HORODYSKIA FROM THE BELT SUPERGROUP (~1.45 GA) OF MONTANA: ANIMAL, MINERAL OR VEGETABLE?

Discovered in siltstones of the Appekunny Formation, Belt Supergroup, of northwestern Montana by the late Robert Horodyski in the 1970s, what he called “problematic bedding plane markings” have continued to be controversial. Fedonkin and Yochelson (2002) settled on a biological origin and named them Horodyskia moniliformis. Recent discoveries in Western Australia, Tasmania, India and China show that similar structures occur worldwide in Precambrian strata of different ages, and to date three ‘species’ have been described. Utilizing petrography, SEM, CT scanning and XRD, the microfabric of the problematic bedding plane markings of the Appekunny Formation was explored in detail. The host plane-laminated and locally wave-ripple cross-laminated argillaceous siltstones were deposited under relatively low-energy conditions in a shallow lake-like sea. Horodyskia consists of illite and chlorite discs 0.5–5 mm wide, which have been compacted to lenses <250 µm thick. The clays are randomly oriented in plan view. Although attention has been drawn naturally to specimens with a ‘string of beads’ aspect, isolated discs are common as well. However, CT scanning reveals that in fact these clay discs occur profusely on virtually every lamina of the host rock, but only some are visible with the naked eye because weathering operated differently on the clays than the surrounding, more porous silt matrix. Additionally, the leaching of Fe from the silty matrix increases the color contrast. We envisage Horodyskia to have formed copiously on the sea floor or possibly in the water column as large clay flocs. It is likely that flocculation and the stringing together of some of the flocs were aided by either a biopolymer or a cyanobacterial mat. Horodyskia moniliformis in the Appekunny Formation is not a eukaryotic alga or tissue-grade organism, but rather a biologically or chemically bound sedimentary feature. Despite the possible role of organic matter in its formation, it belongs in the realm of pseudofossils.