IOWA'S Almost OCEAN . . .
by Raymond R. Anderson
One billion years ago, the Earth's crust split across part of
the North American continent. This tear or rift, known as the
Midcontinent Rift System, extended for 950 miles from what is now
Lake Superior to Oklahoma, and was on its way to becoming a
full-fledged ocean when the process halted. Rocks deposited
during the rift's formation can be seen today surrounding Lake
Superior, including basalts along Minnesota's North Shore and
sandstones along Wisconsin's Bayfield Peninsula. Across most of
Minnesota, Iowa, and part of Kansas, the rift is buried by nearly
one-half mile of younger sedimentary rocks. Studies of exposed
Lake Superior rocks, combined with gravity, magnetic, and seismic
information, have improved our understanding of the Midcontinent
Rift, one of the largest and most spectacular geologic features
in North America.
Research suggests that stresses, generated by deep heat and
pressure differences, pulled at the continent and opened fissures
through its crust. Huge volumes of molten rock flowed up to the
surface and were deposited in the developing rift valley as
dense, dark volcanic rocks, especially basalt and gabbro. As the
rift grew and the valley floor sank, still more volcanic rocks
were deposited, ultimately reaching tens of thousands of feet in
thickness. When the outpouring of volcanic rock ceased, crustal
settling continued, producing a lowland trough into which rivers
flowed, and a large lake formed. The lake filled with gravels,
sands, and silts, setting the stage for the final dramatic
episode in the history of the rift.
Crustal movement that first pulled the continent apart then
reversed and began to push it back together. This compression
forced the dense basaltic rocks upward, producing a range of
mountains along the axis of the rift. The thick lake deposits
were eroded off the steadily uplifting mountain range and were
redeposited in a series of deep sedimentary basins along its
flanks. In Iowa, the sediments were almost completely removed
from the uplifting central block (known as a horst) and the
basaltic volcanic rocks were exposed.
Though now deeply buried in Iowa, the dense rocks of the horst
and the less-dense sedimentary rocks in the flanking basins
produce strongly contrasting gravity signatures. Recent computer
modeling of these gravity data, guided by seismic and drill-hole
data, has given geologists a better picture of the rift in Iowa.
The rift system covers 42,000 square miles of Iowa's geologic
"basement" and is dominated by the central horst (see
map above), bounded by fault zones (heavy lines), and by a series
of flanking basins. Volcanic rocks within the horst are up to 7.5
miles thick, while the adjacent basins reach depths of 5.5 to 6.5
miles. These basins contain an astounding 36,000 cubic miles of
sedimentary rocks, nearly 3 times all the earth materials above
sea level in Iowa!*
In northern Michigan, oil seeps from the rift's sedimentary
rocks, though it has not been found in large enough quantities to
be economically attractive. In 1987, Amoco completed drilling of
a petroleum test well over 3 miles deep in Carroll County, Iowa.
This well penetrated over 2.5 miles of the sedimentary rocks in
one of the flanking basins - the deepest penetration of rift
system rocks anywhere along the trend of the feature. Although no
petroleum was found, evidence suggests that petroleum once formed
there and later migrated from the drilled region. It may lie
trapped in other rift system rocks.
Although much is known about the Midcontinent Rift System,
many aspects of its history remain to be investigated by future
scientists with new data and techniques.
*Iowa's average elevation is 1200 ft above sea level.
Adapted from Iowa Geology 1997, Iowa Department of