New Zealand Journal of Marine and Freshwater Research abstracts

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Sedimentology and evolution of Omaro Spit, Coromandel Peninsula

Graeme P. Marks*

Campbell S. Nelson

Department of Earth Sciences, University of Waikato, Hamilton, New Zealand

Abstract The sedimentary structures, composition, and texture of sediments from the barrier coast complex (Matarangi Beach-Omaro Spit-Whangapoua Harbour) at Whangapoua, Coromandel Peninsula, are described. Sediments are mainly fine sands, rarely muddy or silty, and most are plagioclase feldsarenites, reflecting derivation from a predominantly Tertiary volcanic hinterland. Sediments from each of the modern environments, namely nearshore, foreshore, back-shore, frontal dunes, tidal flats, and tidal channels, are characterised by a particular combination of sedimentary structures and subtle textural parameters. Dune ridge and barrier flat paleoenvironments on Omaro Spit were successfully identified by comparing their lithologic properties with the modern sediments. 'Surficial' sediments of the well-preserved dune ridge system developed immediately inland from Matarangi Beach closely resemble those in the modern frontal dunes, and the 'in depth' dune ridge sediments are more analogous to the present foreshore sands. The barrier flat deposits separating the dune ridge system from Whangapoua Harbour have similar characteristics to the modern tidal flat sediments in the harbour.

Omaro Spit probably began as an offshore bar across the mouth of Whangapoua Harbour, an embayment formed by the post-glacial drowning of a Late Tertiary dislocated fault-block. Tidal flat sedimentation within the harbour formed the ancient barrier flat deposits which rise to at least 2 m above the modern harbour flats, suggesting local sea level at the time was higher than at present. During a subsequent cyclic fall in sea level, supratidal aeolian deposition led to a succession of 15 to 18 parallel dune ridges developed on high-tide berms. Linear regression analyses of dune ridge and swale heights and the height distribution of positive (aeolian) and negative (beach foreshore) skewness values and of contrasting sedimentary structures in dune ridgL paleosediments, together with the stages in dune soil development across the barrier, suggest initial sedimentation occurred from 4000-5000 years ago when local sea level was 2-3 m above present mean high water level. Barrier progradation was interrupted by an important period of coastal erosion during a temporary rise in sea level immediately before deposition in the dune ridge system of a layer of 2000-year-old sea-rafted Leigh Pumice. Sea level probably reached its modern position at Whangapoua about 1000 years ago, since when some evidence suggests the barrier spit may have experienced minor uplift.

New Zealand Journal of Marine & Freshwater Research, 1979, 13 (3): 347-372
Received 14 August 1978; revision received 8 May 1979

PDF file of entire paper: medium quality (6228K); (scanned from paper original: notes about this process)

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