Continental margins

Continental margins mark the transition between the oceans and the continents, and are traditionally defined as the region between the upper limit of the tidal range and the base of the continental slope. They extend from the coastal zone (presented in Chapter 8, “Shallow-water environments”) to the abyssal plains and basins (presented in Chapter 6, “Abyssal plains and polar seas”), and they are roughly divided into three regions: continental shelf, continental slope, and continental rise (

Figure 7.1

). Continental margins are the region on Earth where most of the sediments are deposited (as much as 90% of the sediment generated by erosion on land) (McCave, 2002). It is important, however, to recognize the long-term processes that led to their formation: in the past 1 million years, sea level has only been as high as now for less than 5% of the time, and for the past 7,000 years only (e.g., Thomsen et al., 2002). For the most part, continental shelves were developed sub-aerially by fluvial processes at a lower sea level (as much as 130m lower than now), and their features were smoothed off by wave action during the next sea level rises. For much of the last million years, sediments were fed more directly into the ocean basins, and these variations explain the wide variety of facies observed around the world (see Richards et al, 1998 or McCave, 2002 for more complete descriptions).

Figure 7.1

Schematic representation (not to scale) of the major physiographic provinces, showing the relatively flat continental shelf and the rapidly descending areas of the continental slope and rise.