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Bruce M Simonson - One of the best experts on this subject based on the ideXlab platform.
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spherule layers near the archean proterozoic boundary
2013Co-Authors: B P Glass, Bruce M SimonsonAbstract:As noted in Chapter 7, strata deposited during a relatively short period of time straddling the Archean-Proterozoic boundary contain an unusually high concentration of spherule layers. At least 4 bodies roughly the size of the end-Cretaceous impactor or larger hit the Earth in a span of some 140 million years (Table 8.1). To date, spherule layers from these impacts have only been found in two limited geographic areas where sedimentary strata are unusually well preserved: the Hamersley Basin of Western Australia and the Griqualand West Basin of South Africa. These two successions are thought to be largely contemporaneous and host some of the largest banded iron formations (BIFs) and iron mines in the world (Fig. 8.1; Beukes and Gutzmer 2008; de Kock et al. 2009). Spherule layers are hosted by both BIFs and associated strata and it appears that most can be correlated between the two basins (Simonson et al. 2009a; Hassler et al. 2011; see Sect. 8.4). The Hamersley layers are described first, then those in the Griqualand West Basin, and finally their correlations are discussed.
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using impact spherule layers to correlate sedimentary successions a case study of the neoarchean jeerinah layer western australia
South African Journal of Geology, 2006Co-Authors: Sarah Joneszimberlin, Bruce M Simonson, David Kreisstomkins, Daniel GarsonAbstract:Spherule layers constitute the only trace of impacts by large extraterrestrial bodies in the early Precambrian. Strata in the Hamersley Basin (Western Australia) contain layers from at least three impacts; the older two date to ~2.63 and ~2.54 Ga and occur in the Jeerinah and Wittenoom Formations respectively. A third formation containing one spherule layer, the Carawine Dolomite, is restricted to an area of the basin where the Wittenoom Formation is absent. Upon discovery, the Carawine and Wittenoom layers were attributed to the same impact, but the subsequent discovery of the Jeerinah layer raised questions about this interpretation. To test it, petrographic characteristics of spherules and irregular particles in the Jeerinah layer at Hesta were quantified by point counting. Minor amounts of quartzo-feldspathic sand are present in both the Jeerinah and Carawine layers and their textural characteristics were also quantified for comparative purposes. The textures of the impact melt particles in the Jeerinah and Carawine layers are indeed very similar to one another and differ from those in the Wittenoom layer. The quartz in the Jeerinah and Carawine layers is broadly similar, but its textural characteristics are more variable. Moreover, the feldspar grains in the Jeerinah layer appear to have a more volcanic provenance than those in the Carawine layer. All of the quartz shows textures typical of plutonic to regionally metamorphosed basement rocks instead of shock metamorphism, affirming its earlier interpretation as detrital sand rather than impact ejecta. Correlations have also been proposed between Hamersley layers and an impact spherule layer in the roughly coeval Monteville Formation (Griqualand West Basin, South Africa) that is locally rich in quartzose sand. Petrographically, the melt population in the Monteville layer is remarkably similar to those of the Jeerinah and Carawine layers, including similar diagenetic histories. The quartzose detritus is again broadly similar but not identical to that of the Hamersley layers. In summary, our petrographic data are consistent with the Jeerinah, Carawine and Monteville layers all being products of a single large impact. However, the quartzo-feldspathic detritus appears to have multiple sources, consistent with its being locally derived detritus. These results suggest petrographic analysis can be used to test correlations in well-dated Precambrian successions that contain impact spherule layers.
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geochemical evidence for an impact origin for a late archean spherule layer transvaal supergroup south africa
Geology, 2000Co-Authors: Bruce M Simonson, Christian Koeberl, Iain Mcdonald, Wolf Uwe ReimoldAbstract:A Late Archean layer rich in sand-sized spherules of former silicate melt in the Monteville Formation (Transvaal Supergroup, South Africa) has Ir concentrations as high as 6.4 ppb and is clearly enriched in Ir relative to associated tuffs, carbonates, and shales. The Monteville spherule layer is also enriched in other siderophile elements, including the platinum group elements (PGEs). The PGEs in the spherule layer produce a flat (meteorite like) pattern when they are normalized to chondritic abundances. The abundances of Ir and other siderophile elements are similar to broadly contemporaneous spherule layers in the Hamersley basin of Western Australia. That the mineral compositions, textures, and sedimentary structures of the spherule layers in the Transvaal Supergroup and Hamersley basin are also very similar suggests that they were all formed by the same processes. We think that the best way to explain the high Ir concentrations and other characteristics of the Monteville spherule layer is that it represents distal impact ejecta. There are, however, significant differences between the Monteville spherule layer and Early Archean spherule layers in the Barberton greenstone belt, including much higher average and maximum concentrations of Ir in the latter. The data presented here clearly show that each Precambrian spherule layer is unique and needs to be characterized individually, as is true for the impact spherule layers of the Phanerozoic.
