- Marine terrace
A marine terrace, coastal terrace, raised beach or perched coastline is a relatively flat, horizontal or gently inclined surface of marine origin, mostly an old abrasion platform which has been lifted out of the sphere of wave activity (sometimes called "tread"). Thus it lies above or under the current sea level, depending on its time of formation. It is bounded by a steeper ascending slope on the landward side and a steeper descending slope on the seaward side (sometimes called "riser"). Due to its reasonably flat shape it is often used for anthropogenic structures like settlements and infrastructure.
The platform of a marine terrace usually has a gradient between 1°- 5° depending on the former tidal range with commonly a linear to concave profile. The width is very variable, reaching up to 1000 m, and seems to differ between northern and southern hemisphere. The cliff faces, delimiting the platform, can vary in steepness depending on the relative roles of marine and subaerial processes. At the intersection of the former shore (wave-cut/abrasion-) platform and the rising cliff face it commonly retains a shoreline angle or inner edge (notch) which indicates the location of the shoreline at the time of maximum sea ingression and therefore a paleo sea level. Sub-horizontal platforms usually terminate in a low tide cliff and it is believed that the occurrence of these platforms depends on tidal activity. Marine terraces can extend for several tens of kilometers parallel to the coast.
Older terraces are covered by marine and/or alluvial or colluvial materials while the uppermost terrace levels usually are less well-preserved. While marine terraces in areas of relatively rapid uplift rates (> 1 mm/year) can often be correlated to individual interglacial periods or stages, those in areas of slower uplift rates may have a polycyclic origin with stages of returning sea levels after times of exposure to weathering.
Marine terraces can be covered by a wide variety of soils with complex histories and different ages. In protected areas allochtonous sandy parent materials from tsunami deposits may be found. Common soil types found on marine terraces include Planosols and Solonetz.
The formation of marine terraces is controlled by changes in environmental conditions and by tectonic activity during recent geological times. Changes in climatic conditions have led to eustatic sea-level oscillations and isostatic movements of the earth’s crust, especially with the changes between glacial and interglacial periods.
Processes of eustasy lead to glacioeustatic sea level fluctuations due to changes of the water volume in the oceans and hence to regressions and transgressions of the shoreline. E.g. at times of maximum glacier extend during the last glacial period the sea level was about 100 m lower compared to today. Eustatic sea level changes can also be caused by changes of the void volume of the oceans, either through sedimento-eustasy or tectono-eustasy.
Processes of isostasy involve the uplift of continental crusts including their shorelines. Today the process of glacial isostatic adjustment mainly applies to during Pleistocene glaciated areas. In Scandinavia, for instance, the present rate of uplift reaches up to 10 mm/year.
In general, eustatic marine terraces were formed during separate sea level highstands of interglacial stages and can be correlated to Marine Oxygene Isotopic Stages (MIS). Glacioisostatic marine terraces were mainly created during stillstands of the isostatic uplift. When eustasy was the main factor for the formation of marine terraces, derived sea level fluctuations can indicate former climate changes. This conclusion has to be treated with care, as isostatic adjustments and tectonic activities can be extensively overcompensated by an eustatic sea level rise. Thus, in areas of both eustatic and isostatic or tectonic influences, the course of the relative sea level curve can be complicated. Hence most of today’s marine terrace sequences were formed by a combination of tectonic coastal uplift and Quaternary sea level fluctuations.
Jerky tectonic uplifts can also lead to marked terrace steps while smooth relative sea level changes may not result in obvious terraces and their formations are often not referred to as marine terraces.
Marine terraces often result from marine erosion along rocky coast lines in temperate regions due to wave attack and sediment carried in the waves. Erosion also takes place in connection with weathering and cavitation. The speed of erosion is highly dependent on the shoreline material (hardness of rock), the bathymetry and the bedrock properties and can be between only a few millimeters per year for granitic rocks and more than 10m per year for volcanic ejecta. The retreat of the sea cliff generates a shore (wave-cut/abrasion-) platform through the process of abrasion. A relative change of the sea level leads to regressions or transgressions and eventually forms another terrace (marine-cut terrace) at a different altitude while notches in the cliff face indicate short stillstands. It is believed that the terrace gradient increases with tidal range and decreases with rock resistance. In addition the relationship between terrace width and the strength of the rock is inverse and higher rates of uplift and subsidence as well as a higher slope of the hinterland increases the number of terraces formed during a certain time.
Furthermore shore platforms are formed by denudation and marine-built terraces arise from accumulations of materials removed by shore erosion. Thus a marine terrace can be formed by both erosion and accumulation. However, there is an ongoing debate about the roles of wave erosion and weathering in the formation of shore platforms.
Reef flats or uplifted coral reefs are another kind of marine terrace found in intertropical regions. They are a result of biological activity, shoreline advance and accumulation of reef materials.
While a terrace sequence can date back hundreds of thousands of years, its degradation is a rather fast process. On the one hand a deeper transgression of cliffs into the shoreline may completely destroy previous terraces; on the other hand older terraces might be decayed or covered by deposits, colluvia or alluvial fans. Erosion and backwearing of slopes caused by incisive streams play another important role in this degradation process.
