Magnetic mineral and sediment porosity distribution on a storm-dominated shelf investigated by benthic electromagnetic profiling (Bay of Plenty, New Zealand)

Abstract

Magnetic susceptibility and electric conductivity within the uppermost meter of the seafloor were measured with high resolution along 33 coast-normal profiles in the Bay of Plenty (New Zealand), using the recently developed electromagnetic benthic profiler MARUM NERIDIS III. These parameters are used to determine magnetic mineral concentration and porosity of mostly volcanoclastic sediments between 2 and 35m water depth to investigate distribution and formation of magnetic mineral enrichments on a storm-dominated shelf.In general, magnetic mineral concentration (susceptibility) is inversely correlated to porosity (conductivity). Along profiles, susceptibility maxima and conductivity minima were commonly found on bathymetrical elevations such as dune and ripple crests. Cores and grab samples show the highest degree of enrichment in the uppermost 20cm of the seafloor. Complementing petromagnetic analysis indicates a spectrum of FeTi-oxides in the sediment, ranging from magnetite and hematite over titanomagnetite (TM40 and TM60) to titanohematite (likely TH80 and TH95).Three distinct zones of magnetic mineral enrichment could be identified: a coast-parallel structure with low porosity in recent fine sand between 5 and 20m water depth, a widespread structure with low porosity in older, transgressionally reworked, coarse sand in up to 30m water depth, and third structure coarse sand with high porosity located mostly below 30m. While the fine sand structure is interpreted as the result of an active process, the other two appear to be relics. Sorting by grain size selective entrainment appears to be the major formation mechanism.This study demonstrates how electromagnetic benthic profiling, in combination with environmental magnetic laboratory analysis, provides reliable, highly interpretable data that allow insight into sorting processes within the boundaries of established lithofacies that are difficult to detect using a classical sedimentological approach.