Japan Geoscience Union Meeting 2015

Presentation information


Symbol H (Human Geosciences) » H-CG Complex & General

[H-CG35] Interdisciplinary approach to earth's changing surface

Wed. May 27, 2015 6:15 PM - 7:30 PM Convention Hall (2F)

Convener:*Naofumi Yamaguchi(Center for Water Environment Studies, Ibaraki University), Hajime Naruse(Department of Geology and Mineralogy, Graduate School of Science, Kyoto University), Koji Seike(Atmosphere and Ocean Research Institute, University of Tokyo), Hideko Takayanagi(Institute of Geology and Paleontology, Graduate School of Science, Tohoku University), Masayuki Ikeda(Shizuoka University)

6:15 PM - 7:30 PM

[HCG35-P06] Chemical composition and classification of frutexites in Alwa Formation, Oman mountains, northern Oman

*Ren HONJO1, Kohki YOSHIDA2 (1.Graduate school of science and technology, Shinshu University, 2.Department of Geology, Faculty of Science, Shinshu University)

Keywords:Triassic, limestone, chemical composition, Oman

The frutexites, showing irregular shrub structure, which was described as a micorostromatolites induced by microbial activity by Maslov (1960). The structure with microlayering is interpreted to be formed by microbiotic activity with deposition of Fe and Mn minerals (Jakubowicz et al., 2014). The frutexites were reported from the Lower Triassic Alwa Formation, which is distributed in the southeastern part of the Oman Mountains (Woods and Baud, 2009). This formation consist mainly of red colored carbonate including peloidal wakestones and perodial packstones. The frutexites is occurred from the remarkable black colored carbonate, which is called as sheet crack by Woods and Baud (2009) and is filled by submarine cement in the Alwa Formation. This black colored carbonate encrusts the wall-rock surface of sheet crack. In this study, we aim to describe the microstructure and chemical composition of this frutexites on the basis of microscopic observation and chemical analysis using EDS.
The predominant feature of the frutexites in the Alwa Formation exhibits 0.1-1.5mm long shrub structure consisting of microlayering by Fe-Mn rich carbonate minerals. Under microscope, two distinct layers showing orange and white colors are recognized and the latter is developed in outer marginal layer of the shrub structure of these carbonate microlayers. In addition, the black color shrubs, which are 0.1-0.5mm long and lack to orange colored layers, are also observed in the black carbonate.
The EDS analysis, using JSM-6510A in Shinshu University, shows the compositional difference of the micro layers of these frutexites. The orange and white colored microlayers of frutexites contain Mg, Fe, Mn as a carbonate component, whereas Al and P concentrates in the black colored microlayers. Thus, the frutexites in the Alwa Formation is classified by two types; Mg-Fe-Mn rich and Al-P rich types. The shrub structure grew as a result of mineralization by microbial activity under fluctuating conditions (Jakubowicz et al., 2014). Thus, this compositional differences and variety of frutexites is possibly related to the microbial activity and bottom water condition during their growth stage.