Calcretes at sequence boundaries in upper carboniferous cyclothems of the Sydney Basin, Atlantic Canada

Abstract

Nodular and associated groundwater calcretes in coal-bearing cyclothems of the Sydney Basin mark the boundary between underlying marginal-marine and overlying alluvial deposits. They are inferred to represent lowstand surfaces (sequence boundaries). The host sediments include both sand/siltstones and palustrine limestones. Microfabrics of calcretes with detrital hosts show evidence of replacement, displacement and shrinkage (alpha fabrics), suggesting that the calcretes formed under relatively arid conditions. In contrast, microfabrics of nodular calcretes with limestone hosts exhibit predominantly beta fabrics, with root-induced brecciation and subordinate shrinkage fabrics. The calcretes and host limestones are inferred to have formed in topographic lows. Thick alluvial sediments with red calcic vertisols overlie the calcretes, and formed within the transgressive systems tract under conditions of abundant sediment supply. Coals and associated hydromorphic palaeosols lie near the transgressive maximum and in the highstand systems tract. The presence of calcareous palaeosols on lowstand surfaces suggests that the lowstands were times of relative climatic aridity.