There is evidence from around the world today that the position of the shoreline is not constant, even in the geologically short time-span of historical records: harbours built hundreds or thousands of years ago are in some places drowned, in others left high and dry away from the shoreline. The first obvious cause is tectonic activity that moves the crust vertically, as well as the horizontal movements due to plate motion. This movement of the crust itself up or down relative to the sea level may affect the crust within a few kilometres of a single fault, or may be large-scale ‘thermo-tectonic’ activity that has an effect on whole continental margins. Second, there can be changes in the volume of water in the world’s oceans: this is called ‘eustatic sea-level change’ (eustasy) and is caused by melting and freezing of continental ice caps, among other things. Debates about the effects of global warming on the level of the sea worldwide have brought this phenomenon to the attention of most people. Third, there is the effect of sedimentation: sand, gravel and mud piled up at the shoreline can result in the shoreline moving away from its former position. These three factors – tectonic uplift/subsidence, eustatic sea-level rise/fall, and sedimentation – and how they occur, where they occur, their rates and how they interact are fundamental to sedimentology and stratigraphy. The character of sediment deposited in environments ranging from rivers and floodplains to shorelines, shelves and even the deep seas is in some way influenced by these three factors. The study of the relationships between sea-level changes and sedimentation is often referred to as ‘sequence stratigraphy’. In the following sections the principles underlying the basic concepts are considered and then there is an explanation of some of the terminology that has evolved to describe the relationships between strata under conditions of changing sea level. The causes of sea-level fluctuations and the use of a sea-level curve as a correlative tool are also discussed.
Changes to a shoreline
If the three variables are considered in isolation of each other, five different scenarios can be considered. Consider what will happen to a palm tree growing on a beach and a crab sitting on the sea floor a few hundred metres away.
- Eustatic sea-level rise: the palm tree is drowned and the crab will find itself in deeper water.
- Eustatic sea-level fall: the palm tree will end up growing some distance from the shoreline, and the crab is now in shallower water.
- Uplift of the crust: the palm tree will end up growing some distance from the shoreline, and the crab is now in shallower water.
- Subsidence of the crust: the palm tree is drowned and the crab will find itself in deeper water.
- Addition of sediment at the shoreline: the palm tree will end up growing some distance from the shoreline, and the crab is now in shallower water (providing it is not engulfed by sediment, but instead moves up to the new sea floor).
It is important to note that scenarios 1 and 4 are exactly the same, and viewed from just one point on the Earth’s surface it is not possible to distinguish between these two possible causes. The same is true of scenarios 2 and 3, which are indistinguishable at a local scale, and often the difference between either of these and scenario 5 can be subtle. The controls on sea level fluctuations are considered but because it is difficult to distinguish between uplift and eustatic sea-level fall on the one hand and subsidence and eustatic sea-level rise on the other, it is usually best to refer to changes in ‘relative sea level’ or ‘relative base level’ when looking at strata in one place. The drowning of a palm tree may therefore be considered to be evidence of a ‘relative sea-level rise’, without any implication of the cause, and in the same way, the crab is now in relatively deeper water. The impact that these relative changes have on processes and products of sedimentation is something that will be considered in the next section, along with the importance of the third factor, sedimentation.
Sea level and sedimentation
Although we may find palm trees fossilised in sedimentary rocks, the evidence for the position of the shoreline and the relative depth of water comes mainly from the character of the sediments themselves. The characteristic facies of sediment deposited at different positions relative to the shoreline and in different depths of seawater were considered. If, therefore, we can establish the water depth/position relative to shoreline by examining the sedimentary facies, we can also recognise relative changes in the shoreline/water depth from changes in those facies. In fact, the analysis of strata in terms of relative sea-level changes can be carried out only if a facies analysis is carried out first. Once all the beds in a succession have been analysed and classified according to environment of deposition using the approaches described in earlier chapters in this book, the effects of sea-level changes on their deposition can then be considered.
