EARLY ONSET OF MODERN-STYLE SHELL BEDS IN THE PERMIAN SEQUENCES OF THE PARANÁ BASIN: IMPLICATIONS FOR THE PHANEROZOIC TREND IN BIOCLASTIC ACCUMULATIONS

Abstract

The internal characteristics and stratigraphic distribution of Permian shell beds from the sedimentary sequences of the Parana basin were analyzed quantitatively. Taphonomic, sedimentologic, and stratigraphic data from 23 geological sections and 32 outcrops were amassed for the total of 32 fossil concentrations found in the stratigraphic sequence of the Passa Dois Group (Intervals 1-4 of Rohn's lithobiostratigraphic framework). The analysis of bioclastic concentrations (including clast composition, geometry, thickness/ traceability, close-packing, internal complexity, and taphonomic signatures) reveals that the shell beds group into two distinct biostratinomic styles or modes of Kidwell's classification: Archaic and Modern. Archaic shell beds (29.4%) are thin (>3 cm), two-dimensional, internally simple concentrations (distal tempestites), with disperse, high-organic nacreous aragonitic shells of anomalodesmatan bivalves. Modern shell beds (70.6%) are thick (30-50 cm), fully three-dimensional, internally complex, amalgamated concentrations (bioclastic sandstones or coquinas interpreted as proximal tempestites) with a mixture of densely-packed, crossed calcitic (?) shells of veneroid bivalves, and shells of anomalodesmatan bivalves. Archaic shell beds dominate (100%, n=8) the distal lithofacies of Passa Dois Group (Serra Aha Formation; Interval 1), whereas Modern shell beds are abundant in the more proximal lithofacies (Teresina/Corumbatai Formations) of Intervals 2 (81.25%, n=13), 3 (100%, n=l), and 4 (88.9%, n=8). The increase in shellbed thickness and internal complexity in Intervals 2 - 4 (the latest Late Permian) appears to be tied to lithologic changes (changes in tempestite proximity), suggesting that the trend is controlled primarily by physical factors. A biologic control is also suggested, however, because the thick coquinas of Modern Type are invariably composed of low organic (crossed), veneroid shells of Pinzonella illusa and Pinzonella neotroplca. The high proportion of infaunal bivalves (anomalodesmatans, veneroids) indicates that the Permian benthic association was ecologically post-Paleozoic rather than Paleozoic. The shell beds from the Parana basin suggest that the transition from the Archaic to Modern type of shell beds and the shift from Paleozoic to more modern ecology may have both started already in the late Paleozoic.