© Peter J Reilly, 1997
SYMBOLS USED IN ACCOMPANYING TEXT INCLUDES;
~ .... means 'approximately equal to'.
bp ... means before present ... as in 'years before present'.
Note: You can never be truly precise about Geological dating so the 'years-before-present' is intended only as a guide: -
For Example; some rock formations on planet Earth have been carbon-dated at 4.5-billion years old. Therefore! Planet Earth has been in existence for, at least, 4.5-billion years [or its equivalent 4,500-million years]. - - Ok! By comparison; an 80-year old man has lived for 2,540-million seconds, or thereabouts, at least he had 'last-weekend', when we did the calculations: - Therefore, if you think about it, measuring Geological Time in terms of years, is about twice as absurd as measuring a human's age in terms of seconds: - Nonetheless, modern homo sapient are in the habit of measuring 'Comprehensible Time' in terms of "Earth Orbits around the Sun", and habits are hard to break:
Geologists themselves, however, prefer using names that alludes to the nature of the epoch or eon concerned. Much like referring to the human 'aging' process in terms such as; -Newborn; -Infant; -Toddler; -School age; -Juvenile; -Early Adolescence; -Late Adolescence; -Young adult; -Adult; -Marriageable age; -Parenthood era; -Middle age; -Elderly; -Geriatric etc., etc.: -- And! The gentleman mentioned in the previous paragraph would qualify as an example of "Elderly" and may even fit into a sub-category called "Spritely elderly"! ... Clearly it is impractical if not impossible to 'pin-point' the month, or even the year, that these human transitional changes occur. And! it would be a mathematical impossibility to work out the exact second in which they occur: - Nonetheless, "everyone" knows that there IS a chronological progression to it all; and that elderly occurs a looooonng time after adolescence; - In most cases, at-least! ~ ~
-- As for dear old “Mother Earth”? It seems as though she is currently approaching the end of her “Parenthood era” and, in all probability, in the next few hundred thousand years, or-so, she will be entering into her “Middle age” phase.
A Brief glossary of the geological terms used in the accompanying text appears at the bottom of this page.
GEOLOGICAL SYNOPSIS OF THE MURRAY VALLEY IN SOUTH AUSTRALIA " FOR TOURISTS "
BRIEF OF THE MOST RECENT ERA ( HOLOCENE ERA) - i.e. approx 50,000years~bp till present . . .
Man's evolution !!: - Ape-man evolved into Homo Erectus around 500,000bp, then into Homo Sapient around 200,000bp; - Graves at Roonka and Big Bend, on the River Murray, indicate that Aboriginal Communities have dwelt along these sections of this river, continuously, since at least 35,000bp. - See page on Aboriginal pre-history
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35,000 bp: ... Colder than at present: - Little rainfall and little evaporation. Lakes existing at the time remain constantly full. Conditions "humid" indicating that the water vapour already in the atmosphere remained meteorologically static, i.e. weather conditions "static". - Absence of strong winds is indicated by a noticeable reduction in 'dunning' and a corresponding increase in vegetation.
29,000 bp: ... Dry & windy: - Decline in vegetation. 'Dunning', the result of persistent winds, has re-commenced.
28,000 bp: ... Very dry: - Extensive 'Dunning' indicates very strong-windy conditions. - Another ice age is now in progress in the northern hemisphere.
24,000 bp: ... Local sea levels have dropped. Likely due to build-up of the northern hemisphere ice: ... Strong, windy conditions persist resulting in wild seas pounding the lately exposed coral and shell pulverising it into sand. 'Sandstorms' of calcium-based sand are blown far and wide across the land; - Calcium deposits on land surfaces increase dramatically at this time.
20,000 bp: ... Local sea levels continue to drop, notwithstanding that the northern ice mass briefly recedes.
18,000 bp: ... Start of a local arid spell and a re-increase in northern ice mass. Sea levels locally go as low as 90m (300ft) below our present-day level: - It could be argued that the South Pole was more exposed to the sun than was the North Pole, at this time. Suggesting a substantial wobble in the Planet's north-south axis.
