Eustasy has commonly been invoked to explain peritidal carbonate cyclicity,but is difficult to explain cycles formed in a greenhouse climate when eustasy is minimal.We propose that peritidal cycles on an Early Triassi...Eustasy has commonly been invoked to explain peritidal carbonate cyclicity,but is difficult to explain cycles formed in a greenhouse climate when eustasy is minimal.We propose that peritidal cycles on an Early Triassic isolated carbonate platform in Guizhou,South China,were formed by hierarchical carbonate productivity variations.Most of the 149 shallowing-upward cycles are typically terminated by flooding over intertidal facies and contain rare supratidal facies and no prolonged subaerial exposure.Low-diversity benthos in the platform interior during the post-end-Permian biotic recovery were sensitive to environmental perturbations,which caused variations in benthic sediment productivity in the subtidal carbonate factory.The perturbations may be driven by changes in salinity and degree of eutrophication,or repeated platform mini-drowning by anoxic and/or CO2-charged deep water upwelled onto the banktop.They were modulated by Milankovitch orbitally-driven climatic and oceanographic factors as suggested by the hierarchical stacking pattern and spectral signals of these cycles.A one-dimensional conceptual model shows that hierarchical productivity variations alone may generate hierarchical peritidal carbonate cycles under conditions of constant subsidence and no sea-level fluctuation.展开更多
This paper describes the occurrence of dolomite and the mechanism of dolomitization of the Upper Triassic-Lower Jurassic Klzlloren Formation in the autochthonous Bolkardagl unit of the middle Taurus Mountains in south...This paper describes the occurrence of dolomite and the mechanism of dolomitization of the Upper Triassic-Lower Jurassic Klzlloren Formation in the autochthonous Bolkardagl unit of the middle Taurus Mountains in south western Turkey. Dolomites were analyzed for geochemical, isotopic and crystallographic variation. Dolomites occur as a replacement of precursor carbonate and cement. The dolomite crystals range from 〈10 to -1000 ltm existing as both replacements and cements. Sr concentrations range between 84 and 156 ppm, and the molar Sr/Ca ratios of dolomitizing fluids are estimated to range between 0.0066 to 0.013 ratios. Dolomites are Ca-rich （with average CaCO3 and MgCO3 equal to 56.43 and 43.57 mol%, respectively） and they are non-stoichiometric, with an average Sr=ll6 ppm, Na=286 ppm, Mn=81 ppm, Fe=1329 ppm, and ~lSO and ~lac ranges from -0.6%o to -6.1% Pee Dee Belemnite [PDB], and ＋1.2 to ＋3.9%0 PDB. The North American Shale Composition [NASC]- normalized rare earth element （REE） values of the both limestone and dolomite sample groups show very similar REE patterns characterized by small positive Eu （mean=l.32 and mean=l.42, respectively） and slightly or considerably negative Ce （mean=0.61 and mean=0.72, respectively） anomalies and a clear depletion in all REE species. The Klzlloren Formation dolomites have been formed as early diagenetic from mixing zone fluids at the tidal-subtidal environment and at the late diagenetic from basinal brines at the shallow-deep burial depths.展开更多
Carbonate strata of the Mesoproterozoic Wumishan Formation in the Jixian area near Tianjin are ~3300 m thick and were deposited over some 100 million years(from ~1310±20 Ma to ~1207±10 Ma).Metre-scale cycles...Carbonate strata of the Mesoproterozoic Wumishan Formation in the Jixian area near Tianjin are ~3300 m thick and were deposited over some 100 million years(from ~1310±20 Ma to ~1207±10 Ma).Metre-scale cycles(parasequences) dominate the succession.They are generally of the peritidal carbonate type,and mostly show an approximately symmetrical lithofacies succession with thick stromatolite biostromes and small thromboliteoncolite bioherms constituting the central part and tidal-flat dolomites forming the upper and lower parts.Lagoonal-supratidal dolomitic shales with palaeosol caps make up the topmost layers.The boundaries of the Wumishan cycles are typically exposure surfaces,and there is abundant evidence for fresh-water diagenesis.Widespread 1:4 stacking patterns indicate that the individual Wumishan cycles are sixthorder parasequences,with 4 parasequences constituting one fifth-order parasequence set.Locally,5-8 beds or couplets,can be discerned in some of the cycles.The regular vertical stacking pattern of beds within the sixth-order parasequences,forming the fifth-order parasequence sets,are interpreted as the result of environmental fluctuations controlled by Milankovitch rhythms,namely the superimposition of precession,and short and long-eccentricity.The widespread 1:4 stacking pattern in the cyclic succession,as well as the local 1:5-8 stacking patterns of the beds within the cycles,suggest that the Milankovitch rhythms had similar ratios in the Mesoproterozoic as in the Phanerozoic.Based on the cycle stacking patterns,26 thirdorder sequences can be distinguished and these group into 6 second-order,transgressiveregressive megasequences(or sequence sets),all reflecting a composite,hierarchical succession of relative sea-level changes.展开更多
文摘Eustasy has commonly been invoked to explain peritidal carbonate cyclicity,but is difficult to explain cycles formed in a greenhouse climate when eustasy is minimal.We propose that peritidal cycles on an Early Triassic isolated carbonate platform in Guizhou,South China,were formed by hierarchical carbonate productivity variations.Most of the 149 shallowing-upward cycles are typically terminated by flooding over intertidal facies and contain rare supratidal facies and no prolonged subaerial exposure.Low-diversity benthos in the platform interior during the post-end-Permian biotic recovery were sensitive to environmental perturbations,which caused variations in benthic sediment productivity in the subtidal carbonate factory.The perturbations may be driven by changes in salinity and degree of eutrophication,or repeated platform mini-drowning by anoxic and/or CO2-charged deep water upwelled onto the banktop.They were modulated by Milankovitch orbitally-driven climatic and oceanographic factors as suggested by the hierarchical stacking pattern and spectral signals of these cycles.A one-dimensional conceptual model shows that hierarchical productivity variations alone may generate hierarchical peritidal carbonate cycles under conditions of constant subsidence and no sea-level fluctuation.
