Rate Of Sea-level Fall Research Articles

Delta growth and river valleys: the influence of climate and sea level changes on the South Adriatic shelf (Mediterranean Sea)

Incised valleys across continental margins represent the response of fluvial systems to changes in their equilibrium dynamics, mainly driven by base level fall forced by glacial–eustatic cycles. The Manfredonia Incised Valley formed during the last glacial sea level lowstand, when most of the southern Adriatic shelf was sub-aerially exposed but the outer shelf remained under water. The pronounced upstream deepening of the valley is ascribed to river incision of the MIS5e highstand coastal prism and related subaqueous clinoform under the influence of MIS5-4 sea level fluctuations, while the downstream shallowing and narrowing mainly reflects the impact of increased rates of sea level fall at the MIS3-2 transition on a flatter mid-outer shelf. Until 15 ka BP, the valley fed an asymmetric delta confined to the mid-outer shelf, testifying that continental and deep marine systems remained disconnected during the lowstand. Sea level rise reached the inner shelf during the Early Holocene, drowning the valley and leading to the formation of a sheltered embayment confined toward the land: at this time part of the incision remained underfilled with a marked bathymetric expression. This mini-basin was rapidly filled by sandy bayhead deltas, prograding from both the northern and southern sides of the valley. In this environment, protected by marine reworking and where sediment dispersal was less effective, the accommodation space was reduced and autogenic processes forced the formation of multiple and coalescing delta lobes. Bayhead delta progradations occurred in few centuries, between 8 and 7.2 ka cal BP, confirming the recent hypothesis that in this area the valley was filled during the formation of sapropel S1. This proximal valley fill, representing the very shallow-water equivalent of the cm-thick sapropel layers accumulated offshore in the deeper southern Adriatic basin, is of key importance in following the signature of the sapropel in a facies-tract ideally from the shoreline to the abyss.

Vertebra of a Bowhead whale (Balaena mysticetus) in Late Wisconsinan deglacial marine sediments, Trout River, Newfoundland, and local late-glacial sea-level change

This note reports the geological contexts and radiocarbon and calibrated ages of whalebone and bivalve recovered from emerged deglacial marine sediments at Trout River, Newfoundland (49°28.6’ N, 58°07’ W). The rate of sea-level fall is calculated for the first deglacial millennium, and discrepancies are noted between calibrated ages of bone and shell.

Large‐scale fluvial grade as a nonequilibrium state in linked depositional systems: Theory and experiment

A graded river conveys its sediment load without net aggradation or degradation. Grade is thought to represent the equilibrium state of a river system subject to steady allogenic forcing. We address the concept of grade in linked depositional systems through a combination of mathematical modeling and flume experiments. We develop a moving boundary morphodynamic model of fluviodeltaic sedimentation and use it to quantify the conditions necessary for a state of sustained grade in an alluvial‐deltaic system developing on a uniformly sloping shelf. To test our theory, we conducted a series of flume experiments in which we constructed laboratory‐scale fluviodeltaic systems under specific relative sea level histories. On large spatiotemporal scales, our theoretical and experimental results suggest an alternative view of grade as an intrinsically nonequilibrium state that requires a fall in relative sea level. Under conditions of stillstand or rise in relative sea level, the equilibrium (graded) profile does not exist, and the alluvial river will always aggrade. With a steady sediment supply, alluvial grade can be sustained if relative sea level falls with a specific, square‐root‐of‐time dependence. Common sequence‐stratigraphic conceptualizations of alluvial river response to sea level change suggest that rivers aggrade and degrade in response to relative sea level rise and fall, respectively. We show that rivers can remain aggradational if the rate of sea level fall is sufficiently small. The boundary between aggradational and degradational states is the graded state described here: that associated with a relative sea level fall that varies with the square root of time.

