The Midcontinent Rift

The Midcontinent Rift in Iowa

Mike Sedlacek
March 25, 2010
ES 767: Global Tectonics
Dr. James Aber, Instructor

Introduction Rift Formation
Rift Closure Rocks & Sedimentation
Rodinia's Plan References


Introduction

The Midcontinent Rift, also known as the Keweenawan Rift, is a 1,200 mile-long, 20-40 mile wide (Anderson) rift formation that occurred between 1.0 and 1.1 billion years ago (Ga). The rift runs through Michigan, Minnesota, Wisconsin, Iowa, Nebraska, and Kansas. In addition to the expanse of the rift itself, adjacent basins are filled with over 3,500 cubic miles of sedimentary rocks that are derived from geomorphic processes associated with the rift. The mechanisms by which the rift opened and closed are not known for sure, but it has been hypothesized that both crustal extension and later, compression, is correlated with the formation and breakup of a large supercontinent called Rodinia (Levin, 2005).

One common question arises as to why the rift failed to open completely. As you will see, crustal compression via a nearby orogeny may have played an important role in keeping the North American continent from splitting in two (Anderson). Finally, the remains of the Midcontinent Rift are now buried by younger sedimentary strata--a fair amount of which is derived from post-compression erosion of the rift itself. Due to the presence of these massive sedimentary beds, and also the presence of life, the area is known to contain petroleum deposits (Iowa Geological Survey, 2006). What is known, though, has been obtained through geophysical analysis. Gravity anomaly data, as well as seismic profiling methods have both been used to depict the size, depth, and extent of the Midcontinent Rift and the associated sedimentary strata.

Figure 1: A map showing where the Midcontinent Rift is located in the United States. Map taken from (Anderson).

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Rift Formation

The Midcontinent Rift is believed to have begun forming around 1.1 billion years ago (Ga) during the early Proterozoic (Iowa Geological Survey, 2006), and likely stopped forming around 100 million years later (Green, 1972). Figure 2 shows the timeline for when the Midcontinent Rift formed; Precambrian time. Figure 3 below shows a generalized map of the Midcontinent Rift as it runs from Michigan to Kansas. Green areas represent the rift basin with basalt layers, and yellow areas represent associated sedimentary deposits, such as the Red Clastics formations:

Figure 2: A timeline showing Precambrian time; when the midcontinent rift formed. Note the other geologic processes such as glaciation and orogeny than might have interacted with the rift (Gore, 2004).

Figure 3: A map showing the extent of the Midcontinent Rift; shown in green. (Iowa Geological Survey, 2006).

Why exactly did the rift form? Why did the rift form in the middle of a stable craton? It is believed that the closing of the Grenville Ocean (the present-day Atlantic Ocean) was the process reponsible for causing crustal extension on the North American craton (Anderson). This crustal thinning ultimately led to the development of a normal fault, complete with horsts and grabens. These horsts and grabens were centered between two parallel faults that ran northeast-southwest through western Iowa and several other Midwestern states. After sufficent crustal thinning, the mafic mantle-derived tholeiitic basalt began to flow through fractures and dikes within the downward-dropped graben and solidified (Iowa Geological Survey).

Continued basalt solidification and crustal heating led to the continued subsidence of the graben. Volcanism stopped completely after several million years, causing the basalt to cool and shrink in volume. This action allowed streams to deposit sediment within the rift and the nearby valleys. Over the course of several million more years, a rift lake formed, and even supported life; both bacteria and algae. Due to this, thick layers or organic "ooze" formed at the bottom of the lake in a distince layer of sedimentary strata called the Lower Red Clastics. It is the Lower Red Clastics that now houses petroleum deposits.

Rift Closure

The first stage of the formation of the Midcontinent rift involved crustal extension, rifting, and basalt extrusions. The second stage of formation for this rift zone involves compression and sedimentation (Anderson). Around 1 Ga, the Grenville Orogeny began to occur. The orogeny involved compression between two cratons--Amazonia and Laurentia (Levin, 2005). The Grenville orogeny didn't just effect what is now the U.S. eastern seaboard, but also the North American craton as a whole. It is believed that the collision of those two plates caused continental compression in what is now the midwest along a northwest-southeast axis (Anderson). The compression caused the central graben of the rift to rise as an uplifted block.

The rise of the former rifted graben (now a horst) pushed hundreds of meters of basalt, fluvial, and lacustrine sediments over thirty thousand feet up. In addition, the newly uplifted materials spanned a distance of 20-40 miles. This area of uplift is now called the Iowa Horst. Over the next several million years, the area remained relatively dormant tectonically, and was subjected to geomorphic processes; namely erosion. (Anderson). The upper layers of the Iowa Horst began to erode and deposit in the adjacent valleys; these deposits are now referred to as the Upper Red Clastics (Anderson; Iowa Geological Survey, 2006).

