The EAST NICHOLAS RANGE An oddity

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The EAST NICHOLAS RANGEAn oddity

• A presentation
• By
• David Leaman

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SECRETS DEPEND ON ORIGINS

• An understanding of just how and why the Nicholas
  Range is peculiar or special an oddity
  depends on knowing just how the range came to
  exist, and what forces and processes act upon it.
• Let us turn the clock back a few million years,
  or so

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Beneath the sea

• A subsiding continent allowed a build up of
  sediment beneath a relatively shallow sea which
  was occasionally fairly deep. These sediments
  compacted into sandstones, mudstones, and
  turbidites (mixed up sediments which have
  resulted from sea floor collapses). This occurred
  about 350-450 million years ago (Ordovician and
  Silurian Periods).
• The pile of rocks became several kilometres thick
  before the crust was destabilised and uplift
  began. Some of the rocks were crumpled (folded),
  or were pushed or slid laterally (thrusted) and
  many were thermally altered due to changes in
  heat flow and, in some cases, due to the pressure
  and depth of burial.
• These rocks are known today as the Mathinna Beds
  and some of them can be seen near St Marys as
  well as to the north of the range.

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The Mathinna Beds

• Ordovician
• Silurian rocks

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Really finish the job

• The changes in heat flow due to the formation of
  the deep basin filled with sediment led
  ultimately to total crustal distortion and
  melting. Result a suite of volcanics and
  granites.
• The molten material welled upward (being less
  dense than the now altered sediments) and both
  heated them, and shoved some of aside and upward.
  Who would be a rock?
• The first of these igneous upwellings is known as
  the St Marys Porphyrite and you can see it as
  you drive down St Marys Pass. This process began
  about 390 million years ago and continued for
  about 50 million years during the Devonian Period.

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St Marys Porphyrite

• Devonian

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Ever upward

• But the big changes had only just begun and the
  whole package continued to rise into a mountain
  range.
• By 300 million years it had been extensively
  eroded and denuded and was now covered in ice.
  This was the Permo-Carboniferous ice age.
  Glaciers ravaged the landscape and by the time it
  had all melted the region had been almost
  levelled. (This great planar surface remains
  visible today as the rocks which once buried it
  are removed and it is the reason that so much of
  the landscape to the west has similar maximum
  elevations.)
• The region was once again flooded by a shallow,
  near polar sea (with the odd ice berg) during the
  early Permian Period (say 280 million years ago).

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Permian rocks, flat lying
Erosion surface
unconformity
Mathinna Beds steeply dipping, folded
Unconformity the mark of much erosion and lost
time.
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A different world

• New rocks were laid down on the eroded land which
  had been flooded sandstone, mudstone, limestone,
  siltstone and occasional conglomerates. We drive
  through these in the upper section of Elephant
  Pass and the road up to the saddle on South
  Sister.
• This sequence of rocks is quite thin, in the
  range of a few hundred metres at most.
• Then, about 250 million years ago, the entire
  Earth was transformed. No one is really certain
  what happened but in Tasmania things which had
  been marine became elevated and have stayed that
  way. Nearly every living species died. This was
  the time of the greatest ever extinction of life
  on the planet.
• Nothing would ever be the same again.

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The typical look of the Permian rocks

• The typical style of the Triassic coal measures

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A changed climate

• For a few million years the rock record is
  missing. When it is restored (well into the
  Triassic Period) we see a cold desert landscape
  with sand being swept across plains with dry
  streams which flow only after great storms. These
  fed swamps which, as time passed and the climate
  slowly warmed, spread across the landscape. Great
  forests lived and died. The coals of the region
  date from this time (Triassic-Jurassic, about 200
  million years ago).
• The climate changes were accompanied by tectonic
  changes uplift and increasing volcanism.
• The St Marys-Nicholas Range area has preserved
  the only Tasmanian examples of the lava flows of
  the period.

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Dolerite cap seen from the level of the Triassic
lavas
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Then, catastrophe

• All this action was mere precursor to serious
  business. The continent was in process of
  breaking and rifting apart and the harbinger of
  this was the injection of huge volumes of melted
  material from the lower crust. This happened
  about 180 million years ago (Jurassic Period).
• Another complete transformation uplift,
  disruption and, afterwards, actual commencement
  of the separation of the now southern continents.
• The injected material, dolerite, was once a fair
  depth below the land surface but uplift has
  continued and all the rocks, once above it, have
  been eroded away. This rock now sits firmly and
  dominantly on the top of our landscape.

