Assigment on Sedimentary depositional Enviornment

Assignment: Sedimentary Drepositional Enviornments

Submitted to:

Sir Abdul Hanan

Submitted By:

Falak Sheir                                        BGLF11M028

Class:

                             BS(Geology)6^th Semester

Department of Earth Sciences

    University of Sargodha

Sargodha

Acknowledgement

I would like to thank Allah Almighty for giving me a sense and strength to write this worksheet and Sir Abdul Hanan for sharing his knowledge with me. I never forget the prayers of my parents for success.

Table of content

Abstract

Sedimentary Deposional Enviornment

Ø Introduction

Fluvial environment

Ø introduction

Ø Recognization of fluvial environment

SEDIMENT GRAVITY FLOWS AND THEIR DEPOSITS

Ø Introduction

Ø Classification

Ø Motion

Ø Sedimentary gravity flow deposits

Open shallow Marine deposition

Ø Introduction

Ø Hydrodynamic classification of coasts

·      Deposits of Tide-Dominated Coasts

·      Deposits of Storm-Dominated Coasts

Tidal Flats

BARRIER ISLAND, BEACH, AND LAGOON ENVIRONMENTS

Ø  Introduction

Ø  Beach and Shoreface Deposits

Ø  Tidal-Inlet Deposits

Ø  Lagoonal Deposits

Abstract

The study of sedimentary environment at different places are difficult and have great importance due to different conditions they are developed, which is occurred due erosion, deposition on different rocks. They have different groups Such as marine,  non marine and marginal marine environments.

Sedimentary DEPOSITIONAL ENVIRONMENTS

INTRODUCTION

 What does a sedimentologist mean by environment of deposition? The concept is not as easy to define . Basically, what the conditions were at the site of deposition. This is usually viewed in terms of an overall geographical complex or entity that has a characteristic set of conditions, like a river or a beach.

The term environment is really used in two different ways:

THE environment: The aggregate or complex of physical and/or chemical and/or biological conditions that exist or prevail at a given point or in a given local area at a given time or for a period of time.

AN environment: A distinctive kind of geographic setting characterized by a distinctive set of physical and/or chemical and/or biological conditions.

One can go out and look at all the surface environments in the modern world to help depositional interpretations, but one should keep in mind that only a small subset of environments (on land, that is) are depositional environments: most are erosional environments, and they won’t help much (and maybe even mislead you badly) about depositional environments. The somewhat strange word actualistic is used in geology to describe situations or processes that are represented on the Earth today, as opposed to non-actualistic ones, which are interpreted to have existed at a particular time or times in the past but are not represented on the modern Earth.

Fairly detailed list of depositional environments.

·        nonmarine

·        terrestrial desert

·        loessial

·        subaqueous

·        fluvial lacustrine

·        paludal spelean

·        glacial

·        subglacial

·        glacier-terminus proglacial

·        transitional deltaic

·        lagoonal estuarine

·        marine

·        coastal

·        beach

·        tidal-flat

·        muddy shoreline reefy shoreline prodelta

·        shelf

·        siliciclastic shelf carbonate shelf

·        abyssal

·        continental slope

·        submarine canyon/fan/abyssal plain open deep ocean

There are various problems with this list:

• There’s lots of overlap among the different environments.

• It’s still not a complete list.

• Some of the environments on the list have been much more common and important in the sedimentary record than others.

• The divisions are not entirely natural.

• The extremely important effect of tectonic setting is taken into account to some extent but seriously inadequately.

SPECIFIC ENVIRONMENTS
1. FLUVIAL ENVIRONMENTS
Introduction

Rivers are the main routes by which sediment derived from weathering on the continents reaches the ocean. (Remember that this sediment includes dissolved material as well as particulate material; the discussion here deals only with the particulate material.) Most of this sediment indeed reaches the ocean, but a certain smaller percentage is deposited either within the rivers themselves or where rivers end in basins of interior drainage.

In a sense such storage is temporary, in that it eventually is remobilized in a later geologic cycle, but commonly it is stored for geologically long times, tens to hundreds of millions of years and sometimes even billions of years—long enough for it to be deeply buried and lithified, even metamorphose

 Rivers are enormously varied, in size, geometry, and dynamics. When someone speaks of rivers to you, what image comes to your mind? A rushing mountain stream? The broad and placid Mississippi? These are only two of a great many common manifestations of rivers. So it should not surprise you that fluvial sediments and sedimentary rocks are highly varied as well.

It’s usually considered that most rivers are either meandering or braided; straight rivers are not very common. But meandering and braiding are not mutually exclusive tendencies: either or both can be in evidence in a given river, with each with varying degrees of prominence. And both meandering and braiding show great diversity. This should suggest to you that it's not easy to compartmentalize the sedimentological behavior of rivers into a few neat models.

