Araised beach,coastal terrace,[1] orperched coastline is a relatively flat, horizontal or gently inclined surface of marine origin,[2] mostly an oldabrasion platform which has been lifted out of the sphere of wave activity (sometimes called "tread"). Thus, it lies above or under the currentsea level, depending on the time of its formation.[3][4] It is bounded by a steeper ascending slope on the landward side and a steeper descending slope on the seaward side[2] (sometimes called "riser"). Due to its generally flat shape, it is often used foranthropogenic structures such as settlements andinfrastructure.[3]
A raised beach is anemergent coastallandform. Raised beaches and marine terraces arebeaches orwave-cut platforms raised above the shoreline by a relative fall in thesea level.[5]
Around the world, a combination of tectonic coastal uplift andQuaternarysea-level fluctuations has resulted in the formation of marine terrace sequences, most of which were formed during separateinterglacial highstands that can be correlated tomarine isotope stages (MIS).[6]
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.
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The platform of a marine terrace usually has a gradient between 1°–5° depending on the formertidal range with, commonly, a linear to concave profile. The width is quite variable, reaching up to 1,000 metres (3,300 ft), and seems to differ between thenorthern andsouthern hemispheres.[9] Thecliff faces that delimit the platform can vary in steepness depending on the relative roles of marine andsubaerial processes.[10] At the intersection of the formershore (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.[11] 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.[10] Marine terraces can extend for several tens of kilometers parallel to thecoast.[3]
Older terraces are covered by marine and/oralluvial orcolluvial materials while the uppermost terrace levels usually are less well preserved.[12] While marine terraces in areas of relatively rapid uplift rates (> 1 mm/year) can often be correlated to individualinterglacial periods or stages, those in areas of slower uplift rates may have a polycyclic origin with stages of returningsea levels following periods of exposure toweathering.[2]
Marine terraces can be covered by a wide variety ofsoils with complex histories and different ages. In protected areas,allochthonous sandy parent materials fromtsunami deposits may be found. Common soil types found on marine terraces includeplanosols andsolonetz.[13]
It is now widely thought that marine terraces are formed during the separated high stands ofinterglacial stages correlated tomarine isotope stages (MIS).[14][15][16][17][18]
The formation of marine terraces is controlled by changes in environmental conditions and bytectonic activity during recentgeological times.Changes in climatic conditions have led toeustatic sea-level oscillations andisostatic movements of theEarth's crust, especially with the changes betweenglacial andinterglacial periods.
Processes ofeustasy lead toglacioeustatic sea level fluctuations due to changes in the water volume in the oceans, and hence toregressions andtransgressions of the shoreline. At times of maximum glacial extent during thelast glacial period, thesea level was about 100 metres (330 ft) lower compared to today.Eustaticsea level changes can also be caused by changes in the void volume of the oceans, either through sedimento-eustasy or tectono-eustasy.[19]
Processes ofisostasy involve the uplift ofcontinental crusts along with their shorelines. Today, the process ofglacial isostatic adjustment mainly applies toPleistocene glaciated areas.[19] InScandinavia, for instance, the present rate of uplift reaches up to 10 millimetres (0.39 in)/year.[20]
In general, eustatic marine terraces were formed during separate sea-level highstands ofinterglacial stages[19][21] and can be correlated tomarine oxygen isotopic stages (MIS).[22][23] Glacioisostatic marine terraces were mainly created during stillstands of theisostatic uplift.[19] When eustasy was the main factor for the formation of marine terraces, derived sea level fluctuations can indicate formerclimate changes. This conclusion has to be treated with care, asisostatic adjustments andtectonic activities can be extensively overcompensated by an eustatic sea level rise. Thus, in areas of both eustatic and isostatic ortectonic influences, the course of the relative sea level curve can be complicated.[24] Hence, most of today's marine terrace sequences were formed by a combination of tectonic coastal uplift andQuaternary 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.[11]
Marine terraces often result frommarine erosion along rocky coastlines[2] intemperate regions due to wave attacks andsediment carried in the waves.Erosion also takes place in connection withweathering andcavitation. The speed of erosion is highly dependent on the shoreline material (hardness of rock[10]), thebathymetry, and thebedrock properties and can be between only a few millimeters per year forgranitic rocks and more than 10 metres (33 ft) per year forvolcanic ejecta.[10][25] The retreat of the seacliff generates ashore (wave-cut/abrasion-) platform through the process ofabrasion. A relative change in thesea level leads toregressions ortransgressions and eventually forms another terrace (marine-cut terrace) at a different altitude, while notches in the cliff face indicate short stillstands.[25]
It is believed that the terrace gradient increases withtidal 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 thehinterland increase the number of terraces formed during a certain time.[26]
Furthermore,shore platforms are formed bydenudation and marine-built terraces arise from accumulations of materials removed byshore erosion.[2] Thus, a marine terrace can be formed by botherosion and accumulation. However, there is an ongoing debate about the roles ofwave erosion andweathering in the formation of shore platforms.[10]
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 ofreef materials.[2]
While a terrace sequence can date back hundreds of thousands of years, its degradation is a rather fast process. A deeper transgression of cliffs into the shoreline may destroy previous terraces; but older terraces might be decayed[25] or covered by deposits,colluvia oralluvial fans.[3] Erosion and backwearing of slopes caused by incisive streams play another important role in this degradation process.[25]
The total displacement of the shoreline relative to the age of the associated interglacial stage allows the calculation of a mean uplift rate or the calculation of eustatic level at a particular time if the uplift is known.
