Raised beach
An emergent coastal landform raised above the shoreline by relative sea-level fall.
A raised beach, also known as a coastal terrace or perched coastline, is a relatively flat, horizontal or gently inclined surface of marine origin that has been lifted out of the sphere of wave activity. It is bounded by a steeper ascending slope on the landward side and a steeper descending slope on the seaward side. These emergent coastal landforms are often used for anthropogenic structures such as settlements and infrastructure due to their generally flat shape.
Quick Facts
- Also known as
- coastal terrace, perched coastline
- Type
- emergent coastal landform
- Formation mechanism
- relative fall in sea level (tectonic uplift or eustatic sea-level drop)
- Typical gradient
- 1°–5°
- Typical width
- up to 1,000 m / 3,281 ft
- Key feature
- shoreline angle or inner edge indicating paleo-sea level
Facts from the source article.
Morphology
A marine terrace's platform slopes at 1 to 5 degrees, shaped by the past tidal range, and its profile is usually straight or slightly curved inward. The platform can be up to 1000 meters wide, with noticeable size differences between the northern and southern hemispheres. The steepness of the cliff faces bordering the platform depends on how much marine versus land-based forces shaped them. Where the old wave-cut platform meets the cliff, a notch or shoreline angle often remains, marking the exact spot of the highest ancient sea level.
Sub-horizontal platforms end in a low-tide cliff, a feature thought to rely on tidal action. These terraces can run for tens of kilometers along the coast. Older terraces get buried under marine, river, or landslide deposits, while the highest ones are often poorly preserved. In places rising faster than 1 millimeter per year, terraces usually match specific warm periods between ice ages; in slower-uplift areas, they may have formed from multiple sea-level cycles.
The soils here include planosols and solonetz, and in sheltered spots, tsunami deposits can supply sandy material from elsewhere.
Formation
Marine terraces are believed to have formed during the separate high sea-level stands of interglacial periods, which can be linked to marine isotope stages. Their development depends on shifting environmental conditions and tectonic activity in recent geological history. Climate shifts cause eustatic sea-level changes and isostatic crustal movements, especially when moving between glacial and interglacial times. Eustatic processes drive glacioeustatic sea-level fluctuations by altering the ocean's water volume, leading to shoreline advances and retreats.
At the peak of the last glacial period, sea level sat roughly 100 meters below today's level. Eustatic changes can also come from sedimento-eustasy or tectono-eustasy, which affect the ocean basin's void volume. Isostatic processes lift continental crusts and their coastlines; in Scandinavia, for instance, uplift currently reaches up to 10 millimeters per year. Eustatic marine terraces formed during separate interglacial highstands and correlate with marine oxygen isotopic stages.
Glacioisostatic marine terraces mainly appeared during pauses in isostatic uplift. Where both eustatic and isostatic or tectonic forces are at play, the relative sea-level history becomes complex. Abrupt tectonic uplifts can create sharp terrace steps, whereas gradual relative sea-level changes may not produce clear terraces, and such formations are often not classified as marine terraces.
Correlation and dating
Various methods can be used and combined for dating and correlating marine terraces. The morphostratigraphic approach focuses on altitude as the most important criterion for distinguishing coastlines of different ages, especially in regions of marine regression. Individual terraces can also be correlated based on their size and continuity. Paleo-soils and glacial, fluvial, eolian, and periglacial landforms and sediments may be used to find correlations; on New Zealand's North Island, tephra and loess have been used for this purpose.
The lithostratigraphic approach uses typical sequences of sediment and rock strata to prove sea-level fluctuations based on an alternation of terrestrial and marine sediments, though an unconformity in the sequence can complicate analysis. The biostratigraphic approach uses remains of organisms such as mollusc shells, foraminifera, or pollen to indicate age; molluscs can show specific properties depending on their depth of sedimentation, allowing estimation of former water depths.
Marine terraces are often correlated to marine oxygen isotopic stages and can be roughly dated using their stratigraphic position.
Prominent examples
Raised beaches are found in a wide variety of coastal and geodynamical settings, including subduction zones on the Pacific coasts of South and North America, the passive margin of the Atlantic coast of South America, and collision contexts on the Pacific coast of Kamchatka, Papua New Guinea, New Zealand, and Japan. They also occur on the passive margin of the South China Sea coast, on west-facing Atlantic coasts such as Donegal Bay in Ireland, various sites in Cornwall and Wales, Jura and the Isle of Arran in Scotland, Finistère in Brittany, Galicia in Northern Spain, and at Squally Point in Nova Scotia.
Important sites include Turakirae Head near Wellington, New Zealand, one of the world's best-studied examples, and a well-defined sequence of uplifted marine terraces from the late Quaternary at Tongue Point along Cook Strait. On New Zealand's North Island at the eastern Bay of Plenty, a sequence of seven marine terraces has been studied. An especially prominent marine terraced coastline occurs north of Santa Cruz, near Davenport, California, where terraces have probably been raised by repeated slip earthquakes on the San Andreas Fault.
Hans Jenny researched the pygmy forests of the Mendocino and Sonoma county marine terraces, and the marine terrace's 'ecological staircase' of Salt Point State Park is also bound by the San Andreas Fault.
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