ABSTRACT

Iowa has traditionally been regarded with only passing interest by the petroleum exploration industry operating in the Mid-Continent. This has been primarily due to the lack of structural and stratigraphic information in the area of greatest oil potential, the southwest quarter of the state. Thick deposits of glacial drift and a thick, poorly understood Pennsylvanian section have obscured the character of underlying Paleozoic rock units and restricted exploration incentive and subsurface control in the area. Working with recently acquired geophysical and well-control data, the Iowa Geological Survey has constructed a series of structural and stratigraphic maps that suggest that the oil potential of the Forest City basin in southwestern Iowa should be reevaluated.

Gravity and stratigraphic work along the northeast-trending Thurman-Redfield fault zone suggests vertical displacements of lower Paleozoic rocks of up to 1,000 ft (305 m). The abrupt termination of the large vertical component of the north-trending Humbolt fault against the Thurman-Redfield zone, and the recent recognition of left-lateral movement of the Humbolt fault in Nebraska and Kansas has led to speculation that transcurrent movement and associated structures could provide traps for oil migrating out of the basin. Stratigraphic mapping has also delineated a series of basins and uplifted areas throughout the Paleozoic column in Iowa, possibly associated with isostatic equilibration of the massive Keweenawan flood basalts of the Portage Lake Series, which forms the pronounced geophysical feature known as the Mid-Continent geophysical anomaly. Details of these basins are not clear at this time, but the potential for stratigraphic traps associated with such features in this area should not be ignored.

A series of maps and cross sections of the southwest quarter of Iowa has been prepared illustrating the results of recent and current studies.

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