THE MIDCONTINENT RIFT SYSTEM IN IOWA
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THE IOWA HORST |
The upper crustal level of the Iowa Horst was modelled by Anderson (1992) as mafic volcanic-dominated rocks, capped in some areas by post-volcanic clastic sedimentary rocks (click for map of the MRS in Iowa). At the base of the Paleozoic strata the Iowa Horst appears as a continuous structure, ranging in width from about 16 km (10 mi) at the Minnesota border to 86 km (54 mi) near the center of the state. The basalt flows have obscured the Sheeder Prairie Structural Zone, although the offset of the dense middle and lower crustal levels of the horst is evident as an offset of the positive maximum that dominates anomalies on the Bouguer gravity map of Iowa.
The juxtaposition of the dense mafic volcanic rocks of the Iowa Horst against the flanking, low density clastic rocks, produces a suite of long, linear gravity anomaly contours. The values of these contours increase rapidly, from lows over the clastic basins to highs over the horst. Seismic interpretations and 2-D gravity models have been used to locate the edges of the horst. The northwestern edge of the Iowa Horst, the trend of the Northern Boundary Fault Zone, is coincident with the -35 mgal contour at the Minnesota border. It follows increasingly- lower value gravity contours as it trends southwest, approximating the trend of the -50 mgal contour near the center of Iowa, and finally the trend of the -75 mgal contour as it nears the Nebraska border. The southeastern edge of the horst, the Thurman-Redfield Structural Zone, enters Iowa from Minnesota along the trend of the -40 mgal Bouguer gravity anomaly contour. It begins to rapidly cross increasingly-lower value gravity contour as it trends south, to about -90 mgals at a position just west of the gravity minimum that identifies the center of the Wellsburg Basin. The location of the horst edge in this area is well-constrained by Seismic Profiles 9 and 12. The eastern edge of the Iowa Horst continues in a south southwesterly direction to its intersection with the -50 mgal contour and the Fayette Structural Zone, interpreted as a pre-Keweenawan northeast-trending feature. The edge of the horst bends sharply to the west, following the trend of the Fayette Structural Zone and the -50 mgal contour as it continues to the northern end of the Ankeny Basin. North of the Ankeny Basin the eastern edge of the Iowa Horst bends slowly southward, again moving down gravity anomaly contours almost to the -70 mgal contour just west of the center of the Ankeny Basin. The edge of the horst in this area is well controlled by a series of basement wells on the Redfield Dome in Dallas County. Continuing to the south, the edge of the Iowa Horst trends gradually up-contour between the Ankeny and Shenandoah basins (to the -60 mgal contour) and then back down-contour to the -75 mgal contour near the center of the Shenandoah Basin. It follows this contour to near the Nebraska border.
The general down-contour movement of both edges of the Iowa Horst on the Bouguer gravity anomaly map of Iowa is primarily a function of the over-all southwesterly decrease in the MRS gravity anomaly. This is produced by the thickening of the low-density Paleozoic strata into the Forest City Basin of southwest Iowa.
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