FIGURE 3.5 Atlantic-type (passive) continental margin along the eastern United States. Continental deposits accumulated in fault-bounded basins as the early stages of rifting separated North America from Africa. When the continents separated enough so that seawater could enter to form the Atlantic Ocean, marine deposition commenced.

Deltas and Estuaries

Rivers reach the sea in deltas and estuaries. Deltas form at river mouths when the prevailing current becomes too slow to carry detritus, so that enormous amounts of sediment are dumped close to the continental margin. Where deltaic deposition has continued for tens of millions of years, deltas have extended onto the deep ocean floor. Huge petroleum resources in the Mississippi, Niger, Orinoco, and other deltas are currently being developed, and related understanding has contributed to the way in which deltas are perceived from the complementary resource, environment, and hazard viewpoints.

Estuaries, in contrast to deltas, are broad embayed river mouths that have been flooded since the end of the most recent glaciation when sea level rose and the river's sediment load diminished. Water circulation in estuaries is much more restricted than in the open sea, and, as a result, sensitivity to environmental modification is very high. For this reason estuaries are important targets for interdisciplinary research in biogeochemical dynamics.

Beneath the Sea

The large-scale structure of the ocean basins has been established by the operation of the Earth's internal heat engine, which causes rupture and drift of continents and island arcs, formation of new ocean floor at spreading centers, and establishment of new arc systems where plates converge. The operation of these processes leads inexorably to the opening and closing of oceans, to island-arc and continental collisions, to the assembly of continents, to the addition of new arc material to existing continents, and to recycling of both ocean-floor rocks and continental material into the mantle. Solar heat modifies the ocean floor mainly by deposition of detrital sediment eroded from the land and by precipitation of calcium carbonate and silica from oceanic waters—partly by marine organisms—to form limestone and chert. Together, these processes degrade the thermally generated submarine relief, not so much by erosion, the process dominant above sea level, as by deposition that smooths the topography.

Deposition at Atlantic-Type Margins

Sedimentary deposition below sea level is controlled primarily by the tectonic framework of the ocean basins. The largest volumes of sediments accumulate at the rifted continental margins, called Atlantic-type margins because they are best developed around the Atlantic Ocean (Figure 3.5). The most rapid additions to these types of margins at present come from rivers draining the world's largest areas of high elevation—from the Mississippi and Mackenzie rivers that drain western North

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