SALINE LAKE SYSTEMS OF THE TRIASSIC MERCIA MUDSTONE OF NORTHERN IRELAND: LESSONS FROM TWO CORES

Red beds and evaporites are common in the Permo-Triassic of Pangea, yet their stratigraphy is not well-constrained due to low surficial preservation potential and reliance on rare, well-preserved, high-recovery cores. In addition, lateral correlative relationships are hindered by the rarity of multiple cores from the same units. Here, we describe the sedimentology and stratigraphy of two exceptionally preserved cores of the Triassic Mercia Mudstone Group from County Antrim, Northern Ireland. The Gaelectric Energy Storage Ltd. Carnduff-01 and Carnduff-02 cores, drilled in 2014 only 258 meters apart, provide an exceptional opportunity to better understand red bed and evaporite depositional environments.

Lithologies of the Mercia Mudstone in these two cores include bedded halite, bedded gypsum, displacive halite in siliciclastic mudstone, siliciclastic mudstone, sandstone, and mud-halite breccia. These rocks formed in saline lakes, saline and dry mudflats, and desert soils. Diagenetic features consist mainly of halite replacement of gypsum, sylvite veins, and basaltic intrusions.

Although the two cores are generally similar, sedimentologic and stratigraphic differences are only noted by centimeter-scale observations of the lithologies. Differences include: (1) color variation, with redder siliciclastics in the Carnduff-02 core; (2) presence of igneous intrusions in Carnduff-02 core, but absence in Carnduff-01 core; and (3) variations in the abundances and thicknesses of some lithologies. There are fewer paleosols in the Carnduff-01 core. The evaporite lithologies have offsets of 22.0 to 40.2 meters between the cores; this offset is not related to structural dip, but may be due to topographic relief during deposition. The spatial and temporal correlation of individual beds are not feasible for the Mercia Mudstone over this 258 meter lateral distance. We propose that the Mercia Mudstone was formed by shallow saline lakes and adjacent mudflats that were strongly influenced in the depositional environment by winds, floods, and evapoconcentration, as well as local topography. Over relatively short time periods, these dynamic environments shifted laterally. This study shows that caution is warranted in correlation attempts for continental red beds and evaporites.