Cross-bedded volcanic lithic sandstone (North Shore Volcanic Group, Mesoproterozoic, ~1.1 Ga; Leif Ericson Park, Duluth, Minnesota, USA) 3 : 無料・フリー素材/写真
Cross-bedded volcanic lithic sandstone (North Shore Volcanic Group, Mesoproterozoic, ~1.1 Ga; Leif Ericson Park, Duluth, Minnesota, USA) 3 / 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 | Cross-bedded volcanic lithic sandstone in the Precambrian of Minnesota, USA.Along the northern shore of western Lake Superior are numerous exposures of a lava flow-dominated succession called the North Shore Volcanic Group. This is equivalent to & the same age as the Portage Lake Volcanic Series of northern Michigan's Keweenaw Peninsula (www.flickr.com/photos/jsjgeology/albums/72157632266738191). The North Shore and Portage Lake successions are ~1.1 billion years old and represent basalt lava flows, plus minor sedimentary rocks, that filled up an ancient rift valley. This old rift is the Lake Superior segment of the Mid-Continent Rift System, a tear in the ancient North American paleocontinent of Laurentia (see: minerals.usgs.gov/science/midcontinent-rift-minerals/imag...). Tectonic rifting started along this tear, exactly like the modern-day East African Rift Valley. Laurentia's Mid-Continent Rift System started and then stopped and was subsequently filled and buried. This ancient failed rift is now exposed on either side of Lake Superior in North America's Great Lakes.The outcrop shown above is an interflow sandstone, representing sediment deposition by rivers in the ancient rift valley during pauses in effusive volcanic activity. The grains making up the sandstone are, not surprisingly, volcanic rock fragments. The tilted layers are cross-beds, which are formed in a one-directional current by wind or water - in this case, water (streams).Stratigraphy: interflow sandstone in the Leif Ericson Park Lavas, Upper Southwest Sequence, North Shore Volcanic Group, Keweenawan Supergroup, upper Mesoproterozoic, ~1098 to 1099 MaLocality: Lake Superior shoreline exposure in Leif Ericson Park, just northeast of the city of Duluth, northeastern Minnesota, USA |
| 撮影日 | 2015-06-08 09:26:29 |
| 撮影者 | James St. John |
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