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Keywords:

  • Sills;
  • Peperites;
  • Fluidization;
  • Block lavas;
  • Ordovician;
  • English Lake District

Abstract

Criteria by which such sills can be distinguished from block lavas include discordance and fault-controlled geometry, vesiculation and homogenization of sediments adjacent to upper contacts, and the presence of pillows or peperites at upper contacts. Because sills do not always show these characteristics in the field, an extrusive origin for a sheet should be demonstrated and not assumed. However, it is not always possible to make the distinction, in which case the non-genetic term sheet can be usefully applied.

Field studies suggest that a substantial part of the Borrowdale Volcanic Group is composed of sills. Thus previously erected lithostratigraphic schemes may not correctly indicate the sequence of magmatic events, and the correlation of locally defined stratigraphies will not be possible until the regional importance of high-level intrusive bodies has been established.