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Geochemical Analysis Of Chert For Interpretation Of Depositional Environment In Saraburi Region, Tha

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The Saraburi Group in Saraburi-Pak Chong area can be sub-divided into 8 lithofacies representing the shelf or platform, basin margin and deep basin environments. Lithofacies of shallow marine environment includes thin-to-thick bedded limestone and dolomite, boundstone and dolomite, and sandstone, shale and limestone. The basin margin and deep basin environments comprise crinoidal limestone, micritic limestone, limestone conglomerate, allodapic limestone, shale and cherts (Chonglakmani, 2001, 2005).
The geochemical analysis of major, trace and rare earth elements are used to understand the depositional environment of chert sequence in Muak Lek district, Saraburi province, Thailand. The studied section belongs to the Saraburi Group and comprises shale, thin-bedded cherts and limestone of deep marine facies. The cherts are brown to dark grey and yield moderate and poorly preserved radiolarians. The radiolarian fauna shows that the cherts are accumulated in the Middle Permian to early Late Permian.
The geochemical analysis of cherts and study of sedimentary facies are incorporated to explain the depositional environment of this sequence. The results of major and trace elements analysis of cherts indicate the high ratios of SiO2/SiO2+Al2O3+Fe2O3. Al/(Al+Fe+Mn) ratio are from 0.67 to 0.82 (Adachi et al., 1986) and MnO/TiO2 are 0.14 on the average (Murray, 1994). Al2O3/(Al2O3+Fe2O3) ratio of chert samples are of 0.60 to 0.82 (0.76 on average) (Sugitani et al., 1996). The MnO/TiO2 and Al2O3/(Al2O3+Fe2O3) ratio and the 100×Fe2O3/SiO2 vs. 100×Al2O3/SiO2 and the Fe2O3/(100-SiO2) vs. Al2O3/(100-SiO2) diagrams suggest that cherts are deposited in a continental margin setting. All samples plotted in Al-Fe-Mn are closed to non-hydrothermal origin (Adachi et al., 1986 and Yamamoto, 1987). The Lan/Cen vs. Al2O3/(Al2O3+Fe2O3) discrimination diagram shows that the majority of cherts samples are related to pelagic and continental margin settings. The NASC-normalized REE distribution diagram is profoundly flat pattern while the Ce/Ce* are from 0.18 to 0.61, indicating these cherts were not only formed in the pelagic basin but also reflected a continental margin setting (Feng et al., 2002: Chen et al., 2006 and Udchachon et al.,2011).
The studied section can be interpreted to represent the deposit in a deep marine basin. This facies can be correlated with the Nong Pong and the Nam Duk Formations of the Saraburi Group in northeastern Thailand. From the results of geochemical analysis, cherts of the Saraburi region and the Nan Suture Zone are related to the continental margin settings. In Nan area, the sequence of chert is associated with oceanic island basalt which represents a small ocean with oceanic islands close to continental margin (Yang et al., 2008). However, the chert sequence of studied area belongs to deposit of a deep basin on the continental crust in a continental margin setting.