No description available.

Channing Lake Basin, located west and northwest of Channing, in the Texas Panhandle, is of substantial area and presumably includes lake beds of Pliocene and Pleistocene ages within the Rita Blanca Formation, a member of the Ogallala Group. The foci of this study were a micritic ooid layer and a directly overlying stromatolite layer, which crop out in a canyon approximately 10 kilometers west of Channing. Research was conducted primarily using petrographic and scanning electron microscopy. Significant conclusions include: organic matter was preserved in ooids as filaments and nanoscale spheroids, which served to capture ostracode carapaces within ~10% of micritic cortices; and stromatolites were deposited within an evolving alkaline lacustrine environment producing discernible zones. Potential significance includes improving the understanding the role of organic matter in calcium carbonate precipitation, which has plausible applications in medical, industrial, and academic realms.

The Middle Cambrian Ute Formation includes some 200 m of cyclically alternating carbonates and mud rocks. These are arranged in eight to nine, meter-scale, shallowing-upwards packages, representing deposition under predominantly subtidal conditions. The packages consist of vertical sequences of shale, silty limestone, oncolitic packstone, and oolitic grainstone that exhibit little variance in this general pattern. Small-scale unconformities separate the packages. The inferred depositional environment consists of an intrashelf basin that has a peritidal platform near its margins. The craton, which supplied most of the terrigenous sediment, was situated to the south (Cambrian orientation), and located near the equator. One cycle includes a stromatolite biostrome that is distributed across more than 1500 km2 in northern Utah and southern Idaho. Stromatolites range from mound-like to club-shaped to columnar and reach up to 2 min vertical dimension, and 0.15 min diameter. These large columnar structures were apparently established just basinward of an oolitic shoal. These ancient stromatolites, which are in many ways similar to those stromatolites recently reported from the Bahamas, contain many clues that suggest that they grew in normal marine conditions. These findings require a rethinking of the commonly held belief that Phanerozoic columnar stromatolites are indicators of restricted, hypersaline conditions. Analysis of several orders of laminae in Ute Formation stromatolites indicates periodicity in accumulation from which yearly accumulation rates may be inferred. Values obtained for growth rate range from 4.39-4.88 cm/yr. Such rates of accumulation are in accord with those documented for ancient stromatolites from the Bitter Springs Formation. Thus, even considering the occurrence of hiatal surfaces within the stromatolites, the duration of the columnar-stromatolite horizon probably encompasses 10-2 - 10-3 yr. The biostrome's position in the sequence of cycles and the changes in stromatolite morphology across depositional dip suggest that the biostrome may be essentially isochronous across its outcrop area and, thus, may be viewed as a bioevent horizon. The stromatolites also contribute to a better understanding of the paleogeography of the study area during the Middle Cambrian by providing information on relative energy levels and flow directions. (212 pages)

Modern microbialites are carbonate-precipitating microbial mats and represent the closest living analogues to ancient stromatolites. These ancient carbonate formations are the oldest fossil evidence of life on Earth; however, our comprehension of their relationship to early earth ecosystems relies heavily on understanding the formation of modern microbialites. Research regarding these formation processes has suggested that chemical constraints of CaCO 3 precipitation vary on sub-millimeter spatial scales within the living microbial community. In an attempt to shed light on the importance of these chemical microenvironments, this study focused on understanding the spatial distribution of the organisms and processes involved in the formation of modern microbialites. This was accomplished by isolating five visually distinct layers from the upper 2 – 3 cm of an actively forming microbialite found in the freshwater system of Cuatro Ciénegas, Mexico. Each layer was analyzed using genomic, molecular organic, and stable isotopic techniques. Bacterial diversity was determined by 16S rRNA gene analyses, lipid biomarker content was detected by GC-MS, and carbon isotope composition of organic matter and CaCO 3 were used as indicators of specific microbial processes. Results of the 16S rRNA gene analysis showed that there is little overlap in the community composition of individual layers. Approximately 90% of the ribotypes identified in the microbialite were unique to a single layer. Furthermore, the relative accretion of CaCO 3 at each layer was used to connect the distribution of organisms and processes with two specific zones of CaCO 3 precipitation. The first zone of CaCO3 accretion, which accounted for approximately 55% of total CaCO 3 accumulation, is found in the surface two layers of the microbialites and dominated by photoautotrophic cyanobacteria and algae. The second zone of CaCO 3 precipitation, found at the interior (layers 4 and 5), is composed primarily of heterotrophic proteobacteria and dominated by sulfate-reducing !-proteobacteria. The lipid content of the microbialite reflected the community structure as determined by genomics. Numerous photosynthetic biomarkers were detected and decreased in abundance with depth, indicating the important function of heterotrophic degradation. Additionally, the detection of sulfurized phytol compounds in layer 5 highlighted an important mechanism for the preservation of biogenic signatures, and reflected both the abundance of phototrophic organisms and sulfatereducing bacteria. In combination, these interdisciplinary analyses provided an understanding of microbial community composition and metabolism while indicating the spatial relationship to CaCO 3 formation and the preservation of distinct biochemical signatures. !

