Storm dominated shallow clastic seas


Facies distribution across a storm-dominated shelf

Shoreface

The shallower parts of the shelf and epicontinental sea 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 ofmud inflaser beds deposited during intervals of lower wave energy become more common in the deposits of slightly deeper water further offshore. Wave ripples are less common as the fair-weather wave base is approached in the lower part of the shoreface. Within the shoreface zone sand ridges may be formed by flows generated by eddy currents related to storms and/or wave-driven longshore drift. These ridges occur in water depths of 5 to 15 m and are oriented obliquely to the coastline as oblique longshore bars. They are up to about 10 m high, a few kilometres wide and tens of kilometres in length, occurring spaced about 10 km apart. The sediments are typically well-sorted sands with a basal lag of gravel.

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. Hummocky cross-stratification (often abbreviated to HCS) is distinctive in form, consisting of rounded mounds of sand on the sea floor a few centimetres high and tens of centimetres across. The crests of the hummocks are tens of centimetres to a metre apart. Internal stratification of these hummocks is convex upwards, dips in all directions at angles of up to 108 or 208, and thickens laterally: these features are not seen in any other form of cross-stratification. Between the hummocks lie swales and where concave layers in them are preserved this is sometimes called swaley cross-stratification (abbreviated to SCS). Hummocky and swaley cross-stratification are believed to form as a result of combined flow, that is, the action of both waves and a current. This occurs when a current is generated by a storm at the same time as high-amplitude waves reach deep below the surface. The strong current takes sand out into the deeper water in temporary suspension and as it is deposited the oscillatory motion caused by the waves results in deposition in the form of hummocks and swales. Swaley cross-stratification is mainly formed and preserved in shallow water where the hummocks have a lower preservation potential. One of the characteristics of HCS/SCS is that these structures are normally only seen in fine to medium grained sand, suggesting that there is some grain-size limitation involved in this process. Storm conditions affect the water to depths of 20 to 50 m or more so HCS/SCS may be expected in any sandy sediments on the shelf to depths of several tens of metres. These structures are not seen in shoreface deposits above fair-weather wave base due to reworking of the sediment by ordinary wave processes, so this characteristic form of cross-stratification is found only in sands deposited in the offshore transition zone.

Individual storm deposits, tempestites, deposited by single storm events typically taper in thickness from a few tens of centimetres to millimetre-thick beds in the outer parts of this zone several tens of kilometres offshore. Proximal tempestites have erosive bases and are composed of coarse detritus, whereas the distal parts of the bed are finer-grained laminated sands: hummocky and swaley cross-stratification occurs in the sandy parts of tempestites. An idealised tempestite bed will have a sharp, possibly erosive base, overlain by structureless coarse sediment (coarse sand and/or gravel): the scouring and initial deposition occurs when the storm is at its peak strength. As the storm wanes, hummocky–swaley cross-stratification forms in finer sands and this is overlain by fine sand and silt that shows horizontal and wave-ripple lamination formed as the strength of the oscillation decreases. At the top of the bed the sediment grades into mud. The magnitude of the storms that deposit beds tens of centimetres thick is not easy to estimate, because the availability of sand is probably of equal importance to the storm energy in determining the thickness of the bed. In the periods between storm events this part of the shelf is an area of deposition of mud from suspension. This fine-grained clastic material is sourced from river mouths and is carried in suspension by geostrophic and wind-driven currents, and storms also rework a lot of fine sediment from the sea floor and carry it in suspension across the shelf. Storm deposits are therefore separated by layers of mud, except in cases where the mud is eroded away by the subsequent storm. The proportion of mud in the sediments increases offshore as the amount of sand deposited by storms decreases.

Offshore

The outer shelf area below storm wave base, the offshore zone, is predominantly a region of mud deposition. Exceptional storms may have some effect on this deeper part of the shelf, and will be represented by thin, fine sand deposits interbedded with the mudstone. Ichnofauna are typically less diverse and abundant than the associations found in the shoreface and offshore transition zone. 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 storm-dominated shallow-marine succession

If there is a constant sediment supply to the shelf, continued deposition builds up the layers on the sea bed and the water becomes shallower. Shelf areas that were formerly below storm wave base experience the effects of storms and become part of the offshore transition zone. Similarly addition of sediment to the sea floor in the offshore transition zone brings the sea bed up into the shoreface zone above fair-weather wave base and a vertical succession of facies that progressively shallow upwards is constructed. 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. 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.

Mud-dominated shelves

Some shelf areas are wave- and storm-dominated, but receive large quantities of mud and relatively little sand. They occur close to rivers that have a high suspended load: the plumes of suspended sediment from the mouths of major rivers may extend for tens or hundreds of kilometres out to sea and then are reworked by wind-driven and geostrophic currents across the shelf. Muddy deposits on the inner parts of the shelf are normally intensely bioturbated, except in cases where the rates of sedimentation of mud are so high that accumulation outpaces the rate at which the organisms can rework the sediment. High concentrations of organic matter may make these shelf muds very dark grey or black in colour.
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