Interpreting ambiguous bedforms to distinguish subaerial base surge from subaqueous density current deposits

Abstract

Bedform geometries of volcanogenic sedimentary structures such as dune, low- to high-angle cross-stratification and planar stratification produced in subaqueous and subaerial environments can be very similar. This has historically created difficulties in unambiguously distinguishing primary from reworked deposits, and gas-deposited versus water-deposited ones. The origins of dunes and associated structures within the pyroclastic deposits of basaltic Surtseyan-style eruptions exposed in seacliffs along the Cape Wanbrow coastline of the Northeast Otago region in the South Island of New Zealand are such an example. Careful analysis of contextual information of these deposits, including granulometry, dune geometry, grading, sorting and particle hydraulic equivalence, has been completed to distinguish between major flow types in different environments. To determine the depositional setting of the Cape Wanbrow dune-bearing deposits, a number of related sediments have been examined in more detail including well-described examples of (i) subaerial dry pyroclastic deposits, (ii) subaerial moist pyroclastic deposits, (iii) aeolian deposits, (iv) deposits of unidirectional fluid-gravity water flows (e.g. rivers) and cyclic tidal flows, and (v) aqueous sediment-gravity flow deposits encompassing both pyroclastic material and those with non-pyroclastic material. From this compendium, a framework has been created that allows users to compare the physical controls that shape each example with the factors controlling bedform deposition in that environment. Identifying key characteristics of the Cape Wanbrow dunes and comparing multiple flow types and environments using the designed framework indicates that they were deposited by subaerial dry pyroclastic density currents. This conclusion has wider implications for the entire Surtseyan stack at Cape Wanbrow, because it indicates that at least this volcano (Rua2) became emergent and fully subaerial during its lifespan.