Morphology and facies architecture of a falling sea level strandplain, Umiujaq, Hudson Bay, Canada

AbstractCoastal strandplain deposits near Umiujaq, eastern Hudson Bay, Canada, were formed under falling relative sea level conditions resulting from postglacial isostatic uplift. Ground‐probing radar profiles across the strandplain reveal a lower progradational unit (LPU) discordantly overlain by an upper progradational unit (UPU), which were correlated with stratigraphic sections exposed in incised valley walls. The discordance is a wave erosion surface (WES) that separates fine shoreface sands of the LPU from coarse‐sand and gravel of the UPU. Major basal downlap surfaces can be traced updip into marine terraces and define downstepping wedges. The downstepping is interpreted as representing ‘autocyclic’ morphological reconfiguration rather than a response to changes in the rate of sea level fall. The internal architecture is strongly dependent on the accommodation and thus on antecedent topography. A conceptual model for strandplain deposition under falling sea level incorporates a bipartite shallowing‐upward sandy succession when sufficient accommodation is available. Where accommodation space is limited, a sharp‐based bar‐and‐beach sandbody directly overlies muddy deeper water deposits and the WES resembles a regressive surface of erosion.

Mid-Holocene to recent sea level changes in Peninsular Malaysia: a tectonic implication

This paper tries to analyse the Peninsular Malaysia mid-Holocene to recent relative sea level index points. 56 reliable sea level indicators are scrutinized and their relative sea levels corrected. Attempt is made to explain the relative sea level difference noted between the Peninsular Malaysia West and East coasts. A relative rate of sea level faU of 0.7 and 0.5 mm/year respectively. are indicated from cumulative analysis of the West and East coasts index points. The significant dissimilarity in the rate of sea level fall suggests differential crustal movement between the two coasts. Explanation of hydro- and tectono-isostasy are put forward to explain the observed discrepancy.

Aptian‐Albian sea level history from Guyots in the western Pacific

Relative sea level fluctuations are an important control on patterns of sedimentation on continental margins and provide a valuable tool for regional correlations. One of the main objectives of combined Ocean Drilling Program Legs 143 and 144 was drilling the thick carbonate caps of a suite of seamounts, called guyots, scattered over the northwestern Pacific. The array of drowned Cretaceous banks includes four carbonate banks of Aptian‐Albian age. These particular carbonate banks display emergent surfaces if regional sea level falls faster than the rate of guyot subsidence, or intervals of condensed parasequences and well‐cemented peritidal crypto‐algal flats if the rate of sea level fall is slightly less than guyot subsidence. Rapid rises of sea level following these sequence boundaries are recorded as drowning of the emergent horizons or as pronounced deepening of facies. The cored lithologies and downhole geophysical and geochemical logs were used to identify depositional sequences and surfaces of exceptional shallowing or deepening. A combination of biostratigraphic datums, carbon and strontium isotope curves, relative magnitude of surfaces of emergence, relative thicknesses of depositional sequences, sea level events, and counts of upward shallowing cycles or parasequences were used to correlate sequences among the four sites. After compensating for thermal subsidence rates at each guyot, an identical pattern of major Aptian‐Albian eustatic sea level events is evident throughout this large portion of the Pacific Ocean. There are approximately 12 Aptian and 12 Albian significant sequence boundaries, of which a third were associated with major episodes of emergence. When these events are compared with Aptian‐Albian relative sea level changes observed in European shelf successions, the major sequence boundaries and transgressive surges can be easily correlated, and it appears that both regions also display the same number of minor events. Therefore we can apply the relative timing of these events from the thermal subsidence rates and parasequence counts of the Pacific banks to construct an improved scaling of the associated ammonite zones and biostratigraphic datums in the Aptian‐Albian interval.An electronic supplement of this material may be obtained on adiskette or via Anonymous FTP from KOSMOS.AGU.ORG (LOGINto AGU's FTP account using ANONYMOUS as the username andGUEST as the password. Go to the right directory by typing APEND.Diskette may be ordered from American Geophysical Union, 2000Florida Ave., N.W., Washington, D.C. 20009, $15.00. Payment mustaccompany order.

