Comparative sedimentology and paleoecology of Tertiary giant oyster reefs in New Zealand and Argentina

MACMILLAN, L.; NELSON, C. S.; CASADIO, SILVIO

Oamaru

Congreso; Geosciences '09; 2009

Geological Society of New Zealand

The usefulness of shell concentrations as indicators of relative sea-level changes, systems tracts, and depositional sequence and boundary surfaces has been recognised for more than a decade. Shellbeds can equally act as useful environmental indicators, such as using the orientation and packing of the shells to infer transport conditions and sclerochronological and LA-ICP-MS analysis of periodic skeletal increments to unravel aspects of their life history and growth environment.   Giant oyster reefs and shellbeds occur in Tertiary sequences in the North Island, with similar occurrences in Patagonia, southern Argentina. Study sites in the North Island include the large oysters common in the Oligocene Orahiri Limestone of the Te Kuiti Group found in the vicinity of Waitomo and the oyster beds in the Pliocene Wilkies Shellbed in the Wanganui Basin. In the Waitomo region the oyster Flemingostreini Stenzel can be found in bands up to 9 m thick within highly indurated temperate limestones. Individual specimens reach 15 cm in length, 10 cm in width and 2.5 to 5 cm in thickness. The Wilkies Shellbed, up to 15 m thick, comprises the oyster Crassostrea ingens within a weakly calcareous silty very fine sandstone. Individuals are up to 30 cm long, 7 cm thick and weigh as much as 2 kg. Specimens of Ostrea patagonica in the Late Miocene Puerto Madryn Formation, Pen¨ªnsula Vald¨¦s, Patagonia, are held in a weakly calcareous medium sandstone host. They reach 20 cm in length, 5 cm thick and weigh up to >3 kg.   Early studies suggested that the giant oysters at Waitomo lived in marginal marine conditions. Stable isotope results show ¦Ä18O values from 1.40 to -2.10¡ë and ¦Ä13C values between 0.4 and 2.5¡ë, supportive of fully marine conditions. Wanganui oysters have the largest spread of isotopic values, ¦Ä18O -3.0 to 3.0¡ë and ¦Ä13C -2.6 to 1.8¡ë, suggestive of fluctuating marginal marine to marine conditions. Patagonian samples plot tightly, with ¦Ä18O values -4.4 to -3.2¡ë and ¦Ä13C from -2.0 to -3.2¡ë, supportive of a marginal marine setting.   As seen in extant species, oyster reefs play an important role in the establishment of very diverse community assemblages, providing hard substrata for encrusting and boring organisms. Analysis of these communities show ichnospecies such as Gastrochaenolithes (bivalve), Maeandropolydora (polychaete), Clionolithes (boring algae) and Leptichnus (bryozoan). Chi squared tests have been performed to determine whether predefined parts of the valves were colonised preferentially. Results confirm that polychaetes, sponges, boring algae, boring bivalves, boring bryozoans, serpulids and barnacles do have preferences for specific sectors of valves.