The Old Red Sandstone of Britain and Ireland — a review

Abstract

The Old Red Sandstone (ORS) is an informal term which is given to continental, predominantly siliclastic, strata of late Silurian to early Carboniferous age which were deposited across the continent of Laurussia at sub-tropic to tropical latitudes. The coincidental development of land plants had a major impact on the atmosphere and global climate by lowering atmospheric carbon dioxide levels, which profoundly affected the style of alluvial sedimentation during this interval, by stabilising flood plains and facilitating the development of soils. The ORS also provides examples of syn- to post-orogenic deposition related to the Caledonian Orogeny, which was affected by synchronous tectonism and volcanism. The influence of Variscan tectonics on basin deformation and tectonism is also evident in the ORS sequence.

In October 2014, a symposium was held, organised by the South Wales Geologists’ Association, entitled The Old Red Sandstone: is it Old, is it Red and is it all Sandstone? The event consisted of talks and posters on topics associated with the Old Red Sandstone deposits, principally of Wales and the Welsh Borders and the Scottish Borders in the UK, and included a series of field trips. Seven of the speakers have contributed manuscripts which are presented in this volume. These include papers discussing fossil fish and plant assemblages, the Fforest Fawr Geopark, Old Red Sandstone building stones, and soft sediment deformation. A brief report on the event and acknowledgements is also included.

Introduction

The deposition of the Old Red Sandstone (ORS) occurred during a critical period of Earth history both globally and regionally, with profound innovations in terrestrial ecosystems, and concomitant changes in sedimentological regimes. During this time, complex, terrestrial ecosystems were established (Jeram et al., 1990, Edwards and Selden, 1993, Gray, 1993, Edwards and Wellman, 2001, Shear and Selden, 2001), including the proliferation of land plants which had a major impact on both earth surface processes and atmospheric composition (Retallack, 1997, Berner, 1998, Elick et al., 1998, Algeo et al., 2001, Driese and Mora, 2001). The establishment of surface vegetation impacted the style of alluvial sedimentation, by stabilising flood plains and facilitating the development of soils (Davies and Gibling, 2010b, Gibling et al., 2014). By the late Silurian, vascular plants were well established on land, but were restricted to small forms, typified by Cooksonia which grew up to 2 cm tall and possessed networks of shallow, horizontal, primitive roots (Edwards and Wellman, 2001, Edwards and Richardson, 2004). By the late Silurian (Přídolí) and Lower Devonian, plants had become larger and possessed more complex root systems, and had greater diversity in terms of their morphology and geographic distribution (Davies et al., 2011). Their deeper rooting systems consolidated the soil and contributed organic content, thereby decreasing surface run-off (Davies and Gibling, 2010a, Davies and Gibling, 2010b) and creating new habitats and nutrient sources for terrestrial faunas (Selden and Edwards, 1989; Shear and Selden, 2001). The expanded vegetation cover has also been linked to enhanced rates of pedogenesis (Allan, 1974, Algeo et al., 2001, Brasier, 2011), as plant growth and the incorporation of organic matter raised the concentration of CO₂ in soils. Plant respiration and microbiobial decay facilitated the production of carbonic acid, which in turn dissolved limestone producing biocarbonate ions, which ultimately contributed to calcrete precipitation which is a characteristic of the ORS (Brasier et al., 2014).

Vascular plants spread to upland areas during the Devonian, where their deep root systems contributed to enhanced chemical and physical weathering of silicate rocks (Algeo et al., 2001, Brasier, 2011), enhancing the production and delivery of finer-grained sediments into the sedimentary systems (Davies and Gibling, 2010a, Davies and Gibling, 2010b). The transported sediment included the breakdown products of bacterial and fungal activity on plant detritus, which facilitated algal blooms and the development of anoxic conditions in closed water basins. A mass extinction event in the oceans at this time extinguished approximately 70–80% of shallow marine organisms (Algeo et al., 1998, 2001).

