Sedimentologi Kamal Roslan Mohamed Shallow Sandy Seas

INTRODUCTION Shallow marine environments are areas of accumulation of substantial amounts of terrigenous clastic material brought in by rivers from the continental realm. Offshore from most coastlines there is a region of shallow water, the continental shelf, which may stretch tens to hundreds of kilometres out to sea before the water deepens down to the abyssal depths of ocean basins. Terrigenous clastic material is distributed on shelves by tides, waves, storms and ocean currents: these processes sort the material by grain size and deposit areas of sand and mud, which form thick, extensive sandstone and mudstone bodies in the stratigraphic record. Characteristic facies can be recognised as the products of transport and deposition by tides and storm/wave processes. Deposition in shallow marine environments is sensitive to changes in sea level.

SHALLOW MARINE ENVIRONMENTS OF TERRIGENOUS CLASTIC DEPOSITION The continental shelves and epicontinental seas are important sites of deposition of sand and mud in the world’s oceans and account for over half the volume of ocean sediments. These successions can be very thick, over 10,000 m, because deposition may be very long-lived and can continue uninterrupted for tens of millions of years. They occur as largely undeformed strata around the edges of continents and also in orogenic belts, where the collision of continental plates has forced beds deposited in shallow marine environments high up into mountain ranges.

Sediment supply to shallow seas The supply of sediment to shelves is a fundamental control on shallow marine environments and depositional facies of shelves and epicontinental seas. If the area lies adjacent to an uplifted continental region and there is a drainage pattern of rivers delivering detritus to the coast, the shallow-marine sedimentation will be dominated by terrigenous clastic deposits. The highest concentrations of clastic sediment will be near the mouths of major rivers: adjacent coastal regions will also be supplied with sediment by longshore movement of material by waves, storms and tides.

Sediment supply to shallow seas Shallow seas that are not supplied by much terrigenous material may be areas of carbonate sedimentation, especially if they are in lower latitudes where the climate is relatively warm. In cooler climates where carbonate production is slower, shelves and shallow seas with low terrigenous sediment supply are considered to be starved. The rate of sediment accumulation is slow and may be exceeded by the rate of subsidence of the sea floor such that the environment becomes gradually deeper with time.

Characteristics of shallow marine sands Texturally, the grains of sand will have suffered a degree of abrasion and the processes of turbulent flow during transport will separate the material into different grain sizes. The compositional maturity will probably be greater than the equivalent continental deposits, because the more labile minerals and grains (such as feldspar and lithic fragments) are broken down during transport: shallow marine sands are commonly dominated by quartz grains. Shallow seas are rich in marine life, including many organisms that have calcareous shells and skeletons. The remains of these biogenic hard parts are a major component of shelf carbonate deposits, but can also be very abundant in sands and muds deposited in these seas.

Characteristics of shallow marine sands Shallow seas are environments rich in animal life, particularly benthic organisms that can leave traces of their activity in the sediments. Bioturbation may form features that are recognisable of the activities of a particular type of organism, but also results in a general churning of the sediment, homogenising it into apparently structureless masses. Primary sedimentary structures (wave ripples, hummocky cross-stratification, trough rossbedding, and so on) are not always preserved in shelf sediments because of the effects of bioturbation. Bioturbation is most intense in shallower water and is frequently more abundant in sandy sediment than in muddy deposits.

Facies distribution across shelf Shoreface The shallower parts of the shelf are within the depth zone for wave action and any sediment will be extensively reworked by wave processes. Sands deposited in these settings may preserve wave-ripple cross-lamination and horizontal stratification. Streaks of mud in flaser beds deposited during intervals of lower wave energy become more common in the deposits of slightly deeper water further offshore

Facies distribution across shelf Offshore transition zone In the offshore transition zone, between the fairweather and storm wave bases on storm-dominated shelves, sands are deposited and reworked by storms. A storm creates conditions for the formation of bedforms and sedimentary structures that seem to be exclusive to storm-influenced environments.

Facies distribution across shelf Offshore The outer shelf area below storm wave base, the offshore zone, is predominantly a region of mud deposition. The sediments are commonly grey because this part of the sea floor is relatively poorly oxygenated allowing some preservation of organic matter within the mud.

Characteristics of a shallow-marine succession The offshore facies mainly consists of mudstone beds with some bioturbation. This is overlain by offshore transition facies made up of sandy tempestite beds interbedded with bioturbated mudstone. The tempestite beds have erosional bases, are normally graded and show some hummocky–swaley cross-stratification. The thickness of the sandstone beds generally increases up through the succession, and the deposits of the shallower part of this zone show more SCS than HCS. A schematic graphic sedimentary log of a storm-dominated succession.

Characteristics of a shallow-marine succession The shoreface is characterised by sandy beds with symmetrical (wave) ripple lamination, horizontal stratification and SCS, although sedimentary structures may be obscured by intense bioturbation. Sandstone beds in the shoreface may show a broad lens shape if they were deposited as localised ridges on the shallow sea floor. The top of the succession may be capped by foreshore facies. A schematic graphic sedimentary log of a storm-dominated succession.

Deposition on tide-dominated shelves Offshore sand ridges Near shorelines that experience strong tidal currents large sand ridges are found on modern shelves. The ridges form parallel to the shoreline in water depths of up to 50m and may be tens of metres high, in places rising almost to sea level. The sands are moderately well sorted, medium grained but the deposits may include some mud occurring as clay laminae deposited during slack phases of the tidal flow.

Deposition on tide-dominated shelves Tidal sandwaves and sand ribbons Currents generated by tides affect the sea bed tens of metres below sea level and are strong enough to move large quantities of sand in shallow marine environments. The effects of waves and storms are largely removed by tidal currents reworking the material in macrotidal regimes and only the tidal signature is left in the stratigraphic record.

Deposition on tide-dominated shelves Tidal sandwaves and sand ribbons In seas with moderate tidal effects the influence of tides is seen in shallower water, but storm deposits are preserved in the offshore transition zone in these mixed storm/tidal shelf settings.

A schematic sedimentary log through a tidally influenced shelf succession. A schematic graphic sedimentary log of a storm-dominated succession.

Characteristics of deposits of shallow sandy seas lithology – mainly sand and mud, with some gravel mineralogy: – mature quartz sands, shelly sands texture – generally moderately to well sorted bed geometry – sheets of variable thickness, large lenses formed by ridges and bars sedimentary structures – cross-bedding, cross- and horizontal lamination, hummocky and swaley crossstratification palaeocurrents – flow directions very variable, reflecting tidal currents, longshore drift, etc. fossils – often diverse and abundant, benthic forms are characteristic colour – often pale yellow-brown sands or grey sands and muds facies associations – may be overlain or underlain by coastal, deltaic, estuarine or deeper marine facies.

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