Origin of manganese carbonates in Jurassic red shale, central Japan

KOJI MINOURA, SHU NAKAYA, ATSUSHI TAKEMURA

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8 Citations (Scopus)

Abstract

Manganese carbonate deposits in Japanese Jurassic sedimentary rocks were studied petrogeochemically. The deposits are characteristically composed of spheroidal micronodules, up to 1 mm in diameter, and always contain well‐preserved radiolarian shells. Chemical elemental composition and mineralogical characteristics indicate that the micronodules contain rhodochrosite in a mixed carbonate phase composition (Mn86.7−92.2Ca2.2−2.9Mg2.6−6.7Fe2.6−5.6)CO3 Carbon and oxygen isotope values, which range from −7.99 to −4.78‰ and −4.05 to 0.28‰ relative to PDB, respectively, suggest that the manganese carbonate was precipitated in a suboxic zone. The micronodules closely resemble agglutinated benthic foraminifera in shape. We suggest that agglutinated foraminiferal tests composed of radiolarian shells accumulated selectively on the sediment surface during redeposition of bottom sediments and were replaced by manganese carbonate in suboxic diagenetic conditions of manganese reduction.

Original languageEnglish
Pages (from-to)137-152
Number of pages16
JournalSedimentology
Volume38
Issue number1
DOIs
Publication statusPublished - 1991 Feb

ASJC Scopus subject areas

  • Geology
  • Stratigraphy

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