HORODYSKIA: NOT A FOSSIL BUT DUE TO SEDIMENT–MICROBIAL INTERACTIONS IN THE APPEKUNNY FORMATION (BELT SUPERGROUP, CA. 1.46 GA, WESTERN NORTH AMERICA)

Horodyskia moniliformis Yochelson and Fedonkin 2000 is found extensively in the lower Appekunny Formation (Ravalli Group, lower Belt Supergroup, ca. 1.46 Ga) of northwestern Montana and adjacent southwestern Alberta. The host rock is thin-bedded, planar-laminated, argillaceous and silty, very fine-grained sandstone and argillaceous siltstone. Sedimentation of most of the finer material is envisaged to have involved settling of silt and clay flocs from suspension. First identified as the ‘string-of-beads’ structure due to its appearance on flat bedding planes, Horodyskia was initially suspected to be an inorganic feature. Subsequently it has been attributed to a number of different organic origins, including fungi and colonial endobenthic organisms. The mostly round to ellipsoidal ‘beads’ consist of silty clay lenses that vary in size, shape and spacing, although these are roughly uniform along the chain in individual specimens. Besides eye-catching chains, single ‘beads’ are ubiquitous and visible petrographically and via micro-CT scanning, and X-ray projection. Horodyskia beads, physical sedimentary structures, and inferred organic features in the form of microbially induced sedimentary structures (MISS) seem to show a consistent textural relationship. Microbial mats or biofilms, probably dominated by filamentous cyanobacteria, preferentially bound flocs of various sizes, often leading to the formation of a complex mat topography of small domes, ‘elephant skin texture,’ polygonal ridges, and pinnacles. Serial grinding parallel to lamination shows that flocs incorporated into the mat surface contributed to its topographic development. The chains owe their apparent regularity to the texture of the mat surface. Compaction flattened the ‘beads’ and flocs. During Recent outcrop exposure, differential weathering of bedding planes has accentuated the difference between the ‘strings-of-beads’ and adjacent argillaceous silty matrix and obscured the abundant disseminated flocs. There is no evidence in the Appekunny Formation that Horodyskia was a complex multi-cellular organism. Rather it is a type of microbially induced sedimentary structure formed in certain kinds of argillaceous sediments.