VARIATIONS IN TEXTURAL BANDING OF OOIDS OVER STRATIGRAPHIC HEIGHT OF THE JOHNNIE OOLITE AND THEIR GEOCHEMICAL CORRELATIONS: IMPLICATIONS FOR EDIACARAN PALEOCEAN CHEMISTRY

AbstractThe Stac Fada Member of the Stoer Group, within the Torridonian succession of NW Scotland, is a melt-rich, impact-related deposit that has not been conclusively correlated with any known impact structure. However, a gravity low approximately 50 km east of the preserved Stac Fada Member outcrops has recently been proposed as the associated impact site. We investigate the location of the impact structure through a provenance study of detrital zircon and apatite in five samples from the Stoer Group. Our zircon U–Pb data are dominated by Archaean grains (> 2.5 Ga), consistent with earlier interpretations that the detritus was largely derived from local Lewisian Gneiss Complex, whereas the apatite data (the first for the Stoer Group) display a single major peak at c. 1.7 Ga, consistent with regional Laxfordian metamorphism. The almost complete absence of Archaean-aged apatite is best explained by later heating of the > 2.5 Ga Lewisian basement (the likely source region) above the closure temperature of the apatite U–Pb system (c. 375–450°C). The U–Pb age distributions for zircon and apatite show no significant variation with stratigraphic height. This may be interpreted as evidence that there was no major change in provenance during the course of deposition of the Stoer Group or, if there was any significant change, the different source regions were characterized by similar apatite and zircon U–Pb age populations. Consequently, the new data do not provide independent constraints on the location of the structure associated with the Stac Fada Member impact event.

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Volcanic rocks of the Blake River Group, Abitibi Greenstone Belt, Ontario, and their sulfur content

Canadian Journal of Earth Sciences ◽

10.1139/e77-056 ◽

1977 ◽

Vol 14 (4) ◽

pp. 539-550 ◽

Cited By ~ 6

Author(s):

A. J. Naldrett ◽

A. M. Goodwin

Keyword(s):

Sulfur Content ◽

Greenstone Belt ◽

Geometric Mean ◽

Volcanic Rocks ◽

Smooth Curve ◽

Arithmetic Mean ◽

Arithmetic Means ◽

Basaltic Rocks ◽

Abitibi Greenstone Belt ◽

Stratigraphic Height

Six hundred and ninety samples of volcanic rocks from the Blake River Group of the Abitibi Greenstone Belt have analysed for sulfur on a Leco sulfur analyser. Basaltic rocks have been subdivided into komatiites, Fe-rich tholeiites, Al-rich basalts, and intermediate basalts with more than 1% TiO2 and with less than 1% TiO2. Andesites have been subdivided into Fe-rich types, Al-rich types, and others. All dacites are grouped together as are all rhyolites. Rocks of many of these subdivisions occur at more than one level within the Blake River stratigraphy. Within a given rock subdivision, the sulfur content is distributed log normally. When the geometric mean of the sulfur content of each of the subdivisions outlined above is plotted against the arithmetic mean of the FeO content, a smooth curve is obtained, with sulfur increasing markedly with increase in FeO. The data give no indication of any change in sulfur content of a given rock subdivision with stratigraphic height. The arithmetic mean of the sulfur content of each rock subdivision also increases with the mean FeO content, although less smoothly than the geometric mean. The arithmetic means of sulfur content fall within the scatter of points obtained experimentally for the sulfur content of sulfur saturated basalts, supporting the contention that the Blake River rocks may have been saturated with sulfur at the time of their extrusion.

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GEOCHEMISTRY OF THE YELLOWKNIFE VOLCANIC ROCKS

Canadian Journal of Earth Sciences ◽

10.1139/e66-002 ◽

1966 ◽

Vol 3 (1) ◽

pp. 9-30 ◽

Cited By ~ 36

Author(s):

W. R. A. Baragar

Keyword(s):

