Résumé
Detailed stratigraphic, petrographic, and zircon U-Pb geochronological data are provided for surface outcrops and newly obtained, unweathered drillcore intersections of Earth's oldest fossiliferous sedimentary rocks, the lowermost chert-barite unit of the ca. 3.5 Ga Dresser Formation, Warrawoona Group, Pilbara Craton.
Results show that the similar to 8 m thick unit at the drilling locality consists of thinly bedded, originally micritic carbonates deposited under quiet water conditions that are interbedded with volcaniclastic conglomerates and coarse polymict conglomerates (diamictites) deposited during periods of tectonic instability. The presence of rapid vertical and lateral facies changes, tilted bedding in some members, and internal erosional unconformities, combined with analysis of fault offsets, indicate that tectonically unstable periods were caused by growth faulting. Intense hydrothermal fluid flow accompanied episodes of growth faulting and resulted in pulsed, repeated precipitation of silica +/- barite +/- sphalerite that alternated with precipitation of pyrite. Clasts of hydrothermal minerals in sandstone and coarse, polymict conglomerate beds at several levels within the unit highlight the repeated nature of hydrothermal fluid circulation during sediment accumulation. Hydrothermal fluid circulation caused widespread acid-sulfate alteration of the footwall, extensive replacement of the newly deposited carbonate sediments by hydrothermal precipitates, and crystallization of hydrothermal chert-barite-pyrite in veins perpendicular to bedding in the footwall and parallel to bedding in the sedimentary unit. The combined evidence points to deposition of the chert-barite unit within an active volcanic caldera. A 10 cm thick bed of felsic volcaniclastic tuff within finely bedded carbonates near the top of the unit has yielded a maximum age of deposition of 3481.0 +/- 3.6 Ma (2 sigma uncertainty), confirming earlier Pb-Pb age data for the antiquity of these rocks.
Putative signs of life are present as stratiform, columnar, domical, and coniform stromatolitic laminates at various levels throughout the unit. Petrographic observations show that red- and black-weathering stromatolitic laminates on the surface consist of pyrite in unweathered drillcore material. Observation of local relics of carbonate between pyrite crystals in these laminates indicates a carbonate protolith prior to replacement by hydrothermal pyrite, which provides support for a biological origin of stromatolitic laminates. Further support is provided by clasts of laminated carbonaceous material in thinly bedded, primary micritic carbonates.
Textural analysis of jaspilitic "cherts" near the top of the unit reveal haematite as tiny crystals within recrystallized siderite/dolomite rhombs in carbonate beds affected by hydrothermal silica alteration. The presence of unaltered diagenetic pyrite crystals in the haematite-altered siderite indicates that alteration did not result from oxidizing fluids. Rather, haematite alteration is interpreted as the result of an increase in pH during diagenetic alteration by mildly reducing, silica-rich fluids associated with eruption of overlying basalts, possibly with the influence of microbial activity. This has important implications for the origin of jaspilitic cherts throughout the early Archean record and for atmospheric conditions of early Earth. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.