Sedimentologi Kamal Roslan Mohamed CLASTIC COASTS ESTUARIES INTRODUCTION

Sedimentologi Kamal Roslan Mohamed CLASTIC COASTS & ESTUARIES

INTRODUCTION The morphology of coastlines is very variable, ranging from cliffs of bedrock to gravelly or sandy beaches to lower energy settings where there are lagoons or tidal mudflats. Wave and tidal processes exert a strong control on the morphology of coastlines and the distribution of different depositional facies. Wave-dominated coasts have well developed constructional beaches that may either fringe the coastal plain or form a barrier behind which lies a protected lagoon. Barrier systems are less well developed where there is a larger tidal range and the deposits of intertidal settings, such as tidal mudflats, become important. Estuaries are coastal features where water and sediment are supplied y a river, but, unlike deltas, the deposition is confined to a drowned river valley.

COASTS / PESISIR Coasts are the areas of interface between the land the sea, and the coastal environment can comprise a variety of zones, including coastal plains, beaches, barriers and lagoons. The shoreline / garis pantai is the actual margin between the land the sea. Coastlines can be divided into two general categories on the basis of their morphology, wave energy and sediment budget.

COASTS / PESISIR Erosional coastlines typically have relatively steep gradients where a lot of the wave energy is reflected back into the sea from the shoreline: both bedrock and loose material may be removed from the coast and redistributed by wave, tide and current processes. At depositional coastlines the gradient is normally relatively gentle and a lot of the wave energy is dissipated in shallow water: provided that there is a supply of sediment, these dissipative coasts can be sites of accumulation of sediment. Reflective coasts are usually erosional with steep beaches and a narrow surf zone. Dissipative coasts may be depositional, with sand deposited on a gently sloping foreshore.

Erosional coastlines Exposure of bedrock in cliffs allows both physical and chemical processes of weathering: oxidation and hydration reactions are favoured in the wet environment, and the growth of salt crystals within cracks of rocks sprayed with seawater can play an important role in breaking up the material. Material accumulates at the foot of cliffs as loose clastic detritus and occasionally as large blocks when whole sections of the cliff face are removed. Cliff erosion may result in wave cut platforms of bedrock eroded subhorizontally at beach level. Wave action, storms and tidal currents will then remove the debris as bedload, as suspended load or in solution. This contributes to the supply of sediment to the marine environment, and away from river mouths can be an important source of clastic detritus to the shallow marine realm.

Depositional coastlines A coastline that is a site of accumulation of sediment must have an adequate supply of material to build up a deposit. The sources of this sediment are from the marine realm, either terrigenous clastic detritus reworked from other sources or bioclastic debris. The terrigenous material ultimately comes from rivers, with a small proportion of wind-blown origin and from direct erosion of coastlines. This sediment is brought to a depositional coastline by tidal, wind driven and geostrophic currents that transport material parallel to shorelines or across shallow seas. Wind-driven waves acting obliquely to the shoreline are an important mechanism of transport, creating a shore-parallel current known as longshore drift. Shallow seas are generally rich in fauna, and the remains of the hard parts of these organisms provide an important source of bioclastic material to coastlines.

BEACHES / PANTAI The beach is the area washed by waves breaking on the coast. The seaward part of the beach is the foreshore, which is a flat surface where waves go back and forth and which is gently dipping towards the sea. Where wave energy is sufficiently strong, sandy and gravelly material may be continuously reworked on the foreshore, abrading clasts of all sizes to a high degree of roundness, and effectively sorting sediment into different sizes. Sandy sediment is deposited in layers parallel to the slope of the foreshore, dipping offshore at only a few degrees to the horizontal (much less than the angle of repose). This low-angle stratification of well-sorted, well-rounded sediment is particularly characteristic of wave-dominated sandy beach environments. Grains are typically compositionally mature as well as texturally mature because the continued abrasion in the beach swash zone tends to break down the weaker clasts.

BEACHES / PANTAI At the top of the beach, a ridge, known as a berm, marks the division between the foreshore and backshore area. Water only washes over the top of the berm under storm-surge conditions. Sediment carried by the waves over the berm crest is deposited on the landward side forming layers in the backshore that dip gently landward. These low-angle strata are typically truncated by the foreshore stratification, to form a pattern of sedimentary structures that may be considered to be typical of the beach environment. The backshore area may become colonised by plants and loose sand can be reworked by aeolian processes.

