Long Bay

This site is an excellent place to examine some of the features of the Waitemata Sandstones. Waitemata Sandstones underlie much of the Auckland region and underlie Auckland’s volcanic rocks. The Waitemata Sandstones were deposited on the floor of a 1000-2000 m-deep marine basin (known as the Waitemata Basin) during the early Miocene period, 21-18 million years ago. This deep marine depression was formed by rapid subsidence of the whole Auckland region (22-20 million years ago) as northern New Zealand started to feel the effects of the newly active boundary between the Pacific and Australian plates, with both compression and subduction forces.
Typically the Waitemata Sandstones consist of alternating layers of sandstone and mudstone. The top of each layer is the ancient sea floor that was progressively building upwards as sediment was deposited on top of it. If the layering is no longer flat-lying, it indicates that the ancient seafloor has been tilted or even folded sometime after the layers of sediment were deposited. This may have been soon after deposition during seafloor slumping or some millions of years later as the whole sequence was slowly pushed up by tectonic forces and has subsequently been eroded down to what we see today. Approximately 800 m thickness of Waitemata Sandstones was deposited and in the east today (e.g. Hunua Ranges, Waihake, Motutapu, Tiritiri, Kawau, Leigh) all this thickness has been eroded away, whereas the sequence generally dips down to the west where most of the sequence still exists below the ground level (Waiuku, Huia, Waimauku, Helensville, Kaukapakapa).
Each sand layer was transported in a turbulent slurry of sand, mud and water (called a turbidity current) down a submarine canyon from the northwest and deposited on the floor of the Waitemata Basin as the slurry slowed down and dropped its load. Sedimentary features within the sand layers were formed as the slurry was slowing down. The lower parts are often the coarsest sand that dropped out of suspension first while finer sediment was carried further along the basin floor. The size of the sand particles get finer upwards in the layer (called grading). A little higher the sand bed is often finely layered (laminated) produced by grains being left behind by the slowing speed and power of the slurry - here the grains were being slid and rolled across the sea floor (traction flow). Above the laminated layer you may see a layer of rippled fine-grained sandstone produced by an even slower flow where the sand grains were bouncing along the sea floor and small ripples developed and advanced along the bottom. In these upper parts of the sand layer you often see flames of sand produced by upwards water escape from within the underlying sand layer (just like the liquefaction that happened in the Christchurch Earthquake). The fine sandstone usually then grades upwards into mudstone, which is the finest grains of the slurry which finally dropping out of suspension from the passing muddy cloud.
Between the sandstone layers (beds) there are usually thin layers of mudstone, which is a combination of the mud that was deposited by the tail of the turbidity slurry and also mud that dropped out of suspension through the entire column of sea water in the long time between each turbidity slurry coming down into the basin. Each sand layer was deposited in a matter of hours, whereas each mud layer was deposited over hundreds of years at an accumulation rate of about 1 cm per 100 years.
The sand that flowed down the northwest slopes of the Waitemata Basin was sourced from erosion of an ancient Northland land area that existed at that time. The sand, mud and plant debris was carried down rivers to the shore of the basin in the vicinity of where Dargaville and Wellsford are today, and periodic earthquakes shook piles of sand loose from the top of the submarine slopes to periodically flow down into the basin. If you look carefully you will also see layers rich in black plant material (sometimes even flattened branches) that had been carried down into the sea by rivers during storms, where it became water-logged and sunk to the sea floor and was later carried along in the turbulent sand slurries. Being less dense than the sand grains the carbonaceous plant material was deposited on the sea floor late in the sand layer deposition, usually near the top of each layer with the fine sand ripples.
Within the upper parts of some sand layers you can see burrows left by organisms, such as marine worms, heart urchins and shrimps, that lived on the sea floor at that time and lived in the soft sediment. There are a number of different kinds left by different types of sea floor animal.

Estimate the thickness of mudstone layers in a cliff sequence and using an approximate mud accumulation rate of 1 cm per 100 years, estimate how long it took for the cliff sequence to be deposited on the seafloor, 20 million years ago. Why do we not also include the thickness of the sandstone beds to help estimate the time taken to deposit the sequence?