H. J. Hofmann

Origin of 3.45 Ga coniform stromatolites in Warrawoona Group, Western Australia

A new occurrence of conical and branched pseudocolumnar stromatolites in Archean dolostones in the Pilbara region, Australia, contributes significant new morphologic information on such structures. These remains are interpreted as probably representing, in part, microbially mediated accretionary growth surfaces in an Archean hypersaline depositional basin. The structures comprise laterally linked pseudocolumns of centimeter width and decimeter height, with first-order conical laminae of as much as 15 cm of synoptic relief and apical angles of 30°–80°. The conical laminae are modified by a second-order, centimeter-scale, low-amplitude primary corrugate lamination, with crests and troughs occasionally stacked to form satellitic, obliquely directed pseudocolumns; bedding surfaces exhibit a preferred direction of elongation of the cones, an orientation that is orthogonal (and unrelated) to the trend of younger folding; the microstructure is secondary. The stromatolites are better preserved than those previously known from chert in the Warrawoona succession. The remains exhibit certain distinct morphologic attributes corresponding to those in younger stromatolites, such as displayed by Thyssagetes and Jacutophyton , whose biogenicity is generally accepted (although difficult to demonstrate conclusively); the conical Warrawoona forms may represent the oldest known precursor of these taxa.

Ediacaran remains from intertillite beds in northwestern Canada

A turbidite sequence within the Twitya Formation (Windermere Supergroup) in the ackenzie Mountains, northwestern Canada, has yielded an assemblage composed of simple centimetric annuli and discs, which are interpreted as biogenic and referred to Nimbia occlusa Fedonkin, Vendetta ? sp., and Irridinitus ? sp. They occur below a glaciomarine diamictite about 2.5 km below the base of the Cambrian, and they may represent the oldest Ediacaran fossils now known. The find has implications for models that relate the evolution of the Ediacara fauna to the waning of the Varanger glaciation.

Archean Stromatolites as Microbial Archives

Stromatolites are morphologically circumscribed accretionary growth structures with a primary lamination that is, or may be, biologically influenced (biogenic). They are found in Archean sedimentary carbonate rocks, almost always associated with extensive volcanic sequences. Thirty-two occurrences have been reported from n small regional clusters representing the world’s principal preserved Archean cratons: North America 16, Africa 7, Australia 5, Asia 3, and Europe s; none are presently known from Archean rocks of South America and Antarctica; less than two dozen of the occurrences are viewed as definitely Archean and stromatolitic. The earliest stromatolite records date back to nearly 3.5 Ga, and their worldwide distribution and abundance increase as time progresses.

Ediacaran Biota on Bonavista Peninsula, Newfoundland, Canada

Abstract Newly found fossils in the Conception and St. Johns groups of the Bonavista Peninsula considerably extend the known geographic distribution of the Ediacaran fossils in Newfoundland. They occur in deepwater sediments and are preserved as epireliefs, forming census populations underneath volcanic ash layers throughout a more than 1 km thick turbiditic sequence. The exposed fossiliferous units comprise the Mistaken Point, Trepassey, Fermeuse, and Renews Head formations. The remains are tectonically deformed, with long axes of elliptical discs aligned parallel to cleavage strike; shortening of originally circular bedding surface features is on the order of 30–50% (averaging ∼35%). The assemblage includes Aspidella, Blackbrookia, Bradgatia, Charnia, Charniodiscus, Fractofusus, Hiemalora, and Ivesheadia. These occur throughout the succession, with Aspidella being the most common genus, followed by Charnia and Charniodiscus. Four new taxa are described, with candelabra-like fossils with a Hiemalora-like base referred to Primocandelabrum hiemaloranum n. gen. and sp., bush-like fossils to Parviscopa bonavistensis n. gen. and sp., ladder-like fossils to Hadryniscala avalonica n. gen. and sp., and string-like fossils with basal disc to Hadrynichorde catalinensis n. gen. and sp. The remains also include dubiofossils. The stratigraphic ranges of some taxa on the Bonavista Peninsula are longer than previously reported from the Avalon Peninsula, with Fractofusus spindles present in the Trepassey Formation, Bradgatia, Charnia, Charniodiscus, and Ivesheadia reaching as high as the Fermeuse Formation, and Aspidella extending into the middle of the Renews Head Formation. The spindles in the Trepassey Formation are comparable to those found mainly in the stratigraphically older Briscal Formation on the Avalon Peninsula.

Namacalathus-Cloudina assemblage in Neoproterozoic Miette Group (Byng Formation), British Columbia: Canada's oldest shelly fossils

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.

