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Zircon grains from metasiltstone beds in the Precambrian Ashburn Formation were dated to constrain the age of the stromatolite Archaeozoon acadiense. The largest group of analyzed grains consists of mostly well-rounded detrital zircon no younger than 1060 Ma with significant peaks at about 1200-1100, 1500, 1800, 2050, and 2700 Ma, as well as a few Mesoarchean ages. A second population of apparently euhedral grains gives ages in the range 680-400 Ma. The distribution of detrital zircon ages along with field relationships suggest that the Ashburn and Martinon formations are proximal and distal parts of the Green Head Group, respectively, and hence a previously reported maximum depositional age of ca. 602 [+ or -] 8 Ma for the Martinon Formation also applies to stromatolite in the Ashburn Formation. Paleozoic zircon recovered from the Ashburn Formation cannot be detrital because the rocks were intruded and contact metamorphosed by the Golden Grove Plutonic Suite at ca. 550-525 Ma. The Paleozoic zircon grains were found in four independently processed samples and have also been seen in other studies in the region, showing that they are not due to laboratory contamination. Zircon formed during younger magmatic activity may have been transported in hydrothermal fluids and emplaced into these metasedimentary rocks in veins. A single euhedral zircon recovered from an atypically thick vein gave an age of 659 [+ or -] 33 Ma. Hydrothermally transported zircon might provide a window into younger igneous processes for which evidence is not otherwise preserved in this and other areas. Key words: U--Pb geochronology, zircon, hydrothermal transport, detrital ages, Ashburn Formation. Des grains de zircon provenant de lits de metasiltstone dans la Formation precambrienne d'Ashburn ont ete dates afin de circonscrire l'age du stromatolite Archaeozoon acadiense. Le plus grand groupe de grains analyses est principalement constitue de zircons detritiques arrondis d'un age minimum de 1060 Ma avec des pics significatifs a environ 1200-1100, 1500, 1800, 2050 et 2700 Ma, ainsi que quelques ages mesoarcheens. Une deuxieme population de grains en apparence idiomorphes donne des ages dans la fourchette de 680-400 Ma. La distribution des ages de zircons detritiques combinee aux relations de terrain donne a penser que les Formations dAshburn et de Martinon sont les parties proximale et distale du Groupe de Green Head, respectivement, et qu'un age de depot maximum deja publie d'environ 602 [+ or -] 8 Ma pour la Formation de Martinon s'applique egalement aux stromatolites dans la Formation dAshburn. Des zircons paleozo'iques provenant de la Formation dAshburn ne peuvent etre detritiques puisque les roches ont ete recoupees par la serie plutonique de Golden Grove vers 550-525 Ma et en ont subi un meta-morphisme de contact. Les grains de zircon paleozo'iques ont ete trouves dans quatre echantillons traites de maniere independante et ont egalement ete releves dans d'autres etudes dans la region, ce qui demontre qu'ils ne sont pas dus a une contamination en laboratoire. Des zircons formes durant des episodes magmatiques plus recents pourraient avoir ete transported dans des fluides hydrothermaux et mis en place dans des veines dans ces roches metasedimentaires. Un unique grain de zircon idiomorphe provenant d'une veine inhabituellement epaisse a donne un age de 659 [+ or -] 33 Ma. Des zircons transported par des fluides hydrothermaux pourraient ouvrir une fenetre sur des processus ignes plus jeunes dont d'autres preuves ne sont pas preservees dans cette region et ailleurs. [Traduit par la Redaction] Mots-cles: geochronologie U--Pb, zircon, transport hydrothermal, ages de zircons detritiques, Formation d'Ashburn., Introduction The Saint John area of southern New Brunswick, Canada (Fig. 1), has been a centre of geological investigation since the mid part of the 19th century, when the Steinhammer [...]

