Roll-up Structures: Evidence of In situ Microbial Mats in Late Archean Deep Shelf Environments

In addition to its well-known banded iron formations, the late Archean to Paleoproterozoic Hamersley Group of Western Australia contains substantial volumes of thinly laminated carbonaceous shale and carbonate lutite. Locally, some of the laminae in these carbonates are deformed into highly distinctive folds in pockets whose cross-sections range up to about 2 cm in height and 10 cm in length. Individual folds consistently have circular hinge zones and vary in shape from simple buckles to recumbent, elastica, and even spiral geometries. Based on the strength and flexibility with which they responded to soft-sediment deformation, we infer the deformed carbonaceous laminae were bound by cohesive microbial mats. We propose the name roll-up structures for these features as they differ in subtle but important ways from most previously described soft-sediment deformation structures. Roll-up structures only occur in lutites deposited below wave base and are absent from peritidal deposits, suggesting that the microbial mats involved may not have been based on photosynthesis. Large portions of the deep sea floor could presumably have been stabilized by mats built by heterotrophic or chemoautotrophic microbes prior to the Neoproterozoic, given the lack of both predators and competing algae. Roll-up structures are potentially useful as indicators of deposition below wave base, but very few candidates have been reported anywhere else save for the coeval carbonates of the Transvaal basin of South Africa. The paucity of other examples, particularly in younger strata, means that either roll-up structures have simply not been distinguished from other types of soft-sediment deformation structures, or the microbes and/or environmental conditions required to make them are a rarity in Earth history.