Woodgreen, Northern Territory : explanatory notes
Northern Territory Geological Survey
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Geology; Georgina Basin; Arunta Region
Northern Territory Government; Northern Territory Government
Australia 1:100 000 Geological Map Series
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Northern Territory Government
20 Quartz gravel (Czq) This symbol is used where abundant quartz float is concentrated in areas surrounding outcropping quartz veins, or in one case, is the result of disintegration of a quartz pebblerich conglomerate. Considering the highly stable nature of quartz and the amount of quartz float surrounding a relatively small source, the period of accumulation during denudation may be considerable. Soil over subcrop (Cz/S) This symbol is used for shallow soils developed from the insitu breakdown of granitic and metamorphic rocks (S) and may also include some material being transported radially away from such areas. The soil is commonly sandy and gritty, containing significant feldspar and mica components, as well as quartz float. Such areas are generally slightly elevated above surrounding Quaternary soils and are comparatively sparsely vegetated. They commonly forms halos around mapped outcrops of granite and metamorphic rocks, and often contain small, low outcrops of weathered and/or fresh parent rock that are too small to represent on the map. On aerial photographs and satellite images, they are distinguished by their pale reflective nature and bedrock trends are sometimes visible. These soils are the product of long periods of slow weathering, and although this process is ongoing today, they are categorised as Cenozoic rather than Quaternary. Quaternary sediment (Qc, Qr, Qs, Qp, Qa) The distribution of Quaternary units shown on the mapface has largely been interpreted from aerial photographs and Landsat imagery. The most widespread Quaternary unit in Woodgreen is colluvial red soil (Qr) plains, which are particularly well developed in the western half of the sheet area. Where most developed, they are characterised by arcuate stands of mulga (Acaciaaneura) and other vegetation types, which are clearly visible on aerial photographs and Landsat images. Such vegetation patterns have been referred to as banded mosaic or tiger bush and develop through the progressive separation of bare sheet flood areas from downslope water and silttrapping vegetated zones (WakelinKing 1999). Colluvialdeposits (Qc) include active gravel fans and adjacent gravel outwash around ranges, together with minor talus slopes around areas of steep relief. Alluvial deposits of silt, sand and gravel (Qa) occur on active floodplains and in channels. Red soil plains (Qr) and colluvial deposits (Qc) both have gradational boundaries with alluvium (Qa). Lacustrinedeposits (Qp) comprise silt and clay deposited from standing water within ephemeral playa lakes and swamps, and occur most commonly in the far northeast and northwest of the sheet area. Redaeolian sand (Qs), which forms sheetlike deposits slightly elevated above surrounding areas of red colluvial soil and alluvium, is mainly found in northern Woodgreen. Aeolian sand also forms sloping deposits rimming the southeastern sides of hills and ranges in this area. Although low linear mounds are present in some areas, true dunes are absent from the sheet area. StructurEAndMEtAMorphISM The metamorphic grade of Palaeoproterozoic metasedimentary units in the Arunta Region in Woodgreen ranges from lower amphibolite to granulite facies. The highest metamorphic grade is in the Anira Metamorphics, which have undergone hightemperature, lowpressure granulitefacies metamorphism. There is no pervasive fabric development associated with this metamorphism. The high volume of melt in metapelites is accompanied by a cordierite and spinelbearing melanosome, and this suggests that the rocks underwent the biotite dehydration melting reaction: biotite + sillimanite + quartz = cordierite + spinel + melt. This reaction is common in the hightemperature, lowpressure regional aureole metamorphism evident in a number of locations through the northern Arunta Region, most notably around Mount Stafford (NAPPERBY; Greenfield etal 1996) and in the Deep Bore Metamorphics (HUCKITTA; Scrimgeour etal 200). It is consistent with metamorphic pressuretemperature conditions of 34 kbar and 750800C. It is likely that metamorphism occurred at about 730 Ma and that the granulites form an extension of the easttrending zone of hightemperature, lowpressure metamorphism, north of the Delny Shear Zone in HUCKITTA (Jinka domain; Scrimgeour etal 200). The Delny Metamorphics and Crooked Hole Granite have a strong foliation that most commonly trends northeast and dips steeply to vertically, although east of Ugnada Tank, the foliation dips more moderately to the southsouthwest. These units have been metamorphosed to lowpressure lower amphibolite facies, with biotitemuscovite andalusite assemblages, and with grade increasing to middle amphibolite facies in westernmost outcrops of the Delny Metamorphics in Woodgreen, where sillimanite is present. The Mapata Gneiss has been metamorphosed to middle to upper amphibolite facies. The northern Arunta Region in ALCOOTA and HUCKITTA contains steeply southdipping, westnorthwesttrending reverse faults and shear zones that truncate early Palaeoproterozoic fabrics. These include major structures such as the Delny, Mount Ida and Waite River shear zones. The Delny Shear Zone (Scrimgeour and Raith 2001; formerly the DelnyMount Sainthill Fault Zone of Warren 1978) does not outcrop in Woodgreen, but is interpreted to occur in the subsurface in the southwestern corner of the sheet area. The Waite River Shear Zone separates the Woodgreen Granite Complex in the north from the Delny Metamorphics to the south. The Mount Ida Shear Zone (Shaw and Warren 1975) forms a number of prominent mylonite outcrops and separates garnetbearing constituents of the Woodgreen Granite Complex to the north from garnetabsent constituents to the south. These shear zones are likely to have had a similar evolution to the Delny Shear Zone in HUCKITTA, which underwent ductile deformation that culminated in the late Devonian (365360 Ma) with a southup reverse sense of movement (Scrimgeour and Raith 200). This shear sense is consistent with locally preserved SC fabrics in
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