To estimate vertical uplift, the eustatic position of the considered paleo sea levels relative to the present one must be known as precisely as possible. Currentchronology relies principally onrelative dating based ongeomorphologic criteria, but in all cases, the shoreline angle of the marine terraces is associated with numerical ages. The best-represented terrace worldwide is the one correlated to the last interglacial maximum (MIS 5e).[27][28][29] The age of MISS 5e is arbitrarily fixed to range from 130 to 116 ka[30] but is demonstrated to range from 134 to 113 ka inHawaii andBarbados with a peak from 128 to 116 ka on tectonically stable coastlines. Older marine terraces well represented in worldwide sequences are those related toMIS 9 (~303–339 ka) and11 (~362–423 ka).[31] 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 duringMIS 7.[32][33] 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 theHolocene andLate Pleistocene, these uncertainties don't affect on overall interpretation.
The sequence can also occur where the accumulation ofice 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.
For exact interpretations of the morphology, extensive datings, surveying and mapping of marine terraces are applied. This includesstereoscopicaerial photographic interpretation (ca. 1 : 10,000 – 25,000[11]), on-site inspections withtopographic maps (ca. 1 : 10,000) and analysis of eroded and accumulated material. Moreover, the exact altitude can be determined with ananeroid barometer or preferably with alevelling instrument mounted on a tripod. It should be measured with an accuracy of 1 cm (0.39 in) and at about every 50–100 metres (160–330 ft), depending on thetopography. In remote areas, the techniques ofphotogrammetry andtacheometry can be applied.[24]
Different methods for dating and correlation of marine terraces can be used and combined.
The morphostratigraphic approach focuses especially in regions ofmarine regression on the altitude as the most important criterion to distinguish coastlines of different ages. Moreover, individual marine terraces can be correlated based on their size and continuity. Also, paleo-soils as well asglacial,fluvial,eolian andperiglacial landforms andsediments may be used to find correlations between terraces.[24] OnNew Zealand's North Island, for instance,tephra andloess were used to date and correlate marine terraces.[34] At the terminus advance of formerglaciers marine terraces can be correlated by their size, as their width decreases with age due to the slowly thawing glaciers along the coastline.[24]
Thelithostratigraphic approach uses typical sequences ofsediment androck strata to provesea-level fluctuations based on an alternation of terrestrial andmarine sediments orlittoral and shallow marine sediments. Those strata show typical layers of transgressive and regressive patterns.[24] However, anunconformity in the sediment sequence might make this analysis difficult.[35]
Thebiostratigraphic approach uses remains of organisms which can indicate the age of a marine terrace. For that, oftenmollusc shells,foraminifera orpollen are used. EspeciallyMollusca can show specific properties depending on their depth ofsedimentation. Thus, they can be used to estimate former water depths.[24]
Marine terraces are often correlated tomarine oxygen isotopic stages (MIS)[22] and can also be roughly dated using theirstratigraphic position.[24]
There are various methods for the direct dating of marine terraces and their related materials. The most common method is14Cradiocarbon dating,[36] which has been used, for example, on theNorth Island of New Zealand to date several marine terraces.[37] It utilizes terrestrialbiogenic materials in coastalsediments, such asmollusc shells, by analyzing the14Cisotope.[24] In some cases, however, dating based on the230Th/234U ratio was applied, in casedetrital contamination or lowuranium concentrations made finding a high-resolution dating difficult.[38] In a study in southernItaly,paleomagnetism was used to carry outpaleomagnetic datings[39] andluminescence dating (OSL) was used in different studies on theSan Andreas Fault[40] and on theQuaternaryEupcheon Fault inSouth Korea.[41] In the last decade, the dating of marine terraces has been enhanced since the arrival of the terrestrialcosmogenic nuclides method, particularly through the use of10Be and26Alcosmogenic isotopes produced on site.[42][43][44] These isotopes record the duration of surface exposure tocosmic rays.[45] This exposure age reflects the age of abandonment of a marine terrace by the sea.
