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1.
We use the radiocarbon ages of marine shells and terrestrial vegetation to reconstruct relative sea level (RSL) history in northern Southeast Alaska. RSL fell below its present level around 13,900 cal yr BP, suggesting regional deglaciation was complete by then. RSL stayed at least several meters below modern levels until the mid-Holocene, when it began a fluctuating rise that probably tracked isostatic depression and rebound caused by varying ice loads in nearby Glacier Bay. This fluctuating RSL rise likely reflects the episodic but progressive advance of ice in Glacier Bay that started around 6000 cal yr BP. After that time, RSL low stands probably signaled minor episodes of glacier retreat/thinning that triggered isostatic rebound and land uplift. Progressive, down-fjord advance of the Glacier Bay glacier during the late Holocene is consistent with the main driver of this glacial system being the dynamics of its terminus rather than climate change directly. Only after the glacier reached an exposed position protruding into Icy Strait ca. AD 1750, did its terminus succumb - a century before the climate changes that marked the end of the Little Ice Age - to the catastrophic retreat that triggered the rapid isostatic rebound and RSL fall occurring today in Icy Strait.  相似文献   

2.
We present results from an investigation of relative sea-level changes in the Qaqortoq area in south Greenland from c. 11 000 cal. yr BP to the present. Isolation and transgression sequences from six lakes and two tidal basins have been identified using stratigraphical analyses, magnetic susceptibility, XRF and macrofossil analyses. Macrofossils and bulk sediments have been dated by AMS radiocarbon dating. Maximum and minimum altitudes for relative sea level are provided from two deglaciation and marine lagoon sequences. Initially, relative sea level fell rapidly and reached present-day level at ∼9000 cal. yr BP and continued falling until at least 8800 cal. yr BP. Between 8000 and 6000 cal. yr BP, sea level reached its lowest level of around 6-8 m below highest astronomical tide (h.a.t.). At around 3750 cal. yr BP, sea level has reached above 2.7 m below h.a.t. and continued to rise slowly, reaching the present-day level between ∼2000 cal. yr BP and the present. As in the Nanortalik area further south, initial isostatic rebound caused rapid isolation of low elevation basins in the Qaqortoq area. Distinct isolation contacts in the sediments are observed. The late Holocene transgression is less well defined and occurred over a longer time interval. The late Holocene sea-level rise implies reloading by advancing glaciers superimposed on the isostatic signal from the North American Ice Sheet. One consequence of this transgression is that settlements of Palaeo-Eskimo cultures from ∼4000 cal. yr BP may have been transgressed by the sea.  相似文献   

3.
Southwestern Finland was covered by the Weichselian ice sheet and experienced rapid glacio-isostatic rebound after early Holocene deglaciation. The present mean overall apparent uplift rate is of the order of 4-5 mm/yr, but immediately after deglaciation the rate of crustal rebound was several times higher. Concurrently with land uplift, relative sea level in the Baltic basin during the past more than 8000 years was also strongly affected by the eustatic changes in sea level. There is ample evidence from earlier studies that during the early Litorina Sea stage on the southeastern coast of Finland around 7000 yr BP (7800 cal. yr BP), the rise in sea level exceeded the rate of land uplift, resulting in a short-lived transgression. Because of a higher rate of uplift, the transgression was even more short-lived or of negligible magnitude in the southwestern part of coastal Finland, but even in this latter case a slowing down in the rate of regression can still be detected. We used evidence from isolation basins to obtain a set of 71 14C dates, and over 30 new sea-level index points. The age-elevation data, obtained from lakes in two different areas and located between c. 64 m and 1.5 m above present sea level, display a high degree of internal consistency. This suggests that the dates are reliable, even though most of them were based on bulk sediment samples. The two relative sea-level curves confirm the established model of relatively gradually decreasing rates of relative sea-level lowering since c. 6100 yr BP (7000 cal. yr BP) and clearly indicate that the more northerly of the two study areas experienced the higher rate of glacio-isostatic recovery. In the southerly study area, changes in diatom assemblages and lithostratigraphy suggest that during the early Litorina Sea stage (8300-7600 cal. yr BP) eustatic sea-level rise exceeded land uplift for hundreds of years. Evidence for this transgression was discovered in a lake with a basin threshold at an elevation of 41 m above sea level, which is markedly higher than any previously known site with evidence for the Litorina transgression in Finland. We also discuss evidence for subsequent short-term fluctuations superimposed on the main trends of relative sea-level changes.  相似文献   

