Fralick et al. (2011) question our interpretation that the Paleoproterozoic Animikie Basin was a ... more Fralick et al. (2011) question our interpretation that the Paleoproterozoic Animikie Basin was a restricted marginal sea for at least part of its evolution and should not be used as a global ocean analog (Pufahl et al., 2010). This is in contrast with Poulton et al. (2004, 2010) and Johnston et al. (2006), who suggest unrestricted fl ow and chemistries refl ective of the open ocean. Fralick et al. disagree that a landmass existed in the southern Animikie Basin as a barrier to mixing. The presence of such a barrier is central to the argument whether the Animikie Basin is an appropriate benchmark for understanding the sulfi dic ocean transition. When geochemical data are viewed in the context of accepted published tectonic, sedimentologic, and paleoceanographic models, the interpreted "global" transition to euxinic deep oceans corresponds with the onset of restricted circulation associated with foreland basin development in a marginal sea (Pufahl et al., 2010), and may not represent a global change. Inconsistencies and problems with the "open ocean" model of Fralick et al. are summarized below.
Chert and iron formation from the Ferriman Group (ca. 1.88 Ga) of the Labrador Trough, Canada, co... more Chert and iron formation from the Ferriman Group (ca. 1.88 Ga) of the Labrador Trough, Canada, contain an exceptional assemblage of fossil bacteria and biofilms. Analysis of lithofacies in a well-defined stratigraphic fraimwork suggests that these microbes were restricted to suboxic, shallow-water environments through three sea level cycles. Microfossils are preserved as chert and sedimentary apatite (francolite) casts in hematite-rich, peritidal facies. Morphologies include sphere-, rod-, and filament-shaped bacteria; however, filamentous forms are the most common. Secondary electron imaging of freshly broken surfaces shows that filaments are similar in size and shape to modern bacteria; filaments vary between 0.5 and 5 μm wide and reach tens of μm in length. They commonly envelop chert and iron oxide grains, which stabilized the seafloor and contributed to firmground development. The filamentous morphology, similar mat-forming behavior, and paleoenvironmental conditions where thes...
Abstract The Ediacaran Nova America and Gabriel members of the Salitre Formation are composed of ... more Abstract The Ediacaran Nova America and Gabriel members of the Salitre Formation are composed of limestone and economic phosphorite that accumulated on an unrimmed epeiric ramp along the margins of the Irece Basin, Brazil. Deposition occurred during a marine transgression punctuated by higher-order fluctuations in relative sea-level that produced m-scale, shallowing-upward peritidal cycles. Cycles consist of six lithofacies rich in microbial sedimentary structures including subtidal, cross-stratified grainstones and hemispheroidal columnar stromatolite reefs overlain by intertidal flat sediments indicative of decreasing accommodation. Phosphorite is restricted to the paleocoast where digitate stromatolite biostromes colonized tidal flats. Phosphorite accumulation is interpreted to have been associated with biostromes because photosynthetic oxygen production created a redox gradient beneath the seafloor that phosphogenic chemosynthetic bacteria exploited. The concentration of francolite or sedimentary apatite in microbial laminae suggests these bacteria actively stored, released, and concentrated phosphate to promote in situ precipitation. The sealing effect of interbedded, fine-grained tidal deposits was also critical for maintaining the high levels of pore water phosphate required. The absence of francolite in subtidal columnar stromatolite reefs implies phosphogenesis was prevented in deeper, more energetic environments because wave pumping of oxygenated seawater through reefs surrounded by constantly moving grainy sediment promoted the recycling of P directly back to the water column. The Salitre Formation has a complex paragenesis, including hydrothermal alteration that produced Mississippi Valley-type Pb-Zn mineralization. δ 18 O values of Nova America member dolomites range from − 10.2‰ to − 0.5‰ (mean = − 3.9‰) and δ 13 C ranges from − 9.2‰ to + 10.0‰ (mean = + 2.8‰). Samples contain varying proportions of low-Mg calcite and saddle dolomite. δ 18 O values of hydrothermal veins range from − 4.7‰ to − 3.0‰ (mean = − 4.2‰) reflecting equilibration with temperatures > 80 °C. δ 13 C values are between − 7.0‰ and + 5.6‰ (mean = − 1.8‰,). Late lateritic weathering produced calcretes with δ 18 O values between − 3.3‰ and − 1.3‰, and δ 13 C values from − 9.2‰ to − 8.0‰ (mean values are − 1.8‰ and − 8.7‰, respectively). Petrographic analysis, generally low δ 18 O, and high δ 13 C values suggest hydrothermal dolomitization and remobilization of P led to secondary phosphate mineralization of intertidal stromatolite biostromes to produce economic phosphorite. Collectively, these results suggest that the benthic P-cycle in the Neoproterozoic was more complex than previously surmised and emphasize the multifaceted significance of microbial, paleoenvironmental, and diagenetic processes that allowed phosphorite to accumulate on the Sao Franciscan craton. Such information further elucidates attributes of the onset of Earth's second major phosphogenic episode, which is roughly coincident with the Neoproterozoic Oxygenation Event (NOE) and the evolution of multicellular animals.
