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
Attribution International 4.0 (CC BY 4.0)
Northern Territory Government
5 depth). In drillcore, the Elyuah Formation is dominated by black micaceous shale with minor thin, micaceous fine sandstone interbeds. A basal coarse pebbly unit is developed above an unconformable contact with basement granitoids. The Elyuah Formation unconformably overlies Arunta Region basement and, locally, the Oorabra Arkose and Boko Formation. It is overlain conformably by the Grant Bluff Formation. It is absent at Mount Harper and near the northern boundary of Woodgreen, where attenuated Grant Bluff Formation rests directly on Arunta Region basement. The Elyuah Formation is interpreted to be of marine origin. With the exception of highenergy transgressive sandstone and conglomerate at the base, most of the unit was deposited under quiet conditions below fair weather wave base, punctuated by rare storm events. The Elyuah Formation is very recessive and rarely exposed. The presence of a gap with no outcrop beneath the Grant Bluff Formation is taken as an indicator of the presence of the unit. Small outcrops of red and white weathered shale have been located between the Oorabra Arkose and Grant Bluff Formation in the Kunoth Knob area (436500mE 7535300mN, 434300mE 7535500mN). Rounded quartz pebbles in scree from this same interval hint at the existence of a basal conglomerate over the Oorabra Arkose, as in HUCKITTA (Freeman 1986). At 444300mE 7532500mN, a dam excavation east of the outcropping Grant Bluff Formation also exposes red and white micaceous siltstone and shale. These very minor exposures do not appear on the mapface. Grant Bluff Formation (LPra) The Grant Bluff Formation was defined in HUCKITTA (Walter 1980, Freeman 1986). In the Woodgreen area, First Edition ALCOOTA mapping recognised the Grant Bluff Formation only in the Kunoth Knob area, other outcrops now assigned to this formation being incorrectly mapped as Central Mount Stuart beds (Shaw and Warren 1975). In outcrop, the Grant Bluff Formation is dominated by thinly bedded, tabular, pale fine sandstone (Figure17). It is characteristically very low in all radiometric channels, especially K. Ripple marks and parting lineations are common. Surface weathering often causes outcrops to be reddened to varying degrees and, where this occurs, it can be difficult to distinguish the Grant Bluff Formation from the Central Mount Stuart Formation in isolated outcrops. In some areas, particularly near Kunoth Knob, the Grant Bluff Formation contains several units of coarse, blocky, pebbly white sandstone. Because these units are resistant and ridge forming, scree from them often covers the more characteristic, though recessive, finegrained tabular facies of the Grant Bluff Formation. Incomplete sections and structural complications impede determinations of thickness from outcrop. The formation is 50 m and 27 m thick in drillholes CMS and CMS2, respectively. It thins to the north and has an estimated thickness of only 2030 m near the northern boundary of Woodgreen. The Grant Bluff Formation is interpreted as a shallowmarine deposit, formed mostly above storm wave base. In most areas, the Grant Bluff Formation is assumed to conformably overlie the Elyuah Formation, but the latter is very recessive and is rarely exposed. Drillcore data indicate a gradational contact, with the base of the Grant Bluff Formation taken to be the point where black shale and minor fine sandstone of the Elyuah Formation are replaced upsection by fine sandstone and minor shale. Near the northern boundary of Woodgreen, the attenuated Grant Bluff Formation unconformably overlies Arunta Region granite in the absence of the Elyuah Formation. A lenticular, basal quartzpebble conglomerate is present in this area. A similar relationship is present at Mount Harper, on the western margin of Woodgreen. Outcrops over basement near the northern edge of Woodgreen are continuous with outcrops mapped as the Forster Member of the Central Mount Stuart Formation in BARROW CREEK (Haines etal 1991). It can now be demonstrated that the Forster Member is equivalent to the Grant Bluff Formation and the former term is here abandoned. Central Mount Stuart Formation (LPst, LPsa) Remapping has demonstrated that the unit shown as Central Mount Stuart beds on the First Edition ALCOOTA mapsheet (Shaw and Warren 1975) is a grouping of several Figure17. Grant Bluff Formation. Outcrop showing thickeningupward interval of tabular white sandstone and quartzite. 421402mE 7564646mN, 14 km NW of New Bore.
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