Abstract
AbstractExtensive evaporites in Lower Mississippian successions from palaeoequatorial regions are commonly used as evidence for an arid to semi‐arid palaeoclimate. However, in this article, detailed studies of evaporites and their context refute this interpretation. Detailed sedimentological and petrographical analysis of the Lower Mississippian of northern Britain, is combined with archived log data from more than 40 boreholes across southern Scotland, northern England and Northern Ireland, and published literature from Canada. Two key cores from the Tweed Basin and the northern margin of the Northumberland – Solway Basin contain 178 evaporite intervals and reveal twelve distinct forms of gypsum and anhydrite across seven facies that are associated with planar laminated siltstone and intercalated thin beds of ferroan dolostone. Nodular gypsum and anhydrite, typically in intervals <1 to 2 m thick, are integral components of the succession. Nodular evaporite occurs within about 1 m of a palaeosurface, but most evaporite deposits represent ephemeral brine pans to semi‐permanent hypersaline lakes or salinas on a floodplain that was subjected periodically to storm surges introducing marine waters. Formation of evaporites under a strongly seasonal climate in a coastal wetland is supported by palaeosol types and geochemical proxies, and from palaeobotanical evidence published previously. Although 65% of modern equatorial areas experience a strongly seasonal climatic regime, salinas and sabkhas are a minor component today in comparison with the evidence from these Lower Mississippian successions. This implies that the earliest terrestrial environments were complex and dynamic, providing a diverse range of habitats in which the early tetrapods became terrestrialized and represent a setting that is rarely preserved in the geological record.
Highlights
Other observations from the UK Tournaisian are cited as evidence for an arid or semi-arid climate, including the presence of calcretes in south-east Scotland (Andrews & Nabi, 1998) and southern Britain (Wright, 1982, 1990), and the occurrence of evaporite rocks (Scott, 1986; Bateman & Scott, 1990)
The current study describes a diverse suite of evaporite rocks from the lower Mississippian Ballagan Formation of northern Britain and interpret these within their host sedimentary succession (Bennett et al, 2016, 2017; Kearsey et al, 2016) and regional context, discussing the implications for regional climate interpretations
Small-scale evaporite deposits are a widespread and important facies of the coastal wetlands of equatorial Laurussia. They occur throughout the 13 million years spanned by the Tournaisian Ballagan Formation
Summary
Two contrasting models for the early Mississippian climatic regime of southern Laurussia are presented in the literature: a humid climate (e.g. van der Zwan et al, 1985) with seasonality (e.g. Falcon-Lang, 1999) or an arid/semi-arid climate (e.g. Scott, 1986; Wright, 1982, 1990; Bateman & Scott, 1990; Andrews & Nabi, 1998). In the Tournaisian of south-east Scotland, Kearsey et al (2016) linked the record of repeated river flood events (Bennett et al, 2016) with palaeosol data to conclude that the climate was tropical and markedly seasonal These interpretations are consistent with the association of the reconstructed forest tree Stamnostoma huttonense with well-drained palaeosols (Retallack & Dilcher, 1988) and the presence of growth rings in fossil wood (Falcon-Lang, 1999, 2004). Other observations from the UK Tournaisian are cited as evidence for an arid or semi-arid climate, including the presence of calcretes in south-east Scotland (Andrews & Nabi, 1998) and southern Britain (Wright, 1982, 1990), and the occurrence of evaporite rocks (Scott, 1986; Bateman & Scott, 1990). Other observations from the UK Tournaisian are cited as evidence for an arid or semi-arid climate, including the presence of calcretes in south-east Scotland (Andrews & Nabi, 1998) and southern Britain (Wright, 1982, 1990), and the occurrence of evaporite rocks (Scott, 1986; Bateman & Scott, 1990). Young (1867) first recorded veins of fibrous gypsum cutting Tournaisian strata north of Glasgow, but the widespread occurrence of evaporite rock in this succession was revealed in a suite of boreholes drilled across the region by the British Geological Survey from the late 1960s onward (e.g. Paterson & Hall, 1986)
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