Investigating the formation of pyrite framboids in the Upper Devonian marine black shales of southeast of Central Iran: an approach to evaluation of water oxygen level in paleoenvironments

In this study, formation of pyrite framboids was investigated in two Upper Devonian marine black shale intervals in south of the Tabas Block in southeast of central Iran. In order to identify and study the morphology of the pyrite framboids, polished section slides were prepared from pyrite-bearing black shale samples. The section slides from the shale samples were examined and photographed with a scanning electron microscope (SEM). The identified pyrite framboids are spheroid to sub-spheroid in shape and consist of several euhedral and octahedral-euhedral compact microcrystals. Generally, the diameter of the pyrite framboids range from 5 μm to 18 μm. The relative abundance and size distribution of the pyrite framboids in the studied shale intervals show that the identified pyrite framboids were mainly formed during the early diagenesis below the redox boundary, within the sediments of a nearshore marine substrate, under a slightly dysoxic–oxic water column. The absence of pyrite framboids in the other shale intervals in the studied Devonian succession may indicate fully oxic conditions in the water column.