Windermere Supergroup - Sedimentation Begins: Filling The Basin

Sedimentation Begins: Filling The Basin

Sedimentation began in the Caradoc (upper Ordovician, 455 million years ago). During the Llandovery, the Stockdale Supergroup is marked by a number of oxic-anoxic transitions, with black shales corresponding to transgressions - these may have helped to mitigate a runaway greenhouse effect. The rate of sediment accumulation accelerated with time; it held fairly steady at a low ~50 metres per million years (m/Ma) until the Wenlock (mid Silurian, 424 million years ago), when it increased greatly, eventually reaching over 1000 m/Ma when the record is terminated by erosion in the Pridoli (terminal Silurian, 419 million years ago). This sudden upturn in deposition rate is a result of the increasing proximity of the Avalon mountain belt, which started to depress the plate from the Ordovician, but was not close enough to increase the sedimentary input until the Silurian. The latest phase of sedimentation reflected a change in the basin's state. Instead of being underfilled, and trapping all sediment that flowed into it, it became overfilled. This was reflected by a shallowing of water depth, as the basin silted up. This culminated with a transition to terrestrial conditions in the Přídolí.