Bulk Composition ◽

Volcanic Rocks ◽

River Section ◽

Magma Chambers ◽

Tholeiitic Magma ◽

Composite Form ◽

High Level ◽

Mafic Lavas ◽

Stratigraphic Height ◽

Volcanic Cycles

Results of rapid-method chemical analyses of samples taken at about 500-ft stratigraphic intervals through two sections of Yellowknife Group volcanic rocks are presented in graphical and composite form. The Yellowknife section is about 40 000 ft thick with the base undefined; the Cameron River section, about 45 mi northeast, is about 7 000 ft thick and may be complete.Two aspects of the volcanic chemistry considered are (1) variation in composition with stratigraphic height; (2) bulk composition of the volcanic rocks.Chemical data of the Yellowknife section define two volcanic cycles in each of which mafic lavas show a small but significant increase in sialic components with stratigraphic height culminating abruptly in acidic layers. The Cameron River section shows a similar but less well-defined trend. Iron–magnesium ratios stage a succession of systematic increases, each persisting for a few thousand stratigraphic feet, but no overall systematic variation. The two types of chemical variation correspond to calc-alkali and tholeiitic differentiation trends respectively. The tholeiitic trend is attributed to fractionation in high-level magma chambers, demonstrated for Yellowknife magma by the Kam Point sill, and the calc-alkali trend to contamination of tholeiitic magma by sialic crust.Frequency distribution diagrams show Yellowknife volcanic rocks to be similar to Chayes' circumoceanic basalts in TiO2, CaO, and MgO and to his oceanic basalts in Al2O3. The characteristic rock type is basalt.

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Plinian eruptions and their impact on human settlements: stratigraphy of the 79 AD Vesuvius fall deposits and detailed study of their downwind and substrate-induced variations inside the archaeological excavations of Pompeii and Stabiae (southern Italy)

10.5194/egusphere-egu21-11035 ◽

2021 ◽

Author(s):

Giulia Chiominto ◽

Claudio Scarpati ◽

Annamaria Perrotta ◽

Domenico Sparice ◽

Lorenzo Fedele ◽

...

Keyword(s):

Progressive Collapse ◽

Field Investigation ◽

Complete Sequence ◽

Archaeological Sites ◽

Pyroclastic Material ◽

Plinian Eruptions ◽

Eruptive Centre ◽

Stratigraphic Height ◽

Increasing Load ◽

Fall Deposits

Plinian eruptions are highly energetic events that release cubic kilometres of magma in the form of pyroclastic material (pumice, lithic clasts and ash). These products tend to accumulate near the vent with considerable thickness. The rapid burial of the territory around the eruptive centre makes these eruptions extremely dangerous. For this purpose, the renowned 79 AD Vesuvius eruption, which destroyed the ancient cities of Pompeii and Stabiae (where Pliny the Elder founds his death) located respectively 10 and 15 km from the vent, was studied in detailed. The recent excavations carried out in collaboration with the Archaeological Park of Pompeii, both in Pompeii and in the Stabian villas, have shown the complete sequence of products of the 79 AD eruption that destroyed and covered these Roman cities. The discovery of thick sequences of reworked material accumulated during previous excavations, testifies for the presence of underground tunnels dug for the Royal House of Bourbon. Fall products of the 79 AD eruptive sequence, accumulated during the main Plinian phase and the successive sustained column phases, were studied in detail to investigate their sedimentological characteristics and how these were influenced by anthropic structures. Results from field investigation show that in both archaeological sites, fall deposits consist of white and grey pumice lapilli in the lower part of the eruptive sequence (units A and B), and of thin, lithic-rich layers interstratified to ash products emplaced by pyroclastic currents, in the highest part of the pyroclastic deposit (units D, G1, G3, I). A new thin lithic-rich layer (X2) has been observed near the top of the sequence at Stabiae. The internal structure of the Plinian pumice lapilli deposit appears weakly stratified in open areas, while it is strongly stratified near steep roofs (e.g., impluvium areas), where the deposit thickens. The observed stratification is confirmed by a significant variation of sedimentological parameters with the stratigraphic height (e.g., median ranging from -3.5 to -0.1&#61542;), possibly related to fluctuations in the eruptive parameters. Locally, rolling of pyroclastic clasts on sloped roofs produced a well-stratified deposit with laterally discontinuous layers and rounded clasts. Several roofing-tiles, either intact or in fragments, were recovered at various stratigraphic heights in the pumice lapilli deposit both at Pompeii and Stabiae. &#160;These tiles testify for the progressive collapse of the roofs under the increasing load of the falling lapilli clasts.