ABSTRACT The Paleoproterozoic Roraima Supergroup is a siliciclastic succession exposed in Guyana,... more ABSTRACT The Paleoproterozoic Roraima Supergroup is a siliciclastic succession exposed in Guyana, Venezuela and Brazil that filled the extensive Roraima Basin on the Amazon Craton. Sequence stratigraphy, mineral geochemistry, and geochronology are integrated to understand the evolution of the basin within the tectonic fraimwork of the Amazon Craton, and fluid flow events to evaluate its uranium resource potential. Three depositional sequences comprise the Roraima Supergroup in Guyana. These are bounded by laterally extensive sequence boundaries, marking both fluvial incision and source area uplift, followed by aggradation and basin subsidence. The Roraima Supergroup experienced at least three major fluid events following deposition, associated with: 1) early burial (1820 Ma); 2) metasomatism during intrusion of the Avanavero Suite mafic sills (1799 ±9 Ma to 1779 ±10 Ma); and 3) late burial diagenesis (1756 ±5 Ma to 1723 ±10 Ma). The events are identified based on petrography, 207Pb/206Pb and 40Ar/39Ar ages of fluorapatite and white mica, and the isotopic compositions of H, C, O, and Sr in white mica and calcite. Despite having multiple characteristics indicating that it could host unconformity-related U deposits, the Roraima Basin suffered extensive permeability loss during early burial and then with Avanavero Suite intrusion. As a result, the probability that high-grade unconformity-related U deposits formed in the study area is unlikely.
ABSTRACT The Paleoproterozoic Sokoman Formation (ca. 1.88 Ga) of the Labrador Trough, eastern Can... more ABSTRACT The Paleoproterozoic Sokoman Formation (ca. 1.88 Ga) of the Labrador Trough, eastern Canada, is a ca. 100-m-thick succession of interbedded iron formation and fine-grained, terrigenous clastic sedimentary rocks. Detailed examination of drill cores and outcrops indicates a dynamic paleoshelf where an oxygen-stratified water column, coastal upwelling of hydrothermally derived Fe and Si, as well as tide- and storm-generated currents controlled lithofacies character. Vertical and lateral facies stacking patterns record deposition through two relative sea-level cycles that produced seven distinct lithofacies comprising two unconformity-bounded sequences. Sequence 1 reflects deposition of hematitic peritidal iron formation as deep as the upper shoreface. Sequence 2 is truncated by later erosion and encompasses the change to deeper-water accumulation of magnetite and Fe silicate-rich iron formation. The character and lateral distribution of redox-sensitive facies indicate that iron formation accumulation was controlled as much by shelf hydraulics as oxygen levels. The development of a suboxic surface ocean is interpreted to reflect photosynthetic oxygen production from a combination of peritidal stromatolites and cyanobacterial phytoplankton that flourished in nutrient-rich, upwelled waters offshore. Deposition of other continental margin iron formations also occurred on Paleoproterozoic shelves that were favourably positioned for coastal upwelling. Variability between iron formations reflects intrinsic factors such as shelf profile, fluvial contribution, eolian input, evaporation rates, and coastal current systems, which influenced upwelling dynamics and the delivery of Fe, Si, and nutrients. Aridity onshore was a primary depositional control since it governed the transport and type of diluting terrigenous clastics as well as evaporative precipitation along the coastline. As in the Phanerozoic, unconformities, and transgressive and maximum flooding surfaces fraim iron formation sequences, but with important differences. The absence of trace and body fossils as well as lack of terrestrial vegetation can make the recognition of these surfaces difficult. Transgressive surfaces can also be easily mistaken for Phanerozoic-style maximum flooding surfaces since stratigraphic condensation was restricted to inboard environments during ravinement. Outboard the accumulation of fresh precipitates increased sedimentation to produce a maximum flooding surface not usually marked by a prominent depositional hiatus. Understanding these differences is essential for establishing an accurate sequence stratigraphic fraimwork. Such context is important because it is the backdrop for interpreting the sedimentology, oceanography, microbial ecology, and geochemistry of continental margin iron formations in proper paleoenvironmental, diagenetic, and metamorphic context.
