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Johnson County Geohydrology

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Stratigraphy of Outcropping Rocks, continued

Pennsylvanian System--Virgilian Stage

Douglas Group

Stranger Formation

The Stranger Formation, as defined in Kansas by S.M. Ball (O'Connor, 1963, p. 1877), consists of five members designated, in ascending order: Weston Shale, Iatan Limestone, Tonganoxie Sandstone, Westphalia Limestone, and Vinland Shale. Only the Weston Shale Member was definitely identified in Johnson County. The Stranger Formation conformably overlies the Stanton Limestone.

Weston Shale Member

The Weston Shale Member is grayish-blue and medium-gray clayey shale that weathers yellowish gray to light olive gray. The lower part is more gray than the upper part and contains several zones of light-brownish-gray dense ironstone concretions that are 2 to 12 inches in diameter. The concretions weather to shades of brown and yellowish orange, and occur both in layers and scattered. Upper beds of the Weston locally have carbonized plant fragments along the bedding planes and are siltier and bluer in color than the lower beds. In the area of maximum thickness (T. 14 S., Rs. 21 and 22 E.), the Weston is about 80 to 100 feet thick.

Marine macrofossils are nowhere abundant in the Weston, but thin fossiliferous zones contain fragments of crinoids, brachiopods, and bryozoans. Small mollusks are abundant locally in the basal 5 to 10 feet; for example, in the exposures along Interstate Highway 35 in the NW NW sec. 29, T. 14 S., R. 23 E.

Much of the area underlain by Weston is upland having low relief. The Weston may be deeply weathered and may have a thin cover of colluvial deposits overlying it. Consequently, exposures of Weston are restricted to areas of active stream erosion and manmade excavations.

Other Members

None of the members above the Weston Shale Member, with the possible exception of the Tonganoxie Sandstone Member, are recognized in Johnson County; the units either were eroded or were never deposited. Sandstone and sandy shale beds overlying the Weston are identified as the Ireland Sandstone Member of the Lawrence Formation, chiefly because stratigraphic studies, mapping, and test-hole data immediately to the west in southeastern Douglas County indicate that thick Ireland deposits rest disconformably on shale of the Weston and that the Tonganoxie Sandstone Member is very thin and does not contain massive beds of sandstone (O'Connor, 1960). The Iatan and Westphalia Limestone Members were not identified in outcrops or in the subsurface of Johnson County.

Newell (1935) described and mapped the first sandstone beds above the Weston as Tonganoxie. Although these sandstone beds are considered here as the Ireland Sandstone Member of the overlying Lawrence Formation, they may include, at the base, beds equivalent to the Tonganoxie. However, this relationship cannot be demonstrated, because the bounding Iatan and Westphalia limestones are absent, and the Ireland deposits just west of the county line are more than 130 feet thick in channels eroded into the Stranger Formation to within 30 feet of the Stanton Limestone. The Ireland and Tonganoxie are not lithologically distinct enough to identify one from the other where only a small outcrop of sandstone or sandy shale is observed.

Lawrence Formation

The Lawrence Formation, as designated by S.M. Ball (O'Connor, 1963), includes the Haskell Limestone Member, Robbins Shale Member, Ireland Sandstone Member, and Amazonia Limestone Member. Only the Ireland Sandstone Member has been recognized in Johnson County.

The Lawrence Formation is the youngest unit of Pennsylvanian age exposed in Johnson County.

Ireland Sandstone Member

The Ireland Sandstone Member consists of very fine to fine-grained quartzose micaceous sandstone, but contains minor amounts of shale, conglomerate, and coal. The sandstone beds may contain carbonaceous plant fragments, and locally are silty and argillaceous. Unoxidized samples of the sandstone are light to medium gray, but the beds as seen in exposures are almost always oxidized to shades of olive or greenish gray, yellowish gray, and yellowish orange. In exposures, the sandstone commonly is poorly cemented and may be quite friable; locally, however, the sandstone is well cemented with calcite. The Ireland overlies an irregular surface from which the Robbins Shale and Haskell Limestone Members of the Lawrence Formation, and the Vinland Shale, Westphalia Limestone, Tonganoxie Sandstone, and Iatan Limestone Members of the Stranger Formation either have been eroded prior to deposition of the Ireland or were never deposited. The Ireland exposed in southwestern Johnson County lies chiefly or entirely on an irregular erosion surface cut into the Weston Shale Member of the Stranger Formation. The local relief of the disconformity at the base of the Ireland is about 70 feet in Johnson County, and locally Ireland deposits are separated from the Stanton Limestone by less than 35 feet of Weston.

The maximum thickness of the Ireland recognized in Johnson County is about 49 feet (test hole 14-21E-25aaa).

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Kansas Geological Survey, Johnson County Geohydrology
Web version April 2002. Original publication date Dec. 1971.
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