deep water systems and sequence...
TRANSCRIPT
Deep Water Systems and
Sequence Stratigraphy
By: Matt Kyrias, Chris Majerczyk, Nick Whitcomb, Wesley Vermillion
Contents
Gravity Flow Depositional Elements Defined
References
NOTE: Source uses seismic data but that will not be at our disposal in Ireland. We’ll try to explain to you what the different depositional environments look like in the field.
Gravity-Flow Depositional Elements
Turbidity-flow leveed channels
Channel overbank sediment
Distributary-channel complexes
Crevasse-splay complexes
Debris-flow channels, lobes, and sheets
– ALL elements characterized by direct deposition onto or into (through erosion) the immediately underlying substrate
Turbidity-Flow Leveed Channels: Basics
Commonly form in slope and basin floor environments
Widths range from 200-3000m
Wide range of sinuosity
Channels meander downstream
Extent of contact between sand-prone part of channel and levee is important
Turbidity-Flow Leveed Channels: Cont.
Posamentier and Kolla, 2003
Channel Meander Loops
Uniformitarianism? Deep water vs. alluvial systems
Morphological similarities is a result of processes that involve dynamic interaction of flows, sediments, and terrain gradient
Meandering Channels & AvulsionAvulsion- a sudden cutting off of land by flood, currents, or change
in course of a body of water.
- Change in flow parameters associated with changes upslope or on the staging area.
- Avulsion channels form systematically, beginning down-system and gradually forming further up-system.
- Large-scale avulsion events are relatively rare, but where present may yield good reservoir lithologies.
Overbank/Levee Deposits
- Associated with deep-water channels, and overbank flow out of adjacent channels.
- Often an order of magnitude wider than their associated channel.
- Flow directed away from the channel in close proximity to the levee crest.
- Results in thickened section on the stoss side of the sediment waves
- Levees characterized by sediment wave development, not present in other systems.
- Contain reservoir-quality thin bedded sandstones.
SEDIMENT
WAVES
Frontal Splay Complexes
A process of flaring out from a confined to unconfined system
This causes lobate structures to form
The transition from confined to unconfined can be related to a progressive lowering of levee height down system to where the high density part of the turbidity flow is no longer confined
Such a shift of transition down system would be associated with a progressive overall grain size
decrease
Frontal Splay Complex: Diagram
Frontal Splay Complex: Cont
There is minimal loss of sand within these flows due to the high density part of the flow remains effectively confined by levee walls
Down system there is a gradual increase of net sand deposited on levees, and as the system loses sand the levees loose height
The high density part of the flow eventually breaks through the levee splaying out in a braided pattern. This also leads to channel avulsion
Where the levee is breached crevasse splay complex form. This is similar to formation of avulsion channels. A network of channels form and produce sheet like sand rich deposits
Debris flow: sheets, Lobes, and Channels
Debris flows deposits have a common characteristic of long linear grooves that reflect the basal surface
Crevasse and Overbank Splays
Controls on Deep-Water Sedimentation
Controls on the occurrence of sand-rich deep-water sedimentation
– The presence of sand in the shelf-margin staging area
– Steep slopes immediately after the staging area
– Sea-level falls sufficient to expose most of the shelf
Controls on Deep-Water Sedimentation:
Cont
Controls on the style of deep-water deposition
– Sand to mud ratio within the material delivered to the deep water
– Basin physiography
– Magnitude, duration, and frequency of depositional events
– Sediment delivery mechanisms
– Single source versus multiple smaller sources
– Rheology and concentration of sediment gravity flows
– Bottom-current activity
Sequence Stratigraphic Significance Typical deep-water sequence
– Debris-flow deposits at base
– Splay-complex-dominated turbidite deposits
Period of sea-level fall
– Leveed-channel-dominated deposits
Period of sea-level stillstand and slow rise
– Debris-flow deposits
– Capped by condensed-section deposits
Posamentier and Kolla, 2003
Conclusions
While the paper dealt mainly with seismic data, we discussed what could be relevant to Ireland.
Turbidity-flow leveed channels, meander loops, overbank or levee deposits, frontal splay deposits, crevasse splays or overbank splays, and debris-flows were discussed.
The controls on deep-water sedimentation were also discussed along with a general sequence stratigraphy.
References
Posamentier, H. W. and Kolla, V. (2003). Seismic geomorphology and stratigraphy of depositional elements in deep water. Journal of Sedimentary Research. vol. 73 no. 367-388.