10:15 AM - 10:30 AM
[MIS13-06] Origin of the Lower Triassic deep-sea chert gap
Keywords:Lower Triassic, deep-sea chert gap, conodont, clastic burial flux, biogenic silica burial flux
Detailed geological mapping and reconstruction of lithostratigraphy was carried out at Ogama in the Kuzuu area, Tochigi Prefecture, and in the Tsukumi area, Oita Prefecture, and consequently, several continuous sections were identified. Based on conodont biostratigraphy, the studied sections are correlated to the triangularis-collinsoni Zone, the homeri Zone, the timorensis Zone and the bulgarica Zone of Koike (1981), which corresponds to the Spathian (upper Olenekian; uppermost Lower Triassic) to middle Anisian (lower Middle Triassic). Using the conodont biostratigraphic framework, absolute ages of volcanic tuff beds in South China were projected onto the reconstructed lithostratigraphic columns of the deep-sea sections. Consequently, the lower limit of the linear sedimentation rate (LSR) was obtained for the Spathian and lowermost Anisian siliceous claystone dominant lithofacies.
In addition, major element concentrations were measured for rock samples obtained from the studied sections by X-ray fluorescence (XRF) analysis. The SiO2 and Al2O3 concentrations were were combined with measured rock density data and the lithostratigraphic data of the studied sections to calculate the burial fluxes of clastic material and biogenic silica for the stratigraphic intervals for which LSR was estimated. For the lowermost Anisian in Ogama, the minimum burial fluxes of clastic material and biogenic silica are 1030 g/cm2·m.y. and 1040 g/cm2·m.y., respectively. For the Spathian in the Tsukumi area, the minimum burial fluxes of clastic material and biogenic silica are 960 g/cm2·m.y. and 1130 g/cm2·m.y., respectively. The estimated minimum burial flux of clastic material for the deep-sea chert gap greatly exceeds the burial flux calculated for the Anisian bedded chert lithofacies in the Tsukumi area (80 g/cm2·m.y.), calculated based on LSR by Soda et al. (2015). The estimated minimum burial flux of biogenic silica for the deep-sea chert gap is also higher than that of Anisian bedded chert in the Tsukumi area (390 g/cm2·m.y.), calculated based on LSR by Soda et al. (2015). These results indicate that, during the deposition of sediments in the deep-sea chert gap, burial fluxes of clastic material and biogenic silica were both elevated, but the increase is greater in the former. Hence, the deep-sea chert gap is a result of anomalously high flux of clastic material to the pelagic realm, which diluted biogenic silica in deep-sea sediments.