| Baratal limestone | |
|---|---|
| Stratigraphic range: Ediacaran-Cambrian | |
| Type | Geologic formation |
| Thickness | 8km |
| Lithology | |
| Primary | Limestone |
| Location | |
| Coordinates | 50°15′N 87°53′E / 50.250°N 87.883°E[1] |
| Region | Siberia |
| Country |  Russia |
  Baratal limestone (Russia) | |
Baratal limestone, also known as the Baratal Formation, is a limestone formation in the Altai Republic of Russia that formed during the Cambrian and Ediacaran, presumably on top of an ancient seamount that was later accreted to a continent. It may constitute the oldest known atoll in the world.
Geography and geology
The formation crops out west and northwest of Chagan-Uzun in the Altai Republic, Russia, in particular around Kurai and Akkaya[2] in the Kosh-Agach region.[3] It is also known as the Baratal Formation.[4]
The Baratal limestone of Cambrian age is the oldest known carbonate deposit on a seamount,[5] akin to a present-day atoll.[6] It formed a no more than 500 metres (1,600 ft) thick unit before tectonic processes thickened it to about 8 kilometres (5.0 mi).[7] Rock units include limestone breccia/conglomerates, bedded limestone and ooid mudstones[2] with 0.5 millimetres (0.020 in) ooids,[8] and chert nodules.[9] The limestone is associated with greenstone, which probably constitutes the volcanic basement that the Baratal limestone was emplaced upon.[7] Parts of the unit may be buried by glacial deposits.[10]
Interpretation
The limestone probably formed on a volcanic seamount or oceanic plateau[7] in the open ocean, remote from any continental influence. The ooid mudstones may have developed in lagoonal environments like they are found on Great Bahama Bank,[11] where photic zone[8] waters were intensely agitated by tides[11] and where the climate may have been hot and dry. Small sediment movements and large mass movement took place around the carbonate platforms, traces of this instability are found in the Baratal limestone[12] which contains parts from both the original carbonate platform and debris from landslides off of it.[13] The seamount was located in an ocean between the Siberian and Kazakhstan continents[14] and is known as the Baratal or Kurai palaeoseamount.[15]
The limestone is largely lacking in fossils[6] but fossil stromatoliths have been recovered.[7] They appear in the form of 40 centimetres (16 in) high and 40 centimetres (16 in)[2] wide domes in numerous layers of the limestone,[9] and probably grew in waters less than 50 metres (160 ft) deep.[16] Unidentified fossils are found within the limestone[2] and ooid units. Because of their relatively simple shapes they cannot be confidently identified,[17] but they appear to be of Cambrian age[18] and may be correlative with the Cambrian small shelly fauna and would indicate that it diversified not only in continental shelf areas but also in the open ocean.[19] The stromatolites were probably constructed by microbial reefs that trapped sediments.[12] Proper Cambrian fossils do not occur in the Baratal limestone.[6]
Age
Lead-lead isotope dating has yielded an age of 598±25 million years, placing the limestone in the early Vendian[7]/Ediacaran-Cambrian[20] but the large margin of error and difficulty in reproducing the age make the estimate suspect.[16] Strontium isotope ratios indicate that the basal limestone was emplaced[21] immediately after the end of Snowball Earth, consistent with the lead-lead isotope dates.[12]
The platform on which the Baratal limestone had formed would have eventually ended up in the Siberian trench due to plate tectonics and was amalgamated[12] onto the Kuznetsk-Altai[14] or Uimen-Lebed' island arc.[1] Faulting also took place,[3] and some faults in the region are active to this day.[10] Groundwater flow through the limestone is leaching elements that later precipitate as tufas in the region.[22]
Geological context
The Baratal limestone is found within the 1,500 kilometres (930 mi) wide and 5,000 kilometres (3,100 mi) long Altai-Sayan orogenic belt which makes up the Gorny Altai Mountains. This orogenic belt formed from subduction processes lasting from the Neoproterozoic to the Paleozoic, including Cambrian units in the central Gorny Altai that include the Baratal limestone.[7] Other rock formations in the area include Vendian-Cambrian accretionary complexes, Devonian volcanic rocks, metamorphic rocks and ophiolites.[2] They are part of an accretionary complex called the Kurai accretionary complex.[6]
The Ediacaran-Cambrian transition at the end of Snowball Earth featured a massive change in animal fauna, the so-called "Cambrian radiation". It is mostly known from ancient continental slope regions such as they are preserved in Canada's Burgess Shale and China's Maotianshan Shales while little is known of communities on the ocean floor.[23] No ocean floor on Earth is older than 200 million years; all oceanic crust older than this has been subducted. Pieces of oceanic crust older than this - including sediments and volcanic rocks - however have been amalgamated to accretionary complexes in continents. Such outcrops, which allow the investigation of older seafloor, have been encountered in Japan (Paleozoic to Mesozoic) and Siberia (Cambrian).[5]
References
- 1 2 Buslov et al. 2001, p. 217.
