Removal of Names
Structural features in Illinois were named between 1910 and 1945, when abundant subsurface data became available. Dozens of structural names were introduced in ISGS county reports that emphasized structural targets for oil and gas exploration. Many of these reports included structure maps contoured on shallow units (generally Pennsylvanian coals and limestones). Partly to aid reference from text to map and partly to emphasize exploration potential, authors of county reports commonly named all known or suspected areas of closure as anticlines or domes.
For example, Easton (1942) published a subsurface structure map of the Herrin Coal Member (Pennsylvanian) in Macoupin County, west-central Illinois. Easton named numerous anticlines and domes and discussed them in his text. The anticlines that Easton named are short, curving, or irregularly shaped upwarps, most of which lack closure and have relief of only a few tens of feet. These domes are small, irregular areas of enclosed contours, mapped on the basis of one or a few control points. The reader who has Easton's map at hand will not be misled about the significance of the features he named. When such minor or questionable structures are transferred to regional or statewide maps that lack contour lines and control points, however, a distorted picture develops. Treworgy's (1981) map shows all of the structures named by Easton (1942). The structures appear as a dense swarm of folds, arranged in a semicircular pattern. They present the impression that Macoupin County is a strongly deformed area. In reality, the folds mapped and named by Easton are gentle undulations, probably of nontectonic origin. Remapping of the Herrin Coal in Macoupin County (Nelson 1987b) was based on hundreds of control points unavailable to Easton and revealed a pattern of undulations substantially different from that mapped by Easton. Most of the anticlines and domes that Easton named do not appear on the map in Nelson (1987b).
The names of 167 structural features recommended for removal from stratigraphic records were assigned prior to 1945 and cited only once or twice in the literature (table 2). These structures are indicated in the catalog by the term "discarded," which appears in parentheses after the name. These structural features are not shown on Plate 1. Catalog entries explain specific reasons for discontinuing the use of certain structural names. Decisions to discard a named structural feature were made according to the following criteria.
- Originally the feature was improperly defined (for example, a "dome" with no closure).
- Later studies have shown that the feature does not exist or that its existence is questionable.
- The name refers to a small portion of a larger structure, and no useful purpose is achieved by naming that part separately. In many cases, part of an anticline or fault system was named before continuity of the part with a larger whole was recognized
Care was taken not to remove structures merely because they are small or obscure. For example, the named synclines in Jo Daviess County(A-3 on Plate 1) are tiny compared with most named structures in Illinois, but they reflect regional stress patterns and control mineralization in the Upper Mississippi Valley Zinc-Lead District. Another example is a series of parallel anticlines and synclines called the Peoria Folds, mapped by Wanless (1957) in west-central Illinois (E-2 to D-4, Plate 1). These folds were mapped from little evidence; many do not have closure. Yet a striking match between mapped folds and the regional drainage pattern exists, and the parallelism of the folds indicates an origin in a regional stress field. Structural names such as these should be preserved at least for the purpose of stimulating further research.
