Sedimenatary Structures (Syn Depositional [PDF]

Zitiervorschau

JAWAD (THUNDER) Presents

SEDIMENTARY STRUCTURES (Syn depositional structures)

SEDIMENATARY STRUCTURES Sedimentary structures are important attributes of sedimentary rocks. They occur on the upper and lower surfaces of beds as well as within beds. They can be used to deduce the processes and conditions of deposition, the directions of the currents which deposited the sediments and in areas of folded rocks. Sedimentary structures are very diverse and many can occur in almost any lithology. Sedimentary structures develop through physical and/or chemical processes before, during and after deposition, and through biogenic processes. Sedimentary structures are arbitrarily divided into primary and secondary classes. Primary structures are those generated in a sediment during or shortly after deposition. They result mainly from the physical processes, examples of primary structures include ripples, cross-bedding, and slumps. Secondary sedimentary structures are those that formed sometime after sedimentation. They result from essentially chemical processes, such as those which lead to the diagenetic formation of concretions.

STUDYING AND ANALAYZING SEDIMENTARY STRUCTURES Sedimentary structures can be studied at outcrop and in cliffs, quarries, and stream sections. Large-scale channeling and cross-bedding can also be studied using groundpenetrating radar . They can also be studied in cores taken from wells. Sedimentary structures in cores are the easiest to describe because of the small size of the sample to be observed. There are two basic approaches to observing sedimentary structures. The first approach is to pretend the outcrop is a borehole and to measure a detailed sediment logical log. This records a vertical section of limited lateral extent. The second method is to create a two-dimensional survey of all, or a major part, of the outcrop. This may be recorded on graph paper, using a tape measure and an Abney level for accurately locating inaccessible reference points on cliff faces. This method is aided by photography, especially by on-the-spot Polaroid photos, on which significant features and sample points can be located. Consider now the interpretation of sedimentary structures. They are the most useful of sedimentary features to use in environmental interpretation because, unlike sediment

grains, texture, and fossils, they cannot be recycled. They unequivocally reflect the depositional process that laid down the sediment.

PRIMARY INORGANIC SEDIMENTARY STRUCTURES Primary inorganic sedimentary structure is classified into four major types which are.

1. PRE DEPOSITIONAL • CHANNEL • SCOUR AND FILL • FLUTE MARKS • GROOVE MARKS • TOOL MARKS

2.SYN DEPOSITIONAL • MASSIVE • FLAT BEDDING • GRADED BEDDING • CROSS BEDDING • LAMINATION • CROSS LAMINATION

3.POST DEPOSITIONAL • SLUMP • SLIDE • CONVOLUTE LAMINATION • CONVOLUTE BEDDING • RECUMBENT FORESETS • LOAD STRUCTURE 4.MISCLLEANEOUS • RAIN PRINTS • SHRINKAGE CRACKS

We will not discuss the pre depositional type of sedimentary structure. we jump to Syndepositional structures.

SYNDEPOSITIONAL STRUCTURES: Syndepositional structures are those actually formed during sedimentation. They are therefore, essentially constructional structures that are present within sedimentary beds. DIFFERENCE BETWEEN BEDDING AND LAYERING It is necessary to define and discuss just what is meant by a bed or bedding. Bedding, stratification, or layering is probably the most fundamental and diagnostic feature of sedimentary rocks. bedding is sometimes absent in thick diamictites, reefs, and some very well-sorted sand formations. Nevertheless, some kind of parallelism is present in most sediments. Bedding is due to vertical differences in lithology, grain size, or, more rarely, grain shape, packing, or orientation. Though bedding is so obvious to see it is hard to define what is meant by the terms bed and bedding and few geologists have analyzed this fundamental property.Here are two more arbitrary but useful definitions: • •

Bedding is layering within beds on a scale of about 1 or 2 cm. Lamination is layering within beds on a scale of 1 or 2mm 2 mm.

Bedding and lamination define stratification. Bedding is thicker than 1 cm whereas lamination is thinner than 1 cm. Bedding is composed of beds; lamination is composed of laminae. Parallel (also called planar or horizontal) lamination is a common internal structure of beds.

MASSIVE BEDDING An apparent absence of any form of sedimentary structure is found in various types of sedimentation unit. It is due to a variety of causes. First, a bed may be massive due to diagenesis. This is particularly characteristic of certain limestones and dolomites that have been extensively recrystallized. Secondly, primary sedimentary structures may be completely destroyed in a bed by intensive organic burrowing. FIELD OCCURRENCE AND OBSERVANCE Genuine depositional massive bedding is often seen in fine-grained, low-energy environment deposits, such as some claystones, marls, chalks, and calcilutites. Reef rock (biolithite) also commonly lacks bedding. In sandstones massive bedding is rare. It is most frequently seen in very well-sorted sands, where sedimentary structures cannot be delineated by textural variations.