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discovery of a layer of probable impact melt spherules in the late archaean jeerinah formation fortescue group western australia
Australian Journal of Earth Sciences, 2000Co-Authors: Bruce M Simonson, D Davies, Scott W HasslerAbstract:Individual layers rich in sand-sized spherules interpreted as distal ejecta from Late Archaean to Early Palaeoproterozoic impacts have already been reported from three stratigraphic units in the Hamersley Group of Western Australia: the Wittenoom Formation, the Carawine Dolomite, and the Dales Gorge Member of the Brockman Iron Formation. Here we report the occurrence of a similar layer near the top of the Jeerinah Formation, the uppermost unit in the underlying Fortescue Group. This layer, which we informally name the Jeerinah Impact Layer, was deposited ca 2.63 Ga below wave-base in a marine deep shelf to upper slope environment. As in all other spherule layers, the spherules in the Jeerinah Impact Layer are sand-sized (up to 0.83 mm across), dominantly spheroidal, and show a mix of crystallisation and devitrification textures internally, indicating they are droplets of former low-silica silicate melt. The morphologies of some of the crystallites indicate they were originally plagioclase, yet they now consist of K-feldspar; this, too, is common in all of the other spherule layers. Despite the fact that it is ≤6 mm thick, the Jeerinah Impact Layer pinches and swells laterally and consists of several different subunits internally. This indicates that more than the passive settling of particles in still water was involved in the formation of the Jeerinah Impact Layer, which probably involved traction of coarse sand-sized grains. Despite its thinness, the Jeerinah Impact Layer still represents a thicker accumulation of spherules than all but the most proximal parts of spherule layers formed by some of the largest Phanerozoic impacts, suggesting the Jeerinah Impact Layer represents a major impact. If the Jeerinah Impact Layer and the other spherule layers in the Hamersley Basin did form by impacts, this also means that ejecta from a minimum of three distinct impacts are preserved with age dates suggesting a recurrence interval similar to the inferred rate of impacts in the Phanerozoic. The discovery of the Jeerinah Impact Layer also reconfirms that basinal successions such as black shale are among the most favourable environments for the preservation of distal layers of impact ejecta. It also raises intriguing questions about stratigraphic correlations in the Hamersley Basin of Western Australia and to another spherule layer discovered recently in the Transvaal Supergroup of South Africa.
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Isotopic dating of an Archean bolide impact horizon, Hamersley basin, Western Australia
Geology, 1998Co-Authors: Jon D. Woodhead, Janet M. Hergt, Bruce M SimonsonAbstract:Detailed geologic field work in the Hamersley basin of Western Australia has identified a single horizon that contains predominantly sand-sized spherules similar to those found in impact ejecta such as at the Cretaceous-Tertiary boundary. Evidence suggests that these spherules represent a reworked distal strewn field formed by a large bolide impact in late Archean time. This so-called “spherule marker bed” occurs throughout the main body of the Hamersley basin and in stratigraphically equivalent, but shallower-water lithologies in the northeastern corner. Carbonate constitutes both a matrix component of the spherule marker bed and minor interbeds in the local stratigraphic section. We have utilized carbonate Pb-Pb dating methods to provide, for the first time, an age estimate (2541 +18/−15 Ma) for this important marker bed and therefore of the bolide impact event. Samples from both facies define, within error, an identical isochron age that is also consistent with the few zircon-based age estimates for the Hamersley succession. These findings highlight the great potential of carbonate Pb-Pb geochronology in the dating of Archean and Proterozoic sedimentary rocks.
Scott W Hassler - One of the best experts on this subject based on the ideXlab platform.
