Sedimentary environments of the basinal Llandovery of mid-Wales

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Abstract
Summary: The position of the Ordovician-Silurian junction in the mid-Wales rock sequence remains elusive. There is no evidence of a break in sedimentation between the periods but conditions of deposition changed abruptly about then; presumed Ashgill rocks, mainly argillaceous and disturbed, are overlain by Llandovery strata with even and regular bedding, yielding Glyptograptus persculptus (Salter). The former are barren and accumulated rapidly, near the foot of a slope facing west from the ‘shelf’, synchronously with a marine regression and a glaciation in N Africa. The regresson may therefore have been a sympathetic eustatic fall. Of the succeeding strata, those of lower Llandovery age embody much pelagic mud while the middle and upper Llandovery strata record an invasion by turbidite fans. Current indicators in the turbidites reveal westward (‘lateral’) flow below the Aberystwyth Grits (low turriculatus Zone), later augmented or replaced by northwards (longitudinal) flow. This change stemmed probably from a tectonic event to the south. Characteristic turbidite rhythms have been noted, varieties of which are used to define many of the formations. Such formations are probably highly diachronous.