Guidelines for Naming Structures

No formal code comparable to the North American Stratigraphic Code (1983) governs the naming of structural features. Structural names may be introduced by simple mention in a report or on a map. Vague definition, duplication of names, or multiple names for the same feature are thus common. The only rule that seems to be universally followed is that structures are named for geographic features. There is no agreement on whether all the words in the name of a structure should be capitalized. For example, the U.S. Geological Survey, Geological Society of America, and American Association of Petroleum Geologists capitalize only the geographic component of structural names (i.e. Cottage Grove fault system). The ISGS originally published structural names in that manner. Since 1959, all words in a name have been capitalized (i.e. Cottage Grove Fault System), a practice continued in this report because structural names are proper nouns.

Structural nomenclature was not "formalized," but consistency of names was striven for in this document. Duplication or close similarity of structural names within a basin or region was avoided. For example, the Cottage Grove Fault System of southern Illinois is not likely to be confused with the Cottage Grove Fault of Minnesota. But the Mt. Carmel-New Harmony Fault in southeastern Illinois has been confused with the Mt. Carmel Fault in south-central Indiana; therefore, the name of the Mt. Carmel-New Harmony Fault of Illinois has been changed to the New Harmony Fault Zone. Similarly, the Lincoln Dome (in Logan County) has been renamed the Logan Dome to avoid confusion with the Lincoln Anticline of Calhoun County and adjacent Missouri.

Geologists also are inconsistent in defining structures. Different authors have varying concepts of what constitutes a "dome" versus an "anticline," or a "fault zone" versus a "fault system."

The following definitions are used in this document.

Anticline

An upward fold of strata that has a well defined axis and a length of at least twice its width. Example: Salem and Louden Anticlines. An anticline that lacks closure may be called a nose. Minor, irregular structural highs, especially those that lack closure, should not be named anticlines.

Arch

An elongated structural rise of regional extent; a divide between regional basins. On most arches the width is many times greater than the vertical relief. Typically the sedimentary section is thinner on an arch than in adjacent basins.

Basin

(1) An enclosed, roughly equidimensional structural depression of any size. (2) A regional depression of any shape. Because subsidence occurred during sedimentation, the sedimentary succession is thicker in the basin than on surrounding arches and domes. Example: Illinois Basin.

Disturbance

A localized circular or polygonal area of intense structural deformation, commonly called "cryptoexplosive." In Illinois, the Des Plaines Disturbance and Glasford Structure have been described as probable impact structures. The term "disturbance" does not imply the nature of the deforming agent.

Dome

(1) An upward fold without a well defined axis and with the length less than twice the width; closure must be demonstrated. (2) A roughly equidimensional area of regional extent that remained high while adjacent basins subsided. An area of thinned or absent sedimentary section. Example: Ozark Dome.

Fault

A fracture in rock along which displacement has occurred parallel to the fracture surface.

Fault zone

A set of closely spaced parallel or subparallel faults, generally all of the same type. Example: Sandwich Fault Zone.

Fault system

A group of closely related or interconnected faults that may differ from one another in type and orientation. A fault system typically is wider and more complicated than a fault zone. Example: Cottage Grove Fault System.

Fault complex

A group of two or more fault systems that have similar trend and character. The only example in Illinois is the Fluorspar Area Fault Complex.

Graben

A block downdropped between two subparallel faults or fault zones. Generally the faults bordering a graben are normal faults that developed under extensional stress.

Horst

A block upraised or left high between two subparallel faults or fault zones; the opposite of a graben.

Monocline

A step-like fold having an inclined or vertical flank between more or less horizontal upper and lower limbs. A monocline commonly overlies a buried fault. Example: Du Quoin Monocline.

Shelf

A raised region of nearly horizontal strata within or adjacent to a basin. A shelf typically subsided more slowly than the deep part of the basin and so has a thinner sedimentary section. Example: Sparta Shelf.

Syncline

An elongated downward fold with a well defined axis and generally bordered by anticlines.

Uplift

An arch or dome of regional extent that was raised relative to surrounding areas. The term uplift should be reserved for structures that definitely rose at one or more times in history.

The definitions have not been rigidly applied to cases in which the name of a structural feature is well established from long usage. For example, the Moorman Syncline might better be called a basin, and Hicks Dome could be called a disturbance. Names such as these have not been changed because such change would serve no useful purpose and might cause confusion.