THE HUMPHREY HEAD BOREHOLE - EVIDENCE FOR CARBONIFEROUS VULCANICITY AND PERMIAN DOLOMITIZATION IN THE SOUTHERN LAKE DISTRICT

In south Cumbria, Permo-Triassic breccias and conglomerates ('brockram ') are exposed only at Rougholme Point on the Cartmel peninsula. In 19 73 the Institute of Geological Sciences Humphrey Head borehole penetra ted 257 m of brockram before entering probable Upper Carboniferous sed iments. The brockram consists of pebbles of carbonate, chert and basal t in a matrix of haematite-stained quartz sand. Carbonate and chert fr agments were derived from the upper part of the Carboniferous Limeston e sequence exposed today nearby. Basalt clasts were derived from lavas , which appear to have cooled in a subaqueous environment, at least in part. They were locally derived and are the only certain evidence for Carboniferous volcanic activity in south Cumbria. As volcanic fragmen ts increase in abundance towards the base of the borehole they must ha ve been derived from the top of the succession being eroded and are pr obably of Brigantian age. Carbonate fragments were dolomitized soon af ter incorporation in the brockram, probably by saline fluids derived f rom the evaporative Zechstein Sea. The dolomitization was incomplete, leaving remnant limestone cores to clasts which were subsequently diss olved. The resultant vugs were infilled by dolomite, calcite and gypsu m cements, which have been partially weathered from outcropping brockr am, leaving hollow pebbles.