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321.
Ian J. Ferguson Anne‐Christine Da Silva Nancy Chow Annette D. George 《Basin Research》2021,33(1):312-341
Frasnian reef complexes along the northern margin of the Canning Basin in northwestern Australia evolved during rifting of the Fitzroy Trough. Geological investigations of the Frasnian Hull platform, which developed on an active tilted fault‐block, reveal significant lateral and vertical facies variations superimposed on prominent metre‐scale cyclicity. This study uses numerical analyses of facies and magnetic susceptibility data from three measured sections along the Hull platform to test whether a tectonic signal can be distinguished from eustatic and other signals. Geostatistical analysis of facies variations reveals an exponential distribution of thin (<3 m) facies, characteristic of stochastic depositional processes. Thick subtidal facies predominate in the Guppy Hills (GH) and southeastern Hull Range (SHR) sections near the hangingwall margin, and thick shallow‐subtidal to intertidal facies dominate the Horse Springs drillcore (HD 14) section near the footwall margin. Power and wavelet spectral analyses indicate a strong periodic component; Average Spectral Misfit and spectral optimisation methods confirm the presence of Milankovitch eccentricity signals and suggest the presence of obliquity and precession signals. However, the results also expose strong temporal and spatial variation providing evidence for tectonic control. Spectral analyses show strongest periodicity is recorded in short intervals that are not correlated across the platform and provide evidence of variations in sedimentation rate and hiatuses. Time series for the neighbouring GH and SHR sections show no overall statistical correlation, and Markov analysis indicates weakly ordered vertical facies transitions that do not correlate across the platform. Subtidal to intertidal facies data from HD 14 core suggest that at least 35% of the section is absent, almost obscuring the Milankovitch signal. The results indicate a complex set of controls on deposition on the Hull platform with local tectonic effects having produced spatio‐temporal moderation of the underlying eustatic signals and autogenic processes adding a localised stochastic response. 相似文献
322.
Nancy G. Newberry Donald R. Peacor Eric J. Essene John W. Geissman 《Contributions to Mineralogy and Petrology》1982,80(4):334-340
Magnetite-ilmenite oxidation-exsolution intergrowths from an original titanomagnetite microphenocryst from an ash flow tuff unit have been studied using conventional transmission electron microscopy and analytical electron microscopy. Silicon has been found to be in solid solution in all of the magnetite studied and in some of the coexisting ilmenite. The average value in magnetite is 1.2 wt.% Si, equivalent to solid solution of 9 mole % Fe2SiO4. Silicon is also present in very small silicate inclusions and as unusual Si-rich domains of uncertain origin in magnetite. The inclusions and domains may be irregularly distributed through the magnetite in sizes well below those resolvable with the electron microprobe. Microprobe analyses for Si in magnetite generally reflect these heterogeneities in addition to a component presumably in solid solution. The petrologic implications of the data can be assessed only when relevant thermochemical data become available and the distribution of Si in magnetite is better understood.Contribution No. 376 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan 相似文献
323.
Nancy E. Williams John A. Westgate D. Dudley Williams Anne Morgan Alan V. Morgan 《Quaternary Research》1981,16(2):146-166
Larval caddisfly, chironomid, and beetle remains have been recovered from the Pleistocene Scarborough Formation in the Toronto region of southern Ontario. Three stratigraphic levels were sampled at the northeastern end of the Scarborough Bluffs; the youngest horizon yielded 16 chironomid taxa, 33 caddisfly taxa, and 28 beetle taxa, whereas the two older levels yielded somewhat less diverse assemblages. Only one taxon in each of the caddisfly and chironomid groups was identified from the presumed correlative beds at Woodbridge, Ontario, but numerous beetle fragments were recovered, several of which have been specifically identified and match species found previously in the upper part of the Scarborough Formation. The youngest sampled assemblage in the Scarborough Formation at the northeastern end of the Scarborough Bluffs is interpreted as indicating cool climatic conditions in a boreal forest environment, given the present-day distributions and feeding habits of these river, lake, and terrestrial taxa. The mean July temperature at this time was probably about 15°C, as compared to the present-day value of 20.5°C. The forest was poorer in deciduous species during deposition of the older part of the formation as preserved here. These results agree well with previous interpretations based on plant remains. We suggest that both aquatic and terrestrial insects are good indicators of macroclimate. 相似文献
324.
Regulations for boaters can help mitigate adverse effects on threatened marine mammals. One management tool to protect endangered North Atlantic right whales is a 460-m distance restriction rule for all vessels. This study is the first effort to analyze factors that influence recreational boaters’ intentions to comply with this regulation. Using the theory of planned behavior, we analyzed 362 mail surveys of recreational boaters using the offshore waters of the southeastern United States. We found that two constructs of the theory significantly explained 58% of the intention to comply with the rule: (1) positive attitude toward the rule and (2) stronger belief that other people are complying. Boaters recommended increasing knowledge about whales to improve compliance, but they were divided with respect to increasing fines for violators to increase compliance. This information can be useful for designing outreach strategies to protect whales. 相似文献
325.
Petrogenesis of the Swartruggens and Star Group II kimberlite dyke swarms,South Africa: constraints from whole rock geochemistry 总被引:1,自引:0,他引:1
Nancy Coe Anton le Roex John Gurney D. Graham Pearson Geoff Nowell 《Contributions to Mineralogy and Petrology》2008,156(5):627-652
Thirty-seven samples from the Swartruggens and Star Group II kimberlite dyke swarms, emplaced through the Kaapvaal craton,
have been analysed for their major and trace element and Sr, Nd and Hf isotope compositions. The samples are all MgO-rich
(~12–35 wt%) with high Mg# (0.72–0.90) and Ni (~610–2700 ppm) contents. The kimberlites are strongly enriched in incompatible
elements (Zr = 140–668 ppm; La = 124–300 ppm; Nb = 68–227 ppm; Ba = 1500–7000), and have high and variable chondrite normalised
La/Yb ratios (Swartruggens = 94 ± 21; Star = 202 ± 36). 87Sr/86Sr (0.70718–0.71050) ratios are elevated, whereas εNd (−11.95 to −7.84) and 176Hf/177Hf ratios (0.282160–0.282564) are low. Inter- and intra-dyke compositional variation is significant, and there are systematic
differences between the kimberlites found at the two localities. Intra-locality differences can largely be attributed to a
combination of the effects of alteration, crustal contamination, macrocryst entrainment and phenocryst fractionation. There
is some evidence for distinct parental magmas formed through variable and low degrees (0.5–2%) of partial melting, as illustrated
by crossing rare earth element patterns. The Star kimberlites have derived from a less radiogenic source, with higher LREE
enrichment than the Swartruggens kimberlites. Inferred primary magmas at each locality have high Mg# (~0.83), are Ni-rich
(850–1220 ppm) and are strongly enriched in incompatible elements. Calculated mantle source compositions are strongly enriched
in incompatible elements (La/Ybn ~ 10–50), but refractory in terms of Mg# and Ni contents. Incompatible element ratios such as Ba/Nb (>13.5), La/Nb (> 1.1)
and Ce/Pb (< 22) are unlike those characteristic of Group I kimberlites or ocean island basalts, but indistinguishable from
calc-alkaline magmas. Taken together with extremely low εNd and εHf, these compositional characteristics are used to argue
for derivation of these Group II kimberlite magmas from the deep subcontinental lithospheric mantle, metasomatised during
the Proterozoic by calc-alkaline fluids/melts. 相似文献