Ground penetrating radar imaging of the late Quaternary Pyramid Valley lake deposit, North Canterbury, New Zealand: Correlation with paleoclimatic records and the stratigraphie sequence

Macrofossil and microfossil remains from Quaternary sediments are important archives of palaeoenvironmental and palaeoecological information. Remains in anoxic swamp and lacustrine environments are often well preserved. Several fossil deposits have been located in wetlands in the North Canterbury fold and fault zone, but the fossil remains are often damaged or destroyed by development when their presence is unsuspected. Near-surface geophysical imaging provides, however, a means of investigating potential sites and mapping deposits. As part of ongoing palaeobiological investigations, the lacustrine deposit at Pyramid Valley was imaged by ground penetrating radar (GPR). Because of an unusually dry summer, the lake bed was dry and the water table low, and the high ionic content typical of the waters of the Pyramid Valley lake and lake bed did not interfere unduly with the GPR response. Two sets of GPR data were acquired for 3D analysis. Near the western shore, Site 1 included areas of previous excavations that had yielded macrofossil remains of vegetation and vertebrates, along the lake edge. The GPR survey showed some clearly anomalous responses, one of which was found on excavation to be associated with a buried matai tree (Prumnopitys taxifolia). The radar images also showed undulating layers which can be correlated with continuous stratigraphic features disrupted by dehydration cracks developed in a previously unrecognised desiccation surface. At Site 2, in the middle of the southern basin of the lake bed, some small anomalous responses were obtained, but the major feature recorded was associated with a previous attempt at piston coring the sediments. As is clear from the GPR 3D image, the piston had simply pushed sediments out, causing updoming of the surrounding material.