基金Support from the Graduate School of Natural and Applied Sciences of Selcuk University
文摘This paper describes the occurrence of dolomite and the mechanism of dolomitization of the Upper Triassic-Lower Jurassic Klzlloren Formation in the autochthonous Bolkardagl unit of the middle Taurus Mountains in south western Turkey. Dolomites were analyzed for geochemical, isotopic and crystallographic variation. Dolomites occur as a replacement of precursor carbonate and cement. The dolomite crystals range from 〈10 to -1000 ltm existing as both replacements and cements. Sr concentrations range between 84 and 156 ppm, and the molar Sr/Ca ratios of dolomitizing fluids are estimated to range between 0.0066 to 0.013 ratios. Dolomites are Ca-rich （with average CaCO3 and MgCO3 equal to 56.43 and 43.57 mol%, respectively） and they are non-stoichiometric, with an average Sr=ll6 ppm, Na=286 ppm, Mn=81 ppm, Fe=1329 ppm, and ~lSO and ~lac ranges from -0.6%o to -6.1% Pee Dee Belemnite [PDB], and ＋1.2 to ＋3.9%0 PDB. The North American Shale Composition [NASC]- normalized rare earth element （REE） values of the both limestone and dolomite sample groups show very similar REE patterns characterized by small positive Eu （mean=l.32 and mean=l.42, respectively） and slightly or considerably negative Ce （mean=0.61 and mean=0.72, respectively） anomalies and a clear depletion in all REE species. The Klzlloren Formation dolomites have been formed as early diagenetic from mixing zone fluids at the tidal-subtidal environment and at the late diagenetic from basinal brines at the shallow-deep burial depths.
基金The funding for this research into the Middle and Upper Proterozoic strata of North China was provided by the Natural Sciences Foundation of China (Grant 49802012,40472065)
文摘Carbonate strata of the Mesoproterozoic Wumishan Formation in the Jixian area near Tianjin are ~3300 m thick and were deposited over some 100 million years(from ~1310±20 Ma to ~1207±10 Ma).Metre-scale cycles(parasequences) dominate the succession.They are generally of the peritidal carbonate type,and mostly show an approximately symmetrical lithofacies succession with thick stromatolite biostromes and small thromboliteoncolite bioherms constituting the central part and tidal-flat dolomites forming the upper and lower parts.Lagoonal-supratidal dolomitic shales with palaeosol caps make up the topmost layers.The boundaries of the Wumishan cycles are typically exposure surfaces,and there is abundant evidence for fresh-water diagenesis.Widespread 1:4 stacking patterns indicate that the individual Wumishan cycles are sixthorder parasequences,with 4 parasequences constituting one fifth-order parasequence set.Locally,5-8 beds or couplets,can be discerned in some of the cycles.The regular vertical stacking pattern of beds within the sixth-order parasequences,forming the fifth-order parasequence sets,are interpreted as the result of environmental fluctuations controlled by Milankovitch rhythms,namely the superimposition of precession,and short and long-eccentricity.The widespread 1:4 stacking pattern in the cyclic succession,as well as the local 1:5-8 stacking patterns of the beds within the cycles,suggest that the Milankovitch rhythms had similar ratios in the Mesoproterozoic as in the Phanerozoic.Based on the cycle stacking patterns,26 thirdorder sequences can be distinguished and these group into 6 second-order,transgressiveregressive megasequences(or sequence sets),all reflecting a composite,hierarchical succession of relative sea-level changes.
