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[BCG05-P07] Reconstruction of hydrothermal oceanic sequence by detailed observation of the Dixon Island Formation in 3.2Ga in the Coastal Pilbara terrane, Western Australia
The Dixon Island Formation, which records a hydrothermal oceanic sequence 3.2-3.1 Ga, is located on Dixon Island in the Cleaverville area in the Coastal Pilbara terrane, Western Australia (Kiyokawa et al., 2006, Kiyokawa et al., 2012). This formation has a total thickness of more than 400 m, and is composed of three subgroups: The Komatiite-Rhyolite (KR) Tuff Member (<250 m thick), the Black Chert (BC) Member (7-20 m thick), and the Varicolored (VC) Tuff Member (<250 m thick). The lower KR Member is composed of komatiite and rhyolite, which have undergone significant alteration and penetrating many black chert veins containing organic matter of hydrothermal origin. The BC and VC formations are coincident with the KR formation and were deposited directly above the seafloor where hydrothermal activity was high at that time. At Dixon Island, the strike-slip fault consists of six blocks, DX A to DX F, each of which exposes the above these members. In this study, I focused on DX C, which is the center of hydrothermal activity and retains a continuous stratigraphic sequence from the KR to BC sections, and conducted polarized light microscopy of each layer to elucidate the sedimentary facies directly above the seafloor of the hydrothermal oceanic sequence at that time. A total of 74 flakes were prepared for each stratum from 56 lamina found in 8 m thickness from the bottom of the BC layer for observation.
We classified them into the six lithologies, which are dark green siliceous shale, homogeneous massive black chert, layered black chert, white chert (ex-barite layer), volcanic ash layer, and biomat layer. Based on this classification, a columnar map was created. Unit 1 (150 cm thick): 2 repetitions of dark green siliceous shale and massive black chert with a thickness of 20-50 cm at the bottom. Unit 2 (150 cm thick): Starts with a 25 cm thick white layer of volcanic ash, interbedded with 5 to 40 cm thick dark green siliceous shale and layered black chert. Unit 3 (250 cm thick): Starting from white chert with a thickness of 30 cm, dark green siliceous shale, massive black chert and layered black chert with a thickness of 5 to 30 cm are interlayered. Unit 4 (210 cm thick): 15 to 30 cm thick biomat layer with stromatolitic structure consisting of iron hydroxide particles, overlain by 5 to 30 cm thick interlayered black chert and dark green siliceous shale.
In Unit 1 and Unit 3, massive black chert is deposited and is enriched in fine organic particles. There is no lamina, but homogeneous silica deposits with round organic matter. In Unit 2 and Unit 4, fine parallel laminae containing biomats and organic matter are preserved, and especially in the massive black chert of Unit 1 and the layered black chert of Unit 4, carbonaceous materials are concentrated. The white chert at the boundary of Unit 3 shows crystals in the direction orthogonal to the layer, suggesting that the barite layer associated with the original hydrothermal action may have been silicified. The dark-green siliceous shale in all units is massive, with massive clay beds composed mainly of chlorite and little clastic particles.
Thus, the hydrothermal oceanic sequence field found in the Dixon Island Formation does not contain terrestrial source material, but rather a steady accumulation of clay material. Silica deposits containing organic matter associated with hydrothermal activities and hydrothermal deposits such as microorganisms and barite thriving in the surface layer were found to be interlayered.
We classified them into the six lithologies, which are dark green siliceous shale, homogeneous massive black chert, layered black chert, white chert (ex-barite layer), volcanic ash layer, and biomat layer. Based on this classification, a columnar map was created. Unit 1 (150 cm thick): 2 repetitions of dark green siliceous shale and massive black chert with a thickness of 20-50 cm at the bottom. Unit 2 (150 cm thick): Starts with a 25 cm thick white layer of volcanic ash, interbedded with 5 to 40 cm thick dark green siliceous shale and layered black chert. Unit 3 (250 cm thick): Starting from white chert with a thickness of 30 cm, dark green siliceous shale, massive black chert and layered black chert with a thickness of 5 to 30 cm are interlayered. Unit 4 (210 cm thick): 15 to 30 cm thick biomat layer with stromatolitic structure consisting of iron hydroxide particles, overlain by 5 to 30 cm thick interlayered black chert and dark green siliceous shale.
In Unit 1 and Unit 3, massive black chert is deposited and is enriched in fine organic particles. There is no lamina, but homogeneous silica deposits with round organic matter. In Unit 2 and Unit 4, fine parallel laminae containing biomats and organic matter are preserved, and especially in the massive black chert of Unit 1 and the layered black chert of Unit 4, carbonaceous materials are concentrated. The white chert at the boundary of Unit 3 shows crystals in the direction orthogonal to the layer, suggesting that the barite layer associated with the original hydrothermal action may have been silicified. The dark-green siliceous shale in all units is massive, with massive clay beds composed mainly of chlorite and little clastic particles.
Thus, the hydrothermal oceanic sequence field found in the Dixon Island Formation does not contain terrestrial source material, but rather a steady accumulation of clay material. Silica deposits containing organic matter associated with hydrothermal activities and hydrothermal deposits such as microorganisms and barite thriving in the surface layer were found to be interlayered.