Transgression, regression and forced regression
If there is a relative sea-level rise the shoreline will move landward: this is referred to as transgression. Movement of the shoreline seawards as a result of sedimentation occurring at the coast is called a regression, but if it is due to a relative sea-level fall it is known as a forced regression. The sedimentary response to these changes in shoreline can be preserved in strata as changes in facies going up through a succession, changes that reflect either a landward movement of the shoreline, transgression, or a seaward movement of the shoreline, regression (forced or otherwise). Under conditions of transgression, the shoreline will move to a place that used to be land, and the coastal plain deposits are overlain by beach deposits. Similarly, beach (foreshore) deposits will be overlain by shoreface deposits because the former beach is now under shallow water. The same pattern of changes in facies from shallower to deeper will be seen all the way across the shelf. It is therefore possible to recognise the signature of a transgression in a succession of beds by the tendency for the environment of deposition, as indicated by the facies, to become deeper upwards. If there is a regression, the pattern seen in vertical succession will be the opposite: as the sea becomes shallower, either due to a relative sea-level fall (forced regression) or addition of more sediment (regression), the facies will reflect this: shoreface facies will be overlain by foreshore deposits, offshore transition sediments by shoreface deposits, and so on. In some circumstances, a forced regression may be distinguished from a simple regression by evidence of erosion in the coastal and shallowest marine deposits: as sea level falls, the river may have to erode the older coastal deposits asitcuts a new path to the shoreline. However, this may not always happen, and depends on rates of sediment supply and the slope of the foreshore/shoreface.
The concept of accommodation
Sediment will be deposited in places where there is space available to accumulate material: this is the concept of accommodation (or accommodation space) and its availability is determined by changes in relative sea level. In shallow marine environments an increase in relative sea level creates accommodation that is then filled up with sediment until an equilibrium profile is reached. The equilibrium profile is a notional surface of deposition relative to sea level and sedimentation occurs on any point in the shallow marine environment until this surface is reached: any material deposited above the surface is reworked by processes such as waves and tidal currents. The equilibrium profile is at different positions relative to sea level in different environments: in the foreshore it is at sea level, in the shoreface a few metres below sea level and then progressively deeper through further offshore. Accommodation in shallow marine environments is created by any mechanism that results in a relative rise in sea level, including eustatic sea-level rise, tectonic subsidence and compaction of sea-floor sediments. Accommodation is reduced by the addition of sediment to fill the space or by tectonic or eustatic mechanisms that lower the relative sea level. The rate of change of accommodation is determined by the relative rates of relative sea-level change and sediment supply. Deposits in places where there has been a relative sea-level fall will often be eroded, and this can be considered to be a condition where there is negative accommodation. The ideas of accommodation and equilibrium profiles can also be applied to fluvial environments. A mature river will erode in its upper tracts and deposit in the downstream parts until it develops an equilibrium profile, whereby the main channel is neither eroding nor depositing. Under these conditions erosion still continues in the hill slopes above the main channel valley, but sediment is carried through the river down to the sea. This profile may be disturbed by a fall in sealevel that creates negative accommodation along part of the profile, resulting in erosion, or by sea-level rise generating accommodation that allows sediment to accumulate in the channels and overbank areas until it returns to the equilibrium position. The concept can also be applied to non-marine systems such as lakes and river systems feeding them, where it is the level of the water in the lake that determines the amount of accommodation available. In the following discussion, accommodation is considered in terms of relative sea level, and depositional systems described are either marine or have marine connections. The same principles can be applied to lacustrine systems and the deposits of large lakes can be considered in terms of relative changes in the lake level. Global eustasy does not directly control the level of water in lakes, but climatic controls are important because the balance between precipitation/run-off and evaporation determines the amount of water in the lake and hence its level.
Rates of sea-level change and sediment supply
In a previous section it was stated that if there is a relative sea-level rise, the shoreline will move landwards. In fact, this is not necessarily the case: if the rate at which sediment is supplied is greater than the rate at which the sea level is rising, then the shoreline will still move seawards. Similarly, if the rate of sediment supply and the rate of sea-level rise are in balance, the shoreline position does not change. Several different situations can be envisaged when the sea level is rising (either due to subsidence or eustatic sea-level rise) which give rise to different stratal geometries.
- If the rate of sediment supply is very low then the shoreline will move landward without deposition occurring and with the possibility of erosion.
- With moderate sedimentation rates, but high rates of sea-level rise, deposition will occur as the shoreline moves landward.
- If it is a higher sedimentation rate, then as fast as the sea level rises the space is filled up with sediment and the shoreline stays in the same place.
- At high sedimentation rates, the shoreline will still move seawards, even though the sea level is rising.
- During periods when the sea level is static the addition of sediment causes the shoreline to shift seawards.
- At low rates of sea-level fall and/or high rates of sediment supply deposition occurs as the shoreline moves seawards.
- If the rate of sea-level fall is relatively high and the rate of sedimentation is low, there is no sedimentation, and there may be erosion.
- A coast undergoing rapid erosion during sealevel fall could theoretically fall into this category.