A late ordovician/early silurian non‐depositional slope and perched roll‐over basin along the tywi anticline, mid wales

Ordovician stratigraphy between the Sugarloaf and the Wye valley is summarized with particular attention given to the area northeast of Abergwesyn. New data refine thickness estimates for the Cerriggwynion Formation, the Trembyd Formation (new), and the Cefn Cynllaith Formation (new). The major Bron Rhudd Fault is suggested to have been a synsedimentary down‐to‐basin fault which was later reactivated, folded, and suffered local reversal of throw. Roll‐over into this fault controlled deposition of mass‐flow conglomerates in a perched basin. During the late Ordovician‐early Silurian the axis of the Tywi Anticline underlay a non‐depositional slope which developed by northwest‐directed progradation of a shelf margin, possibly accentuated by flexure over a basement fault zone with contemporaneous basin‐down normal movement. The slope angle cannot be estimated from the geometry of the Ordovician because the contemporaneous throw on the Bron Rhudd Fault cannot be determined, but from the rate of Silurian onlap it was about 5 degrees.