During icehouse times, the oceanic eustatic response to rapidly waxing and waning polar ice sheets is in terms of amplitude excessively high, and time-wise too short, to allow sufficient time for periplatform-margin biogenic buildups to grow into continuous platform-edge barriers. It is the long-term elevated sealevels of greenhouse times, along with their lower fourth-order amplitudes (less than 5-10 metres of sealevel change every 100,000 years), that allow persistent platform-edge reefs and shoals to form (see inset in the figure). With a subsequent slight fall in sealevel, these contiguous platform-edge barriers became subaerial barriers to any hydrographic connection of the back-barrier lagoon to the open ocean. This initiates marine-seepage fed lagoons (salterns) in their hinterland. Periplatform evaporites tend to megasulphate rather than megahalite affinities and today act as regional seals to many of the world’s oil and gas fields (e.g. Hith Anhydrite, Middle Anhydrite marker of the Khuff Fm and the Ferry Lake Anhydrite; Warren, 2016, Chapter 10).