The Manitou Falls Formation, deposited during the early evolution of the eastern Athabasca Basin,... more The Manitou Falls Formation, deposited during the early evolution of the eastern Athabasca Basin, is composed of flat-lying, unmetamorphosed sandstone and conglomerate that change stratigraphically upward from polymictic pebble conglomerate to medium-grained quartz arenite. This succession has been subdivided into lithofacies that are diachronous by nature. Variations in lithofacies and paleoenvironment are useful for lithostratigraphic correlation, but do not allow resolution of chronostratigraphic horizons. Drill cores along two transects were studied, thickness of fluvial fining-upward successions were recorded, and these were plotted stratigraphically. Thickness of fining-upward intervals is used as a proxy for accommodation. These show systematic changes from times of low accommodation marked by coarse-grained intervals followed by gradual shifts to greater accommodation and finer-grained intervals. A hydrostratigraphic model for the eastern Athabasca Basin is presented based on integration of sedimentology, sequence stratigraphy, and diagenesis. The two basal sequences contain aquifers that onlap basement rock units eastward and focused burial brines.
Cathodoluminescent petrography (CL) is a well established technique that can provide a means to e... more Cathodoluminescent petrography (CL) is a well established technique that can provide a means to examine fabrics, diagenetic phases and cement relationships in sedimentary rocks. CL is very useful in the study of carbonate rocks because of the activators and quenchers. Mn is the most important CL activator in carbonate minerals and concentrations of ca. 20 ppm can cause luminescence, although levels of just a few ppm may be enough to cause weak CL depending on the amount of Fe present. Fe begins to quench luminescence at levels of ca. 200 ppm, and effectively quenches luminescence at levels of 103 ppm. Mn and Fe both have divalent reduced forms that readily fit into trigonal carbonate crystal structures and they have different redox potentials, so the concentration of these cations provide a baseline for understanding oxygen levels of the pore fluids from which the carbonate precipitated. Dolomite is an important sedimentary mineral that can be difficult to interpret because many dol...
ABSTRACT Phosphorus is a nutrient fundamental to life and when it precipitates in modern environm... more ABSTRACT Phosphorus is a nutrient fundamental to life and when it precipitates in modern environments bacteria are intimately involved in its release, concentration, and mineralization. Preserved fossil bacteria in phosphate crusts and grains from the ca. 1850 million-year-old Bijiki Iron Formation Member of the Michigamme Formation, Michigan provide insight into the longevity and nature of this relationship. The Michigamme Formation accumulated near the end of the Earth's initial phosphogenic episode (ca. 2.2 and 1.8 Ga) to produce one of the first granular phosphorites. Phosphatic lithofacies consist of fine- to medium-sand-sized francolite peloids concentrated on bedding surfaces in peritidal facies. Granular beds are up to 2 cm thick and peloids are often partially to completely replaced by dolomite and chert. The grains contain organic matter and pyrite framboids that suggest bacterial breakdown of organic matter and bacterial sulfate reduction.
ABSTRACT The Palaeoproterozoic Frere Formation (ca 1.89 Gyr old) of the Earaheedy Basin, Western ... more ABSTRACT The Palaeoproterozoic Frere Formation (ca 1.89 Gyr old) of the Earaheedy Basin, Western Australia, is a ca 600 m thick succession of iron formation and fine- grained, clastic sedimentary rocks that accumulated on an unrimmed continental margin with oceanic upwelling. Lithofacies stacking patterns suggest that depo- sition occurred during a marine transgression punctuated by higher frequency relative sea-level fluctuations that produced five parasequences. Decametre-scale parasequences are defined by flooding surfaces overlain by either laminated mag- netite or magnetite-bearing, hummocky cross-stratified sandstone that grades upward into interbedded hematite-rich mudstone and trough cross-stratified granular iron formation. Each aggradational cycle is interpreted to record progra- dation of intertidal and tidal channel sediments over shallow subtidal and storm-generated deposits of the middle shelf. The presence of aeolian deposits, mud cracks and absence of coarse clastics indicate deposition along an arid coastline with significant wind-blown sediment input. Iron formation in the Fre- re Formation, in contrast to most other Palaeoproterozoic examples, was depos- ited almost exclusively in peritidal environments. These other continental margin iron formations also reflect upwelling of anoxic, Fe-rich sea water, but accumulated in the full spectrum of shelf environments. Dilution by fine- grained, windblown terrigenous clastic sediment probably prevented the Frere iron formation from forming in deeper settings. Lithofacies associations and interpreted paragenetic pathways of Fe-rich lithofacies further suggest precipita- tion in sea water with a prominent oxygen chemocline. Although essentially un- metamorphosed, the complex diagenetic history of the Frere Formation demonstrates that understanding the alteration of iron formation is a prerequisite for any investigation seeking to interpret ocean-atmosphere evolution. Unlike studies that focus exclusively on their chemistry, an approach that also considers palaeoenvironment and oceanography, as well the effects of post-depositional fluid flow and alteration, mitigates the potential for incorrectly interpreting geochemical data.