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The distinctive dolerite of the range cap peeps
over the foothills.
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Another remnant of a lost plateau St Patricks
Head
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The landscape becomes modern

• Much of the cover on the dolerite was removed
  during the Cretaceous Period and by Tertiary
  times (about 60 million years) new forces were at
  work. The final breakups were under way (Tasman
  Sea and New Zealand in the east the Southern
  Ocean and Antarctica in the west and south and
  warping and stretching of Bass Strait in the
  north).
• This activity led to faulting and rifting within
  Tasmania and to warping, tilting and uplift in
  different regions at various times. These impacts
  are important since they account for many of the
  oddities of the region.
• Great rivers drained the rifts formed during the
  Jurassic (note that it was a much bigger land
  mass than now) and these were re-arranged by the
  extra rifting of the Tertiary Period.

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The Break ODay

• The modern Break ODay River is a rather pathetic
  stream in a huge, broad valley which joins the
  South Esk at Fingal.
• Once, however, it flowed out to sea but, as the
  land was tilted and raised, its flow was
  reversed. This is also why the South Esk now
  flows toward Conara. The subsidence toward Bass
  Strait is why it also flows north toward the
  Tamar. These are complete reversals effected in
  the last 15-20 million years.
• This uplift and tilting has not stopped.
• In the midst of all this activity the climate
  stepped back into play.

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A cooling-off period

• Things had been cooling for quite a while (since
  the formation of the Southern Ocean and changes
  in current circulation) but became decidedly cold
  about 3 million years ago. Apart from a few
  warmish periods (including the one we now live
  in) things have been icy.
• During the colder times most of the major
  vegetation has disappeared, the land has been
  partly denuded and soils have either not formed,
  formed slowly, or been lost entirely.
• Some pockets of ancient weathering, thick soils,
  and botanical assemblages survived.
• During these changeable times there was extensive
  erosion and surging rivers. Erosive debris draped
  the slopes of the valleys.

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The dolerite-derived debris (talus and scree)
which drapes most slopes
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Little was stable

• Extreme climate action, debris piled on slopes,
  and variable water flows and content meant that
  few slopes were stable. Landslides were endemic
  and material collapsed or slid to the valley
  floors.
• Parts of the more solid geology were involved in
  some of the larger failures.
• Drier times have held the slopes but any changes
  involving slope changes (like human excavations
  or cuts) and anomalous water changes will permit
  more failures.
• The landscape is barely stable and several risk
  areas are noted on the Mineral Resources Tasmania
  data base along the range.

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Today

• After all this drama we now have the present
  landscape and distribution of rocks and soils.
• An eroded capping of dolerite slopes draped with
  (mainly) dolerite debris wide valley floors with
  alluvials and gravels variable soils.
• Everything appears in a variety of sun aspects,
  on various slopes, and in a range of hydrological
  conditions.
• It is a recipe for variety and a modern
  geological map indicates these factors.
• And a few elements of this history are rarities
  (the Triassic basalts for example) in Tasmanian
  context.

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Geological map of eastern Nicholas Range north of
St Marys
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• Sections across the range
• note the unconformity, lava zone, coals and
  surface drape deposits.

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A final geological thought

• The range appears to stand in isolation and it
  is now isolated but it is really a part of
  Fingal Tier, now separated by the massive valley
  of the Break ODay, a river which does not
  deserve its valley.
• The rest of the great plateau which once extended
  north to Bass Strait is gone but, off in the
  distance other remnants are apparent Mts
  Victoria, Albert.
• So much that was, so much that no longer exists.
  Nothing is forever on this planet it is all
  about change a lesson humans have yet to learn.

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Some climatic realities

• The long history of the region has involved
  some significant changes in climate. There have
  been warm, wet periods and some very cold and dry
  periods and most variants in between. The last
  three million years have been particularly cold
  with a few moderately warm (inter-glacials)
  periods.
• The intense cold, glacial period of 20 000 years
  ago began to lift about 12 000 years ago and we
  have experienced a warm intermission over the
  past 8 000 years.
• The great changes in climate, and ice cover, have
  also determined sea levels.
• The Nicholas Range area has been affected by all
  these changes even though never permanently
  covered with ice.

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Associated changes

• Changes in climate have affected the nature of
  vegetation, and the extent of its coverage. There
  will have been times when the land was barren.
• Land stability is largely determined by the
  nature and extent of precipitation, especially
  when vegetation has been removed.
• Within this variability there have been some
  constants due to the scale of Tasmania, and the
  layout of surrounding seas.
• Westerly systems have predominated, along with
  occasional Southern Ocean depressions, and Bass
  Strait passage depressions much as now.

