Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

STRUCTURAL FEATURES OF SEDIMENTARY ROCKS.

In the deposition of sediments in the sea, or in other bodies of standing water, the coarser portion of the material is usually deposited in the shallow water near the shore where the wave-action is strongest, and the less coarse of various grades is deposited at greater and greater distances from the land, while only extremely fine silt is usually carried out to abysmal depths (see [p. 380]). To this general law of distribution there are important exceptions. Fine sediments are sometimes deposited near the shore, and where currents, tidal agitation, or floating ice are effective, coarse deposits are occasionally carried far out from the shore.

Stratification.—Sedimentary rocks are usually arranged in more or less distinct layers; that is, they are stratified. The stratification consists primarily in the superposition of layers of different constitution or different compactness on one another. Layers of like constitution or compactness are often separated by films of different material which cause the partings between them. The bedded arrangement of stratified rocks is due to various causes, but primarily to the varying agitation of the waters in which the sediment was laid down. Where the depositing waters are agitated to the bottom, coarse sediment is likely to be deposited. Where the waters are quiet at the bottom, fine sediment is the rule. Since the agitation of the waters is subject to frequent change, it follows that coarser material succeeds finer, and finer coarser, in the same place. Hence arise beds, layers, and laminæ. The terms layer and bed are generally used as synonyms, while laminæ are thinner divisions of the same sort. The term stratum is sometimes applied to one layer and sometimes to all the consecutive layers of the same sort of rock. For the latter meaning the term formation is often used. Sometimes bedding seems to have been determined by strong currents which temporarily not only prevented deposition over a given area, but even cut away the loose surface of deposits already made, giving a firm surface from which succeeding deposits are distinct. This sequence of events is sometimes shown by the truncation of laminæ, and by other signs of erosion. The commoner sorts of bedded rock are limestones, shales, sandstones, and conglomerates.

The bedding of limestones is often caused by the introduction of thin films of clayey material which interrupt the continuity of the lime accumulation and cause natural partings. Sometimes, however, bedding arises from variations in the physical condition of the lime sediment itself. Lamination is not usually conspicuous in pure limestone, though it may be well developed in the shaly phases of this rock. Shales are normally laminated as well as bedded, and the lamination is often more notable than the thicker bedding. Bedding in shale may arise from the introduction of sandy laminæ, or by notable changes in the texture of the shale material. Similarly, sandstones are sometimes divided into beds by shaly (clayey) partings, but more often by variations in the coarseness of the sand itself, or by the presence of laminæ that are less coherent than those above and below. Sometimes the layers appear to be determined by the compacting of the surface of sand already accumulated before it was buried by later deposits. Sandstones may be thick- or thin-bedded, and their bedding passes insensibly into lamination.

Sand deposits usually take place in relatively shallow water, and the sand is subjected to much shifting before it finds a permanent lodgment. In the course of this shifting, bars are formed which usually have a rather steep face in the direction in which they are being shifted. The sand carried over the top of the bar finds lodgment on the sloping terrace face. The inclined laminæ thus formed constitute a kind of bedding, but since its planes do not conform to the general horizontal attitude of the formation as a whole, it is called false- or cross-bedding or, more accurately, cross-lamination (see [Fig. 368]). The same structure is developed on delta fronts and generally in water shallow enough to be subject to frequent agitation at the bottom. Sandstone is cross-bedded more commonly than other sorts of sedimentary rock.

The bedding of conglomerate is due chiefly to variations in coarseness. Laminæ or thicker layers of sand are frequently found between layers of coarser material. Conglomerate is likely to be thick-bedded, and cross-bedding is common.

Lateral gradation.—When the varying nature of the agitation of the sea at different depths and along the different parts of the coast-border, and during different phases of the sea-currents, is considered, it will be readily understood that sedimentary beds are affected by many irregularities, and that deposits of one kind grade into others horizontally with great freedom. Thus a bed of conglomerate (gravel) may grade laterally into sandstone, and this into shale or limestone. It is indeed rather more remarkable that the sedimentary strata should be as regular and persistent as they are, than that they sometimes grade into one another.

Fig. 368.—Cross-bedding in sandstone. Dells of the Wisconsin near Kilbourn, Wis. (Bennett.)

Special markings.—The rhythmical action of waves gives rise to undulatory lodgment, known as ripple-marks ([Fig. 324]). They are usually not the direct product of the surface-waves, since they are much too small. They are produced mainly by the vibratory movement of the undertow, but they apparently result from various other phases of vibratory agitation of the bottom waters. They are sometimes made by streams and stream-like currents. Ripple-marks are apparently preserved indefinitely under proper circumstances. They are sometimes found, for example, on very ancient quartzites. Ripples are also made by wind ([p. 37]). Ripple-marks are usually only an inch or two from crest to crest, but in rare instances they attain much greater size. Examples of ripple-marks 30 feet across are known.[211] Occasional ridges and depressions of much greater dimensions are produced which are attributable to the formation of successive bars, or to the building of wave-cusps.[212] Rill-marks are not infrequently produced by the undertow and other currents passing over pebbles, shells, etc. (Figs. [325] and [326]).

Fig. 369.—Mud-cracks in Brunswick Shale, N. J. (Kümmel.)

Sediments are sometimes exposed between tides, or under other circumstances, for periods long enough to permit drying and cracking at the surface. On the return of the waters, the cracks may be filled and permanently preserved. These are known as sun-cracks or mud-cracks (Figs. [328] and [369]). They chiefly affect shales, but are occasionally seen in limestones and fine-grained sandstones. During the exposure of the sediments a shower may pass and rain-drop impressions ([Fig. 370]) be made which are subsequently filled by fine sediment and preserved. The size and depth of rain-drop impressions give some hint as to the meteorological conditions of far-off ages. Wave-marks, which consist of the faint line-ridges developed on a sandy beach at the limit of the incoming wave, are sometimes preserved and may be seen occasionally on layers of rock deposited millions of years ago.

