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MIDCONTINENT RIFT SYSTEM IN IOWA

MIDCONTINENT RIFT SYSTEM IN IOWA

 

by Raymond R. Anderson


The Midcontinent Rift System (MRS) is a billion year old rip in the continental crust that represents an episode of crustal stress that nearly ended in the complete severing of the southeastern part of North America and the formation of a new ocean. Like a giant scar, the MRS stretches for over 1450 kilometers from eastern Lake Superior to southern Kansas, with a possibly related eastern arm extending southeastward across Michigan and into Ohio. Rocks produced during the formation of the MRS are exposed in the Lake Superior area, along the lake shore from Duluth north into Canada and east into Michigan, including Wisconsin's Bayfield and Michigan's Keweenaw peninsulas. The rocks exposed in this area are dominated by basaltic lavas (and related intrusive rocks) (click for more information on MRS volcanic rocks) and reddish sedimentary rocks (sandstones, siltstones and shales), a pre-volcanics, early clastics sequence and a post-volcanics, late clastics sequence. These rock units continue southward beneath a cover of younger sedimentary rocks. In Iowa, the rocks of the MRS are buried to a depth of 1,200 to 5,500 feet. The very dense basaltic lavas and very light sedimentary rocks produce a series of linear gravity anomalies that follow the trend of the MRS and produce the most dramatic gravity anomaly in North America.

Because it lies so deeply buried in Iowa and south, and few wells penetrate into MRS rocks (click for discussion of well data), much of what is known about the characteristics and trend of the MRS in this area has come from interpretations and modeling of geophysical data including gravity, aeromagnetic, and seismic.

In Iowa, the MRS are dominated by a huge, uplifted block (called a horst) of basalt that follows the axis of the rift across Iowa from south-central Minnesota to southeastern Nebraska near Lincoln. This block, named the Iowa Horst, ranges from 20 to 40 miles wide in Iowa and was thrust upward over 30,000 feet during the formation of the rift. Flanking the Iowa horst on both sides are a series of five deep sedimentary rock-filled basins. These basins contain over 35,000 cubic miles of sedimentary rock and cover a total of about 150,000 square miles of Iowa (click for details on the structure of the MRS in Iowa).

The rock units, or stratigraphy, of the MRS in Iowa is very similar to the rocks of the MRS exposed in the Lake Superior area and seen from subsurface samples from Minnesota.

Formation of the Midcontinent Rift System is believed to have formed in response to a sequence of events that began about 1100 Ma (million years ago).

* Initially the region was subjected to tensional stresses that caused a large fracture to form in the crust, and a series of large blocks to subside (called grabens) along the trend of the fracture. Lavas (basaltic) moved up along the fractures from the base of the crust and erupted into the elongate valley formed by the subsiding grabens. Tens of thousands of cubic miles of lava spilled into the continually subsiding MRS central graben.

* As volcanism waned, subsidence of the central graben continued and the crustal rocks flanking the graben were also pulled downward. Rivers flowed into the subsiding rift zone, lakes formed, and thick sequences of sedimentary rocks were deposited along the entire trend of the MRS. In Iowa these sedimentary rocks are called the Lower Red Clastic Group.

* Subsidence apparently ceased abruptly about 1000 Ma when the area was subjected to compressive stresses. These stresses caused the central graben to be forced upward through the pile of sedimentary rocks that had accumulated. Sedimentary rocks eroded from the uplifting central area of the MRS were redeposited above the older sedimentary rocks on the margins of the structure. These rocks are called the Upper Red Clastic Group in Iowa. Eventually the central regions of the MRS were uplifted as much as 30,000 feet in Iowa, with most of the sedimentary rocks eroded from the central horst in Iowa. The entire feature probably formed in about 150 million years.

click for references cited

For more information contact Ray Anderson at (319)335-1575

or

(e-mail: Raymond.Anderson@dnr.iowa.gov)