PRELIMINARY EVIDENCE OF EXTREME ENVIRONMENTS OF PANGEA: RED BEDS AND EVAPORITES OF THE TRIASSIC MERCIA MUDSTONE OF NORTHERN IRELAND
These rocks were deposited in a terrestrial setting characterized by saline pans and paleosols. The main processes in saline pans are flooding, evapoconcentration, and desiccation. Flooding in the Mercia Mudstone Group is indicated by climbing ripple cross-bedding, mud drapes, and halite-filled dissolution pipes. Evapoconcentration is suggested by bedded halite and gypsum. Desiccation is indicated by mudcracks and rip-up clasts. Red mudstones with soil slickenslides, blocky peds, circumgranular cracks, and suspected root features are suggestive of paleosols. The massive red mudstone could have been eolian, flood deposited, or precipitated directly from lake water.
Our preliminary results suggest that the Mercia Mudstone Group formed in acid, saline environments. The presence of halite, gypsum, iron oxides, kaolinite, and the lack of carbonates are characteristic of modern and ancient acid lakes and adjacent environments. Halite is ubiquitous regardless of the facies, showing that surface waters and groundwaters were saline. Scarce fossils of low diversity in the Mercia Mudstone Group suggest extreme environments. Our ongoing studies of petrography, mineralogy, and fluid inclusions will focus on the confirmation of depositional environment, acidity, and salinity of the Triassic of northeastern Pangea.