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).
THE MIDCONTINENT RIFT
SYSTEM IN IOWA