Meandering rivers lend themselves to fairly easy description or characterization, and a widely accepted facies model for the deposits of meandering rivers has been around for a long time. Braided rivers are much more variable and less easy to characterize; there have been attempts to develop facies models for braided rivers, but these have not been nearly as successful or widely used as the meandering model.

This is not the place to present an account of the dynamics and geomorphology of rivers, although I want you to appreciate that understanding of the geomorphology of rivers is crucial to understanding and interpretation of fluvial deposits. The major factor in how a fluvial deposit looks is the geomorphology of the river: the arrangement of channels and overbank areas, and how they change with time during slow buildup of the floodplain. All I’ll do here is present some basic things about fluvial deposition and fluvial deposits, and then outline the meandering fluvial model, with appropriate caveats on its application.

Keep in mind that the same problem of the modern versus the ancient that holds for marine deposits holds for fluvial deposits as well: It’s relatively easy to study sediment movement and disposition in modern rivers, but it’s rather difficult to study the vertical sequence of deposits, because of the generally high water table. In ancient rocks, on the other hand, it’s easy to study the vertical sequence of deposits, but it’s usually impossible to establish the geomorphology of the depositing river itself.

Why Are There Fluvial Deposits?

It’s not obvious why fluvial deposits are an important part of the ancient sedimentary record. After all, rivers drain areas of the continents that are undergoing erosion. It’s true that most rivers, except the smallest, are alluvial rivers: they have a bed, and a floodplain, composed of their own sediments. But in most cases this alluvial valley sediment isn’t very thick. Only in certain cases does the alluvial valley fill become thicker.

Two effects are conducive to deposition in rivers: progradation and crustal subsidence.

Progradation. As a river wears down the land and delivers sediment to the sea, the mouth of the river builds seaward Because the longitudinal profile of a river is anchored by base level at the mouth, this means that there has to be a slight upbuilding in the lowermost reach of the river. This may not seem like a big effect, but even a hundred meters represents a lot of sediment.

Figure by MIT OCW.

Deposition by seaward progradation in the lower reaches of a river system

Crustal subsidence. The only way to get a really thick sequence of fluvial sediment is to drop the crust beneath the river As this happens, slowly, along some reach of the river, there develops a very slight expansion of flow and a decrease in flow velocity and therefore in sediment-moving ability. Just by simple bookkeeping, this must lead to sediment storage along the river: if what comes into a given area of the bed is greater than what goes out, sediment is stored in that area,and the bed builds up. From an anthropomorphic standpoint, the river tries tomaintain its longitudinal profile while the bottom drops out from under it, and it does so by leaving a little of the passing sediment to build up its bed.

How Do You Know It’s Fluvial?

By what criteria might you tell that a sedimentary sequence is fluvial? Here are some, but remember that none is incontrovertible, because each applies to other

environments as well.

absence of marine fossils

presence of plant fossils

red beds

scoured channels

unidirectional-flow cross-stratification broadly unidirectional paleocurrents paleosols

desiccation cracks

plant fossils

1.     SEDIMENT GRAVITY FLOWS AND THEIR DEPOSITS

Introduction

Sediment gravity flows are turbulent flows of sediment–fluid mixtures which are driven by the downslope component of the excess density of the mixture. This is not an entirely satisfactory definition, and some words of explanation are in order.

The sediment–fluid mixture flows beneath a body of fluid, usually taken to be sediment-free; the density of the sediment–fluid mixture is greater than that of the overlying fluid, and that’s what provides the downslope driving force. One of the intentions of the definition is to exclude rivers from the concept; after all, rivers are downslope flows of sediment–fluid mixtures under gravity, so why aren’t they sediment gravity flows? But rivers flow even if they carry no

sediment, whereas the presence of sediment is essential to the existence of sediment gravity flows.

Another sticky point about the definition is that in grain flows, universally considered to be one kind of sediment gravity flow, a sheared dispersion of sediment particles moves downslope under the pull of gravity without the necessary presence of fluid at all, either interstitial or supernatant. There can be grain flows even in a vacuum! But for almost all the sediment gravity flows of sedimentological interest, the concept is clear and useful.

Relatively small turbidity currents have been observed and studied in lakes and reservoirs since late in the nineteenth century, but the existence of large marine turbidity currents came to light only by deductions made by geologists about features seen in the ancient sedimentary record. Soon after the existence of large marine turbidity currents was hypothesized, instances of their occurrence in recent times were recognized and studied, and small-scale laboratory experiments (mainly by geologists!) to study their motion and deposits, together with the evidence from the ancient, convinced most geologists of their existence and importance. Interpretation of turbidity-current deposits was well established by the early 1960s.

Recognition of more “exotic” sediment gravity flows, which would generally be classified as submarine debris flows , was longer in coming. Although subaerial debris flows were well known, it was not until the 1970s that the concept of submarine debris flows was widely invoked to explain the coarse, poorly sorted, and seemingly deep-marine deposits so common in the sedimentary record. Even now our understanding of the dynamics and depositional effects of debris flows does not match that of turbidity currents. Many important questions, among them the following, remain unresolved:

• What’s the nature of flows transitional between turbidity currents and debris flows?

