Stephen Formation

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Stephen Formation

Stratigraphic range: Middle Cambrian
WalcottQuarry080509.jpg
The Walcott Quarry in the Stephen Formation shales
Type Geological formation
Underlies Eldon Formation
Overlies Cathedral Formation
Thickness"Thin" Stephen: < 60 metres (200 ft) [1]
"Thick" Stephen: up to 335 metres (1,100 ft) [2]
Lithology
Primary Shale, limestone
Other Siltstone
Location
Coordinates 51°27′51″N116°19′28″W / 51.46425°N 116.32443°W / 51.46425; -116.32443 (Stephen Formation)
Region Canadian Rockies
CountryFlag of Canada (Pantone).svg  Canada
Type section
Named for Mount Stephen (from George Stephen)
Named by Charles Doolittle Walcott, 1908 [3] [4]

The Stephen Formation is a geologic formation exposed in the Canadian Rockies of British Columbia and Alberta, on the western edge of the Western Canada Sedimentary Basin. It consists of shale, thin-bedded limestone, and siltstone that was deposited during Middle Cambrian time (513 to 497 million years ago). [5] It is famous for the exceptional preservation of soft-bodied fossils: the Burgess Shale biota. [6] [7] 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. [8] 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.

Contents

Sedimentary setting

Satellite image of the area. Burgess Shale, Yoho National Park of Canada.jpg
Satellite image of the area.

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. [9] 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. [1]

The fossiliferous deposits of the Stephen Formation are a sequence of slightly calcareous dark mudstones, about 508 million years old. [10] The beds were deposited on top of and at the base of a cliff about 160 meters (520 ft) tall, [10] below the depth agitated by waves during storms, [11] and thus at a water depth of around 200 m. [12] This vertical cliff was composed of the calcareous reefs of the Cathedral Formation, which probably formed shortly before the deposition of the Burgess shale. [10] 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. [10] Later reactivation of faults at the base of the formation led to its disintegration from about 509  million years ago. [6] 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 produced a vertical cleavage that fractures the rocks, so they split perpendicular to the fossils. [10] 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. [10] Both the thick and thin Stephen formation were deposited below wave base. [1]

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. [13] 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. [10]

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. [9] These two are truncated by an unconformity and covered by the Campsite Cliff member, which contains the Ogygopsis beds. The Wash member, which contains many shelly but no soft-bodied fossils, [9] 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'. [6]

The Wapta member has been redefined into the Raymond Quarry member, Emerald Lake member, Odaray member, Paradox member and Marpole member. [9] The thin Stephen grades conformably into the overlying Eldon formation. [1]

Fossiliferous collection sites

Of the dozen-plus fossiliferous sites in the Stephen formation, [1] the Walcott Quarry is the most famous, bearing the Phyllopod beds. This lies at the base of the Walcott Quarry member, and three other quarries – the Raymond, UE and EZ – lie above it. [6] The UE and EZ quarries herald from the Upper Ehmaniella Zone and EhmaniellaZone, respectively, and belong to the Emerald Lake member. [9] The Campsite Cliff member contains the Ogygopsis-bearing Mount Stephen trilobite beds (both on Mount Stephen), [14] 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 [14] and the Kicking Horse member. [9] The Trilobite Beds, the first Burgess Shale locality to be discovered, [15] mark the southerly extent of fossiliferous exposure on Mount Stephen, although many more sites exist on the inaccessible northeasterly flank of the mountain. [9] The Lower Trilobite Beds, although lower on the mountainside, are in fact stratigraphically higher than the Upper Trilobite Beds. [9]

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. [1] They have been recorded around Odaray Mountain, Park Mountain, Curtis Peak, Natalko Lake and Monarch Cirque, although no major collection at these localities has yet been performed. [9]

Related Research Articles

<span class="mw-page-title-main">Burgess Shale</span> Fossil-bearing rock formation in the Canadian Rockies

The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At 508 million years old, it is one of the earliest fossil beds containing soft-part imprints.

<i>Marrella</i> Extinct genus of Arthropods

Marrella is an extinct genus of marrellomorph arthropod known from the middle Cambrian Burgess Shale of British Columbia. It is the most common animal represented in the Burgess Shale, with tens of thousands of specimens collected. Much rarer remains are also known from deposits in China.

