| Site Description |
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| Highlights: |
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| The photographic record of the Clay-with-Flints in Demesne Quarry, Glenarm includes examples from the present and from previous visits in 2000 when the working face had not extended as far south. |
| Introduction: |
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| Production of chalk at Demesne Quarry commenced in the late 19th century and is still continuing today. During that period the quarry has been deepened to access the 50m thickness of chalk available. At present the quarry is also being extended southwards but, with the gentle regional dip of the chalk to the southwest, the increasing thickness of basalt overburden and the resulting amounts of waste rock are hindering development. |
| Description: |
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| Site Geology: |
| The Clay-with-Flints, and the unconformable contacts with the white chalk below and the black basalt lava flows above, is exposed along the western and southern sides of the quarry. |
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| Ulster White Limestone Formation: Chalk exposed in the quarry is Campanian and includes strata of the Larry Bane Chalk, Ballintoy Chalk, Glenarm Chalk and Garron Chalk members of the Ulster White Limestone Formation (Figures 2 and 3). |
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| Clay-with-Flints: The Clay-with-Flints is exposed in the upper part of the quarry with access provided by the highest bench on the south and west faces (Photograph 36). Along the west face, the preservation of sections in the Clay-with-Flints is assured by the presence of the Tully Road at the edge of the quarry site. On the south side this is not the case and new sections in the Clay-with-Flints are continually being revealed as the quarry is extended in that direction. |
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| Antrim Lava Group: The overlying basalt lava flows belong to the Lower Basalt Formation of the Early Palaeogene Antrim Lava Group. |
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| Quaternary Deposits: Drift deposits of glacial till (boulder clay) overlying rock head are relatively thin but increase in thickness to the south. |
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| The Clay-with-Flints detail: |
| With the continuing southward expansion of the quarry, former exposures of the Clay-with-Flints are lost whilst new exposures are created near the top of that face. In the west wall of the quarry, below the Tully Road, the existing faces and exposures of the Clay-with-Flints are degrading and gradually being concealed by a thin cover of rock debris and plant growth. Some sections in the southwest corner of the quarry have been lost through quarrying, but a photographic record is included to demonstrate some of the more significant features of the Clay-with-Flints. This report was compiled using data from various sections in the quarry. |
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| The topography of the plane of unconformity between the chalk and the Clay-with-Flints varies from relatively flat to one with depressions at least 3m deep in the chalk bedrock (Photograph 37). |
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| These depressions in the chalk surface also appear to underlie depressions in the profile of the surface separating the lower and upper layers in the Clay-with-Flints and thus coincide with a marked thickening of both layers of the deposit. In some parts of the quarry the combined thickness of the two layers forming the Clay-with-Flints is less than 1m whilst, in contrast, in areas overlying depressions in the chalk surface the deposit may be as much as 4m thick (Photograph 38). The full scale of the depressions in the chalk surface is not exposed in the quarry. However, based on the unusual thickness of both layers in areas overlying the depressions in the chalk surface it is tempting to conclude that subsidence of the depressions occurred during and after the accumulation of the Clay-with-Flints. Thus pinnacles in the original chalk land surface, which remained static relative to the areas of depressions during accumulation of the Clay-with-Flints, were still covered by both layers of the Clay-with-Flints but with a much reduced thickness. |
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| The section of the Clay-with-Flints in Demesne Quarry varies in thickness from about 0.8m to at least 4m (Figure 26). Petrographic descriptions and geochemical analyses were performed on samples taken from a section of the Clay-with-Flints in the southwest corner of the quarry that has since been developed. Photographs of the Clay-with-Flints in this quarry illustrate that former section and the current exposure along the south face of the quarry. At both sections the contacts between the Clay-with-Flints and the underlying chalk and overlying basalt lavas are clearly defined. At the previous section the upper level of the quarry in the southeast corner coincided with the top of the Clay-with-Flints and only a thin layer of basalt remained. In that section the top surface of the chalk was originally concealed by waste material but was subsequently cleared and the full thickness of the Clay-with-Flints exposed and measured. |
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| The lower layer of the Clay-with-Flints consists of small to very large flints forming a clast supported framework although there are small patches of predominantly clay matrix. There is no obvious size sorting or grading of clasts. By contrast the upper layer shows a well defined clast zonation from a high density at the base becoming almost clast free in the remaining upper parts. Unusually, the top of the upper layer, just below the base of the basalt lava flows, is characterised in places by a layer, one clast thick, of large flints (Photograph 39). The implication is that the upper layer is the product of flow and that these flints have been transported at the surface of a high density mud lahar with high yield strength. |
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| Petrogenesis: |
| The Clay-with-Flints in Demesne Quarry has been studied in detail in order to obtain as much information as possible on its origins, the nature of the parental volcanicity and to assess any petrographical and geochemical differences that may be apparent between the two separate layers that make up the deposit. |
