Palaeosols as indicators of geomorphic stability in two Old Red Sandstone alluvial suites, South Wales

Abstract

Palaeosols (fossil soils) are abundant in the alluvial late Silurian–early Devonian Moor Cliffs Formation and Rat Island Mudstone (Freshwater West Formation) of south Dyfed, South Wales. The palaeosols occur in thick mudstones deposited on floodplains under a seasonal, semi-arid climate, and represent Vertisol-type soils. Simple, single profiles are rare and most of the mudstone intervals exhibit complex overprinted profiles. However, a major difference exists between the palaeo-Vertisols in each of the two units studied. The Moor Cliffs Formation contains thick floodplain intervals with evidence of frequent erosion and reactivation of earlier soil horizons. The associated coarser channel deposits mainly represent ephemeral streams. The alluvial system was unstable, flashy and prone to extensive stripping of stored floodplain sediment. Arroyos (unstable ephemeral stream channels) probably developed during this interval. Evidence for any marine influence on soil development is missing and previous speculations that the formation may represent deposition in an upper-intertidal or supratidal system are not supported. The Rat Island Mudstone represents a complete contrast in that there was little or no stripping of floodplain sediments. The channel deposits are typically of the fining-upwards type representing more stable, low sinuosity channels and with ephemeral-stream floods. By integrating the palaeosols, a more detailed picture of the geomorphic history of these units can be formulated. Comparison with contemporary drainage basins indicates that a change in climate was a possible cause for the differences between the two units, less frequent and more severe storms occurring during the deposition of the Rat Island Mudstone.

Response of Soils and Soil Ecosystems to the Pennsylvanian–Permian Climate Transition in the Upper Fluvial Plain of the Dunkard Basin, Southeastern Ohio, USA

“…Abundant plant matter and horizontal grayish-green rhizohaloes indicate high soil moisture, e.g., [2,3,32,33]. The abundance of kaolinite also indicates a high soil moisture regime, e.g., [9,25,39]. PT1 plasmic microfabrics indicate some downward illuviation of clay minerals in Ustifluvents, but a lack of illuviation in Epiaquents, e.g., [3].…”

“…Calciusteps are Inceptisols defined by the occurrence of a calcic horizon within 100 cm of the soil surface and Quartzipsamments are Entisols defined by a quartz grain content of >90% in their sand fraction, e.g., [4,36]. PT2 paleosols consist of Bssk, Bsskg, Bkss, and BC horizons truncated by channel sandstone indicating periodic landscape instability ( Figure 7; Table 2) e.g., [2,39]. PT2 paleosols are, therefore, interpreted to be composite and compound (most common) paleosols, e.g., [2,39].…”

“…PT2 paleosols consist of Bssk, Bsskg, Bkss, and BC horizons truncated by channel sandstone indicating periodic landscape instability ( Figure 7; Table 2) e.g., [2,39]. PT2 paleosols are, therefore, interpreted to be composite and compound (most common) paleosols, e.g., [2,39]. The reddish-brown to variegated reddish-brown and grayish-green color indicate variable hydrology, e.g., [3,37].…”

Micromorphology of Paleosols of the Marília Formation and their Significance in the Paleoenvironmental Evolution of the Bauru Basin, Upper Cretaceous, Southeastern Brazil

“…Paleosols have been used both in analysis and paleoenvironmental reconstructions (Andreis, 1981;Catt, 1990;Retallack, 2001;Sheldon and Tabor, 2009;Nascimento et al, 2017) by forming themselves into open systems capable of recording environmental conditions during their training, as in stratigraphic correlation (Brown and Kraus, 1988;Wright, 1992;Marriott and Wright, 1993;Kraus 1997;McCarthy and Plint, 2003), indicating a stable surface and representing moments of pause in erosion and deposition.…”

Rivers through geological time: the fluvial contribution to understanding of our planet

“…From careful recording and various analyses they have been able to recognize the influence of tectonic activity and changes in climate, although the latter was predominantly hot, semi-arid and with seasonal rainfall, giving rise to ephemeral streams with flashy flow regimes. Detailed analyses of sediment facies, palaeosols and palaeogeography, together with comparisons with modern-day dryland river systems, have provided important bases for interpretation (see also Allen, 1974;Channel et al, 1992;Marriott and Wright, 1993;Edwards and Richardson, 2004).…”