Archean hydrothermal oceanic floor sedimentary environments: DXCL drilling project of the 3.2 Ga Dixon Island Formation, Pilbara, Australia

Many place in Archean greenstone belts have been reported of the black chert to Iron rich sediments above volcanic sequence. The chemical sedimentary sequence has been recognized to form by as hydrothermal siliceous sequence. These sediments contain the hint to understand the Archean ocean and earth surface environments. Here, we will focus the Dixon Island and Cleaverville formations, which are one of the best preserved Archean hydrothermal sedimentary sequence in the world, to recognized detail stratigraphy and restored deep ocean environment. We did scientific drilling, which is called ‘DXCL drilling project’, at 2007 summer. This drilling project had been selected two coastal sites; CL site at lower part of the Cleaverville Formation, and another is DX site at the upper Dixon Island Formation. A systematic combinations of geological, sedimentological, geochemical, and geobiological approaches will be applied to the fresh samples. Here we will show the recent result of this sequence, which will be key evidence to understand the nature of the middle Archean (3.2 Ga) marine environment influenced by hydrothermal activity. The 3.2 Ga Dixon Island -Cleaverville formations composed of volcanic rock units and chemical-volcanosedimentary sequence which are identified by accreted immature island arc setting. The ~350m-thick Dixon Island Formation which is overlie by pillow basalt consists mainly of highly silicified volcanic-siliceous sequences that contain apparent microbial mats and bacterial fossil-like structure within black chert and also includes a komatiite-rhyolite sequences bearing hydrothermal veins. The >300m-thick Cleaverville Formation, which conformably overlay pillow basalt, contains a thick unit of reddish shale, bedded red-white chert and banded iron formation. It partly contains chert fragments-bearing pyroclastic beds. In detail lithology from the drill cores, the CL and DX contain different type of organic rocks. The CL 1 and CL2 core samples mainly consist of the organic-rich massive black shale bed (20cm in thickness) with few cross-laminated fine volcanic tuff, and the DX core sample contains thin alteration of very thin laminated black shale and thin pyrite lamination which is first discovery of new lithology of those geologic units. Preliminary Organic Carbon and pyrite S data shows as follows: delta13C: -32 - -26par mil (mostly 28 par mil), TOC: 0.6-1.4%, delta 34S: -6 - +8. There are no MIF signal from delta 33/delta 34 S in this core. The depth of depositional place of upper Dixon Island Formation is anoxic and very clam condition. There is sulfur reducing bacteria activity in this ocean surface environment. After pillow basalt eruption, the Cleaverville Formation deposited slightly shallower condition. Biogenic activity occurred before sedimentation of the Banded Iron Formation and form thick black shale. The shallowing upward sequence from the Dixon Island to Cleaverville formations might be shown the buried history along the deeper hydrothermal setting of submarine floor in the immature island arc.