Sedimentology of the Palaeoproterozoic Barney Creek Formation in DDH BMR McArthur 2, southern McArthur Basin, Northern Territory

The 122 m-long diamond drillcore DDH BMR McArthur 2 was collared in Reward Dolomite approximately 23 km southwest of McArthur River Mine. It intersected 25 m of Reward Dolomite and was terminated in the underlying Barney Creek Formation, comprising 97 m of grey to black dolomitic siltstone and shale. Facies and geochemical characteristics suggest that the lowermost 15 m of the intersection represent the upper part of the W-Fold Shale Member, and the overlying 82 m has a close affinity to the HYC Pyritic Shale Member. Sedimentary-facies-based analysis of the core indicates that the entire succession accumulated in a quiet, reduced, sub-wave-base environment. However, the restriction of key facies to discrete stratigraphic intervals indicates subtle fluctuations in environmental conditions, chiefly water depth, during deposition. A schematic relative water depth curve has been constructed that identifies two periods of increasing water depth (one from the W-Fold Shale Member into the lower part of the HYC Pyritic Shale Member, and one in the upper part of the HYC Pyritic Shale Member), and two periods of shallowing and/or shoreline transgression (one in the middle/upper part of the HYC Pyritic Shale Member and one at the top of the hole, representing a gradational contact between the Barney Creek Formation and the Reward Dolomite).