Abstract

Late Oligocene sediments of the Lough Neagh group in Northern Ireland were deposited in and around a closed, shallow, perennial lake. There are four distinct lithological units present in this group. The lowermost unit is weathered basalt and clay-supported conglomerate, which forms a fining-upward sequence. This unit is overlain by an alternating sequence of clay and lignite units. The clays contain sand, silt and diagenetic ironstone laminae and in places contain fragments of basaltic, rhyolitic, micritic and schistose rocks. The sandstone and siltstone unit forms coarsening-upward and fining-upward sequences in clays above and below the main lignite bed respectively. The lignite occurs in woody and non-woody forms.

Five sedimentary facies are recognised in the Lough Neagh Group, representing 1) a fluvial environment, 2) a fluvio-lacustrine environment, 3) a marginal lacustrine-deltaic environment, 4) a lacustrine-nearshore and open lacustrine environment, and 5) a nearshore lacustrine and swampy environment.

Lacustrine nearshore and open lacustrine environments are dominant, whereas the fluvial and deltaic environments occur locally. The coarsening-upward sequence suggests deltaic deposition and contains the remains of leaves, fruits, rootlets and lizard’s teeth. The fining-upward sequence containing plant and gastropod fossils suggests a fluvio-lacustrine environment. The thick sequence of structureless detrital clays were deposited under lacustrine conditions. The lignite horizons were formed in a swampy environment.

The vertical lithological repetition in the lacustrine and marginal lacustrine facies are attributed to lake-level fluctuations in response to climatic changes, rate of deposition, tectonic conditions of the basin and the source rocks.