THE MIDCONTINENT RIFT
SYSTEM IN IOWA
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THE STRATFORD BASIN |
The Stratford Basin was named for the Hamilton County town near its center and for the Stratford Geophysical Anomaly, studied by Osweiler (1982). She interpreted the feature as an elliptical, clastic-filled basin with an area of 1650 square kms (650 square miles) that formed along the axis of the MRS (click for map of MRS in Iowa). Osweiler argued that an earlier interpretation of the feature as a clastic-roofed granitic lopolith (Cohen, 1966) was not consistent with the gravity and magnetic anomalies and known MRS geology. She suggested that a clastic-filled basin, floored by MRS volcanic rocks, and similar to the Twin City Basin in Minnesota (King and Zietz, 1971), was the best interpretation.
Using a gravity contrast of 0.6 g/cc between the clastics and the underlying mafic volcanic rocks, Osweiler (1982) modelled the basin as elongate, about 56 km (35 miles) long (click here for map of Stratford Basin), trending in a northeasterly direction (along the axis of the MRS), and about 37 km (23 miles) wide. The basin reaches a maximum depth of about 950 m (3000 feet) below the base of overlying Phanerozoic rocks. The basin was here interpreted by Anderson (1992) from seismic Profile 9 (click here for location of profile) where it displays a maximum depth of 0.7 seconds, about 1660 m (5500 feet) assuming a velocity of 4740 m/sec (15,800 feet/sec) for the basin-filling clastics. This depth requires a density of 2.65 g/cc (density contrast of only 0.2 g/cc with the underlying basalts) for successful modelling. The density is higher than the average 2.55 g/cc density assigned to the Lower Red Clastic Group along Profile 9. This density may be increased by a higher percentage of mafic volcanic rock fragments (similar to clasts in the Copper Harbor Conglomerate) in the clastic rocks of the Stratford Basin. This theory is strengthened by a description (Norton, 1928) of the rocks encountered during the drilling of the Ogden City #2 well in 1929 (click here for well locations). Although no samples exist for the basal 2 m (7 feet) of the well, the lowest samples collected, from 854 m (2845 feet), were litharenites with abundant feldspar, mafic igneous rock fragments, and mafic minerals. This lithology is similar to the Copper Harbor Formation and in Iowa is part of the Lower Red Clastic Group. The well encountered the Lower Red Clastic Rocks at a depth of 825 m (2750 feet) and penetrated 31 m (102 feet) of the clastic sequence. Additionally, the Boone City #1 well also apparently penetrates into the clastics of the Stratford Basin. Boone City #1 was drilled in 1890 and encountered a sequence of red shales and soft red sandstones, interpreted as Lower Red Clastic Group, at a depth of about 810 m (2700 feet). A total of 90 m (300 feet) of these rocks was penetrated.
Additionally, deep burial along the axis of the MRS and thorough cementation may combine to form an anomalously dense unit, similar to the dense clastic rocks encountered in the Amerada Schroeder well which lies in a similar position on the Iowa Horst in Nebraska. Using the 0.2 g/cc density contrast, Anderson's model for the Stratford Basin has a width and length similar to values determined by Osweiler (1982), but reaches a greater maximum depth, about 3600 m (12,000 feet). The total volume of Lower Red Clastic Sequence rocks in the Stratford Basin is approximately 2050 cubic kms (500 cubic miles).
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