Comparative sedimentology and paleoecology of fossil giant oyster beds in some tertiary strata of New Zealand and Argentina

摘要

Shell concentrations are useful indicators of relative sea-level changes, systems tracts, and depositional sequence and boundary surfaces. Shellbeds can equally act as archives of paleoenvironmental information and aid in the reconstruction of past environmental and ecological conditions. These considerations are applied in this study to occurrences of giant oyster reefs and shellbeds in Tertiary sequences in North Island, New Zealand, and in Patagonia, southern Argentina. Large recliner morphotype Flemingostreini Stenzel oysters are common in the Oligocene Orahiri Limestone in the vicinity of Waitomo, New Zealand. The oysters occur in tabular beds typically 0.5-2 m, but up to 9 m thick within highly indurated biomicritic limestone. Individual valves reach 15 cm in length, 10 cm in width and 2.5 to 5 cm in thickness. The occurences at Waitomo are characteristic of multi-event shellbeds and can be interpreted as onlap (transgressive lag) and backlap shellbeds within a transgressive systems tract. Shell delta18O (-2.1 to 1.4 per mille) and delta13C (0.4 and 2.5 per mille) values, and minimal seasonal isotopic ranges with little variability, confirm that these oysters lived in a fully marine environment. The environment of Oligocene Flemingstreini Stenzel was probably similar to modern Ostrea chilensis from Foveaux Strait off southern New Zealand, living on coarse shelly and gravelly tide swept substrates in 18-40 m water depth, under normal marine salinity conditions and temperatures around 13oC, forming in situ biostromes of haphazardly packed oyster shells.Specimens of Ostrea patagonica in the Late Miocene Puerto Madryn Formation, Pen nsula Vald s, Patagonia, are held in a weakly calcareous fine sand host. They reach 20 cm in length, 5 cm thick and weigh as much as 3 kg. The reef has a lens (or bioherm) geometry and is composed of articulated, well preserved oysters set in a fine sand matrix. The reef is interpreted as a downlap shellbed within a highstand systems tract. Oyster shell delta18O (-4.4 to -3.2 per mille) and delta13C (-2.0 to -3.2 per mille) values are low and show large seasonal isotopic ranges, with a large amount of variability, collectively supporting a marginal marine setting receiving extensive freshwater input and mixing. The environment of these oysters is comparable with modern Crassostrea gigas reefs at San Blas, Patagonia. Ostrea patagonica occupied a low energy intertidal zone in water depths of only 1-2 m. Temperatures ranged from 20degreesC (summer maximum) to 8degreesC (winter minimum). The oysters were not cemented firmly to the substrate, but reclined on the muddy sediment and formed an in situ bioherm of loosely packed oysters, with the living animals concentrated over time to the outside of the accumulation.The Pliocene Wilkies Shellbed in the Wanganui Basin, New Zealand, comprises oyster accumulations up to 15 m thick involving the extinct oyster Crassostrea ingens. Individuals are up to 30 cm long, 7 cm thick and weigh as much as 2 kg. The shellbed consists of a lower onlap shellbed (transgressive lag) and an overlying backlap shellbed (biostrome). The widespread shell delta18O (-3.0 to 3.0 per mille) and delta13C (-2.6 to 1.8 per mille) isotopic values support a range of marginal marine to marine environmental conditions. The lower onlap shellbed had estuarine influence, while the upper part had affinities with a nearshore (less than 40 m) marine setting of more normal salinity. As the thick backlap shellbed migrated shoreward a depth was maintained in which Crassostrea could live in situ within a weakly calcareous very fine muddy sand in favourable conditions of low turbidity and sedimentation. All species of fossil oysters in this study are easily distinguished by the large size and thickness of their valves. The 'reefs' formed by these oysters provided hard substrata for a diverse community of encrusting and boring organisms which includes the likes of Gastrochaenolites (bivalve), Maeandropolydora (polychaete), Clionolithes (boring algae), Entobia (sponge), Leptichnus (bryozoan) and Radulichnus (gastropod). These communities are comparable to those seen on extant oyster reefs. They can thus be termed autogenic ecosystem engineers. Non-random distribution of euendoliths and epiliths on oyster valves may be accounted for by different survival adaptations of larvae. Shell morphotypes and exterior architectures are inferred to have prompted active rugophilic (groove-seeking), geophobic (anti-gravity) and rheophilic (current-seeking) behaviour of larvae, which enhanced their survival rate. Preference for the external surface of shells suggests that traces were created during the life time of the oysters, while most internal traces are post-mortem features.