Relative Sea-level Curve Research Articles

An Incipiently Drowned Platform Deposit in Cyclic Ordovician Shelf Sequence: Lower Ordovician Chepultepec Formation, Virginia: ABSTRACT

The Chepultepec interval, 145 to 260 m (476 to 853 ft) thick, in Virginia contains the Lower Member up to 150 m (492 ft) thick, and the Upper Member, up to 85 m (279 ft) thick, of peritidal cyclic limestone and dolomite, and a Middle Member, up to 110 m (360 ft) thick, of subtidal limestone and bioherms, passing northwestward into cyclic facies. Calculated long term subsidence rates were 4 to 5 cm/1,000 yr (mature passive margin rates), shelf gradients were 6 cm/km, and average duration of cycles was 140,00 years. Peritidal cyclic sequences are upward shallowing sequences of pellet-skeletal limestone, thrombolites, rippled calcisiltites and intraclast grainstone, and laminite caps. They formed by rapid transgression with apparent submergence increments averaging approximately 2 m (6.5 ft) in Lower Member and 3.5 m (11.4 ft), Upper Member. These submergence increments may have resulted from small (about 1 m; 3.2 ft) relative sea level rise, coupled with subsidence due to loading by water and accumulating sediment. Progradation of tidal flat facies did not occur until subsidence had ceased. Tidal flats were shifted westward from 70 to 380 km (43 to 236 mi) after each submergence event, but in peritidal sequences, had sufficient time to prograde back into the shelf. Deposition during Middle Member time was dominated by skeletal limestone-mudstone, calcisiltite with storm generated fining-upward sequences, and burrow-mixed units that were formed near fair-weather wave base, along with thrombolite bioherms (subwave base to wave agitated shallow water settings). Locally, there are upward shallowing sequences, (subtidal cycles) of basal wackestone/mudstone to calcisiltite to bioherm complexes (locally with erosional scalloped tops). Apparent submergence increments during Middle Member deposition averaged 7.0 m (23 ft), ranging up to 23 m (75 ft) in southeastern belts. Following each submergence, carbonate sedimentation was able to build to sea level prior to renewed submergence. Large submergence End_Page 428------------------------------ events caused tidal flats to be shifted far to the west, and they were unable to prograde out onto the open shelf because of insufficient time before subsidence was renewed, and because the open shelf setting inhibited tidal flat deposition. The Middle Member represents an incipiently drowned sequence that developed by repeated submergence events. Such incipiently drowned shelf sequences are common but poorly documented in the geologic record. They punctuate many aggraded cyclic shelf sequences and may ultimately provide important information for construction of relative sea level curves for Paleozoic. End_of_Article - Last_Page 429------------

Taconic Foreland Basin graptolites: age zonation, depth zonation, and use in ecostratigraphic correlation

Late Middle Ordovician graptoloids are stratigraphically zoned by depth of deposition as well as by age along an 83 km long downslope transect through a roughly four million year long continental-shelf and outer-trench slope sequence in the Mohawk Valley, New York. The distribution of graptoloid assemblages parallels the distribution of sediment types and benthic macroinvertebratc assemblages in showing the general, convergence-related marine transgression, secondary transgressive and regressive pulses, and topographic irregularities related to syndepositional block faulting. The relationship of ordination scores for graptoloid assemblages (which arc based on relative abundances of genera) to the samples' positions on downslope transects along bentonite beds, and to depth-related ordination scores for benthic macroin-vertebrate assemblages in the same samples throughout the sequence, quantitatively demonstrates zonation by depth of deposition. Rather like modern pelagic colonial tunicates, graptoloids were evidently zoned by depth in the water column, with Orthograptus spp. predominating in offshore waters above the oxygen minimum zone, Climacograptus spp. near the top of the minimum zone at 300 to 500 m depths, and Corynoides spp. well within the minimum zone. Destruction of sinking rhabdosomes by decomposers evidently accentuated zonation by depth of deposition, and may have contributed to the strong, broad oxygen minimum zone thought to have been characteristic of graptolitic shale basins. Stratigraphic sections' approximate relative sea level curves, calibrated in terms of graptoloid assemblage ordination score, give ecostratigraphic correlations of 2–3 × 105 year average accuracy when statistically cross-correlated between sections, and when cross-correlated with similar curves based on benthic macroinvertebratc assemblage ordination score. □Graptolithina, age zonation, depth zonation, paleoeco-logy, Middle Ordovician, ecostratigraphy, coenocorrelation.

