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THE MIDCONTINENT RIFT SYSTEM IN IOWA

Red ball iconTHE MIDCONTINENT RIFT SYSTEM IN IOWA

 

THE AMES BLOCK


Although the uniformity of the uplift of the Iowa Horst, as documented by the long, continuous, horizontal reflectors seen in the seismic profiles of the volcanic rocks, is remarkable, several major structural features have been identified on the horst. Key among these is the Ames Block, an elongate feature that trends along the eastern edge of the horst (click for map). The Ames Block, named for the Story County town nea its center, is bounded on the west by the Perry-Hampton Fault Zone and on the east by the Thurman-Redfield Structural Zone. The Perry-Hampton Fault Zone (Anderson, 1992) that bounds the block on the west apparently begins at the Thurman-Redfield Structural Zone, east of Mason City in Cerro Gordon County, and about 48 km (30 miles) south of the Minnesota border. The fault continues in a south/southeasterly direction into Story County, where it abruptly bends to a nearly due westerly direction as it nears the intersection of the Fayette Structural Zone and the MRS. The fault passes just north of the block's namesake, the city of Ames, where it begins to bend increasingly southward, finally paralleling the trend of the Thurman-Redfield Structural Zone through Dallas and Guthrie counties. As the fault moves into Adair County it apparently converges with the Thurman-Redfield Zone.

The Ames Block is evidently floored by upper package Keweenawan volcanic rocks. Above the volcanic rocks, both Lower and Upper Red Clastic Group rocks have been modelled on the block. Both have thicknesses over much of its length nearly identical to those modelled in the flanking Wellsburg and Ankeny basins. The Ames Block has been uplifted with respect to the flanking basins. This uplift has maximum values of about 3 km (2 miles) at its northern (Profile 12) and south-central (Profile 11) (click for map of profiles) regions, to only about 1.6 km (1 mile) between them (Profile 9).

The Ames Block is down-dropped with respect to the main body of the Iowa Horst, west of the Perry-Hampton Fault Zone. Modelling by Anderson (1992) revealed an apparent displacement, ranging from a minimum value of 3.0 km (1.9 miles) along the southern Profile 11, to 6.7 km (4.2 miles) along Profile 9, and 5.6 km (3.7 miles) near the northern end of the block along Profile 12.

The upper surface of the volcanic rocks on the Ames Block displays an apparent synclinal structure. There is no apparent vertical displacement along the Perry-Hampton Fault Zone on the southern end of the structure. Downward displacement of the block increases to the north, to a maximum near Profile 9, then decreases to its northern intersection with the Thurman-Redfield Structural Zone. At this point, near Anderson's (1992) gravity Profile a-a', displacement may still be as much as 4.0 km (2.5 miles).

The Lower Red Clastic Group on the Ames Block ranges in thickness from zero on the southern end of the block, increasing rapidly to about 1.1 km (0.7 miles) along Profile 11 to 3 km (2 miles) along Profile 9. Along much of its eastern margin, the unit is approximately equal in thickness to its thickness in the flanking clastic basin. Like the Lower Sequence rocks of the adjoining basin, it increases in thickness to the west. The approximate model volume of Lower Red Clastic Sequence sediments preserved on the Ames Block is about 4000 cubic kms (1000 cubic miles).

The Upper Red Clastic Sequence on the Ames Block is also similar in thickness and westward thickening to related rocks in the adjoining flanking basin. The Upper group ranges in thickness from zero on the southern end of the block to about 2 km (1 miles) along Profile 12 and about 3 km (2 miles) along Profile 9. Approximately 6000 cubic kms (1500 cubic miles) of Upper Red Clastics are preserved on the Ames Block.

Both the Upper and Lower Red Clastic groups are apparently erosionally beveled along the southern end of the Ames Block, where drill data confirm the presence of volcanic rocks at the Precambrian surface.

Available information suggests that the uplift of the Iowa Horst included reactivation and reversal of the normal faults that bounded the central graben of the MRS in Iowa --the Northern Boundary Fault Zone on the west and the Thurman-Redfield Structural Zone on the east. However, along the central area of the eastern margin of the graben, a new structure formed, the Perry Hampton Fault Zone. The thick Upper Red Clastic Group rocks on the Ames Block suggest that deposition of the unit on the block occurred at the same time as in the adjoining flanking basins. Then, at some late stage, the Ames Block was uplifted, probably along with the remainder of the Iowa Horst. Additional complexity is added by the absence of clastic rocks at the southern end of the block, as confirmed by well data. This southern end probably remained attached to the main portion of the Iowa Horst, as it was moved upwards.

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