The studied succession, located on Mainland Orkney and the island of Westray lies in cyclic flagstones of Middle Devonian lake laminites. The lake bed of each cycle is constrained above and below by marginal lake facies that reflect a transition from open playa to permanent lake and vice versa, referred to as transition sequences. Abundant sedimentary structures in these sequences, previously described as subaqueous shrinkage cracks or subaqueous cracks are reinterpreted as gypsum pseudomorphs. The study shows the small structures to have prismatic, lensoid and clustered forms of gypsum in graded stages of growth and supports a new perception of the palaeoclimate and its close interdependence with periodic gypsum growth. A lake model reconstruction of lake floor morphology and its link to seasonal palaeoclimate illustrates the case for the presence of limited size pseudomorphs. It is suggested that the occurrence of consistently small pseudomorphs is a consequence of a strongly regular, wet/dry tropical monsoon palaeoclimate, coupled with an extremely low gradient lake floor, allowing short-term gypsum growth in strongly evaporating conditions across widely oscillating lake shorelines, the remarkable preservation of pseudomorphs being a consequence of dolomitization.

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