The stratigraphy and diatom content of sediments from Lake Kohangapiripiri

Abstract

Lake Kohangapiripiri is a small coastal lake with a barrier between it and the sea.  The study of the sediment beneath Lake Kohangapiripiri and the microscopic algae within the sediment, has shown that the lake is at least 7000 years old.  The lake is situated in an area where large earthquakes are known to have occurred in the past.  However, the uniformity of the sediment indicates that the lake has not been greatly affected by earthquake events.

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Technical Abstract

Lake Kohangapiripiri is situated in a tectonically active area, almost half way between Turakirae Head and Wellington Harbour.  At both these sites well documented uplift records exist.  This study of the sediment and diatom flora of Lake Kohangapiripiri shows that the lake has been unexpectedly stable.

Sediment beneath the lake is brownish black and >88% mud.  Grain size and organic carbon content are uniform throughout the core sequence.  The diatom flora shows that there has been a fresh-brackish water lake at this site, with a barrier between it and the sea, for the last 7000 years.

Raised beach ridges at Turakirae Head are evidence that at least four large uplift events have occurred in this region in the last 7200 years.  Raised beach ridges have also been documented at Rongotai and Petone.  There is no evidence from the lake sediment or diatom assemblage of any of these events.

It appears from this study that the lake environment is highly stable (in the time scale studied) and is relatively insensitive to earthquake events.  It may also be that uplift that has occurred at the lake, has been balanced or totally overridden by subsidence caused by downward tilting to the west.