Introduction of structure

Introduction

About the study of sedimentary structures has discovered by the geologists for decades. These types of sedimentary structures like cross-bedding and ripple marks were known as early as the late eighteenth century, and sometimes well before. Gradual improvement in identification, description, classification, and reading of sedimentary structures has been especially rapid since the1950s, and the origin of most sedimentary structures is now meaningfully well understood. None the less, empiric study of modern and belonging to sediments and experimental research of the mechanisms which form sedimentary structures resume. How specific sedimentary structures are related to such aspects of ancient depositional environment as relative water energy, water depth, and current flow directions is especially interested in understanding by Geologists. Research of the origin and important of bed forms such as ripples and dunes has been a singularly active site of research. Most of the sedimentary structures comes from by physical processes involving passing water or wind that operate at the time of deposition. Some of the others are formed by physical processes such as gravity slumping or sediment loading that deform unconsolidated sediment after first deposition (soft-sediment distortion). Still other structures are of biogenic origin, which is formed by the burrowing, boring, browsing, or sediment-binding activities of organisms. Some kinds of bedding, the laminated bedding of evaporates which are generated by primary chemical precipitation methods. Some of the other structures, like concretions, produced by chemical processes operating within sediment during burial and diagenesis. Therefore, they are regarded to be secondary in origin. Short discussions of the processes that act to form the major types of sedimentary structures are given in suitable parts of this chapter like how- ever, detailed analysis of the sedimentary processes connected in the production of sedimentary structure is beyond the intended scope of the book. Classification of sedimentary structures and a representation of the major types of structures. Lastly, the methods for studying sedimentary structures and the importance of sedimentary structures in paleo current analysis are studied. The researcher who wish to have more knowledge on sedimentary structures may turn to a species of extra sources. For example, a rigorous treatment of the mechanisms of sediment transport involved in generation of bed forms and other types of structures is given in Allen (198 2). More recent books dealing with physi cally formed sedimentary structure s include Collinson and Thomps on ( 1989 ), Dimicco and Hardie ( 1994 ), and Ricci-Lu cci ( 1995 ).

Major types of sedimentary structures

The classification of Sedimentary structures may be based purely on the basis of their morphological or descriptive characteristics or on the basis of presumed mode of origin. Neither of that methods is entirely confortable. Serving to describe of classification gives little or no information about the genesis of structures; also, it is trying to suit all structures into a few descriptive categories. On the other side, genetic classifications are taking place and can be misleading. Some structures can form by more than one process or by a mixture of processes and hence it can be described under different genetic categories. The classification includes a compromise by listing primary sedimentary structures under both morphological and genetic headings. In any case, this provides a reference point for further discussion of the important types of sedimentary structures. The structures are examined mainly under the descriptive headings; however, some types of structures are further divided for discussion by genetic category.

Nature of bedding

All sedimentary rocks come to pass in beds some kind. Beds are flat or lenticular layers of sedimentary rock containing characteristics that distinguish them from strata above and below. Beds area purpose of and are differentiated by the composing, size, shape, orientation, and packing of sediment. Beds are kept apart from others by bedding planes or bounding planes into units that may range widely in thickness. Beds may be summarized internally by the presence of features such as laminae ,a lens of pebbles, or a band of chert. A distinct discontinuity, such as an erosional surface, that is present between two types of bed of identical composition is called an amalgamation surface.

Laminated bedding

Different kinds of sandstones and shales, as well as some non siliciclastic sedimentary rocks such as evaporites, display inner laminations that are fundamentally parallel to bedding surfaces. Particular laminae in these planar-stratified beds may range in thickness from a few grain diameters to as much as 1cm. The laminae are identified on the basis of differences in grain size, clay and organic matter content, mineral composing, and in uncommon cases microfossil content of the sediment. The color changes may emphasize the presence of some laminae. The laminated bedding forms as a result of suspension settling of fine-size sediment in a variety of stored environments like lakes, tidal flats, subtidal shelves, deep-sea environments. The laminae in many shales and evaporites, for example, may have formed by the suspension settling mechanisms.

Graded bedding

Graded beds are stratum characterized by gradual but often followed by vertical changes in grain size. Beds which display gradation from coarser particles at the base to fine particles at the top are said to have simple grading. Those that grade from finer particles at the base to coarser at the top have reverse grading or inverse grading. Beds of almost any thickness occurs in grading, even in laminae, but is most common in beds ranging from a few centimeters to a few meters.

Massive bedding

The massive bedding is used to beds of sedimentary rock which contain few or no visible internal laminae. Correctly massively bedded sediments are rare. Most of the massive appearing beds have been shown actually to possesses internal structures when examined after etching and staining or by X-radiography techniques. None the less, massive beds do occur, both in graded and nongraded units. They shown to be most repeated in sandstones. The origin of massive beds is not easy to explain. Presumably, massive bedding is generated in the absence of fluid-flow traction transport, either by some kinds of sediment gravity flow or by rapid deposition of material from suspension.

Cross-bedding

Cross-bedding is one of the repeated structures in sedimentary rocks. Although it is most abundant in sandstones, it can occur in any types of rock made up of grains confident of undergoing traction transport. Therefore, cross-bedding has been reported in limestone, salt deposits, ironstones, and phosphorites. Cross-beds, described in the simplest capable of handling terms, are strata in which internal layers, or foresets, dip at a distinct angle to the surfaces that bound the sets of cross-beds. (Cross-bedding is called cross-lamination if thickness of the foresets is less than 10 mm.) The bedding surfaces themselves may be either planar surfaces or surfaces that are curved in some manner. Thus, one common, simple method of classifying cross-bedding is to characterize it as either tabular cross-bedding, having bounding surfaces that are planar, or trough cross-bedding, having bounding surfaces that are curved.

Ripple cross-lamination

Ripple cross-lamination is a type of cross-stratification that has the general appearance of waves when viewed in outcrop sections cut normal to the wave (ripple) crests. Ripple cross-lamination forms when deposition takes place very rapidly during migration of current or wave ripples. A series of cross-laminae is produced owing to superimposition of one rippleon another as the ripples migrate. Their pples succeed one another upward in such a manner that the crests of vertically succeeding ripples are out of phase and appear to be advancing or climbing in a down current direction; thus, this structure is sometimes called climbing-ripple lamination. In outcrop sections cut at orientations other than normal to ripple crests, the laminae may appear horizontal or trough-shaped, depending upon the orientation and the shape of the ripples.

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.