'The rocks' Mannum quarries
COWIRRA - SOUTH AUSTRALIA
MAY 28th 2021 | UPDATED OCT 5th 2021
The Mannum granite forms a section of the Padthaway A-type granite suite, the granite intruded the Adelaide fold belt with the Black Hill gabbroic complex just after the Delamerian Orogeny came to an end during the Ordovician. The Mannum Granite is a part of a extensive post-tectonic A-type granite and lava also associated with mafic rocks in Victoria. The Reedy Creek Pluton has been age dated to 482 million years old.
During the Tertiary Period (Middle Oligocene 33 to 27 to Miocene 23 to 5 to million years ago) the sea progressed inland over the fluvial and lacustrine environments surrounding the south eastern coasts of South Australia, the seas inflow developed the Murravian Gulf. Towards Mannum, the sea was relatively shallow with intertidal zones at depths estimated from 10 to 50 feet using Econoide Fossils.
The sea regressed in the Middle Oligocene, this left behind lakes and other small bodies of water until around 15-10 million years ago. Lots of salt has been left behind in the Mallee region in Vicoria and in South Australia, this creates its own issues with farming and irrigation of those areas of the Murray Basin currently. Between the Lachlan Fold Belt to the east and the Adelaide Fold Belt to the west is the Murray Basin where the current Murray River cuts through.
The Mannum Granite is a small isolated outcrop unit, it tracks eastward through the Murray Basin and is mostly hidden under younger sediments, the granite intudes the Reedy Creek Pluton. A-type granites are more isotopically mantle-like then the syn-tectonic granites in the surrounding areas of South Australia.
Intrusion of the Mannum Granite occurred in two phases, an earlier more fine grained magma injection with very shallow inturions (mariolitic cavities) that occurs at the quarry in angular blocks, and the more noticeable and main type of granite at the quarry which is more course-grained. Both phases of the granite are chemically identical, enclaves of fine grained phase 1 granite formed from magma mingling when both were partially or completely liquid, inclusion in the form of xenoliths are visible where the physical shape of the enclaves are visible with deformed outlines.
The medium to coarse-grained A-type granite intruded after the Delamerian Orogeny mountain building event came to an end placing it as post-orogenic (tectonic) granite, it was intruded during the Ordovician (481± 9 Million years). Modelled temperature-viscosity relationship data focusing on enclaves/Xenoliths show that for the granite to behave in a plastic manner the Mannum granite as magma would have been at a temperature in excess of 930 °C.
Some parts of the Mannum Granite are vicious and solidified slowly which formed larger grains, the non-viscous magma visible in some parts of the Mannum Granite solidified fast and formed small grains. Other minerals that make up the biotite rich Mannum Granite are pink orthoclase feldspar, grey quartz, and hornblende.
An unconformity lies above the granite where wave weathering from the tertiary sea that moved in during the Miocene eroded the granite below. The intertidal area weathered back and smoothed out the Mannum Granite leaving a dome-like shape visible when looking from a distance at the top of the quarries high wall and at some of the granite in the local area.
(Right/Bottom Image: Fine grain Phase 1 granite surrounded by Phase 2 granite)
Diobase (Dolerite) Dikes
The mafic dolerite dikes are igneous in origen and are compositionally equivalent to grabbo and to basalt but the rock type does have a different texture. Dike and Sill intrusions are shallow and the composition is basaltic. Diabase/Dolerite is comprised of plagioclase (Labradorite) feldspar and of pyroxene (Augite). To form dolerite the intrusion has to cool slowly, the dike is smooth, fine-gained and crystalline in texture and was intruded after the Mannum Granite was completely solidified.
The Mannum Formation is a fine-grained orange/brown and sandy yellow limestone with sandstone, calcrete, and minor carbonaceous clay and silt located within the Murray Basin. It was deposited from the Miocene (Middle Oligocene 33 to 27 - 23 to 5 million years ago), the limestone would have been deposited in a shallow intertidal zone when the below granite would have been weathering out.
This would have taken several hundred thousand years for the sediment to accumulate which explains the unconformity as it slowly got covered in newer sediment as the possible island comprised of Mannum Granite vanished with the movement of the sea that pushed far inland. Groundwater has coloured the once white limestone to yellow and newer calcrete is forming on top of the unconformity in the above regolith.
Within the Mannum Granite, there are visible xenoliths that form when magma is pushed upwards into the surrounding country rock, that county rock breaks off and then drifts from the ceiling into the rising magma chamber. A line of xenoliths are visible in the high wall, this would have occurred because of a current of magma moving through the main magma body when it was once liquid, it then solidified in place and was exposed during mining efforts in the early to mid1920s. Xenoliths range in size from 5 to 50cm in diameter.
Mafic xenoliths also could have formed from the draw-up of a lower layer of layered magma in a magma chamber further down in the crust, or by a dike as it intersected the magma body. Both mafic and felsic igneous rocks didn't freely mix because of viscosity differences and by other physical differences between the magma, fluid was constricted to laminar flow only which added to the physical differences.
After a period of time within the magma chamber, the xenoliths start to melt and assimilate, they eventually become a part of the magma itself. Xenoliths that are grey/pink and far less noticeable have been trapped in the Mannum Granite long before the intrusion solidified, they have blurred edges and are almost assimilated. The surrounding rock (country rock), was most likely schist and has been partially assimilated as the layering and foliation would have been destroyed by the heat of the nearby magma body.