16,000 bp: ... Very severe arid conditions exist. Extensive 'dunning' occurs in SA and in Victoria.
14,000 bp: ... Locally, arid conditions are starting to ease.
12,000 bp: ... Glacial thaw occurs but conditions still cooler than at present. Meteorological conditions generating severe wet/dry cycles.
10,000 bp: ... Minor ice age occurs about now, but locally seas are rising.
9,000 bp: ... Warmer humid: - Seas continue to rise.
6,000 bp: ... Arid thermal maximum: - Large deposits of gypsum are formed at this time. As gypsum molecules contain atom/s of sulphur (CaSO4 2H2O), it suggests the presence of sulphur gasses in the local atmosphere. Most likely source of sulphur is volcanic emission of gas, not necessarily lava eruptions, however.
[It is generally believed that the era of Pyramid building in Ancient Egypt occurred at around this time.]
4,700 bp: ... Volcanoes are now erupting in 'southeast' region of SA; - large amounts of ash are emitted.
3,000 bp: ... Seas are 3-7m (10-23ft) higher than at present: - A fjord of the Southern Ocean now filled the Murray Valley as far upstream as Waikerie. The Coorong, Lake Alexandrina & much of the countryside south of the Padthaway Ridge is inundated by the sea. ~ ~ The valley between the Padthaway Ridge & Waikerie, at this time, would've somewhat resembled conditions as they existed during the height of the 1956 flood, except the water would've been ocean salt water subjected to tidal variations. The local high sea level persists for a thousand years or more. - Also at this time, in what we nowadays call the Murray's North-West Bend region, a freshwater flood rose to 12m (40ft) above what we currently regard as normal 'river-level', which would've been 9m above the sea level that existed at the time.
1,400 bp, i.e. 500AD: ... Volcanoes again erupting in the 'southeast' region of South Australia.
~ 200 bp, i.e. about 1780AD - or - 8-years before the "First Fleet" entered Sydney Harbour: ... In SA, in the North-West Bend region, another Murray flood rose to approximately 14m (47ft) above what we nowadays regard as normal river level.
PRESENT - Without doubt, the "wider" Murray Valley was shaped by a river that was vastly larger than the Murray we know today. - The modern Murray is but a mere "trickle", meandering in the riverbed of a river that, in its heyday, would've challenged the Rhine or the Amazon for supremacy.
BEYOND THE HOLOCENE
eeeeefffff
Lower Miocene ~ 26>20Mbp Mid to Upper Miocene ~ 20>9Mbp Pliocene ~ 9>2Mbp
Pleistocene ~ 2Mbp>35,000bp Holocene (Recent) Era
Introduction:
As mentioned above, elsewhere on planet Earth carbon dating has scientifically identified rocks that are upwards of 4-billion (4,500-million) years old. But in the Murray Valley, aside from a few outcrops of rock in the Tailem Bend to Purnong region, where some granite is about 400-million years old, as in the quarry on the east bank 3-Km above Mannum. - All other visible features in the Murray Valley, in SA were formed within the last 30-million years. So, what you see along the Murray is, geologically, a very embryonic landscape.
Nonetheless, geologists from around the world recognise the cliff-face exposures and landform features of the Murray Valley, as classic examples of the Earth's development during the various epochs that make up the later half of the Cainozoic period. Which, in total, embraces the most recent, 50-odd million years, of this planet's history.
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The lower
Murray Basin MUST have been a coral sea for much of this time.
Mannum, Finniss and Morgan deposits were all consecutively laid
down during this era. - evidence;- huge limestone deposits *.
* Mannum Formation: - Coral engendered
limestone, crowded with the remains of tiny heart-urchins, and
other marine fossils: - It rises near Murray Bridge, attains
maximum prominence in the Mannum area, and tapers-off around the
Marne to Big Bend region. - Though originally whitish, ground
water and soakage have long since stained the strata
yellowish/beige.
