Sequence stratigraphy control on fossil occurrence and concentration in the epeiric mixed carbonate-siliciclastic ramp of the Early Permian Irati Formation of southern Brazil

The Early Permian Irati Formation records sedimentation of a mixed carbonate-siliciclastic, storm-influenced ramp in a shallow and restricted epeiric sea. It is formally divided in the dark grey mudstones of the lower Taquaral Member, and alternating dark grey mudstones, bituminous black shales, and carbonates of the upper Assistência Member. The formation crops out from central to southern Brazil on the eastern border of the intracratonic Paraná Basin. Fossils of mesosaurid reptiles, pygocephalomorph crustaceans, and subordinate paleonisciform fish form dense concentrations within carbonate tempestites, interpreted as mass mortality events caused by storms. The present study describes the Passo do São Borja outcrop and five new locations, along with 11 drill cores of the complete Irati interval in the Rio Grande do Sul state, applying methods of facies analysis and sequence stratigraphy. Three fourth-order sequences within the Irati Formation were recognized, termed sequences 1, 2, and 3 (S1, S2, S3). S1 is broadly coincident with the Taquaral Member, and S2 and S3 with the Assistência Member. The upper two sequences are composed of a lowstand systems tract (LST), a transgressive systems tract (TST), and a highstand systems tract (HST), whereas the lower sequence lacks a LST. Sequence boundary SB2 was interpreted as coincident with a third-order sequence boundary. Mesosaurid and pygocephalomorph concentrations occur only in S3. Fossil occurrences and concentration were found to be strongly controlled by facies and sequence stratigraphy. Paleonisciform fish fossils are considerably more common at the early HST of S2 and S3, resulting from balanced sedimentation to accommodation rates. Pygocephalomorphs compose intraclasts concentrated in grainstone proximal tempestites of the LST in S3. They are the result of mass mortality events and background bioclastic input, enhanced by firmground formation (hiatal concentration) and subsequent storm reworking. Mesosaurids are also concentrated in grainstone tempestites of the LST, but in cores they were mostly found in distal tempestites of the TST, becoming more common towards the maximum transgressive surface (MTS). Their accumulation is also the result of mass mortality and background sedimentation, enhanced by sediment omission (hiatal concentration) of a severely sediment starved TST (culminating in a condensed section near the MTS), and subsequent storm reworking.