THE
MIDCONTINENT RIFT
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.