Finniss Clay: - The sea retreated: ... Creeks etc.,
washed clay over the top of the Mannum Formation. Depending on
locality, the clay varies from zero to 2m thick. It is very
conspicuous in excavations at the Mannum Pump House.
* Morgan Limestone: - The sea returned, and
coral proliferated: - Types of fossil, and nature of hard/soft
layers, indicate warm shallow seas, and, probably minor,
variations in sea level. - The "minor variations",
however, gradually accumulated to a hundred metres, or more, of
water. - The strata is up-to 91m thick (300ft) in places, and,
like the Mannum Formation, has been stained yellowish by ground
water. It exists as a minor top layer near Mannum, and rises to
towering prominence in numerous river cliffs between Big Bend and
Overland Corner: - You can't 'miss-it' at
Blanchetown,
and the town of Morgan is 'built' on it:.
Example of Morgan Limestone - Waikerie district, (dated about
20Mbp)
(formed from calcareous deposits laid down around 22 to 20
million years ago; - and slowly converted into limestone during
the 5-10 million years that followed. i.e. time to 'create'
the strata about 10 to 22 Mbp)
FOR THE NUMBER CRUNCHERS AMONG YOU:
In terms of human life, if we stick with the example of the
80-year old gentleman referred to at the top of the page. Then it
was all laid-down over a 2-week-period about 4-n-a-bit-months
ago;
Of course, if you're only 40, then it was laid-down over a 1-week
period 2-months ago.
On the other hand, if you're 20, then it all happened in 3 or 4
days about 4-weeks ago;
And! - if your only 10, of course, it all happened in about 48
hours a fortnight ago:
------------------------
Transition from Lower to Middle Miocene ~ ( 20Mbp).
Atop the Morgan Limestone. Lies a thin greenish-grey glauconitic marl, which contains abundant calcareous tubeworm "pipes", and separates the Morgan layer from a more diminutive layer, known as "Pata Limestone"; - Both the marl, and the Pata Limestone, contain a type of plankton which evolved around 20Mbp; - Though often inconspicuous. Both these deposits can be seen, at scattered locations, in the Morgan to Waikerie district. Identifiable by its greenie colour and the presence of tubeworm "pipes".
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Middle Miocene (20 > 15Mbp). - The sea retreated from the Murray Basin and the region reverted to "dry-land"; - As "Dry Land", the region was subjected to the effects of weathering and erosion. Wind and wave action pulverised the shells and coral, producing massive quantities of calcareous sand and grit.
Upper
Miocene (15 > 9Mbp). - Erosion and weathering
of the exposed surfaces continued. While, at the same time, below
the surface, the vast bulk of what had been coral reefs, was
being compacted, consolidated and turned into limestone;
Surface erosion would have obliterated and reshaped many
features. There is no evidence, therefore, of any notable
sedimentation. Until the sea returned about the start of the
Pliocene era ~ (9Mbp)
------------------------
Pliocene (9 > 2Mbp): Bookpurnong
Beds, Loxton Sands, North West Bend Formation, and Parilla Sands.
Were all laid down during this epoch; - The existence of oyster
shells, indicates the sea again inundated the valley from time to
time.**
** Bookpurnong Beds (9 > 7Mbp) - Impervious
clay-like strata of glauconitic shelly marl, typical of river
delta sediment. Most evident in the Loxton district: - A
distinctive 3m (10ft) layer can be seen at a sharp river bend
1.5-Km above Katarapko Creek outlet. - The marl became integrated
and overlain with a 60cm (2ft) layer of pale calcareous sand (now
sandstone). The stratum is rich in preserved shallow water shells
and plankton.