On the reconstruction of pleistocene ice sheets: A review

Pleistocene ice sheets can be reconstructed through three separate approaches: (1) Evidence based on glacial geological studies, such as erratic trains, till composition, crossing striations and exposures of multiple tills/nonglacial sediments. (2) Reconstructions based on glaciological theory and observations. These can be either two- or three-dimensional models; they can be constrained by ‘known’ ice margins at specific times; or they can be ‘open-ended’ with the history of growth and retreat controlled by parameters resting entirely within the model. (3) Glacial isostatic rebound after deglaciation provides a measure of the distribution of mass (ice) across a region. A ‘best fit’ ice sheet model can be developed that closely approximates a series of relative sea level curves within an area of a former ice sheet; in addition, the model should also provide a reasonable sea level fit to relative sea level curves at sites well removed from glaciation. This paper reviews some of the results of a variety of ice sheet reconstructions and concentrates on the various attempts to reconstruct the ice sheets of the last (Wisconsin, Weischelian, Würm, Devensian) glaciation. Evidence from glacial geology suggests flow patterns at variance with simple, single-domed ice sheets over North America and Europe. In addition, reconstruction of ice sheets from glacial isostatic sea level data suggests that the ice sheets were significantly thinner than estimates based on 18 ka equilibrium ice sheets (cf. Denton and Hughes, 1981). The review indicates it is important to differentiate between ice divides, which control the directions of glacial flow, and areas of maximum ice thickness, which control the glacial isostatic rebound of the crust upon deglaciation. Recent studies from the Laurentide Ice Sheet region indicate that the center of mass was not over Hudson Bay; that a major ice divide lay east of Hudson Bay so that flow across the Hudson Bay and James Bay lowlands was from the northeast; that Hudson Bay was probably open to marine invasions two or three times during the Wisconsin Glaciation; and that the Laurentide Ice Sheet was thinner than an equilibrium reconstruction would suggest.

A comparison of observed and theoretical postglacial relative sea level in Atlantic Canada

Two extreme models of late Wisconsinan ice cover in Atlantic Canada and the northeastern U.S.A. are shown to produce postglacial relative sea level curves that bracket existing field observations at six sites throughout the region. This suggests that the true late Wisconsinan ice distribution is probably intermediate to the two contrasting reconstructions proposed. Both ice models predict the existence of four relative sea level zones: an innermost zone closest to the centre of glaciation in which relative sea level falls continuously throughout postglacial time; an outermost zone in which it rises continuously; and two transitional zones in which it first falls and then rises in varying proportions according to the distance from the ice margin. The distinctive forms of the relative sea level curves are probably representative of each of the zones and are unlikely to be significantly perturbed even by large local ice readvances. They, therefore, establish patterns with which future field data are expected to conform. The form that the geological record of relative sea level change is likely to take within each zone is discussed and promising settings for the collection of new data are proposed. The common practice of separating relative sea level into an isostatic and a eustatic component is analysed and shown to be incorrect as usually applied. The practice is also shown to be unnecessary because the models discussed in this paper predict changes in relative sea level that can be compared directly with the observations.

