GEO PHYSICAL INVESTIGATIONS IN TREMADOC BAY, NORTH WALES

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Abstract
The results of a gravity survey of North Wales ( Powell 1956 ) suggested that Tremadoc Bay, off North Wales, might be a Triassic basin, the eastern boundary being a major fault coinciding in part with the coastline, but nowhere exposed. To test these ideas a seismic refraction survey has been made of Tremadoe Bay, consisting of a long traverse and some short lines, these being supplemented by a number of shorter profiles across the coastal flats of Morfa Dyffryn. A survey of the bay with a proton magnetometer was also undertaken and some closely spaced gravity and magnetic measurements have been made on Morfa Dyffryn. The results of the seismic work indicate the presence of three layers. The top layer has a maximum thickness of about 1800 feet and a velocity of 6500 feet/sec., thins westwards and ultimately dies out. It is considered to be late Mesozoic or Tertiary in age. All the evidence indicates the eastern boundary of this layer to be a major fault or a scarp. The middle layer has a maximum thickness of 6000 feet and a velocity of 13,000 feet/sec. The form and magnitude of the gravity anomaly in the area, after allowing for the effect of the top layer, make it most unlikely that this second layer has a low enough density to be Mesozoic. It is probably Ordovician. The third and lowest layer identified is thought to be Cambrian. The form of the regional gravity low over Cardigan Bay points to a structural origin for the bay, though the alternative theory that the anomaly is due to a major granitic intrusion is considered possible.