Calciclastic submarine fans: An integrated overview

Calciclastic submarine fans are rare in the stratigraphic record and no bona fide present-day analogue has been described to date. Possibly because of that, and although calciclastic submarine fans have long intrigued deep-water carbonate sedimentologists, they have largely been overlooked by the academic and industrial communities. To fill this gap we have compiled and critically reviewed the existing sedimentological literature on calciclastic submarine fans, thus offering an updated view of this type of carbonate slope sedimentary system.Calciclastic submarine fans range in length from just a few to more than 100 km. Three different types can be distinguished: (1) Coarse-grained, small-sized (< 10 km) fans, which are characterized by the abundance of calcirudites and the scarcity of mud. They have relatively long leveed channels and small radial lobes. (2) Medium-grained, medium-sized fans are typified by the abundance of calcarenites and lesser amounts of calcirudites and mud. They have a tributary network of slope gullies, which merge to form a leveed channel that opens to the main depositional site, characterized by extensive lobes and/or sheets, which eventually pass into basinal deposits through a narrow fan-fringe area. These fans are between 10 and 35 km in length. (3) Fine-grained, large-sized fans are rich in calcarenites and mud, but poor in calcirudites. They have wide and long slope channels that feed very extensive calciturbiditic sheets, the total length always exceeding 50 km and generally being close to 100 km. In terms of grain-size distribution the three fan types compare well with sand/gravel-rich, mud/sand-rich and mud-rich siliciclastic submarine fans, respectively. However, they show notable differences in terms of size and sedimentary architecture, a reflection of the different behaviour of their respective sediment gravity flows.Most calciclastic submarine fans were formed on low-angle slopes and were sourced from distally steepened carbonate ramps subjected to high-energy currents. Under these conditions shallow-water loose grainy sediments were transferred to the ramp slope and eventually funnelled into the submarine fan by sediment gravity flows. These conditions seem to have been more easily met on leeward margins in which the formation of reefs was hampered by cool waters, nutrient enrichment or oligophoty. Another circumstance that contributes to the transfer of shallow-water sediments to the distal ramp slope is a low sea level, forcing the carbonate factory closer to the slope break and destabilizing sediments by increased pore-water pressure. However, the most important factor controlling the development of calciclastic submarine fans was the existence of an efficient funnelling mechanism forcing sediment gravity flows to merge downslope and build up a point-sourced sedimentary accumulation. In most cases this occurred through a major slope depression associated with tectonic structures, an inherited topography, or large-scale mass failures.