Lance B. Morrissey

Enigmatic sedimentary structures in the Lower Old Red Sandstone, south Wales, UK: possible microbial influence on surface processes and early terrestrial food webs

Several enigmatic sedimentary structures are present in Upper Silurian and Lower Devonian terrestrial rocks of south Wales. These are millimetre ripples, wrinkle structures, ‘cauliflower’ and ‘molehill’ structures, calcretized matgrounds, ‘pepper-pots’ and ‘fairy rings’. Most of the structures occur on fine-grained sandstone bedding surfaces in both inclined and non-inclined red heterolithic deposits that form a large part of the Lower Old Red Sandstone in south Wales. These deposits consist of conspicuous units up to 3 m thick, comprising millimetre- to centimetre-scale laminasets of alternating mudstone and fine-grained sandstone, interpreted as muddy point bars on ephemeral channel margins, crevasse-splay lobes or distal flood-out deposits from ephemeral flow. They are rich in trace fossils reflecting animal locomotion, burrowing, foraging, ploughing and resting. Millimetre ripples and wrinkle structures are related to rapid growth of cyanobacterial mats on wet sediment surfaces, especially in films of standing water, that may have been a critical constituent of the food chain for terrestrial communities. Pustular ‘cauliflower’ and ‘molehill’ structures are preserved on tuffs and have been linked to algal blooms formed following an increase in nutrients due to ashfall into floodplain ponds. The tuffs are associated with a diverse ichnofauna indicating that opportunist colonizers took advantage of an abundant food source.