- 1 2 3 4 5 Uchio et al. 2004, p. 424.
- 1 2 Uchio et al. 2008, p. 184.
- ↑ Safonova 2009, p. 140.
- 1 2 Uchio et al. 2004, p. 422.
- 1 2 3 4 Nohda et al. 2013, p. 67.
- 1 2 3 4 5 6 Uchio et al. 2004, p. 423.
- 1 2 Uchio et al. 2008, p. 186.
- 1 2 Nohda et al. 2013, p. 69.
- 1 2 Kokh et al. 2017, p. 2.
- 1 2 Uchio et al. 2004, p. 425.
- 1 2 3 4 Uchio et al. 2004, p. 426.
- ↑ Uchio et al. 2004, p. 427.
- 1 2 Nohda et al. 2013, p. 72.
- ↑ Safonova 2008, p. 494.
- 1 2 Nohda et al. 2013, p. 70.
- ↑ Uchio et al. 2008, p. 188.
- ↑ Uchio et al. 2008, p. 191.
- ↑ Uchio et al. 2008, p. 190.
- ↑ Uchio et al. 2008, p. 185.
- ↑ Nohda et al. 2013, p. 74.
- ↑ Kokh et al. 2017, p. 16.
- ↑ Uchio et al. 2008, p. 183.
Sources
- Buslov, M. M.; Saphonova, I. Yu.; Watanabe, T.; Obut, O. T.; Fujiwara, Y.; Iwata, K.; Semakov, N. N.; Sugai, Y.; Smirnova, L. V.; Kazansky, A. Yu. (September 2001). "Evolution of the Paleo-Asian Ocean (Altai-Sayan Region, Central Asia) and collision of possible Gondwana-derived terranes with the southern marginal part of the Siberian continent". Geosciences Journal. 5 (3): 203–224. Bibcode:2001GescJ...5..203B. doi:10.1007/BF02910304. S2CID 128751598.
- Kokh, Svetlana N.; Sokol, Ella V.; Deev, Evgeny V.; Ryapolova, Yuliya M.; Rusanov, Gennady G.; Tomilenko, Anatoliy A.; Bul'bak, Taras A. (15 June 2017). "Post-Late Glacial calcareous tufas from the Kurai fault zone (Southeastern Gorny Altai, Russia)". Sedimentary Geology. 355: 1–19. Bibcode:2017SedG..355....1K. doi:10.1016/j.sedgeo.2017.04.003. ISSN 0037-0738.
- Nohda, Susumu; Wang, Bo-Shian; You, Chen-Feng; Isozaki, Yukio; Uchio, Yuko; Buslov, Michael M.; Maruyama, Shigenori (15 November 2013). "The oldest (Early Ediacaran) Sr isotope record of mid-ocean surface seawater: Chemostratigraphic correlation of a paleo-atoll limestone in southern Siberia". Journal of Asian Earth Sciences. 77: 66–76. Bibcode:2013JAESc..77...66N. doi:10.1016/j.jseaes.2013.08.008. ISSN 1367-9120.
- Safonova, I. Yu. (September 2008). "Geochemical evolution of intraplate magmatism in the Paleo-Asian Ocean from the Late Neoproterozoic to the Early Cambrian". Petrology. 16 (5): 492–511. doi:10.1134/S0869591108050056. S2CID 54668897.
- Safonova, I. Yu. (1 April 2009). "Intraplate magmatism and oceanic plate stratigraphy of the Paleo-Asian and Paleo-Pacific Oceans from 600 to 140 Ma". Ore Geology Reviews. 35 (2): 137–154. doi:10.1016/j.oregeorev.2008.09.002. ISSN 0169-1368.
- Uchio, Yuko; Isozaki, Yukio; Ota, Tsutomu; Utsunomiya, Atsushi; Buslov, Mikhail M.; Maruyama, Shigenori (2004). "The oldest mid-oceanic carbonate buildup complex: Setting and lithofacies of the Vendian (Late Neoproterozoic) Baratal limestone in the Gorny Altai Mountains, Siberia". Proceedings of the Japan Academy, Series B. 80 (9): 422–428. Bibcode:2004PJAB...80..422U. doi:10.2183/pjab.80.422. S2CID 55987965.
- Uchio, Yuko; Isozaki, Yukio; Buslov, Michael M.; Maruyama, Shigenori (1 August 2008). "Occurrence of phosphatic microfossils in an Ediacaran–Cambrian mid-oceanic paleo-atoll limestone of southern Siberia". Gondwana Research. 14 (1): 183–192. Bibcode:2008GondR..14..183U. doi:10.1016/j.gr.2007.12.009. ISSN 1342-937X.