• At what rate do sediment gravity flows lose sediment by deposition as they move?

• How does one distinguish between slowly deposited and rapidly deposited sediment-gravity-flow deposits?

• More generally, how does one interpret the nature of the sediment gravity flow from the record of the deposit?

Classification

Classification of sediment gravity flows is based on the nature of the mechanism or mechanisms by which the sediment is kept supported in the flow. There are considered to be four such mechanisms:

fluid turbulence

matrix strength

dispersive grain collisions fluidization

Sediment gravity flows are accordingly classified into four kinds (with various intergradations) on the basis of the dominant support mechanism:

Turbidity currents (mainly fluid turbulence; grain dispersion might be important near the base, and fluidization by dewatering of deposits from an earlier part of the same turbidity current is certainly important in many jcases)

Debris flows (mainly matrix strength; fluid turbulence is presumably important in less concentrated debris flows, and I myself am inclined to think that there is a continuous gradation from turbidity currents to debris flows)

Grain flows (grain dispersion; fluid turbulence may contribute to grain support in some grain flows)

Fluidized flows (fluidization—that is, the suspension of sediment from uprise of fluid from a source below the flow).Fluidized flows, except as part of a turbidity current, are not considered to be of great sedimentological importance. Turbidity currents and debris flows are certainly important. The role of grain flows has been controversial: some see them everywhere, whereas others think that except for very local situations, like the avalanche faces of dunes, they are not of great sedimentological importance. No one has yet gotten a good “handle” on grain flows, in the sense that they have developed widely accepted criteria for their recognition in sedimentary deposits.

Motion

All the sediment gravity flows I have seen, or have seen movies of, have a fairly well defined head or front, where the flow is thickest and where velocities are highest, and a long body, and a tail, where velocities are lower (Figure 10-7; Walker, R.G., 1984, Turbidites and associated coarse clastic deposits, in Walker, R.G., ed., Facies Models, Second Edition: Geological Association of Canada, Geoscience Canada Reprint Series 1, 317 p. (Figure 1, p. 171)). The implication of this is that sediment gravity flows tend to be dispersive: the head outruns the tail, and the flow stretches out and becomes more diffuse as it flows. Debris flows sometimes show pulses of more active movement along their length: the flow thins and slows or even stops for a while, and then thickens and speeds up again as a pulse from upstream moves by.

Sediment gravity flows may at first incorporate more sediment, by erosion of the substrate, but even if they do (and presumably some are depositional almost from the beginning), when they reach a gentler slope they eventually lose sediment by deposition and therefore weaken. The velocity picture in sediment gravity flows is thus complicated: velocity varies both along the flow at a given time, and with time at any point that’s followed along with the flow. Of course, at any point on the bottom the velocity increases and then decreases as the flow passes by.

The state of internal motion in sediment gravity flows is complex and varied. The low-concentration kinds of sediment gravity flows, like turbidity currents, are certainly turbulent, whereas the higher-concentration kinds of sediment gravity flows, like debris flows, tend to be laminar. Relatively low-concentration sediment gravity flows behave approximately as Newtonian fluids, but relatively high-concentration sediment gravity flows must be non-Newtonian; in fact, very high-concentration flows are thought to have matrix strength (meaning that the applied shear stress must reach a certain value before the mixture starts to deform by shearing), so they may behave as plastics.

Matrix strength is significant for the flow behavior of debris flows: in the interior of the flow, where shear is least, there is likely to be a rigid plug, and as the flow decelerates and the shear within the flow becomes weaker, the rigid-plug zone expands upward and downward, until the flow grinds to a halt by total rigidification. Many lower-concentration debris flows can be seen to be turbulent, however, so matrix strength must be a much less important effect in them.

Sediment-Gravity-Flow Deposits

Features.—Most sediment-gravity-flow deposits are event beds: relatively coarse beds, sandstones or conglomerates, underlain and overlain by finer deposits, siltstones and mudstones. They are mostly marine, although lacustrine sediment-gravity-flow deposits are of non-negligible importance.

The lower contacts of sediment-gravity-flow beds are almost always sharp, and often erosional, reflecting the initially very strong current. Upper contacts are usually gradational, although the gradation is often complete over a small thickness, of the order of a centimeter. Normal grading is characteristic of turbidity-current deposits, reflecting temporal decrease in current velocity, and therefore size of sediment carried. Inverse grading is common at the base of both turbidity-current deposits and debris-flow deposits; the mechanics of its development is not clear.

Thickness of sediment-gravity-flow deposits ranges from a few millimeters, in the case of feather-edge distal turbidites, to well over ten meters, in the case of deposits from the largest debris flows or flows intermediate between turbidity currents and debris flows.