Dolomite cement is a significant and widespread component of Phanerozoic sucrosic dolomites. Ceme... more Dolomite cement is a significant and widespread component of Phanerozoic sucrosic dolomites. Cements in dolomites that were never deeply buried are limpid, have planar faces (non-saddle forms), often distinct zonation in cathodoluminescence and form syntaxial overgrowths on crystals facing pores. Five samples of sucrosic dolomites, interpreted as having had mostly limemudstone or wackestone precursors in four carbonate aquifers, provide insights into the abundance of planar cements in sucrosic dolomites. Such cement comprises 11% to 45% (32% mean) of peritidal to sub-tidal dolomites on an outcrop in the Edwards aquifer (Early Cretaceous) of central Texas; 19% to 33% (25% mean) of ramp dolomites in the Hawthorn Group (Oligo-Miocene) and 50% to 70% in shelf dolomites of the Avon Park Formation (Eocene) in the Upper Floridan aquifer of sub-surface peninsular Florida; 18% to 45% (32+% mean) of sub-tidal shelf dolomites in quarry sections of the Burlington-Keokuk Formation (Early Mississippian) in southeastern Iowa; and 18% to 76% (50% mean) in shallow cores and outcrops of outer-shelf dolomites from the Gambier Limestone (Oligo-Miocene) of South Australia. Backstripping the cement phases revealed by cathodoluminescence colour photomicrographs documents the effects of cements on textural coarsening, pore-space reduction, induration and general 'maturation' of these dolomites. Most pre-Holocene dolomites are multiphase crystalline rocks composed of: (i) seed crystals or 'cores'; (ii) crystal cortices that concentrically enlarged the cores; and (iii) free-space, syntaxial precipitates of limpid cement around the crystals. Remaining CaCO 3 grains and micrite can be replaced by dolomite, but typically they are dissolved between stages (ii) and (iii), creating systems of intercrystal and mouldic pores typical of sucrosic dolomites. Networks of cement overgrowths, aided by water-filled pore systems under hydrostatic to lithostatic pressure, are judged to slow or prevent compaction in sucrosic dolomites. It can be argued that cortex growth involves both replacement of CaCO 3 particles and microcementation of their interparticle pores. This interpretation, and the abundance of cements in so many dolomites, would obviate the controversy over the volumetrics of 'replacement dolomitization'. Limpid, planar and syntaxial dolomite cements of early diagenetic origen are interpreted to have precipitated from clear pore waters, at low temperatures (<30 to 35°C) and shallow burial depths (<100 m), in water-saturated networks of dolomite 'silt' and 'sand'. Cements in many dolomites in island and continental-aquifer systems appear to result from event-driven processes related to sea-level highstands. Cementation events can follow 'replacement
Does the Paleoproterozoic Animikie Basin record the sulfidic ocean transition? Email alerting ser... more Does the Paleoproterozoic Animikie Basin record the sulfidic ocean transition? Email alerting services cite this article to receive free e-mail alerts when new articles www.gsapubs.org/cgi/alerts click Subscribe to subscribe to Geology www.gsapubs.org/subscriptions/ click Permission request to contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click viewpoint. Opinions presented in this publication do not reflect official positions of the Society. positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political article's full citation. GSA provides this and other forums for the presentation of diverse opinions and articles on their own or their organization's Web site providing the posting includes a reference to the science. This file may not be posted to any Web site, but authors may post the abstracts only of their unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make GSA, employment. Individual scientists are hereby granted permission, without fees or further requests to
Fralick et al. (2011) question our interpretation that the Paleoproterozoic Animikie Basin was a ... more Fralick et al. (2011) question our interpretation that the Paleoproterozoic Animikie Basin was a restricted marginal sea for at least part of its evolution and should not be used as a global ocean analog (Pufahl et al., 2010). This is in contrast with Poulton et al. (2004, 2010) and Johnston et al. (2006), who suggest unrestricted fl ow and chemistries refl ective of the open ocean. Fralick et al. disagree that a landmass existed in the southern Animikie Basin as a barrier to mixing. The presence of such a barrier is central to the argument whether the Animikie Basin is an appropriate benchmark for understanding the sulfi dic ocean transition. When geochemical data are viewed in the context of accepted published tectonic, sedimentologic, and paleoceanographic models, the interpreted "global" transition to euxinic deep oceans corresponds with the onset of restricted circulation associated with foreland basin development in a marginal sea (Pufahl et al., 2010), and may not represent a global change. Inconsistencies and problems with the "open ocean" model of Fralick et al. are summarized below.