Preservation potential of transgressive coastal lithosomes on the U.S. Atlantic shelf

Migration of coastal lithosomes across the continental shelf is a response to the latest Quaternary rise in sea level. Variable fractions of the transgressive sequence may be preserved, depending on pre-existing topography, depth of erosion, wave energy, sediment supply, erosion resistance, tidal range, and rate of relative sea-level change. Materials at greater depth in the stratigraphic column are more likely to be preserved. Sediment samples, seismic profiles, and bathymetric observations, indicate better preservation of shoreline elements on the outer shelf and more reworking on the inner shelf. This is hypothesized to be due primarily to the rate of sea-level rise. A relative sea-level curve for Delaware, obtained from radiocarbon dates on basal peats, rises smoothly from 25 m below present sea level 10,000 years B.P. The rate of rise decreases with time. Sea-level rise resulted in rates of coastal retreat of 20 m/yr 10,000 years B.P., 5 m/yr 5000 years B.P., and 1.5 m/yr at present. The long-term average rate of coastal retreat throughout the Holocene was 10 m/yr. In a conceptual model of constant volume of net erosion per unit length of coast, smaller depths of erosion are envisioned during the period of rapid coastal retreat early in the transgression, allowing a greater preservation potential. During the present slower rate of sea-level rise and coastal retreat, depths of erosion are greater, averaging 10 m off Delaware. In this situation there is a decreased potential for preservation of the coastal environmental lithosomes in a continuing transgression. Changes in factors such as wave climate, sediment supply, tidal range, and tectonics affect this simple model.

Submarine terraces and morphology in the Kieler Bucht, Western Baltic, and their relation to late Quaternary events

From analysis of the densest network of echogrammes yet available for the Kieler Bucht, Western Baltic, the submarine erosional terraces have been mapped. In general these correlate well with the features mapped and described by Kolp (1976) for the adjacent Mecklenburger Bucht. The lower terraces are at ‐ 30 m and may reach 2100 m in broadness. Compared to present day conditions, the rates of cliff retreat at the time of formation were evidently much accelerated, due possibly to harsher climatic conditions including a greater intensity of winter lake ice, frosts, wind and rain. Other terraces at ‐ 27, ‐ 24, ‐ 19, and ‐ 14 m were identified, and these are related to syngressions in the various eustatic curves applicable to the Western baltic. A hypsometric curve for the submarine terrain of the Kieler Bucht, when compared to the relative sea level curve shows that 65% of the bay was transgressed in only 700 years. Maximum sedimentation rates in the Kieler Bucht should have occurred at this time, and independent data from dated cores from Vejsnäs Rinne support this prediction.

HOLOCENE MARINE TERRACES AND SEISMIC CRUSTAL MOVEMENTS IN THE SOUTHERN PART OF BOSO PENINSULA, KANTO, JAPAN

Prominent Holocene marine terraces classified into four levels in southern Boss, have recorded the history of seismic crustal deformation by large earthquakes along the Sagami trough. The mode and amount of the crustal movements are discussed based on the relative sea-level curve reconstructed from topographical and chronological data including 36 14 C dates. 1. The relative sea-level curve indicates four times sudden sea-level drops at 6, 150 y. B. P., 4, 350 y. B. P., 2, 850y. B. P. and 250y. B. P., caused by seismic upheavals associated with -earthquakes like that of Genroku Taisho type (Fig. 5). 2. The duration of interseismic period relates to the amount of upheaval associated with the preceding earthquake which measures 5.5m, 5m, 7m and 5.5m respectively. 3. Using the method of least squares, long-term mean uplift rate is calculated 3.0mm/year based on the relative altitude of sea-level (H) and the period (T) just before each earthquake (Fig. 7). 4. Trend of sea-level change deduced from difference between the long-term mean uplift rate and the uplift rate calculated by each earthquake, exhibits general regression toward 2, 700y. B. P. and transgression thereafter. 5. The same kind of seismic crustal movement and trend of sea-level change are also revealed from the data of Holocene emerged coral reefs in Kikaijima, the Ryukyu Islands (Figs. 6 and 7).

