The ordovician‐silurian boundary in the garth area of southwest powys, wales

Abstract The Ordovician‐Silurian rocks at Garth, 30 km northeast of Llandovery, have been remapped. Within the siltstone‐dominated succession, the coarser Cwm Clŷd Formation is regarded as stratigraphically equivalent to the A1 sandstone of Llandovery and has yielded a single specimen of Eostropheodonta hirnantensis from near the base in the northernmost exposures. The Formation rests with a stratigraphical break on fossiliferous Rawtheyan siltstones in the south but is underlain by later Rawtheyan and abundantly fossiliferous Hirnantian sediments (Wenallt Formation) further north where the succession appears continuous. Sedimentary and faunal evidence demonstrates the shallowing of the Ashgill sea before a transgression and deepening started in late Hirnantian times. The implications of the sequence in determining the systemic boundary are discussed.