Caracterização tafonômica das concentrações fossilíferas da Formação Cape Melville, Grupo Moby Dick (Mioceno Inferior), Ilha Rei George, Antártica / Taphonomic characterization of fossil concentrations from the Cape Melville Formation, Moby Dick Group (Early Miocene), King Georg Island, Antactica

Rafael Casati

Works focusing on the taphonomy of fossil concentrations generated in glacial or periglacial environment are rare. In this context, the present dissertation carried out the taphonomic characterization of the fossil concentrations of the Cape Melville Formation, Moby Dick Group (Lower Miocene), King George Island, Antarctica, in order to elucidate the genesis of these deposits. To this end, data relative to a total of 534 specimens were obtained in the richly fossil layers of the four sections, called Pingüineira (PRS), Hard Ground (HGS), Chaminé (CS) and Lava Crag (LCS), investigated at the top of the Melville Peninsula between January and February of 2003. Of these, only the PRS Section presents two distinct fossil layers (PRS-C1 and PRS-C2). The studied fauna is mainly composed of remains of bivalve clams. Less abundant remains of corals, crabs, gastropods and brachiopods, as well as trace fossils, also are present. Taxonomic, ecological and taphonomic analyses allowed similarities to be identified between PRS and HGS and CS and LCS. PRS and HGS Sections are dominantly composed by shallow infaunal deposit-feeding bivalves (Ennucula frigida, Enncula musculosa Yoldia peninsularis), preserved preferentially with closed articulated valves, however out of life position, indicating remobilized fauna; the absence of signs of spalling, abrasion and incrustation indicates that the bioclasts were not affected by intense chemical, physical or biological bioestratinomic processes. The occurrence of pyritized bioclasts in HGS is interpreted as the result of decomposition of the entombed organisms, still alive, in an anoxic environment. The assemblages of CS and LCS Sections are dominantly constituted by deep infaunal suspension-feeding bivalves (Neilo (N.) rongelii), preserved preferentially with disarticulated valves, indicating that the physical biostratinomic processes were more operative; however there are few signs of spalling or other taphonomic signatures such as incrustation and bioerosion. Disarticulated valves are preferentially preserved convex down indicating that bioclasts were placed in suspension and redeposited in a position other than that of greatest hydrodynamic stability; the orientation of these bioclasts in plan view shows a preferential direction, indicating weak currents and a longer time of exposition at the water/sediment interface; the occurrence of rare remains of deep infaunal suspension-feeding bivalves (Panopea (P.) cf. P. regularis) and crabs (Antarctidromia inflata) preserved in life position is indicative that this community was entombed in situ by sediments in suspension that brought the thanatocenosis of disarticulated valves. The results obtained in the present study reiterate the importance of taphonomic and paleoecological studies for the understanding of the depositional dynamics of the past and contribute a great number of data useful in the characterization of glacial and periglacial environments.

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