Coromandel History

Isdale on Tapu part 1

Alistair Isdale: Geological History and Later Land Developments

"Hundreds of millions of years ago, before there was a New Zealand, this region was under the waters of shallow seas, whose floor had been built up by immense deposits of mud and silt and clay from a great continent to the east.” (Own “History of the River Thames,” using data from Government Geological Survey Bulletins, especially bulletin 10 re Thames, by Colin Fraser.)

As eons went by, with the combined effect of pressure on the lower layers and constant vibrations (microseisms) producing a rammed earth effect, these deposits became quite hard rocks.  The clays became argillites, which could be clay coloured and moderately hard, to darker and harder, ending in the very had black argillites, which could be used for tool and weapons by early inhabitants, and able to take a polish.  The muds became greywackes, hard enough in broken up form or as stream gravels to use as road surfacing.  (In at least one instance, near Auckland, the pounding and vibration of road usage in wet conditions brought this mud-rock back to its original mud.)  Fine silts could become slates, likely with a little metamorphic help from nearby hot volcanism which duly came.

The one of most interest for this region, since it determined the main deposits of gold and other metals, was around 60 million years ago, following one around 120 million years ago, in mid cretaceous times, which brought a change from seasons to a world greenhouse effect, with extinctions of species.  In which the world volcanic outburst of around 60 or 65 million years ago was also very efficient, wiping out the dinosaur.

It was a time of big tectonic earth movements.  The Coromandel Range and its southward extensions rose, spouting numerous volcanoes, with great lava flows of andesite as much as 2,000 feet (more impressive in the Bulletins than in metres), thick.  Then a great earth block, around 110 miles long by 12 or more miles wide (say 180 by 20 or so kilometers), sank at least 1500 feet (say over 500 metres), causing a great graben or mammoth ditch, with an arm of the sea extending as far as present day Matamata, where an old beach was found.  This was later filled in with at least 500 cubic miles of material, most of it from the volcanic explosion in comparatively recent times which blasted out the bed of Lake Taupo.  That left largely swampy plains, till drained, and the shallow Firth of Thames.

There had meanwhile been other volcanic episodes, such as around 20 million years ago, but the above general picture is sufficient for the area now under examination, as it is at a meeting place of the first big andesite deposits and a large area of the original sedimentary rocks still showing on the surface, starting between the two Tapu and Te Mata streams, and going on northward to cover quite an extensive area.

This is shown in a 1962 geological map covering the whole Coromandel Peninsular, for the general picture, while for the localized area now under consideration, a map attributed to December 1867, when there was a gold rush to Tapu, give quite an accurate picture of the local scene.

The gold rush was made possible by modifications to the original andesitic lavas ejected during the first great phase of volcanism.  Water, at a temperature of 410°F (nearly double boiling point) at sea level and under a pressure equal to 660ft of sea water, will dissolve silica (quartz) and many other things as well.  As waters with dissolved material near the surface, they lose both temperature and pressure.  In cracks and fissures, some up to 100 feet or so wide, the loss of pressure was particularly sudden, the superheated water flashing into steam and leaving the solids in the form of quartz reefs, with other substances that had been dissolved, preferably gold, though substances like iron compounds were much more common.

Later Land Developments

One that took place comparatively soon after the Coromandel Peninsula emerged with land movements and volcanic build-ups was the coming of vegetation, interrupted by fires and later volcanic outbursts, so that charred logs can be found buried by the fiery clouds of incandescent ash.  Generations of forest followed generations, emerging in stages from successive devastations and climaxing in great stands of kauri trees, which came to cover the greater part of the Coromandel Peninsula, while in the ground were deposits of kauri gum from trees long rotted away.

Like the gold in the quartz reefs, both the standing kauri trees and the buried gum would give rise to important industries.

Not an important industry but of interest in very recent times, was the effect upon the old sediments of having hot volcanic neighbours.  Not only was there a certain amount of metamorphism, but also there were silica materials brought up through the old sedimentaries under somewhat different conditions than through fissured lava beds of andesites of the ‘First Period’.  There were also ‘Second Period’ andesites and dacites with quartz family gemstones of the chalcedony group rather than gold.  Instead of immediate formation of quartz crystals from solutions as pressure fell, there was what seems to have been a slower process, in which there was first concentration into a gel, which hardened to show no visible crystals, resulting in cryptocrystalline quartz or chalcedony in its various forms, such as translucent cornelians and agates.

These appear in the Te Mata stream and nearby, but not in the Tapu, with its gold.  (While unusual, gold in sedimentaries on the Coromandel Peninsula is not impossible, such an association being found at Kuaotunu).  Gold has also been found in opaque chalcedonies of the jasper group in later volcanics behind Hikutaia, but the proportion of gold was too small, and the rugged jasper too difficult to crush, to be profitable.  There is no data to suggest such an association around the Te Mata.

Both the gold – and other materials in quartz – in the volcanics around Tapu, and the quartz family gemstones around the Te Mata, would have taken place in the solfatara or hot water stage after the eruptive phase of volcanism – varied by an occasional belated eruption somewhere, and the later volcanic eruptive phases of different types.