BEACHES / PANTAI Morphological features of a beach comprising a beach foreshore and backshore separated by a berm; beach dune ridges are aeolian deposits formed of sand reworked from the beach. Foreshore-dipping and backshore-dipping stratification in sands on a beach barrier bar.

Beach dune ridges Aeolian processes can act on any loose sediment exposed to the air. Along coasts any sand that dries out on the upper part of the beach is subject to reworking by onshore winds that may redeposit it as aeolian dunes. Coastal dunes form as ridges that lie parallel to the shoreline and they may build up to form dune complexes over 10 m high and may stretch hundreds of metres inland. The limiting factor in beach dune ridge growth is the supply of sand from the beach. They commonly form along coasts with a barrier system, but can also be found along strand-plain coasts. In a sedimentary succession these beach dune ridge deposits may be seen as well-sorted sand at the top of the beach succession. Some preservation of the roots of shrubs and trees that colonised the dune field is possible, but the effect of the vegetation is often to disrupt the preservation of well-developed dune cross-bedding.

Beach dune ridges A beach dune ridge formed by sand blown by the wind from the shoreline onto the coast to form aeolian dunes, here stabilised by grass.

Beach dune ridges A schematic graphic sedimentary log of sandy beach deposits.

Coastal plains and strand plains Coastal plains are low-lying areas adjacent to seas. They are part of the continental environment where there are fluvial, alluvial or aeolian processes of sedimentation and pedogenic modification. Coastal plains are influenced by the adjacent marine environment when storm surges result in extensive flooding by seawater. A deposit related to storm flooding can be recognised by features such as the presence of bioclastic debris of a marine fauna amongst deposits that are otherwise wholly continental in character.

Coastal plains and strand plains Sandy coastlines where an extensive area of beach deposits lies directly adjacent to the coastal plain are known as strand plains. Along coasts supplied with sediment, beach ridges create strand plains that form sediment bodies tens to hundreds of metres across and tens to hundreds of kilometres long and progradation of strand plains can produce extensive sandstone bodies. A wave-dominated coastline with a coastal plain bordered by a sandy beach: chenier ridges are relics of former beach strand plains. The strand plain is composed of the sediment deposited on the foreshore and backshore region. The backshore area merges into the coastal plain and may show evidence of subaerial conditions such as the formation of aeolian dunes and plant colonisation.

BARRIER AND LAGOON SYSTEMS A wave-dominated coastline with a beach-barrier bar protecting a lagoon.

Barriers / Pulau Penghalang Along some coastlines a barrier of sediment separates the open sea from a lagoon that lies between the barrier and the coastal plain. Beach barriers are composed of sand and/or gravel material and are largely built up by wave action. They may be partially attached to the land, forming a beach spit, or wholly attached as a welded barrier that completely encloses a lagoon, or can be isolated as a barrier island in front of a lagoon. Barriers range in size from less than 100 m wide to several kilometres and their length ranges from a few hundred metres to many tens of kilometres.

Barriers / Pulau Penghalang The seaward margin of a barrier island has a beach and commonly a beach dune ridge where aeolian processes rework the sand. Vegetation helps to stabilisethe dunes. On the landward side of the island the layers of sand deposited during storms pinch out intothe muddy marshes of the edge of the lagoon. During storm surges seawater may locally overtop the beach ridge and deposit washovers of sediment that has been reworked from the barrier and deposited in the lagoon. Washover deposits are low-angle cones of stratified sands dipping landwards from the barrier into the lagoon.

The conditions required for a barrier to form are as follows. • an abundant supply of sand or gravelsized sediment is required and this must be sufficient to match or exceed any losses of sediment by erosion. The supply of the sediment is commonly by wave driven longshore drift from the mouth of a river at some other point along the coast. • the tidal range must be small. In macrotidal settings the exchange of water between a lagoon and the sea during each tidal cycle would prevent the formation of a barrier because a restricted inlet would not be able to let the water pass through at a high enough rate. • barrier islands generally form under conditions of slow relative sea-level rise. If there is a well-developed beach ridge, the coastal plain behind it may be lower than the top of the ridge. With a small sea-level rise, the coastal plain can become partially flooded to form a lagoon, and the beach ridge will remain subaerial, forming a barrier. For the barrier to remain subaerial as sea level rises further, sediment must be added to the beach to build it up, that is, the first condition of high sediment supply must be satisfied.