4.
While contributing <1 m equivalent eustatic sea‐level rise the British Isles ice sheet produced glacio‐isostatic rebound in northern Britain of similar magnitude to eustatic sea‐level change, or global meltwater influx, over the last 18 000 years. The resulting spatially variable relative sea‐level changes combine with observations from far‐field locations to produce a rigorous test for quantitative models of glacial isostatic adjustment, local ice‐sheet history and global meltwater influx. After a review of the attributes of relative sea‐level observations significant for constraining large‐scale models of the isostatic adjustment process we summarise long records of relative sea‐level change from the British Isles and far‐field locations. We give an overview of different global theoretical models of the isostatic adjustment process before presenting intercomparisons of observed and predicted relative sea levels at sites in the British Isles and far‐field for a range of Earth and ice model parameters in order to demonstrate model sensitivity and the resolving power available from using evidence from the British Isles. For the first time we show a good degree of fit between relative sea‐level observations and predictions that are based upon global Earth and ice model parameters, independently derived from analysis of far‐field data, with a terrain‐corrected model of the British Isles ice sheet that includes extensive glaciation of the North Sea and western continental shelf, that does not assume isostatic equilibrium at the Last Glacial Maximum and keeps to trimline constraints of ice surface elevation. We do not attempt to identify a unique solution for the model lithosphere thickness parameter or the local‐scale detail of the ice model in order to provide a fit for all sites, but argue that the next stage should be to incorporate an ice‐sheet model that is based on quantitative, glaciological model simulations. We hope that this paper will stimulate this debate and help to integrate research in glacial geomorphology, glaciology, sea‐level change, Earth rheology and quantitative modelling. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

5.
Vertical land motion caused by continuing glacial isostatic adjustment is one of several important components of sea‐level change and is not limited just to previously glaciated regions. A national‐scale analysis for the British Isles shows an ellipse of present‐day relative uplift (relative sea‐level fall), ～1.2 mm a?1, broadly centred on the deglaciated mountains of Scotland. The pattern of three foci of relative subsidence, ～1 mm a?1, results from the additional interactions of the deglacial meltwater load on the Atlantic basin and the continental shelf, and the signal due to far‐field ice sheets. At a local scale, sediment compaction can more than double the rate of relative land subsidence. Relative land‐level change (the negative of relative sea‐level change) is not the same as vertical land motion. There is a spatial pattern in the difference between relative land‐level change and vertical land motion, with differences at present of approximately ?0.1 to ?0.3 mm a?1 around the British Isles and +2.5 to ?1.5 mm a?1 globally. For the wider scientific and user community, whether or not the differences are considered significant will depend upon the location, time frame and spatial scale of the study that uses such information. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

6.
Thirty-six new and previously published radiocarbon dates constrain the relative sea-level history of Arviat on the west coast of Hudson Bay. As a result of glacial isostatic adjustment (GIA) following deglaciation, sea level fell rapidly from a high-stand of nearly 170 m elevation just after 8000 cal yr BP to 60 m elevation by the mid Holocene (~ 5200 cal yr BP). The rate of sea-level fall decreased in the mid and late Holocene, with sea level falling 30 m since 3000 cal yr BP. Several late Holocene sea-level measurements are interpreted to originate from the upper end of the tidal range and place tight constraints on sea level. A preliminary measurement of present-day vertical land motion obtained by repeat Global Positioning System (GPS) occupations indicates ongoing crustal uplift at Arviat of 9.3 ± 1.5 mm/yr, in close agreement with the crustal uplift rate inferred from the inferred sea-level curve. Predictions of numerical GIA models indicate that the new sea-level curve is best fit by a Laurentide Ice Sheet reconstruction with a last glacial maximum peak thickness of ~ 3.4 km. This is a 30–35% thickness reduction of the ICE-5G ice-sheet history west of Hudson Bay.  相似文献   