** Loxton Sands (7 > 5Mbp) - Sea retreated, and
a bright yellow micaceous sand and grit mix was blown, and/or
washed, over the Bookpurnong Beds. Very widespread and usually
unconsolidated; - Pockets of oyster shells exist among these
sands, indicating that, at the time, they formed primeval coastal
sand dunes - i.e., estuarine conditions existed here at the time;
- The stratum is 15m (50ft) thick at the Loxton Pump House.
** North West Bend Formation (5 > 2Mbp) -
Consisting of fossil sandstone, worked over sand/limestone mixes,
and thick oyster beds (same shells as above): - It occurs on top
of Loxton Sands or sometimes directly on Morgan Limestone. It
extends from Tailem Bend to Waikerie; - The result of sediment
left behind from another sea incursion, colder and more suited to
oysters than coral. Which formed a North-South estuary,
more-or-less, coinciding with the current course of the Lower
Murray floodplain; At Waikerie the sandstone is quarried for
building stone and is used in many of Adelaide's old city
buildings.
** Parilla Sands (5 > 2Mbp) - Concurrent with
the above and on the North and East shores of the above estuary,
extending all the way down to Keith and back into Victoria.
Massive quantities of fine-to-medium quartz sand was drifting and
'dunning' far and wide across the landscape: - It was also
accumulating along the creeks, rivers and in lakes that existed
at the time; - This sand is now a semi-consolidated, pale, sandy
stratum, which varies greatly in thickness of up to 15m thick
(50ft) in places; - It appears as a dominant feature in many
river cliffs between Loxton and the NSW border, and is especially
prominent in the Chowilla to NSW border region. It can usually be
identified by it's hard, silicified quartz capping. Which formed
later.
------------------------
Pleistocene (2Mbp > 35,000bp): - The
first half-million or-so years of this epoch
were free of further sedimentation and were characterised by widespread 'silicification' and
'ferruginisation' of the exposed land surface. This indicates that there was enough
light-rainfall, or fog/mist or dew, to wet the landscape, but not enough to
result in the sort of 'run-off' that moves sediment. The combination of repeated
and periodic surface wetting, which dissolves any mineral salts present in
the soil; and high temperature induced rapid re-evaporation, results a thin
concrete-like 'skin' forming on unconsolidated soil surfaces. - Dare
I say, conditions not dissimilar to those we nowadays see in the Simpson Desert,
where the same process continues to create what we have come to know as "claypans".
Eons later, whenever these very-thin but concrete-hard, 'once-upon-a -time' surface stratum are
re-expose. They are, very obviously, very
much better at resisting erosion than are any 'un-concreted' materials that
surround them. - The erosion resistant "skin" that was formed on the land surface
at the onset of the Pleistocene has since been named the
Karoonda Surface and, although it is discontinuous, it is very
widespread and
it conveniently marks the time break
between the older Pliocene, and the younger Pleistocene epochs. The
silicified quartz capping, mentioned above, on the Parilla Sands,
was formed at this time (2 > 1.5Mbp)..
Karoonda Surface clearly exposed
in Headings Cliff - Murtho district:
(As far as our 80 year old friend is concerned, the Karoonda
Surface was formed 12 days ago)
------------------------
The Big Picture, however, shows how the Pleistocene epoch shortly became characterised by global tectonic movement and a series of northern hemisphere ice ages, which caused worldwide sea-level changes as well as major climate/weather changes. All of which combined to shape our planet's geography to that which we see around us today; - All modern plants and animals evolved during the Pleistocene epoch, which, at times, saw sea levels along the SA coast rise as much as 7m, and drop as much as 90m, from the levels we are familiar with today. - Local evidence is: -
1 ... Mt. Lofty Ranges progressively rose, while the Gulf St Vincent
section dropped; - Boreholes indicate that a, once level Miocene
stratum ~ (20Mbp), is now fractured and vertically out-of-line by
up to 300m (1,000ft).
2 ... Fresh water draining into the Murray Basin was temporarily 'dammed' by the high ground associated with the "Marmon Jabuk Range" &
"Padthaway Ridge", in the Bow Hill to Purnong area: - The water
so 'dammed' backed-up to around the Waikerie/Berri latitudes, creating the very
substantial freshwater lake, named "Lake Bungunnia", that endured until around 500,000 years ago,
when the overflow eventually eroded a path through the high ground and cut a
path to the sea.
