Tidal-wetland stratigraphy reveals that great plate boundary earthquakes have caused hundreds of kilometers of coast to subside at the Cascadia subduction zone. However, determining earthquake recurrence intervals and mapping the coastal extent of past great earthquake ruptures in this region are complicated by the effects of many sedimentologic, hydrographic, and oceanographic processes that occur on the coasts of tectonically passive as well as active continental margins. Tidal-wetland stratigraphy at many Cascadia estuaries differs little from that at similar sites on passive-margin coasts where stratigraphic sequences form through nonseismic processes unrelated to coseismic land level changes. Methods developed through study of similar stratigraphic sequences in Europe provide a framework for investigating the Cascadia estuarine record. Five kinds of criteria must be evaluated when inferring regional coastal subsidence due to great plate boundary earthquakes: the suddenness and amount of submergence, the lateral extent of submerged tidal-wetland soils, the coincidence of submergence with tsunami deposits, and the degree of synchroneity of submergence events at widely spaced sites. Evaluation of such criteria at the Cascadia subduction zone indicates regional coastal subsidence during at least two great earthquakes. Evidence for a coseismic origin remains equivocal, however, for the many peat-mud contacts in Cascadia stratigraphic sequences that lack (1) contrasts in lithology or fossils indicative of more than half a meter of submergence, (2) well-studied tsunami deposits, or (3) precise ages needed for regional correlation. Paleoecologic studies of fossil assemblages are particularly important in estimating the size of sudden sea level changes recorded by abrupt peat-mud contacts and in helping to distinguish erosional and gradually formed contacts from coseismic contacts. Reconstruction of a history of great earthquakes for the Cascadia subduction zone will require rigorous application of the above criteria and many detailed investigations.

Identifying coseismic subsidence in tidal‐wetland stratigraphic sequences at the Cascadia subduction zone of western North America

Rock coasts, with particular emphasis on shore platforms

During the last very cold period in the British Isles, which occurred more than 10,000 radiocarbon years ago, glaciers developed in many mountain areas, permafrost was extensive, frost break-up of rocks was severe (its effects including a pronounced coastal platform and cliff), downslope movement of debris was greatly accelerated, many rivers spread coarse debris over low ground, the vegetation was transformed and the fauna was greatly affected.

The Loch Lomond Stadial in the British Isles

Sea-level changes in the Holocene

The errors involved in attempting to correlate evidence of sea-level change gathered from disparate areas are discussed as follows:- 1. Errors due to levelling, especially in deep boreholes and at sea. 2. Problems of identifying the horizon to be levelled, and uncertainties that this horizon does represent the intended relationship to water-table. 3. Uncertainties in relating recognised changes in water-table, as indicated by vegetation or sediments to sea-level. 4. Errors in the estimation of present-day tidal levels, particularly in areas with a large tidal range. 5. Variations in tidal levels over time, especially in areas such as the English Channel and estuaries, where large changes in the coastline may have occurred. 6. The importance of the rare event, e.g. storm surges; and astronomical causes of variations in tidal levels. 7. Consolidation of sediments as a result of gravitational compaction. 8. Changes in the relationship of vegetation zones to sea-level, during periods of rapid sea-level rise. 9. Variations in the closeness of correspondence between sedimentation rates and sea-level rise. 10. Errors in radiocarbon dating. All the available ‘sea-level’ radiocarbon dates for Wales, S.W. England and the Channel Islands are plotted on the same diagram, as error ellipses. No significant difference in sea-level rise is apparent between S.W. England and Cardigan Bay. Only in the case of N. Wales is there a strong suggestion that isostatic uplift has had an appreciable effect in the last 8000 years. Important differences result from the use of M.H.W.S.T. or M.H.W.O.T. as the datum level. Any oscillations in the course of sea-level rise are smaller than the other uncertainties. No evidence of sea-levels higher than the present is seen.

Sea-level changes in southwest England and Wales

Interest in studies of amino acid racemisation as a framework for Quaternary chronostratigraphy has increased recently1–4. These studies have used different materials, but the main two substances have been bone and shell. Different shell genera can have different rates of racemisation5, so the use of new genera requires some study of the kinetics of the reactions before the results from one taxa can be compared to another. We report here the successful application of amino acid racemisation studies on the limpet Patella vulgata as a basis for correlating the scattered raised marine beaches of interglacial age(s) from south-west England and Wales. We show that the epimerisation of D alloleucene: L isoleucene (D allo: L iso) in P. vulgata, provides a useful basis for regional correlation. This mollusc lives attached to rocks above the mean tide level and, hence, spends a good portion of its life living at the ambient air temperature. We suggest that the fossil material may have been primarily exposed to regional atmospheric climate since death, because all available evidence points to sea level being lower than present during much of the past 120,000 yr (ref. 6).

Amino acid ratios and the correlation of raised beach deposits in south-west England and Wales

RELATIVE sea level changes result from eustatic variations in water level and vertical movements of the land, from tectonic or isostatic causes. If these changes are to be properly understood, it is important to define the eustatic component. Devoy1 has presented a relative Holocene sea level curve for the Thames Estuary, an area of known downwarping throughout the Quaternary and earlier time. If predictions of future relative sea levels in South-east England are to be made, the contributions of the worldwide post-glacial rise of sea level and of regional subsidence in the Flemish Bight have to be evaluated. One approach to the problem is to compare results from areas of tectonic instability with those from more stable areas. In the British Isles, the only area which can be regarded as stable is in the south and west, as far removed as possible, on the one hand, from the subsidence of the south-east, and on the other hand from isostatic rebound from ice-loading centred in North-west Scotland. Although theoretical objections to long-term stability, even in this region, can be adduced2, the geomorphological evidence in South-west England suggests that, at least in the Holocene and later Pleistocene, the area can be regarded as stable3. We presented in 1973 (ref. 4) a sea level curve based on detailed work in the Bristol Channel, which we regarded, in the absence of any evidence of Holocene isostatic and tectonic activity, as representative of the eustatic sea level rise. In view of evidence of continuing isostatic rebound in South-west Scotland5, studies were subsequently initiated in Cardigan Bay, particularly in the neighbourhood of Aberystwyth and Borth, to try to circumscribe the areas of stability and those of continuing recovery from ice-loading. The results are compared here with our previous ones.

Holocene eustatic sea level change

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Amino acid ratios and the correlation of raised beach deposits in south-west England and Wales

Holocene eustatic sea level change

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