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Testing for a mass extinction without selecting taxa

Paleobiology ◽

10.1666/0094-8373(2000)026<0647:tfamew>2.0.co;2 ◽

2000 ◽

Vol 26 (4) ◽

pp. 647-650 ◽

Cited By ~ 16

Author(s):

Andrew R. Solow ◽

Woollcott K. Smith

Keyword(s):

Statistical Inference ◽

Mass Extinction ◽

Extinction Event ◽

Mass Extinction Event ◽

Extinction Events ◽

Mass Extinction Events ◽

Stratigraphic Height ◽

Selection Of

Statistical inference about mass extinction events is commonly based on the pattern of fossil finds among a group of taxa. An important issue for existing methods is the selection of taxa for inclusion in the analysis. A common approach is to select taxa on the basis of the stratigraphic height of their uppermost finds. This approach creates a bias in favor of detecting a mass extinction event. This paper describes and illustrates an approach that avoids this problem.

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The Neoproterozoic geomagnetic field: new insights from a high-resolution paleomagnetic study in South China

10.5194/egusphere-egu21-4671 ◽

2021 ◽

Author(s):

Justin Tonti-Filippini ◽

Boris Robert ◽

Élodie Muller ◽

Michael Wack ◽

Xixi Zhao ◽

...

Keyword(s):

South China ◽

Geomagnetic Field ◽

Alternative Explanation ◽

Rotation Axis ◽

Sedimentary Rocks ◽

Polar Wander ◽

True Polar Wander ◽

Paleomagnetic Study ◽

Stratigraphic Height ◽

Independent Motion

The paleomagnetic record during the middle Neoproterozoic (~825-780 Ma) displays rapid apparent polar wander variations leading to large discrepancies in paleogeographic reconstructions. Some authors propose that these data may represent true polar wander events, which correspond to independent motion of the mantle and lithosphere with respect to Earth&#8217;s rotation axis. An alternative explanation might be a perturbation of the geomagnetic field, such as a deviation from a predominantly dipole field or a hyper-reversing field. To test these hypotheses, we sampled 1200 oriented cores over a stratigraphic height of 100 metres in sedimentary rocks of the 820-810 Ma Laoshanya Formation in South China. We will present preliminary paleomagnetic and rock magnetic analyses together with results of petrologic and geochemical experiments to better understand the origin of the paleomagnetic signal.

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Testing for the effects of depositional rates in multiproxy models of environmental and faunal change: the Silurian Lau δ13C excursion

10.5194/egusphere-egu2020-7615 ◽

2020 ◽

Author(s):

Emilia Jarochowska ◽

Madleen Grohganz ◽

Niklas Hohmann ◽

Axel Munnecke

Keyword(s):

Trace Metal ◽

Metal Content ◽

Deposition Rates ◽

Continuous Increase ◽

Primary Element ◽

Trace Metal Content ◽

Measured Element ◽

Temporal Rate ◽

Deposition Models ◽

Stratigraphic Height

The Silurian is characterized by several extinction events, among them the late Ludfordian Lau event, which mainly affected conodont and graptolite communities. This event was followed by the strongest global positive &#948;13C excursion of the Phanerozoic. This event has been first recognized in the classical succession in Gotland, Sweden, where a continuous increase in &#948;13C values of up to nearly 9 &#8240; from the upper N&#228;r Formation to the Eke Formation is observed. It has been attributed to large scale carbon cycle perturbations. This time period is also characterized by a regression and associated changes in sedimentation and deposition rates. This raises the question to what extent these factors contribute to the observed changes in faunal diversity and geochemical proxies.A mechanism linking changes in seawater chemistry and the faunal turnover has been proposed based on a high abundance of malformed acritarchs observed during the onset phase of some Phanerozoic isotope excursions including the Lau isotope excursion. Malformations during the late Silurian Pridoli event coincide with a significant increase in trace metal content measured in fossils and host rock, which suggests the teratology to be caused by metal pollution. However, also in the case on an increase in the trace metal content the contribution of changing depositional rates has not been quantified. &#160;Models developed in the field of stratigraphic paleobiology have demonstrated that changing deposition rates have a substantial influence on the stratigraphic distribution of fossils. In the same sense, element concentrations can be altered by changing deposition rates, which may dilute or condense the primary element signal. For this study, concentrations of different trace elements were measured across a profile in an outcrop Bodudd (Gotland) which exposes the Lau isotope excursion from the upper N&#228;r to the Eke Formation. Using a newly developed statistical method, the effects of changing deposition rates were quantified and the measured element signal corrected for these effects. This method uses a deposition model to transform the measured element signal, which is a function of the stratigraphic height in the outcrop, back into a temporal rate. The temporal rate reflects the primary element signal and is defined as a function of time instead of stratigraphic height. Thus, the effect of changing deposition rates is accounted for. Approximate deposition models were created based on Th concentrations measured across the profile, which act as a proxy for the rate of terrestrial input, and based on the chitinozoan yield, which reflects deposition rates. Pre- and post-transformation element patterns were compared for different deposition models and evaluated with respect to their ability to preserve element peaks.