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discovery of a layer of probable impact melt spherules in the late archaean jeerinah formation fortescue group western australia
Australian Journal of Earth Sciences, 2000Co-Authors: Bruce M Simonson, D Davies, Scott W HasslerAbstract:Individual layers rich in sand-sized spherules interpreted as distal ejecta from Late Archaean to Early Palaeoproterozoic impacts have already been reported from three stratigraphic units in the Hamersley Group of Western Australia: the Wittenoom Formation, the Carawine Dolomite, and the Dales Gorge Member of the Brockman Iron Formation. Here we report the occurrence of a similar layer near the top of the Jeerinah Formation, the uppermost unit in the underlying Fortescue Group. This layer, which we informally name the Jeerinah Impact Layer, was deposited ca 2.63 Ga below wave-base in a marine deep shelf to upper slope environment. As in all other spherule layers, the spherules in the Jeerinah Impact Layer are sand-sized (up to 0.83 mm across), dominantly spheroidal, and show a mix of crystallisation and devitrification textures internally, indicating they are droplets of former low-silica silicate melt. The morphologies of some of the crystallites indicate they were originally plagioclase, yet they now consist of K-feldspar; this, too, is common in all of the other spherule layers. Despite the fact that it is ≤6 mm thick, the Jeerinah Impact Layer pinches and swells laterally and consists of several different subunits internally. This indicates that more than the passive settling of particles in still water was involved in the formation of the Jeerinah Impact Layer, which probably involved traction of coarse sand-sized grains. Despite its thinness, the Jeerinah Impact Layer still represents a thicker accumulation of spherules than all but the most proximal parts of spherule layers formed by some of the largest Phanerozoic impacts, suggesting the Jeerinah Impact Layer represents a major impact. If the Jeerinah Impact Layer and the other spherule layers in the Hamersley Basin did form by impacts, this also means that ejecta from a minimum of three distinct impacts are preserved with age dates suggesting a recurrence interval similar to the inferred rate of impacts in the Phanerozoic. The discovery of the Jeerinah Impact Layer also reconfirms that basinal successions such as black shale are among the most favourable environments for the preservation of distal layers of impact ejecta. It also raises intriguing questions about stratigraphic correlations in the Hamersley Basin of Western Australia and to another spherule layer discovered recently in the Transvaal Supergroup of South Africa.
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precise zircon u pb ages from the marra mamba iron formation and wittenoom formation hamersley group western australia
Australian Journal of Earth Sciences, 1998Co-Authors: A F Trendall, D R Nelson, J R De Laeter, Scott W HasslerAbstract:The Mt Bruce Supergroup of the Pilbara Craton of Western Australia was laid down in the Hamersley Basin, unconformably over a basement of granite‐greenstone terrane, and consists of the Fortescue, Hamersley and Turee Creek Groups. The base of the ∼230 m‐thick Marra Mamba Iron Formation, the lowest formation of the Hamersley Group, marks the onset of the major banded iron‐formation deposition that characterises the group. Ion microprobe U‐Pb isotope analyses of zircons from a tuff band (NS3) within a high‐grade iron orebody in the uppermost (Mt Newman) member show two distinct, and non‐overlapping, age populations. Fifteen grains from the younger group of 16 have a pooled age of 2597±5Ma (95% confidence), interpreted as the age of syndepositional volcanism; individual grains of the older group of 8, with ages between ca 2950 Ma and ca 2820 Ma, are interpreted as xenocrysts derived from basement rocks transected by the rising magma. The conformably overlying Wittenoom Formation, ∼300–600 m thick, consists l...
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carbonate sedimentology of the early precambrian hamersley group of western australia
Precambrian Research, 1993Co-Authors: Bruce M Simonson, Kathryn A Schubel, Scott W HasslerAbstract:Abstract The early Precambrian Hamersley Group of Western Australia contains both major and minor occurrences of well-preserved carbonate sedimentary rock. The Carawine Dolomite and the middle or Paraburdoo Member of the Wittenoom Formation are the two most extensive occurrences. The Carawine Dolomite is restricted to the eastern part of the Hamersley Basin and contains abundant stromatolites, oncolites, and wave ripples, as well as local occurrences of evaporite crystal pseudomorphs and oolitic to pisolitic textures. These carbonate strata were deposited in a shallow-water paleoenvironment herein referred to as the Carawine Platform. In contrast, the Wittenoom Formation is restricted to the centraland western parts of the Hamersley Basin, and such shallow-water features are entirely absent from its carbonate strata. These strata consist largely of thinly laminated lutite, but also include some thin carbonate turbidites. The carbonate sedimentary rocks of the Wittenoom Formation, as well as some in the Carawine Dolomite which display similar characteristics, are therefore interpreted as sediments that were deposited off-platform in deeper-water paleoenvironments. Paleocurrent, thickness, and grain size trends of the carbonate turbidites indicate they were deposited by paleoflows moving south and west. Even though the Carawine Dolomite lies to the northeast of the Wittenoom Formation, stratigraphic correlations using a newly recognized marker bed of probable impact origin suggest that the carbonate strata of the Carawine Dolomite are younger than the Paraburdoo Member of the Wittenoom Formation. If true, the Carawine Platform could not have been a source of sediment for most of the carbonate sedimentary rocks in the Wittenoom Formation, but it may havebeen the sediment source for similar carbonate turbidites in younger formations such as the Mt. McRae Shale and the Dales Gorge Member of the Brockman Iron Formation. This, in turn, suggests that carbonate sediments were accumulating in the shallower parts of the Hamersley Basin while some of the BIFs were being deposited simultaneously in the deeper parts.
