Rip-up clasts in proglacial lacustrine facies (Konnarock Formation, Neoproterozoic, ~750 Ma; Grassy Branch Outcrop - Rt. 603 roadcut, Smyth County, Virginia, USA) 4 : 無料・フリー素材/写真
Rip-up clasts in proglacial lacustrine facies (Konnarock Formation, Neoproterozoic, ~750 Ma; Grassy Branch Outcrop - Rt. 603 roadcut, Smyth County, Virginia, USA) 4 / 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 | Rip-up clasts in proglacial lacustrine sedimentary rock from the Precambrian of Virginia, USA.The Snowball Earth Glaciations during the Neoproterozoic were the most significant ice ages that Earth ever experienced - two or three of them occurred in succession. The most extreme models describing Snowball Earth have glacial ice completely covering all continents and all oceans, even at the equator. Some models, called “Slushball Earth”, have Earth’s equatorial oceanic areas not completely frozen over. Each Snowball Earth Glaciation was followed by a super-greenhouse climate. The resulting sedimentary record of these “freeze-fry” events typically consists of glacial tillites and overlying cap carbonates. These units are preserved at many localities on Earth.The rock shown above is mixed sandstone and argillite (= a very low-grade metamorphic rock formed by slight alteration of shale). The purplish gray-colored material is sandstone. The dark maroon-colored material is argillite.The surrounding outcrop is dominated by rhythmically-laminated argillite. Published research has identified the sediments in the argillite as proglacial lacustrine turbidites - they appear to be varves (rhythmites). Varves are common sedimentary units in Pleistocene proglacial lake settings. This is a Snowball Earth equivalent. The rhythmicity reflects seasonal changes (summer-winter-summer-winter, etc.). Demonstrable annual layering is scarce in the sedimentary rock record.The elongated (really platy) pieces of argillite surrounded by sandstone are rip-up clasts, formed during a relatively high-energy event, probably a turbidity current (= underwater sediment slide).Stratigraphy: Konnarock Formation, Neoproterozoic, ~750 MaLocality: Grassy Branch Outcrop - roadcut on the northern side of Rt. 603, just downstream from the Big Laurel Creek-Grassy Branch confluence, west of the town of Troutdale & east of the town of Konnarock, southern Smyth County, southwestern Virginia, USA (36º 40.900’ North latitude, 81º 33.988’ West longitude) |
| 撮影日 | 2016-10-14 13:57:32 |
| 撮影者 | James St. John |
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