Sediment-gravity-flow deposits range from well stratified (as in most turbidity-current deposits), to wholly nonstratified (as in many debris-flow deposits). Structures range from nonstratified through parallel-laminated to cross-stratified, usually but not always on a fairly small scale; soft-sediment deformation is common as well. Because sediment-gravity-flow deposits are deposited rapidly, you might expect them to have rather loose packing and excess pore water; dewatering structures, mainly dish structures and vertical pipelike structures, are common.

Interpretation.—How do you know that you are dealing with a sediment-gravity-flow deposit? Remember that it’s always an interpretation, because it’s a matter of genesis rather than just description. You have to use some or all of the above features, and more, to make that kind of interpretation on the outcrop. The broader stratigraphic context (what’s the overall nature of the section?) is also useful in making such an interpretation. You need practice on the outcrop.

Submarine Fans.—It stands to reason that the deposit formed by repeated sediment-gravity-flow depositional events at the base of a submarine (or lacustrine) slope would be broadly fan-shaped or cone-shaped—although outcrop in the ancient is seldom if ever good enough to pin down the three-dimensional geometry of the fan. On the other hand, submarine fans, large and small, are well known in the modern; marine geologists have been studying their geometry and surface sediment for many years. (But it’s almost as difficult to study the thick vertical succession of deposits in a modern fan as it is to study the geometry of an ancient fan.) Nowadays, sophisticated seismic reflection techniques allow great insight into the innards of deeply buried submarine fans

During the 1970s, Italian sedimentologists were active in developing a model for the deposition of sediment-gravity-flow deposits as submarine fans. That work drew mainly upon studies in the ancient, but other sedimentologists later integrated that model with what’s known about modern fans.

Keep in mind that, as with any depositional models, application of the submarine-fan model involves a certain leap of faith, because there’s nothing from the outcrop that tells you directly that you are dealing with a fan, only indirectly.

The irregular shifting of distributary channels on a fan surface gives rise to a signal in the event-bed succession, whereby some parts of the section consist mostly or entirely of coarse event beds (this represents the active upbuilding of the fan by the channelized flow) and other parts of the section

consist mostly of finer background deposits (this represents interchannel or overbank deposition away from the areas of active upbuilding by channelized flow).

Packets of event beds in the submarine-fan setting often show a tendency, subtle or pronounced, for thickening and coarsening upward, or thinning and fining upward. Such packets are typically several meters to a few tens of meters thick. The model accounts for this by assuming that as a new area of the fan is being constructed, the event beds become thicker and coarser for two reasons: the new distributary channel gathers discharge slowly, and the local environment becomes more proximal as the deposit progrades. On the other hand, if flow in a given distributary channel is gradually choked off, the sequence of event beds would show a tendency to be thinner and finer upward.

The submarine fan model has been refined to the point where many sub-environments are recognized. I won’t elaborate except to mention three such sub-environments;

• The basin plain, the most distal environment, where the waning sediment gravity flows are no longer strongly channelized but can spread widely to the side, to leave beds that are traceable for long distances in two lateral directions, not just one.

• Throughput channels, where strong sediment gravity flows in fairly proximal positions transport large quantities of sediment past a given reach but leave little or no thickness of sediment. It’s in such environments that thin, coarse, and amalgamated sediment-gravity-flow deposits are common.

• Overbank areas lying adjacent to active distributary channels, where especially large flow events spill over the channel banks to deposit finer suspended sediment (silts and fine sands) from currents of moderate velocities to build broad natural levees.

2.     OPEN SHALLOW MARINE DEPOSITS

Introduction

Today several percent of the area of the world’s oceans is floored by the continental shelves: the submerged shallow margins of the continents. Water depths are seldom greater than about 200 m even at the shelf edge, and relief

is subdued, except where submarine canyons cut into the shelf. Widths range up to a few hundred kilometers. Average bottom slopes are so small that if they drained the ocean and parachuted you blindfolded onto the middle of the continental shelf, you wouldn’t know which way to walk to get back home, just from the lay of the land.

The total area of continental shelves in the world is a very sensitive function of world sea level, because along tectonically stable passive-margin

coasts at least, the inland area is usually a gently sloping coastal plain. Sea level today is not as low as it has been in the geologic past, but not as high, either: at certain times of high eustatic sea level stand, a much larger percentage of the continents was flooded by shallow seas, giving rise to what are called epeiric seas or epicontinental seas. For example, during the Cretaceous, the greater part of the area of North America was covered with water! Today we have no models for such vast shallow seas—so reconstruction and interpretation of depositional environments is hindered by the impossibility of studying them in the modern.

It stands to reason that shelf deposits are generally coarser than deep marine deposits, because coarse sediments (sands and gravels) delivered to the shoreline by rivers, or derived by coastal erosion, need strong currents for dispersal, and such strong currents are generally restricted to shallow water, where the tides and the winds can cause strong water movements throughout the water column. (Sediment gravity flows, are a notable exception to this generalization.)

