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III. Geology of the Midcontinent Rift System (MRS)
The MRS (Wold and Hinze, 1982) is recognizable by a significant elongate magnetic and gravity anomaly extending from south-central Kansas into the Lake Superior region and then southeastward into the Michigan Basin. Van Schmus and Hinze (1985) and Dickas (1984, 1986) provide excellent reviews of the MRS. The rocks that cause the geophysical anomalies are predominantly mafic volcanics interbedded with thousands of feet of clastic sediments deposited in response to subsidence of the MRS. More exotic rocks such as kimberlites and carbonatites of various ages also are present along the rift.
The rift also is characterized by faults up to several hundred kilometers long that more or less define its areal extent. Some faults show signs of tens of kilometers of strike-slip or even transform motion. While substantial normal faulting has occurred on listric to steeply dipping faults, there is also evidence of reverse faulting or even thrusting in some places. The net motion over geologic time has been extensional, however, and the entire thickness of the earth's crust if not the lithosphere has been affected along the length of the rift. The East African Rift System has been suggested by some authors as a modern analog of the MRS (Lee and Kerr, 1984; McSwiggen et al., 1987).
The rift-fill materials are composed in general of three tectono-stratigraphic sequences. First, synrift rocks are dominated by mafic igneous rocks that poured onto the surface in the early stages of rifting. The sedimentary rocks deposited during this phase are controlled by rift-basin architecture. Post-volcanic sedimentary rocks make up the second phase, and they are strongly influenced by the presence of erosional debris from the volcanics and by collapse of underlying magma chambers. Finally, the rift basin slowly subsided as regional cooling occurred, marking a transition to gentler sedimentation processes that are more characteristic of the stable craton.
In Kansas, the sedimentary phase of the rift development has loosely been called the "Rice formation" (Scott, 1966). The volcanic aspects of the rifting are only known in their upper few tens of feet in Kansas, and this has occurred by analysis of cuttings from the bottom of a few tens of wells (Bickford et al., 1979). The Poersch hole gives the first look at deeper units in Kansas that are analogous to rocks that are exposed in the Lake Superior region.
A more complete discussion of the geology of the MRS is included as Appendix 2 near the end of this report.
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Kansas Geological Survey, Texaco Poersch #1 Report
Placed on web March 5, 2010; originally published June 1988.
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