Late Middle to Late Ordovician seismites of Kentucky, southwest Ohio and Virginia: Sedimentary recorders of earthquakes in the Appalachian basin

Synsedimentary ball-and-pillow beds, breccias, and faults in late Middle to Late Ordovician foreland basin rocks of Kentucky, southwest Ohio, and Virginia indicate broad zones of seismicity near the Cincinnati arch and Taconic orogenic front during deposition. Earthquake-induced liquefaction formed seismites, that include ball-and-pillow beds and rare sedimentary breccias that are correlative over large areas (hundreds to thousands of square kilometers). Comparison of these features with liquefaction structures in Holocene sediments indicates that the Ordovician ball-and-pillow beds were probably generated by large earthquakes (magnitudes >6). The Ordovician seismites also provide information about epicenter location and the recurrence interval of large earthquakes in the Ordovician foreland basin. Some were produced by faulting in the foreland and accretionary prism. However, horizons of resedimented lithoclastic breccias in the Jeptha knob cryptoexplosive structure appear to correlate with several ball-and-pillow beds on Jessamine dome, along the Cincinnati arch, suggesting that some of the seismites may be genetically related to this enigmatic structure.

New microfossils from the neoproterozoic (Sinian) Doushantuo Formation, Wengan, Guizhou Province, southwestern China

Well-preserved, three-dimensional microfossils are reported from a 3.7 m thick black phosphorite bed in the Sinian Doushantuo Formation at Wengan Phosphate Mine, Guizhou Province, southwest China. The diverse assemblage, observed in thin sections, comprises planktonic acritarchs, benthic cyanobacteria, bacteria, and thallophytes. Twenty one taxa are recognized, including 1 new genus and 4 new species. Acritarchs include Baltisphaeridium rigidum sp. nov., Vulcanisphaera phacelosa sp. nov., Comasphaeridium magnum Zhang 1984, Cymatiosphaeroides yinii sp. nov., Dicrospinasphaera zhangii gen. et sp. nov., Echinosphaeridium maximum (Yin, 1987) Knoll, 1992, Ericiasphaera sp., ?Ericiasphaera sp., Hocosphaeridium scaberfacium Zang & Walter, 1992, Meghystrichosphaeridium sp., Meghystrichosphaeridium wenganensis Chen & Liu, 1986, Polyhedrosphaeridium sp. The cyanobacteria and bacteria comprise Paratetraphycus giganteus Zhang, 1984, Gloeodiniopsis lamellosa (Schopf, 1968) Knoll & Golubic, 1979, Globophycus sp., Myxoc...

Grain-shaped indices and isometric graphs

ABSTRACT Plotting of grain-shape data on a conventional ternary composition diagram, with long (L), intermediate (I), and short (S) axes of clasts as end members, has advantages over triangular diagrams that employ geometric ratios as primary scales. The isometric (linear) nature of the conventional ternary scale facilitates plotting and interpretation, and the plot provides a basis for comparing the three components over a common denominator (100). Consequently, the graph preserves the information on individual axial values (as a percentage of their sum). The triangle serves as a compilation diagram on which the L-I-S phase space can be subdivided in any number of ways, one of which is the use of selected contour lines of percentage values. In addition, the graph is capable of accommodating t e various grain-shape diagrams (e.g., those of Zingg, Krumbein, and Sneed and Folk) by superposition; this allows for direct, graphic comparison of all such diagrams, and evaluation of their relative efficacy for shape classification. The percentage triangle can be subdivided using a variety of additional functions, such as those that express relative axial uniformity (an entropy-like function), or any other parameter derived from the basic axial values. Employing a scale of percentage values of S versus the form index (L-I) in Cartesian coordinates, superimposed on a conventional isometric composition diagram, yields a ternary plot of great simplicity and high information content.

Archean stromatolites from Abitibi greenstone belt, Quebec, Canada

The oldest fossils in eastern North America are Neoarchean stromatolites discovered near Joutel, in the northern part of the Abitibi Subprovince of the Superior Craton. They are found in allochthonous limestone blocks as much as 2 m across in 2730-2720 Ma volcanic breccia and turbiditic volcaniclastic rocks of the Joutel Volcanic Complex. The stromatolites, which include thyssagetacean and pseudocolumnar forms, represent an assemblage transported to greater depth from biohermal or biostromal complexes formed in shallow-water settings on a submerged volcanic edifice.

Precambrian time units and nomenclature—The geon concept

A geologic calendar using the concept of geon (a time unit of 108 a) is suggested as an alternative to the recently proposed complex nomenclature for Proterozoic periods.