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Raised beach
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Definition
A raised beach or a marine terrace is an emergent coastal landform. Raised beaches and marine terraces are beaches or wave cut platforms raised above the shore line by a relative fall in the sea level.
Around the world, the 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 Oxygen Isotopic Stages (MIS) (for example Johnson, M.E.
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Encyclopedia
Definition
A raised beach or a marine terrace is an emergent coastal landform. Raised beaches and marine terraces are beaches or wave cut platforms raised above the shore line by a relative fall in the sea level.
Around the world, the 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 Oxygen Isotopic Stages (MIS) (for example Johnson, M.E. and Libbey, L.K., 1997. Global review of Upper Pleistocene (Substage 5e) Rocky Shores: tectonic segregation, substrate variation and biological diversity. Journal of Coastal Research,
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.
Origin
It is now widely viewed that marine terraces are formed during the separated highstands of interglacial stages correlated to Marine Oxygen Isotopic Stages (MIS) (James et al., 1971; Chappell, 1974; Bull, 1985; Ota, 1986; Muhs et al., 1990).
Tectonical and or eustatical use of marine terrace sequence
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 eustatical 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. Our chronology relies principally on relative dating based on geomorphologic criteria but in all cases we associated the shoreline angle of the marine terraces with numerical ages. The best-represented terrace worldwide is the one correlated to the last interglacial maximum (MISS 5e) (Hearty and Kindler, 1995; Johnson and Libbey, 1997, Pedoja et al, 2006 a, b, c). Age of MISS 5e is arbitrarily fixed to range from 130 to 116 ka (Kukla et al., 2002) but is demonstrated to range from 134 to 113 ka in Hawaii and Barbados (Muhs et al., 2002) with a peak from 128 to 116 ka on tectonically stable coastlines (Muhs, 2002). Older marine terraces well represented in worldwide sequences are those related to MIS 9 (~303-339 ka) and 11 (~362-423 ka) (Imbrie et al., 1984). Compilations show that sea level was 3 ± 3 meters higher during MISS 5e, MIS 9 and 11 than during the present one and –1 ± 1 m to the present one during MIS 7 (Hearty and Kindler, 1995, Zazo, 1999). Consequently MIS 7 (~180-240 ka; Imbrie et al., 1984) marine terraces are less pronounced and sometimes absent (Zazo, 1999). 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 occurs 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.
Other coastal Quaternary morphologies registering uplift
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 above sea level (Pirazzoli et al., 1996 in Rust and Kershaw, 2000). 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.
World wide occurrence
Raised beaches are found in a wide variety of coast and geodynamical background such as subduction on the pacific coast of South America (Pedoja et al., 2006), of North America, passive margin of the Atlantic coast of South America (Rostami et al., 2000), collision context on the Pacific coast of Kamchatka (Pedoja et al, 2006), Papua New Guinea, New Zealand, Japan (Ota and Yamaguchi, 2004), passive margin of the South China sea coast (Pedoja et al., in press), 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, the Isle of Jura and Isle of Arran in Scotland, Finistère in Brittany and Galicia in Northern Spain.
See also
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