BAROSSA SCULPTURE PARK

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GEOLOGY

SOUTH AUSTRALIA

The area around the Adelaide Fold-thrust Belt in the Barossa Ranges (Kanmantoo Province) along the foreland basin was once affected by a major tectonic collision called the Delamerian Orogeny, this occurred around the Neoproterozoic-Cambrian period (514 to 490 million years ago) and took part in the formation of the Adelaide Geosyncline during the Neoproterozoic-Cambrian (750 to 500 million years ago).

 

The orogeny is whats responsible for the Mount Lofty Ranges, Ikara-Flinders Ranges, and the Vulkathunha-Gammon Ranges. The sedimentary strata was folded and uplifted as the supercontinent Rodinia broke apart and re-merged to form the short-lived Pannotia supercontinent in the Neoproterozoic. 

 

The Barossa and Mount Lofty Ranges are the result of long-term neo-tectonic movement which has affected the pre-tertiary basement rocks. The rocks which make up the Range were most likely involved with crustal stretching followed by subsidence and then deposition during the early to late protozoic. Following the Delamerian Orogeny in the Cambrian-Ordovician (500 million years ago) low-pressure and high-temperature metamorphism occurred, this affected several different rock layers. 

The Barossa Basin below the Sculpture Park formed from the eroding Mount Lofty Ranges as the area was isolated from the ocean during the Cenozoic (59 million years ago to current), the majority of the sediment deposited is terrestrial and formed from the eroding mountain range. Rivers have cut through the current basin and continue to erode the region out, this will continue several million years into the future. 

MORALANA SUPERGROUP

The area resides under the Moralana Supergroup, which is comprised of mostly marine sediments, and metasediments that have been metamorphosed only slightly which includes limestone, conglomerate, and minor volcanics which are also a part of the group. The supergroup was placed in the Early and Middle Cambrian period (541-485 million years ago) which was the first of the Paleozoic Era.   

BACKSTAIRS PASSAGE FORMATION

The Backstairs Passage formation was most likely deposited in a shallow-water marine setting more than likely in a delta. The formation and its members were deposited during the Botomian period in the Lower Cambrian (517 to 510 million years ago).

 

The Backstairs Passage Formation is comprised of metasedimentary rocks like metasiltstone and kaolinised metasandstones which are comprised of a fine whiteish clay kaolinite rock. The base 60 meters of the formation is comprised of laminated grey siltstone with fine lenses of sandstone and fine to medium-grained subarkose siltstones, and a mix of laminated arkoses with interbedded laminated siltstone. Crossbedding with unimodal current directions (one main primary direction) is also visible in some locations. 

 

The higher 45 meters of the formation contain the majority of the siltstone and also comprises laminated siltstone up to 3 meters in height, the addition of intercalated and laminated fine-grained subarkose with beds up to 15cm thick with tabular cross-bedding which also makes up some of the layers in the formation. The Backstairs Passage Formation comprised of 3-35% feldspar, and 60-90% quartz most of the quartz is located at the bottom 100 to 120 meters (328 to 393 feet) of the formation and has usually been recrystallized.  

 

The Backstairs Passage Formation consists of 2 members:

.Campana Creek Member (Not Visible In This Location)

The Campana Creek Member is considered the lowest member and the base of the Backstairs Passage Formation and begins from the first exposed sandstone horizon above the mudstones (now phyllites) of the Blowhole Creek Member below. The below 20 meters at the base of the member is comprised of metasandstone that is fine-grained, mixed with low-angle cross-bedding.

 

The member is also a mix of fine-grained biotite-rich quartz arenite, originally the biotite was once clay that was recrystallized during metamorphism. The top 25 meters of the member are mostly comprised of laminated fine sandstone with only minor siltstone, the member contains more quartz further up in the form of medium-grained, thinly bedded and cross-bedded sandstones. 

.Tungkillo Member (Not Visible In This Location)

The Tungkillo Member is comprised of white Marble, fine-to medium-grained recrystallised white quartz/feldspar with pale/dark green amphibole-pyroxene rich interlayers.

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WEATHERING

The Backstairs Passage Formation is exposed on Menglers Hill by the erosion of top-soil which slowly uncovers the rock layers below, weathering is also very prominent on the rocks as the edges have been softened. 

Physical weathering by wind, chemical weathering by rain, and even biological weathering where trees and plants help break down the rocks are all visible at the sculpture park. This takes hundreds of thousands to millions of years to occur especially with a formation that has been slightly metamorphosed, erosion will continue to tear the old Barossa Range down, this will take another few million years.  

REFEreNCES

Australian Government, Geoscience Australia. (n.d.). Backstairs Passage Formation | Australian Stratigraphic Units Database, Geoscience Australia. Last update: September 3, 2019.

James B Jago, & Gatehouse, C. G. (2009). The Type Section of the Cambrian Backstairs Passage Formation, Kanmantoo Group, South Australia. Transactions of the Royal Society of South Australia, 133(1), 150–163. 

Keith Brown. (2002). The hydrogeology of the Barossa Basin, South Australia. The department of water, land and Biodiversity Conservation & The Government of South Australia. 

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

Victor Tokarev. (2005, February). Neotectonics of the Mount Lofty Ranges (South Australia). Adelaide University - school of Earth Science & Environmental Sciences, Geology & Geophysics. 

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