Stephen Formation
Encyclopedia
The Stephen Formation is a middle Cambrian unit exposed in the Canadian Rockies
of British Columbia
. It is famous for the exceptional preservation
of soft-bodied fossils: the Burgess Shale biota
. The formation overlies the cathedral escarpment
, a submarine cliff; consequently it is divided into two quite separate parts, the 'thin' sequence deposited in the shallower waters atop the escarpment, and the 'thick' sequence deposited in the deeper waters beyond the cliff. Because the 'thick' Stephen Formation represents a distinct lithofacies, some authors suggest it warrants its own name, and dub it the Burgess Shale Formation. The stratigraphy of the Thin Stephen Formation has not been subject to extensive study, so except where explicitly mentioned this article applies mainly to the Thick Stephen Formation.
The fossiliferous deposits of the Stephen Formation are a collection of slightly calcareous dark mudstones, about old. The beds were deposited on top of and at the base of a cliff about 160 metres (524.9 ft) tall, below the depth agitated by waves during storms. This vertical cliff was composed of the calcareous reefs of the Cathedral Formation, which probably formed shortly before the deposition of the Burgess shale. 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 kilometres — away from the reef edge. Later reactivation of faults at the base of the formation led to its disintegration from about . This would have left a steep cliff, the bottom of which would be protected, because the limestone of the Cathedral formation is difficult to compress, from tectonic decompression. 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. The Walcott quarry produced such spectacular fossils because it was so close the Stephen formation — indeed the quarry has now been excavated to the very edge of the Cambrian cliff. Both the thick and thin Stephen formation were deposited below wave base.
It was originally thought that the Burgess Shale was deposited in anoxic conditions, but mounting research shows that oxygen was continually present in the sediment. 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.
beds. The Wash member, which contains many shelly but no soft-bodied fossils, interrupts this sequence in places, and directly underlies the Phyllopod beds, which mark the base of the Walcott Quarry member. This underlies the Wapta member, which is unconformably overlain by 'Tokumm'.
The Wapta member has been redefined into the Raymond Quarry member, Emerald Lake member, Odaray member, Paradox member and Marpole member. The thin Stephen grades conformably into the overlying Eldon formation.
s. This lies at the base of the Walcott Quarry member, and three other quarries – the Raymond, UE and EZ – lie above it. The UE and EZ quarries herald from the Upper Ehmaniella Zone and Ehmaniella Zone, respectively, and belong to the in the Emerald Lake member. The Campsite Cliff member contains the Ogygopsis-bearing Mount Stephen trilobite beds
(both on Mount Stephen
), while the Collins Quarry (containing the Sanctacaris beds) is situated in the Kicking Horse member. The S7 locality on Mount Stephen has been attributed both to the Campsite Cliff member and the Kicking Horse member. The Trilobite Beds, the first Burgess Shale locality to be discovered, mark the southerly extent of fossiliferous exposure on Mount Stephen, although many more sites exist on the inaccessible northeasterly flank of the mountain. The Lower Trilobite Beds, although lower on the mountainside, are in fact stratigraphically higher than the Upper Trilobite Beds.
Fossils have also been collected from the 'thin' Stephen Formation, in the vicinity of the Stanley Glacier, some 40 km from the main collecting sites on Fossil Ridge and Mount Stephen. They have been recorded around Odaray Mountain, Park Mountain, Curtis Peak, Natalko Lake and Monarch Cirque, although no major collections of these localities has yet been performed.
Canadian Rockies
The Canadian Rockies comprise the Canadian segment of the North American Rocky Mountains range. They are the eastern part of the Canadian Cordillera, extending from the Interior Plains of Alberta to the Rocky Mountain Trench of British Columbia. The southern end borders Idaho and Montana of the USA...
of British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
. It is famous for the exceptional preservation
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
of soft-bodied fossils: the Burgess Shale biota
Fossils of the Burgess Shale
The fossils of the Burgess Shale, like the Burgess Shale itself, formed around in the Mid Cambrian period. They were discovered in Canada in 1886, and Charles Doolittle Walcott collected over 60,000 specimens in a series of field trips up from 1909 to 1924...
. The formation overlies the cathedral escarpment
Cathedral escarpment
The Cathedral escarpment was a submarine cliff during the Cambrian period, and is often associated with the exquisite preservation of the Burgess Shale. It runs for around 100 km through and around Yoho national park, British Columbia...
, a submarine cliff; consequently it is divided into two quite separate parts, the 'thin' sequence deposited in the shallower waters atop the escarpment, and the 'thick' sequence deposited in the deeper waters beyond the cliff. Because the 'thick' Stephen Formation represents a distinct lithofacies, some authors suggest it warrants its own name, and dub it the Burgess Shale Formation. The stratigraphy of the Thin Stephen Formation has not been subject to extensive study, so except where explicitly mentioned this article applies mainly to the Thick Stephen Formation.
