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THE EASTERN FLANKING BASINS

Red ball iconTHE MIDCONTINENT RIFT SYSTEM IN IOWA

 

THE EASTERN FLANKING BASINS


Introduction

On the eastern edge of the Iowa Horst, an apparently continuous belt of flanking Proterozoic Red Clastic rocks stretches from Minnesota to Nebraska. It was divided by Anderson (1988) into three basins: from the north the Wellsburg, Ankeny, and Shenandoah basins (click for map), with their centers defined by closed isogal minima on the Bouguer Gravity Anomaly Map of Iowa). The eastern flanking basin is larger than the western basin, with clastics covering an area of 25,300 square kms (9900 square miles). The total volume of Red Clastic Sequence rocks, as modelled by Anderson (1992), in the basins flanking the eastern margin of the Iowa Horst is about 97,200 cubic kms (25,500 cubic miles). This includes about 43,000 cubic kms (10,500 cubic miles) in the Wellsburg Basin, 28,300 cubic kms (6900 mi3) in the Ankeny Basin, and 33,200 cubic kms (8100 cubic miles) in the Shenandoah Basin.


Wellsburg Basin

The northern-most of the three eastern clastic basins, the Wellsburg Basin, extends south from the Minnesota border for about 160 km (100 miles) to central Marshall County where it abuts the Ankeny Basin. The basin covers an area of about 9200 square kms (3600 square miles) and ranges in width from 21 to 75 km (13 to 47 miles), although its eastern zero edge is poorly constrained and may extend up to 16 km (10 miles) further east than is mapped. The model by Anderson (1992) indicates that about 43,000 cubic kms (10,500 cubic miles) of Proterozoic Red Clastic rocks fill the basin, and these rocks are partially penetrated by only one well, #1 Huntley in Butler County. The Red Clastics in the Wellsburg Basin are overthrust by the Iowa Horst, from about 5 km (3 miles) on the north to as much as 13 km (8 miles) in the area of the Ames Block (click for map of Red Clastic thickness).

The Lower Red Clastic Group rocks in the Wellsburg Basin form a belt about 18 to 56 km (13 to 35 miles) wide, cover an area of about 7700 square kms (3000 square miles), and reach a maximum model thickness in excess of 10,000 feet (4800 m) (click for map of Lower Red Clastic Group thickness). Model calculations indicate that about 14,300 cubic kms (3500 cubic miles) of Lower Red Clastics are preserved in the Wellsburg Basin.

The Upper Red Clastic Group rocks in the Wellsburg Basin display a maximum thickness of almost 7500 m (25,000 feet) where the central portions (click for map of Upper Red Clastic Group thickness) of the basin abut the Iowa Horst. Model calculations indicate the presence of 28,700 cubic kms (7000 cubic miles) of Upper Red Clastics in the Wellsburg Basin.

Ankeny Basin

The central of the three eastern flanking MRS basins in Iowa is the Ankeny Basin. The basin covers an area of about 5900 square kms (2700 square miles), extending for about 110 km (70 mi) between the Wellsburg and Shenandoah basins. It ranges in width from about 37 to 80 km (23 to 50 miles), although the eastern zero edge of the clastics is poorly defined. Modelling by Anderson (1992) indicated a maximum thickness in excess of 7500 m (25,000 feet) and a total volume of about 28,300 cubic kms (6900 cubic miles) of Proterozoic Red Clastics are preserved in the Ankeny Basin. The Red Clastic rocks are partially penetrated by only one well, the Story City Town Well #3. The Iowa Horst is thrust from 8 to 13 km (5 to 8 miles) over the Red Clastic rocks in the basin.

Lower Red Clastic Group rocks in the Ankeny Basin range in width from about 40 to 56 km (25 to 35 miles) and cover an area of about 4100 square kms (1600 square miles). The Lower Sequence reaches a maximum thickness of about 4200 km (14,000 feet) near the center of the basin, and the model volume of Lower Red Clastic Group rocks in the Ankeny Basin is about 7400 cubic kms (1800 cubic miles).

The Upper Red Clastic Group in the Ankeny Basin ranges in width from about 40 to 56 km (25 to 35 miles) and cover an area of about 4100 square kms (1600 square miles). The Lower Sequence reaches a maximum thickness of about 4200 km (14,000 ft) near the center of the basin, and the model volume of Lower Red Clastic Group rocks in the Ankeny Basin is about 7400 cubic kms (1800 cubic miles).

Shenandoah Basin

The Shenandoah Basin, the southern-most of the eastern flanking basins in Iowa, has two conspicuous lobes. The lobes are separated by an area of positive relief on the crystalline basement, coincident with the Central Iowa Arch Pluton (Anderson, 1988). The basin covers an area of about 9200square kms (3600 square miles), extending for about 160 km(100 miles) from the Nebraska border to the Ankeny Basin. The basin displays a maximum width, in the northern lobe, of about 75 km (47 miles) and a minimum width of 42 km ( 25 miles) between the two lobes -- although the eastern zero edge is not well defined. Proterozoic Red Clastic Sequence sediments in the Shenandoah Basin range up to about 9000 m (30,000 feet) in thickness, and modelling by Anderson (1992) suggested the presence of about 33,200 cubic kms (8100 cubic miles) of clastic rocks preserved in the basin. The clastic rocks are overthrust from 8 to 16 km (5 to 10 miles) by the Iowa Horst.

Two wells penetrate into the Red Clastics of the Shenandoah Basin including the #1 Wilson in Page County, which sampled 182 m (605 feet) of the sequence, and the Gulf Energy 16-13 Poetker in Mills County, which penetrated 24 m (80 ft) of Red Clastics. Additionally, a third well, the Ohio Oil Company Wisnom #1 penetrated 60 m (200 feet) of quartz arenite at below 945 m (3150 ft). The only quartz arenite in this portion of the geologic column is in the basal Cambrian sandstone. This basal sand, however rarely exceeds 30 m (100 feet) in thickness in this region. The anomalous quartz arenite in the Wisnom well may represent rocks related to the Bayfield Group Hinckley or Devils Island Sandstone of the Lake Superior region.

Lower Red ClasticGroup rocks in the Shenandoah Basin form a belt ranging from 24 to 59 km (15 to 37 miles) in width and covering an area of about 7200 square kms (2800 square miles). The unit reaches a maximum depth of about 3600 m (12,000 feet) in the basin, and model calculations suggest it includes a total of 12,300 cubic kms (3000 cubic miles) of rocks.

Upper Red ClasticGroup rocks in the Shenandoah Basin reach a maximum thickness of almost 6000 m (20,000 feet) near the Iowa Horst in the southern end of the basin. The modelling estimated volume of Upper Red Clastics in the basin is about 20,900 cubic kms (5100 cubic miles).

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