7.
The relative rate of rise of the sea levels measured by a tide gauge is made of a sea and a land component. The first is usually restricted to the global short-term effect of melting icecaps and expansion of water mass due to global temperature change. The second is often limited to the regional long-term effects of glacial isostatic adjustment (GIA). Sometimes, the regional subsidence, due to compaction and ground water withdrawal, is considered. Here we show as this assumption of regional subsidence fails to represent the relative sea level patterns of Sandy Hook, NJ, and The Battery, NY, as well as of Venezia Punta Della Salute, Venezia II, Trieste and Trieste II. The subsidence of the tide gauge instrument may only be addressed by the precise monitoring of the tide gauge vs. a Global Navigation Satellite System (GNSS) antenna, even if the GNSS tracking is only recent and not yet very accurate. The relative sea level records are much more complicated than what is thought.  相似文献   

8.
We conducted archaeological and geophysical surveys in order to investigate the submergence of ancient constructions along the coast at Kekova Island in southern Turkey. Submerged archaeological remains were measured with respect to present sea level, and geological features that may have caused subsidence were determined by seismic investigation. We estimated relative sea‐level change taking into account presumed original elevation of constructions associated with the harbor at the time of occupation and compared our results with eustatic–isostatic sea‐level studies in the Mediterranean. Ceramics found along the submerged quay of Kekova Island provided evidence for the time of last use of the harbor structures and suggest that submergence began during the early Byzantine period. Considering relative sea‐level changes, eustatic–isostatic effects, and the period of submergence, we estimate a rate of coastal tectonic subsidence of at least 1.6 mm/yr over the last 1400 years.  相似文献   

9.
The history of the recording and interpretation of the Fennoscandian uplift illustrates the main history of Earth sciences because the results obtained had (and still have) immediate impact of the interpretation of a large number of fundamental problems in Earth sciences. Thanks to a paper of De Geer in 1888, the glacial isostatic origin was established. Fennoscandia became the classic area of glacial isostasy, and its sea level records were used for geophysical calculations of the properties and dynamics of the mantle and crust. The varve dated sea level curve of Lidén (1938) from the center of uplift provided an exceptionally well dated record. With the radiocarbon method, the records of shorelines and shorelevel displacement curves were drastically improved providing a totally new basis for the understanding of the geodynamics of the Fennoscandian uplift and for the geophysical interpretation of the data obtained. This is especially true in combination with the repeated levelling data obtained during the last decades for Finland and Sweden.The Late Cenozoic long term movements of the Fennoscandian Shield are characterized by a considerable subsidence. The postglacial uplift of Fennoscandia is complex (an exponential and a linear factor) and caused by two different mechanisms. The total absolute movement in relation to the last glaciation is an elliptic uplift cone of 830 m height surrounded by a subsidence through of 170 m height. The mass in the uplift cone and in the subsidence through is as 1:1 with a volume 0.7 × 106km3. The disappearence/appearance of mass give evidence of a mass transfer in a low viscosity asthenosphere. The properties and conditions of the asthenosphere are found to be: 1–10 × 1020 Poises in viscosity, 3 × 10–14 – 3 × 10–16 sec–1 in strain rates, 0.7% of the melting temperature, 3 mm in grain size, and 5–0.4 bar in stress. The main isostatic uplift (the exponential factor) originates from an asthenospheric dislocation glide process which in early-mid Holocene time changed over into a diffusion creep process. The present linear uplift factor (identified through the last 8000 yrs) seems to originate from mesospheric motions under the following approximate conditions: 0.6% of the melting temperature, 2 × 1022 Poises in viscosity, 3 × 10–16 sec–1 in strain rates and 8 bars in stress. Uplift irregularities and neotectonism are frequently established and often reveal an old geodynamic inheritance (e.g. the Pre-Baikalian/Gothian bedrock seam of high geodynamic activity). The peak rates of glacial isostasy are associated with intensive fracturing, faulting and seismic activity.  相似文献   

