Woodgreen, Northern Territory : explanatory notes
Northern Territory Geological Survey
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Geology; Georgina Basin; Arunta Region
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18 similar basal mudstone unit reported from southern BARROW CREEK (Haines etal 1991). The basal mudstone unit may be widespread, but this interval is usually scree covered. In Woodgreen, the Octy Formation is unconformably overlain by a poorly exposed carbonate unit, tentatively identified as the Chabalowe Formation. Regional relationships indicate that this contact is significantly angular, such that in central and eastern Woodgreen, the Octy Formation has been removed and the Chabalowe Formation(?) lies directly on the Central Mount Stuart Formation. Although the thickness of the Octy Formation cannot be accurately determined, due to low angles of dip and faulting, it is estimated to reach a maximum thickness of about 150 m near 421000mE 7556000mN. A moderate Th and U radiometric signal is observed over the Octy Formation west of this location, but not elsewhere, and may arise from a residue of previously overlying Chabalowe Formation, whose radiometric signature is very similar (see chabalowe Formation). The sedimentary facies and ichnofossils indicate a shallowmarine depositional environment. The Octy Formation is correlated, at least in part, with the Mount Baldwin Formation, as mapped in the Jervois Range area of HUCKITTA (see discussion under central Mount Stuart Formation). It can also be correlated (Haines etal 1991) with upper parts of the Arumbera Sandstone (Amadeus Basin) and Yuendumu Sandstone (Ngalia Basin). narpagroup Chabalowe Formation (_Cmc) A poorly preserved unit, marked at the surface by chert and sandstone rubble, and commonly associated with ferricrete, unconformably overlies the Octy Formation or members of the Central Mount Stuart Formation. No overlying unit is preserved at the surface in Woodgreen, but outcrops of the Cambro Ordovician Tomahawk Formation, exposed just east of northern Woodgreen in utoPia, are presumably stratigraphically younger than this unit. The chert has relict textures which indicate that it is a silicified carbonate rock. This is consistent with the moderate Th and U radiometric signal usually observed over outcrops of this unit. Relicts of domal and columnar stromatolites are often preserved (Figure19), although no other fossils have been recognised. Cauliflowerlike chert masses are probably after a sulfate evaporite, such as gypsum or anhydrite. The thickness of the unit is not well constrained, but may reach several hundred metres. Similar outcrops are present in southcentral BARROW CREEK and these were tentatively assigned to the Chabalowe Formation (Haines etal 1991), a formation defined from drillcore data further north (stratotype in NTGS stratigraphic drillhole BC5 in BARROW CREEK; Stidolph etal 1988). Based on a similar surface expression and stratigraphic relationships, the outcrops in Woodgreen are tentatively assigned to the same formation, although it is unlikely that a more definitive stratigraphic assignment can be provided without stratigraphic drilling. From drillcore intersections in BARROW CREEK and ELKEDRA, the Chabalowe Formation comprises interbedded sandstone, dolomitic sandstone, mudstone, dolostone and evaporite (Stidolph etal 1988, Haines etal 1991). Stromatolites are common, but agediagnostic fossils are lacking. The Chabalowe Formation is considered to be of early Late Cambrian age, based on its lateral relationship with the Arrinthrunga Formation (Dunster etal 2007). Deposition probably occurred in a restricted marine or marginal marine sabkha environment. It is possible that the Late Cambrian carbonatedominated Arrinthrunga Formation, which conformably overlies the Chabalowe Formation in northern and eastern BARROW CREEK, and which was tentatively identified in a small outcrop 6 km northeast of the northeastern corner of Woodgreen in luraPulla (Haines etal 1991), is also represented in some outcrops. Alternatively, the Arrinthrunga Formation may be present, but not exposed at the surface, to the east of outcrops of inferred Chabalowe Formation in the far northeast of Woodgreen. cockroachgroup Tomahawk Formation The CambroOrdovician Tomahawk Formation (formalised by Kruse etal 2002) does not outcrop in Woodgreen. Figure19. Chabalowe Formation(?). Chert float with relict stromatolites. 427392mE 7561311mN, 9 km NNW of New Bore.
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