Estromatólitos foram estudados em nove localidades ao sul de Itapeva (SP), principalmente em metacalcários dolomíticos cinza-claro e, secundariamente, em metacalcários calcíticos cinza-escuro, do Grupo Itaiacoca, uma unidade vulcanossedimentar mesoproterozóica da Faixa Ribeira. Foram diferenciados em cinco formas colunares, a mais comum consistindo de colunas coniformes, não ramificadas, de diâmetros e alturas centimétricas a decimétricas, atribuídas a Conophyton. As outras quatro formas, com laminação convexa mas não coniforme, diferem em tamanho, silhueta e estilo/freqüência de ramificação. As diferenças na preservação dos estromatólitos são relacionadas ao comportamentos tectônicos distintos entre o metacalcário dolomítico (mais puro), com comportamento competente, e o calcítico (mais argiloso) que atuou de forma mais plástica. Nas melhores exposições desta área os estromatólitos estão agrupados em bioermas de Conophyton, sem indícios de exposição ou retrabalhamento sub-aéreo ou por ondas, evidenciando um ambiente calmo e relativamente profundo, provavelmente abaixo do nível de base de ondas, de talvez até algumas dezenas de metros de profundidade. Conophyton de Itapeva é semelhante a estromatólitos coniformes próximo a Abapã (PR), também no Grupo Itaiacoca, a cerca de 100 km de Itapeva, mas difere de outras formas, incluindo Conophyton cylindricum e C. metulum, de unidades proterozóicas associadas a margem oeste do Cráton do São Francisco. O Conophyton do Grupo Itaiacoca é semelhante a formas na ex-União Soviética que são geralmente encontrados no Mesoproterozóico ou Neoproterozóico inferior, que é consistente com as datações radiométricas disponíveis que colocam esta unidade próximo ao final do Mesoproterozóico. / Stromatolites were studied at nine localities south of Itapeva, São Paulo, Brazil, generally in light-gray metadolostones and secondarily in dark-gray metalimestones of the Itaiacoca Group, a Mesoproterozoic volcanosedimentary unit of the Ribeira Belt. Five columnar forms were distinguished, the most common consisting of unbranched, coniform columns, with centimetric to decimetric diameters and heights, attributed to Conophyton. The other four forms exhibit convex, but not coniform lamination and differ in size, silhouette and style/frequency of branching. Differences in stromatolite preservation are related to the differing tectonic behavious of the purer and more competent metadolostones and the more argillaceous metalimestones which behaved more plastically. In the best exposures in this area the stromatolites are grouped into Conophyton bioherms, without any evidence of subaerial exposure or reworking by waves, which suggests that they formed in a calm and relatively deep setting (perhaps up to several tens of meters in depth), probably below the base of fairweather water. Conophyton from Itapeva is similar to other coniform stromatolites in the Itaiacoca Group near Abapã (Paraná), about 100 km SW of Itapeva, but differs from other forms, including Conophyton cylindricum and C. metulum, from Proterozoic successions associated with the western margin of the São Francisco Craton. The Conophyton from the Itaiacoca Group is most similar to forms in the ex-Sovietic Union that are usually found in the Mesoproterozoic or lowest Neoproterozoic, which is consistent with available radiometric age dates that place this unit near the end of the Mesoproterozoic.

No description available.

