Cryogenian-Ediacaran GSSP (Elatina Formation-Nuccaleena Formation boundary, upper Neoproterozoic; Enorama Creek, Brachina Gorge, Flinders Ranges, South Australia) : 無料・フリー素材/写真
Cryogenian-Ediacaran GSSP (Elatina Formation-Nuccaleena Formation boundary, upper Neoproterozoic; Enorama Creek, Brachina Gorge, Flinders Ranges, South Australia) / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Cryogenian-Ediacaran GSSP (global stratotype section and point) in Brachina Gorge, Flinders Ranges, South Australia.On the geologic time scale, the Cryogenian (850 to 650 million years ago) and the Ediacaran (650 to 544 million years ago) are the last two geologic periods of the Proterozoic Eon. During the Cryogenian, Earth experienced the most significant Ice Age ever - all of Earth, or nearly all of Earth, was covered in glacial ice. Two or three separate global Ice Ages occurred during the Cryogenian - they are called Snowball Earth Glaciations.The dark reddish rock below the boundary marker in the above photo is a Snowball Earth tillite in the Elatina Formation. Tillites are glacial conglomerates - poorly sorted polymict conglomerates of glacial origin (lithified glacial till). Numerous large boulders and smaller clasts that occur in a dark red, fine-grained matrix can be observed by walking along the creek bank outcrop seen here. This Elatina tillite exposure represents the very top of the Cryogenian.The light brown rock conformably overlying the dark red unit is a strange carbonate unit - the Nuccaleena Formation. The Snowball Earth glaciations ended because volcanogenic greenhouse gases accumulated in the atmosphere over long periods of time (plate tectonics doesn't stop just because Earth's surface is covered in ice!), to the point that trapped solar heat started melting the global ice sheet. After the Ice Age was over, the high greenhouse gas concentration resulted a supergreenhouse climate. Very high CO2 levels resulted in high dissolved CO2 levels in seawater, which resulted in lots of carbonate precipitation on the seafloor. The Nuccaleena Formation represents marine deposition during this post-Ice Age supergreenhouse climate. Odd carbonate units are frequently found directly atop Snowball Earth tillites in many places on Earth - they are called "cap carbonates". The Nuccaleena Formation here is horizontally-laminated dolostone, with some barite and teepee structures (not shown in this photo).The marker at the Elatina Formation-Nuccaleena Formation boundary in the outcrop shown above is the GSSP for the Cryogenian-Ediacaran boundary. "GSSP" means global stratotype section and point. This is the equivalent of a holotype specimen in taxonomy. This boundary at this outcrop is the global definition for the Cryogenian-Ediacaran boundary (unfortunately, the marker was placed incorrectly - the boundary was supposed to be a bedding plane just above the marker's). The drill holes were made by geology researchers from Yale University and done with the permission of local Australian aborigines, but before the Cryogenian-Ediacaran GSSP was ratified.Locality: Enorama Creek, Brachina Gorge, Flinders Ranges, South Australia |
| 撮影日 | 2006-08-19 21:42:30 |
| 撮影者 | James St. John |
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