Imbrication, Paleocurrents and Other Sedimentary Features in the Lower New Red Sandstone, Devonshire, England

Abstract

ABSTRACT Red breccias, sandstones and siltstones of the New Red Sandstone, from late Carboniferous to Triassic in age, are exposed along the South Devon coast and extend inland to the north in an irregular outcrop area. They overlie earlier rocks which had been folded and intruded during the Variscan Orogeny, and from which they were derived. The coarse beds in the sequence evidently formed as alluvial fan deposits under a semi-arid climatic regime, deposition taking place in a series of cuvettes opening into a larger basin to the eastward. Paleocurrents are mainly deduced from the direction of imbrication of phenoclasts in the breccias, although contours of average roundness of limestone phenoclasts give useful supporting evidence. Two intergrading types of imbrication, termed contact nd isolate, can be related to the mode of deposition, with two or possibly three processes acting to produce such a fabric. The bedding style of the breccias is mainly flat, with no definite evidence of any original depositional dip; low-angle trough cross-bedding is present in down-fan areas, probably produced in braided stream channels. Large boulders of volcanic rocks in the axial zone of the cuvettes were probably transported by successive sheetfloods in which energy had been augmented by topographic focusing. Cross-bedding of both eolian and aqueous origin are present in the sandstones, the latter largely conforming to the general pattern of alluvial fan deposition. Wind directions shown in the middle part of the succession (Permian) were from the south-southeast and of unidirectional character, but near the base (Carboniferous) a north-northeast direction is indicated. Non-directional features found in the breccias include a group of sediment-injection structures which can be attributed to de-watering of enclosed quicksand layers. Rain prints, dessication cracks and large annelid burrows are present at a few localities.