Estromatólitos foram estudados em nove localidades ao sul de Itapeva (SP), principalmente em metacalcários dolomíticos cinza-claro e, secundariamente, em metacalcários calcíticos cinza-escuro, do Grupo Itaiacoca, uma unidade vulcanossedimentar mesoproterozóica da Faixa Ribeira. Foram diferenciados em cinco formas colunares, a mais comum consistindo de colunas coniformes, não ramificadas, de diâmetros e alturas centimétricas a decimétricas, atribuídas a Conophyton. As outras quatro formas, com laminação convexa mas não coniforme, diferem em tamanho, silhueta e estilo/freqüência de ramificação. As diferenças na preservação dos estromatólitos são relacionadas ao comportamentos tectônicos distintos entre o metacalcário dolomítico (mais puro), com comportamento competente, e o calcítico (mais argiloso) que atuou de forma mais plástica. Nas melhores exposições desta área os estromatólitos estão agrupados em bioermas de Conophyton, sem indícios de exposição ou retrabalhamento sub-aéreo ou por ondas, evidenciando um ambiente calmo e relativamente profundo, provavelmente abaixo do nível de base de ondas, de talvez até algumas dezenas de metros de profundidade. Conophyton de Itapeva é semelhante a estromatólitos coniformes próximo a Abapã (PR), também no Grupo Itaiacoca, a cerca de 100 km de Itapeva, mas difere de outras formas, incluindo Conophyton cylindricum e C. metulum, de unidades proterozóicas associadas a margem oeste do Cráton do São Francisco. O Conophyton do Grupo Itaiacoca é semelhante a formas na ex-União Soviética que são geralmente encontrados no Mesoproterozóico ou Neoproterozóico inferior, que é consistente com as datações radiométricas disponíveis que colocam esta unidade próximo ao final do Mesoproterozóico. / Stromatolites were studied at nine localities south of Itapeva, São Paulo, Brazil, generally in light-gray metadolostones and secondarily in dark-gray metalimestones of the Itaiacoca Group, a Mesoproterozoic volcanosedimentary unit of the Ribeira Belt. Five columnar forms were distinguished, the most common consisting of unbranched, coniform columns, with centimetric to decimetric diameters and heights, attributed to Conophyton. The other four forms exhibit convex, but not coniform lamination and differ in size, silhouette and style/frequency of branching. Differences in stromatolite preservation are related to the differing tectonic behavious of the purer and more competent metadolostones and the more argillaceous metalimestones which behaved more plastically. In the best exposures in this area the stromatolites are grouped into Conophyton bioherms, without any evidence of subaerial exposure or reworking by waves, which suggests that they formed in a calm and relatively deep setting (perhaps up to several tens of meters in depth), probably below the base of fairweather water. Conophyton from Itapeva is similar to other coniform stromatolites in the Itaiacoca Group near Abapã (Paraná), about 100 km SW of Itapeva, but differs from other forms, including Conophyton cylindricum and C. metulum, from Proterozoic successions associated with the western margin of the São Francisco Craton. The Conophyton from the Itaiacoca Group is most similar to forms in the ex-Sovietic Union that are usually found in the Mesoproterozoic or lowest Neoproterozoic, which is consistent with available radiometric age dates that place this unit near the end of the Mesoproterozoic.

No description available.

Den övergripande tanken med projektet är att ta reda på vad som hände på Västra Gräddmanhällan,utanför Kalix kust, för 2 miljarder år sedan. Det görs genom att ta reda på vad det är för bergarter ochstrukturer som hittats på området och hur de korrelerar till varandra. De två huvudsakliga delarna somprojektet fokuserar på är de stromatolitiska strukturerna som finns i de sedimentära bergarterna på VästraGräddmanhällan och de magmatiska intrusionerna som går genom de sedimentära bergarterna. De skaidentifieras med hjälp av tre olika metoder som ska komplettera varandra. Alla tre metoder kräver attprover från platsen hämtas till Uppsala Universitet där de kan slipas ned till tunnslip. Det kommer finnastotalt åtta prover tagna från olika stromatolitiska strukturer, både från land och Västra Gräddmanhällanoch två stycken tunnslip från de magmatiska intrusioner som tidigare antagits vara antingen kimberliteller lamprofyr. Alla tunnslip kommer att studeras i ett optiskt mikroskop och då kommer vissa tunnslipväljas ut för att undersökas i SEM och sedan Elektronmikrosond. Det finns vissa egenskaper i de olikabergarterna som är avgörande för resultatet. I de stromatolitiska strukturerna är spår av paleobiologiviktigt. De saker som kommer att kunna ge en indikation på tidigare liv i en stromatolit är bland annatorganiskt kol och pyrit. Fynd av antingen opaka mineral, organiskt kol eller andra intressanta struktureri det optiska mikroskopet kommer därför påverka projektets gång. För de magmatiska intrusionerna blirdet istället mineralsammansättning och eventuella karbonater som blir viktiga för hur synen på Sverigesgeologiska historia kommer se ut i fortsättningen. / The overall idea of the project is to find out what happened on the Western Gräddmanhällan, off thecoast of Kalix, two billion years ago. This is done by finding out what rocks and structures that are foundin the area and how they correlate to each other. The two parts that this project focuses on is thestromatolitic structures found of the Western Gräddmanhällan and the igneous intrusions that passthrough the sedimentary rocks. They must be identified using three different methods that willcomplement each other. All three methods require samples from the site to be brought to Uppsalauniversity and then be ground down to thinsections. There will be a total of eight thin sections takenfrom different stromatolitic structures, both from the coast and the western Gräddmanhällan and twopieces of thin sections from the magmatic intrusions that are already assumed to be either kimberlite orlamprophyre. All thin sections will be studied in an optic microscope and then certain thin sections willbe selected to be examined in SEM and then Electron Microprobe. There are certain properties in thedifferent rock types that are decisive for the result. In the stromatolitic structures, traces of paleobiologyare important. The things that will be able to give an indication of past life in a stromatolite includeorganic coal and pyrite. Findings of either opaque mineral, organic carbon or other interesting structuresin the optical microscope will therefore affect the progress of the project. For the magmatic intrusions,it will instead be the mineral composition and possible carbonates that will be important for how theview of Sweden's geological history will look in the future.

No description available.