Surficial sediments of Raglan Harbour

Abstract

Raglan Harbour is a drowned river valley system lying in a structurally depressed fault block. Much of the upper reaches of the harbour consist of tidal flats formed by sediment veneered shore platforms, and dissected by relatively deep channels. Estuarine conditions occur in the tidal reaches of major streams entering the harbour.

Sediment textures reflect a gradual decrease in energy conditions passing, up the harbour from clean, well-sorted sands near the harbour entrance to mainly muddy sands and sandy muds that characterise the tidal flats. Tidal currents result in highly variable energy conditions. Modes of sediment transport and deposition and a generalised scheme of current patterns and relative current strengths throughout the harbour are interpreted from textural analyses.

The coastal iron-sands are the principle source heavy minerals in sediments throughout Raglan Harbour. Bulk sediment mineralogies and clay mineralogies indicate detrital inheritance from hinterland rocks as the main source of terrigenous sediment supplied to the harbour as fluvial sediment load and by shoreline erosion. Benthonic organisms supply most of the carbonate and organic matter in the sediments, as well as causing considerable sediment reworking. Phosphatic concretions found in certain areas of the harbour appear to be of Recent to Sub-recent diagenetic origin.

Much of the present aerial extent of Raglan Harbour is probably the result of shore platform development within the harbour during the last 8-10,000 years. The bulk of the material eroded during this process has been removed from the sedimentary system of the harbour and deposited on the continental shelf.