Short Papers on Research in 1968

Abstract

The Lincoln Limestone Member (Cenomanian) of the Greenhorn Limestone (Upper Cretaceous) is the oldest formally named member of the formation. Originally known as the "Lincoln Marble" (Logan, 1897), the member has been known in more recent years as the "Lincoln Limestone Member" (Rubey and Bass, 1925, p. 27). The Member is recognized across the entire outcrop of central and western Kansas and enjoys formal status also in New Mexico and Colorado. The member extends also into Cimarron County, Oklahoma, where it is poorly exposed in an outlier of the Colorado Group near the former town of Mineral (Kauffman, Powell, Hattin, manuscript). Logan did not designate a type section for the Lincoln and he failed to indicate whether the name is derived from the city of Lincoln or from Lincoln County. The base of the member is poorly exposed on the east side of Kansas Highway 14 about 1 mile north of Lincoln, but I know of no completely exposed section in the vicinity of that city. Parts of the member are well exposed in cuts on Highway 14 in northern Lincoln County and also north of Sylvan Grove, Lincoln County, on Kansas Highway 18. Partial sections of the Lincoln are exposed at a number of additional localities in Lincoln County, in road cuts and gullies, and I have measured a complete, but poorly exposed, section on Interstate Highway 70 in S/2 sec. 36, T. 13 S., R. 7 W., Lincoln County. However, the only complete, extensively exposed, fresh exposure in Lincoln County known to me is in a road cut on an east-west county road, approximately 1.5 miles east of the Wilson Dam and located in SE SE sec. 31, T. 12 S., R. 10 W. and in NE NE sec. 6, T. 13 S., R. 10 W. The cut was made when road improvements were undertaken during construction of Wilson Dam. The cut is approximately 100 yards long and the faces are relatively steep. It is likely that the Lincoln will remain well exposed here for many years, and probable that the cut will be regraded from time to time because of the heavy flow of reservoir-bound traffic. Because of the general desirability of having a reference section for any stratigraphic unit, and especially one where the unit is well exposed, of characteristic lithologic development, and of typical thickness, I am designating the exposure on the north side of this cut as the reference section for the Lincoln Limestone Member of the Greenhorn Limestone. Description of this section is presented herein because this material will not be included in a forthcoming publication on the Greenhorn Limestone (Hattin, in preparation).

In the Kansas outcrop the Lincoln Limestone Member ranges in thickness from 16.2 feet (western Kearny County) to 32.4 feet (southeastern Mitchell County), averaging 23.1 feet for 11 measurements including two that are composites. At the reference section the thickness is 24.1 feet. Through much of central Kansas the base of the member lies at a widespread disconformity (Hattin, 1968) that separates mostly noncalcareous or only weakly calcareous shales in the upper part of the Graneros Shale from basal Greenhorn skeletal limestones that are overlain by shaly chalk or chalky shale and other carbonate rocks. In westernmost Kansas and in the northern part of central Kansas the contact is gradational and, in a section that grades upward from noncalcareous shales to calcareous strata, is placed at the base of the first bed or zone of abundant lenses of skeletal limestones, or bed of chalky limestone, above which the section consists predominantly of shaly chalk and skeletal limestone. The upper contact is located at the top of the uppermost bed, group of beds, or zone of abundant lenses of skeletal limestones above which the section is largely devoid of such rock. In most of central Kansas this contact lies from 0.91 to 11.68 feet below a prominent, widespread marker bed that contains the Sciponoceras gracile assemblage of invertebrate fossils and that characteristically shatters readily upon weathering. In general, the interval increases in thickness from northeast to southwest. In the southwestern part of the central Kansas outcrop and along the Arkansas River (in Kearny and Hamilton counties) the contact is located much lower in the section, owing to the lateral gradation of the upper part of the Lincoln into a predominantly shaly chalk section having characteristics typical of the Hartland Shale Member.

As thus defined the Lincoln Limestone Member consists principally of shaly chalk through which are scattered abundant lenses and mostly irregular thin beds of calcite-cemented skeletal limestones, including rare calcilutites, calcarenites, and calcirudites, as well as numerous seams of bentonite and a few thin, commonly discontinuous, beds of chalky limestone. In the northern part of central Kansas the lower part of the Lincoln contains some calcareous shale and chalky shale, especially in Mitchell, Cloud, and Washington counties. Molds of ammonites, especially including species of Calycoceras, Dunveganoceras?, Borissiakoceras, and Stomohamites, have been collected at many Lincoln localities, but the most abundant and characteristic fossil is an early form (fide E. G. Kauffman) of Inoceramus pictus J. de C. Sowerby. Ostrea beloiti Logan and Exogyra columbella Meek occur locally in the basal part of the member. An undescribed species of Exogyra is abundant in the lower part of the Lincoln at most central Kansas localities, and this species occurs in the middle and/or upper parts of the member in Hodgeman, Ford, and Kearny counties.

At the newly designated reference section, lithologic and stratigraphic character of the Lincoln is typical for the member as developed in Lincoln County, as well as for nearby counties to the south and west (i.e., Russell, Ellsworth, and Ellis counties), with the exception that the lowermost few feet contains some calcareous and chalky shale. North and northeast of Lincoln County, such rocks are common in the lower part of the Lincoln. The upper and lower contacts are typical of sections exposed in or directly adjacent to Lincoln County. Although the reference section contains few ammonites, the rest of the fauna is typical of the Lincoln of central Kansas. The reference section is illustrated in Figure 1 and described in detail below.

Figure 1--Graphic representation of reference section of the Lincoln Limestone Member of the Greenhorn Limestone.