5. Sea Level Processes and Effects of Sea Level Change Importance of sea level positions


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5. Sea Level Processes and Effects of Sea Level Change

  • Importance of sea level positions

  • Sea Level processes and indicators

  • Coastal morphology and the Recent rise in sea level

  • Ice-driven sea level fluctuations

  • Tectonically driven sea level fluctuations


Importance of sea level positions

  • Important to know the position of sea level

  • Depth of deposition dominates the major facies patterns of the material accumulating on it

  • Also …

  • the size distributions of clastic sediments,

  • the chemistry of biogenous and authigenic matter,

  • the distribution of benthic organisms.



Two kinds of sea level fluctuations

  • Global (eustatic)

  • Regional (tectonic)



Global (eustatic) Fluctuations

  • produce contemporaneous transgressions and regressions on the shelves of all continents

  • originate from changes in the volume of ocean water or in the average depth of the ocean basin

  • Examples are changes in ice volume, or changes in sea floor spreading rates



Regional (tectonic) Fluctuations

  • consist in transgressions and regressions on one particular shelf

  • produced by regional sinking or uplift of the shelf.



Effects on a smaller scale

  • Where the sea level intersects the continental margin, physical, chemical, and biological processes are of high intensity

  • Waves, tides, and currents show maximum activity

  • The productivity is great

  • Sea level is the baseline of erosion and deposition: exposed areas are eroded and submerged areas build up; determine the coastal morphology.



On a larger scale

  • 1) determines the degree to which shelves are submerged

  • Submerged shelves absorb much more sunlight than exposed ones.

  • Climate is mild during times of high sea level, harsh during times of retreat of the ocean into its basins.

  • closely tied to paleoclimatic evolution.

  • 2) the rates of erosion of the continents and the sites of deposition in the ocean

  • How sediment is transported to the deep ocean is controlled by sea level.

  • During high stands, the transport of sediment by turbidity currents will be reduced.

  • Turbidity currents depend on supply of mud to the outer edge of the shelf.

  • This supply is highest when sea level is low, and is greatly reduced during high stands when the submerged shelves trap the material delivered by rivers.

  • Thus, the types of sediment bodies found at the base of the ocean margins should depend on the history of sea level fluctuation.





wave-cut terraces and beach deposits

  • wave-cut terraces and beach deposits

  • sea cliffs as the result of wave action



Sea Level Processes and Indicators; Wave Action

  • Waves transport and rework the sediment, changing its texture by sorting and influencing its structure (ex. ripple marks)

  • Thick enrichment of heavy minerals (placers) on a beach

  • Well-sorted skeletal remains (shell pavements or coquina)



Wave Action

  • Waves usually influence the sea floor only down to about 10 to 20m.

  • Responsible for producing certain types of sediments such as polished beach shingles and calcareous oolites.





Diurnal & Semidiurnal Tides



Tides and storm action: the intertidal zone



Small sea level changes can quickly lead to submergence or emergence.

  • Small sea level changes can quickly lead to submergence or emergence.

  • Storm play an important role in producing such change.

  • The storm flood created new access for the tides into the salt marshes and moors lying behind natural barriers, inland from the intertidal flats.

  • Slight changes in sea level, and the action of heavy storms, bring marked changes in the type of sediment deposited..

  • The record which is preserved on the slowly sinking floor consists of an intercalation of peat, salt marsh deposits, marine muds, and the sands and shells of the beach.

  • Storm deposits are common within the intercalation.



Sedimentary structures in intertidal flats

  • Desiccation cracks

  • rain drop impressions

  • pseudomorphs of cubic halite crystals

  • precipitation of gypsum,

  • tracks of land animals on sediments with marine organisms.



Photosynthesis

  • Photosynthesis can only proceed when sufficient light is available.

  • The light intensity drops to 1% of the surface value anywhere between 10 and 200 m water depth, depending on the clearness of the water.

  • Sessile plants, such as the geologically important calcareous algae and algal mats, generally occur no deeper than 100 m or so.

  • Animals living in symbiosis with algae also indicate shallow water.

  • large foraminifera, stone corals, and certain molluscs.



Photic zone & aphotic zone



The recent rapid rise in sea level which began about 15000 years ago and lasted till about 7000 years ago.

  • The recent rapid rise in sea level which began about 15000 years ago and lasted till about 7000 years ago.

  • The sea level rose in response to the melting of glacial ice.

  • The maximum addition of water to the ocean occurred between 13000 and 9000 years ago.



General effects of recent sea level rise

  • The sea level changed by approximately 130 m on the whole; that is, it rose from the average depth of the shelf edge to the present position.

  • In temperate humid regions, coastal peat bogs grew upward and became flooded by salt water and covered by marine sediments.

  • Dunes were eroded away by the approaching surf.

  • Resistant matter left from the erosional process collected as a transgression conglomerate.

  • The basal conglomerate typical of many marine transgression sequence in the geologic record.



Example for the effects of the rise of sea level

  • A coast of drowned rivers.

  • Longshore sand transport tends to close off the estuaries which fill with sediment to make marshlands.

  • Relict sediments



River Mouths

  • Why a delta forms and not an estuary depends of many factors.

  • for instance, low tidal activity as in land-surrounded seas (Mississippi and Nile delta), high sediment load due to strong seasonal rains, and high erosion rates in the mountain hinterland (the Indus, Ganges, and Irrawadi deltas).



Tidal motion can reach far into the estuaries.

  • Tidal motion can reach far into the estuaries.

  • Salt water intrudes along the bottom.

  • This intrusion of the marine realm brings marine sediments upriver.

  • Why then are the estuaries not filled in?

  • River floods, tidal action, and especially the young age of estuaries are the cause.