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| Geochemistry |
| Samples of the clay matrix from each of the two layers that constitute the Clay-with-Flints (Figure 26) in Demesne Quarry were selected for major and selected trace element analysis. Immobile element ratios, when plotted on a Zr/TiO2 – Nb/Y discrimination diagram, show that the lower sample plots in the andesite field while the higher sample is less evolved and plots in the basaltic andesite field (Figure 4). |
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| Petrography |
| Two clay samples, one each from the two layers in the Clay-with-Flints, were investigated petrographically (Photograph 39). |
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| Sample 1: In general the matrix clay of the lower layer of the Clay-with-Flints is stained a deep orange-brown but in the area around each of the flint clasts, it assumes a greyish brown colour. The sample, from this lower layer, consisted of a predominantly grey clay-rich matrix with small angular fragments of grey flint. In thin section the rock is composed of opaque clasts, detrital flints and small, angular quartz crystals in a fine-grained/cryptocrystalline matrix. The opaque clasts reach up to 5mm, have ragged outlines and are possibly heavily oxidised lithic fragments. The flint clasts are smaller, up to 3mm, sub-rounded, and range from colourless to rusty coloured suggesting varying degrees of alteration. Quartz crystals are small, predominantly angular and occasionally show strained extinction. The matrix, which was probably originally composed of glassy fragments, and may or may not have been welded, has now completely devitrified and been replaced by a fine-grained/cryptocrystalline intergrowth of quartz and clay minerals. Primary compositional differences in the matrix are defined by different shades of colour that represent the outline of deformed fragments wrapping the clasts suggesting flowage of the deposit. This sample appears to be of an altered volcaniclastic rock, possibly a lithic tuff. |
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| Sample 2: A sample of deep reddish brown clay matrix from the upper layer of the deposit shows clear laminations or flow banding, although this is not seen at the thin section level. Clasts include rounded, slightly lobate heavily altered fragments of lava (up to 1.5mm), rare sub-angular flints up to 0.3mm and angular quartz crystals/crystal fragments. In detail, some of the quartz fragments are polycrystalline and show signs of recrystallisation such as strain extinction and internal sutured margins and contain |
| small (<0.01mm) opaque oxide granules. The matrix is composed of reddish brown structureless clay and cryptocrystalline (? quartz, clay and hematite) material and is cut by hairline veins of colourless zeolite. Regions of different optical density within the matrix suggest some primary structure. The sample is judged to be another possible example of an altered volcaniclastic rock, although it is finer grained overall and contains fewer clasts than the sample from the lower layer. |
| Conclusions: |
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| In Demesne Quarry, in places, the unusually thick development of the Clay-with-Flints was a response to episodes of contemporaneous subsidence in the chalk land surface. Based on variations in colour, flint content and sedimentological structures, the Clay-with-Flints is divided into two layers. Petrographical analyses of samples of the clay matrix demonstrate that both layers originated as volcaniclastic deposits. While it appears that the lower layer was probably a lithic tuff there is no sedimentological evidence of flow having occurred during deposition. In contrast, the upper layer shows clear evidence that the flints were transported during later stages of the volcanic eruption. Geochemical analyses, using immobile element ratios, indicate that there is a transition in the volcanic component with the lower layer being of andesitic composition while the upper layer is less evolved and of basaltic andesite composition. |
| Notes: |
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The full series of Clay-with-Flints records is as follows: Record no. 1300 Introduction to Clay-with-Flints of Northern Ireland |
| Record no. 1301 Belshaw’s Quarry – Clay-with-Flints |
| Record no. 1302 Gibson’s Quarry, Portadown – Clay-with-Flints |
| Record no. 1303 Devlin’ Quarry, Moneymore – Clay-with-Flints |
| Record no. 1304 Glenshane Pass – Clay-with-Flints |
| Record no. 1305 Donald’s Hill – Clay-with-Flints |
| Record no. 1306 Tircrevan Burn – Clay-with-Flints |
| Record no. 1307 Ballycastle – Clay-with-Flints |
| Record no. 1308 Demesne Quarry, Glenarm – Clay-with-Flints |
| Record no. 1309 Magheramorne Quarry, Larne – Clay-with-Flints |
| Record no. 1310 Bellevue, Belfast, Co Antrim – Clay-with-Flints |
| Record no. 1311 Soldierstown Quarry, Moira – Clay-with-Flints |
| References |
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| Cleland, A. M. I. 1928. Red flints. Irish Naturalists’ Journal. 7, 5-8.
Lamplugh, G., Kilroe, J. R., M’Henry, A., Seymour, H. J., Wright, W. B. and Muff,
H. B. 1904. The Geology of the Country Around Belfast. Memoirs of the Geological
Survey of Ireland.
Jukes-Browne, A. 1906. The Clay-with-Flints; its origin and distribution. Quarterly
Journal of the Geological Society, London, 62, 132-164.
Mitchell, W. I., Cooper, M. R., Hards, V. L. and Meighan, I. G. 1999. An occurrence
of silicic volcanic rocks in the early Palaeogene Antrim Lava Group of Northern
Ireland. Scottish Journal of Geology, 35, 179-85.
Patterson, E. M. 1956. Vent agglomerates in relation to lava stratigraphy. Proceedings
of the British Association, Section C (Geology), Sheffield.
Portlock, J. E. 1843. Report on the Geology of the County of Londonderry and of
parts of Tyrone and Fermanagh. HMSO, Dublin.
Simms, M. J. 2000. The sub-basaltic surface in northeast Ireland and its significance
for interpreting the Tertiary history of the region. Proceedings of the Geologists
Association, 111, 331-336.
Smith, B. J. and McAlister, J. J. 1995. Mineralogy, chemistry and
palaeoenvironmental significance of an Early Tertiary Terra Rossa from Northern
Ireland: A preliminary review. Geomorphology, 12, 63-73. |