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Rocks & Sedimentation

Perhaps the most interesting (and lasting) features involved with the formation and closing of the Midcontinent Rift involve post-compression sedimentation. Figure 4 shows the modern-day bedrock of Iowa. One can clearly see that the Paleozoic and Mesozoic sea regressions and transgressions played a pivotal role in hiding the failed midcontinent rift. Large beds of limestone, dolomite, shale, and sandstone (varying by geologic periods) deposits can be found. Total, they are several thousand feet deep, anbd they conceal the rift in western Iowa (Anderson). Interestingly enough, newer sedimentary deposits are exposed in the western part of the state due to the southwestern dip of regional beds (Iowa Geological Survey, 2006).

Figure 4: A bedrock map of Iowa. The Midcontinent Rift is located under a few thousand feet of Paleozoic, Mesozoic, and Cenozoic sedimentary beds (Iowa Geological Survey, 2001).

Drill core records indicate that the Red Clastics beds are anywhere from 500-1700 meters below the current surface in western Iowa (Anderson). As was mentioned before, the Lower Red Clastics, which were deposited during the rifting phase, were constantly interrupted by random basalt extrusions, though the extent was centralized near the main rift axis (Anderson, 2006). Currently, the extent of the Red Clastics group covers an area of over 3,500 cubic miles; or nearly 6% of the state of Iowa. The extent of the Red Clastics group can be found below in Figure 5.

Figure 5: The total thickness of the Upper and Lower Clastics sedimentary beds. Note how the thickness of the beds decreases with increased distance away from the rift (Anderson, 1992).

The hill-like appearance from the ridge axis can be attributed to both uplift and the fluvial erosion of the Upper Red Clastics down into the adjacent valleys (Iowa Geological Survey, 2006). Exactly how deep are these former basins that hold the Red Clastics group? Geophysical data show that they are as deep as sixteen thousand feet at the edges of the rift margins (Iowa Geological Survey, 2006).

As was mentioned earlier, the Red Clastics group contains petroleum deposits (Iowa Geological Survey, 2006). The lake that formed along the rift axis, Nonesuch Lake, is directly responsible for creating an environment for petroleum production. The lake, which encompasses an area over 100 miles wide along the length of the rift, was covered by the lake, while the Lower Red Clastics were being deposited. These deposits are made of feldspathic sandstones that are interbedded with red-brown colored siltstone and shale.

At approximately 14.5-16.5 thousand feet below the surface, 600 feet of black shales and black siltstones can be found with occasional bands of pyrite (Iowa Geological Survey, 2006). These deposits are a result of erosion and deposition in both fluvial and lacustrine environments, with the black siltstones and shales having formed in a low-oxygen lacustrine environment. After compression and uplift occurred at the rift, erosion of the topmost sedimentary beds began. The Upper Red Clastics are also made predominantly of feldspathic sandstones, but contain no pyrite bands. It is believed that the weakly cemented, highly porous Upper Red Clastics acts as a reservoir rock, or storage area, where biological material was deposited and altered into petroleum (Iowa Geological Survey, 2006). In addition, it is believed that the alternating layers of less permeable siltstone and shale acted as cap rocks.

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Rodinia's Plan

Levin (2005) has hypothesized that the Midcontinent Rift was the first stage of a Wilson cycle. A Wilson cycle is defined as one complete supercontinent cycle; that is a supercontinent breaks up, an ocean forms, complete with dispersed continents; an ocean closes, and brings all the continents back into one large supercontinent. The internal workings of a Wilson cycle are not completely understood, but it is believed that the breakup of a supercontinent promotes crustal extension--such could be the case of the Midcontinent Rift. What is understood is that the Midcontinent Rift thinned to the point where it was only 1/4 the original thickness after rifting (Cannon and Green, 1989). Most of what we know about the buried rift comes from seismic profiling data (Van Schmus and Hinze, 1985). Gravity and aeromagnetic maps show a series of linear anomalies (shown in Figure 6) that extend from central Kansas to Lake Superior, and even as far east as central Michigan.

Figure 6: This gravitational anomaly map shows variations in the composition of rocks in the vicinity of the Midcontinent Rift. Red and orange represent a positive gravity anomaly, and blue and green represent a negative anomaly. Picture taken from the (Iowa Geological Survey).

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References

Website created by Mike Sedlacek: March 25, 2010

ESU Earth Science Department homepage

For more information on the Midcontinent (Keweenawan) Rift, visit the Iowa Geological Survey website.
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