Early Mississippian evaporites of coastal tropical wetlands

Abstract

Extensive 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.

Palaeogeography of tropical seasonal coastal wetlands in northern Britain during the early Mississippian Romer's Gap

“…The analysis presented here of the Ballagan Formation extends the detailed picture that has emerged from recent studies of the Norham cores and the coastal section at Burnmouth by Bennett et al (2016,2017), Kearsey et al (2016) and Millward et al (2018) (Millward et al 2018). A persistent, dominantly meandering, fluvial system in the east of the region hosts the greatest diversity of terrestrial habitats with the riparian strip extensively vegetated as indicated by a diverse suite of palaeosols ranging from ephemeral plant colonies to marsh and forest (Retallack & Dilcher 1988;Kearsey et al 2016).…”

“…Floodplain siltstones and thin dolostones are interbedded with the Whita sandstone units, but there are very few palaeosols. Surface exposures reveal little evidence of evaporite deposits in this belt, with the exception of the Hoddom Borehole, near Ecclefechan, which proved abundant layers and nodules of gypsum deposited in a perennial brine pan (Millward et al 2018).…”

A lungfish survivor of the end-Devonian extinction and an Early Carboniferous dipnoan radiation

“…Such environments were not exclusively freshwater as indicated by Kearsey et al (2016) as well as trace-fossils (Bennett et al. 2017) and gypsum/anhydrite evaporites (Millward et al 2018). Together these data provide evidence of marine-to-brackish water input onto the floodplain.…”

Multi‐proxy constraints on the significance of covariant δ13C values in carbonate and organic carbon during the early Mississippian

“…Evaporite deposits have been documented from the Middle Ordovician through to the Tournaisian in regions around the Transcontinental Arch and the Williston Basin, and describe depositional environments ranging from sabkha‐type environments to evaporite–solution breccias (Johnson, ). Mapping of evaporitic facies in the time‐equivalent Ballagan Formation in Nova Scotia and Scotland shows the presence of gypsum nodules in siltstone, dolostone with gypsum and halite in seven of the eight studied sections in depositional environments described as coastal tropical wetlands (Millward et al ., ). The observation of evaporites around the globe during this period demonstrates that evaporite deposition during this period was not a locally restricted phenomenon, and is likely to reflect conducive palaeoclimatic conditions (Millward et al ., ).…”