FLAT BEDDING One of the simplest intrabed structures is flat- or horizontal bedding. This, as its name

implies, is bedding that parallels the major bedding surface. It is generally deposited horizontally. Flat-bedding grades, however, via sub horizontal bedding, into crossbedding. The critical angles of dip that separate these categories are undefined. FIELD OCCURRENCE AND OBSERVANCE

Flat-bedding occurs in diverse sedimentary environments ranging from fluvial channels to beaches and delta fronts. It occurs in sand-grade sediment, both terrigenous and carbonate. Flat-bedding is attributed to sedimentation from a planar bed form. This occurs under shooting flow or a transitional flow regime with a Froude number of approximately 1. Sand deposited under these conditions is arranged with the long axes of the grains parallel to the flow direction. PARTING LINEATION Moderately well-indurated sandstones easily split along flat-bedding surfaces to reveal a preferred lineation or graining of the exposed layer . This feature is termed parting lineation, or primary current lineation. It is important to remember that, like many of the bed sole markings previously described, parting lineation will not be seen in friable unconsolidated sands, nor in low-grade metamorphic sediments.

GRADED BEDDING

The term "graded bed" is normally applied to beds measurable in centimeters or decimeters. "Varves," typical of lacustrine deposits, are measurable in millimeters. The term "upward-fining sequence" is normally applied to intervals of several beds whose grain size fines up over several meters. A graded bed is one in which there is a vertical change in grain size. Normal grading is marked by an upward decrease in grain size. Reverse grading is where the bed coarsens upward. There are various other types . Graded bedding is produced as a sediment settles out of suspension, normally during the waning phase of a turbidity flow. FIELD OCCURRENCE AND OBSERVANCE The lower part of a graded bed is normally massive, the upper part may exhibit the Bouma sequence of sedimentary. Normally graded beds generally represent depositional environments which decrease in transport energy as time passes, but also form during rapid depositional events. They are perhaps best represented in turbidite strata, where they indicate a sudden strong current that deposits heavy, coarse sediments first, with finer ones following as the current weakens. They can also form in terrestrial stream deposits.

CROSS BEDDING Cross-bedding is one of the most common and most important of all sedimentary structures. Cross bedding, as its name implies, consists of inclined dipping bedding, bounded by sub horizontal surfaces. Each of these units is termed a set. Vertically

contiguous sets are termed as cosets. The inclined bedding is referred to as a foreset. Foresets may grade down with decreasing dip angle into a bottomset or toeset. At its top a foreset may grade with decreasing dip angle into a topset. In nature toesets are rare and topsets are virtually nonexistent. Foresets may be termed heterogeneous if the layering is due to variations in grain size, or homogeneous if it is not. Two other descriptive terms applied to foresets are avalanche and accretion. Avalanche foresets are planar in vertical section and are graded toward the base of the set. Accretion foresets are ungraded, homogeneous, and have asymptotically curved toesets. Basically, two main types of cross-bedding can be defined by the geometry of the foresets and their bounding surfaces: Tabular planar cross-bedding and trough crossbedding (McKee and Weir, 1953). In tabular planar cross-bedding, planar foresets are bounded above and below by subparallel subhorizontal set boundaries (Fig. 5.15). In trough cross-bedding, upward concave foresets lie within erosional scours which are elongated parallel to current flow, closed upcurrent and truncated downcurrent by further troughs. FIELD OCCURRENCE AND OBSERVANCE It appears that much cross-bedding is formed from the migration of sand dunes or megaripples. Flume experiments showed how these bed forms migrate downcurrent depositing foresets of sand in their downcurrent hollows. If sedimentation is sufficiently great, then the erosional scour surface in front of a dune will be higher than that of its predecessor and a cross-bedded set of sand will be preserved. Tabular planar crossbedding will thus form from straight crested dunes. In river channels, especially those of braided type, the course consists of an alternation of shoals and pools through which the axial part or parts of the channel (termed the "thalweg") make their path. Where the thalweg suddenly enters a pool there is a drop in stream power and a subaqueous sand delta, termed a braid bar, is built out. Given time, sufficient sediment, and the right flow conditions, this delta may completely infill the pool with a single set of cross-strata . A second important way in which cross-bedding forms is seen in channels. A channel may be infilled by cross-bedding paralleling the channel margin. Alternatively cross-bed deposition occurs on the inner curves of meandering channels synchronous with erosion on the outer curve. A third important variety of cross-bedding is that formed by antidunes in upper flow regime conditions. It has been pointed out that at very high current velocities sand dunes develop that migrate upcurrent . These deposit upcurrent dipping foresets. The foresets of these antidunes, as they are called, are seldom preserved. As the current wanes prior to net sedimentation, antidunes tend to be obliterated as the bed form changes to a plane bed or dunes.

