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Accessibility: EASY
East of Army Bay, tilted, folded and faulted sandstones enclose a thick volcano-derived conglomerate
WHAT'S HERE?
Tilted beds are cut off by a low angle fault and overlain by flat lying beds including Parnell Grit
East of Army Bay, Early Miocene sandstones are offset by low and high angle faults and tightly folded. They enclose a thick bed of conglomerate to coarse-grained sandstone ("Parnell Grit"), with pebbles and boulders of andesite lava and limestone, and some fossil fragments.
LEARN ABOUT IT
A thick "Parnell Grit" bed of volcano-derived conglomerate.
Sandstones and mudstones which form cliffs around Auckland Harbour were deposited in a deep marine basin (1000-2000 m deep). Geologists describe them as belonging to the Waitemata Group. The sandstones are turbidites deposited from gravity-driven underwater currents. The thick bed (or beds) of grey-green conglomerate to coarse-grained sandstone with many pieces of pebble to boulder-sized andesite lava is an example of the Parnell Grit. It was deposited by submarine landslide and debris flows and was probably derived from huge active volcanoes that were erupting 40 to 65 km away. These were along the western side of the Waitemata sedimentary basin.
HAVE A LOOK!
Large block of lava carried into deep water by a submarine landslide and debris flow
There are small fossil fragments in the Parnell Grit - mainly bryozoa, but also bivalves. The pebbles, cobbles and boulders are mainly andesite and basalt from the western volcanoes but there are other types of rock too. Look at high tide level in the stony beach deposits eroded from Parnell Grit. Are there limestone boulders? How many sorts?
Turbidites commonly show "sedimentary structures" such as graded bedding - a sandstone is coarser grained near the base and may 'grade' (ie gradually get finer) upward into the overlying (and younger) mudstone. This is because when the suspended material was deposited, the larger pieces settled down first followed by finer and finer particles that slowly floated down and settled on top. This means we can check whether a layer is the original way up or if it has been overturned. Have a look at the turbidites here. What is your verdict?
It is impressive to imagine these layers of tilted, folded and faulted rock strata originally lying flat on the sea floor.
Turbidites commonly show "sedimentary structures" such as graded bedding - a sandstone is coarser grained near the base and may 'grade' (ie gradually get finer) upward into the overlying (and younger) mudstone. This is because when the suspended material was deposited, the larger pieces settled down first followed by finer and finer particles that slowly floated down and settled on top. This means we can check whether a layer is the original way up or if it has been overturned. Have a look at the turbidites here. What is your verdict?
It is impressive to imagine these layers of tilted, folded and faulted rock strata originally lying flat on the sea floor.
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Directions/Advisory
Follow the signs on Whangaparaoa Road to Shakespear Regional Park but stop at the Army Bay car park on the left of the road by the park turn-off, before the gates to the army base. There is a large parking area, and there are public toilets.
Keep an eye on the cliffs for loose blocks which may fall.
Google Directions
Accessibility:
EASY
Walk east from the car park along the beaches and over the extensive rock shore platforms. Only accessible at low tide or half tide.
Features
Sedimentary
Rock Deformation
Geological Age
Early Miocene
Zealandia Evolution Sequence
Māui Supergroup (Emergence): 25 – 5 million years ago
Links