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Origin of manganese carbonates in Jurassic red shale, central Japan
Author(s) -
MINOURA KOJI,
NAKAYA SHU,
TAKEMURA ATSUSHI
Publication year - 1991
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1111/j.1365-3091.1991.tb01859.x
Subject(s) - geology , rhodochrosite , manganese , diagenesis , carbonate , foraminifera , geochemistry , mineralogy , sedimentary rock , benthic zone , oil shale , paleontology , calcite , chemistry , oceanography , organic chemistry
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 (Mn 86.7−92.2 Ca 2.2−2.9 Mg 2.6−6.7 Fe 2.6−5.6 )CO 3 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.