10.
Late-Quaternary sea level changes in the eastern Aleutian Islands are of paramount importance in the reconstruction of the migrations and environment of the ancient Aleuts. A radiocarbon-dated ash stratigraphy provides the chronology into which geomorphic events can be fitted. These provide evidence for the sea level changes. Deployment of beach material and coastal configuration intimate that sea level was about 2–3 m above the present level about 8250 radiocarbon yr BP. Beach deposits suggest that sea level remained high until about 3000 radiocarbon y.a. when it gradually dropped to its present position. It is concluded that the ancient Aleuts that settled Anangula about 8400 y.a. used boats; all major passes in the eastern Aleutians were flooded, and did not have winter ice. Those ancient Aleuts did not have available the major year-around food resources of the present strandflats as they were cut during the high sea level stand 8250–3000 yr BP. The ancient Aleuts must have been marine oriented, for land-based food resources would have been limited.The cause of relative sea level changes on Umnak Island is considered indeterminate with present data. Eustatic, glacial isostatic, water isostatic, tectonic, and volcanic causes are considered the main possible controls in combinations such that a basic eustatic sea level curve and likely a glacial-water isostatic curve must be common to any solution. Representative solutions are given to illustrate some of the problems.  相似文献   

11.
The Cyprina Clay at Mommark, southern Denmark, contains a unique record of a shallow threshold-controlled fjord environment through an entire interglacial. Today, the area suffers from periodic severe hypoxia, and the Eemian mollusc faunas (dominated by the hypoxia-tolerant Corbula gibba and Arctica islandica ), the sediments' varying content of pyrite, calcite and gypsum, as well as evidence of shell-dissolution all show that the area also suffered from oxygen stress during the Eemian. A comparison with previous faunal studies on Cyprina Clay indicates that the entire southwestern Baltic region suffered from oxygen stress of varying degree throughout the Eemian. At Mommark, the severity of hypoxia varies inversely with water depth, indicating control by sea level and water depth over the basin threshold. After an early phase of eustatic sea level rise came a prolonged period of isostatic emergence and regression. Although this isostasy pattern is different from the Holocene, the present isostatic uplift of the Baltic entranceways may have the same effect and add to the anthropogene hypoxia threat in the Baltic region.  相似文献   

12.
Two previously undocumented Pleistocene marine transgressions on Wrangel Island, northeastern Siberia, question the presence of an East Siberian or Beringian ice sheet during the last glacial maximum (LGM). The Tundrovayan Transgression (459,000–780,000 yr B.P.) is represented by raised marine deposits and landforms 15–41 m asl located up to 18 km inland. The presence of high sea level 64,000–73,000 yr ago (the Krasny Flagian Transgression) is preserved in deposits and landforms 4–7 m asl in the Krasny Flag valley. These deposits and landforms were mapped, dated, and described using amino acid geochronology, radiocarbon, optically stimulated luminescence, electron spin resonance, oxygen isotopes, micropaleontology, paleomagnetism, and grain sizes. The marine deposits are eustatic and not isostatic in origin. All marine deposits on Wrangel Island predate the LGM, indicating that neither Wrangel Island nor the East Siberian or Chukchi Seas experienced extensive glaciation over the last 64,000 yr.  相似文献   

