Kansas Geological Survey, Bulletin 187, pt. 1, originally published in 1967
Originally published in 1967 as part of "Short Papers on Research in 1966," Kansas Geological Survey Bulletin 187, part 1, p. 17-19. This is, in general, the original text as published. The information has not been updated.
Joint sets were examined in the Plattsmouth and Leavenworth limestone members of the Oread Limestone and in the Ervine Creek and Rock Bluff limestone members of the Deer Creek Limestone. These joint sets trend about N 60° E and N 35° W, respectively. The long, planar northeast-trending shear joints are more common than the short, rough-surfaced northwest-trending tension joints. Short, planar north-trending joints also are present in these beds.
Joints are abundant in limestone beds of northeastern Kansas, but they have received little examination and have not been well described. This study is concerned with the general character and distribution of joints in four members of the Shawnee Group (Virgilian, Upper Pennsylvanian) in parts of Douglas, Shawnee, Jefferson, Osage, and Atchison counties, Kansas.
Well-jointed limestones were examined at 10 localities (Fig. 1), mainly in highway cuts. Most of the joints are vertical. This study was concentrated on the trends and interrelations of the joint sets, and the effects of lithology on development of joints. Only well-defined joints were studied, in an effort to eliminate fractures that could have been developed by human activity, e.g., blasting for highway construction.
Figure 1--Map of northeastern Kansas showing approximate locations of exposures studied.
Trends of joints are shown by strike-frequency diagrams (Figs. 2, 3, 4, 5). Radial scales differ (cf. Fig. 2 and 3), so that diagrams constructed from only a few measurements can be easily compared by inspection with those constructed from many measurements.
The Ervine Creek Limestone Member of the Deer Creek Limestone and the Plattsmouth Limestone Member of the Oread Limestone where examined are about 10 and 15 feet thick, respectively. These members are gray, weathering buff, fossiliferous, and are wavy-bedded with thin shale partings. Small northeast- and northwest-trending joints cut thin single beds of these members, but dominant joints trend about N 60° E to N 65° E (Figs. 2, 3). The northeast-trending joints are long, planar, and distinct and cut completely through these multiple-bedded members. Northwest-trending joints are short, irregular fractures that generally trend about N 30° W and commonly do not penetrate the entire thickness of the Ervine Creek and Plattsmouth. Northeast-trending joints are more abundant than northwest-trending joints (Figs. 2, 3).
Figure 2--Strike-frequency diagram of the Ervine Creek Limestone Member of the Deer Creek Limestone. Total number of observations is 244, of which 199 trend northeastward. Radius of small semicircle represents 10 observations.
Figure 3--Strike-frequency diagram of the Plattsmouth Limestone Member of the Oread Limestone. Total number of observations is 59, of which 46 trend northeastward. Radius of small semicircle represents five observations.
The Rock Bluff Limestone Member of the Deer Creek Limestone and the Leavenworth Limestone Member of the Oread Limestone are single beds about 2 feet thick and are bluish-gray, fossiliferous, hard, and brittle. The dominant joints in these units trend about N 55° E (Figs. 4, 5), are abundant, long, and planar, and many faces have feather-fracture markings.
Figure 4--Strike-frequency diagram of the Rock Bluff Limestone Member of the Deer Creek Limestone. Total number of observations is 350, of which 205 trend northeastward and 48 trend almost north. Radius of small semicircle represents 10 observations.
Figure 5--Strike-frequency diagram of the Leavenworth Limestone Member of the Oread Limestone. Total number of observations is 228, of which 113 trend northeastward and 49 trend northward. Radius of small semicircle represents five observations.
Northwest-trending joints are not as abundant, and they trend about N 30° W to N 40° W. They commonly extend only from one northeast-trending joint to the next, and their surfaces generally are quite rough. Both the Rock Bluff and Leavenworth contain a set of short, smooth-surfaced, north-trending joints that are tightly closed, and many of the joints do not entirely penetrate the beds. At some places, joints that are smooth and unweathered parallel joints that are marked by solution channels and joints that are coated with calcium carbonate. Figures 4 and 5 show orientations of joints measured in the Rock Bluff and Leavenworth limestones.
