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Table Mountain basalt

A cache by PowerDime Send Message to Owner Message this owner
Hidden : 5/5/2008
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Size: Size: not chosen (not chosen)

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Geocache Description:

Located in Oroville, Table Mountain can be accessed via Cherokee Road. Parking is available at the following coords: N39°35.750 / W121°32.500. There is a short hike to the posted coordinates.

Local Oroville Geology:
The geology of the Oroville area is fascinating. Oroville sits on the eastern rim of the Great Valley, defined today by the floodplains of the Sacramento River and its tributaries. Around Oroville these sediments are dominated by thick fans of Feather River sediments, but just east of this there is a thin, N-S band of late Cretaceous sediments. These sit on top of the Sierran basement, which beneath eastern Oroville comprise greenschist-facies metavolcanic rocks of Jurassic age, giving way to granites of the Sierra batholith to the east. These are manifestations of a vigorous island arc sequence, built out over an east-dipping subduction zone of mid- to late Mesozoic age. The gold veins lace this ancient arc, remobilized by Mesozoic shearing and intrusions of igneous rock. The crystalline foothills are locally overlain by a Cenozoic sequence of Eocene clean beach sands overlain by Neogene volcanics, including the Diamond Head-like profile of Table Mountain.

Table Mountain Geology:
Along the North Fork of the Feather River, about 3 miles northwest of Oroville, anyone who has traveled through the Stanislaus National Forest or the Sierra National Forest will see a familiar site: a Table Mountain, created by a lava flow going down an ancient streambed.
Table Mountain is the proper name given to these geologic features. The lava flow near Oroville occurred about 50 million years ago, paving the streambed with basalt. The hardened basalt survived the erosion process over time and now stands as a flat-top mountain.
Along State Highway 70, there is evidence of rocks much older than the Table Mountain. Slates and schists, which probably were deposited as marine layers in the Pacific Ocean 200 million years ago, can be found along road cuts on Highway 70.

The "Table" is a massive columnar basalt referred to as the Lovejoy Basalt (LJB). The LJB flowed into and filled what was then a "paleovalley" roughly 16 million years ago, but over time, the less resistant sediments of the valley walls eroded away, leaving the basalt high above its surroundings. So what appears to be the top of a hill today is actually the bottom of a former valley. The Basalt's age of 16 million years was determined using new isotopic dating techniques, revealing that it is much younger than the 50-million-year-old date attributed to it previously.

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