Introduction

The continental margin is the submerged shelf and slope forming the outer edge of a major landmass. The differences between oceanic and continental crust composition are fundamental in margin development. The transition between the two crustal types lies under the area of the continental slope with the shelf over continental crust and oceanic crust below the continental rise. The difference in densities and isostatic balance of the continental to oceanic crustal blocks is expressed dramatically by the presence of a continental slope, which is the great declivity from the shelf break downward to deep ocean depths. It is a continuous and impressive geomorphic feature -- an escarpment of more than three kilometers height and 15 to 50 km width that extends for more than 350,000 kilometers. ^{Hedberg, 1970}

The continental shelf is the submerged part of the continent lying between the coastline and the shelf break, which is the change in inclination marking the boundary between continental shelf and slope. The average gradient of the shelf is only seven minutes of inclination, and off coastal plain shorelines, the shelf is a submerged continuation of the plain that has alternated between submergence and emergence during geologic time.

Burk and Drake ¹⁹⁷⁴ emphasized that plate tectonics provides the framework for interpreting the history and character of the continental margins. Plate movements and the basic difference in the density of oceanic and continental crustal units initiate the structural pattern of continental margins and result in a tectonic classification of coastlines as active (Pacific, leading edge) or passive (Atlantic, trailing edge) margins, each of which have certain fundamental characteristics.

Active continental margins, where plates are converging, coincide with plate boundaries, where the continental and oceanic crust are separated by a subduction zone. These margins are active tectonically and have less width and sediment input than passive margins. They are also marked by the addition of blocks from distant sources to the continental mass at the subduction zone. Passive margins are within plates and are separated from the oceanic ridge plate margin by an expanse of oceanic crust that was generated after rifting. Oceanic and continental crust meet in a region of low tectonic activity that does however experience a broad pattern of subsidence. Passive margins are generally wide and may receive a large influx of terrigenous sediments or intrabasin carbonate sedimentation from local sources.