Abstract

Magadi-type cherts occur in lacustrine rocks of the Middle Devonian (Middle Old Red Sandstone) Orcadian Basin, Scotland. Chert nodules are replacive within organic-rich laminites, the deposits of a stratified lake. They exhibit soft-sediment deformation, including sediment injection along shrinkage fractures within the nodules. The thefts preserve stellate quartz pseudomorphs after an evaporite mineral, which may have been a sodium carbonate mineral (?trona). These are features common to other documented occurrences of Magadi-type cherts. Nodules were nucleated about vertical cracks in the host sediment and show polygonal or parallel-aligned patterns. Precipitation of the magadiite precursor occurred from alkaline groundwaters rather than in the lake water column.

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