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At Capitol Reef National Park, Utah, 5-m-high stromatolites are present locally on interdune carbonate lenses in the Early Jurassic Navajo Sandstone. The stromatolites display both finely laminated and fenestral internal fabrics, and grew along south-facing interdune margins. These stromatolites formed during a high-water-table episode engendered by a dune-dammed paleodrainage in a stabilized Navajo erg. These stromatolites, and the thick interdune section associated with them, suggest a hiatus in erg accumulation and the presence of a super bounding surface. Keywords: stromatolite, eolian, Navajo Sandstone, Utah.

The three well-documented occurrences of three-dimensional stromatolites older than 3.2 Ga meet most criteria for biogenicity except the presence of fossil bacteria. However, they also show features more consistent with nonbiological origins. Small conical structures in the Strelley Pool chert in the upper part of the Warrawoona Group (3.5-3.2 Ga), Western Australia, lack the structure typical of stromatolites and probably formed mainly through evaporitic precipitation. A domal structure from the North Pole chert, Warrawoona Group, formed by soft-sediment deformation of originally flat layers. Laminated chert containing domal and pseudocolumnar structures in the Onverwacht Group (3.5-3.3 Ga), Barberton Greenstone Belt, South Africa, extends downward into veins and cavities, where it formed through inorganic precipitation. Although bacterial communities were widespread on Earth prior to 3.2 Ga, these particular three-dimensional structures are probably abiotic in origin and do not provide information on the paleobiology or paleoecology of early organisms. The paucity of Archean stromatolites older than 3.2 Ga probably reflects the paucity of known and possibly extant carbonate deposits of this age.

Upper Silurian (Ludlovian) limestones from the Heceta Formation of southeastern Alaska Alexander terrane) contain skeletal stromatolite reefs and stromatolite mud mounds that were colonized by sphinctozoan sponges. Internal growth cavities, synsedimentary marine cements, and stromatolite debris flows in slope deposits indicate that these reefs grew with relief at the seaward margin of the carbonate platform. The biotas under study have evolutionary significance because they contain the earliest widespread reef-building sphinctozoans and represent a previously unrecognized stage in the evolution of Phanerozoic microbial-metazoan (predominantly poriferan) reefs. On the basis of the presence of Silurian stromatolite-sphinctozoan deposits in southeastern Alaska, southwestern and west-central Alaska, an4 the Urals, this study also establishes a Late Silurian paleobiogeographic connection between the Alexander terrane, cratonic northwestern North America, and the Uralian region.

Thick laminar to columnar stromatolitic growths encrust typical high-energy coralgal reef communities in a French Polynesian outer-reef edge. The reef communities accumulated during vertical growth from 5870 plus or minus 100 yr B.P. to the present, at depths never exceeding 5 m below mean sea level. Oxygen and carbon isotope records and high calcification rates of stromatolite-encrusted corals strongly suggest that for about the past 6000 yr, the local environmental conditions were optimal for reef development. Accordingly, stromatolites can grow in healthy Holocene reefs. Because similar stromatolites have been recorded in other Cenozoic reefs, we suggest that microbial structures may have played a more prominent role in Cenozoic reef growth than is currently recognized.

The 25- to 30-m-thick Algal Member of the Mescal Limestone (middle Proterozoic Apache Group) contains two distinct stromatolitic units: at the base, a 2- to 3-m-thick unit composed of columnar stromatolites and above, a thicker unit of stratiform and pseudocolumnar stromatolites. Columnar forms from the first unit belong to the Group Tungussia, and two new Forms are described: T. mescalita and T. chrysotila. Among the pseudocolumnar stromatolites of the thicker unit, one distinctive new taxon, Apachina henryi, is described. Because of the low stromatolite diversity, the biostratigraphic value of this assemblage is limited. The presence of Tungussia is consistent with the generally accepted isotopic age for the Apache Group of 1200 to 1100 Ma. The Mescal stromatolites do not closely resemble any other known Proterozoic stromatolites in the southwestern United States or northwestern Mexico. Analyses of sedimentary features and stromatolite growth forms suggest deposition on a stable, flat, shallow, subtidal protected platform during phases of Tungussia growth. Current action probably influenced the development of columns, pseudocolumns, and elongate stromatolitic ridges; these conditions alternated with phases of relatively quiet water characterized by nonoriented stromatolitic domes and stratiform stromatolites. Stable conditions favorable for development of the Mescal stromatolites were short-lived and did not permit the development of thick, stromatolite-bearing units such as those characteristic of many Proterozoic sequences elsewhere.

The 80-m-thick Egan Formation preserves sediment deposited during the younger of two episodes of glaciation recorded in the Neoproterozoic succession of the Kimberley region, northwestern Australia. Like many terminal Proterozoic glaciations recorded in Australia and elsewhere, the glacial strata of the Egan Formation are associated with carbonate rocks of likely warm-water affinity, but they are sedimentologically distinct from the marker 'cap carbonate' horizons that overlie glacial strata in other Neoproterozoic successions. The carbonate strata comprise a wide range of facies indicative of shallow-water patch reef, shoal, and lagoonal deposition. Detailed facies analysis of the Egan Formation indicates interruption of the carbonate system by glaciation and subsequent resumption of warm-water conditions. This sedimentological analysis allows a reassessment of the regional stratigraphic correlations proposed for the Egan Formation, which is here considered to record a glacial event younger than the widespread Marinoan glaciation of central and South Australia and, therefore, a speculated third global glaciation in terminal Proterozoic time. Keywords: carbonate rocks, glaciation, Neoproterozoic, stromatolite, Western Australia.

We report for the first time micrometer-scale correlation of geologic and microbial processes in modern marine stromatolites. Precipitation of micritic laminae in these stromatolites was studied by comparing microstructure, as observed in petrographic thin sections, with microbial sulfate-reduction activity. Two-dimensional mapping of sulfate-reduction rates was implemented by incubating a vertical section of a stromatolite face on silver foil coated with [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. Our results show that sulfate-reduction activity is high in zones of Ca[CO.sub.3] precipitation and indicate that microbial activity produces lithified micritic laminae near the surface of the stromatolites. Similarities with micritic laminae in ancient stromatolites suggest that sulfate reduction may also have been an important mechanism of carbonate precipitation in these fossilized structures. Keywords: stromatolites, calcium carbonate precipitation, sulfate reduction, bacteria.