Important reservoirs if associated with faults
Deltas are likely the most important agent to deliver sediment across the shelf & to deepwater beyond. The shelf edge is the staging area for this delivery
Regime of the shelf-edge delta is important
Somme et al., 2009
SHELF-EDGE DELTAS ARE REGULAR DELTAS BUT OCCUPY SHELF-EDGE SITES
Fluvial Dominated Wave Dominated Tide Dominated
River-dominated deltas: most efficient to deliver sediments basinwards. Wave-dominated deltas: lowest rate of sediment transfer basinwards. The dispersal currents are shore parallel Tidal-dominated deltas: sediment dispersal currents are shore normal
Courtesy C.Olariu
SPITSBERGEN
Porebski & Steel 2003
Log signatures of deltas on the shelf
River & coastal plain reservoirs
Deltas & other shoreline reservoirs
Shelf-Edge Delta
Slope mudstones and slope channels
Basin-floor fan reservoirs
RESERVOIRS
They tend to be thick (100m +) because of increased water depth at the shelf edge
They are often turbidite fronted if delta is strongly river-driven
Alternating beds of soft-deformed and undeformed mouth-bar deposits occur in rollover area
Porebski & Steel 2003
Washakie Basin, S Wyoming
landward basinward 80 km
TURBIDITE-FRONTED SHELF EDGE DELTAS
Distributary channels
Delta-front turbidites 60m
WASHAKIE BASIN/FOX HILLS: RIVER-DOMINATED SHELF-EDGE DELTAS
Downcutting distributary channels
15m
SPITSBERGEN
Shelf-edge incised channel
Thin-bedded delta-front strata
Mellere et al 2003
Ocean waves tend to be large at the shelf break, causing deltas there to be wave dominated. This hinders the deltas from delivering sand to deepwater
Orinoco shelf, Trinidad Steel et al 2007
WAVE-DOMINATED SAND BELTS AT THE SHELF EDGE
Orinoco shelf margin
The conventional model is that falling sea level drives them across the shelf edge, producing deepwater sand accumulation at lowstand
In the last 4-5 years we have been given many examples where cross-shelf transits of the deepwater delivery system happen with rising sea level, ie highstand fans. This requires a strong fluvial drive on the deltas
VARIABLES AFFECTING SHELF TRANSIT TIME (TRAVEL TIME BY DELTA TO SHELF EDGE) •Shelf width & gradient
•Coastal-plain gradient
•Sediment flux
•Sea level behaviour
Muto & steel 2001
WHEN SL IS RISING AT HOLOCENE RATES, SOME DELTAS ARE NOT ABLE TO REACH
THEIR SHELF EDGE
THERE IS A CATEGORY OF DELTA THAT NEEDS SL FALL TO REACH THE SHELF EDGE
Shelf-transit time…………
THE OTHER CATEGORY COULD BE TERMED ‘SUPPLY DRIVEN’ AND CAN DO IT WITH RISING SL
SPITSBERGEN
WASHAKIE BASIN, WY
Shelf edge trajectory shows alternating flat and rising segments (My)
Sydow et al 2003
Saller et al., 2007
Courtesy Josh Dixon, UT Austin
It can work with falling sea level, as in the conventional lowstand model
It can work with rising sea level if supply is sufficient (though much sediment is stored also on shelf
In either case it may not work if delta regime is not strong fluvial drive
Top Related