Page images
PDF
EPUB

brown color, altho the clay of Table No. 4 is black. Ries (1914, p. 109) points out that

Carbonaceous matter often serves as a strong coloring agent of raw clays. If present in small amounts it tinges them gray or bluish gray, while larger quantities cause a black coloration. Indeed, so strong may this be that it masks the effect of other coloring agents such as iron. If the blue stone contains a greater quantity of carbonaceous matter than the buff, one should naturally expect the same relationship to exist in the residual clays. If the color of the buff stone is secondary, due to an alteration of the iron and carbon content, the question next arises as to whether or not the stylolitc-seams originated before or after the color change in the rock took place (see p. 59). If the stylolites all originated in the blue rock, the writer would believe the stylolite-clays all to have been originally black, with a good percentage of organic matter. The brown clays of today, then, result from a subsequent oxidation and volatilization of the carbon content, the process accompanying the color change of the parent limestone.

RELATION BETWEEN THE THICKNESS OF THE CLAY CAPS AND THE SIZE AND COMPOSITION OF THE STYLOLITES. In addition to the important relationships. between the chemical composition of the clay caps and the associated limestones, field investigations lead to the following conclusions:

1. The thickness of the clay caps varies in direct proportion to the length of the stylolites.

2. The thickness of the clay caps varies in inverse proportion to the purity of the limestone.

These two conclusions are logical if the clay partings are the solution residue of the dissolved limestone. In the Salem limestone, the smallest stylolites bear clay caps which are extremely thin, while larger stylolites have clays proportionately thicker. Invariably, this relationship is observed. That the thickness of the clay varies in inverse proportion to the purity of the limestone is especially striking in comparing the clays of the stylolites of the impure Harrodsburg limestone. with those of the pure Salem limestone (see pp. 80-81). For example, six-inch stylolites of the former have as much as an inch or more of clay, while the same sized columns of the latter bear caps as thin as an eighth of an inch. Analyses

FIG. 34. Stylolites of the Salem limestone, showing a double clay parting, separated by a thin layer
of limestone, at the top of the wide column on the left. The other columns have single clay caps.
Note the well-defined striations on the sides of the columns at the right. From a quarry of the
Consolidated Stone Company, Dark Hollow district, Lawrence County, Ind.

[graphic]

show the Harrodsburg limestone to contain as much as six to eight times the amount of insoluble constituents as the Salem limestone (compare Table No. 5 with No. 6).

That the clay partings and the associated stylolites always show a definite chemical and physical relationship is certainly not a coincidence. It is conclusive proof that the clay is a residue from the solution of the limestone.

OCCURRENCE OF CORRODED FOSSIL FRAGMENTS IN THE CLAY RESIDUE. The presence of corroded fossil fragments in the clay caps speaks for itself. Altho often only microscopically visible, they are to be found. They are the partially dissolved remains of the original limestone, and make up a considerable portion of the subordinate calcium carbonate content of the residual clay.

SUBORDINATE FEATURES OF THE CLAY CAPS. Stylolite caps often present a compressed and semi-laminated appearance. Since the circulation of ground waters would be variable, one should not always expect a uniform, even rate of solution to take place. A retardation or pause in the solution would produce a consequent pause in the deposition of the residue and thus give a laminated appearance to the deposit. Altho the line of contact between the clay caps and the ends of the columns is usually sharply defined, a few examples were found which show a slight gradation resulting from a partial solution of the limestone column itself.

Occasionally, stylolites are found which have what might be termed a "double cap", where the end of the column is marked by two layers of clay separated by a thin layer of limestone (see Fig. 34). In such a case the solution has been divided between two crevices, and the combined thickness of the clay of the two partings of the one column is equal to that of the single cap of the adjacent column. Analogous to this, a stylolite frequently contains one or more small, subordinate stylolite-seams crossing it at right angles (usually near the end), while the surrounding columns show none. This is nothing more than subordinate solution which has taken place along crevices of this one major projection and has produced within it minor stylolite-seams (see Fig. 26).

3. Stratigraphic Evidence which Precludes the Pressure

Theory and Supports the Solution Theory

OCCURRENCE OF STYLOLITES ONLY IN SOLUBLE ROCKS. Investigation of the geologic distribution of stylolites reveals indirect evidence that the phenomenon is one of solution. The fact that stylolites occur only in carbonate rocks-varieties of limestones, dolomites, and marbles-suggests solution as a factor, or otherwise they would not be limited to soluble rock strata (see p. 13). If the pressure theories of Marsh, Gümbel, Rothpletz, and others explain their origin, why should stylolites not be found in shales, sandstones, etc.? Could not Gümbel's experiment (see p. 27) be applied to rocks other than soluble ones?

OCCURRENCE OF ANGULAR STYLOLITE-SEAMS. It is interesting to note that in undisturbed strata, such as the southern Indiana limestones, the direction of stylolite penetrations is vertical (with but few exceptions), resulting from the static pressure of the overlying mass; and the direction of the stylolite-seams is usually horizontal, or nearly so, and parallel with the planes of stratification. However, in some instances, stylolites have developed along angular crevices which cut across the stratification (see Fig. 17). A normal fault surface, in one case, was stylolitic. In disturbed or metamorphic strata, where lateral pressure has been active, such as in the Muschelkalk of Germany and the Tennessee marble, stylolites run in all directions, the occurrence of vertical seams even being common. Stylolite-sutures which cross one another are observed. Since the pressure theory considers the clay caps of the columns as an original deposit of clay laid down in due order with the rest of the sediments, how can it explain the clay partings of angular to vertical stylolite-seams which cut across the stratification of the rock at various angles? These partings are undoubtedly not original deposits of clay, for they are by no means limited to the stratification planes of the stone (see pp. 44, 54, 67).

OCCURRENCE OF BRANCHING STYLOLITE-SEAMS. Commonly there are found two or more parallel stylolite-seams which converge and join into one larger seam, producing what might be called a "branching stylolite-seam" (see Figs. 35 and 36). This major, single seam sometimes continues some distance, and then branches again. The subordinate branches, after

FIG. 35.-Diagram of a branching stylolite-seam in the Mitchell limestone. Note that the combined length of the columns of the two minor, branching seams is equivalent to that of the stylolites of the major, single seam. Threefourths natural size.

[graphic]

FIG. 36.-Branching stylolite-seam in the Salem limestone. Note that the combined thickness of the black clay of the two branching seams is equivalent to that of the major, single seam.

« PreviousContinue »