Fig. 370.—Rain-drop impressions. (Brigham.)

Concretionary structure.—Various sedimentary formations contain nodules or irregularly shaped masses of mineral matter unlike the rock in which they occur. When these nodules consist of matter aggregated about some center, they are called concretions. They are common in sedimentary rocks, and here it may sometimes be seen that the aggregation has taken place about a shell, a leaf, or some other organic relic. The nuclei are, however, not always organic. The material of the concretion may have come from the immediately surrounding rock, having been first dissolved by water and then deposited about the nucleus, or it may have been introduced from without, likewise by the agency of water. In the first case, the mineral matter of the concretion is usually one of the minor constituents of the rock. Thus the commonest concretions in limestone are composed of impure silica (chert, [Fig. 361]); in shale, of lime carbonate or iron sulphide; in sandstone, of iron oxide. The concretion may be made up almost wholly of concentrated matter, in which case the matter originally in the place of the concretion has been crowded aside; or it may involve much of the material of the imbedding rock. Thus the concretion of lime carbonate in shale may be nearly pure, or it may involve much of the earthy matter of the shale, while the concretion of iron oxide in sandstone commonly includes much sand. In extreme cases, indeed, the concentrated matter of the concretion merely cements the material involved into distinct nodules. Occasionally the rock substance itself takes on a concretionary form, all or most of its material being involved.

Fig. 371.—Discoid calcareous concretions from post-glacial clays. Ryegate, Vt. (Photo. by Church.)

Fig. 372.—Irregular calcareous concretions. Ryegate, Vt. (Photo. by Church.)

Fig. 373.—Calcareous concretions, some of them showing bilateral symmetry. Ryegate, Vt. (Photo. by Church.)

Fig. 374.—Irregular tubular silicious concretions in Arikaree clays. Northwest of Wildcat Mountain, Banner Co., Neb. (Darton, U. S. Geol. Surv.)

In size, concretions may vary from microscopic dimensions to huge masses, 8, 10, or even more feet in diameter. The variations in shape are also great. They may be spherical, elliptical, discoid, or they may assume more irregular and complex forms (Figs. [371] and [372]). The conditions of growth have much to do with the form. Thus a concretion which starts as a sphere may find growth easier in one plane than another, when it becomes discoid. Two or more concretions sometimes grow together, giving rise to complicated forms. Some of the most complex and fantastic forms are perhaps to be explained in this way. Concretions sometimes take the form of tubes. Some minute tubular concretions were formed about rootlets, but the larger ones appear to owe their form to other influences ([Fig. 374]).

Fig. 375.—Section of a concretion (septarium) the cracks of which have been filled by matter deposited from solution. About half natural size. (Photo. by Church.)

Fig. 376.—Section of a concretion, the cracks in which have been filled by deposition from solution. The filling appears to have wedged the parts of the original concretion apart. The fillings are veins. Some of them show that the vein-material was deposited on both walls. About half natural size. (Photo. by Church.)

One of the most extraordinary features of some concretions of complex form is their symmetry. This may be of various phases; in exceptional cases there is a bilateral symmetry almost as perfect as in the higher types of animals. This is especially true of certain calcareous concretions developed in plastic clays ([Fig. 373]).

Fig. 377.—Septarium from Cretaceous clays near the east base of the Rocky Mountains in Montana. (Photo. by Church.)

Concretions sometimes develop cracks within themselves, and these may then be filled with mineral matter differing in composition or color from that of the original concretions (Figs. [375] and [376]). Concretions the cracks of which have been filled by deposition from solution, are called septaria. They are especially abundant in some of the Cretaceous shales and clays. In not a few cases the filling of the cracks appears to have wedged segments of the original concretion farther and farther apart, until the outer surface of the septarium is made up more largely of vein-matter than of the original concretion ([Fig. 377]). Such concretions are often popularly known as “petrified turtles.”

Concretions of the sort indicated above often develop after the enclosing sedimentary rock was deposited. This is shown, among other things, by the fact that numerous planes of lamination may sometimes be traced through the concretions.

Concretions also form in water during the deposition of sedimentary rock. Exceptionally, sedimentary rock is made up chiefly of concretions. The chemical precipitates from the concentrated waters of certain enclosed lakes sometimes take the form of minute spherules which resemble the roe of fish. From this resemblance the resulting rock is called oolite ([Fig. 357]). Oolite is now forming about some coral reefs, presumably from the precipitation of the lime carbonate which was temporarily in solution. Considerable beds of limestone are sometimes oolitic. The calcium carbonate of such rock may be subsequently replaced by silica, so that the oolitic structure is sometimes found in silicious rock. If the concretions become larger, say as large as peas, the rock is called pisolite instead of oolite ([Fig. 378]).

Fig. 378.—Pisolite. Half natural size. (Photo. by Church.)

Fig. 379.—Columnar structure, “Devil’s Post Pile.” Upper San Joaquin Canyon, Sierra Nevada Mountains.

Beds of iron ore are likewise sometimes concretionary. Thus in the Clinton formation there are widespread beds of “flaxseed” ore made up of concretions of iron oxide which, individually, resemble the seed which has given the ore its name. The nucleus in this case is usually too small for identification.

Secretions.—When cavities in rock are filled by material deposited from solution, the result is sometimes called a secretion. Secretions therefore grow from without toward a center, while concretions follow the opposite order. Crystal-lined cavities (geodes, [Fig. 359]) and agates ([Fig. 358]) are examples of secretions. Crystal-lined cavities and veins are the same in principle.