Fractional Preservation of Transgressive Coastal Lithosomes on Atlantic Continental Shelf: ABSTRACT

Migration of coastal environmental lithosomes across the continental shelf is a response to the latest Quaternary rise of the sea. Preservation of fractions of the transgressive sequence is dependent on depth of erosion, which is a function of impinging wave energy, sediment supply, resistance to erosion, and rate of relative sea-level change. Materials deeper in the column have a greater potential for preservation. The relative sea-level curve for Delaware, based on C14-dated basal peats, rises smoothly from 25 m below present from 10,000 years B.P. to the present at a decreasing rate with time. Shells and peats 9,000 to 10,000 years old on the shelf are 40 m deeper, suggesting an east-southeast shelf tilt, tectonically or hydro-isostatically induced. Sea-leve rise results in rates of coastal retreat of 102 m/year for 10,000 years B.P., 101 m/year for 5,000 years B.P., and 100 m/year at present. In a model of constant volume of net erosion per unit length of coast, a much smaller depth of erosion applies early in the transgression, allowing a greater preservation potential. Changes in wave climate, sediment supply, and downwarping across the shelf also apply. Recovered sediments, seismic profiles, and recognized morphic features indicate better preservation of shoreline elements on the outer shelf, and more planing off and reworking on the inner shelf. Similar analysis of Delaware Bay indicates that it too follows such a model, in changing from a dendritic fluvial system to a broad estuary. End_of_Article - Last_Page 675------------

Flandrian sea level changes and vegetational history of the lower Thames estuary

A biostratigraphic study, investigating the interleaved Flandrian biogenic and inorganic deposits of the lower Thames Estuary, has been carried out between central London and the Isle of Grain. The vegetational and environmental history, showing the relation of the biogenic deposits to former sea level has been deduced from pollen, diatom and other microfossil studies. Radiocarbon dating has been used to establish an objective chronology. From this evidence the height of relative sea level movements, seen in marine transgression and regression surfaces, have been determined. These are plotted against time to show the rate of relative sea level change and subsidence trends for the Thames Estuary and southern England. Diatom studies show the early importance of marine and brackish water influences, at the beginning of Flandrian sedimentation in the Thames. Pollen and macrofossil analyses demonstrate the strong local effects of the saltmarsh and fen environment upon the vegetational history. The rise of Alms pollen is seen to occur before 8100 years b.p ., probably reflecting local physiographic conditions and valley flooding consequent upon the rising sea level. Elements of the regional vegetation development are recorded however, with the Ulmus and Tilia pollen declines shown. Five regression phases (Tilbury I-V), represented by the biogenic deposits and four marine transgressions (Thames I-IV), together with the possible existence of a fifth (Thames V), are recognized. The relative sea level for mean high water mark of spring tides (m.h.w.s.t.) is shown to rise at about 8500 b.p . from —26.5 m o.d . to above present Ordnance Datum (Newlyn) by about 1750 years b.p . Relative sea level curves for the Thames during Flandrian times correlate well with the form and rate of relative sea level changes shown for northwest Europe. Plotting these graphs against each other has allowed subsidence trends to be shown. Within the Thames, possible differential downwarping of approximately 1.5 m has been identified between Crossness and Tilbury for the Flandrian. The regional trends of west to east and north to south downwarping are supported. The amount of subsidence for southeast England, formerly given as 6.1 m since 6500 b.p ., is not confirmed. The figure for the Thames area relative to the Bristol Channel lies closer to 2-3 m since 7000 b.p., although rates of downwarping vary with the type of environment studied, making generalizations tenuous. Sea level only shows relative subsidence trends and is not as yet seen to provide an accurate fixed datum from which one can give precise figures for land subsidence.

Holocene eustatic sea level change

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.

