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THOR VOLCANIC GROUP

Red ball iconTHE MIDCONTINENT RIFT SYSTEM IN IOWA

THOR VOLCANIC GROUP


In Iowa, 44 drill holes encountered mafic volcanic rocks correlated by Anderson (1992) with the upper volcanic sequence rocks of the Lake Superior region. These include 36 wells with cutting samples that penetrated 116 m (388 ft) of basalt rock, and 8 cores totalling 51 m (170 ft). Most of these samples are currently reposited at the Iowa Department of Natural Resources Geological Survey's Oakdale Research Facility. The most complete and informative basalt samples are from the Northern Natural Gas #1 Sharp core, drilled in Webster County near the town of Thor, which encountered mafic volcanic rocks at a depth of 647 m (2156 ft) below the surface, and penetrated 17 m (57 ft) of the unit. All or portions of three individual basalt flows can be observed in the core, separated by weathered, vesicular flow tops. The rocks in this core are the "type" for the informal name "ThorVolcanic Group" that is applied to upper volcanic package rocks in Iowa (click to view depositional model for Thor volcanics).

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