CASADIO Silvio Alberto
congresos y reuniones científicas
Oysters as paleoenvironmental keys. An example from the Upper Cretaceous of Scania, Sweden
Congreso; Congreso Argentino de Paleontología y Bioestratigrafía; 2006
Institución organizadora:
Academia Nacional de Ciencias-Cipal
Oysters are frequent elements in fossil mollusk assemblages. However, their relative importance in these assemblages is usually enhanced by their calcite shells, which improves their chances of fossilization. Taxonomic and taphonomic studies suggest that - regionally at least - they may be good biostratigraphic and paleoenvironmental tools (Casadío, 1998; Parras and Casadío, 2006). The aim of this study is to fine-tune paleoenvironmental reconstructions for the early Campanian of NE Scania (Sweden) using an assemblage of mollusks and fish dominated by Ostrea curvirostris. This assemblage was recorded at Åsen (56º 09’ 08’’, 14º 30’ 20’’), 20 km NE of Kristianstad. The Upper Cretaceous of NW Europe is characterized by a predominance of carbonatic sedimentation (white chalk). These facies represent the central areas of basins, while in the poorly represented coastal areas the chalk is replaced by a glauconitic white chalk, bryozoan packstone mounds or skeletal sands (Surlyk, 1997). In the study area, the better known locality is Ivö Klack (56º 08’ 25’’, 14º 24’ 11’’), a limestone and kaolin quarry where one of the best examples of a fossil rocky shore is exposed. Over the surface of the Precambrian gneiss a very diverse community of encrusting organisms (i. e., Ostrea haliotoidea, Spondylus labiatus, Crania stobaei and two serpulid species) has been recorded, showing a very clear zonation. These organisms are restricted to the wave-cut platform constituting the lowermost 5-10 m of rocky shoreline. Above this bed, the shoreline is much steeper and is covered by large basement boulders representing the submarine slope. The boulders carry no epifauna, while the sediments covering them are mainly shells. Among these, the dominant species is the oyster Rastellum diluvianum, which is highly bioeroded and encrusted, with many valves articulated. Very subordinate are disarticulated valves of O. haliotoidea, O. curvirostris, Spondylus labiatus, and Crania spp. According to Surlyk and Christensen (1974) the early Campanian transgression modeled the lower part of the rocky slope at Ivö Klack and the action of waves originated a relatively wide shelf. The slope was then submerged and the protected shelf was colonized by the first encrusting organisms.  A high diversity oyster bank community developed and built upwards and outwards on the rocky slope. Beyond the slope shells were deposited as skeletal gravel sands and mud. The presence of hardgrounds in this area suggests periods of low to null sedimentation. The shallow water environment points towards a periodic lack of sedimentation primarily due to strong wave and current action (Surlyk and Christensen, 1974). Contrarily to the setting described for the section at Ivö Klack, equivalent beds comprising fine sandstone exposed at Åsen, seven kilometers to the East, show a very different assemblage. It includes abundant specimens of Ostrea curvirostris with articulated valves and scarce shark and rayfish teeth. Ninety percent of the oysters show attachment surfaces in the form of subcylindrical grooves (Φ < 1.5 cm), suggesting that the ostreid grew on littoral plants. The marks observed on the left valves are similar to those presently recorded in oysters living attached to pneumatophores or roots of plants growing in wave-protected coastal environments. The absence of encrusting and boring organisms on the oysters suggests that salinity was below or above normal. Available information allows us to infer normal marine conditions and higher exposure to waves and littoral currents at Ivö Klack, while at Åsen the environment was probably more restricted and subject to strong salinity changes.