The Dorothy bentonite: an extraordinary case of secondary thickening in a late Campanian volcanic ash fall in central Alberta
| Autor: | J. F. Lerbekmo |
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| Rok vydání: | 2002 |
| Předmět: | |
| Zdroj: | Canadian Journal of Earth Sciences. 39:1745-1754 |
| ISSN: | 1480-3313 0008-4077 |
| DOI: | 10.1139/e02-079 |
| Popis: | The late Campanian (~73 Ma) Dorothy bentonite outcrops in the marine Bearpaw Shale for 20 km along the Red Deer River valley east of Drumheller, Alberta, and is up to 13.5 m thick. An isopach map based upon 230 sub surface and surface thickness measurements illustrates an elongated southwestnortheast lobe with maximum dimensions of about 300 km by 50 km. The volume is ~57 km3 distributed within an area of ~11 000 km2. This bentonite is the altered product of what is believed to be a short-lived Plincan-type eruption from part of the Howell Creek Instrusives in southeastern British Columbia. Fortuitous preservation of the original ash in an up to 2.5 m thick calcite-cemented tuffaceous zone near the middle of the bentonite shows the original ash to have been >99% glass shards and pumice. The remainder of the ash is a crystal component consisting mostly of plagioclase and biotite. The silica content of the isotropic glass shards of about 77%, and a refractive index of 1.503 ± 0.001, suggest a magma of rhyolitic composition. |
| Databáze: | OpenAIRE |
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