TABULAR PLANAR CROSS BEDDING

TROUGH CROSS BEDDING The term hummocky cross-stratification was first applied by Harms (1975) to a particularly distinctive type of cross-bedding. Each unit contains several sets of irregular convex-up cross-beds, some 10-15 cm thick (Fig. 5.22). Hummocky cross-bedding tends to occur in regular sequences about 0.5 m thick (Dott and Bourgeois, 1982, 1983). The base of each unit is generally a planar erosional surface with a lag gravel, often with bioclasts. The upper contact is sharp or gradational.

Cross-lamination and cross-bedding Cross-stratification forms either a single set or several/many sets (then termed a coset) within one bed . On size alone, the two principal types of crossstratification are cross-lamination, where the set height is less than 6 cm and the thickness of the cross-laminae is only a few millimetres, and cross-bedding, where the set height is generally greater than 6 cm and the individual cross-beds are many millimetres to 1 cm or more in thickness. Ripples are a wave-like bed form that occurs in fine sands subjected to gentle traction currents. Migrating ripples deposit cross-laminated sediment. Individual cross-laminated sets seldom exceed 2-3 cm in thickness, in contrast to crossbedding, which is normally >50 cm thick. It is hard to define arbitrarily the set height that separates cross-lamination from cross-bedding. In practice, the problem seldom arises,

because sets 5-50 cm thick are rare in nature. Ripple marking in modern and ancient sediments has attracted the interest of many geologists.

FIELD OCCURRENCE AND OBSERVANCE Ripples, however, and the cross-lamination they give rise to, are commonly produced by local flow directions which do not reflect the regional palaeoslope. In turbidite beds, for example, cross-lamination within the bed may vary considerably in orientation and differ substantially from the palaeocurrent direction recorded by the sole structures. The crosslamination forms when the turbidity current has slowed down and is wandering or meandering across the seafloor. In spite of their shortcomings, if there is no other, more suitable directional structure present (cross-bedding or sole structures), it is always worth recording the orientation of the ripples and cross-lamination. Wave-formed ripples are small-scale structures which record local shoreline trends and wind directions; their crest orientation should be measured, or if visible the direction of dip of internal cross-lamination Simplest of all are the straight-crested ripples; these include ripples with both symmetric and asymmetric profiles. Straight-crested or rectilinear ripples can be traced laterally for many times further than their wavelength. They are oriented perpendicular to the direction of wave or current movement that generates them. Sinuous ripples show continuous but slightly undulating crest lines. The second main group of ripples, as seen in plan, are those whose crest lengths are generally shorter than their wavelength. These are exclusively asymmetric current ripples. Two important varieties can be recognized. Lunate ripples have an arcuate crest, which is convex upcurrent. Linguoid ripples have an arcuate crest, which is convex down current.

A third main group of ripples can be recognized from their appearance in plan. These are interference ripples, which, as their name suggests, consist of two obliquely intersecting sets of ripple crests. Interference ripples result from the modification of one ripple train due to one set of conditions by a later train, generated by waves or currents with a different orientation. Ripples occur today in many different environments, ranging from the backs of eolian sand dunes, through rivers and deltas, to the ocean bed. It has already been pointed out that ripples are closely related to a given set of flow conditions and that these may be encountered in diverse environments.

POST DEPOSITIONAL SEDIMENATRY STRUCTURES

The third main group of sedimentary structures is a result of deformation. These may be termed post depositional because, obviously, they can only form after a sediment has been laid down. A great variety of deformational structures exist, many of which are ill defined and strangely named.

Slumps and Slides Slump structures, involve the penecontemporaneous plastic deformation of sand and mud. Slump folds, however, commonly show clear evidence of extensive lateral movement in a consistent direction. Where a sediment mass is internally deformed during downslope movement, then the term slump is more appropriate. A slumped mass typically shows folding; recumbent folds, asymmetric anticlines and synclines, and thrust folds are common, on all scales. FIELD OCCURRENCE AND OBESERBANCE Slump folds are commonly associated with penecontemporaneous faulting and with major lowangle zones of decollement termed "slide planes." Large masses of sediments are lat erally displaced along slide surfaces. In rare, but fascinating cases, the top of a slump bed may be covered by volcanoes of sand complete with axial vents and bedded cones. These are formed from sand carried up during dewatering of the slump after it came to rest.