Acraman Impact Layer (Bunyeroo Formation, Neoproterozoic, ~580 Ma; Brachina Gorge, Flinders Ranges, South Australia) 6 : 無料・フリー素材/写真
Acraman Impact Layer (Bunyeroo Formation, Neoproterozoic, ~580 Ma; Brachina Gorge, Flinders Ranges, South Australia) 6 / 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 | Impact deposit in the Precambrian of Australia.This bed is a distal impact ejecta layer exposed in the Flinders Ranges of South Australia. The horizon, named the Acraman Impact Layer, consists of volcanic rock material (the largest clasts are football-sized) - it was originally interpreted to be a volcanic ash bed. The Acraman layer has clasts that are 1.59 billion years old, which is much older than the adjacent Ediacaran-aged formations. The volcanic grains are identical to the Gawler Range volcanics (felsic to intermediate). The impact site is Lake Acraman, 310 kilometers west-southwest of this outcrop. Debris from the impact is also found in the Officer Basin of Australia, about 600 kilometers from Lake Acraman. The impacting ojbect is estimated to have been ~4 to 5 kilometers in size. In terms of the impact's effects on life forms, acritarch microfossils experience an increase in diversity and size from below to above the impact layer. The Acraman Impact is not well dated, but is estimated to be at ~580 million years ago.At this locality, the impact layer is about 1 decimeter thick, but elsewhere it can be as thin as a few centimeters. Larger clasts occur at the base of the weathering-resistant bed. Weathered, altered, corroded pyrite framboids are present at the top of the bed. Obvious reworked shale clasts occur within the bed. Greenish-colored, fine-grained siliciclastics (siltstones) occur above and below the impact layer - a reducing band. Reddish-colored siltstones occur above and below the greenish rocks.Local stratigraphy:red silstone--------------------green siltstone--------------------coarse-grained impact layer--------------------fine-grained impact layer--------------------clasts--------------------green siltstone--------------------red siltstoneThe impact may have had climatic effects - it possibly led to the short-lived Gaskiers Glaciation.Stratigraphy: Acraman Impact Layer (~80 meters above the top of the ABC Range Quartzite), Bunyeroo Formation, middle Ediacaran, upper Neoproterozoic, ~580 MaLocality: low exposure on the southern side of Brachina Gorge, west of the ABC Range Quartzite outcrop & east of the Rawnsley Quartzite outcrop, Flinders Ranges, South Australia (vicinity of 31° 20.030’ South latitude, 138° 35.503’ East longitude) |
| 撮影日 | 2006-08-19 23:58:58 |
| 撮影者 | James St. John |
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