<span class="mw-page-title-main">Charles Doolittle Walcott</span> American paleontologist and 4th Secretary of the Smithsonian (1850–1927)

Charles Doolittle Walcott was an American paleontologist, administrator of the Smithsonian Institution from 1907 to 1927, and director of the United States Geological Survey. He is famous for his discovery in 1909 of well-preserved fossils, including some of the oldest soft-part imprints, in the Burgess Shale of British Columbia, Canada.

<i>Waptia</i> Cambrian arthropod

Waptia is an extinct genus of arthropod from the Middle Cambrian of North America. It grew to a length of 6.65 cm (3 in), and had a large bivalved carapace and a segmented body terminating into a pair of tail flaps. It was an active swimmer and likely a predator of soft-bodied prey. It is also one of the oldest animals with direct evidence of brood care. Waptia fieldensis is the only species classified under the genus Waptia, and is known from the Burgess Shale Lagerstätte of British Columbia, Canada. Specimens of Waptia are also known from the Spence Shale of Utah, United States.

<i>Choia</i> Genus of sponges

Choia is a genus of extinct demosponge ranging from the Cambrian until the Lower Ordovician periods. Fossils of Choia have been found in the Burgess Shale in British Columbia; the Maotianshan shales of China; the Wheeler Shale in Utah; and the Lower Ordovician Fezouata formation. It was first described in 1920 by Charles Doolittle Walcott.

<i>Helmetia</i> Genus of arthropods (fossil)

Helmetia is an extinct genus of arthropod from the middle Cambrian. Its fossils have been found in the Burgess Shale of Canada and the Jince Formation of the Czech Republic.

A number of assemblages bear fossil assemblages similar in character to that of the Burgess Shale. While many are also preserved in a similar fashion to the Burgess Shale, the term "Burgess Shale-type fauna" covers assemblages based on taxonomic criteria only.

The fossils of the Burgess Shale, like the Burgess Shale itself, are fossils that formed around 505 million years ago in the mid-Cambrian period. They were discovered in Canada in 1886, and Charles Doolittle Walcott collected over 65,000 specimens in a series of field trips up to the alpine site from 1909 to 1924. After a period of neglect from the 1930s to the early 1960s, new excavations and re-examinations of Walcott's collection continue to reveal new species, and statistical analysis suggests that additional discoveries will continue for the foreseeable future. Stephen Jay Gould's book Wonderful Life describes the history of discovery up to the early 1980s, although his analysis of the implications for evolution has been contested.

<i>Eldonia</i> Extinct genus of soft-bodied animals

Eldonia is an extinct soft-bodied cambroernid animal of unknown affinity, best known from the Fossil Ridge outcrops of the Burgess Shale, particularly in the 'Great Eldonia layer' in the Walcott Quarry. In addition to the 550 collected by Walcott, 224 specimens of Eldonia are known from the Greater Phyllopod bed, where they comprise 0.43% of the community. Species also occur in the Chengjiang biota, Siberia, and in Upper Ordovician strata of Morocco.

<span class="mw-page-title-main">History of the Burgess Shale</span>

The Burgess Shale, a series of fossil beds in the Canadian Rockies, was first noticed in 1886 by Richard McConnell of the Geological Survey of Canada (GSC). His and subsequent finds, all from the Mount Stephen area, came to the attention of palaeontologist Charles Doolittle Walcott, who in 1907 found time to reconnoitre the area. He opened a quarry in 1910 and in a series of field trips brought back 65,000 specimens, which he identified as Middle Cambrian in age. Due to the quantity of fossils and the pressures of his other duties at the Smithsonian Institution, Walcott was only able to publish a series of "preliminary" papers, in which he classified the fossils within taxa that were already established. In a series of visits beginning in 1924, Harvard University professor Percy Raymond collected further fossils from Walcott's quarry and higher up on Fossil Ridge, where slightly different fossils were preserved.

The Phyllopod bed, designated by USNM locality number 35k, is the most famous fossil-bearing member of the Burgess Shale fossil Lagerstätte. 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 soft-bodied fossils representing over 150 genera have been recovered from the Phyllopod bed alone.

