The Torridonian Mudstones: Implications for Early Life and Extraterrestrial Sedimentology Geology from the Neoproterozoic Diabaig Formation

Mudrock is a crucial geological lithology, both as an archive of sedimentary environments and a regulating component of the Earth system (sequestering chemically-weathered clays). Ongoing research suggests the deposition of mudrock throughout Earth history has not been uniformitarian: the evolution of land plants in the Palaeozoic provided the means of both producing more mud through weathering, and retaining more mud in sedimentary conduits (rivers) through binding and baffling. Non-marine mudrock is rare before the evolution of vegetation, but insights into such exceptional occurrences are key to understanding how non-uniformitarian the ancient mudrock record is, how and where mud may have been deposited on other (presumably abiotic) planets (e.g., Mars) and the environmental context of some of Earth's oldest fossils. Sedimentary rocks that accumulated in ancient lake basins are crucial archives of such material, and this research will create a robust sedimentological framework for the Diabaig Formation (a mudrock-rich interval of the Precambrian "Torridonian Sandstones", Scotland); producing a critical case study for developing an understanding of deep time mudrock deposition.

Why is there so little mudrock in the Precambrian non-marine sedimentary rock record? An answer may be found within the "Torridonian Sandstones": a classic succession in British regional geology. Despite being the focus of investigation for 200 years, intervals of the succession remain poorly understood. One such unit is the Diabaig Formation, well-known for its microfossils and microbial sedimentary structures, but poorly characterized from a sedimentological standpoint. The current model is that the unit records lacustrine facies, but its lithology is variable and may reflect very localized deposition within small topographic basins in the underlying bedrock. Detailed sedimentological and stratigraphic contextualization of the unit is overdue: this project will explain where and why Torridonian mudrock accumulated, with potential implications for Earth history and planetary geology.

The student will undertake a number of field seasons in NW Scotland, visiting every outcrop of the Diabaig Formation to characterize the unit through mapping, logging and architectural analysis. Sedimentological evidence for deposition and microbial activity will be synthesised with an understanding of local bedrock topographic controls that appear to have acted a 'trap' for mudrock on land in the Precambrian. Samples will be collected for petrographic analysis back in Cambridge, and the study will be supported by the compilation and analysis of a detailed database of the sedimentary character all other mudrock-rich pre-vegetation successions globally (as well as those recently reported from Mars). Secondary fieldwork will be undertaken within other mudrock intervals of the Torridonian (e.g., the Cailleach Head Formation) and younger, clearly lacustrine facies (e.g., the Devonian basins of the Orkney Islands) to fully contextualize how unique the Diabaig Formation is, and better understand its potential implications.