By Howard G. O'Connor
Outcrops of igneous and metamorphic rocks occur in Riley, Woodson, and Wilson counties (State Geol. Survey Kansas, 1964). The igneous rocks are chiefly peridotite and granite; the metamorphic rocks are quartzite and hornfels. Merriam (1963) and Franks (1966) have summarized information about the origin and age of these igneous and metamorphic rocks.
Hills Pond Peridotite.--Outcrops of sill-like intrusive bodies of olive-gray, medium-grained peridotite are found in parts of sec. 29, 32, and 33, T. 26 S., R. 15 E., Woodson County. The peridotite is of late Cretaceous age (Zartman et al., 1966) and is in contact with rocks of late Pennsylvanian age (Lansing Group and Douglas Group) at the surface.
Wagner (1954) has described the Hills Pond Peridotite as a dike-like intrusion, along a fault, which spread laterally at several horizons in the Pennsylvanian strata as sill-like bodies. The igneous rock intruded into the Pennsylvanian strata has caused a doming effect in the surface rocks. The name "Silver City dome" is applied to the surface structure.
Granite.--Medium- to coarse-grained granite or granitoid igneous rock is found at the surface on Rose Dome in the SE sec. 13, T. 26 S., R. 15 E., Woodson County, and in parts of adjacent sections in association with peridotite and other ultramafic rock (Franks, 1965, 1966).
The upper Pennsylvanian shales into which the granite is intruded are mildly metamorphosed. K-feldspar from a sample of the granite has been dated (Muehlberger, Denison, and Lidiak, 1964) by the Rb-Sr method as 1,220 m.y. and is therefore Precambrian in age. Emplacement of the granite as xenoliths in an intrusive peridotite plug of possible Cretaceous age has been suggested (Merriam, 1963; Franks, 1965, 1966).
Alkaline ultramafic rocks.--Five pluglike intrusives (sec. 27 and 35, T. 6 S., R. 6 E.; sec. 22, T. 8 S., R. 5 E.; sec. 23, T. 8 S., R. 6 E. and sec. 6, T. 9 S., R. 5 E.) of basic igneous rock crop out in association with lower Permian rocks in Riley County. One of these intrusives, at Bala, has been described as a dark-green carbonatized, porphyritic, peridotite breccia (Moore and Haynes, 1920). Another intrusive near Stockdale, having a mineralogy similar to that of the Bala intrusive (Rosa and Brookins, 1966 ), has been described as a kimberlite pipe.
The age of the intrusives is uncertain, but they are post-early Permian and probably late Cretaceous or early Tertiary.
Metamorphic rocks exposed at the surface in Kansas are associated with the Riley and Woodson county intrusives. Lower Permian limestones and shales have been slightly altered by contact metamorphism in Riley County. In the area of Rose Dome in Woodson County and Silver City Dome in Woodson and Wilson counties Pennsylvanian shales and limestones have been altered to dense hornfels composed largely of feldspar and mica. Pennsylvanian sandstones have been converted to a greenish-gray quartzite. The quartzite in Woodson and Wilson counties is known informally as the "Silver City quartzite."
Kansas Geological Survey, Stratigraphic Succession in Kansas
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Web version August 2005. Original publication date Dec. 1968.