Geochemical study on a limestone/marlstone alternation, Bajocian, Mecsek Mountains, Southern Transdanubia, Hungary

A geochemical study of the Bajocian portion of the Komló Calcareous Marl Formation (Mecsek Mountains, southern Hungary) was undertaken in order to characterize the rhythmic alternations of carbonate-rich and carbonate-poor layers and to investigate their possible origin. 45 samples of Komló Calcareo...

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Bibliographic Details
Main Authors: Raucsik Béla
Szabó Gyula
Borbély-Kiss Ildikó
Format: Article
Published: University of Szeged, Department of Mineralogy, Geochemistry and Petrology Szeged 1998
Series:Acta mineralogica-petrographica 39
Kulcsszavak:Kőzettan, Ásványtan, Földtan
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Online Access:http://acta.bibl.u-szeged.hu/24872
Description
Summary:A geochemical study of the Bajocian portion of the Komló Calcareous Marl Formation (Mecsek Mountains, southern Hungary) was undertaken in order to characterize the rhythmic alternations of carbonate-rich and carbonate-poor layers and to investigate their possible origin. 45 samples of Komló Calcareous Marl Formation collected from six segments of the outcrops of Püspökszentlászló II. and Kecskegyür, road cut were examined by PIXE analysis. Concentrations some of the trace elements are higher than can be explained by a pure detrital clastic source. Excess concentrations (over detrital) of these trace elements may be derived from seawater and likely associated with the organic and clay mineral fraction as well as with the carbonate phase. Limestone semicouplets are characterized by good oxygenation as expressed by the pervasive bioturbation, by the Fe/Mn parameter and by the lack of preserved organic matter. The abundance of silica, phosphorous and strontium in the absence of high terrigenous input suggests that during limestone deposition surface waters were rather highly fertile due to an efficient recycling of nutrients from deeper waters. The enhanced fertility was coupled with a current system at the well-oxygenated seafloor which prevented the accumulation of organic matter and of the organic matter-bound trace elements, such as Zn, Cu and V. During early diagenesis, Mn should migrated from weaker oxic parts of sediment column and precipitated as carbonate and/or as oxide, oxihydroxide coatings on biogenic tests resulting Mn-enrichment in the carbonate-rich semicouplets. Marly semicouplets deposited under moderately oxic, probably dysoxic conditions (indicated by the high Fe/Mn parameter), which does not allow the preservation of abundant organic matter and the appearance of sedimentary structure as lamination. High TÍO2 an SiOi values indicate that marly semicouplets received a substantial contribution from a terrigenous source. Differences in the solubility of reduced iron and manganese could lead to sedimentary fractionation of these elements across redox boundaries resulting Fe-enrichment during deposition of carbonate-poor semicouplets. Trace metals (Zn, Cu, V) were carried as portions of organometal complexes and as adsorbed ions on clay minerals in the water column. Disoxic conditions in the sediment mass and at the seafloor were favorable to preservation and accumulation a part of these trace elements. Mainly the V, Zn and Cu abundances in the carbonate-poor layers seem to be accounted by diagenetic enrichment; Ni enrichment is affiliated with the increased terrigenous supply, although some degree of redox-controlled diagenetic modification cannot be excluded. The lower phosphorous abundance in the carbonate-poor semicouplets suggests that the depositional environment should be characterized by lower surface-water fertility and productivity during their deposition. According to these data and results of formerly stable isotope measurements, the rhythmic organization of couplets should represent climatic changes. Palaeoceanographic conditions alternated from efficiently mixed, highly-fertile surface waters and well-oxygenated seafloor, to enhanced water runoff and/or decreased evaporation with decreased productivity and moderately oxygenated bottom waters. Moderately oxidizing (niost likely dysoxic) conditions at the bottom may be caused by water mass stratification. This stratification could have resulted from the presence of a less saline, l80-depleted surface-water. The corresponding climatic conditions thus alternated from more arid to more humid.
Physical Description:107-138
ISSN:0365-8066