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Cryptic variations of minor elements Al, Cr, Ti and Mn in Lower and Critical Zone orthopyroxenes of the Western Bushveld Complex

Mineralogical Magazine ◽

10.1180/minmag.1993.057.387.08 ◽

1993 ◽

Vol 57 (387) ◽

pp. 257-264 ◽

Cited By ~ 8

Author(s):

H. V. Eales ◽

B. Teigler ◽

W. D. Maier

Keyword(s):

Electron Microprobe ◽

Bushveld Complex ◽

Major Elements ◽

Critical Zone ◽

Basaltic Rocks ◽

Minor Elements ◽

Compositional Variations ◽

Ultramafic Cumulates ◽

Stratigraphic Height

AbstractCompositional variations with respect to minor elements A1, Cr, Ti and Mn, and major elements Fe and Mg, in orthopyroxenes along ca. 160 km of strike of the Lower (LZ), Lower Critical (LCZ) and Upper Critical (UCZ) Zones are reviewed on the basis of 1900 analyses by electron microprobe. AI increases with stratigraphic height and declining Mg/(Mg + Fe2) ratios (hereafter MMF ratios) through the LZ and LCZ, reaching peak values close to the base of the UCZ, where the first cumulus plagioclase appears in the succession. Above this, Al contents decline as MMF ratios decline. Through the same interval, subdued increase in Ti occurs through >1000 m of ultramafic cumulates, but this increase accelerates within the ca. 450 m UCZ sequence. Mn increases linearly with declining MMF ratios through the entire succession, Cr levels are highest in orthopyroxenes of the ultramafic LZ and LCZ, and olivine norites of the UCZ, but decline in more evolved norites and associated anorthosites of the UCZ.This pattern of cryptic variations, displayed by a thick succession of cumulates, is consistent with the model of Bence and Papike (1972) and Grove and Bence (1977) for basaltic rocks, which links the levels of minor elements in pyroxenes with entry of plagioclase into the paragenesis.

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An ocean-ridge type magma chamber at a passive volcanic, continental margin: the Kap Edvard Holm layered gabbro complex, East Greenland

Geological Magazine ◽

10.1017/s001675680001952x ◽

1992 ◽

Vol 129 (4) ◽

pp. 437-456 ◽

Cited By ~ 22

Author(s):

Stefan Bernstein ◽

Minik T. Rosing ◽

C. Kent Brooks ◽

Dennis K. Bird

Keyword(s):