Mapping and surveying
For exact interpretations of the morphology extensive datings, surveying and mapping of marine terraces is applied. This includes stereoscopic aerial photographic interpretation (ca. 1 : 10,000 - 25,000), on-site inspections with topographic maps (ca. 1 : 10,000) and analysis of eroded and accumulated material. Moreover the exact altitude can be determined with an aneroid barometer or preferably with a levelling instrument mounted on a tripod. It should be measured with the accuracy of 1 cm and at about every 50 - 100 m, depending on the topography. In remote areas technics of photogrammetry and tacheometry can be applied.
Correlation and dating
Different methods for dating and correlation of marine terraces can be used and combined.
The morphostratigraphic approach focuses especially in regions of marine regression on the altitude as the most important criterion to distinguish coast lines of different ages. Moreover individual marine terraces can be correlated based on their size and continuity. Also paleo-soils as well as glacial, fluvial, eolian and periglacial landforms and sediments may be used to find correlations between terraces. On New Zealand’s North Island, for instance, tephra and loess were used to date and correlate marine terraces. At the terminus advance of former glaciers marine terraces can be correlated by their size, as their width decreases with age due to the slowly thawing glaciers along the coast line.
The lithostratigraphic approach uses typical sequences of sediment and rock strata to prove sea level fluctuations on the basis of an alternation of terrestrial and marine sediments or littoral and shallow marine sediments. Those strata show typical layers of transgressive and regressive patterns. However, an unconformity in the sediment sequence might make this analysis difficult.
The biostratigraphic approach uses remains of organisms which can indicate the age of a marine terrace. For that often mollusc shells, foraminifera or pollen are used. Especially Mollusca can show specific properties depending on their depth of sedimentation. Thus they can be used to estimate former water depths.
There are various methods for the direct dating of marine terraces and their related materials including 14C radiocarbon dating, which is the most common one. E.g. this method has been used on the North Island of New Zealand to date several marine terraces. It utilizes terrestrial biogenic materials in coastal sediments such as mollusc shells analyzing the 14C isotope. In some cases dating based on the 230Th/234U ratio was applied though in case of detrital contamination or low Uranium concentrations a high resolution dating was found to be difficult. In a study in southern Italy paleomagnetism was used to carry out paleomagnetic datings and luminescence dating (OSL) was used in different studies on the San Andreas Fault and on the Quaternary Eupcheon Fault in South Korea.
In order to calculate the eustatic sea level for each dated terrace it is assumed that the eustatic sea-level position corresponding to at least one marine terrace is known and that the uplift rate has remained essentially constant in each section.
Relevance for other research areas
Marine terraces play an important role in the research on tectonics and earthquakes. They may show patterns and rates of tectonic uplift and thus may be used to estimate the tectonic activity in a certain region. In some cases the exposed secondary landforms can be correlated with known seismic events such as the 1855 Wairarapa earthquake on the Wairarapa Fault near Wellington/New Zealand which produced a 2.7 m uplift. This figure can be estimated from the vertical offset between raised shorelines in the area.
Furthermore with the knowledge of eustatic sea level fluctuations the speed of isostatic uplift can be estimated and eventually the change of relative sea levels for certain regions can be reconstructed. Thus marine terraces also provide information for the research on climate change and trends in future sea level changes.
When analyzing the morphology of marine terraces it must be considered, that both eustasy and isostasy can have an influence on the formation process. This way can be assessed, whether there were changes in sea level or whether tectonic activities took place.
Marine terraces can be found on many geodynamically influenced coastlines around the world.
Important sites include various coasts of New Zealand, e.g. Turakirae Head south of Wellington being one of the world’s best and most thoroughly studied examples. Also along the Cook Strait in New Zealand there is a well-defined sequence of uplifted marine terraces from the late Quaternary at Tongue Point. It features a well preserved lower terrace from the last interglacial, a widely eroded higher terrace from the penultimate interglacial and another still higher terrace, which is nearly completely decayed. Furthermore on New Zealand’s North Island at the eastern Bay of Plenty a sequence of seven marine terraces has been studied.
Along many coasts of mainland and islands around the Pacific, marine terraces are typical coastal features. An especially prominent marine terraced coastline can be found north of Santa Cruz, near Davenport, California, where terraces probably have been raised by repeated slip earthquakes on the San Andreas Fault. But also along the coasts of South America marine terraces are present, where the highest ones are situated where plate margins lie above subducted oceanic ridges and the highest and most rapid rates of uplift occur. At Cape Laundi, Sumba Island, Indonesia an ancient patch reef can be found at 475 m above sea level as part of a sequence of coral reef terraces with 11 terraces being wider than 100m. The coral marine terraces at Huon Peninsula, New Guinea, which extend over 80 km and rise over 600 m above present sea level are currently on UNESCO’s tentative list for world heritage sites under the name Houn Terraces - Stairway to the Past.
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