Other more assimilated xenoliths show interesting patterning called Rapakivi, this forms when K-spar is coarsely unmixed perthite, rims of plagioclase form around them. Quartz cristalization occurs later, they also contain biotite and Fe-rich hornblende, occasional fluorite forms as well. Rapakivi is predicted to have formed by extended fractional crystallization of parent mafic rocks.
(Middle Image: Rapakivi, largest is less then one cm in size)
The lightly coloured smokey quartz visible in the quarry is most likey vein quartz and was possibly injected after the magma body cooled down and cracked, the colour origintes from natural radiation emitted from the surrounding rock formation. The radiation activates colour centers around aluminium impurities within the quartz which leads to the dark colourization.
The Mannum Quarries began operation in 1913 In the town of Mannum, that same granite was used as building materials. By the end of operations over half a million tonnes of Mannum Granite had been carted from the quarry and crushed then sent upstream to build locks along the River Murray.
From Newspaper Articles:
"On December 6, the paddlewheel steamer Murray left Milang, towing two barges recently built there to the granite quarries a couple of miles above Mannum, there they will be used for conveying the crushed rock to the different locks now being constructed. These barges (Fairy and Active), are the last of a fleet of 12 built for the government." - The Observer Adelaide, 23rd of December 1922.
"The S.S. Mannum (Captain Good) with barge Cowirra in tow arrived on Tuesday morning laden with 770 tons of granite from Mannum quarries." - Murray Pioneer and Australian River Record, 5th of October 1926.
"The S.S. Murrabit (Capt. N. Prom-nitz) put into Renmark on her way down stream on Tuesday. She has delivered in all about 1,800 tons of granite from the Mannum quarry to the No. 6 lock. Her heaviest load was over 500 tons. When loaded the vessel draws seven feet of water. On her last trip up stream she never touched the bottom." - Murray Pioneer and Australian River Record, 18th of November 1927.
"The steamer Captain Sturt passed through Renmark yesterday, with three barges of crushed granite from the Mannum quarries, weighing 800 tons, for No. 6 Lock. Captain Harry Payne was in charge." - Chronicle Adelaide, 4th of August 1928.
"From the time of her commissioning in 1917 until 1935, the Captain Sturt delivered loads of granite, from a quarry near Mannum, the home of the PS Marion, for the construction of the locks and weirs from Blanchetown to lock 9 at Kulnine." - Times Victor Harbor, 22nd of December 1995.
The last documented year the quarry was active was in1928, after that, no more catalogued daily newspapers mentioned the quarry again. One record of Captain Sturt still delivering granite from Mannum remains, placing its possible closure in1935 but after that one reference in 1995, the quarry isn't mentioned again. This doesn't mean the quarry fell into abandonment but it does mean pinpointing the exact date and year of its closure very difficult as the records may be lost entirely.
Today the quarry is a stop on the Murry River Scenic Drive, in low water the jetty is still visible from its time operating. Further past the high wall is a broadcast site that services digital TV channels in the current Mannum region. East Front Road (Access to the Quarry on the Right) has been closed since 2010 and still remains blocked off due to riverbank slumping, walking to the quarry is still possible.
Fossils found at 'The Rocks' Mannum Quarries:
Scallops (Semipallium foulcheri)
Abundant fossils exist within the Mannum Formations limestone not just in the quarry but also in road cuts visible all around the town and river cliffs. The most commonly visible fossil at the quarry are Scallops (Semipallim Foulcheri) which were suspension feeders, the last recorded appearance of this fossil was in the Miocene when the sea regressed. Sea Urchins (Lovenia Forbesi) are also in the Mannum Formation at the quarry, and are very abundant aroud the town of Mannum.
Mannum Digital TV Channels — Broadcast Site Quarry Reserve adj to East Front Rd Mannum transmitter site. (n.d.). Mannum Digital TV Broadcast Site.
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Mindat.org. (n.d.). Semipallium foulcheri.
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Observer Adelaide, SA: 1905 - 1931. (1922, December 23). From Milang to Mannum. Towing two lock barges. Humour and scenes on the way. Trove.
Chronicle Adelaide, SA: 1895 - 1954. (1928, August 4). Locking the Murray, Granite from Mannum. Trove.
Murray Pioneer and Australian River Record, Renmark, SA: 1913 - 1942. (1926, October 5). Shipping at Lock 4. Trove.
State Library of South Australia. (Degital 2001, Original 1920). Mannum quarries (1920) [Photograph]. State Library of South Australia.
Times Victor Harbor, SA: 1987 - 1999. (1995, December). Captain Sturt helpers needed. Trove.
Murray Pioneer and Australian River Record, Renmark, SA: 1913 - 1942. (1927, November 18). Murrabit delivering granite to lock six. Trove.
MG. E. Wilford, & P. J. Brown. (2017). Maps of late Mesozoic-Cenozoic Gondwana break-up: some palaeogeographical implications, History of the Australian Vegetation: Cretaceous to Recent. Adelaide University Press.
Australian Government, Geoscience Australia. (n.d.). Mannum Granite | Australian Stratigraphic Units Database, Geoscience Australia. Updated September 3, 2019.
John Foden. (2021, 18 September). Stop 05: Quarry at Mannum on the eastern shore of the Murray River. [Field Guide handed out on the excursion day]. Geological Society of Australia: Saturday 18th September 2021.
(1996). Geology: Early Ordovician post-tectonic A-type, monzogranite with mafic enclaves. Minerology and Petrology 56: 147-169.
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