Early Permian post-glacial bivalve faunas of the Itarare Group, Parana Basin, Brazil: Paleoecology and biocorrelations with South American intraplate basins

摘要

The uppermost portion of the Taciba Formation, Itarare Group, Parana Basin, Brazil, records a succession of depositional environments tied to the demise of late Paleozoic glaciation. In the study area, Teixeira Soares county, state of Parana, the unit is dominated by massive to laminated diamictites with inclusions of sandstones and other coarse-grained lithotypes, representing re-sedimented material in proximal areas. These are succeeded by fine to medium-grained sandstones with tabular cross-stratification and pectinid-rich shell pavements, interpreted as nearshore deposits. Above this, laminated and intensely bioturbated siltstones with closed articulated bivalve shells are recorded, probably deposited in inner shelf settings. Fine to very fine sandstones/siltstones with hummocky cross-stratification and intercalated mudstones, including infaunal in situ shells, are interpreted as stacked storm deposits, generated in distal shoreface environments. These are succeeded by fossil-poor, massive to laminated siltstones/ mudstones or gray shales (=Passinho shale) that are inferred to be outer shelf deposits, generated in organic-rich, oxygen-deficient muddy bottoms. In this sedimentary succession dropstones or ice-rafted debris are missing and locally the Passinho shale marks the maximum flooding surface of the Itarare succession. These are capped by the fluvio-deltaic deposits of the Rio Bonito Formation (Sakmarian -Artinskian). Six facies-controlled, bivalve-dominated assemblages are recognized, representing faunal associations that thrived in aerobic to extreme dysaerobic bottoms along a nearshore-offshore trend. Within these assemblages, nineteen bivalve species (three of them new) were recorded and described in detail. The presence of Myonia argentinensis (Harrington), Atomodesma (Aphanaia) orbirugata (Harrington) and Heteropecten paranaensis Neves et al. suggests correlation with bivalve assemblages of the Eurydesma-bearing Bonete Formation, Pillahuinco Group, Sauce Grande-Colorado Basin (Buenos Aires Province), Argentina, indicating a possible Asselian age for this diverse post-glacial bivalve fauna. Despite that, typical members of the icehouse-style Eurydesma-Trigonotreta biota (stricto sensu) have not yet been found in the studied bivalve assemblages.