13.
The coastal zone of Norrbotten, northern Sweden, was gradually inundated by the Ancylus Lake following the retreating ice margin and forming a highest coastline approximately 210 m above the present sea level. The succeeding shore displacement is reconstructed based on lithological investigations and radiocarbon datings of identified isolation sequences from 12 cored lake basins. The highest lake basins, along with two basins above the highest shoreline, suggest ice-free conditions already at 10 500 cal. yr BP. This is at least 500 years earlier than previously thought and implies rapid ice-sheet break-up in the Gulf of Bothnia. The shore displacement (RSL) curve represents a forced regression of successively decreasing rate through the Holocene, from 9 m/100 yr to 0.8 m/100 yr. During the first 1000-1200 years, the isostatic uplift is exponentially declining, followed by a constant uplift rate from c. 9500 cal. yr BP to 5500-5000 cal. yr BP. The last 5000 years seem to be characterized by a low but constant rebound rate. The development of the Ancylus Lake stage of the Baltic may also be discerned in the Norrbotten RSL curve, suggesting that the chronology of the Ancylus Lake stages may have to be revised. The Littorina transgression is also reflected by the RSL curve shape. In addition, a series of early to mid-Holocene beach terraces were OSL-dated to allow for comparison with the 14C-dated shore displacement curve. Interpretations of these ages and their relation to former sea levels were clearly more problematic than the dating of the lake basin isolations.  相似文献   

14.
Pham  Dat T.  Switzer  Adam D.  Huerta  Gabriel  Meltzner  Aron J.  Nguyen  Huan M.  Hill  Emma M. 《Natural Hazards》2019,98(3):969-1001

With sea levels projected to rise as a result of climate change, it is imperative to understand not only long-term average trends, but also the spatial and temporal patterns of extreme sea level. In this study, we use a comprehensive set of 30 tide gauges spanning 1954–2014 to characterize the spatial and temporal variations of extreme sea level around the low-lying and densely populated margins of the South China Sea. We also explore the long-term evolution of extreme sea level by applying a dynamic linear model for the generalized extreme value distribution (DLM-GEV), which can be used for assessing the changes in extreme sea levels with time. Our results show that the sea-level maxima distributions range from ~?90 to 400 cm and occur seasonally across the South China Sea. In general, the sea-level maxima at northern tide gauges are approximately 25–30% higher than those in the south and are highest in summer as tropical cyclone-induced surges dominate the northern signal. In contrast, the smaller signal in the south is dominated by monsoonal winds in the winter. The trends of extreme high percentiles of sea-level values are broadly consistent with the changes in mean sea level. The DLM-GEV model characterizes the interannual variability of extreme sea level, and hence, the 50-year return levels at most tide gauges. We find small but statistically significant correlations between extreme sea level and both the Pacific Decadal Oscillation and El Niño/Southern Oscillation. Our study provides new insight into the dynamic relationships between extreme sea level, mean sea level and the tidal cycle in the South China Sea, which can contribute to preparing for coastal risks at multi-decadal timescales.

  相似文献   

15.
Marine transgression and regression, i.c. movements of the sea relative to the land, occurred in south-western Scotland during the Holocene epoch. Wigtown Bay may have been more extensive than now, and the (now abandoned) Lochar Gulf may have been in existence at the beginning of the Epoch. Holocene marine transgression in S. Ayrshire began after 8400 B.P. Commencement of marine transgression from place to place along the northern shore of the Solway Firth was diachronous: between 9400 and 7200 B.P. Culmination also was diachionous; the Lochar Gulf was abandoned by the sea by 6600 B.P., but transgression continued in the eastern Solway Firth until about 5600 B.P., and near the head of Wigtown Bay until 5000 B.P. or later. A pause in regression of the sea from its maximum occurred about 2000 B.P. Comparison of curves for relative movements of sea level suggests that land uplift in S.W. Scotland between 9200 and 6000 B.P. was rapid compared with contemporaneous land uplift in N.W. England, Sweden, and the Netherlands.  相似文献   