Hydrodynamic Classification of Coasts

Coasts can usefully be classified in many ways, but one way that’s especially useful for sedimentology is by the dominant hydrodynamic effects that give rise to sediment-moving currents. Here’s a list of types of coasts by hydrodynamics:

Tide-dominated coasts. Along coasts with a large tidal range of a few meters of more, strong tidal currents, often exceeding a meter per second near the bed, move sands and even gravels along complex transport paths governed by coastal and sea-floor topography. Thick accumulations of sand can be deposited even on the outer shelf.

Storm-dominated coasts. Along coasts exposed to passage of intense storms (hurricanes and migratory extratropical cyclones), combinations of strong currents and powerful wave motions affect the sea floor now and then.

Current-dominated coasts. Some coasts lie in the path of the margins of major deep-ocean currents, which produce steady movement of coastal sediment parallel to the coastline.

Wave-dominated coasts. Along some coasts, the strongest water movements are nothing more than oscillatory flows produced by impingement of swell from distant ocean storms. This has a strong effect on beaches, where the large waves finally break, but much less effect on offshore areas of the shelf.

Of these four types, the first two, tide-dominated and storm-dominated, are the most important in the sediment record.

Deposits of Tide-Dominated Coasts

 Two things you should know about tidal currents are that:

§  in nearshore areas (and in offshore areas too, if there's any large-scale relief) tidal currents tend to be channelized into largely bidirectional currents rather than rotary, as tidal dynamics would predict, and

§  such bidirectional tidal currents almost always show some degree of asymmetry, in that the flow in one direction is stronger than the flow in the other direction during the tidal cycle.

 These two facts together imply that sand deposits shaped by tidal currents tend to show one-way cross-stratification—especially because sediment transport rate is such a steeply increasing function of current strength. The moderately high current velocities, together with the fairly deep water depths, lead to large-scale cross stratification, ranging from planar-tabular (resulting from movement of 2D dunes) to trough (resulting from movement of 3D dunes). Dunes in modern tidal currents can be up to tens of meters high; set thickness in ancient sands thought to be of tidal origin are usually less, but in some cases can be up to several meters, or even more.

 Several special features of cross stratification produced by tidal currents are characteristic:

Herringbone cross stratification:It seems logical that if currents reverse, then the cross stratification produced by bed forms moving under the influence of the reversing current should show vertical sequences with cross sets dipping opposite each other. And that’s true, especially on fairly small scales of up to a few decimeters within the deposits of larger bed forms that on average move in one direction. But beware of making such an interpretation just on the basis of an outcrop face that seems to show 180° reversal of current direction: usually in such cases, the angular difference is much less, and the visual effect is caused by the section view.

Figure by MIT OCW.

Herringbone cross stratification

Reactivation surfaces: The time-varying flow strength can cause periodic degradation and then reconstruction of the crests of large bed forms, resulting in characteristic internal truncation surfaces called reactivation surfaces. Beware of automatically making a tidal interpretation, however, because similar reactivation surface can be produced by changes in bed-form geometry in a flow that’s steady in the large, probably because of the mutual interactions among neighboring bed forms in a train of inherently changeable bed forms.

Figure by MIT OCW.

 Streamwise cross section through a subaqueous dune, showing a reactivation surface

Tidal bundles: The two-week spring–neap cycle causes substantial periodic changes in tidal current velocities, and, a fortiori, sediment transport rates. A large tidal dune that feels effectively one-way flow and sand movement might therefore be expected to show different surface features at different times in the spring–neap cycle. This spring–neap inequality is great enough in many cases that sand movement is strong during the spring part of the cycle but ceases entirely during the neap part of the cycle, causing the dune to be draped or mantled with finer sediment, most but not necessarily all of which is stripped from the dune surface during the spring part of the cycle. The resulting cyclic interbedding of sand and mud, especially on the lower and middle parts of the lee sides of the dunes, is called tidal bundling, and is considered to be definitive evidence of tidal origin.

Figure by MIT OCW.

Figure 10-12: Cross section through a subaqueous dune in a tidal environment, showing tidal bundles

Tidal deposition is just as important, and probably more so, in protected nearshore environments than on the open shelf. There will be more to say in a later section about tidal deposits in protected environments.

Deposits of Storm-Dominated Coasts

 On shelves unaffected by really strong water movements except during occasional strong storms, one might expect that most areas, except fairly near shore, would be floored by mud. Think of that mud as the normal quiet-water deposit, building up very slowly by fallout from suspension during long periods between unusually large storms. Many deposits interpreted as offshore shelf deposits show only such muds. The sediment may be well stratified, or it may be partly or entirely homogenized by bioturbation.

