Lower Border Group (Tournaisian) limestones from the Northumberland Basin

Abstract

Limestones in the Lower Border Group form parts of deltaic clastic-carbonate cycles. They contain a variety of grain types, including ooliths, algal grains and their derivatives and invertebrate bioclastic debris. A Q-mode cluster analysis of samples defines six facies: oolite–intraclast–peloid; oolite; bivalve–ostracod–peloid; intraclast; oncolite and brachiopod–crinoid–bivalve. An additional micrite facies is subjectively defined. The sediments were deposited in nearshore and offshore environments of the gulf-like Northumberland basin. Those of shallow subtidal origin are associated with stromatolites. Northward shelving of the gulf sea and bathymetric changes subsequent to periodic delta abandonments were the major controls upon the lateral and vertical successions of facies. Cements are dominantly of ferroan calcite composition, reflecting mudstone intercalations between carbonate members. Patchy ferroan dolomite is of late-diagenetic origin.