Sedimentary setting
The Stephen Formation formed at a low-latitude miogeoclinic continental margin, at the western limit of a continental craton. Detrital sediments were washed in by rivers from the continent, over the limestone reefs which formed the shallow sea floor. At the top of sequence-stratigraphic cycles, oncoids were sometimes washed in to the Thin Stephen formation from the shallower waters closer to the shore.The fossiliferous deposits of the Stephen Formation are a collection of slightly calcareous dark mudstones, about old. The beds were deposited on top of and at the base of a cliff about 160 metres (524.9 ft) tall, below the depth agitated by waves during storms. This vertical cliff was composed of the calcareous reefs of the Cathedral Formation, which probably formed shortly before the deposition of the Burgess shale. 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 kilometres — away from the reef edge. Later reactivation of faults at the base of the formation led to its disintegration from about . This would have left a steep cliff, the bottom of which would be protected, because the limestone of the Cathedral formation is difficult to compress, from tectonic decompression. 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
Cleavage (geology)
This article is about rock cleavage, for cleavage in minerals see Cleavage Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism. The degree of deformation and metamorphism along with rock type determines the...
that fractures the rocks, so they split perpendicular to the fossils. The Walcott quarry produced such spectacular fossils because it was so close the Stephen formation — indeed the quarry has now been excavated to the very edge of the Cambrian cliff. Both the thick and thin Stephen formation were deposited below wave base.
It was originally thought that the Burgess Shale was deposited in anoxic conditions, but mounting research shows that oxygen was continually present in the sediment. 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.
Subdivisions
The formation is made up of the Kicking Horse member, which includes the Alalcomenaeus–Sanctacaris beds; this underlies and interdigitates with the unfossiliferous Yoho River member. These two are truncated by an unconformity and covered by the Campsite Cliff member, which contains the OgygopsisOgygopsis
Ogygopsis is a genus of trilobite from the Cambrian of Antarctica and North America, specifically the Burgess Shale. It is the most common fossil in the Mt. Stephen fossil beds there, but rare in other Cambrian faunas...
beds. The Wash member, which contains many shelly but no soft-bodied fossils, interrupts this sequence in places, and directly underlies the Phyllopod beds, which mark the base of the Walcott Quarry member. This underlies the Wapta member, which is unconformably overlain by 'Tokumm'.
The Wapta member has been redefined into the Raymond Quarry member, Emerald Lake member, Odaray member, Paradox member and Marpole member. The thin Stephen grades conformably into the overlying Eldon formation.
Fossiliferous collection sites
Of the dozen-plus fossiliferous sites in the Stephen formation, the Walcott Quarry is the most famous, bearing the Phyllopod bedPhyllopod bed
The Phyllopod bed, designated by USNM locality number 35k, is the most famous fossil-bearing member of the Burgess shale fossil lagerstatte. It was quarried by Charles Walcott from 1911–1917, and was the source of 95% of the fossils he collected during this time;tens of thousands of...
s. This lies at the base of the Walcott Quarry member, and three other quarries – the Raymond, UE and EZ – lie above it. The UE and EZ quarries herald from the Upper Ehmaniella Zone and Ehmaniella Zone, respectively, and belong to the in the Emerald Lake member. The Campsite Cliff member contains the Ogygopsis-bearing Mount Stephen trilobite beds
Mount Stephen trilobite beds
The Mount Stephen trilobite beds are a series of fossil strata on Mount Stephen, British Columbia that contain exceptionally preserved fossil material...
(both on Mount Stephen
Mount Stephen
Mount Stephen is a mountain located in the Kicking Horse River Valley of Yoho National Park, ½ km east of Field. The mountain was named in 1886 for George Stephen, the first president of the Canadian Pacific Railway....
), while the Collins Quarry (containing the Sanctacaris beds) is situated in the Kicking Horse member. The S7 locality on Mount Stephen has been attributed both to the Campsite Cliff member and the Kicking Horse member. The Trilobite Beds, the first Burgess Shale locality to be discovered, mark the southerly extent of fossiliferous exposure on Mount Stephen, although many more sites exist on the inaccessible northeasterly flank of the mountain. The Lower Trilobite Beds, although lower on the mountainside, are in fact stratigraphically higher than the Upper Trilobite Beds.
Fossils have also been collected from the 'thin' Stephen Formation, in the vicinity of the Stanley Glacier, some 40 km from the main collecting sites on Fossil Ridge and Mount Stephen. They have been recorded around Odaray Mountain, Park Mountain, Curtis Peak, Natalko Lake and Monarch Cirque, although no major collections of these localities has yet been performed.