16.
Shennan  Ian  Tooley  Michael  Green  Frances  Innes  Jim  Kennington  Kevin  Lloyd  Jeremy  Rutherford  Mairead 《Geologie en Mijnbouw》1998,77(3-4):247-262
Analyses of geomorphologically contrasting sites in Morar, NW Scotland, describe the forcing mechanisms of coastal change. Isolation basins (i.e. basins behind rock sills and now isolated from the sea following isostatic uplift) accumulated continuous marine and freshwater sediments from c.12 to 2 ka BP. Raised dune, marsh and wetland sites register breaching, migration and stability of dunes from c. 9 to 2 ka BP. High-resolution methods designed to address issues of macroscale and microscale sea-level changes and patterns of storminess include 1-mm sampling for pollen, dinocyst and diatom analyses, infra-red photography, X-ray photography and thin-section analysis. The data enhance the record of relative sea-level change for the area. Major phases of landward migration of the coast occurred during the period of low sea-level rise in the mid-Holocene as the rate of rise decreased from c. 3 to < 1 mm/year. Relative sea-level change controls the broad pattern of coastal evolution at each site; local site-specific factors contribute to short-term process change. There is no record of extreme events such as tsunami. Within a system of dynamic metastable equilibrium, the Holocene records show that site-specific factors determine the exact timing of system breakdown, e.g. dune breaching, superimposed on regional sea-level rise. The global average sea-level rise of 3 to 6 mm/yr by AD 2050 predicted by IPCC would only partly be offset in the Morar area by isostatic uplift of about 1 mm/yr. A change from relative sea-level fall to sea-level rise, in areas where the regional rate of uplift no longer offsets global processes, is a critical factor in the management of coastal resources.  相似文献   

17.
We present new results for relative sea‐level change for southern Greenland for the interval from 9000 cal. yr BP to the present. Together with earlier work from the same region this yields a nearly complete record from the time of deglaciation to the present. Isolation and/or transgression sequences in one lake and five tidal basins have been identified using lithostratigraphic analyses, sedimentary characteristics, magnetic susceptibility, saturated induced remanent magnetisation (SIRM), organic and carbonate content, and macrofossil analyses. AMS radiocarbon dating of macrofossils and bulk sediment samples provides the timescale. Relative sea level fell rapidly and reached present‐day level at ～9300 cal. yr BP and continued falling until at least 9000 cal. yr BP. Between 8000 and 6000 cal. yr BP sea level reached its lowest level of around ～10 m below highest astronomical tide. At around 5000 cal. yr BP, sea level had reached above 7.8 m below highest astronomical tide and slowly continued to rise, not reaching present‐day sea level until today. The isostatic rebound caused rapid isolation of the basins that are seen as distinct isolation contacts in the sediments. In contrast, the late Holocene transgressions are less well defined and occurred over longer time intervals. The late Holocene sea‐level rise may be a consequence of isostatic reloading by advancing glaciers and/or an effect of the delayed response to isostatic rebound of the Laurentide ice sheet. One consequence of this transgression is that settlements of Palaeo‐Eskimo cultures may be missing in southern Greenland. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

18.
The phenomenon of the rise of the Scandinavian shield during the Holocene and the concomitant fall in level of the Baltic Sea has been investigated for centuries. Already in medieval times, there were reports about the coastlines of the Gulf of Bothnia that are full of relevant observations. During the eighteenth century, scientists such as Celsius and Linnaeus collected observations such as these. The result was that the search for the possible explanations of this rise-and-fall phenomenon intensified. The generally favoured explanation was that there was an active sinking of sea level in the Baltic rather than an active rising of the land surface in Fennoscandia. This was because water was seen as mobile, in contrast to a „terra firma“. The relevant discussion was often emotional, and here, we try to illustrate it using material from the Geologenarchiv Freiburg (von Hoff, von Buch and Goethe). No more than a few decades later, it became obvious by the theory of Ice Age that both the sea level and the land could be mobile (eustatic sea level changes—glacial isostasy). Additionally, of course, plate tectonics had some influence: Norway is situated at the western end of the Eurasian plate and is part of a passive continental margin. There are still open research problems, many of which can be addressed using modern methods of satellite-based geophysics and geodesy. Some other aspects as the permanent uplift trend of Scandinavia since the Cambrium or the rhythmic to and fro of magma in the upper mantle during the Pleistocene are mentioned.  相似文献   

