Woodgreen, Northern Territory : explanatory notes
Northern Territory Geological Survey
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Geology; Georgina Basin; Arunta Region
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Australia 1:100 000 Geological Map Series
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Northern Territory Government
17 The Tops Member contains a body fossil assemblage referred to as the Mount Skinner fauna (Wade 1969) and rare ichnofossils (Haines etal 1991). The Mount Skinner fauna includes several softbodied organisms, most characteristically, two species interpreted as scyphozoan medusoids (jellyfish'), Hallidaya brueri and Skinnera brooksi, which were described by Wade (1969) from the Mount Skinner area. Elements of the Mount Skinner fauna have also been located in the lower Arumbera Sandstone of the Amadeus Basin (Wade 1969) and are believed to be of Ediacaran age (Haines etal 1991). The fossils are most common in the Mount Skinner area where they are found over an interval broadly coincident with the green cupriferous bands (fossils are found in both the red and green lithofacies). Macrofossil symbols on the map record Wade's (1969) original collection localities and additional sites located during this study. AdneraMember(LPsa) The Adnera Member comprises redbrown, purple and white crossbedded, medium sandstone and quartzite. There are intervals of tabular, fine to medium sandstone interbedded with redbrown mudstone that closely resemble parts of the Tops Member. The Adnera Member conformably overlies the Tops Member and the boundary is likely to be diachronous. It is distinguished by the fact that the sandstones are mostly significantly cleaner, better silicified and more resistant than the underlying Tops Member. The unit is mostly devoid of positive radiometric expression, except for exposures at and near Mount Skinner that exhibit a moderate Th signal. The unit as a whole is more resistant to erosion than the Tops Member and, for this reason, the contact is often most successfully picked on aerial photograph expression. The member has not been intersected in drillholes. It is estimated to reach about 200 m thickness in centralnorthern Woodgreen. Unconformities below younger units indicate that the member has been locally removed in eastern areas. The Adnera Member was deposited under shallowmarine and deltaic conditions, similar to those interpreted for the upper Tops Member. Haines etal (1991) reported ichnofossils from the Adnera Member in BARROW CREEK, although as yet, none has been found in Woodgreen. The ichnofossil assemblage includes forms that are more characteristic of the late Neoproterozoic than the Cambrian and, importantly, the most characteristic Early Cambrian elements, such as Treptichnus pedum, are absent. While it is possible that the EdiacaranCambrian boundary lies within the Adnera Member, it is considered more likely that the boundary lies within the hiatus marking the disconformity at the top of the Central Mount Stuart Formation. Cambrian Shadow Group Octy Formation (_Clo) During previous mapping (Shaw and Warren 1975), strata now recognised as Octy Formation were included in the Central Mount Stuart beds'. The type section of the Octy Formation is in BARROW CREEK (Haines etal 1991). The formation comprises predominantly white to brown, crossbedded medium sandstone and quartzite, with subordinate mudstone. In places, the sediments are glauconitic. On lithological grounds, the formation is only differentiated from the underlying Adnera Member with difficulty; however it is awarded formation status on the basis that mapping in BARROW CREEK demonstrated a disconformity to lowangle unconformity at its base. The Octy Formation is also distinguishable from the Adnera Member in Landsat 7 imagery, particularly band 7. The most distinctive feature is the common presence of definitive Early Cambrian ichnofossils, which are absent from the underlying succession. Ichnofossils include a diversity of forms, among them Treptichnus(Phycodes')pedum, used to define the base of the Cambrian (Landing 1994). The lower formation contact is not well exposed in Woodgreen, but does not appear to be significantly angular. At 409500mE 7563600mN, an exploration costean exposes a recessive interval at the base of the Octy Formation. This interval comprises bioturbated green, purple and red mudstone and interbedded sandstone, and is probably equivalent to a Figure 18. Tops Member of Central Mount Stuart Formation. Exploration costean exposing characteristic redbrown sandstone and mudstone, with green mudstonedominated interval in foreground (beneath hammer at bottom right). Such green intervals typically display minor malachite staining at surface. 429223mE 7543511mN, 3 km N of Mount Skinner.
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