KEWEENAWAN SUPERGROUP CLASTIC ROCKS OF THE MIDCONTINENT RIFT SYSTEM IN IOWA
Keweenawan Supergroup Clastic Rocks in Iowa
Ojakangas, R., Dickas, A.B., and Green, J.C., eds.
Proceedings of the 10th International Conference on Basement Tectonics
Kluwer Academic Publishers, The Netherlands, 1995, p. 55-59.
(reproduced with kind permission from Kluwer Academic Publishers)
The Midcontinent Rift System (MRS) of North America is a failed rift that apparently formed in response to region-wide stresses associated with the Grenville Orogeny about 1100 million years ago. The Iowa portion of the MRS is buried by about 660 m to 1650 m of Paleozoic and Mesozoic sedimentary rocks and Quaternary glaciogenic deposits. The structural configuration, nature of the major clastic rock packages, and geologic history of the MRS in Iowa were investigated by examining and interpreting limited well samples (including the 5355 m-deep M.G. Eischeid deep petroleum test well), gravity and magnetic anomaly data, and petroleum industry reflection seismic profiles. These data were used to produce a series of two-dimensional gravity profiles coincident with the seismic profiles. These studies, described in detail by Anderson (1992) and reviewed by Anderson (1995), reveal the MRS in Iowa (Fig. 1) to be characterized by a central horst (the Iowa Horst) dominated by mafic volcanic rocks, thrust over its contacts with the thick sequences of younger Keweenawan Supergroup clastic rocks that fill flanking basins (Fig. 2). This investigation demonstrated that MRS rocks in Iowa are primarily restricted to middle and upper crustal depths. The MRS displays a relatively symmetrical structure, with sedimentary basins generally deepening toward the axis of the rift, on both flanks of the central horst. Opposing clastic basins are approximately similar in depth and configuration, overthrust by the central horst near the axis of the rift and displaying beveled erosional edges at their outboard limits (Fig. 2). Two basins were documented northwest of the Iowa Horst, the Duncan and Defiance basins (Fig. 1). Southeast of the horst three basins were identified, the Wellsburg, Ankeny, and Shenandoah basins. Three clastic rock-filled basins were identified on the Iowa Horst, the Stratford, Jewell, and Mineola basins.
The clastic rocks in the Defiance basin were completely penetrated by the M.G. Eischeid well which encountered 4468 m of strata, with cuttings samples supplemented by four short cored intervals totalling about 19 m.. These rocks were grouped into two major groups and named by Witzke (1990); one (the Lower Red Clastic Sequence) apparently was deposited shortly after the cessation of rift volcanism, and the second (the Upper Red Clastic Sequence) probably was deposited during and shortly after the uplift of the Iowa Horst. The two Red Clastics groups were subdivided into seven informal formations and described by Witzke (1990). The Lower Red Clastic Group is composed of three formations; they are, from the base up, Unit B, Unit C, and Unit D. Unit B is dominated by white to red fine- to very fine-grained sandstone and is the most quartz-rich Red Clastic formation (Table 1). Unit C is the most distinctive of these units, due to its dominant dark gray to black siltstone, mudstone, and shale, and bears a close resemblence to the Nonesuch Formation of the Lake Superior region. Unit D is a generally upward fining sequence of red sandstone and siltstone. The Upper Red Clastic Group includes four formations; they are, from the base up, Unit E, Unit F, Unit G, and Unit H. Unit E is characterized by red to brown sandstone to siltstone and displays the highest percentage of lithic fragments (Table 1). Unit F is dominated by fining-upward sequences of red to gray sandstone to siltstone. Unit G is dominated by gray to red fine to medium sandstone, siltstone, and shale. Unit H is composed of clear to red fine to coarse sandstone and red micaceous siltstone. Borehole dip logs and observations of cores indicate that the Upper Red Clastic Group is uniformly horizontally bedded. The Lower Red Clastic Group includes beds from horizontal to steeply dipping and overturned (Witzke 1990). Comparable lithologies, depositional settings within the MRS, and similar structural histories led Anderson (1992) to relate the rocks of the Lower Red Clastic Group to the Oronto Group of the Lake Superior region, and the rocks of the Upper Red Clastic Group to the Bayfield Group. The lithologic disparity between the basal unit of the Oronto Group, the mafic volcanic clast-dominated Copper Harbor Conglomerate, and Unit B of the Lower Red Clastic Group, a quartz-dominated sandstone, was explained by Anderson (1992). He noted that the Copper Harbor was deposited as fanglomerate and alluvium within the early MRS axial graben, with mafic rocks derived by erosion of MRS lavas flows on the footwalls of the graben-bounding faults (Daniels 1982). Unit C is an alluvial rock, composed of grains derived from weathered quartz-rich granitic rocks in which the MRS of Iowa developed, and deposited outside of the axial graben. The rivers that deposited Unit C probably flowed into the MRS axial graben, and may have subsequently encountered and eroded MRS volcanic rocks near the graben and deposited Copper Harbor-like lithologies within the graben in Iowa.
This research provided sufficient information to estimate the volumes of the Upper and Lower Red Clastics Groups in Iowa Horst basins and MRS flanking basins in Iowa. A total of about 162,700 km3 of MRS clastic rocks are preserved in Iowa (Table 2).
Anderson, R.R., 1992, The Midcontinent Rift of Iowa: Ph.D. thesis, Iowa City, The University of Iowa, 324 p.
Anderson, R.R. (In press) Keweenawan Supergroup clastic rocks in Iowa, in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Late Precambrian and Cambrian rifting, Midcontinent of North America: Geological Society of America Special Paper.
Daniels, P.A., 1982, Upper Precambrian sedimentary rocks: Oronto Group, Michigan-Wisconsin, in Wold, R.J., and Hinze, W.J. (eds), Geology and tectonics of the Lake Superior Basin: Geological Society of America Memoir 156, 107-134.
Witzke, B.J., 1990, General stratigraphy of the Phanerozoic and Keweenawan sequences, M.G. Eischeid #1 drillhole, Carroll Co., Iowa, in Anderson, R.R., ed., The Amoco M.G. Eischeid #1 deep petroleum test, Carroll County, Iowa: Iowa Department of Natural Resources Geological Survey Bureau, Special Report Series 2, 39-58.
Table 1. Framework grain composition of MRS Red Clastics rocks in the M.G. Eischeid well.
|Unit||Thickness||Mean Framework Grain Composition|
|Upper Red Clastic Group|
|Lower Red Clastic Group|
Table 2. Estimated volume of MRS Red Clastics rocks in Iowa
|volume of clastics (km3)|
|basin||Lower Red||Upper Red||Total Red|
|Clastics Group||Clastics Group||Clastics|
|TOTAL RED CLASTICS||99,100||63,600||162,700|
Figure 1. Structural
components of the Midcontinent Rift System in Iowa (modified from
NBFZ=Northern Boundary Fault Zone, TRSZ=Thurman-Redfield Structural Zone,
PHFZ=Perry-Hampton Fault Zone; DfB=Defiance Basin, DcB= Duncan Basin,
ShB=Shenandoah basin; AkB=Ankeny Basin; WB=Wellsburg Basin;
AB=Ames Block; StB=Stratford Basin;
JB=Jewell Basin; MB=Mineola Basin.
Figure 2. Two-dimensional
gravity model of MRS gravity Profile 13 and observed and
calculated Bouguer gravity anomalies.
Open squares represent the observed Bouguer gravity anomaly.
Filled triangles represent the calculated (model) Bouguer gravity anomaly values.
Vertical exaggeration of model = 0.9 X.
From Anderson (1992).