Lagoons are coastal bodies of water that have very limited connection to the open ocean. Seawater reaches a lagoon directly through a channel to the sea or via seepage through a barrier; fresh water is supplied by rainfall or by surface run-off from the adjacent coastal plain. If a lagoon is fed by a river it would be considered to be part of an estuary system. They are typically very shallow, reaching only a few metres in depth.

Lagoons generally develop along coasts where there is a wave-formed barrier and are largely protected from the power of open ocean waves. Waves are generated by wind blowing across the surface of the water, but the fetch of the waves will be limited by the dimensions of the lagoon. Ripples formed by waves therefore affect the sediments only in very shallow water. The wind may also drive weak currents across the lagoon. Tidal effects are generally small because the barrier– lagoon morphology is only well developed along coasts with a small tidal range.

Lagoons Fine-grained clastic sediment is supplied to lagoons as suspended material in seawater entering past the barrier and in overland flow from the adjacent coastal plain. Organic material may be abundant from vegetation which grows on the shores of the lagoon. In tropical climates, trees with aerial root systems (mangroves) colonise the shallow fringes of the lagoon. Mangroves cause the shoreline to prograde into the lagoon as they act as sites for accumulation of sediment and organic matter along the water’s edge. In more temperate climates, saline-tolerant grasses, shrubs and trees may play a similar role in trapping sediment. Coarser sediment may enter the lagoon when storms wash sediment over the barrier as washover deposits, which are thin layers of sand reworked by waves. Sand is also blown into the water by onshore winds picking up material from the dunes along the barrier.

Lagoons An important characteristic of lagoons is their water chemistry. Due to the limited connection to open ocean, it is common for lagoon water to have either higher or lower salinity than seawater. Low salinity, brackish water will be a feature of lagoons in areas of high rainfall, local run-off of fresh water from the coastal plain or small streams. Mixing of the lagoon water with the seawater is insufficient to maintain full salinity in these brackish lagoons. In more arid settings the evaporation from the surface of the lagoon may exceed the rate at which seawater exchanges with the lagoon water and the conditions become hypersaline, that is, with salinities higher than that of seawater. If salinities become very elevated, precipitation of evaporite minerals will occur.

Lagoons A lagoonal succession is typically mudstone, often organic-rich, with thin, wave-rippled sand beds. The deposits of lagoons can be difficult to distinguish from those of lakes with similar dimensions and in similar climatic settings. Two of evidence can be used to identify lagoonal facies: • the fossil assemblage may indicate a marine influence, and specifically a restricted fauna may provide evidence of brackish or hypersaline water. • the association with other facies is also important: lagoonal deposits occur above or below beach/barrier island sediments and fully marine shoreface deposits.

TIDES AND COASTAL SYSTEMS Microtidal coasts Under microtidal conditions wave action can maintain a barrier system that can be more or less continuous for tens of kilometres. Exchange of water between the lagoon and the sea may be very limited, occurring through widely spaced inlets and as seepage through the barrier. Coarse sedimentation in the lagoon will be largely restricted to washovers that occur during storms. There is a strong likelihood of the lagoon waters becoming either brackish or hypersaline, depending upon the prevailing climate.

TIDES AND COASTAL SYSTEMS Mesotidal coasts With the increased tidal range of mesotidal conditions, more exchange of water between the lagoon and the sea is required, resulting in more inlets forming, breaking up the barrier into a series of islands. These inlets are the pathways for the tidal flows and the currents within them can be strong enough to redistribute sediment. On the lagoon side of the barrier sediment washed through the channel is deposited in a flood-tidal delta. Ebb-tidal deltas form on the seaward margin of the tidal channel as water flows out of the lagoon when the tide recedes.

TIDES AND COASTAL SYSTEMS Macrotidal coasts Coasts that have high tidal ranges do not develop barrier systems because the ebb and flood tidal currents are a stronger control on the distribution of sediment than wave action. A depositional coast in a macrotidal setting will be characterised by areas of intertidal mudflats that are covered at high tide and exposed at low tide. Water flooding over these areas with the rising tide spreads out and loses energy quickly: only suspended load is carried across the tidal flats, and this is deposited when the water becomes still at high tide.

