Terraces – Terraces are “An isolated (or group of) relatively flat horizontal or gently inclined surface(s), sometimes long and narrow, which is (are) bounded by a steeper ascending slope on one side and by a steeper descending slope on the opposite side” (IHO, 2008).  Terraces are only mapped in this study on the continental slope.  A total of 1,230 terraces were identified, covering an area of 2,303,490 km2, equal to 0.64% of the oceans and 11.6% of the area of the continental slope.  Terraces are most common on the slopes of passive continental margins, especially in the Arctic and Indian Oceans, where they characterise over 21% of the continental slope.  Terraces occupy less than 6% of the slope in the Mediterranean and Black Seas, the North Pacific and the South Pacific Oceans. The largest terrace is on the North West Shelf of Australia, which covers an area of 104,470 km2.

The origins of terraces are many and often complex.  The classic “wave-cut terrace” documented in many regions as the result of sustained wave erosion over geologic timescales to produce an erosional rocky ledge, is perhaps the most well known formation process (eg. Kennet, 1982).   Such wave-cut terraces formed during the Pleistocene (ice age) period, when sea level was as much as ~120 below its present position, are now found submerged at depth along some continental margins (eg. Galparsoro et al., 2010).  Some terraces are formed in coral reef provinces behind drowned shelf edge to upper slope barrier reefs, when back-reef lagoons were submerged by rising Pleistocene sea levels (Harris and Davies, 1989; Blanchon et al., 2002).  Other terraces are associated with tectonic processes and slumping of the margin.  Slumping and faulting along the continental margin of southern Australia has resulted in large (dimensions of >10 km) bocks of material to create terraces (eg. James et al., 1999).

Although terraces are commonly characterized by sand and gravel substrate with sparse benthos, there are localized exceptions.  For example, in Australia’s Great Barrier Reef, Beaman et al (2012) reported that smaller reef substrate pinnacles and scarps superimposed upon terraces are sites for dense patches of octocorals. However, little is known about the biotic associations characteristic of most continental slope terrace features.

Statistics on Terraces (from Harris et al., 2014). The largest terrace is on the North West Shelf of Australia, which covers an area of 104,470 km2.  Other terraces of notable size include one 62,850 km2 in area, located on the western shelf of Myanmar and Malaysia, and another 60,850 km2 terrace on the Kerguelen Plateau (in the southern Indian Ocean).

OceanArea km2Terraces Area%%Area of slope that is TerraceNumberMean terrace area km2
Arctic Ocean224,9801.7324.6544,170
Indian Ocean896,7301.2621.43462,590
Mediterranean & Black Sea 50,6301.675.5871710
North Atlantic Ocean343,4100.7671,860
North Pacific Ocean274,5700.3359.751851,340
South Atlantic Ocean286,4000.7085.662051,780
South Pacific Ocean188,4800.21618.0161960
Southern Ocean38,2900.1885.811961,530
All Oceans2,303,4900.6366.22251,870

Blanchon, P., Jones, B., Ford, D.C., 2002. Discovery of a submerged relic reef and shoreline off Grand Cayman; further support for an early Holocene jump in sea level. Sedimentary Geology 147, 253-270.

Galparsoro, I., Borja, Á., Legorburu, I., Hernández, C., Chust, G., Liria, P., Uriarte, A., 2010. Morphological characteristics of the Basque continental shelf (Bay of Biscay, northern Spain); their implications for Integrated Coastal Zone Management. Geomorphology 118, 314-329.

Harris, P.T., Davies, P.J., 1989. Submerged reefs and terraces on the shelf edge of the Great Barrier Reef, Australia: morphology,occurrence and implications for reef evolution. Coral Reefs 8, 87-98.

Harris, P.T., MacMillan-Lawler, M., Rupp, J., Baker, E.K., 2014. Geomorphology of the oceans. Marine Geology 352, 4-24.

IHO, 2008. Standardization of Undersea Feature Names: Guidelines Proposal form Terminology, 4th ed. International Hydrographic Organisation and Intergovernmental Oceanographic Commission, Monaco, p. 32. http://www.iho.int/iho_pubs/bathy/B-6_e4_EF_Nov08.pdf

James, N.P., Collins, L.B., Bone, Y., Hallock, P., 1999. Subtropical carbonates in a temperate realm: Modern sediments on the southwest Australian Shelf. Journal of Sedimentary Research Section B: Stratigraphy and Global Studies 69, 1297-1321.

Kennett, J., 1982. Marine Geology. Prentice-Hall, Englewood Cliffs, N.J.