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.