Submarine Fan Systems Fining and thinningupward or coarsening

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