Recent subsurface and outcrop studies of the Middle and Upper Ordovician rocks in Iowa show that facies changes are of considerable importance in the interpretation of the geologic history and in the correlation of the strata, which are oil-bearing in adjoining states. From the area of outcrop in northeastern Iowa the beds dip generally southwest below the surface of the remainder of the state.

The generalized lithologic character of these rocks is, in descending order:

Table

Facies maps and isopachous maps show that: (1) the Neda zone of oolitic hematitic shale is at the top of the Maquoketa where the latter is thickest; (2) the Maquoketa and Galena formations thin in

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southeastern Iowa, and the Maquoketa is absent in part of that area; (3) the Maquoketa thins in the zinc-lead district of northeastern Iowa and adjacent states; (4) the upper part of the Galena and the lower part of the Maquoketa in southwestern Iowa are cherty dolomite similar to the Viola limestone of the Mid-Continent; (5) the Dubuque, Stewartville, and Prosser members of the Galena outcrop area of northeastern Iowa are not easily recognizable in the subsurface; (6) in western Iowa the lower part of the Galena grades laterally into Decorah-like strata; (7) in southwestern Iowa the lower part of the Galena, and the Ion and Guttenberg members of the Decorah, are sandy; (8) the Spechts Ferry is sandy in southeastern Iowa; (9) in west-central Iowa the Platteville is Decorah-like green hale and limestone (similar to the Simpson group); (10) the McGregor of the eastern bulge of Iowa gains a cherty unit at the top; and (11) in southeastern Iowa an unnamed sandstone lies above the Glenwood and thickens toward the southeast.

Anticlines are present in many parts of the state; however, potential oil finds are likely only in the Forest City basin structures (southwestern Iowa), although the wedge-out of the sandstone unit of the Platteville formation in southeastern Iowa should not be overlooked as a possible trap.