THE HELL’S MOUTH GRITS: CAMBRIAN GREYWACKES IN ST. TUDWAL’S PENINSULA, NORTH WALES

Abstract

The paper is an attempt to reconstruct the nature and position of the source, the mode of transportation and the envi ronment of deposition of the alternating grits and mudstones of the Hell’s Mouth Grits of the St. Tudwal’s Peninsula. The grit beds are very uniform in lithology and in thickness when traced along the outcrops, with a gradual thinning southwards. They exhibit the characteristic textures and structures of greywackes but differ from the normal type in being relatively well sorted and commonly laminated. They are dominantly lithic (low-rank) greywackes with a small percentage of feldspathic greywackes. The directional structures––current-markings and cross-lamination–indicate a northerly source, the composition suggests a source like the Pre-Cambrian of Anglesey and the texture implies transportation by turbidity currents and deposition below wave-base. The intercalated mudstones are more variable both in thickness and in lithology. They contain laminated mudstones rich in sponge remains, white-weathering pyriteless mudstones, small lenses of a peculiar mud-pellet rock and thin intercalations of lenticular siltstone and grits. The lenticular nature of the bedding and the variable nature of the cross-lamination suggest deposition above wave-base. The contradictory nature of the evidence is not fully understood but a somewhat similar association of features can be inferred from the results of recent work on deep-sea sediments.