CHARACTER AND VARIABILITY OF SHELF EVENT STRATA ON THE WAIPAOA AND MISSISSIPPI RIVER MARGINS

Floods and storms are key events driving sediment supply and transport in shelf depositional systems. Although the general stratigraphic patterns of many dispersal systems are reasonably well understood, the emplacement and preservation of individual layers by discrete events has been less studied. Therefore, our ability to relate specific beds to their formative processes is limited. Research has been undertaken on the Waipaoa and Mississippi river margins to determine the character of layers deposited in response to flood and storm events as well as how their stratigraphic complexion changes with time. The Waipaoa River margin, New Zealand, affords the opportunity to study how the signal from a relatively small but mud-rich river (15 Mt/y) propagates into a morphologically complex and energetic shelf system. In contrast, the Mississippi River discharges an order of magnitude more sediment annually (~200 Mt/y), but the oceanographic environment is considerably calmer, except during powerful tropical cyclones, e.g., Hurricane Katrina. X-radiographs, grain-size and radiochemical data reveal significant spatial variability in both event types and systems, but layers are locally preserved.