Origin of apatite-bearing calcitic nodules from the shell-concentrated bed in Ohkuchizawa quarry, Azumino City, central Japan

<p>A thin shell-concentrated bed containing abundant apatite-bearing calcitic nodules is exposed at the Ohkuchizawa quarry in Azumino City, Nagano Prefecture. The major rock distributed at the quarry is sandy mudstone and no calcitic nodules were found except the shell-concentrated bed. The nodules occurring in sandy mudstone are dolomite- and siderite-nodules. The carbon and oxygen isotope ratios of various nodules in the Ohkuchizawa quarry were measured, and the reason that calcitic nodules occur only in the shell-concentrated bed was studied. The isotopic ratios of calcite and dolomite have been modified from the values at the time of crystallization during early diagenesis by the infiltration of groundwater after the uplift of the Aoki Formation. However, it was possible to estimate the initial isotopic ratios of calcite of pre-modification by comparing the measured isotopic ratios with those of calcite in equilibrium with groundwater at 15 °C . The estimated δ¹³C of calcite in apatite-bearing calcitic nodules（type 1）ranges from −5 to + 2 ‰, indicating that the carbonate ion of calcite in type 1 nodules are derived not only from organic matter but also from the carbonate shell of seashells. Until the early stage of sulfate reduction, interstitial water was low in pH. Because of low in pH, the carbonate shell in the shell-concentrated bed was partially dissolved. This resulted in the increase in Ca/Mg ratio of the interstitial water. Apatite crystallized at this diagenetic stage. Then, with the progress of sulfate reduction, the alkalinity and pH of the interstitial water increased, which resulted in the precipitation of calcite. When SO₄ ²⁻ in the interstitial water was depleted, organogenic dolomite of methane fermentation origin started to precipitate, forming dolomite nodules and thin beds of dolomite. The reason that the calcitic nodules were formed only in the shell-concentrated bed is due to the increase of Ca/Mg ratio of the interstitial water, which resulted from the dissolution of carbonate shells in the very early stage of diagenesis of the Aoki Formation. </p>