Submarine Fan Systems
Fining and thinning-upward or coarsening and thickening-upward successions
are common in submarine fan environments
Cyclicity may result from:
Autocyclic processes (inherent to the depositional system,
not driven by external forces)
Allocyclic processes (driven by external forces, primarily
sea level, tectonics, or climate)
Autocyclic processes
Possible causes of fining-upwards successions:
1) channel fill and abandonment
2) lateral migration of lobe (from channel to margin)
3) successive deposition of thinner beds on levee
Possible causes of coarsening-upwards successions:
1) Progradation of fan (e.g., lobe over basin plain)
2) lateral migration of lobe (from margin towards channel)
Allocyclic processes
Submarine fans always have accommodation space, but sea level affects
proximity of sediment source and accommodation in more proximal areas
Negative accommodation
results in erosion of sediment
Nearby source supplies
sediment directly to slope
Allocyclic processes
Sediment entry points at
shoreline far from shelf edge
High sea level results in stable
conditions near shelf edge
Deep-water systems vary with:
1) Grain size of available sediment
2) Nature of supplying system (point source, multiple source…)
Not a universal model!
Gravel-rich systems
Small, high-gradient
depositional systems
Dominated by debris flow
conglomerates and/or breccias
Poorly organized lobes,
unchannelized
Mud-rich systems
Channels very long and highly sinuous
Slumps more frequent
Channel position
more stable
Deposits thin but extremely laterally extensive
Point source
submarine fan
Decreasing source stability
Increasing overlap of multiple source lobes
Decreasing facies organization
Multiple source
slope ramp
Linear slope
apron
Sediment gravity flows are not the only depositional process
Bottom currents (1-20 cm/s) are found in many places in the deep ocean,
and are able to rework and transport fine sand
Mud-draped current ripples, 3091 m depth
Contour-parallel currents deposit or rework sediment into “contourite” units,
typically fine-grained (<v. fine sand) with faint laminations or ripples
Indistinctly laminated contourite siltstone
Turbidite mudstone