Many shelf sequences show an interbedding of quiet-water muds and sand beds interpreted to be event beds deposited by major, even catastrophic storm-generated flow events in which enormous quantities of sand, usually very fine to fine, are transported from sites nearer shore and spread as an extensive mantle over shelf muds. These sand beds characteristically show hummocky cross stratification, suggesting strong and complex oscillatory flows. The sand beds may be amalgamated, especially in areas nearer shore, where the frequency of strong bottom water movements is greater than farther offshore.

The nature of the sand-transporting currents and the mechanisms and site of sand entrainment by the current are still controversial:

• Some people are believers in shelf turbidity currents. In this view, great masses of sediment and water are mobilized at the shoreline during certain major storms, perhaps by liquefaction caused by cyclic loading by storm waves, and then move offshore as a density underflow.

• Other people deny the existence of such shelf turbidity currents, and appeal instead to a kind of current that might be called a storm-surge-relaxation current, whereby water piled against the coast by strong winds tries to flow seaward, only to be turned parallel to shore by the Coriolis force. Such shore-parallel currents, which are in the nature of geostrophic currents, are commonly observed along modern shelves, and can attain speeds in excess of a meter per second.

 Various problems remain unresolved. Here are what we consider three of the most difficult problems:

• How is the sand moved offshore?

• How can the currents carry enough sand to deposit extensive beds that are in some cases over a meter thick?

• How can the necessary unidirectional delivery of the sand be reconciled with the existence of sedimentary structures that seem to be produced by dominantly oscillatory flows?

4. TIDAL FLATS

Tidal flats occur on open coasts of low relief and relatively low energy and in protected areas of high-energy coasts associated with estuaries, lagoons, bays, and other areas lying behind barrier islands. The conditions necessary for development of tidal flats include an efective tidal range and the absence of strong wave-induced currents.

The extent of tidal flats along modern coastlines varies greatly and includes small, locally restricted areas of several hundred square meters or regional features extending over hundreds of square kilometers. One of the best studied siliciclastic depositional environments that has extensive tidal flats is the coastline of the Netherlands, Denmark, and Germany. In the case of carbonates, the best developed tidal flats occur along the coast of the western Persian Gulf and on the western, leeward flank of Andros Island in the Bahamas.

Some confusion in terminology occurs because tidal flats may carry local geographic names such as lagoon, bay, or salt marsh. Studies of tidal flats became very popular in the late 1950s and early 1 960s and helped to clarify this confusion. It is clear that there is great variability in tidal flats, depending on sediment types and availability, presence or absence of vegetation, tidal range, and coastal energy and morphology.

Tidal flats are subdivided into intertidal and subtidal environments which control facies distribution. Parts of the tidal flat lying between high and low tide range, the intertidal zone, make up the major areal extent of the tidal flat. If a noticeable variation in sediment type is present throughout the flat, for example muds and sands, the intertidal area commonly possesses alternating layers of both textures. Where sand forms lenses in mud, the texture is called lenticular bedding, and where mud forms lenses in sand, the texture is called flaser bedding. Most tidal flats have a third zone, developed above the intertidal zone, called the supratidal zone. Supratidal sediments are deposited above normal or mean high tide and exposed to subaerial conditions most of the time because they are flooded only by spring and storm tides; spring tides occur twice each month, and storm tides, the largest of all, occur occasionally during certain seasons.

 Subtidal areas are important to the understanding of this environment because they are the part of the tidal flat most likely to be preserved. Most tidal-flat deposition results from lateral accretion in association with progradation of the flat and the point bar associated with meandering tidal channels. Therefore, a

major part of the sedimentary record for most tidal-flat successions includes features associated with channel fills and tidal point bars.

Animals and plants play a significant role in tidal-flat environments. They are influential in trapping sediment, forming sediment particles as fecal pellets, and generating biogenic sedimentary structures as a result of the processes of feeding, dwelling, and moving. Tidal flats commonly preserve the details of sedimentary structures owing to the alternation of sand and mud layers.

 Intertidal and subtidal parts of tidal flats are continuously affected by tidal currents as well as wind-induced wave currents. Current velocity can be highly variable in different settings of in the same area under different conditions. Regional and local geomorphology, tide range, and strength and direction of local winds are factors controlling waves and currents. For example, consider the difference between the U.S. Gulf Coast, where the tidal range is less than 0.5 m (microtidal) and the Bay of Fundy, where a normal tide may range greater than 10 m (macrotidal). Mesotidal ranges are intermediate, and range from one meter to a few meters.

 Tidal flats that developed under progradational conditions, as in the case of shallowing upward cycles in carbonates, are characterized by a fining-upward succession, consisting of coarse sediments at the base and progressively finer sediments toward the top in an uninterrupted vertical sequence. This common relationship reflects decreasing energy in a progression from subtidal to intertidal parts of the tidal flats. Tidal channels are commonly substantially coarser than laterally equivalent parts of the tidal flat system. Where best developed, facies progression in a vertical sequence can be represented by: (1) a dominantly sandy subtidal zone of channel-fill, point-bar, and shoal sediments; (2) a mixed sand and mud intertidal flat deposit; (3) a muddy upper intertidal flat or salt-marsh deposit.