Integrated Late Eocene‐Oligocene Stratigraphy of the Alabama Coastal Plain: Correlation of Hiatuses and Stratal Surfaces to Glacioeustatic Lowerings

We integrated strontium and oxygen isotopic, biostratigraphic, and magnetostratigraphic studies of two upper Eocene‐Oligocene boreholes drilled near Bay Minette and St. Stephens Quarry (SSQ), Alabama. Continuous coring provided fresh, unweathered material for magnetostratigraphic studies, minimizing problems reported from nearby outcrops. Difficulties with each technique were encountered because of diagenesis, absence of marker fossils, and the presence of unconformities; however, by integrating results from isotopic stratigraphy, biostratigraphy, and magnetostratigraphy, we correlated these relatively shallow‐water deposits to the geomagnetic polarity time scale (GPTS). At the SSQ borehole, the upper Eocene to lower Oligocene section is apparently complete within our stratigraphic resolution (0.2–0.5 m.y.), allowing us to estimate the ages of several stratal surfaces. Late Eocene Sr isotope age estimates are as expected at the SSQ borehole, but Oligocene ages are ∼1 m.y. older than expected due to diagenesis. At the Bay Minette borehole, a latest Eocene‐earliest Oligocene and a late early Oligocene hiatus were detected. We correlate these two hiatuses and stratal surfaces at SSQ with global δ18O increases inferred to represent glacioeustatic lowerings and with evidence for hiatuses on other continental margins: (1) a distinct disconformity at the base of the Chickasawhay Limestone at both boreholes and a hiatus at Bay Minette correlates with a global δ18O increase; we revise the age of this surface (equivalent to the TB 1.1 sequence boundary) making it ∼2 m.y. older than previously reported; and (2) a surface at the top of the Shubuta Member (lowermost Oligocene) has been interpreted both as a condensed section and a disconformity; this surface at SSQ and a hiatus at Bay Minette correlate with a sharp global δ18O increase and with hiatuses on the New Jersey and Irish margins. The timing of the hiatuses and stratal surfaces correlates with the inflection of the δ18O increases and not with the maximum values, supporting models that indicate that unconformities form during the maximum rates of sea level fall.

The effect of sea level changes on the morphology of mountain belts

We have examined quantitatively the effect of large (>200 m) long‐period (>50 m.y.) sea level changes on the development of the landscape of old mountain belts. On the basis of the available data we have assumed that the rate of denudation of these old landscapes is proportional to the average regional elevation by a constant (K ≈ 10−4/1000 years). The results are as follows: (1) In the absence of large sea level changes it will take in excess of 300 m.y. and more likely more than 450 m.y. to degrade a Himalayan size mountain belt ( ) to a peneplain ( ). For an old mountain belt that has been eroded to a regional elevation of 500 m, it will take an additional 220 m.y. and more likely 390 m.y. to degrade to a peneplain. (2) In a system in which the river valleys are graded with respect to base level, the rate of downcutting is enhanced by the rate of uplift caused by the isostatic response to regional denudation. With sea level (base level) rising the rate of downcutting is decreased by the rate of rise. However, transgression of the river valleys cannot take place until the rate of river valley uplift caused by isostatic response to the denudation is equal to the rate of sea level rise. Under usual conditions of sea level rise (nonglacial) this will not occur until the landscape has been degraded to a regional elevation of 60 m or less. However, under these conditions the decrease in the rate of downcutting in the river valleys greatly enhances the rate of degradation. As a consequence, older mountain bells already eroded to a regional elevation of 500 m or less may be further degraded to a peneplain ( ) during an episode of persistent sea level rise at rates of several tenths of a centimeter per 1000 years for several tens of millions of years. With sea level falling, the landscapes evolve toward an equilibrium surface at which point the rate of degradation is equal to the rate of sea level fall. (3) We have applied these concepts to the post Triassic evolution of the ridge and valley province of the Appalachians. Sea level probably rose 300 m during the interval from 205 to 95 Ma, which was sufficient to degrade those areas with a regional elevation of 500 m or less to a peneplain. Subsequent to this time, from 80 to 10 Ma, sea level has fallen approximately 300 m, sufficient to cause emergence of the former peneplain to a regional elevation ( ) of 215 m which is approximately equal to the present‐day regional elevation of the AppalachianS since the ridge and valley province near Harrisburg. We have made a further test of this by attempting to restore the remanent peneplain surface (i.e., the Schooley Mountain surface) to its former elevation in the upper Cretaceous and thus estimate the height that sea level had to be at that time. By infilling the valleys with country rock up to 457 m, which is the present elevation of the presumed former peneplain remanent surface, and making the appropriate isostatic adjustments, we may conclude that at a minimum, sea level was 277 m above present in the upper Cretaceous. Finally, we conclude that large‐scale peneplanation can only take place during episodes of significant sea level rise (>250 m) over long periods of time (>50 m.y.). Thus episodes of extensive peneplanation are indicative of sea level changes of this magnitude.

