Shore Geology and South Coast Habitats
Greywacke cliffs across the South Coast
Geology, Waves and Currents
The hard rock of the Wellington region comprises greywacke, a sedimentary rock laid down about 200 million years ago as sands and muds settled at the bottom of deep oceanic troughs. Eons of layering, tectonic uplift, folding and erosion have formed the greywacke rocks which comprise most of the North Island. Across the Wellington south coast greywacke rocks are extensively exposed giving geologists access to study the age, layers, chemistry and form of these rocks. There are only minor intrusions – on the eastern side of Island Bay can be seen a narrow belt of red greenish volcanic rock, possibly from an ancient seamount. This belt runs out southwards past the eastern side of Taputeranga Island.
Volcanic intrusion in foreground contrasts with rusty brown greywacke beyond
The rock types provide a character for the region, and through their interaction with the sea, air, storms and currents, greatly influence the habitats and the components of the biodiversity of the Cook Strait area.
Form of the Shore
The south coast at and above sea level is described as an uplifted rock platform, showing erosion of all but the most resistant rocks. The most recent, significant uplift occurred in the major 1855 earthquake, when the beaches on the south coast were lifted 1.5 m on average. The uplift is seen clearly on the western side of Owhiro Bay, at the Sirens, Taputeranga Island, and at Princess Bay. These platforms lifted above sea level are in fact part of extensive submarine reefs running south to the limits of the marine reserve at about 40 m depth and 2.3 km out to sea. The south coast landforms of the reserve are also affected by the Happy Valley fault, and also the Lambton fault which runs through Island Bay. The word ‘dynamic’ applies to the formation of the south coast shorelines, with often abrupt and rapid change related to tectonics, tsunamis (in the longer run) and the tides, winds, currents and storms in the short term. The rock platforms form great rock pools to explore.
Eroded rock platform at the Sirens
Waves and Currents
The major influence in waves is from the deep oceanic swells brought in from southerly storms, intensified by the narrowing of the Cook Strait deeps, shallows and land forms. In southerly conditions the average swells are 1.2 to 2.2 m with a periodicity of about 10 seconds. The largest swells are usually around 5 m but the highest recorded at the Harbour entrance was 12 – 14 m in 2002. Although northerlies and NW winds prevail in the Wellington area, they do not usually generate significant wave heights. The tidal range is 1.0 m at mean spring tides, and the east west currents associated with tidal ebb and flow can be up to 70 m/sec out to sea.
Wave erosion is the most important dynamic force affecting the shape of coastal shorelines, acting on the relative hardness/softness of the sandstone/argillite components of the greywackes. Waves produce strong shock effects through the compression of air in the crevices of the most exposed rocks, resulting in a fretwork of shapes on the seaward edges of the shore platforms between tides. There is also abrasive break down from particles as strong swells lift the sands and other abrasive elements from the seabed. Chemical weathering is also an influence, turning the intertidal rocks a rusty iron brown colour.
Shock waves erode rocks at Owhiro Bay in swells
The subtidal seabeds are in virtually constant motion, with waves, currents and tides. Sands and muds are particularly stirred up, although the seabeds appear to show some longevity in the nature of deposition. There are extensive fine sands at Wellington Harbour entrance and Lyall Bay, but less further west at Owhiro Bay, and inshore of Taputeranga at Island Bay. These reflect the increasing distance from Wairarapa and Wainuiomata River sources There are boulders, cobbles and gravels deposited to the east of Island Bay, such as at the Snorkel Trail area, the eastern side of Owhiro Bay, and south of the Sirens submarine rock platform. These are probably detritus from the eroded reefs, and show rounded nature with continued movement in the high energy coast. The extensive hard rock, submarine reefs running to the limits of the reserve were noted above. The dynamic nature of the south coast means that the boundaries between these major elements change constantly, though their essential nature changes more slowly. Dynamism is expected only to increase with climate change and sea level rise.
Sand dunes consolidated by community action, western side of Island Bay
This note is a summary from several chapters in ‘The Taputeranga Marine Reserve’, edited by Gardner and Bell, Second Edition, Victoria University, 2008. Thanks to authors Rodney Grapes, Hamish Campbell, Agnes Goh, and Lionel Carter. Their work deserves better treatment than this limited summary. Photos – MH.