THE MIDCONTINENT RIFT
SYSTEM IN IOWA
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THE WESTERN FLANKING BASINS |
The thick sequence of Middle Proterozoic Red Clastics that flank the Iowa Horst on the west forms a belt from 35 to 51 km (22 to 32 miles) wide (click for map). Although the belt is apparently a continuous blanket of sedimentary rocks, it is divided into two basins, best delineated on the Bouguer Gravity Anomaly Map of Iowa. The central areas of these basins, the Duncan Basin on the north and the Defiance Basin on the south, are located by closed gravity minima. The Manson Impact Structure is located on the saddle that separates the basins. Modelling by Anderson (1992) suggests that a total of about 62,200 cubic kms (13,500 cubic miles) of Red Clastics are present in the western basins, about 23,700 cubic kms (9500 cubic miles) in the Duncan Basin, and 38,900 cubic kms (4000 cubic miles) in the Defiance Basin.
Duncan BasinThe northwesternmost of the MRS clastic basins flanking the Iowa Horst in Iowa, the Duncan Basin, extends in a southwesterly direction for about 110 km (70 miles) from the Minnesota border. The basin ranges in width from about 27 to 32 km (17 to 20 miles), an area of about 3800 square kms (1500 square miles), although the zero edge of the clastics is difficult to define and may extend as much as 16 km (10 miles) beyond the mapped limits. The Basin reaches a maximum modelled depth of 7500 km (25,000 feet) (click for map of Red Clastic thickness) against the Iowa Horst. Modelled overthrusting of the clastic sequence by the volcanic rocks of the Iowa Horst varies from 5 to 13 km (3 to 8 miles), generally greater on the southern end of the basin.
Lower Red Clastic Group rocks in the Duncan Basin display a relatively uniform model width of about 32 km (20 miles), cover an area of about 3300 square kms (1300 square miles), and have a maximum thickness of about 3000 m (11,000 feet) (click for map of Lower Red Clastic Group thickness) adjacent to the Iowa Horst. Model calculations by Anderson (1992) indicate the presence of about 13,500 cubic kms (1500 cubic miles) of Lower Red Clastics in the Duncan Basin.
The Upper Red Clastic Group, overlying the Lower Red Clastics in the Duncan Basin, is present as a belt of relatively uniform width from 35 to 41 km (22 to 26 miles), and reaching a maximum thickness of 5100 m (17,000 feet) (click for map of Upper Red Clastic Group thickness) adjacent to the Iowa Horst. A total of about 10,200 cubic kms (2500 cubic miles) of Upper Red Clastics are modelled in the Duncan Basin. No wells penetrate Duncan Basin clastic rocks in Iowa.
Defiance BasinThe southernmost of the western flanking basins, the Defiance Basin, includes an area of about 9000 square kms (3500 square miles), extending northwest for about 190 km (120 miles) from the Nebraska border. The basin ranges in width from about 35 km (22 miles) on the north to about 54 km (34 miles) in more southerly areas. The zero edge is difficult to define and may extend as much as 16 km (10 miles) west of its model location. The Middle Proterozoic Red Clastic rocks that fill the basin reach a modelled maximum thickness (Anderson, 1992) of about 4200 m (14,000 ft) near the Iowa Horst in the southern portions of the Basin. The volcanic rocks of the Iowa Horst are thrust 8 to 16 km (5 to 10 miles) over Defiance Basin clastics. Only two wells penetrate into these clastic rocks. One well, the M.G. Eischeid #1, has presently the most complete section of MRS clastic rocks. The second well, the Martin Augustine #1, produced only poor samples.
The Lower Red Clastic Group rocks in the Defiance Basin form a belt ranging in width from 27 to 45 km (17 to 28 miles), cover an area of about 6900square kms (2700 square miles), and display a maximum model thickness of about 3900 m (13,000 feet) in the southern area of the basin near the gravity minimum. Model calculations suggest that about 14,300 cubic kms (3500 cubic miles) of Lower Red Clastic Group rocks are preserved in the Defiance Basin.
The Upper Red Clastic Group in the Defiance Basin ranges in width from 35 to 56 km (22 to 34 miles) and reaches a maximum model depth of about 6000 m (20,000 ft). Model calculation suggests that about 24,600 cubic kms (6000 cubic miles) of Upper Red Clastics are preserved in the Defiance Basin in Iowa.
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