Cause of Northern North Sea Jurassic Unconformities: ABSTRACT

Thirteen unconformities and their correlative conformities (sequence boundaries) divide the strata of the Jurassic of the northern North Sea into twelve cycles of coastal onlap. Comparison of charts showing regional relative change of coastal onlap, unconformity age, stratal patterns, and facies relations from the northern North Sea with global charts of relative changes of coastal onlap and eustatic sea level indicates that the unconformities are global and are caused by eustatic changes of sea level. Nine of the global unconformities are believed to be caused by rapid eustatic falls of sea level, three by slow falls followed by rapid rises, and one by an increased rate of sea level fall. In addition, four marine hiatuses were identified that are interpreted to be relate to rapid rises of sea level. Fault block rotation or differential block subsidence occurred almost continuously throughout the Jurassic, causing tilting of beds. Unconformity recognition is enhanced by periodic truncation of tilted strata by lowstand erosion and/or onlap during the subsequent rises, but the tectonics do not cause the unconformities. Northern North Sea unconformities and coastal onlap are demonstrated on two seismic sections tied to well control. One section is from the United Kingdom part of the north Viking graben; the second is from the inner Moray Firth. Sequence boundaries and charts showing chronostratigraphy, relative change of coastal onlap, and ages of unconformities are shown for each seismic section. End_of_Article - Last_Page 1003------------

Relationship between eustacy and stratigraphic sequences of passive margins

It is commonly thought that transgressive or regressive events that may have occurred simultaneously on geographically dispersed continental margins have been caused by worldwide sea-level rise or fall, respectively. Instead, it is shown here that these events may be caused by changes in the rates of sea-level rise or fall. The subsidence of an Atlantic-type (passive) margin may be modeled as a bordering platform rotating downward about a landward hinge line. The rate of subsidence is greatest at the seaward side of the platform and decreases landward to zero at the hinge line. With the exception of sea-level changes due to glaciation, dessication, and flooding of small ocean basins and other sudden events, the rate of subsidence at the seaward edge of the platform (shelf edge) is greater than the rate at which sea level may possibly rise or fall. Thus, if sea level is falling, the shoreline will seek that point on the subsiding platform at which the rate of sea-level fall is equal to the rate of subsidence minus the sedimentation rate. If the rate of sea-level fall decreases, the shoreline will move landward; if the rate increases, the shoreline will migrate seaward. If sea level is rising, the shoreline will move to that point where the rate of sea-level rise is equal to the sedimentation rate minus the subsidence rate. Thus, if the rate of sea-level rise decreases, the shoreline will move seaward; if the rate increases, the shoreline will move landward. The position of the shoreline is also a function of the sedimentation rate. These relationships have been quantified so that the position of the shoreline and the thickness of the sediments deposited during discrete time intervals may be computed as a function of the rate of sea-level change and the sedimentation rate. A sea-level curve, based on volume changes of the mid-oceanic ridge system, has been computed. Sea level is seen to fall persistently from Late Cretaceous to middle Miocene time, but transgressions occur in Eocene and early Miocene time because the rate of sea-level fall is slower for these periods. It is concluded also that the presence of the shoreline seaward of the shelf edge of an Atlantic margin should be symptomatic of events that may cause rapid sea-level fall, such as glacial build-up or the sudden flooding of large deep basins.