THE ROLE OF CLAY MINERALS IN THE PRESERVATION OF PRECAMBRIAN ORGANIC-WALLED MICROFOSSILS

Clay minerals have been suggested as important controls on the formation of soft-bodied fossils. Consistent with this, shales containing Burgess Shale-type fossils tend to have high concentrations of berthierine, a diagenetic product of kaolinite. Precambrian organic-walled microfossils (OWMs) might also be preserved through interactions with clay minerals. Recent microanalyses show an aluminum-rich mineral, likely kaolinite, directly surrounding individual microfossils from several exceptionally-preserved assemblages. Given that exceptionally-preserved assemblages are rare, we test if clay mineralogy plays a broader role in OWM preservation from a variety of fossil assemblages that vary in preservational quality.

We compared OWM presence and preservational quality to the mineral composition of 223 shales from seventeen units and four paleocontinents that span late Paleoproterozoic to late Neoproterozoic time. We found no significant difference in clay mineral assemblage between barren and fossiliferous samples. The probability of a sample to contain well-preserved microfossils, measured by the level of pitting on fossil surfaces and the quality of wall margins, increased with increasing illite concentrations and decreased with increasing berthierine concentrations. This is in contrast to Burgess Shale-type fossils, which positively correlate with berthierine and negatively correlate with illite concentrations. We hypothesize that preservational quality of OWMs is not directly controlled by clay minerals but rather by burial rate, provided that berthierine forms in areas of low sediment input and some types of illite are detrital. If correct, this suggests that, while preservation of particular microfossils (e.g. Proterocladus) in exceptionally-preserved assemblages might depend on the presence of specific clay minerals, most OWMs do not, possibly reflecting a more robust organic composition.