3 ... Stranded beach dunes in South-East SA, indicate that the
sea has recently stood at least 3m (10ft), and possibly 7m (25ft)
above the present level.
4 ... A local ice age caused glaciers to form in the
Victorian/NSW highlands and Tasmania. The ice did not directly
affect the Murray Basin, but related changes to climate, sea
level, and run-off water, certainly did.
5 ... South of Kangaroo Island, the drowned riverbed of the
Murray can be traced to a depth of 90m (300ft) below present-day sea level. - Backstairs Passage and Gulf St Vincent would've been
dry land at that time. There is undersea evidence that a river flowed out of the
"dry" St Vincent Gulf and
joined the Murray in the vicinity of "Backstairs Passage". Where these
rivers flowed off the Continental Shelf, into the "Pleistocene Ocean",
they have eroded a series of three massive canyons, each one of which could
comfortably accommodate "The Grand Canyon", USA., for each one is
twice as deep as the "Grand Canyon"..
6 ... During this era the sand/dust debris of exposed coral and seashell,
pulverised from wave action, was periodically blown far and wide over inland
S.A.'; covering the
entire breadth of the State, the Nullarbor and into western
Victoria; - Subsequent wet/dry weathering of this calcium-rich
sandy-surface has created a widespread patchwork of wind-resistant surface "calcrete"... "Calcrete" produced in
this manner is roughly similar to old style lime-mortar.
------------------------
At about 0.5 million years into the Pleistocene (~ 1.5M-bp). Torrential downpours began falling on what had previously been an arid/semi-arid, wind-shaped landscape. The resulting floodwaters triggered a significant increase in water-borne sediment 'flooding' into the greater Murray Basin. -- Some of the resulting, more notable, formations that ought to be recognisable, IF they are present in your location, are: -
Blanchetown Clay; - Around 1.5M > 1Mbp: - Floodwater carried large quantities of high-silicate-content sediment
(which when immersed in water becomes hydrated and forms clay) into the greater Murray Basin. - In the SA/NSW border regions
there is a 15m (50ft) layer of this clay lying directly atop the Karoonda Surface and it exists, to a greater or lesser degrees,
throughout the rest of the Murray Basin and into Victoria. - Characteristically the strata appears as "pale beigeish" at
the top, grading to "Uluru-red" at the bottom; - At the same time as this clay was being deposited in the greater
Murray Basins, so-called "Ape-men" inhabited the planet.
...
Bungunnia Limestone; - Around 1.2 > 0.7Mbp: - Northern ice masses increased drawing water away from the
world's oceans. Sea levels dropped along the SA coastline and wave action, again, pulverised the exposed coral and seashell beds, as it
had done in the Miocene epoch, and the prevailing winds blew the debris far and wide over SA. Within the Murray Basin, much of it
ended-up in Lake Bungunnia, and its associated creeks and swamps, where it was converted it into a coarse freshwater-limestone.
This chalk-like stratum varies greatly in thickness & location, and is usually atop, or interleaved with, Blanchetown Clay; - It
extends from as far south as Murray Bridge all the way up to Renmark - i.e., an area that corresponds roughly, in size and shape, to
the prehistoric Lake Bungunnia: - Which means the 'Old' lake, although wholly located north of Murray Bridge, was likely about twice,
or perhaps three times, the size of the modern-day "Lake Alexandrina".
Rippon Calcrete; - Concurrent with the above. The dry land surfaces now rich in calcium, from the same coral/seashell
debris cited above, was subjected to repeated wet/dry weathering and the resulting calcrete surfaces, so formed, was very widespread; -
So! By 100,000bp, a wind resistant calcrete surface, whitish and rather lumpy in appearance, covered the entire southern half of South
Australia. - It has been named the Rippon Surface.
