About Deformation structures

Several types of sedimentary structures are recognized that visualize the features of bedding or stratification, but which do not display the regular stratification of the structures discussed. Most of these structures seen to have formed from regular bedding or stratification that was altered during or after deposition but prior to consolidation (penecontemporaneous deformation). Deformation occurs by various means like physical processes that involve soft-sediment slumping, loading, squeezing, or partial liquefaction. The structures of this putative comes are commonly said to as deformation structures. Other irregular stratification structures appear to have formed as a result of erosion of unconsolidated beds followed by an episode of sedimentation. Structures of that kinds are called simply erosion structures. Among all one of irregular stratification structure, stroma to lites, is made as an original bedding structure across the activities of organisms and is thus a biogenic structure

About Deformation structures

Convolute bedding and lamination

The name which is applied to complexly folded or intricately crumpled beds or laminations is Convolute bedding, or convolute lamination which are commonly, although not invariably, confined to a single sedimentation unit. Strata below and above this unit may display little or no suspect of deformation. In cross-section, the deformed strata seen as small-scale anticlines and synclines. Axial planes of some folds may lean in the paleo current direction, as examined by other structures in the deposit.

Commonly confined to beds are less than around 25cm in thickness, convoluted units ranging from several meters thick have been submitted in both subaqueous and eolian sediments. The lateral extent of units possessing convolute lamination can apparently be considerable. For example, a convoluted unit only 12cm thick is submitted to occur over an area of about 750km2. Turbidites is particularly common in convolute lamination but can occur also in a variety of another sediments, containg intertidal-flat, deltaic, river-floodplain, point-bar, and eolian deposits.

Flame structures

Flame-shaped projections of mud are known as flame structures which are extend upward from a shale unit into an overlying bed of several composition, commonly sandstone. Single “flames” may range in its height from a some millimeters to several centimeters. For examples of the structure, the flames extend more or less directly upward into the overlying layer. But in others, the crests of the flames are overturned or bent downward, commonly most of them or almost all are in same direction. Flame structures are probably results of squeezing of low-density, water-saturated muds upward into denser and layers owing to the weight of the sand. The oriented, overturned crests of some flames gives that slight horizontal means: downslope or down current; movement or drag may occurs between the mud and sand layers during the process of loading and squeezing. The orientation direction of overturned flame crests is world widely consistent with the paleo current direction gives by other, related sedimentary structures.

Ball and pillow structures

The name which is used to the basal portion of sand stone beds is known as ball and pillow structure, which over lie shales, that are broken into masses of several sizes which are packed vertically and laterally in a mud matrix. These sand masses somewhat contains the structure of pillow lava sand may have“pillow,”“hassock,”“kidney,”or“ball” shapes. The underlying shale is squeezed between pillows and may extend as tongues into the overlying sandstone. In most of the deposits, the sand masses may become detached from the overlying and be fully surrounded by shale, give results of laterally extensive layer of nearly similar-size sand balls that may superficially resemble concretions. These isolated masses are commonly known as pseudonodules

Synsedimentary folds, faults, and rip-up clasts

Unconsolidated sediment may transfer downslope under the action of gravity as slumps, slides, or flows. Structures which are produced by such penecontemporaneous deformation may be result of two types of movement. A décollment type of movement in which the lateral displacement is concentrated along a sole, giving beds that are tightly folded and piled into the nappe-like structures is first type of movement. Such structures are said to as synsedimentary folds. Slump structure of this type may contain sediment sin several beds, and the structures are commonly faulted. Thicknesses of units characterized by slump structures which may range from less than 1 m to more than 50 m. Slump units are commonly covered above and below by strata that display no evidence of deformation. Such units must be observed with care, however, to be sure that they are indeed the product of penecontemporaneous deformation and which are not the result of deformation of incompetent shale or other beds caught between competent sandstone or carbonate beds at the same time of tectonic folding. Slump structures which are commonly occur in muds tones and sandy shales and less commonly in sandstones, limestones, and evaporites.

They form in sediments deposited typically in environments where fast sedimentation and over steepened slopes lead to in stability like glacial sediments; varved silts and clays of lacustrine origin; eoliandune sands; turbidites; delta and reef-front sediments; subaqueous dune sediments; and sediments from the heads of submarine canyons, continental slopes, and the walls of deep-sea trenches. The another type of penecontemporaneous deformation formed by slumping or flowing is the product of pervasive motion that involves the interior of the transported mass. This motion produces a chaotic mixture of different kinds of sediments, like broken mud layers embedded in sandy sediment. If muddy and sandy sediment are both capable of flowing during such transport, there sultis as treaked, “migmatitic” mixture.

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.