Shoshonite lava flows capping North Table Mountain (Denver Formation, Cretaceous-Tertiary; Golden, Colorado, USA) 9 : 無料・フリー素材/写真
Shoshonite lava flows capping North Table Mountain (Denver Formation, Cretaceous-Tertiary; Golden, Colorado, USA) 9 / 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 | 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 Danian (Early Paleocene). 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 to earliest Tertiary, perhaps with patches of jungle growth.Locality: North Table Mountain, next to the town of Golden, north-central Colorado, USA |
| 撮影日 | 2007-11-01 12:38:05 |
| 撮影者 | James St. John |
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