Raised beach

Source: Wikipedia, the free encyclopedia.
"Marine terrace" redirects here. For the street in Fremantle, see Marine Terrace, Fremantle.
Raised beach and marine terraces at Water Canyon beach
A raised beach, now at 4 metres (13 ft) above high tide, formed King's Cave, Arran, below an earlier raised beach at around 30 metres (98 ft) height.

A raised beach, coastal terrace,

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.[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 for anthropogenic structures such as settlements and infrastructure.[3]

A raised beach is an emergent coastal landform. Raised beaches and marine terraces are beaches or wave-cut platforms raised above the shoreline by a relative fall in the sea level.[5]

Relict sea-cliffs at King's Cave on Arran's south-west coast

Around the world, a combination of tectonic coastal uplift and

marine 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.

Morphology

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) old shore (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 level I, 12) paleo-sea level II. – after various authors[1][3][7][8]

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 1,000 metres (3,300 ft), and seems to differ between the northern and southern hemispheres.[9] The cliff faces that delimit the platform can vary in steepness depending on the relative roles of marine and subaerial processes.[10] 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.[11] Sub-horizontal platforms usually terminate in a low tide cliff, and it is believed that the occurrence of these platforms depends on tidal activity.[10] Marine terraces can extend for several tens of kilometers parallel to the coast.[3]

Older terraces are covered by marine and/or alluvial or colluvial 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 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.[2]

Marine terraces can be covered by a wide variety of

planosols and solonetz.[13]

Formation

It is now widely thought that marine terraces are formed during the separated highstands of

marine isotope stages (MIS).[14][15][16][17][18]

Causes

Sea Level Reconstruction
Comparison of two sea level reconstructions during the last 500 Ma. 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

eustatic sea-level oscillations and isostatic movements of the Earth's crust, especially with the changes between glacial and interglacial
periods.

Processes of

Eustatic sea level changes can also be caused by changes in the void volume of the oceans, either through sedimento-eustasy or tectono-eustasy.[19]

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.[19] In Scandinavia, 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 of

climate changes. This conclusion has to be treated with care, as isostatic adjustments and tectonic activities can be extensively overcompensated by a 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.[24] 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.[11]

Processes

Marine terraces often result from

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[10]), the bathymetry, and the bedrock properties and can be between only a few millimeters per year for granitic rocks and more than 10 metres (33 ft) per year for volcanic ejecta.[10][25] 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.[25]

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.[26]

Furthermore, shore platforms are formed by denudation and marine-built terraces arise from accumulations of materials removed by shore erosion.[2] 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.[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 of reef 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 completely destroy previous terraces; but older terraces might be decayed[25] or covered by deposits, colluvia or alluvial fans.[3] Erosion and backwearing of slopes caused by incisive streams play another important role in this degradation process.[25]

Land and sea level history

The total displacement of the shoreline relative to the age of the associated interglacial stage allows calculation of a mean uplift rate or the calculation of eustatic level at a particular time if the uplift is known.

In order 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. Current

11 (~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 during MIS 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 the Holocene and Late Pleistocene
, these uncertainties have no effect on overall interpretation.

Sequence can also occur where the accumulation of ice sheets have depressed the land so that when the ice sheets melts the land readjusts with time thus raising the height of the beaches (glacio-isostatic rebound) and in places where co-seismic uplift occur. In the latter case, the terrace are not correlated with sea level highstand even if co-seismic terrace are known only for the Holocene.

Mapping and surveying

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 is applied. This includes

levelling instrument mounted on a tripod. It should be measured with the accuracy of 1 cm (0.39 in) and at about every 50–100 metres (160–330 ft), depending on the topography. In remote areas, the techniques of photogrammetry and tacheometry can be applied.[24]

Correlation and dating

Different methods for dating and correlation of marine terraces can be used and combined.

Correlational dating

The morphostratigraphic approach focuses especially in regions of

New Zealand's North Island, for instance, tephra and loess were used to date and correlate marine terraces.[34] 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.[24]

The

marine sediments or littoral and shallow marine sediments. Those strata show typical layers of transgressive and regressive patterns.[24] However, an unconformity in the sediment sequence might make this analysis difficult.[35]

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.[24]

Marine terraces are often correlated to

marine oxygen isotopic stages (MIS)[22] and can also be roughly dated using their stratigraphic position.[24]

Direct dating

There are various methods for the direct dating of marine terraces and their related materials. The most common method is

cosmogenic isotopes produced on site.[42][43][44] These isotopes record the duration of surface exposure to cosmic rays.[45]
This exposure age reflects the age of abandonment of a marine terrace by the sea.

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.[2]

Relevance for other research areas

Marine terraces south of Choapa River in Chile. These terraces have been studied among others by Roland Paskoff.

Marine terraces play an important role in the research on

raised shorelines in the area.[48]

Furthermore, with the knowledge of eustatic sea 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 on climate change and trends in future sea level changes.[10][50]

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.

Prominent examples

Tongue Point New Zealand
Quaternary marine terraces at Tongue Point, New Zealand

Raised beaches are found in a wide variety of coast and

Squally Point in Eatonville, Nova Scotia within the Cape Chignecto Provincial Park
.

Other important sites include various coasts of

New Zealand's North Island at the eastern Bay of Plenty, a sequence of seven marine terraces has been studied.[12][37]

marine terraces California
Air photograph of the marine terraced coastline north of Santa Cruz, California, note Highway 1 running along the coast along the lower terraces

Along many coasts of mainland and islands around 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

world heritage sites under the name Houn Terraces - Stairway to the Past.[57]

Other considerable examples include marine terraces rising up to 360 m (1,180 ft) on some Philippine Islands[58] and along the Mediterranean Coast of North Africa, especially in Tunisia, rising up to 400 m (1,300 ft).[59]

Related coastal geography

Uplift can also be registered through tidal notch sequences. Notches are often portrayed as lying at sea level; however notch types actually 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 that some notches may not contain a tectonic component in their formation.

See also

References

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