Lateral variations in the lithology and organic chemistry of a black shale sequence on the Mesoarchean seafloor affected by hydrothermal processes: The Dixon Island Formation of the coastal Pilbara Terrane, Western Australia

The Dixon Island Formation of the coastal Pilbara Terrane, Western Australia is a 3.2Ga volcanic-sedimentary sequence influenced by syndepositional hydrothermal activity formed in an island-arc setting. We documented lateral variations in stratigraphy, hydrothermal alteration, and biological activity recorded in the sedimentary rocks (over several kilometers), with the aim of identifying areas of biological activity and related small-scale structures. The Dixon Island Formation comprises volcaniclastics, black chert, and iron-rich chert within seven tectonic blocks. Based on detailed geological mapping, stratigraphic columns, carbon isotope composition, and organic carbon (C _org) content, we found lateral (>5km) variations in stratigraphy and carbon isotope compositions in a black chert sequence above the Mesoarchean seafloor with hydrothermal activity. Two felsic tuff layers are used as stratigraphic marker beds within a black chert sequence, which was deposited on altered volcanic rocks. The black chert sequence in each tectonic block is 10-20m thick. Thickness variations reflect topographical undulations in the paleo-ocean floor due to faulting. Early-stage normal faults indicate extensional conditions after hydrothermal activity. Black chert beds in the topographically subsided area contain higher C _org contents (about 0.4wt%) than in areas around the depression (<0.1wt%). Carbon isotope compositions for the black chert vary from -40 to -25‰, which are similar to values obtained for a black chert vein within the komatiite-rhyolite tuff sequence (underlying the black chert sequence). Those for other rock types in the Dixon Island Formation are -33 to -15‰. Results indicate that deformation occurred soon after the final stages of hydrothermal activity. After this early-stage deformation, organic-rich sediments were deposited over an area several kilometers across. The organic-rich sediments indicate stagnant anoxic conditions that resulted in the deposition of siliceous and organic matter from hydrothermal vein systems. When hydrothermal activity terminated, normal faulting occurred and organic matter was deposited from the sea surface and silica from the seafloor.