Informace o publikaci

Shallow-water platform carbonates at the Devonian/Carboniferous boundary - an interdisciplinary study in Ertocoun and Nanbiancun sections (Southern China)



Rok publikování 2013
Druh Konferenční abstrakty
Fakulta / Pracoviště MU

Přírodovědecká fakulta

Popis The Nanbiancun and Ertocoun sections, located in the vicinity of the city of Guilin, represent Devonian/Carboniferous boundary intervals of shallow-water platform carbonates. In the Ertocoun section platform interior to platform margin facies are exposed with a deepening upward trend from microbial mounds into the talus microfacies being replaced by an upward shallowing succession higher in the section. Marginal slope facies exposed at the Nanbiancun section show different trends. At the base, a thick talus layer is exposed and overlain by a upward thinning succession of tempestite beds, which is interpreted as a deepening trend. Field gamma-ray spectrometry (GRS) as well as low-field magnetic susceptibility (XLF) measurements were performed in both sections. In addition, laboratory rock magnetic analyses have been applied in order to determine the nature of magnetic components in these sections. Sixty-two and eight-four samples were collected and measured along Ertocoun and Nanbiancun sections, respectively. A decreasing trend in potassium (K) and thorium (Th) concentrations is noted in the Famennian interval of the Ertocoun section, which is followed by a slight K and Th enrichment just below the D/C boundary. A high and positive Th-K correlation (r = 0.81) suggests that both elements are present in fine-grained aluminosilicate minerals, which are diluted in non-radioactive carbonate. The lowermost Carboniferous is characterised by a second Th decreasing trend. In the middle part of the Ertocoun section, across the D/C boundary, a significant increase in low-field magnetic susceptibility (XLF) is noted. A strong positive correlation between XLF and anhysteretic remanent magnetization (ARM; r = 0.93) suggests that fine magnetite grains are the main source of the magnetic susceptibility signal. The predominance of magnetite is especially visible in the interval just above the D/C boundary. Because the lowermost Carboniferous part of the section has extremely low K and Th concentrations it is not likely that the magnetite is land-derived eolian dust particles. It might be a result of specific environmental redox conditions in the basin. The section is characterized by inhomogeneous magnetic mineralogy and fluctuations in the concentration of high- and low coercivity minerals, which may indicate diagenesis as the major driving mechanism for the magnetic susceptibility signal. In the Nanbiancun section quite a similar, large-scale pattern is visible, however a prominent magnetic susceptibility peak, which coincides with the K and Th peak, is present in the lowermost Carboniferous. Unlike the Etcoucon section, this peak is presumably driven by detrital paramagnetic phyllosilicates. The petrophysical patterns across the biostratigraphic D/C boundary show some misfit, which may point to some diachronism between the inner platform and maginal slope sedimentary events. Alternatively, they might indicate some discrepancy in the biostratigraphy of the conodont-barren, shallow-water Etcoucun section.
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