Global changes in postglacial sea level: A numerical calculation

The sea-level rise due to ice-sheet melting since the last glacial maximum was not uniform everywhere because of the deformation of the Earth's surface and its geoid by changing ice and water loads. A numerical model is employed to calculate global changes in relative sea level on a spherical viscoelastic Earth as northern hemisphere ice sheets melt and fill the ocean basins with meltwater. Predictions for the past 16,000 years explain a large proportion of the global variance in the sea-level record, particularly during the Holocene. Results indicate that the oceans can be divided into six zones, each of which is characterized by a specific form of the relative sea-level curve. In four of these zones emerged beaches are predicted, and these may form even at considerable distance from the ice sheets themselves. In the remaining zones submergence is dominant, and no emerged beaches are expected. The close agreement of these predictions with the data suggests that, contrary to the beliefs of many, no net change in ocean volume has occurred during the past 5000 years. Predictions for localities close to the ice sheets are the most in error, suggesting that slight modifications of the assumed melting history and/or the rheological model of the Earth's interior are necessary.

A reconnaissance survey of late Quaternary sea levels, Bella Bella/Bella Coola region, central British Columbia coast

A preliminary relative sea level curve that covers the last 10 200 years is derived for the area of the islands and outer mainland centered on Bella Bella and Namu, the central coast of British Columbia. The curve shows postglacial emergence of 17 m over this period. The rate of emergence was ~0.6 m/100 year about 9000 BP, and present sea level was attained between 7000 and 8000 BP. Relative sea level continued to fall until the last few hundred to one thousand years BP when a marine transgression led to a rise of sea level and resultant erosion of many coastal Indian middens. Marine limits on the outer islands may reach 120 m asl, whereas in the middle part of the fiord country observed delta surfaces are lower (54–75 m asl). Elevations of raised deltas then attain ~150 m at fiord heads. A readvance of the ice front ≤ 12 210 ± 330 BP (GSC-1351) is suggested by the stratigraphy of one section.

HOLOCENE-EMERGED CORAL REEFS AND SEA-LEVEL CHANGES IN THE RYUKYU ISLANDS

The numerous coral reefs in the Ryukyu Islands provide detailed evidence of the relationship between reef development and late Holocene sea-level changes. The islands belong to the tectonically active island arc and have experienced many destructive earthquakes in the historical period. It is, therefore, assumed that the Holocene reefs have been terraced at various elevations, due to sudden uplift associated with major earthquakes, at various sites during the period of the post-glacial eustatic sea-level changes. In this paper, the authors have attempted to restore the late Holocene sea-level curve as follows. First, cross-profiles of various emerged reefs were constructed, and fossil corals were collected for 14C dating (Figs. 2, 3, 5, 7, 8, 10 and 11). Secondly, 58 fossil corals were dated by the 14C method (Table 1), and relative sea-level curves were constructed for Kodakarajima, Takarajima and Kikaijima islands (Figs. 13, 14 and 15). An attempt was then made to restore the late Holocene sea-level curve, corrected for intermittent seismic uplifts, using the Kikaijima relative sea-level curve (Fig. 15). The amounts of intermittent seismic uplift were calculated using a constant long-term up-lift rate (1.87m/1, 000yrs.) which minimized the difference between pre- and post-uplift sea-levels, the rate being calculated by least squares methods. The restored sea-level curve (Fig. 15) resembles the Shepard curve, which shows a gradual rise of the sea-level towards the present sea-level, rather than the Fairbridge curve, which oscillates considerably during the past 6, 000 years including several stands higher than the present.

