Miranda chenier plain

BY BRUCE HAYWARD (GEOLOGIST)
Accessibility: WHEELCHAIR ACCESS
In storms waves sweep shells across the spits and into the mud flats behind.
New Zealand's best example of a chenier plain. Actively growing sand spit advancing over intertidal flats.
In storms waves sweep shells across the spits and into the mud flats behind.
A chenier is a beach ridge made of gravel, shell or sand, resting on top of mud and separated from the shore by a belt of intertidal mudflats. The southwest corner of the Firth of Thames is fringed by a 15 km-long and up to 2 km-wide coastal plain that accumulated in the last 4000 yrs. It is one of the best examples in the world of a combined gravel and shell chenier plain that has built up by the progressive addition of gravel and shell beach ridges on top of tidal mud flats. The gravel beach ridges form the narrower northern part of the plain.
The southern 2/3 of the plain is made of shell ridges/spits. The shell ridges originated intertidally, as sand and shell banks, during major northeast storms, which swept away the mud and concentrated the coarse material into a heap. These offshore banks were driven shoreward by later storms and eventually attached to the existing shell beach at their northern ends. From here, they developed into narrow shell spits that grew southwards, parallel to the coast, separated from the previous shell beach by a 50-200 m-wide depression. In the shelter of the shell spit, this elongate depression progressively filled with intertidal mud that may have been colonised by mangroves and eventually by high-tidal salt marsh, thereby adding new land to the plain. During high spring tides, some storm waves washed over the shell spit spreading feathery lobes of shell into the mud depression. These characteristic feathery lobes can still be seen in many places on the grazed older parts of the chenier plain today.
Between 7000 and 4000 years ago, when sea level was 1.5-2 m higher than today, the coastline at Miranda was up to 2 km further west, right up against the foot of the hills. Between 4000 and about 1000 yrs ago, as sea level dropped to its present level or slightly lower, much of the shell chenier plain, between Miranda and Kaiaua, was constructed. Each successive shell ridge was added seaward of the preceding one and the plain built out across the intertidal mud. The 4000 yr-old chenier up against the hill is 1.5-2 m higher than the 1000 yr-old one, and those of today. Together they document the decline in sea level since 4000 yr ago. In the last 1000 yr there has been little growth of the gravel plain, but in the south, additional shell spits have been added, as the chenier plain has grown southwards rather than seawards. This is believed to be the result of the stable and more recently rising sea level.
A new shell spit (chenier) is forming across the intertidal flats (1983).
The shorebird centre is built on some of the younger part of the chenier plain (younger than 1000 years).
The oldest part of the plain is against the hills. How rapidly did the plain grow between 4000 and 1000 years ago?
Where and when do you think the next shell ridge will form?
What kinds of shells are found in the shell beaches/ridges? Where did they come from?
Directions/Advisory

Watch out for cars in carparks and roads.
Shell spits on coast best seen when tide is lower, but this is not essential.

Accessibility: WHEELCHAIR

Start by visiting the Pukorukoru Miranda Naturalists Trust shorebird centre which has displays about the chenier plain as well as the wading birds of this coast. The shore bird centre is located on East Coast Rd about 2.5 km north of Miranda and 7 km south of Kaiaua.
Drive around this area visiting several beaches and viewing the plain back to the old cliff line. Particularly visit the bird hide and new spit at Robert Findlay Wildlife Reserve, 2 km south of the shorebird centre.

Features
Sedimentary Landform
Geological Age
Holocene, built in the last 4000 years.
Zealandia Evolution Sequence
Pākihi Supergoup: 5 million years ago – present
Links
See http://www.miranda-shorebird.org.nz/home/chenier-plains-an-internationally-significant-landform See Hayward, B.W., 2017. Out of the Ocean into the Fire. History in the rocks, fossils and landforms of Auckland, Northland and Coromandel. Geoscience Society of New Zealand Miscellaneous Publication 146, p.260-261, box 50.