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
2 tremolite occurs throughout. The age of the intrusion is unknown, but the lack of evidence for tectonic deformation suggests that it postdates regional metamorphism. The ultramafic rock has a high magnetic susceptibility (ca 6000 x 105 SI). pegmatite (peg) Pegmatite bodies of a range of scales are present in most Arunta Region areas and are probably of a variety of ages. Most form relatively small dykes and sills, but some larger areas of pegmatite (hundreds of metres in scale) are present within the Delny Metamorphics. Most pegmatites are leucocratic, and if present at all, micas are usually restricted to muscovite. Biotitebearing pegmatites intrude the Crooked Hole Granite, near the southern boundary of Woodgreen. Black tourmaline is abundant in some pegmatites associated with southern parts of the Mollie Granite Complex, but elsewhere, tourmaline is a minor accessory only. neoProterozoic to Palaeozoic georgina Basin Cryogenian Keeperagroup Boko Formation (LPbo) The Boko Formation is a lenticular unit of massive glacial diamictite, with cobble and boulder clasts up to 2 m in size supported by a redbrown mudstone matrix (Figure12). Clasts comprise quartzite, sandstone, conglomerate, granite, various metamorphic rocks and rare volcanic rocks. They commonly display facets and striations characteristic of glacial action (Figure13). Good exposures of the matrix are restricted to two localities northwest of New Bore (428590mE 7554789mN, 427704mE 755976mN). Elsewhere exposure is poor, with the formation being recognised by the presence of rises and low hills covered by loose cobbles and boulders of quartzite, granite and other rock types. The clast assemblage and the presence of glacial facets and striations serves to distinguish such outcrops from conglomeratic units within the Oorabra Arkose and from old dissected Cenozoic fan deposits. The Boko Formation unconformably overlies rocks of the Arunta Region and is overlain, with possible conformity, by the Oorabra Arkose, or unconformably, by the Elyuah Formation. A small outcrop of the unconformity surface over the Utopia Quartzite at 428621mE 7554960mN (Figure14) has a distinctly polished and weakly striated (eastwest trend) appearance and may represent a small relict of a glaciated pavement. A similar pavement is locally preserved beneath the oorabra Arkose. The Boko Formation is lenticular and only locally preserved, probably in depressions on the unconformity surface. Its thickness ranges up to about 30 m. The type area of the Boko Formation is in BARROW CREEK (Haines etal 1991). In Woodgreen, diamictites that were previously mapped as part of the former Central Mount Stuart beds (Shaw and Warren 1975), were subsequently recognised as a distinct, although unnamed unit by Walter (1980) and are now recognised as the Boko Formation. From comparisons with other Australian Neoproterozoic successions, the Boko Formation is considered to correlate with the younger of two widespread glacial episodes, the ca 600 Ma Marinoan glaciation (Haines etal 1991). It is interpreted as a nonmarine moraine (till) deposit. Oorabra Arkose (LPor) The Oorabra Arkose was defined in HUCKITTA and has had a complex history of usage and redefinition (Joklik 1955, Smith 1964, Walter 1980, Freeman 1986). The unit has not previously been recognised beyond that sheet area. Walter (1980) recognised a similar arkose unit at Poomingie Waterhole in utoPia, just east of Woodgreen, but was reluctant to extend the geographic range of the Oorabra Arkose, because of uncertainties about stratigraphic position and correlation. A similar unit is here recognised in Woodgreen, where it can be demonstrated to occupy the same stratigraphic position as the Oorabra Arkose in HUCKITTA, with which it also shares lithological and Figure12. Boko Formation. Exposure near top of small hill shown in Figure14. Clasts of quartzite, granite and other rock types are supported by dark redbrown mudstone matrix; bouldersize granite clast above and to right of hammer. 428590mE 7554789mN, 3 km NNW of New Bore.
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