Belshaws Quarry
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Geology & landscape
Geological Feature: Karst · Mining / Industrial: Quarry · Geological Period: Cretaceous, Paleogene · Rock Type: Basalt, Chalk, Flint, Limestone
Landscape processes: Karst, Volcanic / Igneous
Description
Belshaws Quarry Clay-With-Flints Belshaw’s Quarry is of importance because of its geology. It is one of a series of sites that describes 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 limestone 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 remobilisation of clays from volcanic deposits, by high-density mudflows at local and regional scale. Chalk extraction took place at Belshaw’s Quarry until about 1950, and in recent years the site was designated as a Nature Reserve. The main face of the quarry is clean and well exposed, but the CwF is obscured in most places except for a section at the western end of the quarry, just beside the top section of the steps leading to the bottom of the quarry. At Belshaw’s Quarry, the Late Cretaceous Ulster White Limestone Formation is just over 10m thick, and represents the Portrush and the Ballymagarry Chalk Members of the formation. The CwF series overlies the chalk, and is best exposed in the western end of the quarry. The upper part of the quarry comprises sub-horizontal Palaeogene basalt flows of the Lower Basalt Formation of the Antrim Lava Group. At the southwest end of the quarry a vertical Palaeogene dyke intrudes the basalt lava flows and continues to the SSE baking the chalk and the CwF. The CwF section is just over 80cm thick and reveals three divisions. In the lowest 40cm of the bed, the clay matrix is a dark grey and contains scattered small to medium flints with no internal staining. In the succeeding 30cm the clay matrix is lighter grey in colour and large whole flints are present, which have developed internal purplish pink staining. The top 20cm is much darker grey, with small to medium sized flints that are still stained internally and are matrix supported. Palynological examination of two samples of clay from the lower 40cm of the CwF at Belshaw’s Quarry yielded abundant organic residues and palynofloras, especially dinoflagellate cysts. The overlapping age ranges of the dinoflagellate cysts Alisocysta circumtabulata, Neoeurysphaeridium glabrum, and Odontochitina operculata are indicative of an Early Maastrichtian age Belemnitella lanceolata Zone. However, the youngest chalk at Belshaw’s Quarry is latest Campanian in age and is therefore older than the dinoflagellate cysts. This seems to indicate that chalk of the B. lanceolata Zone, of Early Maastrichtian age was formerly present but was eroded in later Cretaceous times, prior to the formation of the CwF. The two contrasting lower lithologies at Belshaw’s Quarry support the concept that the CwF is the product of a two-stage process. The lower of these lithologies formed by in-situ accumulation of insoluble flint nodules during karstic weathering of the exposed chalk landscape and is confirmed by the presence of dinoflagellate cysts from now eroded chalk layers. The upper lithology is more difficult to interpret but field observations suggest that the clay was deposited by high density mudflows associated with contemporaneous volcanism. https://www.daera-ni.gov.uk/publications/belshaws-quarry-assi