COASTAL SUCCESSIONS The patterns of sedimentary successions built up at a coast are determined by a combination of sediment supply and relative sea-level change. Prograding / maraan barriers and strand plains are those that build out to sea through time as sediment is added to the beach from the sea. A barrier will become wider, and the inner margins may become more stabilised by vegetation growth. A prograding strand plain will result in a series of ridges parallel to the coastline, chenier ridges, which are the relicts of former beaches that have been left inland as the shoreline prograded.

COASTAL SUCCESSIONS Retrograding barriers form where the supply of sediment is too low to counteract losses from the beach by erosion. Removal of sediment from the front of the barrier reduces its width and, in turn, its height. By this process the beach system will gradually move landward. Through time these transgressive barrier systems will build up a succession from coastal plain deposits at the base, overlain by lagoon facies and capped by beach deposits of the barrier system. A similar transgressive situation at a strand plain will result in coastal plain deposits overlain by beach deposits. A schematic graphic sedimentary log of a transgressive coastal succession.

ESTUARIES An estuary is the marine-influenced portion of a drowned valley. A drowned valley is the seaward portion of a river valley that becomes flooded with seawater when there is a relative rise in sea level (a transgression). They are regions of mixing of fresh and seawater. Sediment supply to the estuary is from both river and marine sources, and the processes that transport and deposit this sediment are a combination of river and wave and/or tidal processes. An estuary is different from a delta because in an estuary all the sedimentation occurs within the drowned valley, whereas deltas are progradational bodies of sediment that build out into the marine environment. A stretch of river near the mouth that does not have a marine influence would not be considered to be an estuary.

Wave-dominated estuaries Distribution of depositional settings in a wave-dominated estuary. An estuary developed in an area with a small tidal range and strong wave energy will typically have three divisions: - the bay-head delta, - the central lagoon - the beach barrier.

Wave-dominated estuaries Bay-head delta The bay-head delta is the zone where fluvial processes are dominant. As the river flow enters the central lagoon it decelerates and sediment is deposited. The form and processes of a bay-head delta will be those of a riverdominated delta because the tidal effect is minimal and the barrier protects the central lagoon from strong wave energy. A coarsening-up, progradational succession will be formed, with channel and overbank facies building out over sands deposited at the channel mouth, which in turn overlies fine-grained deposits of the central lagoon.

Wave-dominated estuaries Central lagoon The lowest energy part of the estuarine system is the central lagoon, where the river flow rapidly decreases and the wave energy is mainly concentrated at the barrier bar. The central lagoon is therefore a region of fine-grained deposition, often rich in organic material, similar to normal lagoonal conditions. When the central lagoon becomes filled with sediment it becomes a region of salt-water marshes crossed by channels. In wave-dominated estuaries, parts of the lagoon that receive influxes of sand may be areas where wave-ripples form and these may also be draped with mud.

Wave-dominated estuaries Beach barrier The outer part of a wave-dominated estuary is a zone where wave action reworks marine sediment (bioclastic material and other sediment reworked by longshore drift) to form a barrier. The characteristics of the barrier will be the same as those found along clastic coasts. An inlet allows the exchange of water between the sea and the central lagoon, and if there is any tidal current, a flood-tidal delta of marine-derived sediment

Wave-dominated estuaries Successions in wave-dominated estuaries The sedimentary succession deposited in the estuary will reflect the three divisions of the system, although they may not all occur in a single vertical succession. The relative thicknesses of each will depend on the balance between fluvial and marine supply of sediment: if fluvial supply is greatest, the bay-head delta facies will dominate, whereas the barrier deposits will be more important if the marine supply is higher. A graphic sedimentary log of wavedominated estuary deposits.

Tide-dominated estuaries Distribution of depositional settings in a tidally dominated estuary. Tidal processes may dominate in mesotidal and macrotidal coastal regimes where tidal current energy exceeds wave energy at the estuary mouth. The funnel shape of an estuary tends to increase the floodtidal current strength, but decreases to zero at the tidal limit, the landward extent of tidal effects in an estuary. The river flow strength decreases as it interacts with the tidal forces that are dominant. Three areas of deposition can be identified: - tidal channel deposits, - tidal flats - tidal sand bars.