New data gathered during mapping of c. 3490-3240 Ma rocks of the Pilbara Supergroup in the Pilbara Craton show that most bedded chert units originated as epiclastic and evaporative sedimentary rocks that were silicified by repeated pulses of hydrothermal fluids that circulated through the footwall basalts during hiatuses in volcanism. For most cherts, fossil hydrothermal fluid pathways are preserved as silica [+ or -] barite [+ or -] Fe-bearing veins that cut through the footwall and up to the level of individual bedded chert units, but not above, indicating the contemporaneity of hydrothermal silica veining and bedded chert deposition at the end of volcanic eruptive events. Silica [+ or -] barite [+ or -] Fe-bearing vein swarms are accompanied by extensive hydrothermal alteration of the footwall to the bedded chert units, and occurred under alternating high-sulphidation and low-sulphidation conditions. These veins provided pathways to the surface for elements leached from the footwall (e.g., Si, Ba, Fe) and volcanogenic emissions from underlying felsic magma chambers (e.g., C[O.sub.2], [H.sub.2]S/H[S.sup.-], S[O.sub.2]). Stratigraphic evidence of shallowing upward and subsequent deepening associated with the deposition of Warrawoona Group cherts is interpreted to relate to the emplacement of subvolcanic laccoliths and subsequent eruption and/or degassing of these magmas. Heat from these intrusions drove episodes of hydrothermal circulation. Listric normal faulting during caldera collapse produced basins with restricted circulation of seawater. Eruption of volcanogenic emissions into these restricted basins formed brine pools with concentration of the volcanogenic components, thereby providing habitats suitable for early life forms. Fossil stromatolites from two distinct stratigraphic units in the North Pole Dome grew in shallow water conditions, but in two very different geological settings with different morphologies. Stratiform and domical stromatolites in the stratigraphically lower, c. 3490 Ma, Dresser Formation of the Warrawoona Group are intimately associated with barite and chert precipitates from hydrothermal vents, suggesting that component microbes may have been chemoautotrophic hyperthermophiles. Evidence of shallow water to periodically exposed conditions, active growth faulting and soft sediment deformation indicates that the volcanogenic emissions were erupted into a shallow water, tectonically active caldera and concentrated therein to produce an extreme habitat for early life. Widespread conical and pseudocolumnar stromatolites in the c. 3400 Ma, Strelley Pool Chert at the base of the unconformably overlying Kelly Group occur in shallow marine platform carbonates. Silicification was the result of later hydrothermal circulation driven by heat from the overlying, newly erupted Euro Basalt. The markedly different morphology and geological setting of these only slightly younger stromatolites, compared with the Dresser Formation, suggests a diversity of microbial life on early Earth. The biogenicity of putative microfossils from this and younger hydrothermal silica veins in the Warrawoona Group remains controversial and requires further detailed study. Keywords: Archaean; cherts; hydrothermal processes; stromatolites; microfossils; Pilbara Craton

The uppermost part of the Miette Group (Windermere Supergroup) in eastern British Columbia has yielded shelly macrofossils in cliff-forming biostromal carbonates (Byng Formation). The biostromes are made up of two principal elements: intergrowths of complex sinuous, plate-like stromatolites (cf. Platella), and intervening planar to curviplanar pockets filled with packstone and wackestone crowded with Namacalathus and Cloudina, presumed calcified metazoans of uncertain biological affinities. Preservation is best in limestone, and the shells are mostly obliterated where the carbonate is dolomitized. This assemblage was previously known only from the Nama sequence in Namibia. The new find in the antipodal Miette Group in the Canadian Rocky Mountains greatly extends its geographic range, and suggests a more widespread distribution in similar facies in intermediate areas. Both assemblages constitute the earliest occurrences of shelly fossils in their respective regions. Keywords: Cloudina, Namacalathus, stromatolites, biostromes, Miette Group, Ediacaran.

Reef-forming stromatolites have been discovered along the windward shoreline of Chetumal Bay, Belize, just south of the mouth of the Rio Hondo. The reefs and surrounding sediment are formed by the precipitation of submicrocrystalline calcite upon the sheaths of filamentous cyanobacteria, principally Scytonema, under a seasonally fluctuating, generally brackish salinity regime (0%-10%). Well-cemented, wave-resistant buttresses of coalesced stromatolite heads form arcuate or club-shaped reefs up to 42 m long and 1.5 m in relief that are partially emergent during low tide. Oncolitic rubble fields are present between well-developed reefs along the 1.5 km trend, which parallels the mangrove coastline 40-100 m offshore. The mode of reef growth, as illustrated by surface relief and internal structure, changes with increasing water depth and energy, proximity to bottom sediments, and dominant cyanobacterial taxa. Sediment trapping and binding by cyanobacteria are of limited importance to reef growth, and occur only where stromatolite heads or oncolites are in direct contact with the sandy sea floor. Radiocarbon-dated mangrove peat at the base of the reef suggests that it began to form about 2300 yr B.P., as shoreline encrustations that were stranded offshore following storm-induced retreat of the mangrove coast.

Combination of automated ribosomal intergenic spacer analysis (ARISA) and 16S rRNA gene sequencing are used to compare the microbial diversity of the artificial microbialites and that of natural stromatolites. Bacterial clone libraries generated from each of the three artificial microbialite layers and natural stromatolites indicated that the cyanobacterial and crust layers most closely resembled the ecotypes detected in the natural stromatolites and are dominated by Proteobacteria and Cyanobacteria.

Low-magnesium calcite stromatolites and pisoids were found to have precipitated within the leachate collection system piping of a Palm Beach County, Florida, landfill. The stromatolites and pisoids formed in an aphotic and anoxic environment that was at times greatly supersaturated with calcite. The stromatolites are composed of branching cylindrical bundles of concentrically laminated radial fibrous crystals. The pisoids consist of concentric layers of radial fibrous and microcrystalline calcite. Bacteria, likely sulfate reducing, appear to have acted as catalysts for calcite crystal nucleation, and thus the formation of the stromatolites and pisoids. The leachate system stromatolites provide a recent example of stromatolites that formed largely by cement precipitation. By acting as catalysts for calcite nucleation, bacteria may cause more rapid cementation than would have occurred under purely abiotic conditions. Rapid calcite precipitation catalyzed by bacteria has interfered with the operation of the Palm Beach County landfill leachate collection by obstructing pipes and may be an unrecognized problem at other landfill sites. Keywords: stromatolite, landfill, leachate, bacteria.