SUMMARY 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 Northumberland 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 entirely of non-marine origin, with any marine influences confined to the westernmost part of the basin. The shaly siltstone was proved in the Alnwick Borehole drilled through the upper section of the Fell Sandstone in an otherwise sandstone-dominated succession. The presence of marine influenced conditions more than 60 km inland from the coeval shoreline reflects contemporaneous intrabasinal tectonics and the location of the Alnwick Borehole at the northeastern end of the central graben within the Northumberland Basin. Marginal fault activity, contemporaneous with Fell Sandstone deposition, caused gentle tilting of the graben floor, thus allowing marine conditions to penetrate farther inland than previously known. The ostracod-bearing marine shaly siltstone, which lies above and below braided river sandstones, provides a key sequence 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 immediately overlying erosion surface records low stand incision, facies tract dislocation and the development of a possible sequence boundary, with the overlying stacked, cross-bedded, coarse pebbly braided channel sandbodies referred to the lower and middle parts of a transgressive systems tract that spans the boundary between the Fell Sandstone and succeeding, more marine influenced Scremerston Coal Group. The lower, finer-grained, fluvial succession beneath the transgressive surface is tentatively assigned to part of an incompletely exposed transgressive systems tract.