Snowball Earth Under the Microscope

Micromorphology is a well-established technique adopted by Quaternary scientists which has received wide application in the study of glacial sediments and in soil science. One area where the approach is far less developed is in the analysis of ancient successions and, in particular, rocks of Cryogenian age that are purported to have been deposited during snowball Earth conditions. Here, we integrate micromorphology with outcrop data in the analysis of three contrasting diamictite-bearing successions of Cryogenian age. The successions include two units in Namibia (the Chuos Formation and the Ghaub Formation: diamictites of Sturtian and Marinoan age, respectively), and a formation in Scotland (the Macduff Slates Formation). In the Chuos Formation, which has been metamorphosed to lower greenschist facies, a well expressed primary stratification is clearly developed in clastic diamictites, in spite of a fracture cleavage that crosscuts the lamination. In the Ghaub Formation, which is dominated by carbonate diamictites, local incorporation into fault zones has resulted in a greater postdepositional overprint during tectonic uplift. At the macro-scale, dropstone textures are well expressed, in addition to highly attenuated, fabric-forming clasts. At the micro-scale, sedimentary boudins, load structures, and galaxy structures are observed which imply shearing and deformation of soft sediment. Strain dissolution has an effect on clast morphology, and in some cases carbonate obstacle clasts show sutured contacts with one another. In our third example, at Macduff, micromorphological analysis reveals granule-size ice-rafted debris (IRD) in fine-grained laminites, but the origin of soft-sediment deformation structures in overlying diamictite is inconclusive. We conclude that the technique has the potential to unravel the origins of soft-sediment deformation structures (e.g., whether deformation structures record glaciotectonic origins or nonglacial origins), but its application must take into account the variability in stratified and unstratified sediments, alongside the later overprint of tectonic deformation. Bizarrely, in the Ghaub Formation, both tectonic fabrics (dissolution cleavage) and sedimentary fabrics (e.g., galaxy structures) survive side-by-side in the same thin sections. Thus, a full awareness of the effects of tectonic overprint is essential in examining Snowball Earth under the microscope.