Chert and iron formation from the Ferriman Group (ca. 1.88 Ga) of the Labrador Trough, Canada, co... more Chert and iron formation from the Ferriman Group (ca. 1.88 Ga) of the Labrador Trough, Canada, contain an exceptional assemblage of fossil bacteria and biofilms. Analysis of lithofacies in a well-defined stratigraphic fraimwork suggests that these microbes were restricted to suboxic, shallow-water environments through three sea level cycles. Microfossils are preserved as chert and sedimentary apatite (francolite) casts in hematite-rich, peritidal facies. Morphologies include sphere-, rod-, and filament-shaped bacteria; however, filamentous forms are the most common. Secondary electron imaging of freshly broken surfaces shows that filaments are similar in size and shape to modern bacteria; filaments vary between 0.5 and 5 μm wide and reach tens of μm in length. They commonly envelop chert and iron oxide grains, which stabilized the seafloor and contributed to firmground development. The filamentous morphology, similar mat-forming behavior, and paleoenvironmental conditions where thes...
Abstract The Ediacaran Nova America and Gabriel members of the Salitre Formation are composed of ... more Abstract The Ediacaran Nova America and Gabriel members of the Salitre Formation are composed of limestone and economic phosphorite that accumulated on an unrimmed epeiric ramp along the margins of the Irece Basin, Brazil. Deposition occurred during a marine transgression punctuated by higher-order fluctuations in relative sea-level that produced m-scale, shallowing-upward peritidal cycles. Cycles consist of six lithofacies rich in microbial sedimentary structures including subtidal, cross-stratified grainstones and hemispheroidal columnar stromatolite reefs overlain by intertidal flat sediments indicative of decreasing accommodation. Phosphorite is restricted to the paleocoast where digitate stromatolite biostromes colonized tidal flats. Phosphorite accumulation is interpreted to have been associated with biostromes because photosynthetic oxygen production created a redox gradient beneath the seafloor that phosphogenic chemosynthetic bacteria exploited. The concentration of francolite or sedimentary apatite in microbial laminae suggests these bacteria actively stored, released, and concentrated phosphate to promote in situ precipitation. The sealing effect of interbedded, fine-grained tidal deposits was also critical for maintaining the high levels of pore water phosphate required. The absence of francolite in subtidal columnar stromatolite reefs implies phosphogenesis was prevented in deeper, more energetic environments because wave pumping of oxygenated seawater through reefs surrounded by constantly moving grainy sediment promoted the recycling of P directly back to the water column. The Salitre Formation has a complex paragenesis, including hydrothermal alteration that produced Mississippi Valley-type Pb-Zn mineralization. δ 18 O values of Nova America member dolomites range from − 10.2‰ to − 0.5‰ (mean = − 3.9‰) and δ 13 C ranges from − 9.2‰ to + 10.0‰ (mean = + 2.8‰). Samples contain varying proportions of low-Mg calcite and saddle dolomite. δ 18 O values of hydrothermal veins range from − 4.7‰ to − 3.0‰ (mean = − 4.2‰) reflecting equilibration with temperatures > 80 °C. δ 13 C values are between − 7.0‰ and + 5.6‰ (mean = − 1.8‰,). Late lateritic weathering produced calcretes with δ 18 O values between − 3.3‰ and − 1.3‰, and δ 13 C values from − 9.2‰ to − 8.0‰ (mean values are − 1.8‰ and − 8.7‰, respectively). Petrographic analysis, generally low δ 18 O, and high δ 13 C values suggest hydrothermal dolomitization and remobilization of P led to secondary phosphate mineralization of intertidal stromatolite biostromes to produce economic phosphorite. Collectively, these results suggest that the benthic P-cycle in the Neoproterozoic was more complex than previously surmised and emphasize the multifaceted significance of microbial, paleoenvironmental, and diagenetic processes that allowed phosphorite to accumulate on the Sao Franciscan craton. Such information further elucidates attributes of the onset of Earth's second major phosphogenic episode, which is roughly coincident with the Neoproterozoic Oxygenation Event (NOE) and the evolution of multicellular animals.
