Editing Burgess Shale (section)

== Geological setting ==
{{Main|Stephen Formation}}
[[File:Burgess Shale, Yoho National Park of Canada.jpg|thumb|[[Satellite]] image of the area.]]

The fossil-bearing deposits of the Burgess Shale correlate to the [[Stephen Formation]], a collection of slightly calcareous dark mudstones, about {{Ma|burgess shale|million years}} old.<ref name=fotbs /> The beds were deposited at the base of a cliff about 160&nbsp;m tall,<ref name=fotbs /> below the depth agitated by waves during storms.<ref name=Gabbott2008>{{cite journal
 | last1 = Gabbott | first1 = S.E.
 | last2 = Zalasiewicz | first2 = J.
 | last3 = Collins | first3 = D.
 | year = 2008
 | title = Sedimentation of the Phyllopod bed within the Cambrian Burgess Shale Formation of British Columbia
 | journal = [[Journal of the Geological Society]]
 | volume = 165
 | issue = 1
 | pages = 307–318
 | url = http://jgs.lyellcollection.org/cgi/content/abstract/165/1/307
 | doi = 10.1144/0016-76492007-023
| bibcode = 2008JGSoc.165..307G| s2cid = 128685811
 }}</ref> This vertical cliff was composed of the calcareous reefs of the [[Cathedral escarpment|Cathedral Formation]], which probably formed shortly before the deposition of the Burgess Shale.<ref name=fotbs /> The precise formation mechanism is not known for certain, but the most widely accepted hypothesis suggests that the edge of the Cathedral Formation reef became detached from the rest of the reef, slumping and being transported some distance – perhaps kilometers – away from the reef edge.<ref name=fotbs /> Later reactivation of faults at the base of the formation led to its disintegration from about {{Ma|509}}.<ref name="Collom2009">{{cite journal| year = 2009| pages = 63–85| doi = 10.1016/j.palaeo.2009.02.012| last2 =   Johnston| last3 =   Powell| volume = 277| issue = 1–2| journal =   Palaeogeography, Palaeoclimatology, Palaeoecology | first1 = C. J.| first2 = P. A.| first3 = W. G.| title = Reinterpretation of 'Middle' Cambrian stratigraphy of the rifted western Laurentian margin: Burgess Shale Formation and contiguous units (Sauk II Megasequence); Rocky Mountains, Canada| last1 = Collom | bibcode = 2009PPP...277...63C}}</ref> This would have left a steep cliff, the bottom of which would be protected from tectonic decompression because the limestone of the Cathedral Formation is difficult to compress. This protection explains why fossils preserved further from the Cathedral Formation are impossible to work with – tectonic squeezing of the beds has produced a vertical cleavage that fractures the rocks, so they split perpendicular to the fossils.<ref name=fotbs /> The Walcott quarry produced such spectacular fossils because it was so close to the Stephen Formation – indeed the quarry has now been excavated to the very edge of the Cambrian cliff.<ref name=fotbs />

It was originally thought that the Burgess Shale was deposited in [[anoxic event|anoxic]] conditions, but mounting research shows that oxygen was continually present in the sediment.<ref name=Powell2009>{{cite journal| year = 2009| last1 = Powell| doi = 10.1016/j.palaeo.2009.02.016| pages = 127–140| issue = 1–2| title = Comparison of Geochemical and Distinctive Mineralogical Features Associated with the Kinzers and Burgess Shale Formations and their Associated Units| journal =   Palaeogeography, Palaeoclimatology, Palaeoecology| volume = 277| first1 = W.| bibcode = 2009PPP...277..127P}}</ref> The anoxic setting had been thought to not only protect the newly dead organisms from decay, but it also created chemical conditions allowing the preservation of the soft parts of the organisms. Further, it reduced the abundance of burrowing organisms – burrows and trackways ''are'' found in beds containing soft-bodied organisms, but they are rare and generally of limited vertical extent.<ref name=fotbs /> Brine seeps are an alternative hypothesis; see [[Burgess Shale type preservation]] for a more thorough discussion.