DEAD MAN'S PASS

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GEOLOGY

SOUTH AUSTRALIA

 

Younger Tertiary rocks are what make up the majority of the sedimentary deposits in Dead Man's Pass, this includes the Rowland Flat Sands and Quaternary Alluvial/Fluvial Sediments. Under the Basin sediment is the basement rocks which include the Woolshed Flat Shale and the unofficial 'Gawler Bluestone' which are the highlights of the park.

 

Gawler is located in the St. Vincent Basin which is mostly comprised of alternating marine and non-marine sediments, the basin itself is less than 3,000 feet deep. Basin filling has been a subject of debate as some ideas have pointed out fault-related movement, erosion, and past drainage systems.

 

During the Tertiary period, the area was mostly flat with low lying hills to the east, west, and south. Rivers deposited sands locally, coarse gravel was taken down from the hills which built deltas, those deltas covered coal swamps which were in some places formed in depressions at the base of fault-line scraps. The ocean entered from the East, this covered the old eroded rocks in the Yorke Peninsula and Kangaroo Island, the ocean continued to expand inland until the Adelaide Plains were covered by it.

WARRINA SUPERGROUP

The Warrina Supergroup is comprised of minor volcanics, fluvial and marine sediments and evaporitic deposits. Breccia formed from diapir intrusions of plasticity and low-density minerals like salt, gypsum, and magma which are also visible in the group. 

WOOLSHED FLAT SHALE

The Woolshed Flat Shale was deposited in the Neoproterozoic (1,000 to 541 million years old) and is comprised of black Shale, dark grey to black dolomitic siltstone, dolomite, and grey laminated siltstone.  

Layers of yellow shale are visible at the entrance to Dead Man's Pass from Gawler, the colour comes from limonite which is essentially just weathered particles of hematite, magnetite, pyrite and other minerals containing iron.

 

The yellow shale changes to a darker dolomitic siltstone and further in the park it changes to slate (Gawler Bluestone). 

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'GAWLER BLUESTONE'

The formation in Dead Man's Pass is mostly slate, although it still resides under the name "Woolshed Flat Shale", the bluestone is tilted at an angle upwards and the ridgeline is verging to the north-south. The dark grey colour of the Slate is visible from 1-2% of organic materials in an oxygen-deficient environment before metamorphism. 

 

The quarries listed below are not in Dead Man's Pass:

Bluestone was used as a building material in the historic town centre of Gawler. One of the quarries in town on lot 319 switched owers twice once owned by James Pile from 1864 to 1885 and local contractors Edward and Samuel Hanna between 1886 and 1925. Another quarry on lots 348 and 349 was actually kept in the Pile Family until 1907, Lot 351 was once owned by Edward Clement who donated bluestone for the building institute in 1870. 

All of the historic quarries are now on private property and are completely inaccessible to anyone but are still listed on the Gawler Heritage Survey.   

ROWLAND FLAT SANDS

The Rowland Flat Sands is a large unit located in the Barossa Basin, it is comprised of Fluviolacustrine sand, and silt in the Dead Man's Pass area. The unit was deposited in the Late Oligocene (33 to 23 million years ago) to the very beginning of the Miocene which began at 23 million years ago and ended 5 million years ago. 

 

At its highest, the Rowlandflat Sands is 110 meters (360 feet) but is not that thick in Dead Man's Pass. The 'formation' was possibly deposited in a river and flood plain environment as other locations have thick conglomerate visible along with siltstone and sand. 

 

Some locations have recorded lignite which is formed from compressed peat and is classed as low-quality coal and also suggests a swamp/flood plain environment just not in Dead Man's Pass. 

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Quaternary alluvial/fluvial sediments

The Quaternary Alluvial/Fluvial sediments were deposited during the Pleistocene and Holocene so they are very young compared to the other visible formations in Dead Man's Pass.

The sediments were placed by river flows that once existed in that location, small slate pebbles and larger chunks of quartz have been trapped in the sand and silt forming conglomerate, only to be cut into by newer seasonal flows which repeat the process again.

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HISTORIC SAND QUARRIES

The quarry scars are visible along the edge of the gully, these weren't large operations as only a few quarries were dug into the hillside and none of them were that organized. 

 

Sand mining began in the early 20th century, mostly in the 1920s where around 40 people worked at loading sand into horse carts by hand. Once the sand was loaded it could be taken into Adelaide and elsewhere by train and used for construction materials. 

ARROYO/DRY WASH

Some of the gullies leading into the low flood plain are classed as an arroyo or dry wash, this is where water only flows seasonally or after thunderstorms/rain pass through the region or upstream. Flash floods cut through at high speeds while taking clastic sediment out with it and converging with the South Para River lower in the valley. 

 

The alluvial/fluvial sediment lining the cliffs along its edge has been cut by water and has made it relatively steep, deeper in the wash more bluestone is visible as the Woolshed Flat Formation re-emerges. The slate visible at the back of the wash has been eroded by water which has softened the edges of the rock and along parallel planes where fractures are most common.

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REFEreNCES

South Australian Government. (n.d.). SARIG geological map. SARIG. 

Australian Government, Geoscience Australia. (n.d.). Rowland Flat Sands | Australian Stratigraphic Units Database, Geoscience Australia. Updated August 20, 2018.

King, H. M. (n.d.). Shale: Sedimentary Rock - Pictures, Definition & More. Geology.Com.

Glaessner, M. F., & Wade, M. (1958). IX. The St. Vincent Basin. Journal of the Geological Society of Australia, 5(2), 115–126. 

Danvers Architects, Peter Jensen Planning & Urban Design, & Anna Pope. (1998). Gawler Heritage Survey. (P. 73) The Corporation of the Town of Gawler.

Australian Government, Geoscience Australia. (n.d.). Woolshed Flat Shale | Australian Stratigraphic Units Database, Geoscience Australia. Updated September 3, 2019.

Town of Gawler & Australian Government. (n.d.). 06 | Farming and Mining from the river [Photograph, and information]. Dead Man’s Pass Walking Trail.

Town of Gawler & Australian Government. (n.d.). | Blue Stone and Native Grassland [Photograph, and information]. Dead Man’s Pass Walking Trail.

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