Much more recent in geological time is a development affecting sea level

Around 10,000 years ago, or a fraction of a second geologically, an Ice Age, which used to be put a million years or so back, came to an end.  There was very extensive flooding, hence flood ‘legends’ all over the world, with the dates confirmed by a study of ancient Australian lake levels, but also a marked rise in sea levels, which had been as 200 to 300 metres lower than today, varying with gravitational and isostatic factors in different places.

At the time of the rise, there was quiet a cliff face where the great earth block had slid down, into what now became even deeper water.  Then when Taupo blew its top or whatever, enormous volumes of rhyolite debris mixed with silt did an enormous infill job, so that the Te Mata and Tapu now discharge into shallow waters.

From the head of the Thames Valley, a natural ‘grade’ was formed.  The slope of a grade depends on the type of material, as noted in ‘Rivers of Hawkes Bay’ by George Nelson, in a newspaper series 1920’s-30’s, using old authorities.  (In Hawkes Bay, settlement and clearing caused accelerated erosion, resulting in shallowing and shingling of the rivers, a change from silt to gravel changing the grade and filling originally deep water course).

And interesting point would be what happened in the Te Mata and Tapu outfalls and bordering and nearby silt deposits a) when they exited by steep slopes or cliffs into a much lower sea.  b) when they exited  into even deeper water, but the sea now more or less at present levels  c) after the great Taupo explosion, whose high velocity hurricanes of ash lapped the Bombay Hills and in filled the till then deep arm of the sea.

Once a) b) c) would have required quite a program of drilling.  Now we have electronic-sonic means of determining layers of different materials below the surface, such as have enabled us to make accurate maps of the actual surface of Antarctica beneath the ice, also determining what is now below sea level, and ready to come up with the isostasy effect if weight of ice is removed, and what is at present above sea level beneath the ice, in places half a mile thick.

Boreholes have been made in and around Thames, and their results give us some idea what to expect, including pre-a)b) and c).  One thing that was established, not so much by boreholes, but principally by a maze of underground workings, was that the original face of the graben was very steep, 45º, or up to Matterhorn standard.  Exposure to erosion has cut this back to an easier grade, though along the sea face, sea erosion as usual, tended to make cliffs.

Initial erosion, before vegetation was established on still hot and shuddering beds of lava and other ejecta, would have been tremendous, as illustrated a t present by the Philippines, with loose material coming down in great mud slides or lahars I have seen, with great boulders dancing like corks.  Adding to the pre-vegetation massive erosion, during the hot water or solfatara stage after eruptions, and in between successive eruptions, the hot water under pressure passing through the lava not only fills fissures, but passes through the body of the lava as if it was so much mist, dissolving out some chemicals and substituting others.  Even, some geologists have surmised, dissolving out gold and suchlike already in the rock and adding it to the deposits in the fissures – the theory of “lateral secretion.”  In any case the process softens the hard black andesite lava to different colours and grades of softness, to the ‘sandstone’ of the miners, sometimes very soft and white and friable indeed.  These would be readily eroded, and gouged by mudslides with their dancing boulders.

When these poured over the 45º sides of the graben into the waters below – at varying sea levels over the millions of years – the sea would tend to clear away the muds and leave the boulders on its bed below the outfalls.  At Thames, some distance out from the 45º  slope of the boundary of the great depth, the boring went down easily through 1.020 feet of silt, and then started wandering and getting jammed among boulders.  Out on the Hauraki Plains, once swamp, a bore suggested the actual bottom was around 1500 feet down, giving a build up of 300 feet or so of boulders at the out-falls during the pre-vegetation phase of massive erosion millions of years ago, with volcanoes of loose material, or softened by ‘hydrothermal’ action or hot water under pressure, being bodily removed, till only their lava cores were left, to take strange steep shapes like Little Kaitarakihi.

If the outfalls started as falls, they would end up being gouged into deep canyons going some distance back.

  1. Could therefore be expected to have deep canyon exits going some distance back, with boulder deposits down in the sea.
  2. In recent geological times, with one.of those phases of higher seas, the canyons would infill, with quieter exit into a higher sea level.
  3. The infilling would be even greater, with a graben choked with silt to within a few feet of the Surface.  The "fall" from near Matamata to the sea averages around 6 inches to the mile, dead flat to the eye

This continues undersea and from the shores to the middle, as a kind of underwater mudflat, so that at Tararu north of Thames proper, the deepest in the middle is around 20 feet, and around 60 feet opposite Coromandel. Opposite Tapu-Te Kata would be about 30 feet, with quite a tidal range, with the compressing effect of the Firth already piling up, being a 12 feet range at Thames instead of 6 feet as at Coromandel.

In any case alluvial deposits of a fairly uniform flatness in the two valleys and watercourses, and along the shoreline (narrow, but wider Just outside the area with the Tapu flat, conceal a deep and turbulent geologic­al history. Top

 

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