The Tilje Formation (Early Jurassic; 120–300 m thick) consists predominantly of heterolithic deposits and is thought to have accumulated in tide-dominated estuarine and deltaic environments in an active rift setting. Anomalously thick (>0.5 cm) and internally structureless mudstone layers, which are interpreted to represent fluid-mud deposits, are widespread and occur in three different environmental settings: (1) in the basal part of upward-fining tidalfluvial channels where they generate upward-sanding successions: (2) in the deposits of mouth bars and terminal distributary channels where they are associated with the coarsest sands and the least-bioturbated sediments, suggesting deposition during tidally modulated river floods; and (3) in delta-front successions where they immediately overlie thick, wave-generated storm beds, suggesting that these fluid-mud deposits result from wave resuspension of previously deposited mud. These observations provide criteria for the recognition of ancient fluid-muds and for interpreting their origin. The tectonic setting may be responsible for their abundance.

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