Dougherty, A.J. and Nichol, S.L., 2007. 3-D Stratigraphic models of a composite barrier system, northern New Zealand. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), 922 – 926. Gold Coast, Australia, ISSN 0749-0208

This paper presents for the first time, detailed three-dimensional stratigraphic models of both the Holocene and Pleistocene components of a double barrier using ground-penetrating radar. The study focuses on a composite Quaternary barrier system located in Bream Bay, north-east North Island, New Zealand. This coastal compartment maintains a mesotidal environment with low to moderate wave energy. The Holocene barrier has prograded over the last 6500 years during a relative sea-level still stand, or at most 2 metre fall, resulting in a hummocky dune morphology. In contrast, the Pleistocene barrier was formed during rapid sea-level fall towards the end of the Last Interglacial (oxygen isotope 5e to 5a), which resulted in a forced regression producing linear beach ridges. The sublithified nature of these barrier sediments is conducive to utilising both coring and ground-penetrating radar techniques. This study uses standard sedimentological analysis to ground-truth multiple GPR transects collected over the width of both barriers. This method was employed in order to reconstruct the subsurface stratigraphy in detail and to compare barrier structure over the timescale of successive sea-level highstands. The resulting two- and three-dimensional GPR records all show detailed stratigraphy of a regressive facies succession with similar architecture. However, slight differences in spacing and geometry between beach facies were detected in association with the different dune morphologies from each timeframe. While both beach facies are laterally continuous and dipping in a seaward direction, the Holocene stratigraphy exhibits a more closely-spaced, curvilinear cross-sectional geometry consisting of medium sand and shells; whereas the Pleistocene barrier beachface is highlighted by heavy mineral-rich, fine-grained sands showing a similar concave shape in the large scale, but at a more regularly-spaced interval associated with individual beach ridges. The replication of this stratigraphic form at successive sea-level highstands within the beachface indicates that present-day intermediate nearshore conditions are similar to those of the last interglacial highstand. The overall evolutionary sequence for each barrier differs in width, morphology and stratigraphy due to a combination of factors, including variations in sediment supply, accommodation space, and relative sea level within successive highstands.