Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Kiyokawa, S. Articles by Kitajima, F. Search for Related Content GeoRef GeoRef Citation

Middle Archean volcano-hydrothermal sequence: Bacterial microfossil-bearing 3.2 Ga Dixon Island Formation, coastal Pilbara terrane, Australia

Shoichi Kiyokawa^,1, Takashi Ito², Minoru Ikehara³ and Fumio Kitajima⁴

¹ Department of Earth and Planetary Science, Kyushu University, 6-10-1 Hakozaki Higashiku, Fukuoka, Japan
² Faculty of Education, Ibaraki University, 2-1-1 Bunkyo Mito-city, Ibaraki, Japan
³ Marine Core Research Center, Kochi University, 2-5-1 Akebono-cho, Kochi-city, Japan
⁴ Department of Earth and Planetary Science, Kyushu University, 6-10-1 Hakozaki Higashiku, Fukuoka, Japan

The 3.2 Ga Dixon Island Formation in the Cleaverville Group of the coastal Pilbara terrane, Australia, is one of the most complete and best-preserved examples of middle Archean oceanic stratigraphy and contains possible microbial material. Field observations and geochemical evidence suggest that this formation contains a low-temperature hydrothermal vent system with a biogenic microbial colony from the Archean ocean. The Dixon Island Formation is 350 m thick and consists of the Rhyolite Tuff, Black Chert, and Varicolored Chert Members, in ascending order. The Rhyolite Tuff Member contains many vein swarms, such as quartz and black chert veins, and highly altered rhyolite tuff layers, which are identified as an underground bypass zone for circulating hydrothermal fluid. Many black chert vein swarms in the Rhyolite Tuff Member imply intensive low-temperature hydrothermal activity during deposition of the Black Chert Member, which is 10–15 m thick. The Black Chert Member is composed of massive black chert, laminated black chert, dark-greenish siliceous shale and tuffaceous laminated chert, which are mainly composed of very fine quartz. Abundant pseudomorphs of silica after aragonite, barite, and gypsum, and a distinctly continuous, stromatolite-like biomat layer (10–20 cm thick), are preserved within the laminated black chert bed. The stromatolite-like biomat bed is formed of fine iron or iron-coated quartz pisolite within fine-grained silica. The absence of detrital sediment of continental origin and the many vein injections imply that this sedimentary facies represents a pelagic hydrothermal environment at 500–2000 m paleodepth, and may have been on the slope of an immature island arc.

Microbial material has been preserved well in the lower part of Black Chert Member. The massive black chert has carbonaceous peloids (0.3–2.0 mm in diameter), which are similar to those in the black chert veins. The massive black chert contains spiral-, rod, and dendrite-shaped bacterial material. The total organic carbon (TOC) value of massive black chert in the lower part of the Black Chert Member is higher (TOC = 0.15–0.45%) than that of the overlying laminated chert section (TOC = 0.02–0.15%) and the black chert vein (TOC = 0.1–0.13%), and the carbon isotope (¹³C) values of this lithology (–33 to –27) are also lighter than for the black chert veins (–29 to –26) and the laminated black chert in the upper part of the Black Chert Member and the Varicolored Chert Member (–27 to –13). This evidence suggests that the carbonaceous grains and bacteria-shaped material in the lower part of the Black Chert Member are of bio-genic origin and were formed close to a low-temperature hydrothermal vent system. The microbial colony may have been rapidly fossilized by silicification related to hydrothermal activity. Laminated black chert in the upper part of the Black Chert and the Varicolored Chert Members may have formed by cyanobacterial sedimentation from the ocean surface.

Key Words: Middle Archean • greenstone belt • hydrothermal • immature island arc • black shale • microfossil

This article has been cited by other articles:

				S. M. Bonny and B. Jones Experimental Precipitation of Barite (BaSO4) Among Streamers of Sulfur-Oxidizing Bacteria Journal of Sedimentary Research, May 1, 2008; 78(5): 357 - 365. [Abstract] [Full Text] [PDF]

				J. W. Schopf The First Billion Years: When Did Life Emerge? Elements, August 1, 2006; 2(4): 229 - 233. [Abstract] [Full Text] [PDF]

				J. W. Schopf Fossil evidence of Archaean life Phil Trans R Soc B, June 29, 2006; 361(1470): 869 - 885. [Abstract] [Full Text] [PDF]