Oncolitic limestone (Chambless Formation, Lower Cambrian; southern Marble Mountains, California, USA) 2 : 無料・フリー素材/写真
Oncolitic limestone (Chambless Formation, Lower Cambrian; southern Marble Mountains, California, USA) 2 / 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 | Oncolitic limestone from the Cambrian of California, USA. (cut surface)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 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 sample is from a Cambrian-aged shallow marine deposit in southern California's "Trilobite Wilderness". The fossil microbes making up the oncolites have been assigned to Girvanella.Stratigraphy: lower Chambless Formation, Dyeran Stage, upper Lower CambrianLocality: outcrop in the southern Marble Mountains, Mojave Desert, southern California, USA |
| 撮影日 | 2022-04-08 10:58:35 |
| 撮影者 | James St. John |
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