Date of Graduation
Eberly College of Arts and Sciences
Geology and Geography
Kathleen C Benison
Timothy R Carr
Joseph J Donovan
The Triassic Mercia Mudstone Group from the Carnduff 02 core from County Antrim, Northern Ireland was deposited by shallow perennial saline lakes and associated environments such as saline mudflats, dry mudflats, siltflats, sandflats, and desert soils. This setting contained shallow acid brines that underwent flooding and evapoconcentration cycles in an arid climate. The redbeds and evaporites of the group have a complicated diagenetic history.;The Mercia Mudstone Group is composed of five main lithologies: mudstone, siltstone, displacive halite, bedded halite, and bedded gypsum. Other less abundant lithologies include sandstone, mud/halite conglomerate, and igneous intrusive rocks. The mudstone and siltstone are composed of quartz, hematite, kaolinite, and halite. Some mudstone and siltstone contain evidence of sheetfloods, including ripple cross-bedding and dewatering structures, as well as evidence of subaerial exposure, such as mudcracks and rip-up clasts. Other mudstones and siltstones are massive and may have been deposited by wind. Later pedogenesis developed soil slickensides, blocky peds, and circumgranular cracks. In areas where these mudstones and siltstones are saturated with saline groundwater, randomly oriented displacive halite crystals formed, leading to the displacive halite lithology. Red mudstones without displacive halite crystals formed in dry mudflats where the groundwater table was low. Likewise, displacive halite crystals formed in saline mudflats, adjacent to saline lakes where saline water table was high. Saline lakes are represented by bedded halite and bedded gypsum. Most bedded halite contains chevron crystals indicating bottom-growth from a shallow surface brine. Bedded gypsum consists of swallow-tailed bottom-growth crystals, also indicative of surface brines. Bedded halite and bedded gypsum both contain dissolution pits/pipes and dissolution surfaces that are commonly covered by mud drapes. The saline lakes underwent cycles of flooding and evapoconcentration.;The diagenetic history of the Mercia Mudstone Group is complex. Mudstones and siltstones contain early reduction spots, early rare displacive halite crystals, and late sylvite veins. Displacive halite units contain abundant early halite grown in shallow groundwater and also host rare late sylvite veins. Bedded halite and bedded gypsum contain early diagenetic features such as dissolution pipes/pits. Another late diagenetic feature is gypsum replaced by halite. Tertiary intrusive rocks cross cut all Mercia Mudstone Group lithologies.;The saline nature of the Mercia Mudstone Group is interpreted based on high abundance of evaporites throughout the group such as bedded halite, bedded gypsum, and displacive halite. Lack of carbonates, as well as abundance of hematite and kaolinite in lake facies, suggests acidity. In summary, the early Triassic of Northern Ireland was characterized by shallow acid perennial saline lakes and red desert siliciclastics.
Andeskie, Anna Sofia, "Depositional and Diagenetic History of the Triassic Redbeds and Evaporites from the Mercia Mudstone Group in the Carnduff 02 Core of County Antrim, Northern Ireland" (2016). Graduate Theses, Dissertations, and Problem Reports. 5105.