Ferruginization as One of the Main Modes of Preservation of Ediacaran Fossils in the Southeastern White Sea Area (russia)

The Flinders-style preservation is one of the most widespread among the known modes of preservation of Ediacaran fossils. In the type localities of South Australia, it is characterized by body imprints located on bedding planes of siliciclastic rocks. The White Sea variant of the Flinders style is significantly more diverse. Here, the imprints are supplemented by remains of the original substance of the deceased organisms in the form of carbonaceous compressions and impregnation of the host rock by mobile organic compounds along the outline of the decomposed body. Additionally, several forms of ferruginization are widely distributed in the White Sea localities. Primarily, this includes pyritization, such as: replacement of the original organic matter by pyrite, crustification along the surfaces of organic structures, and formation of microshrinkage halos around fossilized remains. During surficial weathering, the iron sulfide in all these forms is replaced by various hydroxides. In most cases, the pyrite is represented as framboids about 4–10 microns in diameter and radial aggregates up to 20 microns in diameter with a framboid core. Much less frequently, isolated crystals and irregular aggregates occur. In the Flinders-Belomorian burial event horizons, pyritized remains are located on the base of the bearing sandstone layer and mainly within its thickness near the base. Pseudomorphs apparently formed in places of greatest concentration and increased density of the original organic matter – in body folds, skeletal consolidations, protective integument, and walls of living chambers. The original structure of these pseudomorphs consists of clusters of unconnected framboids. When fossil remains are found inside the rock, these clusters mostly determine the splitting trajectory and formation of joint planes. Thanks mainly to them, we are able to find such fossils. Crustifications occurred on the inner walls of tubes and on imprint surfaces. The structure of these forms consists of individual crystals and various crystal aggregates. Halos are formed by individual framboids located between sandstone grains, creating a sparse cloud above the imprint. Pyrite pseudomorphs occur on separate areas of almost all White Sea macrofossils. For example, structural consolidations on the crowns of Charniodiscus are preserved this way; as well as body folds of Yorgia, sculpture ridges of the shield of Tribrachidium, thalli of filamentous algae, and trichomes of cyanobacteria. The remains of presumed food-gathering pockets of Dickinsonia are made up of framboids; tube walls of Calyptrina, Saarina, and chamber walls of Orbisiana and Palaeopascichnus also consist of them. We observed crustification crusts in the central part of deep Kimberella imprints; possibly, they also fill the internal cavities of Corumbella(?) tubes. Halos are usually located above the surface impression of microbial mats; rarely are they formed around the parts of macroorganisms raised into the sediment. It is important to note that there is generally not much pyrite in the White Sea sandstones; it does not form cement or continuous coatings anywhere. Therefore, the hypothesis of J. Gehling (1999) about a pyrite death mask as a mode of formation of Ediacaran fossils is not applicable to the White Sea localities.