‘Pseudobreccias’ revealed as calcrete mottling and bioturbation in the Late Dinantian of the southern Lake District, UK

Abstract

AbstractTwo types of ‘pseudobreccia’, one with grey and the other with brown mottle fabrics, occur in shoaling‐upward cycles of the Urswick Limestone Formation of Asbian (Late Dinantian, Carboniferous) age in the southern Lake District, UK. The grey mottle pseudobreccia occurs in cycle‐base packstones and developed after backfilling and abandonment of Thalassinoides burrow systems. Burrow infills consist of a fine to coarse crystalline microspar that has dull brown to moderate orange colours under cathodoluminescence. Mottling formed when an early diagenetic ‘aerobic decay clock’ operating on buried organic material was stopped, and sediment entered the sulphate reduction zone. This probably occurred during progradation of grainstone shoal facies, after which there was initial exposure to meteoric water. Microspar calcites then formed rapidly as a result of aragonite stabilization. The precipitation of the main meteoric cements and aragonite bioclast dissolution post‐date this stabilisation event. The brown mottle pseudobreccia fabrics are intimately associated with rhizocretions and calcrete, which developed beneath palaeokarstic surfaces capping cycle‐top grainstones and post‐date all depositional fabrics, although they may also follow primary depositional heterogeneities such as burrows. They consist of coarse, inclusion‐rich, microspar calcites that are always very dull to non‐luminescent under cathodoluminescence, sometimes with some thin bright zones. These are interpreted as capillary rise and pedogenic calcrete precipitates. The δ18O values (−5‰ to −8‰, PDB) and the δ13C values (+2‰ to −3‰, PDB) of the ‘pseudobreccias’ are lower than the estimated δ18O values (−3‰ to −1‰ PDB) and δ13C values of (+2‰ to +4‰ PDB) of normal marine calcite precipitated from Late Dinantian sea water, reflecting the influence of meteoric waters and the input of organic carbon.