Chattian and Miocene elements in the modern landscape of western Britain and Ireland

Abstract

The best preserved denudation surfaces which are benched into the margins of the pre-Permian oldland massifs of western Britain and Ireland are those comparatively close to sea-level. Such surfaces have frequently been regarded as being of late Pliocene or Pleistocene age and of marine origin. Recent discoveries reviewed in this work reinforce a growing view that these low-level planations are much older and may be of terrestrial origin. Evidence from Wales and southwest England suggests that these little-modified planation elements represent landscapes some 15 Ma old. This paper reviews the palaeobotanical, sedimentological and geomorphological evidence for the close association of several bodies of non-marine Chattian and Miocene sediment and saprolite with some of the better known planation features. In these areas, the former vegetation comprised mixed coniferous/deciduous forest, extinct species of modern north temperate genera, well known from contemporaneous deposits in north Europe. Evidently, differential relief in those times was appreciably less than that of the present and it seems probable that much of what is now the western half of the British Isles was then a wide, forested extension of a previous North European Plain, which varied in altitude by no more than a few decametres.

Pre-Quaternary geomorphological history and geoheritage of Britain

“…4 km of sediments had to be removed to expose the basement (Arthur 1989). However, as reviewed before, the adjacent onshore area in Cornwall and South Wales seems to have had very low rates of Neogene denudation (Battiau-Queney 1984, Walsh et al 1999.…”

“…In fact, a series of benches has been identified along the coast and attributed to progressively lower transgressions in the Pliocene and Quaternary. This marine interpretation has been challenged by Walsh et al (1987Walsh et al ( , 1999 who showed the presence of deeply weathered rock and Oligocene (Chattian) and Miocene sediments within this regional surface. Similarly, Battiau-Queney (1984) reports that deeply weathered rock (up to 20 m) underlies this surface in Pembrokeshire in Wales.…”

“…In the British context, the usefulness of the etchplanation concept to explain landform origin was first outlined by Thomas (1978). Environmental conditions favourable to etchplanation have also been championed by Battiau-Queney (1984), most notably in the context of Wales, then by Hall (1987) with reference to Scotland, and more recently by Jones (1999a) and Walsh et al (1999) for southern England.…”

“…Deep weathering profiles of a thickness up to 15 m have probably formed in less than 1 Ma. In North Wales and Derbyshire the Mio-Pliocene Brassington formation consists of pockets of what appear to be products of tropical weathering (Walsh et al 1972), while in Cornwall Walsh et al (1987Walsh et al ( , 1999 believe they have identified pockets of Miocene saprolite on an etchplain of tropical and subtropical origins. Similarly, Battiau-Queney (1984) claims to identify inselbergs, tropical planation surfaces and tropical weathering products in Wales.…”

The Quaternary of the British Isles: factors forcing environmental change

“…Soils and shallow marine systems (SSMS): precession forcing at ca 22 ka cycles Geology This system is represented by the shelly, shallow marine Red and Norwich Crags of East Anglia (Zalasiewicz and Mathers, 1985) and the rare patches of coastal and river deposits elsewhere in Britain (Nettlebed river deposits in the Thames (Turner, 1983); St Erth marine deposits in west Cornwall (Mitchell et al, 1973b); Brassington river muds, silts, sands and gravels in the south Pennines (Walsh et al, 1972)). Residual deposits such as the claywith-flints of the chalk and Tertiary bedrocks (Jones, 1999;Quesnel et al, 2003), the cherty clayey gravels of the Cretaceous Greensand bedrock and occasional patches of deeply weathered parent material elsewhere in upland England, Wales and Scotland (Walsh et al, 1999) also reflect the processes that operated in this system. All these deposits are characterised by limited ranges of clast lithologies reflecting local derivation or in situ weathering.…”