Shelves have previously been classified according to a wide range of criteria, such as tectonic, morphological, climatic, and process-based classifications. Here, the formation of shelves is discussed in the context of conditioning from sedimentary and tectonic processes. A three-fold division of shelves is proposed: sedimentary shelves, combined structural–sedimentary shelves, and structural shelves. With a definition of a shelf as a shallow-marine surface of large areal extent located around the margin of a deeper basin (relief hundreds to thousands of meters), most cases of shelf formation can be explained by means of sedimentation, with the only contribution from tectonics being long-term accommodation provided by basinal subsidence. These sedimentary shelves are formed by virtue of differential sediment deposition basinwards in combination with nucleation and propagation of a break in slope around or close to the shoreline. Additional conditions for the formation of sedimentary shelves include (1) deep frontal waters; (2) a hinterland that can deliver a sediment budget large enough to prograde the margin; and (3) transgressions that periodically flood back across the low-gradient coastal and alluvial plains.
A structural shelf is a shelf where the shelf edge is a subaqueous structural feature (e.g., a fault escarpment), not propagated by sedimentation, usually sediment starved. Commonly, sediment has draped and infilled smaller-scale topography. Combined structural–sedimentary shelves have a direct structural nucleation of the shelf–slope break. This initial break is then blanketed and propagated by sediments retaining a shelf–slope-break topography which is displaced relative to its structural heritage.