File:Shoshonite lava flows capping North Table Mountain (Denver Formation, Upper Cretaceous; Golden, Colorado, USA) 10.jpg
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English: This is North Table Mountain in Golden, Colorado. The stratigraphic section consists of the Denver Formation (latest Cretaceous to earliest Tertiary), a siliciclastics-dominated succession with shale, siltstone, and occasional coarser-grained beds like conglomerate, deposited at a time of low relief. Rocky Mountains uplift and deformation had not yet reached this area at that time. The overlying unit is the Green Mountain Conglomerate, which was deposited near some local relief, probably from movement along the Golden Fault.
The cliff-forming unit at the top of Table Mountain consists of lava flows. Four flows occur in the section - flow # 3 and flow # 4 occur in the top cliff. Flow # 1 forms a minor cliff below that. Flow # 2 only occurs on one side of Table Mountain. Flows # 1 and 2 were small-volume, channel-filling lava flows. The lava flows are composed of porphyritic shoshonite, which is a splitter's term for potassic basalt. The lava flows date to 63-64 Ma, during the late Maastrichtian. All four flows together are about 0.75 cubic kilometers in volume. Northwest of North Table Mountain is the vent area, consisting of two nested plugs - Ralston Dike is the larger of the two. The lava flows here are crudely columnar-jointed. Columnar jointing forms as lava cools and contracts slightly, forming column-like structures having polygonal shapes in plan view - often pentagonal or hexagonal. Based on Denver Formation fossils, this area was probably a savannah during the latest Cretaceous, perhaps with patches of jungle growth. Locality: North Table Mountain, next to the town of Golden, north-central Colorado, USA |
| Date | |
| Source | https://www.flickr.com/photos/47445767@N05/52747179952/ |
| 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@N05/52747179952. It was reviewed on 15 March 2023 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 16:30, 15 March 2023 | | 3,008 × 1,304 (4.24 MB) | Ser Amantio di Nicolao (talk | contribs) | Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/52747179952/ with UploadWizard |
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