Depositional Features of Dittonian Rocks: Pembrokeshire compared with the Welsh Borderland

Abstract

The Dittonian Stage in Pembrokeshire comprises the Lower Marl Group (higher beds), the Sandstone-and-Marl Group, and the Upper Marl Group (at least lower half). These formations cannot be distinguished lithologically from Dittonian strata of the Welsh Borderland 100 miles away along the depositional strike.Cyclothems of intraformational conglomerate (scoured surface below) -> sandstone ->• siltstone with concretions recur vertically in each area. The conglomerates in both districts consist of intraformational siltstone and concretionary debris. Flat-bedding, primary current lineation, planar cross-bedding, trough crossbedding, and ripple-drift bedding are common to the sandstones. Suncracked siltstones abound in both Pembrokeshire and the Welsh Borderland. Slumped bedding, sandstone pipes, and animal burrows are the structures penecontemporaneous with deposition in each area.The Dittonian strata of the Welsh Borderland are shown to be probably floodplain deposits, by reason of their close similarity to the modern sediments of the Colorado Delta floodplain and Colorado River. The occurrence of an identical facies in Pembrokeshire suggests the even wider extent of this floodplain in early Lower Old Red Sandstone times, when the sea lay far to the south of both areas.

Storm surge turbation

“…The sedimentary structures from Brittany produced by storm surge are related to convolute bedding as this term is used by Allen (1963), Einsele (1963), Dzulynski & Walton (1965), and Potter & Pettijohn (1977. Convolute bedding in the sense it is used by these authors is defined inter aliu by the circumstance that the deformation, consisting of broad sags and sharp-crested anticlines, dies out downwards and in some cases upwards as well.…”

Computer simulation of sedimentation in meandering streams

“…Various aspects of the model can also be compared with the coarse members of the fluviatile 'fining upwards' cycles known abundantly from ancient sediments, and which are ascribed to processes of lateral deposition (e.g. Allen, 1963cAllen, , 1964Allen, , 1965bAllen & Friend, 1968;Beutner, Flueckinger & Gard, 1967;Leeder, 1973;Moody-Stuart, 1966;McGowen & Garner, 1970;Potter, 1967;Visher, 1965a, b).…”

“…Several models of fluviatile sedimentation have been published and have formed a fundamental stsrting point in the recognition and interpretation of ancient sediments (Allen, 1963a(Allen, , 1964(Allen, , 1965a(Allen, , 1970a(Allen, , b, 1971Beerbower, 1964;Moody-Stuart, 1966;Potter, 1967;Potter & Blakely, 1967;Visher, 1965a, b). Most of these models are qualitative, and more than one of them embodies concepts that are physically suspect or oversimplified.…”

Primary Current Lineation in the Lower Old Red Sandstone (Devonian), Anglo‐welsh Basin

“…In the Lower Old Red Sandstone this facies is conveniently referred to as thejlut-bedded sandstone facies. It is a recurrent one in the formation, contributing at very many localities to the cyclothems that are widely present (ALLEN, 1962a(ALLEN, , 1963ALLEN and TARLO, 1963 ;METRE, 1963).…”