Geophysical Study of the Keota Dome—Keokuk and Washington Counties, Iowa
The presence of a Paleozoic high in the vicinity of Keota, Iowa, is indicated on structural maps of the Iowa Geological Survey. Stratigraphic test drilling by Natural Gas Pipeline Company of America defined a “butterfly”-shaped structural dome which covered about 30 sq miles. A small amount of petroleum has been produced from the northeast segment of the dome. Closure on this segment is about 120 ft over a length of 3 miles. A geophysical study reveals ground-magnetic and gravity anomalies having amplitudes of from 200 to 400 gammas and from 2 to 3 mgals, respectively, associated with the dome. Polynomial trend-surface analysis of the geophysical data indicates a near-coincident relationship between the low-order residual anomalies and the closure of Keota dome. Interpretation of these data suggests that the structure may possibly be due to recurrent movement during Paleozoic time along a Precambrian fault which was also the locus of Precambrian intrusive igneous activity. The possibility of draping of sediments over a Precambrian topographic high should not be discounted. On the basis of geophysical data and Stratigraphic considerations, the top of the crystalline complex is estimated to be between 3000 and 3500 ft below the surface. It is concluded that ground-magnetic and gravity surveys can be used to locate similar structures in southeastern Iowa. One such structure might occur in the vicinity of the magnetic high south of the village of Talleyrand.