Cyclic Accumulation of Oolitic Ironstones in Epicontinental Seas

Ulf Bayer

Oolitic ironstones commonly accumulate at the top of coarsening-upward cycles, temporally equivalent to or succeeded by a sedimentary unconformity. In places, the iron-oolitic facies unit can be traced from proximally condensed beds to distal thickening-upward cycles of tempestites. To some extent, these sequences parallel global unconformities and, thus, are probably related to global sea level changes.

The depositional proximity gradient is paralleled by the structure of ooliths, which resemble the classical carbonate ooid-oncoid sequence. However, a particular feature of the ironstones is their general deposition within a muddy background sedimentation. The occurrence of redeposited mud balls and interbedded clay layers distinguishes ironstones from their carbonate equivalents.

However, occasionally, temporal successions from iron ooliths to calcareous ooliths occur, illustrating that ferrugination increases with repeated reworking. Mainly, aragonite particles are ferruginated, and ferrugination parallels the porosity of the particles; for example, it is especially strong in particles that were affected by microborings.

Considering ferrugination of carbonate particles, the features of ooliths, and their depositional framework, early diagenetic ferruginations under conditions of repeated reworking may be considered one possibility for the formation of iron ooliths.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.