Magma Chamber ◽

Continental Margin ◽

Basaltic Magma ◽

Olivine Gabbro ◽

Restricted Range ◽

Layered Series ◽

Field Evidence ◽

Mineral Compositions ◽

Intrusive Suite ◽

Stratigraphic Height

AbstractThe gabbros of the Tertiary Kap Edvard Holm Layered Serieshave a stratigraphic thickness of more than 5000 m. Earlier work has shown that the range in cumulus mineral compositions is restricted (plagioclase An81—An51; olivine Fo85—Fo66; pyroxenes Ca43Mg46Fe11 to Ca43Mg37Fe20). Field evidence of magma injections is common, which together with the restricted range in mineral chemistry suggests that the magma chamber was frequently replenished by a less fractionated magma. A detailed study of a 600 m section (900–1500 m) in the Lower Layered Series reveals a period of crystallization when the magma chamber behaved as a closed system (900–1300 m). The rocks formed during this periodare well-laminated olivine–gabbros (900–110 m), which evolved to well-laminated oxide-gabbros (1100–1300 m). Compositional trends in the cumulusminerals are towards more evolved compositions (plagioclase An64—An58, pyroxene Mg# from 80 to 76) with stratigraphic height. From 1300 m to 1500 m, granular olivine-gabbros dominate, with moreprimitive mineral compositions (plagioclase An67—An76, pyroxene Mg# from 78 to 82). The transition olivine–gabbro to oxide-gabbro at 1100m is a consequence of fractional crystallization, and it is shown how changes in activities of FeO and Fe203 in the magma are reflected in the total iron content of plagioclases.The transition from oxide-gabbro to olivine-gabbro at 1300 m results from replenishment by less evolved basaltic magma. On the basis of calcic pyroxene chemistry and the mineral crystallization sequence it is concluded that the Kap Edvard Holm Layered Series crystallized from a tholeiitic magma similar to MORB. Melanogabbroic units occur throughout the intrusion as discordant to subconcordant sill-like bodies 0.2–2.0 m thick. The melanogabbroic units consist of Cr-rich augite-olivine-plagioclase heteradcumulates and contain deformed mica crystals of pre-emplacement origin. These units crystallized from a wet, MgO-rich magma which was injected into the layered host gabbros after the formation of the cumulus pile, but before the magma was completely solidified.The Kap Edvard Holm Layered Series has several parallels with the plutonic part of ophiolite sequences. These include: cumulus mineral assemblage, compositions of the minerals and the restricted range in compositions with stratigraphic height; field evidence of repeated replenishment of basaltic magma; dyke swarms overlying the roof zone of the magma chamber; and the existence of a late intrusive suite of wet, MgO-rich magma. These parallels suggest that the processes involved in the formation of the Kap Edvard Holm Layered Series were similar to those involved in the formation of the crustalpart of many ophiolites and beneath present-day spreading ridges. The Kap Edvard Holm Layered Series is therefore believed to represent a shallow-level magma chamber which acted as a reservoir for basaltic flows at the continental margin during the opening of the North Atlantic Ocean.

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Architecture of a river-dominated, wave- and tide-influenced, pre-vegetation braid delta: Cambrian middle member of the Wood Canyon Formation, southern Marble Mountains, California, U.S.A.

Journal of Sedimentary Research ◽

10.2110/jsr.2020.023 ◽

2020 ◽

Vol 90 (9) ◽

pp. 1011-1036

Author(s):

Jason G. Muhlbauer ◽

Christopher M. Fedo

Keyword(s):

Mojave Desert ◽

Fluid Mud ◽

Delta Front ◽

Sand Waves ◽

Motion Models ◽

Facies Associations ◽

Surface Measurements ◽

Marine Influence ◽

Middle Member ◽

Stratigraphic Height

ABSTRACT Across the Mojave Desert of southeastern California, outcrops of the Cambrian middle member of the Wood Canyon Formation preserve the deposits of pre-vegetation braided-fluvial and braid-delta environments. One 78-meter-thick section in the southern Marble Mountains, documented here through detailed stratigraphic logging, facies analysis, architectural panels based on “Structure-from-Motion” models, and a suite of paleocurrent and accretion-surface measurements, provides insight into the development of a river-dominated, wave- and tide-influenced braid delta at the distal end of a continent-scale braidplain. In contrast to other pre-Devonian braid-delta strata, in which mudrock is largely absent, the greater part of the middle member system contains over 5% mudstone. Four facies associations, FA4–7, constitute the middle member section and represent (in order of stratigraphic height) a braidplain-to-delta transition (FA4), proximal braid delta (FA5), distal braid delta (FA6), and upper braid-delta front (FA7). The 20 meters of braidplain-to-delta transition strata are largely similar to those of fluvial middle member sections, containing approximately 2% mudstone, unimodal north-northwest paleoflow, and vertical, downstream, and downstream-lateral accretion elements representing compound barforms and channel fills. Above, each braid-delta facies association (FA5–7) preserves high-sinuosity paleocurrent indicators, 6–12% mudstone, and symmetrical, wave-formed sand waves. Decimeter-thick fluid-mud deposits found chiefly in FA6 and less commonly in FA7 indicate the presence of a turbidity-maximum zone that records brackish-water conditions in the distal braid delta. Trace fossils concentrated in FA7 suggest that metazoans were confined to the upper braid-delta front and could not tolerate the variable salinity of the braid delta. Increased marine influence with stratigraphic height requires gradual transgression during deposition of the middle member of the Wood Canyon Formation, possibly as part of a lowstand systems tract.

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