The proliferation of land plants is thought to have had a major impact on global atmospheric carbon dioxide (CO₂) and oxygen (O₂) concentrations (Retallack, 1997, Berner, 1998, Elick et al., 1998, Algeo et al., 2001, Driese and Mora, 2001). High levels of atmospheric CO₂ during the earlier Palaeozoic was followed by a significant drop at the Silurian-Devonian boundary. This is attributed to the rise of vascular land plants, which caused increases in the rates of chemical weathering of silicates and burial of organic matter which caused a rise of O₂ and drop in atmospheric CO₂ concentrations (Berner and Kothavala, 2001, Berner, 1991, Berner, 1998, Berner, 2006). As atmospheric concentrations of CO₂ are thought to be a primary control of global temperatures (Royer et al., 2004, Mann et al., 1998, Crowley and Berner, 2001) this would have contributed to global cooling (Algeo and Scheckler, 1998).

Rocks of the ORS preserve a diverse assemblage of animal trace fossils, consistent with the diversification of terrestrial faunas/enhanced complexity of terrestrial ecosystems. Examples from the Anglo-Welsh Basin are mainly preserved in fine-grained, alluvial facies. The most common trackways are those generated by arthropods, including arachnids, eurypterids, myriapods and Diplopodichnus. Other traces include worm burrows and faecal/feeding pellets (Morrissey et al., 2012, Davies et al., 2011, Davies and Gibling, 2010a, Davies and Gibling, 2010b).

The appearance of land plants had a dramatic and lasting effect on alluvial geomorphology and sedimentology (Davies et al., 2014). Prior to the Devonian, alluvial networks were predominantly coarse-grained and braided, with laterally-accreted channel sediments and floodplain muds and silts which subsequently developed soils to become the calcrete palaeosols. By the Early Devonian, the river networks diversified to include discrete channel lenses and the establishment of coastal plains (Morrissey et al., 2012).

The sediments of the ORS also provide opportunity to investigate syn- to post-orogenic deposition associated with a major orogeny—the Caledonian Orogeny. These sediments are modified by synchronous tectonism and volcanism. The influence of Variscan tectonics on basin formation and deformation is also recorded within its sediments (Hillier and Williams, 2006).

The ORS is known to be of late Silurian to early Carboniferous age (Friend and Williams, 2000, Becker et al., 2012), but the precise ages of its upper and lower boundaries within Britain and Ireland remains contentious. This is primarily because it is dominated by continental facies which have poor fossil preservation potential. Palynomorphs, vascular plant fossils and fish are useful, but problems remain with their correlation with schemes used in coeval marine facies (Becker and Kirchgasser, 2007). There are a number of schemes which utilise palynomorhs. These include Streel et al. (1987), Richardson and McGregor (1986), Richardson et al. (2000) and Steemans (1989). Vascular plant based biozonal schemes have been provided by Banks (1980) and subsequent work by Edwards et al. (2000). Biozonal schemes based on fossil fish (the principal macro-fossils) and other microvertebrate remains, are summarised by Blieck and Janvier (1989), Blieck and Turner (2000) and Vergoossen (2000) (Becker et al., 2012).

This special issue of the Proceedings of the Geologists’ Association has been compiled following a symposium, held in Brecon, south Wales in October 2014, entitled The Old Red Sandstone: is it Old, is it Red and is it all Sandstone? The symposium was held at the Elim Church Centre in Brecon and was convened jointly by the South Wales Geologists’ Association and the Fforest Fawr Geopark, supported by the Palaeontological Association and the Geologists’ Association. The aim was to bring together researchers with diverse interests in the Old Red Sandstone (ORS) in order to share ideas. Discussion covered many aspects, but mainly focused on the Midland Valley of Scotland and the Anglo-Welsh Basin. The present paper presents a summary of the Old Red Sandstone in these regions, and is intended to provide context to the themed collection of papers, summaries of which are provided at the end. See Appendix A at the end of this paper, for an acknowledgement to all those involved in making the meeting such a success.