Soom Shale

The Soom Shale is a member of the Late Ordovician (Hirnantian) Cederberg Formation (Table Mountain Group) in South Africa, renowned for its remarkable preservation of soft-tissue in fossil material.[1] Deposited in still waters, the unit lacks bioturbation, perhaps indicating anoxic conditions.[1]

Soom Shale
Stratigraphic range: Hirnantian
Typical lithological features of the Soom shale, note the frequent beds of white quartz
Unit ofCederberg Formation, Table Mountain Group
UnderliesDisa Siltstone
OverliesPakhuis Tillite
Thickness10–15 m (33–49 ft)
Lithology
PrimaryShale
Location
Coordinates33.0°S 19.0°E / -33.0; 19.0
Approximate paleocoordinates28.5°S 153.7°E / -28.5; 153.7
RegionWestern Cape
Country South Africa
Soom Shale (South Africa)

It overlies the Pakhuis tillite and is overlain by the Disa Siltstone.[1]

It contains typical Ordovician microfossils, such as chitinozoa, acritarchs and spores, and its shelly fauna is also typical of this time period.[1]

Its macrofauna comprises pelagic organisms that sank rapidly to a barren sea floor.[2] These include brachiopods,[3] eurypterids, conodonts,[4] nektaspids, trilobites and orthoconic cephalopods.

Environment of formation

The environment of deposition is considered to be anoxic and occasionally euxinic cold bottom waters, as indicated by the lack of bioturbation and most epibenthic fauna, besides a small amount of transiently colonizing brachiopods during temporary oxic conditions. It was deposited in the immediate aftermath of the Hirnantian glaciation, the most likely primary cause of the Ordovician–Silurian extinction event.[5]

Preservation

The preservation of fossils in the Soom Shale is the opposite of that of normal fossil deposits. Things that in typical preservation conditions are not preserved in the fossil record like soft tissue are preserved in exquisite detail. While hard parts (exoskeleton, etc.) that are normally preserved in the fossil record are demineralized, and are therefore by comparison poorly preserved, usually being preserved in the form of a mould. There is almost no taphonomic distortion, with little evidence of transport, current alignment and no evidence of scavenging. The process of preservation for soft tissues is under debate, but possibly could involve replacement by aluminosilicates, but this could just be an artifact of diagenesis and metamorphism.[5]

Biota
Genus Type of organism Images
Promissum Conodont
Promissum
Notiodella Conodont
Onychopterella Eurypterid
Soomaspis Liwiid Nektaspid
Mucronaspis Dalmanitid Trilobite
Soomicaris Archaeostracan Phyllocarid
Myodoprimigenia Myodocope Ostracod
Palaeoglossa Obolid Brachiopod
Trematis Trematid Brachiopod
Kosoidea Discinid Brachiopod
Plectothyrella Rhynchonellid Brachiopod
Siphonacis Incertae sedis
Undescribed Agnathan jawless fish

References
  1. Gabbott, S. E. (1999). "Orthoconic cephalopods and associated fauna from the late Ordovician Soom Shale Lagerstatte, South Africa". Palaeontology. 42: 123–148. doi:10.1111/1475-4983.00065.
  2. Aldridge, R.J.; Gabbott, S.E.; Theron, J.N. (2001). "The Soom Shale". Palaeobiology II. p. 340. doi:10.1002/9780470999295.ch79. ISBN 9780470999295.
  3. Bassett, Michael G.; Popov, Leonid E.; Aldridge, Richard J.; Gabbott, Sarah E.; Theron, Johannes N. (2009). "Brachiopoda from the Soom Shale Lagerstätte (Upper Ordovician, South Africa)". Journal of Paleontology. 83 (4): 614. doi:10.1666/08-136.1.
  4. Aldridge, Richard J.; Murdock, Duncan J. E.; Gabbott, Sarah E.; Theron, Johannes N. (2013). "A 17-element conodont apparatus from the Soom Shale Lagerstätte (Upper Ordovician), South Africa". Palaeontology. 56 (2): 261. doi:10.1111/j.1475-4983.2012.01194.x.
  5. Gabbott, Sarah E.; Browning, Claire; Theron, Johannes N.; Whittle, Rowan J. (2017). "The late Ordovician Soom Shale Lagerstätte: an extraordinary post-glacial fossil and sedimentary record" (PDF). Journal of the Geological Society. 174 (1): 1–9. doi:10.1144/jgs2016-076. ISSN 0016-7649.
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