Raised beach

Did You Know?

• For exact interpretations of morphology, surveying and mapping of marine terraces can include stereoscopic aerial photographic interpretation at scales of about 1:10,000–25,000.
• Notch types form a continuum from wave notches formed in quiet conditions at sea level to surf notches formed in more turbulent conditions as much as 2 m above sea level.

Morphology

A marine terrace’s platform typically slopes at 1 to 5 degrees, shaped by the ancient tidal range, and its profile is usually straight or slightly curved inward. Its width varies greatly, sometimes reaching a kilometer, and it tends to differ between the northern and southern hemispheres. The steepness of the cliff walls bordering the platform depends on how much marine versus land-based erosion shaped them. Where the old wave-cut platform meets the cliff, a notch or inner edge often remains, marking the shoreline at the highest point of the sea’s advance and thus a past sea level. Nearly flat platforms often end in a low-tide cliff, likely due to tidal action. These terraces can stretch for dozens of kilometers along the coast. Older terraces get buried under marine, river, or slope debris, while the highest ones are usually more worn away. In places rising faster than a millimeter per year, terraces often match specific warm periods between ice ages; in slower-uplift zones, they may result from multiple cycles of sea-level returns after weathering. Soils on these terraces include planosols and solonetz, and in sheltered spots, sandy material from tsunamis can be found as a parent layer.

Formation

Raised beaches, also known as marine terraces, are now understood to form during the separate high sea-level stands of interglacial periods, which can be linked to marine isotope stages. Their creation is driven by shifts in environmental conditions and tectonic activity in recent geological time. Climate changes cause eustatic sea-level oscillations and isostatic crustal movements, especially between glacial and interglacial phases. Eustatic processes, such as glacioeustasy, alter ocean water volume, leading to shoreline regressions and transgressions. At the peak of the last glacial period, sea level was roughly 100 meters lower than today. Eustatic changes can also stem from sedimento-eustasy or tectono-eustasy. Isostatic processes lift continental crust and its shorelines; in Scandinavia, for instance, uplift currently reaches up to 10 millimeters per year. Eustatic marine terraces generally formed during separate interglacial highstands, while glacioisostatic terraces developed during pauses in isostatic uplift. Most modern marine terrace sequences result from a mix of tectonic coastal uplift and Quaternary sea-level fluctuations. Abrupt tectonic uplifts can create distinct terrace steps, whereas gradual relative sea-level changes may not produce clear terraces.

Correlation and dating

Various methods can be used and combined for dating and correlating marine terraces. The morphostratigraphic approach relies on altitude as a key criterion to distinguish coastlines of different ages, especially in regions of marine regression. Individual terraces can also be correlated based on their size and continuity, and paleo-soils, glacial, fluvial, eolian, and periglacial landforms and sediments may aid correlation. On New Zealand's North Island, tephra and loess have been used to date and correlate marine terraces. The lithostratigraphic approach uses typical sequences of sediment and rock strata to demonstrate sea-level fluctuations through alternations of terrestrial and marine sediments, though unconformities can complicate analysis. The biostratigraphic approach employs remains of organisms such as mollusc shells, foraminifera, or pollen to indicate age; molluscs, in particular, can show properties depending on their depth of sedimentation, allowing estimation of former water depths. Marine terraces are often correlated to marine oxygen isotopic stages (MIS) and can be roughly dated using their stratigraphic position.

Prominent examples

Raised beaches occur in a wide variety of coastal and geodynamical settings, including subduction zones on the Pacific coasts of South and North America, passive margins such as the Atlantic coast of South America, and collision contexts on the Pacific coast of Kamchatka, Papua New Guinea, New Zealand, and Japan. They are also found on west-facing Atlantic coasts in Ireland, Cornwall, Wales, Scotland, Brittany, and Northern Spain, as well as at Squally Point in Nova Scotia. Notable 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 were probably 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 ecological staircase of Salt Point State Park is also bound by the San Andreas Fault.