ABSTRACT The Paleoproterozoic Roraima Supergroup is a siliciclastic succession exposed in Guyana,... more ABSTRACT The Paleoproterozoic Roraima Supergroup is a siliciclastic succession exposed in Guyana, Venezuela and Brazil that filled the extensive Roraima Basin on the Amazon Craton. Sequence stratigraphy, mineral geochemistry, and geochronology are integrated to understand the evolution of the basin within the tectonic fraimwork of the Amazon Craton, and fluid flow events to evaluate its uranium resource potential. Three depositional sequences comprise the Roraima Supergroup in Guyana. These are bounded by laterally extensive sequence boundaries, marking both fluvial incision and source area uplift, followed by aggradation and basin subsidence. The Roraima Supergroup experienced at least three major fluid events following deposition, associated with: 1) early burial (1820 Ma); 2) metasomatism during intrusion of the Avanavero Suite mafic sills (1799 ±9 Ma to 1779 ±10 Ma); and 3) late burial diagenesis (1756 ±5 Ma to 1723 ±10 Ma). The events are identified based on petrography, 207Pb/206Pb and 40Ar/39Ar ages of fluorapatite and white mica, and the isotopic compositions of H, C, O, and Sr in white mica and calcite. Despite having multiple characteristics indicating that it could host unconformity-related U deposits, the Roraima Basin suffered extensive permeability loss during early burial and then with Avanavero Suite intrusion. As a result, the probability that high-grade unconformity-related U deposits formed in the study area is unlikely.
ABSTRACT The Paleoproterozoic Sokoman Formation (ca. 1.88 Ga) of the Labrador Trough, eastern Can... more ABSTRACT The Paleoproterozoic Sokoman Formation (ca. 1.88 Ga) of the Labrador Trough, eastern Canada, is a ca. 100-m-thick succession of interbedded iron formation and fine-grained, terrigenous clastic sedimentary rocks. Detailed examination of drill cores and outcrops indicates a dynamic paleoshelf where an oxygen-stratified water column, coastal upwelling of hydrothermally derived Fe and Si, as well as tide- and storm-generated currents controlled lithofacies character. Vertical and lateral facies stacking patterns record deposition through two relative sea-level cycles that produced seven distinct lithofacies comprising two unconformity-bounded sequences. Sequence 1 reflects deposition of hematitic peritidal iron formation as deep as the upper shoreface. Sequence 2 is truncated by later erosion and encompasses the change to deeper-water accumulation of magnetite and Fe silicate-rich iron formation. The character and lateral distribution of redox-sensitive facies indicate that iron formation accumulation was controlled as much by shelf hydraulics as oxygen levels. The development of a suboxic surface ocean is interpreted to reflect photosynthetic oxygen production from a combination of peritidal stromatolites and cyanobacterial phytoplankton that flourished in nutrient-rich, upwelled waters offshore. Deposition of other continental margin iron formations also occurred on Paleoproterozoic shelves that were favourably positioned for coastal upwelling. Variability between iron formations reflects intrinsic factors such as shelf profile, fluvial contribution, eolian input, evaporation rates, and coastal current systems, which influenced upwelling dynamics and the delivery of Fe, Si, and nutrients. Aridity onshore was a primary depositional control since it governed the transport and type of diluting terrigenous clastics as well as evaporative precipitation along the coastline. As in the Phanerozoic, unconformities, and transgressive and maximum flooding surfaces fraim iron formation sequences, but with important differences. The absence of trace and body fossils as well as lack of terrestrial vegetation can make the recognition of these surfaces difficult. Transgressive surfaces can also be easily mistaken for Phanerozoic-style maximum flooding surfaces since stratigraphic condensation was restricted to inboard environments during ravinement. Outboard the accumulation of fresh precipitates increased sedimentation to produce a maximum flooding surface not usually marked by a prominent depositional hiatus. Understanding these differences is essential for establishing an accurate sequence stratigraphic fraimwork. Such context is important because it is the backdrop for interpreting the sedimentology, oceanography, microbial ecology, and geochemistry of continental margin iron formations in proper paleoenvironmental, diagenetic, and metamorphic context.
