Introduction

In order to compensate for lithospheric depression beneath a crustal load, the surrounding area may rise creating a forebulge. These principles are demonstrated by the Amazon delta, where huge sediment loading has created a central depression that is surrounded by peripheral uplifts--see Fig. 9-1.

In the cases of ice sheets, the amount of forebulge uplift varies from negligible to a few 100 m at most--see Fig. 9-2. The presence of a forebulge has important implications for melt-water drainage. A drainage moat may be created between the ice margin and forebulge, in which proglacial lakes, seas, or ice-marginal streams are confined.

As an ice sheet begins to shrink, rebound of the depressed region then takes place during deglaciation and for several 1000 years thereafter--see Fig. 9-3. The rate of rebound is initially quite rapid and slows progressively. The magnitude of postglacial rebound is nicely documented in Sweden and eastern Canada--see Figs. 9-4 to 9-6. Five lines of evidence support the idea that recent uplift of formerly glaciated regions is the result of isostatic rebound following deglaciation (based on Flint 1971).

• The outer limit of rebound coincides with the limit of latest ice-sheet coverage in Europe and North America.

• Isobases (contour lines) showing distribution of rebound are concentric around central areas of glaciation. The area of former maximum ice thickness shows the greatest rebound.

• Rates of rebound are almost the same in eastern Canada and Scandinavia--rapid during deglaciation and progressively slower toward the present.

• Negative gravity anomalies exist in central zones of glaciation in both Europe and North America. Maximum values are located around Hudson Bay and the northern Baltic area. These regions have a net deficiency of crust/mantle mass.

• Evidence for postglacial rebound is found in every area of ice-sheet or ice-cap glaciation during the Pleistocene.

Beach ridges on coast of Novaya Zemlya, arctic Russia. Such ridges are formed by pushing of sea ice; older ridges have been uplifted inland (to left) as a result of Holocene glacio-isostatic rebound. Photo © by J.J. Zeeberg; used here by permission.

Skansen bog, island of Askøy, western Norway. Inge Aarseth (left) and assistant display coring tool used to recover samples of sediment within the bog. During late glacial time, this site was depressed below sea level and the basin accumulated marine sediment. With postglacial rebound, the site was uplifted, and lake and bog sediments have filled in. The age of lowest non-marine sediment in the core dates the time the basin rose above sea level. Photo date 5/87; ©; by J.S. Aber.

The margins of many continents slope very gently into the sea forming wide, shallow continental shelves. The Atlantic and Gulf coasts of the United States are good examples--see Fig. 9-10. In such situations, shoreline positions migrated over 10s to 100s of km as sea level rose and fell during glacial cycles. These sea-level cycles could be preserved as intervals of marine deposits (high sea level) separated by surfaces of erosion (low sea level). A complete sedimentary record of past sea-level changes would ideally be developed in stable coastal areas far removed from glaciation.

Much of the sedimentary record for Quaternary sea level is, unfortunately, not directly accessible; it is underwater on the continental shelves--see Fig. 9-11. In some places older deposits have been uplifted tectonically, but the magnitude of uplift cannot be determined from independent evidence. As a result of these problems, a global long-term record of Quaternary sea level is not yet firmly established.

Massive coral (Pavona clavus) exposed in 1954 by tectonic uplift in the Galapagos Islands. Coral reefs of this kind form in shallow tropical seas just beneath low-tide level. They are a record of past sea level. Image obtained from the World Data Center-A for Paleoclimatology, © Educational Slide Project.

An excellent Late Quaternary record is preserved in South Florida (Enos and Perkins 1977). Six episodes of high sea level (interglaciation) are marked by coral reefs and other marine deposits.

1. Recent (Holocene) sea level, last few 1000 years.
   
2. Sangamon sea level, peak about 130,000 years ago.
   
3. High sea level with peak about 180,000 years ago.
   
4. High sea level with peak about 236,000 years ago.
   
5. High sea level with peak about 324,000 years ago.
   
6. Earliest preserved high sea level, age uncertain.

Section in Key Largo Limestone exposed in canal across Key Largo, Florida. A large branching coral is preserved in growth postion. Key Largo is a fossil reef that grew when sea level was 5-8 m higher than today, about 130,000 years ago. Section is about 5 m high. Photo date 12/75; ©; by J.S. Aber.

Miami Oolite, the limestone formation that underlies much of Miami, Florida. The limestone was deposited on a shallow marine bank, much like the modern Bahama Bank. Conspicuous cross-bedding is visible at base of exposure. The Miami Oolite is a northern continuation of the Key Largo Limestone trend; they were deposited simultaneouly during high sea level about 130,000 years BP. Photo date 12/75; © by J.S. Aber.

The highest sea level achieved during any of these cycles was by the Sangamon (Eemian) sea. Sea level stood 5-8 m higher than at present; all of southern Florida was submerged, and the shoreline was located approximately 250 km (150 miles) farther north across the Florida peninsula. In many other parts of the world, the Sangamon Sea is marked by marine deposits or erosional beach terraces a few m above present sea level.

Wembury Bay, near Plymouth southwestern England. Bedrock platform in foreground is a strandflat a few m above present sea level. It was eroded by wave action during the Eemian high stand of sea level about 130,000 years ago. Photo date 8/88; ©; by J.S. Aber.

Crustal rebound then took place; the coastal region was uplifted, and parts of the present shelf were eventually exposed as dry land--see Fig. 9-15. Soil development and forest growth took place about 10,500 years BP. Meanwhile fjords to the east were still depressed well below sea level. Between 10,500 and 9000 years ago, most remaining glaciers on the mainland melted and eustatic sea level rose. The results were rebound of the fjord-head region (emergence) and drowning of the shelf area (submergence).

The Missisippi Delta exhibits another complicated situation for isostasy and eustasy. During times of major glaciation, when sea level was low, the Mississippi River incised a deep valley across the delta and transported sediment into the sea beyond the delta. During interglacial times, in contrast, high sea level allowed sediment accumulation and infilling of the valley. These substantial changes in sediment mass induced crustal subsidence and rebound exceeding 9 m (30 feet) along the Mississippi valley (Blum et al. 2008). Lesser depression and rebound extended >100 km to the east and west along the coast.

Mississippi River and Delta region, southern Louisiana and Mississippi, March 2001, prior to major hurricanes of 2005 and 2008. Note distinctive patterns of suspended sediment in coastal waters. MODIS image from the Visible Earth.

The timing of rebound coincided with release of melt water from continental ice sheets to the north. At present, high sea level combined with sediment accumulation and compaction lead to crustal depression and delta sinking. This example demonstrates the importance of glacially related processes for understanding delta behavior worldwide.

Related sites