AN ACID PERENNIAL SALINE LAKE SYSTEM FROM THE TRIASSIC MERCIA MUDSTONE GROUP IN THE CARNDUFF 02 CORE, NORTHERN IRELAND

The Triassic Mercia Mudstone Group of Northern Ireland was once a shallow perennial saline lake system. Petrographic observations of the Gaelectric Carnduff 02 core from County Antrim recognized four main lithologies: bedded halite, bedded gypsum, displacive halite, and red mudstones. Bedded halite strata contain chevron crystals. Bedded gypsum is comprised of bottom-growth gypsum pseudomorphs, now composed of halite. Bedded halite and bedded gypsum contain dissolution pits, pipes, and surfaces and are interpreted as shallow perennial saline lake lithofacies. The displacive halite lithology contains blocky, randomly-oriented halite crystals in a red mudstone matrix; it represents a saline mudflat lithofacies. Mudstone types are composed of quartz, hematite, and kaolinite. Some mudstone units are massive. Some contain dewatering structures, climbing ripple cross-bedding, and mudcracks, which suggest deposition in dry mudflat that undergoes occasional flooding. Other mudstones contain blocky peds, soil slickensides, and circumgranular cracks. These mudstones are interpreted as paleosols.

Flooding events in shallow lakes and surrounding mudflats are interpreted from dissolution pipes/pits, dissolution surfaces, and climbing ripple cross-bedding. The presence of bedded evaporites is evidence of evapoconcentration. Chevron halite crystals suggest shallow water depths. There is no evidence of desiccation within shallow saline lake lithofacies. Lake waters were saturated with respect to Na, Cl, Ca, and SO₄. Shallow groundwaters in saline mudflats were NaCl-rich, interpreted from displacive halite. Mudcracks, dewatering structures, circumgranular cracks, and soil slickensides are interpreted products of wetting and drying within the dry mudflat lithofacies. The massive mudstone deposits may have been deposited by eolian processes or may have chemically precipitated from lake waters. The lack of carbonates and abundance of halite, gypsum, hematite, and kaolinite are lithologically similar to known acid saline lake systems. A related studied on bedded halite of the Mercia Mudstone Group suggests lake pH less than 1.

Terrestrial redbeds and evaporites may be good analogs for Mars strata. More work is needed on redbed and evaporites on Earth to better understand the past lakes on Mars.