Geologia USP. Série Científica (Oct 2010)
Taphonomy of Bivalve Mollusks in Oolitic Limestones from Teresina Formation (Paraná Basin, Middle Permian, Prudentópolis, PR)
Abstract
Herein, it is presented the first detailed taphonomic study on bivalve mollusk shells preserved in the oolitic limestonesof the Teresina Formation (probably Kungurian-Roadian, Lower-Middle Permian) in the eastern margin of the Paranábasin. The selected beds are located in two quarries (informally named PRU 1 and PRU 2) in Prudentópolis municipality(Center-South Paraná State), and positioned approximately in the middle of the formation and probably in the Pinzonellaillusa Zone. The PRU 1 limestone (~30 cm thick), which is partially silicified and intercalated with predominantly peliticrocks, is classified as a bivalve oolitic grainstone. The basal contact is erosive and the top shows symmetrical ripple marks,which are draped by shale with mud cracks. There are two fining-upwards successions characterized by dense to dispersedpacking of the shells, which are usually disarticulated, randomly oriented (many nested/stacked) and mixed with some Formapeliticintraclasts. Microhummocky cross-stratification occurs a little below the top of the bed. The PRU2 bed is classified as ooidbivalverudstone (~5 cm thick), where all shells are disarticulated and fragmented, showing dense packing. The bivalves probablyinhabited a muddy substrate and were mixed (as parautochtonous and allochthonous bioclasts) with ooids during high-energy stormevents, including posterior shell displacement as a result of bioturbation. Thus, the calcareous beds represent amalgamated proximaltempestites with a complex taphonomic history, strong temporal/spatial mixing of bioclasts and limited paleoecological resolution. They are a typical example of shell beds generated in a huge epeiric sea, which was not necessarily connected to the ocean and where very low depositional-slope gradient, very slow subsidence and minimum sediment accommodation space caused frequent sedimentreworking by storm related processes.
