THE MIDCONTINENT RIFT
SYSTEM IN IOWA
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THE MINEOLA BASIN |
The southernmost of the clastic basins interpreted on the Iowa Horst in Iowa is the Mineola Basin (click for map of MRS in Iowa), named for the Mills County town near its center. It was originally referred to as the Mineola Graben, after early gravity modelling suggested a clastic-filled graben with its center near the northern Mills County town of Mineola. However, interpretation of seismic Profile 13 by Anderson (1992) displays a basin structure with no prominent bounding faults. On seismic Profile 13 the basin appears to be about 14 km (9 miles) wide, with a maximum depth of 0.8 seconds. Anderson (1992) reported that matching observed gravity values by modelling along Profile 13 proved difficult using Lower Red Clastic Group densities in a basin 1800 m (6000 feet) deep, as required by the seismic interpretation. However, using Upper Red Clastic Group densities in a basin about 1500 m (5000 feet) deep (assuming Upper Clastic velocity of 3960 m/sec (13,200 feet/sec), the model fit the observed data very well. Therefore, he concluded that the Mineola Basin is filled with sediments displaying Upper Red Clastic Group densities.
The limits of the Mineola Basin are best delineated on the Aeromagnetic Map of Iowa (click to view aeromagnetic anomaly map of Iowa). Interpretation suggests that the basin is about 65 km (40 miles) long, extending up the axis of the rift from the Nebraska border to eastern Pottawattamie County, Iowa). The basin displays a relatively continuous width of about 14 km (9 miles) over most of its length, and a total area of about 540 square kms (210 square miles. With a maximum depth of about 1500 m (5000 feet) the basin contains approximately 500 cubic kms (120 cubic miles) of Upper Red Clastic Group sediments.
The presence of Upper Red Clastic Group sediments in the Mineola Basin suggest that the feature must have formed after the uplifting of the Iowa Horst and the unroofing of horst volcanics by the erosion of Lower Red Clastic Group sediments. This implies that the Mineola Basin is the latest MRS structural feature on the Iowa Horst. The much younger Paleozoic (Pennsylvanian) Glenwood Syncline (Hershey et al., 1960) is nearly coincident with the Mineola Basin and may represent structural reactivation of the feature.
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