High-resolution imaging of sedimentary basins in New Zealand’s fjords using boomer-sourced multi-channel seismic reflection data

Fiordland, on the southwest coast of New Zealand’s South Island, hosts 15 distinct fjords that extend up to 40 km inland from the Tasman Sea into mountainous terrain consisting primarily of hard crystalline igneous and metamorphic rocks. All of these fjords have seaward entrance sills, and most have glacially eroded and overdeepened basins that contain sediments deposited following the retreat of the glaciers that carved the valleys out. These sedimentary basins preserve a record of post-glacial environments that can be used to evaluate changes in regional sea level, climate, vegetation and other conditions. For example, in cases where the entrance sills were higher than the last glacial maximum sea level, the present-day fjords would have previously been isolated glacial lakes prior to marine ingression due to post-glacial sea level rise; this lacustrine-marine transition is recorded in the fjord sediments, e.g., as flooded deltas and beaches.

Over the last 10 years, we have collected high-resolution boomer-sourced seismic reflection data in most of the fjords of Fiordland using a 75-m-long 24-channel Geometrics MicroEel array recording signals from an acoustic boomer source (initially a Ferranti system and then, more recently, one from Applied Acoustics). Processing has been undertaken using commercial (GLOBE Claritas) seismic processing software. We present a summary of this work, showing profiles along a number of the fjords including, from north to south, Milford, Nancy, George, Thompson/Bradshaw, Doubtful, Dagg, Dusky and Long sounds. Seismic sections show a wide variety of sediment accumulations in the fjords depending on periglacial conditions, sill depth, catchment size, catchment rock types and vegetation history, etc. Sediment thicknesses are observed to exceed several hundred metres in some of the basins – which supports an interpretation of interbedded strata of muds, silts, and fine sands. The depositional history of the sedimentary units imaged by these data, in conjunction with additional seafloor mapping, direct seafloor sampling and shallow cores, will be confirmed by deep drilling efforts in the fjords.