Palaeoclimatic significance of calcretes in the Dinantian of the Cockburnspath Outlier (East Lothian–North Berwickshire)

Abstract

Synopsis Calcretes are present at two horizons in the Dinantian Inverclyde and Strathclyde Groups of the Cockburnspath Outlier. The lower calcrete, probably uppermost Courceyan to Chadian in age (at the top of the Hurker Member, Ballagan Formation), was formerly interpreted as a tectonic breccia. Petrology shows that the bed is a calcrete with associated root casts. Pyrite within this calcrete suggests that it underwent marine hydromorphism, probably caused by rising marine groundwater associated with relative sealevel rise. The upper calcrete, probably Asbian in age (in the Cove Harbour Member, Aberlady Formation), is mature (stage 3) and has undergone in situ gratification caused by desiccation and shrinkage The Hurker Member calcrete (and other palaeoclimatic indicators) suggest that semi-arid climatic conditions prevailed in Scotland throughout the Lower Carboniferous until Chadian times. This is consistent with palaeoclimatic indicators elsewhere in Euramerican rocks. The Cove Harbour Member calcrete probably correlates with calcretes in the Sandy Craig Formation in Fife. However, it occurs in a rock sequence dominated by humid climatic indicators (coals, sideritic mudstone palaeosols and oil-shales), implying a regionally significant semi-arid period, during otherwise humid conditions. This finding is consistent with fluctuating humid to semi-arid palaeoclimatic phases proposed for the Dinantian of England and Wales.