File:Limestones (Big Horse Limestone Member, Orr Formation, Middle Cambrian-Upper Cambrian boundary; Little Horse Canyon, House Range, Utah, USA) 2 (44837826135).jpg
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Limestones in the Cambrian of Utah, USA. The red notebook is resting on the Marjuman Stage-Steptoean Stage contact, which is also the boundary between the Middle Cambrian and Upper Cambrian of Laurentia (= ancestral North America). The rocks are gray limestones of the Orr Formation. The Marjuman-Steptoean stage boundary is near the top of the Big Horse Limestone Member. "Round-head" trilobites occur below the boundary and "square-head" trilobites occur above the boundary. The contact has been placed at the change in trilobite faunas. Such sudden changes in trilobite faunas are caused by extinction events. In the Cambrian, these extinction horizons are called biomere boundaries. After the extinction, deeper-water environments occur, followed by a shallowing-upward succession. Shallowing-upward successions are repeated many times in this section, but they don't affect the extinction boundary. The biomere boundary (trilobite extinction) corresponds with a change from oncolitic limestones to oolitic limestones. A subtle solution surface is present at the boundary. A small facies change does occur here, but later transgressive-regressive cycles don't affect the trilobite faunal composition - the extinction of trilobites is independent of facies. The limestones contain polymeroid trilobites and agnostoid trilobites. In addition, acid processing of rocks from here produce phosphatic brachiopods and chancelloriid sclerites. A microbial/algal bioherm is also present in this area. Stratigraphy: Marjuman Stage-Steptoean Stage boundary, near the top of the Big Horse Limestone Member, Orr Formation, Middle Cambrian-Upper Cambrian boundary Locality: dry wash in the middle branch of Little Horse Canyon, eastern flanks of the House Range, Millard County, western Utah, USA (vicinity of 39˚ 12' 23.79" North latitude, 113˚ 18' 20.60" West longitude) |
| Date | |
| Source | Limestones (Big Horse Limestone Member, Orr Formation, Middle Cambrian-Upper Cambrian boundary; Little Horse Canyon, House Range, Utah, USA) 2 |
| Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/44837826135 (archive). It was reviewed on 7 November 2018 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
James St. John
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| current | 00:27, 7 November 2018 | | 1,280 × 960 (741 KB) | Lu Brito (talk | contribs) | Transferred from Flickr via #flickr2commons |
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