Bovevagh

Also within

Londonderry County

Habitats

Fast flowing river and stream

Geology & landscape

Geological Period: Quaternary

Landscape processes: Glacial

Description

Bovevagh Pleistocene Bovevagh is an example of a shallow deltaic setting, where gravelly sediments prograded northwards into fossiliferous glaciomarine clays. If the sequence is in situ, it records the development of major lowland ice masses during the Early Midlandian resulting in marked isostatic depression. While the Bovevagh sequence is open to two possible interpretations, it is clearly of major significance. This association of large lowland ice sheets and isostatic depression during the early part of the last cold stage, has important palaeoenvironmental and palaeoclimatic implications for Quaternary research in north-west Europe. No other site known in Ireland demonstrates this setting during the Early Midlandian and its preservation is probably a result of weak glacial activity in the Dungiven basin during the Late Midlandian. The site near Bovevagh Old Church occurs 12km inland at a height of about 9Sm O.D; it is therefore of critical scientific importance with respect to its location and altitude. The site was described as early as 1843, since when these fossiliferous muds have been viewed as reworked seafloor deposits as elements within glacial tills. More recent sedimentological and palaeoecological work suggests that they are more complex than hitherto realised, and are a remnant of a low angle, prograded glaciomarine delta. The exposure consists of poorly-stratified gravels, sands and sandy diamicts which interfinger laterally with massive, fossiliferous mud. Poorly-sorted gravel at the base of the exposed sequence consists of amalgamated beds of cobble to pebble gravel, which are generally ungraded and massive. At several horizons in the gravels, strong imbricated fabrics indicate a palaeoflow to the north. Thin beds 10cm of silty-clay within the gravels thicken northwards and pass below the base of the exposure. The basal gravels are overlain abruptly by a flat-lying bed of silty clay, which is highly variable internally and ranges from massive to faintly-laminated. This unit is interbedded with the fossiliferous marine muds towards the end of the section. It contains comminuted marine shell fragments. The contact with the overlying pebble and cobble gravel is sharp and marked by a discontinuous stone lag. The upper unit of gravels is similar in clast composition and sedimentological characteristics to the lower gravel unit. The upper 10m to lSm of the sequence is not exposed, though small slumps immediately behind the graveyard suggest that a brown, massive to stratified diamict is present. The interbedded relationships between massive grey mud at the northern end of the sequence and the coarse clastic units at the southern end of the sequence, show that both were deposited contemporaneously. The most common fossil is the Tower Shell Turritella communis, which occurs in profusion, with about 50 of the specimens being perfectly preserved. Some mud samples contain low concentrations of poorly-preserved foraminifera. Others, however, contain very distinct, well-preserved, foraminiferal assemblages. One assemblage is boreal in aspect and is similar to modern environmental settings of western Norway. The other reflects shallow water glaciomarine conditions with a distinct Arctic aspect. These samples require further processing before more definite statements can be made. Two AMS dates on a whole shell of Aporrhais pespelicani >34ka B.P. and several whole shells of Turritella communis >44ka B.P. are infinite. Amino acid ratios indicate an Early Midlandian age. Flat-lying beds together with the absence of obvious glacitectonic structures at this site suggest that the sequence is in situ and has not been transported by ice. Rapid interbedding of facies and the preservation of many primary sedimentary structures and bedding planes are also indicative of an undisturbed succession. However the exposure is small and could be part of a much larger deformed unit which is not exposed. The marked lateral and vertical facies changes at the site reflect a range of processes. Coarse-grained gravels are the result of mass flow and traction activity with a northwards palaeoflow. Grain size variability shows that flows ranged from fluidal to more dense flows, with debris flow events also evident. The delicate, interbedded relationship between these flow units and the mud shows that both are contemporaneous. The most likely depositional setting is where a stream formed a low-angle delta, which prograded into a shallow water glaciomarine setting dominated by plume deposition from suspension. If the muds are in situ then the succession records deep and prolonged isostatic depression and glaciomarine conditions in the Dungiven basin. The implications are that extensive ice masses developed, probably during the Early Midlandian in north-west Britain. An alternative explanation involves ice transport of sea floor muds, followed by reworking in a shallow proglacial water body, which would explain the sediment geometry of the section. In this case, large onshore ice masses are required during the Early Midlandian leading to ice-damming, but this does not require deep isostatic depression to explain the present height of the muds and their inland position. The second model would explain the presence of mixed faunas in a simpler way than the first, which would require sustained isostatic compensation during the deglacial phase and boreal-type environmental settings. Further work is in progress to resolve this problem. https://www.daera-ni.gov.uk/publications/bovevagh-assi

Plant communities