A hemipelagic fades from the Silurian succession in Wales consists of finely laminated, dominantly silt-grade sediment. It can be interbedded with sand- or mud-grade turbidites, or can occur as an independent facies. Its occurrence and character was controlled mainly by low bottom water oxicity. The pelagic fauna of the laminated hemipelagite supports this view. Many of the silt-grade particles have an aggregate structure and probably represent the faecal pellets of oceanic zooplankton. The fine lamination is thought to have formed by alternations of planktonic blooms and increased discharges of silt into the basin. By comparison with modern examples this alternation was likely to have been annual or seasonal.

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