Cambrorhytium is an enigmatic fossil genus known from the Latham Shale (California), and the Chengjiang (China) and Burgess Shale lagerstätte. 350 specimens of Cambrorhytium are known from the Greater Phyllopod bed, where they comprise 0.7% of the community.

Eiffelia is an extinct genus of sponges known from the Middle Cambrian Burgess Shale as well as several Early Cambrian small shelly fossil deposits. It is named after Eiffel Peak, which was itself named after the Eiffel Tower. It was first described in 1920 by Charles Doolittle Walcott. It belongs in the Hexactinellid stem group. 60 specimens of Eiffelia are known from the Greater Phyllopod bed, where they comprise 0.11% of the community.

<i>Pagetia</i> Genus of trilobites

Pagetia Walcott, 1916. is a small genus of trilobite, assigned to the Eodiscinid family Pagetiidae and which had global distribution during the Middle Cambrian. The genus contains 55 currently recognized species, each with limited spatial and temporal ranges.

<i>Pirania</i> Extinct genus of sponges

Pirania is an extinct genus of sea sponge known from the Middle Cambrian Burgess Shale and the Ordovician Fezouata formation. It is named after Mount St. Piran, a mountain situated in the Bow River Valley in Banff National Park, Alberta. It was first described in 1920 by Charles Doolittle Walcott. 198 specimens of Pirania are known from the Greater Phyllopod bed, where they comprise 0.38% of the community.

Wapkia is an extinct genus of sea sponge with radial sclerites, known from the Middle Cambrian Burgess Shale. It was first described in 1920 by Charles Doolittle Walcott. 32 specimens of Wapkia are known from the Greater Phyllopod bed, where they comprise 0.06% of the community.

The Mount Stephen trilobite beds are a series of fossil strata on Mount Stephen, British Columbia that contain exceptionally preserved fossil material. Part of the same stratigraphic unit as the Burgess Shale deposit, many non-mineralized parts are preserved; in addition, a high density of trilobite fossils is present.

<span class="mw-page-title-main">Walcott Quarry</span> Quarry in British Columbia, Canada

The Walcott Quarry is the most famous quarry of the Middle Cambrian Burgess Shale, located in the Canadian Rockies of British Columbia, bearing the Phyllopod beds. This lies at the base of the Walcott Quarry member, on a ridge between Wapta Mountain and Mount Field, and three other quarries – the Raymond, UE and EZ – lie above it. The quarry's proximity to the Cathedral escarpment led to the preservation of spectacular fossils.

<span class="mw-page-title-main">Cathedral Formation</span>

The Cathedral Formation is a stratigraphic unit in the southern Canadian Rockies of Alberta and British Columbia, on the western edge of the Western Canada Sedimentary Basin. It is a thick sequence of carbonate rocks of Middle Cambrian age. It was named for Cathedral Mountain in Yoho National Park by Charles Doolittle Walcott, the discoverer of the Burgess shale fossils.

The Mount Whyte Formation is a stratigraphic unit that is present on the western edge of the Western Canada Sedimentary Basin in the southern Canadian Rockies and the adjacent southwestern Alberta plains. It was deposited during Middle Cambrian time and consists of shale interbedded with other siliciclastic rock types and limestones. It was named for Mount Whyte in Banff National Park by Charles Doolittle Walcott, the discoverer of the Burgess shale fossils, and it includes several genera of fossil trilobites.