Certain sedimentary structures are characteristic of tidal-flat environments: desiccation cracks, thin lamination, commonly disrupted to some extent by bioturbation, and microbial laminites, often with fenestrae (owing to the exclusion of grazing snails in the tidal zone, microbial communities are free to flourish), and intraclasts.

3.     BARRIER ISLAND, BEACH, AND LAGOON ENVIRONMENTS

Introduction

 Wave-dominated sandy shorelines in interdeltaic and nondeltaic coastal regions are characterized by elongate, shore-parallel sand deposits. Barriers and beaches are prominent depositional features of modern coasts, and sandstone bodies of similar origin are represented in the stratigraphic record. In contrast to rivers and deltas, the geometry of barrier islands is molded almost entirely by marine processes.

 For our purposes, barriers are defined as sandy islands or peninsulas elongate parallel with the shore and separated from the mainland by lagoons or marshes. Some major environments associated with barrier island systems are: (1) beach and shoreface environments on the seaward side of barriers and strand plains, (2) inlet channels and tidal deltas, separating barriers laterally, and (3) washover fans on the landward or lagoonward side of barriers. Seaward or longshore migration of these environments results in facies successions constituting much of the volume of many coastal sand bodies. For example, the emergent parts of many barrier-island successions are often underlain by progradational beach and shoreface facies.

Beach and Shoreface Deposits

 Successions formed by the seaward progradation of beach and shoreface (nearshore) deposits account for a major part of the volume of Holocene barriers and strandplains. One famous example is Galveston Island, located along the Texas portion of the U.S. Gulf Coast.The beach is commonly divided into a backshore, which consists of a nearly level berm, and a foreshore, which slopes seaward fromthe berm edge or crest. The foreshore includes the beachface and, on some beaches, one or more elongate bars and intervening troughs called ridge-and-runnel systems. The shoreface, as is commonly thought of, extends from the beach offshore to a depth of 5 to 20 m, where there is commonly a change of gradient from the gently sloping shoreface to the nearly level shelf.

Sediment transport on the beach and shoreface is dominated by waves and wave-induced currents, although tidal currents may be locally important near tidal inlets and estuaries. As waves move toward the shore they begin to “feel bottom” on the seafloor near the base of the shoreface, become progressively oversteepened, and collapse to form breakers on the upper shoreface. As the surf runs up the beach, it forms a thin rush of water called the swash, followed by an even thinner return flow called the backwash. When waves approach the shoreline with an oblique orientation, one direction of longshore current predominates. Sand is transported very rapidly along shore under such conditions, a situation which no jetty can help remedy.

Beachface and foreshore deposits commonly consist of low-angle, seaward dipping, planar lamination, formed by the swash–backwash process, which occur as wedge-shaped sets. Upper-shoreface deposits are commonly highly variable, owing to the extremely complex hydraulic environment of the surf zone. Such a regime gives rise to a complex sequence of multidirectional sedimentary structures and variable sediment textures characteristic of these deposits. Gravels are often concentrated in this zone, because of winnowing of finer-grained deposits. Trough cross bedding is also developed, but low-angle bidirectional planar cross-bedded sets and subhorizontal

Tidal-Inlet Deposits

Tidal inlets are more or less permanent passages between barrier islands that allow tidal exchange between the open sea and lagoons, bays, and tidal marshes behind the islands. Inlet channels are generally deepest between the tips of the islands and shallow into tidal deltas both lagoonward (flood delta) and seaward (ebb delta). The length of barrier is commonly increased by accretion along the tip of one island and erosion of the tip of the adjacent island. Lateral accretion results in growth of spits.

The spacing of tidal inlets is closely correlated with tidal range. As you might expect, macrotidal zones produce barrier-island systems with many inlets, whereas microtidal regimes generate very continuous barriers, as along the U.S. Gulf Coast. Also, the wave energy of the coastline determines whether or not ebb tidal deltas are as well developed as flood tidal deltas. The higher the energy, the more destructive the system, and the less likely that an ebb tidal delta projecting out into the open         oceanisdeveloped

Tidal inlets migrate laterally as the spit on the end of a barrier island grows. The thickness of sediments deposited by migration of tidal inlets and associated environments may be as great as the depth of the tidal channel itself.

LONGSHORE BAR                             LOW TIDE TERRACE LONGSHORE TROUGH

 

Figure by MIT OCW.

