Other structures

Other structures

Sandstone dikes and sills

Tabular bodies of sandstone are Sandstone dikes which fill fractures in any kind of host rock. They differs in thickness from a few centimeters to 10 meters or more than it. Among them most sandstone dikes want internal structures except oriented mica flakes and elaborated particles which may be aligned analog to the dike walls. They are seemingly formed from liquefied sand forcefully shoot upward into fractures, although some of the examples are known where sand seems to have been injected downward into fractures. Sandstone sills are features of similar visual aspect and origin except that they are sand bodies that have been shoot between beds of other rock. Sandstone sills may be difficult to separate from normally deposited sandstone beds till they can be followed into sandstone dikes or traced far enough to display a cross-cutting relation with other beds. For example, Archer (1984) represent sandstone sills in Ordovician deep-sea store of western Ireland and they are virtually same to normally deposited beds. She discusses also some of the criteria that can be used to distinguish clastic sills from normal beds. The sands which can form sandstone dikes and sills must have been in a highly water- conentrated, liquefied form at the period of injection, and injection visualizes to have been importantly instantaneous. Some sandstone dikes and sills may have been shooted into host rocks before they were fully consolidated, some others could have formed much later – considerably later than the period of lithification of the host. The host rocks that have been injected early tend to be contorted, whereas those injected after lithification are sharp-edged and straight-walled was suggested by Pettijohnetal (1987). Sandstone dikes and sills appear to have comparatively little sedimentological importance, although they may be of some value in working out the tectonic history of a region; e.g. sand-filled cracks imply episodes of tensional deformation.

The structures of secondary origin

Different types of structures are discussed in previous sections, with the elision of some sand stone dikes and may be convolute lamination, seems to have form editor very shortly after deposition of the host sediment. Therefore, these structures are commonly known as primary sedimentary structures. Some kinds of structures come about in sedimentary rocks that distinctly postdate deposition and are thus secondary sedimentary structures. The bulk of these secondary structures are of chemical origin, produced by precipitation of the mineral substances in the pores of the semi consolidated or consolidated sedimentary rock or by the means of chemical replacement processes. Some of the secondary structures seems to form through processes affecting pressure and solution. Concretions are may be the most common secondary structure. That types of structures are typically composed of calcite. However, concretions composed of other minerals like dolomite, hematite, siderite, chert, pyrite, gypsum etc. are generally known also. Concretions which is formed by precipitation of minerals around a center, building up a globular mass. Among them a little types of concretions have a distinct nucleus, such as a shell or shell fragment, but many do not possesses. When cut open then the concretions may display concentric banding around the center or may show little or no internal structure of them. The shape of the body rages from spherical bodies to disc-shaped, cone-shaped, and pipe-shaped bodies; and in size from less than a centimeter to as much as 3 m. So that some concretions may be syndepositional in origin, the process of growing in the sediment as it accumulates; however, most concretions are probably post depositional, as shown in many cases by original bedding structures, such as laminations, that pass through the concretions. Some of the early workers investigated that concretions grow displacively in sediments, pushing aside sedimentary layers as they grow. Many concretions, still, seems to have formed simply by precipitation of minerals in the pore spaces of the sediment. Concretions are particularly common in sandstones and shales but can possesses in other sedimentary rocks.

Nodules are nearly concerned to concretions. They are small, irregularly rounded bodies that normally contain a warty or knobby surface. They do not have internal structure except the preserved remnants of original bedding or fossils. The common minerals which is used to make up nodules include chert, apatite, anhydrite, pyrite, and manganese. Therefore some called nodules like manganese and phosphorite nodules which are forming now on the seafloor and are syndepositional in origin.

The sand crystals are huge euhedral or subhedral crystals of calcite, barite, or gypsum which are generously supplied with detrital sand inclusions. They seems to form during diagenesis by growth in incompletely cemented sands. The radially symmetric, sand-filled crystalline aggregates or clusters of crystals are generally known as Rosettes which somewhat resemble the shape.

The color banding, sometimes mentioned to as Liesegang banding, which is a kind of rhythmic layering showing from the precipitation of iron oxide in fluid-saturated sediments to form thin, closely spaced, ordinarily curved layers. Layers having different kinds of shades of red, yellow, or brown alternate with white or cream layers.

Stylolites are suture or stylus like seams, as display in cross-section, in commonly homogeneous, thick-bedded sedimentary rocks. The seams outcome from the irregular, interlocking penetration of rock on each and every side of the suture. Which are particularly only a few centimeters thick, and they are generally noted by concentrations of difficultly mixable constituents like clay minerals, iron oxide minerals, and fine organic matter. limestones are most common in Stylolites, but possesses also in sandstones, quartzites, and cherts.

The type of structure rarely mentioned in recent sedimentological literature is generally known as Cone-in-cone structure. Which consists of nested sets of small concentric cones, made up of, in most examples, of calcium carbonate, with mortal cones ranging in height mainly from 10 mm to 1cm. The sides of the cones are slightly ribbed or fluted, and some of them have fine striations that resemble slickensides. Cone-in-cone structure commonly possess in thin, persistent layers of fibrous calcite, commonly in related with concretions it is also called as pyrite concretions. The shales and marly limestones are most common in the structures. Considerable difference of view about the origin of cone-in-cone structure proceed from early workers. Most of the recent workers seems to believe that cone-in-cone is an early diagenetic structure which forms by growth of fibrous crystals in the enclosing sediment while it is still in a plastic state.

References

Collinson, D J and B D Thompson. Sedimentary structures. Delhi: CBS Publishers and Distributors, 1989.

Ehlers, E G and H Blatt. Petrology: Igneous, sedimentary and Metamorphic. New Delhi: CBS Publishers and Distributors, 1987.

JR, Sam Boggs. Petrology of Sedimentary structures. New York: Macmillan Publishing company, 1989.

Pettijohn, F J. Petrology of Sedimentary rocks. New Delhi: CBS Publishers and Distributors, 1984.