Tide-dominated estuaries Tidal channels In the inner part of the estuary where the river channel is influenced by tidal processes, the low-gradient channel commonly adopts a meandering form. Point bars form on the inner banks of meander bends in the same way as purely fluvial systems, but the tidal effects mean that there are considerable fluctuations in the strength of the flow during different stages of the tidal cycle: when a strong ebb tide and the river act together, the combined current may transport sand, but a strong flood tide may completely counteract the river flow, resulting in standing water, which allows deposition from suspension. The deposits in the point bar are therefore heterolithic, that is, they consist of more than one grain size, in this case alternating layers of sand mud

Tide-dominated estuaries Tidal flats Adjacent to the channels and all along the sides of the estuary there are tidal flat areas that are variably covered with seawater at high tide and subaerially exposed at low tide. These are typically vegetated salt marsh areas cut by tidal creeks that act as the conduits for water flow during the tidal cycles. The processes and products of deposition in these settings are the same as found in macrotidal settings.

Tide-dominated estuaries Tidal bars The outer part of a tide-dominated estuary is the zone of strongest tidal currents, which transport and deposit both fluvially derived sediment and material brought in from the sea. In macrotidal regions the currents will be strong enough to cause local scouring and to move both sand gravel: bioclastic debris is common amongst the gravelly detritus deposited as a lag on the channel floor. Dune bedforms are created and migrate with the tidal currents to generate cross-bedded sandstone beds. Evidence for tidal conditions in these beds may include mud drapes, reactivation surfaces and herringbone cross-stratification

Tide-dominated estuaries Successions in tide-dominated estuaries A succession formed in a tide-dominated estuary will consist of a combination of tidal channel, tidal flat and tidal bar deposits. The base of a tidal channel is marked by a scour and lag, and will typically be followed by a fining-upwards succession of crossbedded sands, which may show mud drapes, inclined heterolithic stratification. Channel and bar deposits may also show bidirectional palaeocurrent indicators. Muddy tidal flat deposits rich in organic material may contain sandy sediment deposited within tidal creeks, at the highest tides and during storms. A graphic sedimentary log of tidal estuary deposits.

Recognition of estuarine deposits: There are many features in common between the deposits of deltas and estuaries in the stratigraphic record. Both are sedimentary bodies formed at the interface between marine and continental environments and consequently display evidence of physical, chemical and biological processes that are active in both settings (e. g. an association of beds containing a marine shelly fauna with other units containing rootlets).

Recognition of estuarine deposits: The key difference is that a delta is a progradational sediment body, that is, it builds out into the sea and will show a coarsening-up succession produced by this progradation. In contrast, estuaries are mainly aggradational, building up within a drowned river channel. The base of an estuarine succession is therefore commonly an erosion surface scoured at the mouth of the river, for example, in response to sea level fall. It may be difficult to distinguish between the deposits of a tidal estuary and a tidedominated delta if there is limited information and it is difficult to establish whether the succession is aggradational and valley-filling or progradational.

Characteristics of coastal and estuarine systems Beach/barrier systems. lithology – sand conglomerate. mineralogy – mature quartz sands and shelly sands. texture – well sorted, well rounded clasts. bed geometry – elongate lenses. sedimentary structures – low-angle stratification and wave reworking. palaeocurrents – mainly waveformed structures. fossils – robust shelly debris. colour – not diagnostic. facies associations – may be associated with coastal plain, lagoonal or shallow-marine facies

Characteristics of coastal and estuarine systems Lagoons. lithology – mainly mud with some sand. mineralogy – variable. texture – fine-grained, moderately to poorly sorted. bed geometry – thinly bedded mud with thin sheets and lenses of sand. sedimentary structures – may be laminated and wave rippled. palaeocurrents – rare, not diagnostic. fossils – often monospecific assemblages of hypersaline or brackish tolerant organisms. colour – may be dark due to anaerobic conditions. facies associations – may be associated with coastal plain or beach barrier deposits

Characteristics of coastal and estuarine systems Tidal channel systems. lithology – mud, sand l ess commonly conglomerate. mineralogy – variable. texture – may be well sorted in high energy settings. bed geometry – lenses with erosional bases. sedimentary structures – cross-bedding and crosslamination and inclined heterolithic stratification. palaeocurrents – bimodal in tidal estuaries. fossils – shallow marine. colour – not diagnostic. facies associations – may be overlain by fluvial, shallow marine, continental or delta facies.

Characteristics of coastal and estuarine systems Tidal mudflats. lithology – mud and sand. mineralogy – clay and shelly sand. texture – fine-grained, not diagnostic. bed geometry – tabular muds with thin sheets and lenses of sand. sedimentary structures – ripple cross- amination and flaser/lenticular bedding. palaeocurrents – bimodal in tidal estuaries

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