Kaipara South Head cross-bedded sandstone

Accessibility: MODERATE
Burrows of marine animals in sand beds (= trace fossils).
Excellent exposures of flood-tide delta cross-bedded sandstone.
Marine trace fossils.
Folded strata in foreset beds.
Several levels of delta growth.
Mosquito Beach. Botryoidal limonite from iron pan produced within sequence by podsolization.
This is near the northern tip of South Kaipara Peninsula which is a huge sand barrier that was built halfway across the entrance to the wide open Kaipara Bay that existed on the west coast 2-3 million years ago. Creation of the North and South Kaipara peninsulas created New Zealand's largest harbour - the Kaipara. The barriers are believed to be a result of a major increase in sediment supplied to the west coast coastal system by voluminous eruptions in Taranaki and the central North Island in the last 2 million years.
In the low cliffs on either side of Mosquito Beach and best seen to the south we can recognise sand that was deposited in a flood-tide delta. We normally think of deltas being deposited at the mouth of a stream or river where the fast flowing water slows down and deposits the sediment it is carrying. Deltas can also be deposited on the inside and outside of major harbour entrances. The tide flows in very strongly and out very strongly, twice every 24 hours. As the tide is flowing fast it carries along a lot of sand, often bouncing or dragging along the seafloor. Tidal currents slow down near the turn of the tide and no sand moves either way at these times. In the harbour entrance the sand is moved along the relatively shallow floor by strong currents but as the entrance opens out into the harbour the water is deeper and the front of the growing delta has a steeply sloping front where the sand slides down into the deeper water.. This is rather like a sand dune. The sand is moved along on the near-flat shallow floor and deposited on the steeply sloping front where it is no longer moved by the current above and gradually the sloping front grows into the harbour (flood-tide delta) or out into the sea (ebb-tide delta).
In the cliffs south of Mosquito Bay we can see eroding ancient examples of the near-flat seafloor passing into the steeply sloping (cross-bedded) front of the growing delta. They look superficially like sand dune deposits except we can in some places see features like water escape structures, current cross-bedding and marine animal burrows that indicate that this sand was deposited under the sea. Stand back from the cliff and in some places you can see several levels up the cliff where one flood-tide delta layer has been deposited over the top of an earlier one.
In the last 2 million years when these sand barriers were formed sea level cycled up and down by 40-130 m during warm and cold periods of the Ice Ages. Clearly these ancient flood tide delta sands were deposited during a warm period when sea level was high. There is evidence in this area that there has also been some slow uplift in the last 1 million years.
The exact age of the delta deposits at Mosquito Bay is not known but they are low in the sand barrier sequence and possibly 1 million or more years old. Their presence here shows that by that time Kaipara Harbour had been formed and its entrance was being modified, like it is today, by sand being moved in and out by the strong tidal currents through the entrance.
Slump folding within steep dipping sands on front of advancing flood-tide delta.
If you were here when the delta sands were being deposited, what direction was the entrance and in which direction was the incoming tide carrying the sand?
How many levels of cross-bedded sand (flood-tide delta) can you make out in the cliffs?
It is unlikely the sands forming higher parts of the peninsula around here were deposited in a delta. So how do you think the higher land areas of sand were built up?
Look out for pieces of limonite (iron ore) that have a lumpy ("botyoidal") appearence. This was formed within a soil layer when the iron was leached from near the surface to be precipitated in a layer lower down.

Drive along South Head Rd from Helensville (Parakai) to its very end at a farm gate. Park there and walk down the steep driveway to Mosquito Beach.

Steep driveway could be slippery, especially if wet.
Visit close to low tide and be aware of incoming tide if you are around the rocky shore.

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Accessibility: MODERATE

Walk down along partly sealed driveway to beach. Walk south (right) along beach (200 m) and view the rocks from here for a further 200 m until you can go no further. Returning up the steep driveway will challenge the unfit.
Best visited 2-3 hours either side of low tide.

Sedimentary Landform
Geological Age
Pleistocene, last 2 million years.
Zealandia Evolution Sequence
Pākihi Supergoup: 5 million years ago – present