NORMAL SALINITIES FOR PENNSYLVANIAN BASINS IN NORTH-WEST EUROPE: EVIDENCE FROM NEW AMMONOID-BEARING HORIZONS IN THE CARBONIFEROUS SHANNON BASIN, IRELAND

The Shannon Basin, located in Western Ireland, developed in Early Carboniferous times and is floored by shallow and deep-water Dinantian limestones. The basin fill succession consists of deep-water shales which blanket the basin floor, succeeded by basin floor turbidite sandstones (Ross Sandstone Formation), followed by slope and delta deposits in an overall shallowing-upward trend. Ammonoids and thin-shelled bivalves have long been known from widespread, black, carbonaceous shales known as “marine bands”. These faunas have proven to be excellent biostratigraphic tools in the Shannon Basin and elsewhere in NW Europe.

The apparent restriction of the fauna to these intervals of sediment starvation has often been explained by high stands of sea level affecting small, intracratonic basins, which were otherwise cut off from the open ocean. Periodical flooding of these basins linked to high-frequency, eustatic changes in sea level controlled by fluctuations in ice-sheet extent in Gondwana is thought to have resulted in normal marine fauna populating the basin waters and salinity reaching typical marine values. At low stands of sea level, when basin connections to the open ocean were more restricted, it has been proposed that salinity was reduced due to substantial influx of fresh water from large river systems.

However, recent evidence from the Ross Sandstone Formation shows that waters were not devoid of normal marine fauna during the times of higher sedimentation rates between the ‘marine bands’. Ammonoids have been found in muddy siltstone horizons outside the black shale horizons at four localities in the Loop Head Peninsula, Ireland. It is suggested that more will be found upon further inspection even though the faunas might be expected to be sparse due to the high sedimentation rates. The presence of normal marine faunas outside the traditional “marine bands” suggests that salinity values of the Shannon Basin and, by implication, of other deep water basins were normal in between periods of sediment starvation.