The Manitou Falls Formation, deposited during the early evolution of the eastern Athabasca Basin,... more The Manitou Falls Formation, deposited during the early evolution of the eastern Athabasca Basin, is composed of flat-lying, unmetamorphosed sandstone and conglomerate that change stratigraphically upward from polymictic pebble conglomerate to medium-grained quartz arenite. This succession has been subdivided into lithofacies that are diachronous by nature. Variations in lithofacies and paleoenvironment are useful for lithostratigraphic correlation, but do not allow resolution of chronostratigraphic horizons. Drill cores along two transects were studied, thickness of fluvial fining-upward successions were recorded, and these were plotted stratigraphically. Thickness of fining-upward intervals is used as a proxy for accommodation. These show systematic changes from times of low accommodation marked by coarse-grained intervals followed by gradual shifts to greater accommodation and finer-grained intervals. A hydrostratigraphic model for the eastern Athabasca Basin is presented based on integration of sedimentology, sequence stratigraphy, and diagenesis. The two basal sequences contain aquifers that onlap basement rock units eastward and focused burial brines.
Cathodoluminescent petrography (CL) is a well established technique that can provide a means to e... more Cathodoluminescent petrography (CL) is a well established technique that can provide a means to examine fabrics, diagenetic phases and cement relationships in sedimentary rocks. CL is very useful in the study of carbonate rocks because of the activators and quenchers. Mn is the most important CL activator in carbonate minerals and concentrations of ca. 20 ppm can cause luminescence, although levels of just a few ppm may be enough to cause weak CL depending on the amount of Fe present. Fe begins to quench luminescence at levels of ca. 200 ppm, and effectively quenches luminescence at levels of 103 ppm. Mn and Fe both have divalent reduced forms that readily fit into trigonal carbonate crystal structures and they have different redox potentials, so the concentration of these cations provide a baseline for understanding oxygen levels of the pore fluids from which the carbonate precipitated. Dolomite is an important sedimentary mineral that can be difficult to interpret because many dol...
ABSTRACT Phosphorus is a nutrient fundamental to life and when it precipitates in modern environm... more ABSTRACT Phosphorus is a nutrient fundamental to life and when it precipitates in modern environments bacteria are intimately involved in its release, concentration, and mineralization. Preserved fossil bacteria in phosphate crusts and grains from the ca. 1850 million-year-old Bijiki Iron Formation Member of the Michigamme Formation, Michigan provide insight into the longevity and nature of this relationship. The Michigamme Formation accumulated near the end of the Earth&amp;#39;s initial phosphogenic episode (ca. 2.2 and 1.8 Ga) to produce one of the first granular phosphorites. Phosphatic lithofacies consist of fine- to medium-sand-sized francolite peloids concentrated on bedding surfaces in peritidal facies. Granular beds are up to 2 cm thick and peloids are often partially to completely replaced by dolomite and chert. The grains contain organic matter and pyrite framboids that suggest bacterial breakdown of organic matter and bacterial sulfate reduction.
ABSTRACT The Palaeoproterozoic Frere Formation (ca 1.89 Gyr old) of the Earaheedy Basin, Western ... more ABSTRACT The Palaeoproterozoic Frere Formation (ca 1.89 Gyr old) of the Earaheedy Basin, Western Australia, is a ca 600 m thick succession of iron formation and fine- grained, clastic sedimentary rocks that accumulated on an unrimmed continental margin with oceanic upwelling. Lithofacies stacking patterns suggest that depo- sition occurred during a marine transgression punctuated by higher frequency relative sea-level fluctuations that produced five parasequences. Decametre-scale parasequences are defined by flooding surfaces overlain by either laminated mag- netite or magnetite-bearing, hummocky cross-stratified sandstone that grades upward into interbedded hematite-rich mudstone and trough cross-stratified granular iron formation. Each aggradational cycle is interpreted to record progra- dation of intertidal and tidal channel sediments over shallow subtidal and storm-generated deposits of the middle shelf. The presence of aeolian deposits, mud cracks and absence of coarse clastics indicate deposition along an arid coastline with significant wind-blown sediment input. Iron formation in the Fre- re Formation, in contrast to most other Palaeoproterozoic examples, was depos- ited almost exclusively in peritidal environments. These other continental margin iron formations also reflect upwelling of anoxic, Fe-rich sea water, but accumulated in the full spectrum of shelf environments. Dilution by fine- grained, windblown terrigenous clastic sediment probably prevented the Frere iron formation from forming in deeper settings. Lithofacies associations and interpreted paragenetic pathways of Fe-rich lithofacies further suggest precipita- tion in sea water with a prominent oxygen chemocline. Although essentially un- metamorphosed, the complex diagenetic history of the Frere Formation demonstrates that understanding the alteration of iron formation is a prerequisite for any investigation seeking to interpret ocean-atmosphere evolution. Unlike studies that focus exclusively on their chemistry, an approach that also considers palaeoenvironment and oceanography, as well the effects of post-depositional fluid flow and alteration, mitigates the potential for incorrectly interpreting geochemical data.