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Anomalous weather

• The region is noted for its irregular and
  anomalous weather. Clearly, the arrangement of
  broad valleys, offset high ground, and coastal
  location is important. Onshore cloud situations
  can apply.
• The geological signals agree with the rainfall
  data this is a dry area with quite exceptional
  rainfall patterns.
• Rain surges can happen at any time of year and be
  among the most intense in Australia. Monthly
  rainfall data and averages tend to be rather
  meaningless in this region. Rainfall and seasonal
  patterns are not predictable something which
  shows up in river data.
• Up to 255 mm has fallen in 7 hours, or 352 mm in
  16 hours, or 508 mm in 24 hours (March 22, 1974
  Cullenswood and German Town).

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Contrasting river behaviour abnormal v normal
Note the absence of a seasonal pattern with a
winter surge in the Break ODay case
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Rain at German Town

• An indication of the variable and erratic nature
  of rainfall on the range is provided by data from
  German Town.
• The monthly mean ranges from 65 mm (Jan) to 97 mm
  (Nov) with all other months in excess of 83 mm.
  The monthly median ranges from 54 to 81 mm (the
  amount most likely to fall). The monthly minima
  range from zero to 19 mm and can happen in any
  month. The monthly maxima can also occur in any
  month, and have ranged from 216 to 596 mm.
• Other local sites, such as Gray, present similar
  patterns but all such character is restricted to
  a zone with a radius of a few kilometres of the
  pub in St Marys.
• The only differences are, that down in the
  valley, the rainfall totals are likely to be at
  least 300 mm per year less than on the adjacent
  ranges.

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Where does the water go?

• Large rain surges lead to flood surges. The river
  shows this direct pattern. There is no
  significant seasonal flow or base flow.
• The catchment yields an average of 2.25 ML/ha
  with a total of 34 as baseflow feed at
  Killymoon. About 80 of all water falling in the
  catchment is used directly (evaporation and
  transpiration) with about 70 rate in winter and
  90 rate in summer.
• Plantation trees (contrasted with native tree
  use) lift usage by about 1 ML/ha.
• Long term climate changes have reduced rainfall
  by about 8 since 1975 and led to a catchment
  loss of more than 16 in the same period. (due to
  changes in temperature and evaporation)
• The catchment is not stressed (in terms of
  changed usage) at the present time (2004
  analysis).

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Water within

• The top of the range is rocky and dry. Water
  either evaporates or infiltrates into the rock
  mass and slope debris. Stream development is not
  apparent until about mid slope. High level run
  off is not general although sheet flow may occur
  temporarily in the extreme rain events.
• Water migrates within the rock mass (dolerite at
  top, coal measures below) and may reach surface
  at many locations dependent on cover thickness,
  hydraulic properties of slope materials and
  rocks, and structures within the rocks.
• Springs are common but not predictably found.
• The range thus stores a huge volume of water, but
  releases it quite slowly to obvious surface
  systems.

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• Water is stored in pores within talus but within
  fractures and bedding surface in the rock mass.
• Various pathways may control flow within the
  materials or determine outlets to surface.
• Disturbance of materials may affect such pathways
  and alter flow conditions.

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Linkages
Diagram showing water circulation in the Nicholas
Range Land stability depends on water transfer
and volumes and may involve surface materials or
bedrock.
Diagram stresses flow controlled by coal measures
and basalt, and major fractures or failure
surfaces. Cross-unit flow is minimal.
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A bore at St Marys

• The hydrology of the range, its history, and the
  origin of the valley, are matters which combine
  to provide the present water supply to the town
  of St Marys.
• The town bores near the railway seem normal and
  are taken for granted. This supply is quite
  unusual.
• It depends, in equal amounts, on the three rock
  types drilled (Permian, basalt, Triassic) and two
  of these yield water here in abnormal volumes.
  The basalt is a rare extra and may be the supply
  link to the water stored in the range. The local
  fault-fracture system connects all the elements.

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The odd range

• The geological construction of the range is
  unique in Tasmania. The basalt ensures this.
• The geomorphology of the range is unusual
  isolated, elevated and coastal.
• The range stands in a peculiar climatic zone
  which should be, and is, dry but which receives
  exceptional rainfalls and patterns.
• The combination of elements is unique in Tasmania
  and we should expect an interesting and varied
  ecology developed on it as a result.

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THE END