19.
Deglacial sea‐level index points defining relative sea‐level (RSL) change are critical for testing glacial isostatic adjustment (GIA) model output. Only a few observations are available from North Wales and until recently these provided a poor fit to GIA model output for the British‐Irish Ice Sheet. We present results of an integrated offshore geophysical (seismic reflection), coring (drilling rig), sedimentological, micropalaeontological (foraminifera), biostratigraphical (palynology) and geochronological (AMS 14C) investigation into a sequence of multiple peat/organic sediment horizons interbedded within a thick estuarine–marine sequence of minerogenic clay‐silts to silty sands from the NE Menai Strait, North Wales. Ten new sea‐level index points and nine new limiting dates from the Devensian Late‐glacial and early Holocene are integrated with twelve pre‐existing Holocene sea‐level index points and one limiting point from North Wales to generate a regional RSL record. This record is similar to the most recent GIA predictions for North Wales RSL change, supporting either greater ice load and later deglaciation than in the GIA predictions generated before 2004, or a modified eustatic function. There is no evidence for a mid‐Holocene highstand. Tidally corrected RSL data indicate initial breaching of the Menai Strait between 8.8 and 8.4 ka BP to form a tidal causeway, with final submergence between 5.8 and 4.6 ka BP. Final breaching converted the NE Menai Strait from a flood‐dominated estuary into a high energy ebb tidal delta with extensive tidal scouring of pre‐existing Late‐glacial and Holocene sequences. The study confirms the value of utilising offshore drilling/coring technology to recover sea‐level records which relate to intervals when rates of both eustatic and isostatic change were at their greatest, and therefore of most value for constraining GIA models. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

20.
To elucidate the signature of isostatic and eustatic signals during a deglaciation period in pre‐Pleistocene times is made difficult because very little dating can be done, and also because glacial erosion surfaces, subaerial unconformities and subsequent regressive or transgressive marine ravinement surfaces tend to amalgamate or erode the deglacial deposits. How and in what way can the rebound be interpreted from the stratigraphic record? This study proposes to examine deglacial deposits from Late‐Ordovician to Silurian outcrops at the Algeria–Libya border, in order to define the glacio–isostatic rebound and relative sea‐level changes during a deglaciation period. The studied succession developed at the edge and over a positive palaeo‐relief inherited from a prograding proglacial delta that forms a depocentre of glaciogenic deposits. The succession is divided into five subzones, which depend on the topography of this depocentre. Six facies associations were determined: restricted marine (Facies Association 1); tidal channels (Facies Association 2); tidal sand dunes (Facies Association 3); foreshore to upper shoreface (Facies Association 4); lower shoreface (Facies Association 5); and offshore shales (Facies Association 6). Stratigraphic correlations over the subzones support the understanding of the depositional chronology and associated sea‐level changes. Deepest marine domains record a forced regression of 40 m of sea‐level fall resulting from an uplift caused by a glacio‐isostatic rebound that outpaces the early transgression. The rebound is interpreted to result in a multi‐type surface, which is interpreted as a regressive surface of marine erosion in initially marine domains and as a subaerial unconformity surface in an initially subaerial domain. The transgressive deposits have developed above this surface, during the progressive flooding of the palaeo‐relief. Sedimentology and high‐resolution sequence stratigraphy allowed the delineation of a deglacial sequence and associated sea‐level changes curve for the studied succession. Estimates suggest a relatively short (<10 kyr) duration for the glacio‐isostatic uplift and a subsequent longer duration transgression (4 to 5 Myr).  相似文献   

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