Clasts derived from rhizocretions in shallow-marine Miocene clastic deposits of northern Hungary: an example of zombie biogenic structures

Disc-and cy lin dri cal-shaped clasts of fine grained cal car e ous and ferruginous rock, each with a cen tral tun nel, oc cur in shallow ma rine brack ish Mio cene sandy de pos its of the Egyhazasgerge For ma tion in Hun gary. Pre vi ously, these have been in terpreted as enigmatic biogenic (?) structures. After field and laboratory examination and comparisons with sub-recent rhizoclasts in sub soils de vel oped on Qua ter nary fine-grained de pos its in SE Po land, they are re-in ter preted as re de pos ited rhizocretions pos si bly washed out of the co eval con ti nen tal de pos its of the Salgo tarjan Lig nite For ma tion. Most are fragmented and abraded. They are termed rhizoclasts and are pre sented as an ex am ple of zom bie struc tures in her ited from an other en vi ron ment where they played a dif fer ent role. Such rhizoclasts can be con sid ered as an in di ca tor of the source of the clastic ma te rial trans ported from a veg e tated land mass on which mod er ate or poorly drained soils de velop and plant roots pen e trate the fine-grained sub strate. In such soils, iron was mo bi lized, then fixed by ox i da tion, as the wa ter ta ble and ox y gen levels fluctuated.