Graham A Logan - One of the best experts on this subject based on the ideXlab platform.
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a reconstruction of archean biological diversity based on molecular fossils from the 2 78 to 2 45 billion year old mount bruce supergroup hamersley basin western australia
Geochimica et Cosmochimica Acta, 2003Co-Authors: Jochen J Brocks, Roger Buick, Roger E Summons, Graham A LoganAbstract:Abstract Bitumens extracted from 2.7 to 2.5 billion-year-old (Ga) shales of the Fortescue and Hamersley Groups in the Pilbara Craton, Western Australia, contain traces of molecular fossils. Based on a combination of molecular characteristics typical of many Precambrian bitumens, their consistently and unusually high thermal maturities, and their widespread distribution throughout the Hamersley Basin, the bitumens can be characterized as ‘probably of Archean age’. Accepting this interpretation, the biomarkers open a new window on Archean biodiversity. The presence of hopanes in the Archean rocks confirms the antiquity of the domain Bacteria, and high relative concentrations of 2α-methylhopanes indicate that cyanobacteria were important primary producers. Oxygenic photosynthesis therefore evolved > 2.7 Ga ago, and well before independent evidence suggests significant levels of oxygen accumulated in the atmosphere. Moreover, the abundance of cyanobacterial biomarkers in shales interbedded with oxide-facies banded iron formations (BIF) indicates that although some Archean BIF might have been formed by abiotic photochemical processes or anoxygenic phototrophic bacteria, those in the Hamersley Group formed as a direct consequence of biological oxygen production. Biomarkers of the 3β-methylhopane series suggest that microaerophilic heterotrophic bacteria, probably methanotrophs or methylotrophs, were active in late Archean environments. The presence of steranes in a wide range of structures with relative abundances like those from late Paleoproterozoic to Phanerozoic sediments is convincing evidence for the existence of eukaryotes in the late Archean, 900 Ma before visible fossil evidence indicates that the lineage arose. Sterol biosynthesis in extant eukaryotes requires molecular oxygen. The presence of steranes together with biomarkers of oxygenic photosynthetic cyanobacteria suggests that the concentration of dissolved oxygen in some regions of the upper water column was equivalent to at least ∼1% of the present atmospheric level (PAL) and may have been sufficient to support aerobic respiration.
Andrew Y Glikson - One of the best experts on this subject based on the ideXlab platform.
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iridium anomalies and shocked quartz in a late archean spherule layer from the pilbara craton new evidence for a major asteroid impact at 2 63 ga comment and reply comment
Geology, 2005Co-Authors: Andrew Y GliksonAbstract:[Simonson (1992)][1], [Simonson and Hassler (1997)][2], and [Simonson et al. (2000)][3] discovered extraterrestrial impact ejecta in the Hamersley and Fortescue Groups, Pilbara Craton, Western Australia, and noted the absence of shocked quartz grains ([Simonson et al., 1998][4]). [Rasmussen and
Wolf Uwe Reimold - One of the best experts on this subject based on the ideXlab platform.
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geochemical evidence for an impact origin for a late archean spherule layer transvaal supergroup south africa
Geology, 2000Co-Authors: Bruce M Simonson, Christian Koeberl, Iain Mcdonald, Wolf Uwe ReimoldAbstract:A Late Archean layer rich in sand-sized spherules of former silicate melt in the Monteville Formation (Transvaal Supergroup, South Africa) has Ir concentrations as high as 6.4 ppb and is clearly enriched in Ir relative to associated tuffs, carbonates, and shales. The Monteville spherule layer is also enriched in other siderophile elements, including the platinum group elements (PGEs). The PGEs in the spherule layer produce a flat (meteorite like) pattern when they are normalized to chondritic abundances. The abundances of Ir and other siderophile elements are similar to broadly contemporaneous spherule layers in the Hamersley basin of Western Australia. That the mineral compositions, textures, and sedimentary structures of the spherule layers in the Transvaal Supergroup and Hamersley basin are also very similar suggests that they were all formed by the same processes. We think that the best way to explain the high Ir concentrations and other characteristics of the Monteville spherule layer is that it represents distal impact ejecta. There are, however, significant differences between the Monteville spherule layer and Early Archean spherule layers in the Barberton greenstone belt, including much higher average and maximum concentrations of Ir in the latter. The data presented here clearly show that each Precambrian spherule layer is unique and needs to be characterized individually, as is true for the impact spherule layers of the Phanerozoic.
