Abbildungen der Seite
PDF
EPUB

integral parts. The texture of the stone varies from the exceedingly fine-grained lithographic form, thru a typical oölite, to brecciated limestone, ending with an edgewise conglomerate. Thin shale and sandstone partings are to be found.

GENERAL DESCRIPTION OF INDIANA STYLOLITES

The Indiana limestones afford abundant opportunity for a study of stylolites and stylolite-seams of various sizes, types, and complexities. The largest stylolites observed by the writer are 13 inches in length (see Fig. 12); some are so small as to be seen distinctly only under a hand-lens. The length of the seams is in proportion to the size of the stylolites— the larger stylolites constituting partings of greater extent than small stylolites. All stylolite-seams, instead of ending abruptly, grade into smaller and smaller sutures, and finally disappear as a barely noticeable line.

In what might be called the most perfect, but not the most common stylolite-seams, the interlocked parts are more or less columnar in shape, the downward and upward projections alternately interpenetrating with much regularity. The assumption, suggested by Marsh, Gümbel, and Rothpletz-that few stylolites extend from the upper layer into the lowerwill not hold for the Indiana limestones. If one considers the upward-penetrating columns as stylolites, then the adjacent downward-pointing parts must be considered as reciprocal stylolites. In the larger sutures are often found these most perfectly column-shaped stylolites. They are especially sharply defined in the Salem limestone of the Dark Hollow district (see Figs. 2 and 11). The side-surfaces are often parallel; are always well striated, often having a polished or slickensided appearance, especially if there is a thin deposit of calcite on them; and are often covered with a very thin coating of clay, drawn from the clay cap to the base of the column. The ends of these columns are convex, fitting closely into the concave openings of the penetrated rock, and separated from them by the caps of clay.

Many common variations from this above-described, ideal type of stylolite are found. In their more common and typical development, the interpenetrating parts occur with less regu

For a description of the Mitchell limestone, reference should be made to: Beede, J.W., 1915, pp. 206-212.

[graphic]

ՈՂ

FIG. 11.-Diagrammatic sketch of the large, perfectly formed stylolites of the Salem limestone, such as are found in the Dark Hollow district, Lawrence County, Ind. In this specimen the block has been broken along the side-surfaces of the upward-penetrating columns, showing the striations; and thru the downward-pointing columns, exposing the lamination and texture of the rock. One-fourth natural size.

[graphic]

FIG. 12. Thirteen-inch stylolite in the buff Salem limestone. From a quarry of the Consolidated Stone Company, Dark Hollow district, Lawrence County, Ind.

larity, are more or less irregular in shape and size, and present an extremely jagged suture (see Figs. 3, 13, 14, and 15). The parting is often just an undulating seam with only occasional interpenetrating perfect columns. A few extensive partings, with only a slight stylolitic structure, are to be found. The length and shape of the penetrations are quite variable. Short columns, between longer ones, are frequent. Some columns are very broad, compared with their length, and have flat or undulating ends. The ends of others show a subordinate interpenetration of minor columns (see Fig. 24). The sides of the interpenetrating parts, instead of being parallel, frequently converge towards the end, sometimes coming to a sharp point. This is especially true of smaller penetrations. Along a fractured surface of a block of stone, the jaggedness of the stylolite-seams is often exaggerated because of the irregularity of the fracture (see Fig. 12). The true seam is to be seen on the face of a sawed block which has been cut parallel to the direction of the penetrating parts (see Fig. 15). Stylolite-surfaces, which have been exposed by the splitting of the strata along the suture, present a dark, irregular, pinnacled appearance (see Figs. 4, 5, and 6). The dark color is a result of the clay deposit.

In addition to the larger types of stylolite-partings, hundreds of small, sharply intertoothed sutures are found. They are sometimes so small as to present a barely noticeable line (see Fig. 16). These correspond to the so-called "Drucksuturen" of the German investigators. That no distinction can be made between these and the larger stylolite-partings is unquestionable, since the latter always grade into this small type of seam, and finally disappear as a barely noticeable crevice. These small sutures have all of the fundamental characteristics of the larger ones. The interpenetrating parts are irregular and are usually more or less conically pointed. In some, however, the penetrations are minutely columnar in shape and occur with marked regularity. These are especially noticeable in a fairly fine, even-grained limestone. Occasionally are found two or three closely parallel small partings which abruptly join and continue as one larger seam (see Figs. 16 and 35), the combined amount of penetration of the smaller seams being equivalent to that of the larger one.

Regardless of the size and the character of the styloliteseam, the striated sides of the penetrations and the clay part

ing are ever-present features. The clay comes to rest as a thin cap at the end of the columns. Its thickness is as variable as the size of the stylolite. In the smallest sutures, the clay

[graphic]

FIG. 13.-Complex stylolite-seam of the Salem limestone. From a quarry of W. McMillan and Son, Peerless district, Lawrence County, Ind.

FIG. 14.-Diagram of a small, jagged stylolite-seam of the lower Harrodsburg limestone. Note the variety of shapes of the interpenetrating parts. Natural size.

is only microscopically visible, but, nevertheless, is present. In the largest sutures, it is sometimes as much as an inch in thickness. Shorter stylolites between longer ones bear caps

[graphic][subsumed]

FIG. 15.-Typical stylolite-seam as it appears on a sawed surface of the Salem limestone. irregularity in size and shape of the interpenetrating parts. About one-half natural size.

Note the

« ZurückWeiter »