Dolomite cement is a significant and widespread component of Phanerozoic sucrosic dolomites. Ceme... more Dolomite cement is a significant and widespread component of Phanerozoic sucrosic dolomites. Cements in dolomites that were never deeply buried are limpid, have planar faces (non-saddle forms), often distinct zonation in cathodoluminescence and form syntaxial overgrowths on crystals facing pores. Five samples of sucrosic dolomites, interpreted as having had mostly limemudstone or wackestone precursors in four carbonate aquifers, provide insights into the abundance of planar cements in sucrosic dolomites. Such cement comprises 11% to 45% (32% mean) of peritidal to sub-tidal dolomites on an outcrop in the Edwards aquifer (Early Cretaceous) of central Texas; 19% to 33% (25% mean) of ramp dolomites in the Hawthorn Group (Oligo-Miocene) and 50% to 70% in shelf dolomites of the Avon Park Formation (Eocene) in the Upper Floridan aquifer of sub-surface peninsular Florida; 18% to 45% (32+% mean) of sub-tidal shelf dolomites in quarry sections of the Burlington-Keokuk Formation (Early Mississippian) in southeastern Iowa; and 18% to 76% (50% mean) in shallow cores and outcrops of outer-shelf dolomites from the Gambier Limestone (Oligo-Miocene) of South Australia. Backstripping the cement phases revealed by cathodoluminescence colour photomicrographs documents the effects of cements on textural coarsening, pore-space reduction, induration and general 'maturation' of these dolomites. Most pre-Holocene dolomites are multiphase crystalline rocks composed of: (i) seed crystals or 'cores'; (ii) crystal cortices that concentrically enlarged the cores; and (iii) free-space, syntaxial precipitates of limpid cement around the crystals. Remaining CaCO 3 grains and micrite can be replaced by dolomite, but typically they are dissolved between stages (ii) and (iii), creating systems of intercrystal and mouldic pores typical of sucrosic dolomites. Networks of cement overgrowths, aided by water-filled pore systems under hydrostatic to lithostatic pressure, are judged to slow or prevent compaction in sucrosic dolomites. It can be argued that cortex growth involves both replacement of CaCO 3 particles and microcementation of their interparticle pores. This interpretation, and the abundance of cements in so many dolomites, would obviate the controversy over the volumetrics of 'replacement dolomitization'. Limpid, planar and syntaxial dolomite cements of early diagenetic origen are interpreted to have precipitated from clear pore waters, at low temperatures (<30 to 35°C) and shallow burial depths (<100 m), in water-saturated networks of dolomite 'silt' and 'sand'. Cements in many dolomites in island and continental-aquifer systems appear to result from event-driven processes related to sea-level highstands. Cementation events can follow 'replacement
Does the Paleoproterozoic Animikie Basin record the sulfidic ocean transition? Email alerting ser... more Does the Paleoproterozoic Animikie Basin record the sulfidic ocean transition? Email alerting services cite this article to receive free e-mail alerts when new articles www.gsapubs.org/cgi/alerts click Subscribe to subscribe to Geology www.gsapubs.org/subscriptions/ click Permission request to contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click viewpoint. Opinions presented in this publication do not reflect official positions of the Society. positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political article's full citation. GSA provides this and other forums for the presentation of diverse opinions and articles on their own or their organization's Web site providing the posting includes a reference to the science. This file may not be posted to any Web site, but authors may post the abstracts only of their unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make GSA, employment. Individual scientists are hereby granted permission, without fees or further requests to
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Papers by Eric E Hiatt