References

  1. 1 2 3 4 5 6 Caron, J. -B.; Gaines, R. R.; Mangano, M. G.; Streng, M.; Daley, A. C. (2010). "A new Burgess Shale-type assemblage from the "thin" Stephen Formation of the southern Canadian Rockies". Geology. 38 (9): 811. Bibcode:2010Geo....38..811C. doi:10.1130/G31080.1.
  2. ↑ Lexicon of Canadian Geologic Units. "Stephen Formation" . Retrieved 2010-01-02.
  3. ↑ Walcott, C.D., 1908a. Nomenclature of some Cambrian Cordilleran formations; Smithsonian Miscellaneous Collections, vol. 35, no. 1.
  4. ↑ Walcott, C.D., 1908b. Cambrian geology and paleontology: Cambrian sections of the Condilleran area. Smithsonian Miscellaneous Collections, vol. 53, no. 5, pp. 204-208.
  5. ↑ "Lithological Unit Search". Lexicon of Canadian Geologic Units . Natural Resources Canada . Retrieved 1 August 2018.
  6. 1 2 3 4 Collom, C. J.; Johnston, P. A.; Powell, W. G. (2009). "Reinterpretation of 'Middle' Cambrian stratigraphy of the rifted western Laurentian margin: Burgess Shale Formation and contiguous units (Sauk II Megasequence); Rocky Mountains, Canada". Palaeogeography, Palaeoclimatology, Palaeoecology. 277 (1–2): 63–85. doi:10.1016/j.palaeo.2009.02.012.
  7. ↑ Slind, O.L., Andrews, G.D., Murray, D.L., Norford, B.S., Paterson, D.F., Salas, C.J., and Tawadros, E.E., Canadian Society of Petroleum Geologists and Alberta Geological Survey (1994). "The Geological Atlas of the Western Canada Sedimentary Basin (Mossop, G.D. and Shetsen, I., compilers), Chapter 8: Middle Cambrian and Early Ordovician Strata of the Western Canada Sedimentary Basin" . Retrieved 2018-07-13.{{cite web}}: CS1 maint: multiple names: authors list (link)
  8. ↑ Fletcher, T. P.; Collins, D. (1998). "The Middle Cambrian Burgess Shale and its relationship to the Stephen Formation in the southern Canadian Rocky Mountains". Canadian Journal of Earth Sciences. 35 (4): 413–436. Bibcode:1998CaJES..35..413F. doi:10.1139/cjes-35-4-413.
  9. 1 2 3 4 5 6 7 8 9 Fletcher, T.P.; Collins, D. (July 1, 2009). "Geology and Stratigraphy of the Burgess Shale Formation on Mount Stephen and Fossil Ridge". In Caron, Jean-Bernard; Rudkin, David (eds.). A Burgess Shale Primer—History, Geology and Research Highlights. The Burgess Shale Consortium. pp. 33–54. ISBN   978-0-9812885-0-5.
  10. 1 2 3 4 5 6 7 Briggs, D. E. G.; Erwin, D. H.; Collier, F. J. (1995), Fossils of the Burgess Shale , Washington: Smithsonian Inst Press, ISBN   1-56098-659-X, OCLC   231793738
  11. ↑ Gabbott, S.E.; Zalasiewicz, J.; Collins, D. (2008). "Sedimentation of the Phyllopod Bed within the Cambrian Burgess Shale Formation of British Columbia". Journal of the Geological Society. 165 (1): 307. Bibcode:2008JGSoc.165..307G. doi:10.1144/0016-76492007-023.
  12. ↑ Piper, D. J. W. (1972). "Sediments of the Middle Cambrian Burgess Shale, Canada". Lethaia. 5 (2): 169–175. doi:10.1111/j.1502-3931.1972.tb00850.x.
  13. ↑ Powell, W. (2009). "Comparison of Geochemical and Distinctive Mineralogical Features Associated with the Kinzers and Burgess Shale Formations and their Associated Units". Palaeogeography, Palaeoclimatology, Palaeoecology. 277 (1–2): 127–140. Bibcode:2009PPP...277..127P. doi:10.1016/j.palaeo.2009.02.016.
  14. 1 2 Caron, J.-B.; Scheltema, A.; Schander, C.; Rudkin, D. (2006). "A soft-bodied mollusc with radula from the Middle Cambrian Burgess Shale" (PDF). Nature. 442 (7099): 159–163. Bibcode:2006Natur.442..159C. doi:10.1038/nature04894. hdl: 1912/1404 . PMID   16838013.
  15. ↑ Collins, D. (July 1, 2009). "A Brief History of Field Research on the Burgess Shale". In Caron, Jean-Bernard; Rudkin, David (eds.). A Burgess Shale Primer—History, Geology and Research Highlights. The Burgess Shale Consortium. pp. 15–32. ISBN   978-0-9812885-0-5.
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