Age of the Cretaceous Basal Conglomerate at Murlough Bay, Co. Antrim

CONGLOMERATE AT MURLOUGH BAY, Co. ANTRIM SIR,-The Cretaceous basal conglomerate seen in the coastal area between Ballycastle and Cushendun, in the north-eastern corner of Co. Antrim, is of doubtful zonal position owing to its lack of diagnostic contents.At first sight, it seems to be simply a basal facies of the succeeding Chalk, or white Limestone, of B. mucronata age.This, however, is not the case.As typically developed it does not contain B. mucronata, and its junction with the chalk above is distinctly disconformable.Nor, apparently, does Gonioteuthis occur; yet this belemnite is common in the upper part of the pre-1?, mucronata White Limestone developed at Whitepark Bay, eleven miles to the west of Murlough Bay at the north-western end of the coastal conglomerate outcrop.Thus the conglomerate appears to be older than the G. quadrata age assumed by Hume (1897), and to be separated from the succeeding White Limestone by a nonsequence covering one to several zones.On the other hand, the find by the Geological Survey (Anderson, 1957) of a Lewesiceras cf.sharpei Spath in the conglomerate at Torr Head shows that this cannot be older than Turonian.In a recent discussion (Reid, 1958) of the Upper Cretaceous Hexactinellida of Co. Antrim and their bearing on stratigraphical problems, I suggested that the basal conglomerate of this district may pass laterally southwards into the glauconitic chalk with sponges developed south of Carnlough to the Larne-Islandmagee area.This is again a problematical deposit.It is evidently Senonian in age, but lacks distinctive marker fossils; the only belemnite present is Actinocamax verus Miller, which occurs at Whitepark Bay in the flintless Marsupites chalk.The upward junction with White Limestone with B. mucronata is thus again evidently disconformable, though no marked lithological break is apparent.This glauconitic chalk is typically rich in phosphatised remains of hexactinellid sponges, occurring' particularly in a dense concentration which is roughly coincident with the upward passage in this area from glauconitic sandstone into glauconilic chalk, and some distance below the limiting disconformity; the main sponge band may be roughly one foot thick, and followed by a similar or rather greater thickness of hard, less glauconitic chalk with fewer sponges.My suggestion that this facies passes laterally into basal conglomerate of similar age was based on discovery of typical sponge bed Hexactinellida in a pebbly glauconitic chalk seen in slipped blocks at Red Bay, east of Glenariff, between the areas where the two main types are exposed.Hume's assessment of the fauna in general (1897, p. 568, tab. 1) makes the glauconitic chalk with sponges of Marsupites age, in the wide sense including the zones of Uintacrinus and Marsupites s.s.; a similar view has recently been expressed by McGugan (1957), who has studied the problem in conjunction with Dr. J. M. Hancock.Study of the sponges again supports this view, but leads to an additional suggestion.Sponges have generally been regarded as of