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An Occurrence of Pillow Lavas in the Ordovician of County DownAuthor(s): E. N. SharpeSource: The Irish Naturalists' Journal, Vol. 16, No. 10 (Apr., 1970), pp. 299-301Published by: Irish Naturalists' Journal Ltd.Stable URL: http://www.jstor.org/stable/25537406 .

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299

AN OCCURRENCE OF PILLOW LAVAS IN THE ORDOVICIAN OF COUNTY DOWN

E. N. Sharpe

Synopsis

The first outcrop of Ordovician pillow lavas to be found in Co. Down is described. The pillow lavas are associated with metadolerite and tuffaceous

agglomerate. Overlying graptolitic shale is Glenkiln in age and the correlation with the Scottish succession is considered.

Introduction

The Lower Palaeozoic rocks of the Southern Uplands of Scotland continue south-westwards along their strike across the North Channel to compose a large part of Co. Down. On both sides of the Channel there are similar successions of greywackes, siltstones and black shales. On the Scottish side evidence of

contemporary vulcanism is widespread (Mercy in Craig, 1965) but there appears to

be no record of Ordovician volcanic rocks in Down.

The discovery of pillow lavas, associated tuffaceous agglomerate and

metadolerite in the Ordovician of North Down therefore enables a more direct

correlation to be made. The pillow lavas outcrop at the western end of Helen's

Bay beach (Irish grid J 460830) on the north Down coast, eight miles north-east

of Belfast and three miles west of Bangor. The area is described in the Memoirs of the Geological Survey of Ireland

(Hull, et al., 1871, Lamplugh, et. al, 1904). A large number of graptolite species were collected from black shales cropping out at Ballygrot, just north of the

pillow lavas. The tuffaceous agglomerate is described in the 1871 Memoir (page

22) as "a conglomerate formed of rounded fragments of felstone in a pale purple and greenish arenaceous paste". The pillow lavas and associated metadolerite

are described as "pale greenish-grey massive grits", and they were considered

by Du Noyer to be possibly Cambrian in age but the other members of the

Survey did not support this view. The revised one-inch sheet published in 1968

does not show any of the igneous rocks.

Succession

The rocks of the area are generally overturned and dip to the south. The shape of ithe pillows on Horse Rock (Fig, 1 and Plate 5) indicates that this is also true for the igneous rocks, and therefore the stratigraphic succession is

from south to north. Part of the succession is, however, covered by a concrete

slipway and swimming pool. (Fig. 1). South of this slipway there is about 150 feet of metadolerite. The Memoir

of 1871 states (page 22) that in the area now covered by the house and slipway there are two faults, striking east-west, with graptolitic shale between them.

Discontinuous outcrops behind the house suggest that this outcrop of shale is only about 70 feet wide and that there is another 50 feet ofi metadolerite hidden by the

swimming pool. The total thickness of the metadolerite is therefore about 200 feet.

It also appears that about 20 feet of the pillow lavas are hidden by the

northern side of the slipway, giving them a total thickness of about 70 feet.

The pillow shapes are best seen near the outer end of Horse Rock. Despite the

fact that the junction between the main masses of the pillow lava and the



300

metadolerite is not seen, it appears that the two are closely related as the latter is extensively veined by a fine grained material similar to the pillow lava.

Some shearing occurs on the well exposed sinuous contact between the lava and the tuffaceous agglomerate along the northern edge of Horse Rock but Jhe amount of movement is very small. The maximum thickness of the

agglomerate is 20 feet and it is succeeded conformably to the north by graptolitic shale.

Lithologies

Metadolerite: This is predominantly dark blue-green or dark grey in colour and is in general much shattered and faulted. A fresh surface shows that it is fairly coarse grained with white or light green laths of plagioclase feldspar

surrounding dark green ferromagnesian minerals. In thin sections the texture is

ophitic with laths of plagioclase surrounding skeletal pyroxene. The plagioclase, which is almost entirely altered to calcite, chlorite and sphene, appears to have

straight extinction and is probably albite or oligoclase. There appears to be only one pyroxene, partly altered to a colourless fibrous material which is probably uralite. The mesostasis is pale green chlorite and calcite. There are two large

sedimentary xenoliths, very much altered, in the main mass of metadolerite.

Intruded into the metadolerite is a light coloured, fine-grained rock

very similar in appearance to the pillow-lava material. In places this intrusion

forms a rubbly surface with irregularly cracked kidney-shaped lumps of rock

resting on a surface of metadolerite. On a fresh surface the intrusion is light grey

green and very fine grained. It shows a chilled contact against the metadolerite.

From south to north the outcrop of metadolerite becomes more shattered and the

amount of fine-grained rock increases. West of the house (Fig. 1), the rock, most

of which appears to be fine grained material, is in an extremely brecciated state.

