Английская Википедия:Baratal limestone
Шаблон:Infobox Rockunit 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 AkkayaШаблон:Sfn in the Kosh-Agach region.Шаблон:Sfn It is also known as the Baratal Formation.Шаблон:Sfn
The Baratal limestone of Cambrian age is the oldest known carbonate deposit on a seamount,Шаблон:Sfn akin to a present-day atoll.Шаблон:Sfn It formed a no more than Шаблон:Convert thick unit before tectonic processes thickened it to about Шаблон:Convert.Шаблон:Sfn Rock units include limestone breccia/conglomerates, bedded limestone and ooid mudstonesШаблон:Sfn with Шаблон:Convert ooids,Шаблон:Sfn and chert nodules.Шаблон:Sfn The limestone is associated with greenstone, which probably constitutes the volcanic basement that the Baratal limestone was emplaced upon.Шаблон:Sfn Parts of the unit may be buried by glacial deposits.Шаблон:Sfn
Interpretation
The limestone probably formed on a volcanic seamount or oceanic plateauШаблон:Sfn 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,Шаблон:Sfn where photic zoneШаблон:Sfn waters were intensely agitated by tidesШаблон:Sfn 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 limestoneШаблон:Sfn which contains parts from both the original carbonate platform and debris from landslides off of it.Шаблон:Sfn The seamount was located in an ocean between the Siberian and Kazakhstan continentsШаблон:Sfn and is known as the Baratal or Kurai palaeoseamount.Шаблон:Sfn
The limestone is largely lacking in fossilsШаблон:Sfn but fossil stromatoliths have been recovered.Шаблон:Sfn They appear in the form of Шаблон:Convert high and Шаблон:ConvertШаблон:Sfn wide domes in numerous layers of the limestone,Шаблон:Sfn and probably grew in waters less than Шаблон:Convert deep.Шаблон:Sfn Unidentified fossils are found within the limestoneШаблон:Sfn and ooid units. Because of their relatively simple shapes they cannot be confidently identified,Шаблон:Sfn but they appear to be of Cambrian ageШаблон:Sfn 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.Шаблон:Sfn The stromatolites were probably constructed by microbial reefs that trapped sediments.Шаблон:Sfn Proper Cambrian fossils do not occur in the Baratal limestone.Шаблон:Sfn
Age
Lead-lead isotope dating has yielded an age of 598±25 million years, placing the limestone in the early VendianШаблон:Sfn/Ediacaran-CambrianШаблон:Sfn but the large margin of error and difficulty in reproducing the age make the estimate suspect.Шаблон:Sfn Strontium isotope ratios indicate that the basal limestone was emplacedШаблон:Sfn immediately after the end of Snowball Earth, consistent with the lead-lead isotope dates.Шаблон:Sfn
The platform on which the Baratal limestone had formed would have eventually ended up in the Siberian trench due to plate tectonics and was amalgamatedШаблон:Sfn onto the Kuznetsk-AltaiШаблон:Sfn or Uimen-Lebed' island arc.Шаблон:Sfn Faulting also took place,Шаблон:Sfn and some faults in the region are active to this day.Шаблон:Sfn Groundwater flow through the limestone is leaching elements that later precipitate as tufas in the region.Шаблон:Sfn
Geological context
The Baratal limestone is found within the Шаблон:Convert wide and Шаблон:Convert 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.Шаблон:Sfn Other rock formations in the area include Vendian-Cambrian accretionary complexes, Devonian volcanic rocks, metamorphic rocks and ophiolites.Шаблон:Sfn They are part of an accretionary complex called the Kurai accretionary complex.Шаблон:Sfn
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.Шаблон:Sfn 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).Шаблон:Sfn
References
Sources
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite journal
