Acid-saline-lake Systems of the Triassic Mercia Mudstone Group, County Antrim, Northern Ireland

Abstract

The Triassic Mercia Mudstone Group of County Antrim, Northern Ireland, is dominated by red beds and evaporites. After only limited study, both marine and continental environments have been proposed previously. Here, we describe these rocks for the first time from core and petrographic observations to interpret depositional environments and conditions, such as water depth, salinity, and aridity. Bedded halite, bedded gypsum, displacive halite, and red siliciclastic mudstone lithologies comprise most (~80%) of the 591.6-m-thick Mercia Mudstone Group in the Gaelectric Carnduff-2 core. Bedded halite consists of chevron and cornet crystals, indicating bottom-growth from shallow surface brines. Bedded-gypsum lithology is composed of halite-replaced pseudomorphs after swallowtail, bottom-growth gypsum crystals. Both bedded halite and bedded gypsum contain dissolution features and are commonly overlain by mud drapes. Bedded-halite and bedded-gypsum lithologies are interpreted to have formed in shallow saline lakes influenced by flooding and evapoconcentration. The displacive-halite lithology is composed of subcubic halite crystals, randomly oriented in mudstone, and represents deposition in a saline mudflat. Some mudstones contain ripple cross-lamination, dewatering structures, mudcracks, and rip-up clasts, suggesting shallow surface water and desiccation. Other mudstones are massive (structureless) and may have been deposited by wind. Both of these mudstone units were likely deposited in dry mudflats. Other red mudstones contain soil slickensides, blocky peds, and circumgranular cracks and are interpreted as paleosols. Our observations indicate that the Triassic Mercia Mudstone Group was formed by shallow perennial saline lakes and associated continental environments in an arid climate. Furthermore, lack of carbonates, lack of fossils, and paucity of organic matter suggest acid saline lakes and groundwaters. The Triassic Mercia Mudstone Group is similar to some other Pangean red beds and evaporites. Therefore, we hypothesize that the supercontinent was an arid barren landscape hosting acid-saline lakes.

The Physical and Chemical Sedimentology of Two High-Altitude Acid Salars in Chile: Sedimentary Processes In An Extreme Environment

“…Ephemeral acid saline lakes have been documented in Western Australia (e.g., Mann 1988;McArthur et al 1991;Alpers et al 1992;Benison et al 2007;Bowen and Benison 2009) and northwestern Victoria (e.g., Alpers et al 1992;Long et al 1992aLong et al , 1992bMacumber 1992). Permian and Triassic acid saline lake deposits in the U.S. midcontinent and Northern Ireland have also been identified (Benison et al 1998;Benison and Goldstein 2000;Andeskie et al 2018). Although similar in water chemistry and mineral assemblage to modern Australian and Permo-Triassic acid saline environments, salars Gorbea and Ignorado have a different geological setting and a different set of sedimentary processes and products than any acid brine environments previously studied.…”

Dryland continental mudstones: Deciphering environmental changes in problematic mudstones from the Upper Triassic (Carnian to Norian) Mercia Mudstone Group, south‐west Britain

“…The geochemistry of mudstones within the MMG succession has also been discussed, with some favouring marine or marine derived fluids as the source of Mg in the clay minerals and carbonates (Taylor, ; Leslie et al ., ; Leslie & Tucker, ), whereas others have favoured continentally evolved fluids as the source (Wright & Sandler, ). The most recent investigation, of a halite‐dominated succession in Northern Ireland (Andeskie et al ., ), attributes deposition entirely to continental processes, with evaporation, aeolian process and periodic flooding created acid–saline lakes, mudflats and desert palaeosols.…”

“…The relative importance of mixed continental/marine fluids and continentally evolved brines for dolomitization are difficult to assess in the absence of geochemical data. However, this study has found no direct evidence, such as marine biota, tidal features or significant stratigraphic breaks (Andeskie et al ., ), to indicate significant marine influence in the MMG successions of the Severn and Bristol Channel basins. Instead the presence of widespread pedogenic features within alluvial, aeolian and playa‐lacustrine facies is consistent with deposition in a non‐marine, continental setting.…”

“…Gypsum nodules in the blocky claystone beds are considered pedogenic on account of their close association with slickenside planes, blocky structure, mud cracks and displacive fabrics. Nodule precipitation was likely driven by capillary evaporation of Ca sulphate‐enriched meteoric waters during palaeosol development (Warren, ), possibly with acid‐saline lakes and playas supplying sulphates through groundwater flow and/or aeolian transfer (Andeskie et al ., ).…”

“…Debate also surrounds the nature and cause of cyclicity recognized in parts of the MMG, which is variously attributed to basin subsidence (Wills, ), eustasy (Arthurton, ), climate (Porter & Gallois, ) or a combination of these (Tucker, ). The most recent investigation (Andeskie et al ., ) interpreted MMG mudstones and evaporites from the Larne Basin, Northern Ireland, as being developed in perennial saline lakes and mudflats with palaeosols, but the cored succession lacked recognizable stratigraphic units with which to attempt a wider regional correlation.…”