Tufa in Oga Limestone Plateau, Okayama Prefecture: Its developmental conditions and annual lamination

Hori Masako, Kano Akihiro

doi:10.5575/geosoc.112.491

Tufas are largely developed in a ca. 1200-m-long stream at Nagaya in the northern Oga Limestone Plateau (Takahashi City, Okayama Prefecture, SW Japan). The water mainly issues from the spring located at the thrust boundary between the Nakamura Limestone and the lower lying shale and sandstone unit, and deposits tufa along the steep stream. These conditions allow the water being supersaturated with respect to calcite by increasing Ca²⁺ content (>60 mg/L) and decreasing PCO₂ (<2.0 matm). Variation in chemical properties of the springwater, such as PCO₂ and SIc, resembles to the seasonal change, which has been previously described in other tufa localities in SW Japan. However, the springwater of Nagaya is characteristic in low Ca²⁺ content in summer and large seasonal variation in temperature, which suggest a small water mass in the underground system. Some of the tufa specimens display very regular annual laminations, which display clear boundary between summer dense laminae and winter porous laminae. Cyanobacteria and diatoms densely occur in the porous laminae, whereas the dense laminae is poor in microbial remnants and characterized by large rhombic calcite crystals. The textural change of the annual lamination is inconsistent to inorganic precipitation rate (PWP-rate) calculated by chemical data, but is rather ascribed to change in water flow due to agricultural drainage and seasonal rainfall pattern.<br>

Tufa in Oga Limestone Plateau, Okayama Prefecture: Its developmental conditions and annual lamination

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