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.