Raised beach - WikiHQ

thumb|Raised beach and marine terraces at [[Water Canyon beach]]

thumb|A raised beach, now at above high tide, formed [[King's Cave, Arran, below an earlier raised beach at around height.]]

A raised beach, coastal terrace, 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 the time of its formation. It is bounded by a steeper ascending slope on the landward side and a steeper descending slope on the seaward side

thumb|[[Relict (geology)|Relict sea-cliffs at King's Cave on Arran's south-west coast]]

Around the world, a combination of tectonic coastal uplift and Quaternary sea-level fluctuations has resulted in the formation of marine terrace sequences, most of which were formed during separate interglacial highstands that can be correlated to marine isotope stages (MIS).

A marine terrace commonly retains a shoreline angle or inner edge, the slope inflection between the marine abrasion platform and the associated paleo sea cliff. The shoreline angle represents the maximum shoreline of a transgression and therefore a paleo-sea level.

Morphology

thumb|upright=1.5|alt=marine terraces|Typical sequence of [[erosional marine terraces. 1)low tide cliff/ramp with deposition, 2)modern shore (wave-cut/abrasion-) platform, 3)notch/inner edge, modern shoreline angle, 4)modern sea cliff, 5) oldshore (wave-cut/abrasion-) platform, 6)paleo-shoreline angle, 7)paleo-sea cliff, 8)terrace cover deposits/marine deposits, colluvium, 9)alluvial fan, 10)decayed and covered sea cliff and shore platform, 11)paleo-sea levelI, 12)paleo-sea levelII.after various authors]]

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 quite variable, reaching up to , and seems to differ between the northern and southern hemispheres. The cliff faces that delimit 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 the platform commonly retains a shoreline angle or inner edge (notch) that 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 the tidal activity. 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 following periods of exposure to weathering.

Formation

It is now widely thought that marine terraces are formed during the separated high stands of interglacial stages correlated to marine isotope stages (MIS).

Causes

thumb|upright=1.35|alt=Sea Level Reconstruction|Comparison of two [[Sea-level curve|sea level reconstructions during the last 500Ma. The scale of change during the last glacial/interglacial transition is indicated with a black bar.]]

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 in the water volume in the oceans, and hence to regressions and transgressions of the shoreline. At times of maximum glacial extent during the last glacial period, the sea level was about lower compared to today. Eustatic sea level changes can also be caused by changes in the void volume of the oceans, either through sedimento-eustasy or tectono-eustasy.

Processes of isostasy involve the uplift of continental crusts along with their shorelines. Today, the process of glacial isostatic adjustment mainly applies to Pleistocene glaciated areas.

In general, eustatic marine terraces were formed during separate sea-level highstands of interglacial stages and can be correlated to marine oxygen isotopic stages (MIS). Glacioisostatic marine terraces were mainly created during stillstands of the isostatic uplift. 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. The retreat of the sea cliff generates a shore (wave-cut/abrasion-) platform through the process of abrasion. A relative change in 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.

Furthermore, shore platforms are formed by denudation and marine-built terraces arise from accumulations of materials removed by shore erosion. The age of MISS 5e is arbitrarily fixed to range from 130 to 116 ka but is demonstrated to range from 134 to 113 ka in Hawaii and Barbados with a peak from 128 to 116 ka on tectonically stable coastlines. Older marine terraces well represented in worldwide sequences are those related to MIS 9 (~303–339 ka) and 11 (~362–423 ka). Compilations show that sea level was 3 ± 3 meters higher during MIS 5e, MIS 9 and 11 than during the present one and −1 ± 1 m to the present one during MIS 7. Consequently, MIS 7 (~180-240 ka) marine terraces are less pronounced and sometimes absent. When the elevations of these terraces are higher than the uncertainties in paleo-eustatic sea level mentioned for the Holocene and Late Pleistocene, these uncertainties don't affect on overall interpretation.

The sequence can also occur where the accumulation of ice sheets has depressed the land so that when the ice sheets melt the land readjusts with time thus raising the height of the beaches (glacial-isostatic rebound) and in places where co-seismic uplift occurs. In the latter case, the terrace is not correlated with sea-level highstands even if co-seismic terraces are known only for the Holocene.

Mapping and surveying

thumb|alt=Tongue Point New Zealand|Aerial photograph of the lowest marine terrace at Tongue Point, [[New Zealand]]

For exact interpretations of the morphology, extensive datings, surveying and mapping of marine terraces are applied. This includes stereoscopic aerial photographic interpretation (ca. 1 : 10,000 – 25,000 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 coastline.

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. which has been used, for example, on the North Island of New Zealand to date several marine terraces. It utilizes terrestrial biogenic materials in coastal sediments, such as mollusc shells, by analyzing the 14C isotope. 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 the last decade, the dating of marine terraces has been enhanced since the arrival of the terrestrial cosmogenic nuclides method, particularly through the use of 10Be and 26Al cosmogenic isotopes produced on site. These isotopes record the duration of surface exposure to cosmic rays. This exposure age reflects the age of abandonment of a marine terrace by the sea.

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. and thus may be used to estimate the tectonic activity in a certain region. 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 influence on the formation process. This way can be assessed, whether there were changes in sea level or whether tectonic activities took place.

Prominent examples

thumb|right|alt=Tongue Point New Zealand|[[Quaternary marine terraces at Tongue Point, New Zealand]]

Raised beaches are found in a wide variety of coast and geodynamical backgrounds such as subduction on the Pacific coasts of South and North America, passive margin of the Atlantic coast of South America, collision context on the Pacific coast of Kamchatka, Papua New Guinea, New Zealand, Japan, passive margin of the South China Sea coast, on west-facing Atlantic coasts, such as Donegal Bay, County Cork and County Kerry in Ireland; Bude, Widemouth Bay, Crackington Haven, Tintagel, Perranporth and St Ives in Cornwall, the Vale of Glamorgan, Gower Peninsula, Pembrokeshire and Cardigan Bay in Wales, Jura and the Isle of Arran in Scotland, Finistère in Brittany and Galicia in Northern Spain and at Squally Point in Eatonville, Nova Scotia within the Cape Chignecto Provincial Park.

Other important sites include various coasts of New Zealand, e.g. Turakirae Head near 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. Hans Jenny famously researched the pygmy forests of the Mendocino and Sonoma county marine terraces. The marine terrace's "ecological staircase" of Salt Point State Park is also bound by the San Andreas Fault.

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. The coral marine terraces at Huon Peninsula, New Guinea, which extend over and rise over above present sea level are currently on UNESCO's tentative list for world heritage sites under the name Houn Terraces - Stairway to the Past.

Other considerable examples include marine terraces rising to on some Philippine Islands and along the Mediterranean Coast of North Africa, especially in Tunisia, rising to .

Uplift can also be registered through tidal notch sequences. Notches are often portrayed as lying at sea level; however, notch types form a continuum from wave notches formed in quiet conditions at sea level to surf notches formed in more turbulent conditions and as much as above sea level. As stated above, there was at least one higher sea level during the Holocene, so some notches may not contain a tectonic component in their formation.

References

External links

Notes at NAHSTE
US Geological Survey Marine Terrace Fact Sheet - Wikimedia link, USGS link