The Marginal Triassic deposits of South Wales: Continental facies and palaeogeography

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Abstract
Marginal Triassic deposits in Glamorgan, South Wales, occur beneath and laterally equivalent to the Keuper Marl. They consist of coarse elastics, locally interbedded with finer sediments, limestones and replaced evaporites. Fluviatile sediments are chiefly sorted conglomerates and cross‐bedded sandstones of stream‐flood origin. Other conglomerates and sandstones, associated with siltstone and calcrete, are interpreted as the deposits of semi‐permanent streams with well‐developed floodplains. Ill‐sorted breccias accumulated as screes against former cliffs. Matrix‐supported conglomerates of mudflow origin are a minor facies. Thin graded beds were deposited from sheet floods and other finer clastic sediments formed on floodplains and playas. Lacustrine shore‐zone elastics, limestones and evaporites are also present.The distribution of the various facies is related to the Trassic geomorphology, which was controlled by the structure of the underlying Palaeozoic. The South Wales Coalfield formed an upland area bounded to the south by an escarpment. Stream‐flood conglomerates of the Radyr‐Llandaff area were part of an alluvial fan which formed at the foot of this escarpment, perhaps at the mouth of a canyon on the site of the present‐day Taff gorge. South of the escarpment semi‐permanent streams with well‐developed floodplains occupied a broad valley, draining eastwards and cut into the axial region of the plunging Cowbridge anticline. South of the anticline, stream‐flood and sheet‐flood deposits interdigitated with wave‐rippled and desiccated siltstones deposited on playas. The latter, together with the shore‐zone sediments, were marginal to the lake in which the Keuper Marl accumulated. With rising base‐level during the Norian, the Keuper Marl facies spread north and west into the Vale of Glamorgan, covering the Marginal Triassic and inliers of Carboniferous Limestone.