------------------------
On top of the above, marginal and fragmentary layers continue to form: - Forces of wind, rain, drought and flood, constantly move these deposits, mixing and/or redepositing old material on top of new. Churning the whole into a topographical hotchpotch that can change in consistency from year to year. In this synopsis we will simply call the hotchpotch Conglomerate, for it's rarely more than a metre or so thick, and often only centimetres. Which, in addition to the above, is also subject to the aberrations of native flora and fauna, as well as horticulture and agriculture activities.
------------------------
- G L O S S A R Y -
calcareous - coated
with, containing, or like calcium carbonate, chalky; - coral,
shell and bone all qualify..
calcrete - the crust developed on calcareous deposits by
the chemical interaction with rain-water and atmospheric gasses;
notably carbon dioxide..
clay - earthy material, essentially hydrated silicates of
igneous rock debris. Turns plastic when wet, and is deposited in
river beds, lake beds and flood plains..
consolidated - to make solid, to bring together compactly,
as in one mass..
delta - a nearly flat plain where a river divergs and
fans-out before entering the sea..
dune - hill or ridge formed by wind-blown sand, usually in
deserts or by lakes and oceans..
dunning - the forming of dunes..
estuarine - a landscape containing an estuary or
estuaries..
estuary - the part of a river where it's current meets
with and is effected by the ocean's tides; - an arm, or inlet of
the sea..
eon - n. aeon. - a very long period of time, with no
distinct beginning or end..
epoch - a period of time; distinguishable by a particular
series of events..
ferruginisation - when water containing dissolved iron
salts, repeatedly evaporates from a surface, the iron residue builds-up in a
'concreting' effect that makes the surface progressively harder..
glauconitic - a greenish micaceous mineral; essentially a
hydrous silicate of potassium, aluminium and iron, occurring in
clays, sandstones etc..
gypsum - a chalk-like mineral; essentially a hydrous
sulphate of calcium (CaSO42H2O)..
hydrated - when a material's molecules have chemically
combine with water..
hydro - used to describe material that has been hydrated..
igneous - pertaining to intense heat or fire; as in rock
formed by volcanic action on the earth's surface, or within it's
crust..
limestone - a rock consisting wholly or chiefly of calcium
carbonate, consolidated by its chemical reaction with impure,
mainly acidic water, as in acid-rain..
loam - a loose soil composed of clay and sand and
containing organic material..
micaceous - consisting of hydrous disilicates, with
elements such as potassium, iron, magnesium and lithium, which
crumbles readily into tough grain like bits..
marl - a soil deposit consisting of clay and calcareous
mix..
plankton - small animal/plant organisms that drift in
water, primarily near the surface..
quartz - one of the most common minerals, silicon dioxide;
occurs as small crystals right up to massive rocks, occurs in a
myriad variety of colours, lustres, etc..
sand - more or less the fine debris of rock, as in small
loose grains often of quartz..
sandstone - the rock formed by the consolidation of sand,
the grains being held together by a cement of silica, iron and
other mineral salts; Type of salts, and number of wet/dry cycles,
determines the colour and hardness of the deposit..
silicification - when water, containing dissolved silica
salts, is repeatedly evaporated from a surface, the silica
residue builds up in a 'concreting' effect and the layer becomes progressively harder. It
may, in time, be turned into quartz..
soil - that portion in which plants grow, a mixture of
organic and inorganic material, containing living organisms..
stratum - pl. strata. - layer of material, in this case
formed naturally; often one of a number of parallel layers,
placed one upon another..
tectonic - refers to conditions and forces within the
earth which cause movement of the Earth's crust; such as
earthquakes, folds, faults and the like..
topography - the analysis of the surface details of a
location or district..
unconsolidated - opposite to consolidated; i.e. loose
grains that separate readily in wind or rain..
©
Peter J REILLY 1997 ... E-mail Inquiries
GeologyRmv.doc /980215...