NORTH PARA RIVER TANUNDA
The Barossa Basin slopes to the south and to the west away from the Barossa Range, the North and South Para rivers have cut deep gorges into the surrounding valley and take part in the erosion of sediment.
The rather close north-south trending Stockwell Fault activated during the Early Cainozoic (65 Million years ago), uplift in the eastern side of the fault also created subsidence on the western edge. The fault movement formed a north-south trending trough, this created a fault-related depression which has been filled in by younger sediment.
The lack of marine sediment in the basin suggests the area was isolated from the ocean for the majority of the Cainozoic, this is also because of the deposition of only terrestrial sediments. The area around Tanunda is mostly comprised of Quaternary sediments but smaller amounts of Tertiary rocks have also been recorded in the area.
To the east of the North Para River non-carbonaceous sands and red-brown clays have been deposited and are all considered Quaternary outwash sediments most of which were deposited during the Holocene (10,000 years ago to present).
The Bungarider Subgroup is comprised of siltstone deep-water dolomite, and sandstone and is Neoproterozoic (1,000 to 545 million years old) in age. Several metamorphosed rock formations like the Saddleworth Formation are a part of this widespread subgroup.
The Saddleworth Formation visible in Tanunda is not slate like the nearby Chateau Yaldara & 1847 winery but is phyllite which is just another more metamorphosed stage of slate and was formed from a slightly different host rock, on some occasions (shale with added siltstone). Both slate and phyllite are classed as low-grade foliated metamorphic rocks.
The phyllite has rust like colouring formed from iron-bearing (ferruginous) clays, these are sometimes patchy in colourization, nearby schist outcrops also follow this pattern of iron-bearing colourization.
The Saddleworth Formation visible in the Barossa Valley was deposited in the Neoproterozoic (Adelaidean) in a shallow to deep water setting in an ocean region moving away from a tectonic margin. The formation is only visible on maps as a small long and very thin outcrop following the North Para River for a short period of time and is not connected with other sections of the Saddleworth Formation.
Undifferentiated Quaternary rocks
Surrounding the cliff-forming outcrop by the river is a mix of undifferentiated quaternary rocks which are very young only being placed in the Pleistocene and Holocene 2.6 million years ago to current.
Very thin layers of calcrete less then a centimetre in thickness are visible in the exposed roadcut, it forms from calcite that is dissolved in groundwater when the environment dries out and is precipitated as water evaporates from the surface. This occurs in semiarid and arid regions due to climatic changes over time. Unconsolidated sand and silt are also present in that location and make up most of the exposure.
Australian Government, Geoscience Australia. (n.d.). Saddleworth Formation | Australian Stratigraphic Units Database, Geoscience Australia. Last update: September 3, 2019.
Preiss, W. V. (2000). The Adelaide Geosyncline of South Australia and its significance in Neoproterozoic continental reconstruction. Precambrian Research, 100(1–3), 21–63.
C.R Dalgarno. (1961, June). Geology of the Barossa Valley. The University of Adelaide. [This will download as a PDF].
Australian Government, Geoscience Australia. (1988 January). Bungarider Subgroup | Australian Stratigraphic Units Database, Geoscience Australia. Updated September 3, 2019.
South Australian Government. (n.d.). SARIG geological map. SARIG.
Keith Brown. (2002). The hydrogeology of the Barossa Basin, South Australia. The department of water, land and Biodiversity Conservation & The Government of South Australia.