MARINE OSTRACODS IN THE LOWER CARBONIFEROUS FLUVIATILE FELL SANDSTONEGROUP - EVIDENCE FOR BASE-LEVEL CHANGE AND MARINE FLOODING OF THE CENTRAL GRABEN, NORTHUMBERLAND BASIN

Seven species of ostracod have been identified from the coarser, more silty laminae within a thin shaly siltstone interval within the Lower Carboniferous Fell Sandstone Group in the eastern part of the Northumb erland Basin, all of which indicate a quiet, nearshore, shallow water marine environment. This is the first documented occurrence of marine ostracods from the Fell Sandstone which has always been regarded as en tirely of non-marine origin, with any marine influences confined to th e western-most part of the basin. The shaly siltstone was proved in th e Alnwick Borehole drilled through the upper section of the Fell Sands tone in an otherwise sandstone-dominated succession. The presence of m arine influenced conditions more than 60 km inland from the coeval sho reline reflects contemporaneous intrabasinal tectonics and the locatio n of the Alnwick Borehole at the northeastern end of the central grabe n within the Northumberland Basin. Marginal fault activity, contempora neous with Fell Sandstone deposition, caused gentle tilting of the gra ben floor, thus allowing marine conditions to penetrate farther inland than previously known. The ostracod-bearing marine shaly siltstone, w hich lies above and below braided river sandstones, provides a key seq uence stratigraphic surface across this part of the basin. The base of the unit is a transgressive surface with the shale itself equivalent to a maximum flooding to truncated shale prone early high stand. The i mmediately overlying erosion surface records low stand incision, facie s tract dislocation and the development of a possible sequence boundar y, with the overlying stacked, cross-bedded, coarse pebbly braided cha nnel sandbodies referred to the lower and middle parts of a transgress ive systems tract that spans the boundary between the Fell Sandstone a nd succeeding, more marine influenced Scremerston Coal Group. The lowe r, finer-grained, fluvial succession beneath the transgressive surface is tentatively assigned to part of an incompletely exposed transgress ive systems tract.