Murray Valley Geology.
© 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, therefore, prefer to use 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; -Pre-school; -School age; -Teenager; -Juvenile; -Adolescent; -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'; he 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 in exactly which second they occur. - Nonetheless, 'everyone' knows there IS a chronological progression to it all; and that elderly occurs a looonng time after adolescence; in most cases, we hope!
-- As for dear old 'Mother Earth'? It seems that she is currently approaching the end of her 'Parenthood era' and, in the next few hundred thousand years or-so, 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 the present . . .
Man's evolution!!: - Ape-man evolved into Homo Erectus at around 500,000bp, then into Homo Sapient at around 200,000bp. - Graves at Roonka and Big Bend, on the River Murray in SA, indicate that Aboriginal communities have existed in that region since at-least 35,000bp:- See -'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('dunning' = formation of sand dunes) and a corresponding increase in vegetation.
29,000 bp: ... Dry & windy: ... Decline in vegetation and dunning has re-commenced, the result of persistent winds.
28,000 bp: ... Very dry: ... Extensive dunning indicates very strong, windy conditions. Another ice age is in progress in the northern hemisphere.
24,000 bp: ... Sea levels have dropped, likely the result of the ice build-up in the northern hemisphere: ... Strong, windy conditions produce wild seas, which pound the lately exposed coral and shell it into calcium-rich sand. 'Sandstorms' of the calcium-rich 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 the northern hemisphere ice mass. Sea levels locally go as low as 90m (300ft) below 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: ... A minor ice age occurs about now, but locally seas are rising.
9,000 bp: ... Warmer and more humid. - Seas level 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 the southeast region of SA and large amounts of volcanic-ash are deposited on the land.
3,000 bp: ... Sea level are now at 3-7m (10-23ft) higher than at present and remains high for a thousand or more years:- A fjord of the Southern Ocean now filled the Murray Valley as far upstream as what we nowadays call the 'Waikerie' district. The 'Coorong', 'Lake Alexandrina' and much of the countryside south of the 'Padthaway Ridge' is inundated by sea water. ~ ~ The 'Murray Valley', between the Padthaway Ridge & Waikerie, would've 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. ~ ~ Also at this time, in what we nowadays know as the North-West Bend region of The Murray, a freshwater flood rose to 12m (40ft) above what we currently regard as normal 'river-level', which would've been some 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, a 'Murray River' 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, could've challenged The Mississippi, The Yangtze or The Mekong for supremacy.
BEYOND THE HOLOCENE
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Lower Miocene ~ 26>20Mbp Mid to Upper Miocene ~ 20>9Mbp Pliocene ~ 9>2Mbp
Pleistocene ~ 2Mbp>35,000bp Holocene (Recent) Era
Introduction:
As previously mentioned, elsewhere on planet Earth, carbon dating has scientifically identified rocks that are upwards of 4.5-billion (4,500-million) years old. In the Murray Valley, however, 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 in the Murray Valley today is, geologically speaking, a very embryonic landscape.
Nevertheless, 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 which make up the later half of the Cainozoic period; i.e. the most recent, 50-odd million years, of our planet's history.
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; the evidence lies in the region's copious limestone deposits
.** Mannum Formation: - Coral, stranded by retreating sea-levels, was 'weathered' into limestone. It is crowded with the remains of tiny heart-urchins and other marine fossils. - It rises near Murray Bridge, attains maximum prominence in the Mannum region and tapers-off around the Marne/Big Bend section. - Though originally whitish, ground water and soakage have long since stained the strata a yellow/beige colour.
Finniss Clay: - Rivers and creeks washed clay over the top of the stranded coral beds (nowadays know as Mannum Limestone). Depending on the location, the clay varies from zero to 2m thick. It is particularly evident in the excavation work 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, it too has been stained yellowish by ground water. It exists as a minor top layer near Mannum and rises to towering prominence in numerous riverside cliffs between Big Bend and Overland Corner. - You simply can't miss it at Blanchetown, where 90-odd metre high cliff-faces exposures of it demand your attention; further upstream the town of Morgan is built on top of it. - Between Murbko and Cadell, atop of the Morgan Limestone, but still part of it, there is a somewhat unique layer identified as the Cadell Formation, in which numerous specimens of fossilized cowry shells, dated at around 20Mbp, have been found. -
Example of Morgan Limestone - Waikerie district, (dated about 20Mbp)
(formed from calcareous deposits consecutively laid down over period of between
26 to 22 million years ago and slowly converted into limestone over another two-to-three million years; or-so!)
FOR THE NUMBER CRUNCHERS AMONG YOU:
In terms of human life, if we stick to the example of the 80-year old
gentleman referred to at the top of the page. Then it was all laid-down and consolidated, over a 2-week-period,
about 4-n-a-bit-months ago.
Of course, if you happen to be only 40, then it was laid-down and consolidated, over a 1-week period, 2-months ago.
On the other hand, if you're 20, then it was laid-down and consolidated, over a 3 to 4 day period, about 4-weeks ago.
And! - if you happen to be just 10 years-old, it was laid-down and consolidated, in a little under 2 days, a fortnight ago.