The northeast-trending sets of joints in the Ervine Creek, Plattsmouth, Rock Bluff, and Leavenworth members are clean-cut in both single- and multiple-bedded units. They account for 64 percent of 881 observations and occur as a single set in each unit studied. In the single-bedded Rock Bluff and Leavenworth members, feather-fracture markings are commonly developed on faces of the northeast-trending joints. In contrast, northwest-trending joints have rough surfaces in both types of limestone, and these sets make up only 25 percent of the measurements. They are not as straight as northeast-trending joints, and few of them have feather-fracture markings or completely penetrate the multiple-bedded limestones. North-trending joints generally are short, smooth-sided, and tightly closed. They make up 11 percent of all the joints measured in this study. (North-trending joints probably make up more than 11 percent of the joints in the Rock Bluff and Leavenworth. Locally they are more abundant in the Leavenworth than even northeast-trending joints. Because most of these occur as short, tightly closed cracks in the rock, bearings of only the largest and most distinctive of them were measured.)
Northeast-trending joints in these four limestone units are similar to a set of joints in Paleozoic rocks in parts of New York and Pennsylvania. These joints, described by Parker (1942, p. 395-397), are dominantly vertical, have plane surfaces, and cut many kinds of rock, including crossbedded rocks, rocks deformed during deposition, conglomerate pebbles, and even hard concretions in shales. The joints are numerous, locally consistent in trend, and some joint surfaces have feather-fracture markings. Parker concluded that these are shear joints.
Feather-fracture markings, similar to those present on many northeast-trending joints in the Rock Bluff and Leavenworth, were studied by Roberts (1961) in rocks of South Wales, Great Britain, that range in age from Devonian to Jurassic. Roberts concluded that feather-fracture markings seem to be restricted to surfaces of shear joints, although the process of feather fracturing appears to be a tension phenomenon. Roberts showed that small shears could be enlarged by tensional forces during release of stored energy, with feather-fracture markings being produced in the process. Hence, northeast- and northwest-trending joints in the Ervine Creek, Plattsmouth, Rock Bluff, and Leavenworth limestone members seem to be shear and tension joints, respectively.
Results of this study show only the general nature of jointing in four rock units of northeastern Kansas. A detailed study of jointing in limestones of this region should yield a great deal of evidence as to the genesis of the jointing in these rocks. Some of the points discussed below merit special consideration.
If northeast-trending joints are, in fact, shear joints, then perhaps other rocks of Pennsylvanian age in this area may contain two sets of shear joints rather than the single set observed in the four limestone units included in this study. North-trending joints have only been briefly discussed in this paper. They resemble shear joints, but as pointed out previously, they are common only in the Rock Bluff and Leavenworth and are not as well developed as northeast-trending joints. Unweathered joints in the Rock Bluff and Leavenworth parallel joints that show definite evidence of weathering, indicating that jointing has occurred at different times. These fresh joints have been observed in road cuts where they presumably were caused by lateral expansion of the limestone. If this interpretation is correct, then the Rock Bluff and Leavenworth limestones should contain directional zones of weakness that are likely to become the locations of joints when stresses on the rock are properly oriented. The common termination of tension joints at points of contact with shear joints also could be evidence of a difference in ages of joints, with shear joints being the older of the two sets.
Parker, J. M., III, 1942, Regional systematic jointing in slightly deformed sedimentary rocks: Geol. Soc. America Bull., v. 53, p. 381-409.
Roberts, J. C., 1961, Feather-fracture, and the mechanics of rock-jointing: Am. Jour. Sci., v. 259, no. 7, p. 481-492.
Kansas Geological Survey, Short Papers on Research in 1966
Placed on web July 25, 2011; originally published in Feb. 1967.
Comments to firstname.lastname@example.org
The URL for this page is http://www.kgs.ku.edu/Publications/Bulletins/187_1F/index.html