Similar brecciated rock crops out on the northern edge of Irish Hill (Irish grid J 451820) half a mile south-west of Helen's Bay village. There the ridge formed

by the igneous material is only 10 feet wide but continuous for at least a half

mile along itsf strike. This second outcrop is not on strike from, that on the coast

and any correlation implies folding or faulting. Pillow lava*. At the shoreward end of Horse Rock the rock is massive

with no pillow shapes. It contains patches of agglomerate up to eight feet thick

and 15 feet long. On a fresh surface the rock is light grey-green or purple and

very fine grained. The pillows on the outer half of Horse Rock are of the same

material. They show typical shapes (Plate 5) and are separated by layers of shale

and chert. The rock is not vesicular. In this section the rock sho^s a basaltic

texture and is composed almost entirely of small altered plagioclase laths, iron

ore and serpentinous material. The plagioclase laths again appear to show straight extinction indicating a sodic plagioclase and a spilitic composition for the rock.

Tuffaceous agglomerate: The agglomerate consists of sub-rounded pebbles of spilite, mostly about 4 inches in diameter, embedded in a laminated light

hlue-green and purple tuff. The composition of the rock varies from a pure tuff

to a conglomerate of spilite pebbles.

Graptolitic shale: Above the agglomerate, black graptolitic shale inter

fingers with greywacke, grey shale and black chert. Graptolites were found 40

feet north of the base of the shale. Climacograptus antiquus (Hall), C. bicornis

(Hall), Orthograptus calcaratus (Lapw.), Amplexograptus perexcavatus (Lapw.),

Nemazraptus gracilis (Hall), and Dicellograptus cf. sextaris (Hall) were tentatively



^^^^^^^ Bally grot A

ss ^rz^^^z^^^ Ho rse T

^ Wz^^^\ j?0*^^\ KEV

^^ ^^^^W i^?^^^-*-:'-L?r:^y Mffih graptolitic shale, -? / tm^s .^^^l^^^^^iiij^^^i/

y""sa black chert and -?7^ >^liii^ greywacke

T^Zys ^y^^r^^^^^ W5Bg agglomerate

"^^^/^_^1^X ^p \ :::::::: pillow lavas

\ -C^^

"~" slipway \ VF . \ ^VA metadolerite

j ~ / V^V

' | 100 feet j

Fig. 1. Map showing the igneous materials at the western end of Helen's Bay beach.



(a)

BJ8PPl!J^M

(b)

F/are 5

(a) Pillow lavas dipping at about 30 to the south

(b) Pillows showing typical shape



301

identified. These suggest that the shales are of the N. gracilis zone (Basal Glenkiln). The spihte and agglomerate are probably little older.

Conclusions

The compositions of the spilite and metadolerite are very similar and so

they may be closely related. The coarse grain size and the presence of sedimentary xenoliths m the metadolerite indicate that it is an intrusion but there is no evidence

of the rock into which the dolerite was intruded.

In Scotland there is widespread evidence of igneous activity in Glenkiln

Hartfell times (Walton in Craig, 1965). From structural and stratigraphic evidence

it appears that the Ordovician of N Down correlates readily with that of the

Rhinns of Galloway. In the Portslogan coastal section Kelling (1961) found two

bands of graptolitic shale, both m the N. gracilis zone, separated by spilite,

agglomerate and greywacke. The lower band of shale rests on black chert.

Further south, at Morroch Bay, the N. gracilis zone has no volcanic horizon but it rests on cherts, tuffs and mudstones. The spilites and agglomerate of Helen's

Bay also underly N. gracilis zone shales. Structurally and stratigraphically?

Ballygrot seems to correlate well with Morroch Bay and therefore the pillow lavas are probably the stratigraphic equivalents of the cherts and tuffs of Morroch Bay rather than the spilitic horizon of Portslogan.

Acknowledgements

The author would like to express his thanks to Dr T. B. Anderson for his continued help and guidance in the preparation of the manuscript, to Professor

A. Williams for suggesting the project and to Dr J. Preston for his help with the

petrographic descriptions. The field work for this study was carried out with the aid of a grant, which is here gratefully acknowledged, from the Down County

Education Committee.

Department of Geology, Queen's University, Belfast, 7

REFERENCES

Craig, G Y. (1965) The Geology of Scotland Oliver and Boyd, Edinburgh Hull, E., et al (1871) Explanatory Memoir to accompany sheets 37, 38 and part of 29.

Mem. Geol Surv. Ireland. ^_

Kellmg, G (1961) The Stratigraphy and Structure of the Ordovician rocks of the Rhinns of Galloway Q J Geol. Soc, London, 117; 37-75.

Lamplugh, G W., et. al. (1904). The Geology of the country around Belfast Mem. GeoL Surv. Ireland.

SPATIAL VARIATIONS IN IRISH SUMMER WEATHER DURING 1968 AND 1969

A. H. Perry

The summers of 1968 and 1969 will long be remembered in Ireland for

the protracted spells of fine warm weather that characterized them, particularly as they followed the generally poor summers of many years in the early 1960s.

While 1968 produced a dismal summer over much of the British Isles, and

particularly so in eastern parts of England, the weather in the period June-August, 1969, was more generally fine and warm. When examined in detail considerable

regional variations can be found in the weather patterns within Ireland in both

years.



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