Karstification and pedogenesis on a late Dinantian carbonate platform, Anglesey, North Wales

Abstract

SUMMARY The late Dinantian (Lower Carboniferous) carbonate succession on Anglesey, North Wales, comprises over 20 transgressive/regressive shoaling upwards cycles. Cycle tops are marked by distinctive hummocky surfaces, interpreted as palaeokarstic, commonly overlain by green bentonitic clay palaeosols with fossil rootlets, and locally by red terra-rossa clays. Limestones immediately below the karstic surfaces display structures and fabrics typical of calcretes, including rhizoliths, calcrete ooids, micrite pellets, micritised grains and micritic vadose cements. These pedogenically altered limestones are commonly capped by laminar calcrete crusts of accretionary origin. The effects of contemporary brecciation and growth of microspar further contribute to a variety of emergent profiles developed at minor cycle tops. Profile development was in two stages. During the early syn-lithification stage, closely following emergence, plants and soil-dwelling faunas colonised unconsolidated platform carbonates. Micritic coatings and cements were deposited and wind-blown volcanic fines accumulated at the surface. During the ensuing post-lithification stage, roots and downward percolating rain waters could no longer penetrate the underlying case-hardened carbonates. Micrite was deposited as sheets at the soil/rock interface, building laminated crusts rich in rhizoliths, whereas elsewhere, or at other times, sub-soil dissolution led to the formation of the characteristic karstic morphology. Preferential dissolution within broad karstic depressions generated localised pockets of reddened insoluble residues. Brecciation re-exposed lower parts of some profiles to secondary phases of calcretisation and, with concomitant dissolution and downward infiltration of clay palaeosol material, created deep, regolith-like rubbly profiles within which there was extensive replacive and displacive growth of microspar.