Middle Triassic radiolarian bedded cherts in the Mino Belt, central
Japan, include a
sequence showing an abrupt facies change from the lower to the upper, where
grey–black bedded
cherts enriched in carbonaceous matter and framboidal pyrite are overlain
by brick-red hematitic bedded
cherts. Brownish-yellow chert enriched in goethite and purple-red chert
occur at the boundary
between the grey–black bedded cherts and the brick-red bedded cherts.
This facies change is in accordance
with stratigraphic variations of geochemical characteristics; the lower
section grey–black
bedded cherts, compared with the upper section brick-red bedded cherts,
are enriched in Ctot and Stot,
and are characterized by lower MnO/TiO2, higher FeO/Fe2O3*
(total iron as Fe2O3) and more variable
Fe2O3*/TiO2 values. Some of the
lower section samples, in addition, are characterized by an enrichment
in some transition metals (Ni, Cu, and Zn). The covariation of mineralogical
and geochemical
characteristics indicates that sedimentary environments and diagenetic
processes were different
between the lower and the upper section bedded cherts. During the deposition
of the lower section
bedded cherts, the sedimentary environment was anoxic and bacterial sulphate
reduction occurred
during the early diagenetic stage. In contrast, the upper section bedded
cherts were subjected to less
reducing diagenetic processes; active sulphate reduction did not occur.
The change of sedimentary
environment and diagenetic process at the site of deposition is likely
to be attributed to the fluctuated
concentration of dissolved oxygen in the water mass of a semi-closed marginal
ocean basin, which was
potentially caused by sea-level change that occurred during Middle Triassic
time.
Sedimentary environments and basin development of the Jinzu Group in the boarder area between Toyama and Gifu prefectures, central Japan
