File:Oncolitic limestone (North Horn Formation, Upper Cretaceous-Paleocene; Birdseye Marble Quarry, south of Thistle, Utah, USA) 1.jpg
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English: Oncolitic limestone from the Cretaceous-Tertiary of Utah, USA.
Limestone is a biogenic sedimentary rock composed of the mineral calcite (CaCO3), which bubbles in acid. Many geologically young limestones are composed of aragonite (also CaCO3). Numerous varieties of limestone exist (e.g., fine-grained limestone/micritic limestone/lime mudstone, coquina, chalk, wackestone, packstone, grainstone, rudstone, rubblestone, coralstone, calcarenite, calcisiltite, calcilutite, calcirudite, floatstone, boundstone, framestone, oolitic limestone, oncolitic limestone, etc.). Most limestones represent deposition in ancient warm, shallow ocean environments. Oncolitic limestones have oncolites - macroscopic, concentrically layered, ~irregularly spheroidal masses of variable size. The oncolites are biogenic in origin - they grew in successive layers by the active or passive precipitation of calcium carbonate by cyanobacterial films (oncolites can be considered as mobile varieties of stromatolites). Oncolites are usually attributed to “algae”, and are often called “algal balls”. Referring to cyanobacteria as “algae” is a common error - they are frequently called “blue-green algae”. Cyanobacteria are photosynthetic, as algae are, but they are not algae. This oncolitic limestone is from the nonmarine North Horn Formation of Utah, an upper Mesozoic to lower Cenozoic succession of mostly fluvial siliciclastic sedimentary rocks. Intervals of oncolitic limestones are probably nearshore lacustrine deposits. Notice that some of the oncolites are fractured and broken. North Horn oncolites vary in shape - some are developed around gastropod shells. Stratigraphy: North Horn Formation, Upper Cretaceous to Paleocene Locality: Birdseye Marble Quarry, east of Route 89, ~5.2 miles ~south of the town of Thistle, southeastern Utah County, north-central Utah, USA (apparently at 39° 55’ 21.80" North latitude, 111° 31’ 20.38" West longitude) |
| Date | |
| Source | https://www.flickr.com/photos/47445767@N05/50536147167/ |
| 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@N05/50536147167. It was reviewed on 27 October 2020 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 14:00, 27 October 2020 | | 2,608 × 2,591 (4.33 MB) | Ser Amantio di Nicolao (talk | contribs) | Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/50536147167/ with UploadWizard |
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