Transition from Lower to Middle Miocene ~ ( 20Mbp).
Atop the Morgan Limestone lies a thin, greenish-grey glauconitic marl, which contains many calcareous tubeworm 'pipes' and separates the Morgan Limestone from a more diminutive layer known as 'Pata Limestone'. - Both the 'marl' and the Pata Limestone contain a type of plankton that evolved around 20Mbp. - Both of these deposits are non-continuous, in that they do not cover the entire landscape, but they can be spotted at numerous locations within the Morgan to Waikerie section and can be identified by its greenish-grey colour and the presence of the 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. Along the coastline, wind and wave action pulverised the exposed shells and coral, producing massive quantities of calcareous (calcium-rich) 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.
There can be no doubt that erosion would have obliterated and/or reshaped much of the surface topography at this time. There is
no evidence, therefore, of any notable, further sedimentation until the sea again returned at about the start of the Pliocene ~ (9Mbp).
** Bookpurnong Beds (9 > 7Mbp). - An impervious clay-like stratum, consisting of glauconitic shelly marl, typical of river delta sediment and a clear indication of a Pliocene coastline. - It is most evident in the Loxton district, where a distinctive 3m (10ft) layer can be seen at a sharp river-bend 1.5Km (1.0ml) upstream of Katarapko Creek outlet. - The marl has become 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/grit mix was blown and/or washed over the Bookpurnong Beds. It is very widespread, is only partially consolidated and varies greatly in thickness. There is a very obvious 15m (50ft) layer adjacent to the Loxton Pump House - At the time, the landscape would've resembled a Simpson Desert type landscape of Loxton Sand extending all the way down to the Pliocene ocean, with the sea intruding, estuarine fashion, between the sand ridges, as evident from the pockets of oyster shell that can be found among, what would at the time have been, primeval coastal sand dunes.
** 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 Waikerie down to Tailem Bend. - The result of sediment left behind after the most recent sea incursion, which was colder and more suited to oysters than coral, and which formed a North-South estuary, more-or-less, coinciding with the current shape of the Lower Murray floodplain. - At Waikerie the sandstone is quarried as 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, in rivers and in the 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 (50ft) thick 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 its hard, silicified quartz capping, which was 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 any sort of 'run-off' of the
type that can moves sediment. The combination of repeated and periodic surface wetting, which dissolves any mineral salts present in
the soil, followed by high-temperature induced rapid re-evaporation, results in 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 know as 'claypans' .
Eons later, whenever these very-thin but concrete-hard, ancient surfaces 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).
However, The Big Picture
, shows how the Pleistocene 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 nowadays recognise. - All modern plants and animals evolved
during the Pleistocene, which, at times, saw sea levels along the SA coastline rise as much as 7m, and drop as much as 90m,
from the level we are familiar with today.
Local evidence of this tectonic upheaval includes: -
Click to see how, during this time, the Australian continent evolved from the Sahul Continent.
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).
At around 0.5 million years into the Pleistocene (~ 1.5M-bp). Torrential downpours began falling on what had, until now, been a semi-arid, wind-shaped landscape. The resulting floodwaters triggered a significant increase in the water-borne sediment 'flooding' into the Murray/Darling Basin. -- Some of the more notable formations so-formed ought to be recognisable, IF they are present in your location: -
Blanchetown Clay: - Around 1.5M > 1Mbp: - Floodwater carried huge 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 now 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 stratum appears as 'pale beige' colour 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-man' inhabited the planet.
Contentious issues! The origin of Blanchetown Clay & the riddle of the Rainbow Cliffs:
Bungunnia Limestone: - Around 1.2 > 0.7Mbp - Northern ice masses once again increased draining water away from the world's oceans. Sea levels dropped and wave action pounded the exposed coral and shell beds. Prevailing winds then blew the coral/shell debris far and wide over S.A.. ~ ~ In the Murray Basin much of it ended-up in Lake Bungunnia and in the many creeks and swamps that existed at the time, where it was converted into a coarse freshwater-limestone. This chalk-like stratum varies greatly in thickness and location, it usually lies on top of or is 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' indicating how the ancient lake, although wholly located some ways north of 'Murray Bridge', covered an area equivalent to twice the size of Gulf St. Vincent.
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. ... 'Calcrete'produced in this manner commonly resembles great 'dollops' of old style lime-mortar.
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On top of the above marginal and fragmentary layers continue to form. Forces of wind, rain, drought and flood can repeatedly move these deposits about, mixing and/or redepositing old material on top of new, churning the whole into a hotchpotch that can change from year to year. In this synopsis we will simply call the hotchpotch 'Conglomerate' for it is rarely more than a meter or so thick, oftentimes only centimeters and which, in addition to the above, is also subject to the aberrations of native flora and fauna, as well as agriculture and horticulture activities.
- G L O S S A R Y -
calcarous ... 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, 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 beside 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 ... 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 ... hydrous disilicates of 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 ... 'silicon dioxide' a very common mineral. Occurs as small crystals right up to massive rocks, in various 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/or
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/980215...