A late Ordovician (Hirnantian) karstic surface in a submarine channel, recording glacio‐eustatic sea‐level changes: Meifod, central Wales

Abstract

The growth and decay of the end-Ordovician Gondwanan glaciation is globally reflected by facies changes in sedimentary sequences, which record a major eustatic fall and subsequent rise in the Hirnantian Stage at the end of the Ordovician. However, there are different reported estimates of the magnitude and pattern of sea-level change.Particularly good evidence for end-Ordovician sea-level change comes from a sequence at Meifod in central Wales, which has a karstified limestone unit within a channel incised into marine shelf sediments. Pre-glacial (Rawtheyan) mudstones have a diverse fauna suggesting a mid-to-deep-shelf water depth of c. 60 m. The channel, 20 m deep, was incised into these mudstones and partially filled with a mixture of fine sand and detrital carbonate. The taphonomy of bioclasts and intraclasts indicates that many had a long residence time on the sea floor or suffered diagenesis after shallow burial before being resedimented into the channel. The presence of carbonates on the Welsh shelf is atypical and they are interpreted as having accumulated as patches during a minor regression prior to the main glacio-eustatic fall. Comparison of the carbon stable-isotopic values of the bioclast material with the global isotopic record confirms that most of the material is of Rawtheyan age, but that some is Hirnantian. The resedimented carbonates lithified rapidly and formed a limestone, several metres thick, in the deepest parts of the channel. As sea-level fell, this limestone was exposed and eroded into karstic domes and pillars with a relief of over 2 m. The overall, glacioeustatic, sea-level fall is estimated to be in excess of 80 m.A succeeding sea-level rise estimated to be 40-50 m is recorded in the laminated crust that mantles the karstic domes and pillars. The crust is formed of encrusting bryozoans, associated cystoids, crinoid holdfasts and clusters of the brachiopod Paromalomena, which is normally associated with mid-shelf environments. Fine sands buried the karst topography and accumulated to fill the channel. In the sandstones at the base of the channel there is a Hirnantia fauna, while in the sandstones high in the channel-sequence there is cross-stratification characteristic of mid-shoreface environments. This would indicate a fall of sea-level of c. 30 m. The subsequent major transgression marking the end of the glaciation is not recorded at the Meifod locality, but nearby exposures of mudstones suggest a return to mid-to-deep-shelf environments, similar to those that prevailed before the Hirnantian regression.The Meifod sequence provides strong evidence for the magnitude of the Hirnantian sea-level changes and by implication confirm larger estimates for the size of the ice sheets. Smaller oscillations in relative sea-level seen at Meifod may be local phenomena or may reflect eustatic changes that have not been widely reported elsewhere.

The search for ‘hot shales’ in the western Kufra Basin, Libya: geochemical and mineralogical characterisation of outcrops, and insights into latest Ordovician climate

“…from the Yangtze platform of South China (e.g., Xu et al 2004). This biofacies change was accompanied by a climate change, with a cold and dry climate during the Hirnantian glaciation and increasing temperature (global warming) throughout the latest Hirnantian into the early Silurian (e.g., Brenchley et al 2003Brenchley et al , 2006. It seems that the studied Tanezzuft Formation sections in Jabal Eghei span an icehouse to greenhouse transition.…”

Evolution of the Late Ordovician plaesiomyid brachiopod lineage in Laurentia

“…The associated eustatic sea level drop has been estimated at 50-100 m in magnitude (Brenchley and Newall 1980), with more recent observations suggesting it to be in the upper part of this range (Brenchley et al 2006). Sea levels would still have been higher than today ; see Figure 1.3) -around 140 m above present levels.…”

Hirnantian (latest Ordovician) glaciations and their consequences for the Oslo Region, Norway, with a revised lithostratigraphy for the Langøyene Formation in the inner Oslofjorden area

“…On Anticosti island, two erosional subaerial surfaces were later flooded during the end Hirnantian (Ghienne et al, 2014). These must be drowned shorelines, whereas repeated erosion and channelling of the shelf due to glacio-eustatic changes are reported from the Czech Republic (Štorch, 2006) and Meifod, central Wales (Brenchley et al, 2006). The wellstudied incised channels found in the Oslo area were mainly river-cut valleys.…”

“…Kröger et al (2015) linked deep karstic surfaces from the Boda limestone in Sweden to the Hirnantian glaciations. Karstic surfaces of Hirnantian age are also found in submarine channels in Meifod, central Wales (Brenchley et al, 2006). Incised valleys are described from the Upper Ordovician in Iowa (Johnson & Baarli, 2007).…”

“…sea-level fluctuations on many continents at different palaeolatitudinal positions (Kaljo et al, 2001;Bergström et al, 2006;Brenchley et al, 2006;Štorch, 2006;Johnson & Baarli, 2007;Davies et al, 2009;Ghienne et al, 2014;Demski et al, 2015;Kröger et al, 2015). The Oslo Region was situated in a subtropical position during the Ordovician (Torsvik & Cocks, 2013), and the effects of sea-level changes are also recorded there.…”