A Geophysical Survey of the Derwent Estuary

Abstract

The Derwent Estuary is a drowned river valley system that lies within a complex NNW trending structural zone called the Derwent Graben. The graben preserves a poorly understood record of sedimentation and volcanism that extends from at least the early Tertiary to the Holocene. High-resolution marine geophysical data were acquired throughout the estuary to investigate the structural and sedimentary history of the graben. Magnetic data clearly delineate the geometry of Jurassic dolerite basement units, basement controls on the development of the Graben and the distribution of previously unrecognised Tertiary basaltic centres. Modelling suggests that in excess of 800m of Cainozoic sediments may be present in the centre of the graben adjacent to the Cascades Fault in the lower portion of the estuary. High-resolution (boomer) seismic data has for the first time provided an insight into Tertiary, Pleistocene and Holocene sedimentation in the basin. Highly reflective Tertiary sediments are extensively disrupted by normal faulting and are overlain by a complex package of Quaternary sediments. The Pleistocene portion of the section is marked by numerous internal unconformities with deep channels cut during periods of fluvial erosion during sea level lowstands and filled by marine deposition during highstands. An important outcome of the seismic survey was recognition of extensive zones of acoustic turbidity that correlate closely with the inferred late Pleistocene course of the Derwent River. These zones result from high concentrations of biogenic methane in the shallow marine sediments and are indicative of organic-rich anaerobic conditions. The presence of natural anaerobic zones significantly complicates interpretation of recent anthropogenic effects on the estuary.