Holocene Relative Sea-Level Changes and Coastal Stratigraphic Units on the Northwest Flank of the Baltimore Canyon Trough Geosyncline

ABSTRACT A new local curve of relative sea-level change for the Delaware coastal area is based on 88 radiocarbon dates. The curve is smooth with relatively narrow potential limits of variation in amplitude. It has a somewhat steeper slope than published eustatic sea-level curves and other local relative sea-level curves. Holocene radiocarbon isochrons in Delaware marsh sections are horizontal. Interpretations strongly support the concept that Holocene sea level rose slowly and continuously relative to the Delaware coast; the favored hypothesis is that eustatic rise of sea level was responsible. No seaward tilting is indicated for the Delaware segment of the Atlantic coastal plain or inner shelf. However, radiocarbon-died shallow-water samples of comparable age suggest a strong seaward tilt of the outer continental shelf. The outer shelf over the Baltimore Canyon trough geosyncline has subsided approximately 40 m in the last 10,000 years.

Eustatic changes during the last 8,000 years in view of radiocarbon calibration and new information from the Kattegatt region and other northwestern European coastal areas

In view of the calibration of the non-absolute radiocarbon chronology and the accumulation of new field data and radiocarbon dates, the calculated eustatic and isostatic factors responsible for the shore-level displacement in the Kattegatt region are revised for the period covering the last 8,000 sidereal years. The revision does not lead to any drastic changes: some levels are changed by some decimeters and the age of the transgression maximum PTM-7 is now regarded as younger than previously considered. The revised eustatic curve agrees (even in details) very well with all northwestern European relative sea-level records. The tidal effect in the Kattegatt is less than 1 dm. Possible meteorological influences cannot yet be accounted for. The Kattegatt record can be used to test local relative sea-level curves and supposed eustatic curves. Considering possible horizontal and vertical changes of the main geoid pattern, and other factors influencing the surface distribution of the ocean water, regional eustasy (like the present NW European curve) may not be globally valid in detail. The eustatic amplitude variations reflect the Earth's glacial history. The transgression/regression sequence reflects cyclic variations in the world climate of irregular frequency.

Sea-Level Changes during the Last 9000 Years in North-West England

This paper presents a summary of the results of systematic and detailed biostratigraphical investigations within the lagoonal, tidal flat, perimarine and sand dune zones of north-west England and proposes the establishment of nine distinct periods of marine transgression. A relative sea-level curve is described and tentative correlations are made between periods of relative high sea-level and of climatic amelioration inferred from both marine and terrestrial evidence. The implications of these data for the development of coastal areas are briefly considered.

Late Quaternary Sea-Level Change and Crustal Rise at Boston, Massachusetts, with Notes on the Autocompaction of Peat

The compression of peat beneath its own weight (autocompaction) is discussed, and it is shown that because of this process radiocarbondated samples of salt-marsh peat or peaty sediment, other than very thin samples cut from the base of the deposit, cannot be correlated with sea level without construction of a sea-level change curve from other types of data. With rising sea level there is a maximum thickness of salt-marsh peat for any given productivity of marsh grass. Because of this limitation, most marshes older than about 5500 years B.P. have been drowned. An important effect of peat autocompaction is the intrusion of wood into older peat horizons and the juxtaposition of wood of different ages. Wood, therefore, should be avoided in the dating of peat profiles. The roots of salt-marsh plants generally descend a foot or more beneath the rhizomes, contaminating older horizons insofar as radiocarbon dating is concerned. This contamination tends to make dates from marsh samples err on the young side. Sixteen radiocarbon dates from the Boston area are used to construct a sea-level curve going back to 14,000 years B.P. Relative sea level was at +60 feet or higher 14,000 years B.P., dropping sharply to approximately −70 feet about 10,000 years B.P. From a low of −70 feet, sea level rose steadily to about −2 feet approximately 3000 years B.P. Since then sea level appears to have kept close to its present level, probably fluctuating about a foot during the course of the stillstand. A crustal movement curve, based on the relative sea-level curve for Boston and the eustatic curve, indicates that about 290 feet of crustal rise occurred between 14,000–6000 years B.P., with a maximum rate of uplift of about 0.2 foot per year at 12,750 years B.P., and that from 6000 to 3000 years B.P., crustal subsidence occurred at Boston.