  • The drowned river valleys produced by the transgression have not yet come to equilibrium with the sediment load.







Recent Rise in Sea Level

  • If sea level falls, the regression is accelerated.

  • Conversely, a rise of sea level or a sinking coastline will ideally produce a transgressive sequence.

  • The recent rise of sea level resulted in the predominance of transgressive sequences on the top of the deltas of the world.



Lagoons and Barriers

  • A facies zonation parallel to the coastline.

  • Offshore bars, or barrier islands occur together with a sand beach.

  • The beach may be backed by dunes, which the wind pile up using sand from the beach.

  • This beach-dune complex may form a barrier for interior lagoons.



Lagoons and Barriers

  • Rivers emptying into the lagoons may cut on or several channels through such barriers, especially when flooding, chopping them up into a series of barrier islands.

  • From the seaward side, storm waves may break through the barrier, forming overwash fans on the lagoon beach.

  • Tidal action keeps such channels open, building deltas on both sides.



How does barrier-and-lagoon morphology reflects changes in sea level?

  • How does barrier-and-lagoon morphology reflects changes in sea level?

  • The balance between the rise of sea level relative to the land and the supply of sediment to the coast is the crucial factor.

  • Since the sea level stabilized, about 6000 years ago, supply of materials has been important.



During the recent rise of sea level, the various facies zones paralleling the coast migrated landward.

  • During the recent rise of sea level, the various facies zones paralleling the coast migrated landward.

  • Landward invasion of the mangrove swamps.

  • Expansion of mangrove swamps during deglaciation due to a rising sea level, must have produced an organic-rich layer on many tropical shelves.

  • Such layers will coalify.



5.4 Ice-Driven Sea Level Fluctuations - The Würm Low Stand

  • Sea level has been constantly changing over the last several hundred thousand years as a result of the waxing and waning of large continental ice masses.

  • About 17,000 years ago, the last major ice age (the Würm in Europe, the Wisconsin in North America), enough water was tied up in the continental glaciers to depress sea level by some 130 m.

  • Large shelf areas fell dry as a result of the ice-caused regression of the sea.

  • Rivers crossed the shelves and entered the sea at the shelf edge, cutting backward into the shelf.

  • Their sediment load was dropped in a narrow zone at the upper slope, became unstable there and slid, starting turbidity flows which rushed down the undersea canyons.



Major transgressions were much more rapid than any of the regressions.

  • Major transgressions were much more rapid than any of the regressions.



The ratio of the two isotopes in the shells is in equilibrium with the ratio of the isotopes in the water in which the shells grew.

  • The ratio of the two isotopes in the shells is in equilibrium with the ratio of the isotopes in the water in which the shells grew.

  • If the 18O/16O ratio in the water changes, it will change similarly in the shell.

  • Every time the sea level drops, the 18O/16O ratio of the water increases, because the glacial ice is made of water which is impoverished in δ18O.

  • Seawater is enriched in δ18O during glacials.

  • The effect of temperature on the isotopic signal can be neglected in the present context.



Effects on Reef Growth

  • Each highstand of the sea level resulted in a build-up of reef carbonates, while the lowstands resulted in erosion.

  • Results show that the uplifted corals grew during several high sea level stands, namely 124,000 years ago, at 103,000 years, and at 82,000 years.



5.5 Tectonically Driven Sea Level Fluctuations

  • Sea level fluctuated considerably all through the Phanerozoic, even during periods when apparently no ice was present.

  • As to the present sea floor, fluctuations since the Jurassic are of special interest.

  • The thick sediment stacks in the "passive" continental margins have been much studied for economic reasons.

  • When a margin sinks more or less continuously, coastal sediment bodies must reach great thickness, provided the sediment supply keeps pace with subsidence, and deposition remains locked in to the sea level.



The significance of the sand bodies.

  • The significance of the sand bodies.

  • They can retain great quantities of water, petroleum, or gas.

  • In as much as the sea level variations within this geologic period were not driven by the growth and decay of ice caps, they were not reflected in the isotopic composition of seawater.

  • We cannot, therefore, find them in the isotopic composition of the foraminifera in the manner indicated earlier.

  • How then can we measure these fluctuations?



Reconstruction of sea level changes

  • The intensive world-wide exploration of continental margins by seismic profiling.

  • The sediment-stacking patterns in margins of different ocean basins are quite similar.

  • They must be due to global sea level variation.

  • Vail and Mitchum derive sea level fluctuations from the geometry of sediment layers, as recognized on seismic reflection records.









Reconstruction of sea level changes

  • The "Vail sea level curve" is a useful tool for the correlation of seismic stratigraphies of continental margin sediments.

  • To what extent this curve reflects the true global sea level variations is not known.

  • One problem is that the rate of sediment supply must play an important role in controlling the geometry of the sediment bodies.

  • Another is that the rate of sinking of the margins must be considered.



The Causes of Change

  • This can be done by increasing the total mass of new lithosphere formed per year, that is, by increasing sea floor spreading rates or by increasing the length of the Mid-Ocean Ridge and the trenches, or both



There are indications, from the magnetic stripes on the sea floor, that global spreading rates in the Late Cretaceous may have been much higher than now.

  • There are indications, from the magnetic stripes on the sea floor, that global spreading rates in the Late Cretaceous may have been much higher than now.

  • It has been proposed that the high sea level stand of the Late Cretaceous were caused by such a fast spreading rate.



Other Causes

  • Mountain-building with shallow ocean crust, or with continental crust, is one way

  • Shallow crust (which is stacked up within mountain rages) is removed and replaced by deeper sea floor which will cover itself with a thicker layer of water, drawing down the general sea level.












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