Submarine Fan Systems

The architectural elements described are found in various proportions and are made up of different grain sizes of material depending on the characteristics and volume of the sediment supplied to the submarine fan. Any combination is possible, but it is convenient to consider four representative models: gravel-dominated, sandy, mixed sand and mud, and muddy, with the usual caveat that any intermediate form can exist. Submarine fan systems are commonly divided into upper fan (inner fan), mid-fan and lower fan (outer fan) areas: in these schemes the upper fan is dominated by channel and levee complexes, the mid-fan by depositional lobes and the lower fan by sheets. Although this works well for some examples (e.g. sandy and mixed systems) the divisions are not so appropriate for gravelly and muddy systems.

Gravel-rich systems

Coarse sediment may be deposited at the edge of a basin in coarse-grained deltas supplied by braided river or alluvial fans. The deeper parts of these deltas can merge into small submarine fans of material forming wedge-shaped bodies at the base of the slope. The gravel is mainly deposited by debris flows and sands are rapidly deposited by high-density turbidity currents. These fan bodies tend to pass abruptly into thin-bedded distal turbidites and hemipelagic mudstones.

Sand-rich systems

A submarine fan system is considered to be sand-rich if at least 70% of the deposits in the whole system are sandy material. They are usually sourced from sand-rich shelves where waves, storms and tidal currents have sorted the material, removing most of the mud and leaving a sand-rich deposit that is reworked by turbidity currents. Sand-rich turbidity currents have a low efficiency and do not travel very far, so the fan body is likely to be relatively small, less than 50 km in radius. Deposition is largely by high-density turbidity currents and the fan is characterised by sandy channels and lobes. The inner fan area is dominated by channels with some lobes, while the mid-fan area is mainly coalesced lobes, often channelised. Due to the low transport efficiency the transition to finer grained sheet deposits of the lower fan is abrupt. Inactive areas of the fan (abandoned lobes) become blanketed by mud. Strata formed by these systems consist of thick, moderately extensive packages of sandy highdensity turbidites separated by mud layers that represent the periods of lobe abandonment.

Mixed sand–mud systems

Where a river/delta system provides large quantities of both sandy and muddy material, a mixed sand–mud depositional system results; these systems are defined as consisting of between 30% and 70% sand. These higher efficiency systems are tens to hundreds of kilometres in diameter and consist of well-developed channel levee systems and depositional lobes. Deposits in the channels in the inner and mid-fan areas include lags of coarse sandstone, sandy, high-density turbidite beds and channel abandonment facies that are muddy turbidites. They form lenticular units flanked by levee deposits of thin, fine-grained turbidites and muds. The depositional lobes of the mid-fan are very variable in composition, including both high- and lower density turbidites, becoming muddier in the lower fan area. In a sedimentary succession, the lobe deposits form very broad lenses encased in thin sheets of the lower fan and muds of the basin plain.

Muddy systems

The largest submarine fan systems in modern oceans are mud-rich, and are fed by very large rivers. These large mud-rich fans include the Bengal Fan fed by the Ganges and Brahmaputra rivers and the large submarine fan beyond the mouth of the Mississippi. These submarine fan systems are over 1000 km in radius and consist of less than 30% sand. Channels are the dominant architectural element of these systems and when some modern submarine fan channels are viewed in plan they are seen to follow a strongly sinuous course that looks like a meandering river pattern. The channels deposits are sandy while some sand and more mud are deposited on the channel margins as well-developed levees. Depositional lobes are rather poorly developed and thin: the outer fan area consists mainly of thin sandstone sheets interbedded with mudrocks of the basin plain. In a succession of mud-rich fan deposits the sandstone occurrences are limited to lenticular channel units and isolated, thin lobes and sheets in the lower fan.

Ancient submarine fan systems

Successions of turbidites are found in places where deep-sea deposits have been uplifted by tectonic forces and are now exposed on land: this occurs at ocean margins where accretionary prisms form and around mountain belts where foreland basin deposits are thrust to the surface. The beds are commonly quite deformed, having been folded and faulted during the process of uplift, so interpretation of the successions, which may be many thousands of metres thick, is not always easy. The type of depositional system can be assessed by considering the ranges of the grain sizes of the material and the distribution of channel, levee, lobe and sheet facies. Because of the size of most submarine fan systems, the beds exposed will often represent only a very small part of a whole system, even if the outcrop extends for tens of kilometres or more. The palaeogeography of the system can be established by using the distribution of the different facies, and by using indicators of palaeo flow. Scouring during the flow of a turbidity current leaves marks on the underlying surface that are filled in as casts when deposition subsequently occurs. These scour and tool marks can be very abundant on the bases of turbidite sandstone beds and measurement of their orientation can be used to determine the direction of flow of the turbidity current: flute marks indicate the flow direction while groove marks show the orientation of the axis of the flow. Cross-lamination in the Bouma ‘c’ division can also be measured and used as a palaeocurrent indicator. Palaeoflow indicators in turbidites provide reliable information about where the source area was (back-tracking along the flow directions), except where a turbidity current encounters an obstacle and is diverted or in small basins where they may flow all the way across to the opposite margin and rebound back again. Through time a deep-water basin may be wholly or partially filled up with the deposits of a submarine fan. During this process the fan system will prograde as it builds out into the basin. This means that the deposits of the lower fan will be overlain by mid-fan deposits and capped by upper fan facies, but the succession is unlikely to be as simple as presented in this diagram.
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