Regolith landforms of Bimbowrie Station
Aaron D Brown (School of Earth and Environmental Sciences, University of Adelaide)
Bimbowrie Station is in the NE of South Australia, ~100 km west of Broken Hill, and was purchased by the Department for Environment and Heritage in May 2004 WR SURYLGH D VLJQL¿FDQW FRQWULEXWLRQ WR the protection of the natural and cultural heritage of the region (Fig. 1).
As part of PIRSA’s recent prospectivity analysis of Bimbowrie Station, a three-month project was undertaken to characterise the regolith materials and landforms and to produce a regolith landform map of the station. The results of this preliminary evaluation of the regolith and landforms at Bimbowrie have implications for mineral exploration, landscape evolution studies, and environmental rangeland management.
Physical setting Climate
Bimbowrie has a semi-arid to arid climate with hot to extremely hot dry summers and mild to cool, dry winters, with a mean daily maximum temperature of 33 °C in January, and a mean daily minimum of 4 °C in July. The area
experiences irregular low rainfall, which is characterised by low frequency, high intensity rainfall events. Evaporation far exceeds precipitation with mean annual rainfall of 236 mm at nearby Yunta and 254 mm at Broken Hill, whilst average annual evaporation in the region is 2500 mm.
Due to the sparse vegetation cover, rainfall events can result in widespread physical dispersion of erosional products DORQJ GUDLQDJH OLQHV DQG DFURVV VKHHWÀRZ SODLQV 7KH FDWDVWURSKLF ÀRRGV LQ WKH Olary area provide an extreme example of effects of water in an arid landscape. 7KH ÀRRGLQJ ZDV FDXVHG E\ WKXQGHUVWRUP activity which included 200–300 mm of rainfall in the area between Manna Hill and Cockburn, whilst neighbouring Outalpa Station received 274 mm of rain in just four hours. This intense downpour UHVXOWHG LQ VKHHWÀRZ DQG JXOO\LQJ RQ ULVHV DQG SODLQV ZLWK VLJQL¿FDQW HURVLRQ and sediment transport along alluvial FKDQQHOV $V ÀRRGZDWHUV HVFDSHG DOOXYLDO channels, sediments were deposited onto ÀDQNLQJ DOOXYLDO DQG GHSRVLWLRQDO SODLQV
rocks comprise metasiltstones, quartzites, glacially derived diamictites and conglomerates, and sedimentary ironstones.
*OREDOO\ VLJQL¿FDQW HYHQWV LQFOXGH Palaeoproterozoic sedimentation approximately coeval with the formation of the syngenetic Pb–Zn– Ag mineralisation at Broken Hill, early Mesoproterozoic FeO–Cu–Au mineralisation of similar timing to that at Olympic Dam deposit in the adjoining Gawler Craton, and Neoproterozoic glaciation (Conor 2004; Conor et al., 2005). The Willyama sediments were deformed and metamorphosed during the Olarian Orogeny (1600 Ma), and both the Willyama and Adelaidean rocks were affected by the Delamerian Orogeny (~500 Ma).
Regolith landforms Weathered bedrock
Despite the range of lithologies present within Bimbowrie, the bedrock exposures (SS units, Fig. 2; Fig. 3) are only slightly weathered, with little variation in weathering grade at the mapped scale.
CALLABONNA SUB BASIN
Cainozoic and Mesozoic
Adelaidean and Cambrian sediments
BROKEN HILL DOMAIN
Figure 1 Location of Bimbowrie Station and key geological features.
The geology of Bimbowrie records JOREDOO\ VLJQL¿FDQW HYHQWV DQG KRVWV widespread mineralisation. The potential for Zn, Pb, Au, Cu and U mineralisation, and the thin nature of the regolith in the north and west, continues to attract exploration.
Rocks include the Palaeoproterozoic Willyama Supergroup and Mesoproterozoic granites of the Willyama ,QOLHUV DQG WKH ULIW¿OO 1HRSURWHUR]RLF Adelaidean strata of the MacDonald Corridor. Lithologies vary considerably: the Kalabity and Outalpa Inliers are composed of Palaeoproterozoic metasediments and igneous rocks that include psammitic, psammopelitic and pelitic schists, quartzites, albites, calcalbites and marbles, and also felsic volcanics, subvolcanic granites and amphibolite sills. Mesoproterozoic granites are voluminous and there are ODWHU PD¿F G\NHV 7KH 1HRSURWHUR]RLF
7KLQ VXU¿FLDO ZHDWKHULQJ PLQRU LURQ oxide staining, and open fractures are the most widespread weathering features. At a local scale lithological variations result in variable weathering and landform expression with more easily weathered lithologies such as amphibolitic, micaceous, and pelitic units grading towards a moderate weathering grade.
Weathered bedrock units are expressed in the landscape as rises, low hills and hills. The presence of widespread mineralisation raises the possibility RI VLJQL¿FDQW SK\VLFDO DQG FKHPLFDO dispersion of mineralised geochemical signatures into the lower lying landscape units.
Colluvial sediments are present over a range of landforms including erosional and depositional plains, erosional rises
MESA Journal 41