Terminology for beach morphology, shown in a cross section normal to the beach

 Sedimentary structures in tidal inlets are often very complex, owing to the alternating flow directions created during one tidal cycle . There is much variability among different tidal channel complexes that have been studied. However, there is some evidence that in the deeper parts of channels, say below about 4–5 m, sands are coarsest and characterized by mainly ebb-oriented, tabular, planar cross strata, with reactivation surfaces formed by flood-oriented tidal currents. Sands of the channel margin have trough-shaped sets of cross strata formed by both flood-oriented and ebb-oriented dunes, as well as large foresets formed by the lateral migration of the spit. The uppermost part of the channel profile is flattest and exhibits swash stratification, similar to the beachface, but oriented generally along the longshore dip direction.

 Lagoonal Deposits

 Lagoonal successions commonly contain interbedded sandstone, shale, siltstone, and coal facies characteristic of a number of overlapping depositional environments. Sand facies include washover sheet deposits and sheet and channel-fill deposits of flood-tidal-delta origin. Fine-grained sediments include those of the lagoon and tidal flats, which are situated adjacent to the barrier or on the landward side of the lagoon abutting the hinterland marsh and swamp flatlands.

 Generally the lagoon is fed with marine waters that run through the numerous channels in the barrier-island system. However, where lagoons are developed adjacent to rivers and estuaries, lagoonal waters may often be brackish to nearly fresh.

 Because of the inherent low energy of most lagoons (little current activity of any kind), fine-grained sediments are common. Often lagoons are the site of prolific production of plants and burrowing organisms that feed on the decaying organic matter. As a result, lagoonal sediments are rich in organics, are highly bioturbated, and may form coal seams in the geologic record.

 Other than the sands that are swept into the lagoon adjacent to ebb tidal deltas, the only dominant source of sand is from the growth of washover fans. These form during storms, when water is piled up against the beachface on the oceanic side of the barrier island. Commonly, the barrier island is breached at low points between the dune fields that form the top of the island, the water pours through and entrains abundant sand en route to flushing it through to the lagoon. Over the course of several storms, washover fans may actually prograde out

plane beds may also occur. The lower shoreface may include abundant plane beds, wave-oscillation ripples, intercalated with finer silty or muddy layers. However, structures are commonly obliterated by bioturbation.

Refrences

Wood, L. J., Ethridge, F. G., and Schumm, S. A. (1993). The effects of rate

of base-level fluctuations on coastal plain, shelf and slope depositional

systems: an experimental approach.

In Sequence Stratigraphy

and Facies Associations (H. W. Posamentier, C. P. Summerhayes,

B. U. Haq and G. P. Allen, Eds.), pp. 43–53. International

Association of Sedimentologists Special Publication 18.

Wright, V. P. (1994). Paleosols in shallow marine carbonate

sequences. Earth-Science Reviews, Vol. 35, pp. 367–395.

Wright, V. P., and Marriott, S. B. (1993). The sequence stratigraphy

of fluvial depositional systems: the role of floodplain sediment

storage. Sedimentary Geology, Vol. 86, pp. 203–210.

Cant, D.J. (1982) Fluvial facies models. In: Sandstone Depositional Environments (Eds Scholle, P.A. & Spearing, D.).American Association of Petroleum Geologists Memoir, 31,115–138.

Nichols, G.J. (1987) Syntectonic alluvial fan sedimentation,

southern Pyrenees. Geological Magazine, 124, 121–133.

Nichols, G.J. (2005) Sedimentary evolution of the Lower

Clair Group, Devonian, west of Shetland: climate and sediment

supply controls on fluvial, aeolian and lacustrine

deposition. In: Petroleum Geology: North West Europe and

Global Perspectives – Proceedings of the 6th Petroleum Geology

Conference (Eds Dore´, A.G. & Vining, B.A.); 957–967.

Nichols, G.J. & Fisher, J.A. (2007) Processes, facies and

architecture of fluvial distributary system deposits. Sedimentary

Geology, 195, 75–90.

Nichols, G.J.&Thompson, B. (2005) Bedrock lithology control

on contemporaneous alluvial fan facies, Oligo-Miocene,

southern Pyrenees, Spain. Sedimentology, 52, 571–585.

Nichols, G.J. & Uttamo, W. (2005) Sedimentation in a

humid, interior, rift basin: the Cenozoic Li Basin, northern

Thailand. Journal of the Geological Society, London, 162,

333–348.

Nickling, W.G. (1994) Aeolian sediment transport and

deposition. In: Sediment Transport and Depositional Processes

(Ed. Pye, K.). Blackwell Science, Oxford; 293–350.

Ninkovich, D., Shackleton, N.J., Abdel-Monem, A.A.,

Obradovich, J.D. & Izett, G. (1978). K–Ar age of the late

Pleistocene eruption of Toba, North Sumatra. Nature,

276, 574–577.

North American Commission on Stratigraphic Nomenclature

(1983) North American Stratigraphic Code,

American Association of Petroleum Geologists Bulletin, 67,

841–875.

Nurmi, R.D. & Friedman, G.M. (1977) Sedimentology and

depositional environments of basin centre evaporites,

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