To calculate the eustaticsea 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.[2]
Marine terraces play an important role in the research ontectonics andearthquakes. They may show patterns and rates oftectonic uplift[40][44][46] and thus may be used to estimate thetectonic activity in a certain region.[41] In some cases the exposed secondary landforms can be correlated with known seismic events such as the1855 Wairarapa earthquake on theWairarapa Fault nearWellington,New Zealand which produced a 2.7-metre (8 ft 10 in) uplift.[47] This figure can be estimated from the vertical offset betweenraised shorelines in the area.[48]
Furthermore, with the knowledge of eustaticsea level fluctuations, the speed of isostatic uplift can be estimated[49] and eventually the change of relative sea levels for certain regions can be reconstructed. Thus, marine terraces also provide information for the research onclimate change and trends in futuresea level changes.[10][50]
When analyzing the morphology of marine terraces, it must be considered, that botheustasy andisostasy can influence on the formation process. This way can be assessed, whether there were changes in sea level or whethertectonic activities took place.
Raised beaches are found in a wide variety of coast andgeodynamical backgrounds such assubduction on thePacific coasts ofSouth andNorth America,passive margin of theAtlantic coast of South America,[51] collision context on the Pacific coast of Kamchatka,Papua New Guinea,New Zealand,Japan, passive margin of theSouth China Sea coast, on west-facing Atlantic coasts, such asDonegal Bay,County Cork andCounty Kerry inIreland;Bude,Widemouth Bay,Crackington Haven,Tintagel,Perranporth andSt Ives inCornwall, theVale of Glamorgan,Gower Peninsula,Pembrokeshire andCardigan Bay inWales,Jura and theIsle of Arran inScotland,Finistère inBrittany andGalicia inNorthern Spain and atSqually Point inEatonville, Nova Scotia within theCape Chignecto Provincial Park.
Other important sites include various coasts ofNew Zealand, e.g.Turakirae Head nearWellington being one of the world's best and most thoroughly studied examples.[47][48][52] Also along theCook Strait inNew Zealand, there is a well-defined sequence of uplifted marine terraces from the lateQuaternary at Tongue Point. It features a well-preserved lower terrace from the lastinterglacial, a widely eroded higher terrace from thepenultimate interglacial and another still higher terrace, which is nearly completely decayed.[47] Furthermore, onNew Zealand's North Island at the easternBay of Plenty, a sequence of seven marine terraces has been studied.[12][37]
Along many coasts of the mainland and islands around thePacific, marine terraces are typical coastal features. An especially prominent marine terraced coastline can be found north ofSanta Cruz, nearDavenport,California, where terraces probably have been raised by repeated slip earthquakes on theSan Andreas Fault.[40][53]Hans Jenny famously researched thepygmy forests of theMendocino andSonoma county marine terraces. The marine terrace's "ecological staircase" ofSalt Point State Park is also bound by the San Andreas Fault.
Along the coasts ofSouth America marine terraces are present,[44][54] where the highest ones are situated whereplate margins lie above subducted oceanic ridges and the highest and most rapid rates of uplift occur.[7][46] At Cape Laundi,Sumba Island,Indonesia an ancientpatch reef can be found at 475 m (1,558 ft) abovesea level as part of a sequence of coral reef terraces with eleven terraces being wider than 100 m (330 ft).[55] The coral marine terraces atHuon Peninsula,New Guinea, which extend over 80 km (50 mi) and rise over 600 m (2,000 ft) above presentsea level[56] are currently onUNESCO's tentative list forworld heritage sites under the nameHoun Terraces - Stairway to the Past.[57]
Other considerable examples include marine terraces rising to 360 m (1,180 ft) on somePhilippine Islands[58] and along theMediterranean Coast ofNorth Africa, especially inTunisia, rising to 400 m (1,300 ft).[59]
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 2 m (6.6 ft) above sea level.[60] 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.
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