretaceous, Paleogene · Rock Type: Basalt, Chalk, Flint, Limestone

Landscape processes: Coastal, Karst, Volcanic / Igneous

Description

Bellevue Clay-With-Flints Bellevue is of importance because of its geology. It is one of a series of sites that de-scribes the Clay-with-Flints CwF, a unique deposit whose exposure is mostly confined to the margins of the Antrim Plateau. Stratigraphically, at most localities, the deposit occurs between older white chalk of the Late Cretaceous Ulster White Limestone Formation 83 to 72 million years old below, and lava flows of younger black basalt of the early Palaeogene Antrim Lava Group 63 to 60 million years old above. The period of time within which the CwF formed spans about 10 million years of the Late Cretaceous and Early Palaeogene. Following the uplift of the former seabed and associated marine limestones the Ulster White Limestone the limestone surface underwent a prolonged period of weathering, including karstification. This dissolution of the limestone by acidic surface and ground water produced a topographically varied karstic surface on which the CwF horizon was developed. While the limestone could be easily weathered, the more robust flints accumulated, being very resistant to erosion. The highly varied palaeotopography exhibited by the karstified land surface at sites exposing the CwF series, has provided evidence to inform the debate about the relative compaction and hardness of the Ulster White Limestone UWL when compared to similarly aged limestone in Britain. It was generally thought that the UWL owed its character from either thermal alteration and/or compaction resulting from the enormous thickness of the overlying Palaeogene Antrim Lava Group. No equivalent igneous rock series overlies the Cretaceous limestone in Britain. The nature of some of the karstic features found at these sites in Northern Ireland suggests that the lime-stone was already hardened and more compact prior to the development of the CwF series. The origin of the CwF has been a subject of debate for generations of geologists both in Ireland with regard to the contemporaneous deposit in England. It was initially thought by most scientists that the origins of the CwF and its two main components, clay and flint nodules, were as a result of sub-aerial weathering and dissolution of the underlying chalk. With respect to the flint content of the deposit this is clearly correct. However, the origins of the clay component have been much more difficult to demonstrate conclusively. There have been many proposals for the origins of the clay. These have included being an autochthonous by-product of weathering of the chalk and originating from an external volcanic source, eruptions linked to the opening of the Atlantic Ocean, but preceding the main period of basaltic volcanism in the north of Ireland. Most recently, new evidence suggests that the origin of the clay may have been derived from the remobilization of clays from volcanic deposits, by high-density mudflows at local and regional scale. Bellevue consists of a line of disused quarries exposing both the Ulster White Limestone and the Antrim Lava Group. Just over 6m of chalk of Campanian age Late Cretaceous Ulster White Limestone is visible at the main cliff face, represented by the Ballymagarry Chalk Member. CwF is exposed at a number of places along the section – its presence and thickness being determined by the palaeotopography of the chalk surface. Basalt lava flows form the upper part of the disused quarry face and belong to the Lower Basalt Formation of the Palaeogene Antrim Lava Group. At the south end of the section, two Palaeogene dykes offset and cross cut both the underlying Ulster White Limestone and the CwF. The topography of the plane of unconformity between the chalk and the CwF varies from being relatively flat, to one with depressions karstified hollows at least 2m deep in the chalk. Due to this, the thickness of the CwF varies from only a few centimetres, to at least 2m where it fills in depressions. Unusually there are two main divisions in the CwF at Bellevue. A lower unit composed of a soft, pale brown clay matrix with scattered pisoliths small, rounded concretions, surrounds small to large flint nodules that are clast-supported. While most of the nodules are whole, they show evi-dence of internal fracturing due to compaction. Internally, the flint nodules are grey in colour. At the location of the palaeo-doline depression this layer is confined to the lower part of the hollow. The upper unit it characterised by compact, dark brown to red clay matrix with abundant pisoliths. Small fragmentary to very large flint nodules, some up to 40cm long, form a clast-supported framework that in places appears to be reverse graded the size of nodules increases upwards. All flint nodules in the upper unit show vivid internal staining in shades of purplish-red to red and pink. At Bellevue there is an additional sub-layer found at the very top of the upper division of the CwF consisting of a dark grey to green clay that contains localised plant fragments. This layer resembles lignite and is 5 to 10cm thick. The two contrasting lithologies seen at Bellevue support the concept that the CwF is the product of a two-stage process. The lower lithologies formed by in-situ accumulation of insoluble flint nodules during karstic weathering of the exposed chalk landscape. The upper lithology is more difficult to interpret but field observations suggest that the clay was deposited by high density mud-flows associated with contemporaneous volcanism. The presence of a layer of lignite-like deposits would indicate that there has been a period of quiescence prior to the eruption of the Antrim Lava Group, allowing for the development of soils supporting plant material. In this section, the variable thickness of the CwF and the distribution of the lower of the two divi-sions were controlled by the pre-existing topography that formed as a result of karstic weathering. The upper unit is interpreted to have formed by later episodes of reworking of the lower division by coeval volcanism which also resulted in the introduction of large volumes of ash and volcanic debris. https://www.daera-ni.gov.uk/publications/bellevue-assi