A polygenetic palaeosol from the Silurian (Wenlock) of southwest Ireland

Abstract

A problematic palaeosol unit caps a thick basalt at the top of the Clogher Head Formation, Dunquin Group (late Wenlock, Silurian) in the Dingle Peninsula, southwest Ireland. The main part of the unit consists of an altered calc-alkaline basalt with a fine clay assemblage of chlorite, corrensite and smectite probably resulting from weathering during Wenlock times. The uppermost part of the unit consists of detrital silicates which were admixed by bioturbation and well-defined burrow-fills are present. The underlying, weathered basalt was possibly truncated before the detrital horizon was deposited, and finally the whole unit underwent a phase of surface-water gleying. A catenary relationship existed locally with the pseudo-gley developed lower in the landscape while contemporaneous basalts in topographically higher, better drained areas, largely escaped the effects of gleying; these now exhibit reddened tops. The complex history and the possiblity of the incompleteness of the profile makes accurate assessment of the soil/climate function difficult. However, the occurrence of calcrete-bearing palaeosols in the overlying Mill Cove Formation supports the view that the basalt was weathered under a sub-humid or semi-arid climate. Considering the alteration of the Dunquin Group to actinolite-prehnite grade, the preservation of the smectite is highly unusual. There is no evidence for its origin due to more recent weathering or interstratal neoformation; it is thus interpreted as a relict product of the original weathering phase.