Fossils of The Burgess Shale - Geology


Shale Limestone Dolomite Walcott's "Phyllopod bed" "Thin" Stephen Formation "Thick"
Stephen Formation

Burgess Formation Cathedral
160 metres (520 ft) Geology of the Burgess Shale

The Burgess Shale is a series of sediment deposits spread over a vertical distance of hundreds of metres, extending laterally for at least 50 kilometres (30 mi). The deposits were originally laid down on the floor of a shallow sea; during the Late Cretaceous Laramide orogeny, mountain-building processes squeezed the sediments upwards to their current position at around 2,500 metres (8,000 ft) elevation in the Rocky Mountains.

The rocks containing the fossils are on the border between two partially overlapping bands of rock that run along the western face of the Canadian Rockies. On the eastern side of this border is the Cathedral Formation, a platform of limestone formed by algae. The western surface of the Cathedral Formation is steep and consists of the resistant rock dolomite, which was originally part of the limestone platform, but between the Mid Silurian and Late Devonian was transformed by hydrothermal flows of brine at up to 200 °C (400 °F), which replaced much of the limestone's calcium with magnesium. A layer of shale lies partly on top of and partly to the west of the Cathedral Formation. This shale layer used to be called the "thin" Stephen Formation where it lies above the Cathedral Formation and the "thick" Stephen Formation where it lies to the west; but the "thick" Stephen Formation is now generally known as the Burgess Formation.

The shale is made of alternating fine-grained layers of siliceous mudstone (compressed, hardened mud originally made of ground-up silicate rock) and calcisiltite originally animal shells). The calcisiltite layers contain relatively unremarkable shells and occasional non-biomineralized fossils (such as priapulid tubes). The soft-bodied organisms for which the Burgess Shale is famous are fossilized in the mudstone layers, which are between 2 and 170 millimetres (0.079 and 6.7 in) thick, averaging 30 millimetres (1.2 in), and have well-defined bases. Opinions vary about how the mudstone layers were produced: perhaps by mudslides from the top of the "Cathedral" limestone platform, when its edge collapsed occasionally; or possibly by storms that created back-currents that abruptly washed large volumes of mud off the platform. Each mudstone layer is the result of one such catastrophe. The Greater Phyllopod Bed, a 7 metres (23 ft) thick sequence consisting of Walcott's famous "Phyllopod Bed" plus the 5 metres (16 ft) below that, contains at least 36 layers, deposited over 10 to 100 thousand years, during which the environment was essentially stable.

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