Early Permian playa deposits of Sardinia, Italy, with reference to their ichnofauna

Luca Giacomo Costamagna

Member of the Collaboration Group

2022-01-01

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Abstract

The East sector of the post-Variscan Mulargia-Escalaplano Basin of central Sardinia, Italy preserves a 150- to 200-m-thick succession of continental sedimentary and volcanic deposits, pertaining to the red bed Upper Rotliegend Group (latest Carboniferous? to Middle Permian?). This group is formed by two superposed alluvial-lacustrine depositional cycles, respectively the Mulargia Formation and the Pegulari Formation, each one of them followed in turn by volcanic rocks represented by the Sa Fossada Rhyolitic Unit and by the Mataracui Andesitic Unit. The study of a well-exposed succession at the Pegulari locality showed that the intermediate part of the Permian Pegulari Formation in the Escalaplano area is characterized by cyclic siliciclastic, carbonate and minor evaporite deposits, with evidence of inorganic and organic deposition in a playa mud-flat to playa lake environment (possibly a recharge playa). Lake-shore shifts were probably influenced by seasonal fluctuations in an extremely dry climate. These deposits contain large burrow systems, which are tentatively attributed to the activity of tetrapods. They are associated with grooves along the upper bedding surface of a sandstone bed, which connect to burrow systems and show features of bioerosion. These grooves represent Sulcolithos aff. variabilis and were probably produced by freshwater polychaetes. This trace-fossil association from the Early Permian represents the first occurrence of Sulcolithos from a continental environment, beside evidence of macrobioerosion. If produced by crayfish, it would support phylogenetic assumptions that crayfish started to diverge in the Carboniferous–Permian, and polychaetes colonized freshwater to schizohaline environments with significant salinity changes. The evidenced depositional environment in the Sardinian Pegulari Formation shows close analogies with the Lower to Middle Permian Salagou Formation of the French Lodève Basin, which differs from the former by its abundant fossil remains and trace fossils.