ENVIRONMENT AND CLIMATE CLUES FROM EARLY DIAGENETIC FEATURES IN THE TRIASSIC MERCIA MUDSTONE GROUP OF NORTHERN IRELAND
Some diagenetic events occurred so early that they can be considered syn-depositional. Displacive halite, which composes 25% of the Mercia Mudstone Group in the core, is an early diagenetic feature that consists of blocky halite crystals that are randomly oriented within a mudstone matrix. Displacive halite precipitated from shallow groundwaters in brine-saturated mudflats within a dry climate. The abundance of displacive halite suggests that the climate must have been significantly arid for long periods of time. Other early diagenetic features include dissolution surfaces, pipes, and pits on bedded halite and bedded gypsum, which form during flooding of shallow saline lakes. Early halite cement is pervasive; it fills dissolution pits and lithifies red mudstones. Like displacive halite, the halite cements in dissolution pits and red mudstones likely formed as shallow saline groundwater underwent evapoconcentration. Gray circular reduction spots with black centers were observed in some red mudstones and likely formed by local reduction around rare organic material.
There are multiple other diagenetic features that are clearly post-depositional. Bedded bottom-growth gypsum was replaced by halite under subsurface pressures and temperatures. Orange, tangy-tasting sylvite veins cross-cut displacive halite, mudstones, and siltstones but not bedded halite or bedded gypsum. A rare basal mud/halite conglomerate, composed of randomly oriented, laminated gray and red mud clasts and blocky halite cement, is poorly constrained within the paragenetic sequence. This could have formed as a fault breccia, a flood deposit, or evaporitic karst. These later diagenetic features were a product of water-rock interactions that occurred in the subsurface.
The early diagenetic features in the Mercia Mudstone Group, such as displacive halite, are unique to the specific environment and climate. These arid, saline environments were long-lived in northeastern Pangea.