4. Shallow marine clastic environments a.k.a. shelves Flashcards
Define shallow water
Definition 1:
Above storm wave-base (deepest storms and tsunami) - influence on wave and tidal activity and their products.
Typically above 50m water depth in the oceans - much shallower (few m’s), even in the largest lakes
Definition 2:
The low gradient (<0.05degrees) continental shelf
Generally 100-140m deep (the amplitude of Quaternary sea-level changes)
Few 100s m wide to 100s km (perp to coastline)
‘‘Shelf break’’ happens at greater depths than this
What are the modes of sediment transport in shallow seas?
- Dissolved load wash load (ions in solution - pollution)
- Suspended load
- Fine particles (sand, salt & clay)
- Turbulent eddies pick up, carry upward if vel. > settling vel. - Bedload
- on/near bed; rolling, bouncing (saltating) etc.
Suspended and bedload transport increase rapidly with flow strength (nonlinear relationship)
What 3 things drive sediment transport in shallow seas? And what do they create?
1) Power of rivers continues into the sea
- creates lobe based coastlines called deltas
2) Dominance of wave action - causes linear coastlines creating strandplains, spits, beaches and lagoons
3) Tide dominated - Creates embayed coastlines creating estuaries and tidal flats
These processes are not mutually exclusive
What is the importance of shallow sea environments?
- 33% of the worlds population lives at the interface of land and sea
- These areas are most sensitive to (relative) sea level change, and river, wave and tidal processes are the agents of erosion and deposition
- These environments are supremely important for food (fishing) and infrastructure (windfarms, tidal barriers) etc.
- The shelf is a major (often ‘‘permanent’’) sink of sediment. Particulate pollutants will end up there
- It is important we understand the changes that happen here in response to sea-level and other environmental change, to mitigate for the future
How does sea level change relate to shorelines?
There is not a linear correlation
- Sea level rise DOES NOT EQUAL transgression
The change in the shorelines is mainly dependent of the amount of sediment being introduced into the system.
Shorelines change their shape depending on if they are prograding or retrograding.
How does deposition effect shoreline shape?
Deposition in a subsidising basin accelerates transgression (shoreline retreats) and retards regression (flooding happens fast, building-out happens slow)
- Transgressive deposits are thin or absent in the rock record
Describe the ‘‘shallowing upward’’ succession
Extremely characteristic feature of shallow marine deposits: repeated coarsening-up, thickening-up shallowing-up packages, seperated by sharp (‘‘flooding surfaces’’)
- only really have progradation moment preserved as they happen for longer
What are deltas and how do they form?
- Delta are protuberances of the land into the sea
- They form where rivers deliver sediment faster than it can be reworked and transported away by wave and tidal processes
- It’s the main means by which shorelines prograde: deltas are fundamentally regressive
What drives flow in deltas?
- The fundamental mechanism drving flow in deltas is the pre-existing inertia of the river as it enters the sea (or lake)
- Jet theory explains how turbulent mixing of the jet (river) on entering the standing body of water leads to energy diffusion
- The turbulent ‘‘jet’’ (current) loses energy and begins to deposit bedload and suspended load at the river mouth
Describe hyperpycnal flow
In a delta when the jet density > ambient density (e.g. high suspended load entering fresh water), the jet hugs the bed
- sediment hyperconcentration suppresses jet turbulence, so inertia of the flow is maintained (especially where there is a steep gradient)
- Jet is ‘‘hyperpycnal’’
- Makes gravity deposits not deltas
Describe hypopycnal flow
- When density of the jet < ambient density (e.g. sluggish, low suspended load entering salt water), the jet detaches from the bed
- Jet is ‘‘hypopycnal’’
- Cannot drive bedload - coarse load rapidly deposited at the river mouth (mouth bars)
- Suspended load carried out to the shelf, or reworked by tide and wave processes - shelf muds
What is homopycnal flow?
When the jet density = ambient density
- jet expands and decelerates rapidly but the jet remains in contact with the bed
- High bed friction slows flow so rapid deposition of bedload and suspended load
- Because once the first bar has been deposited in from of the river mouth it drastically slows down the flow resulting in more sediment being deposited on either side of the bar
- Creates radial ‘fan’ deltas at river mouth
Describe the two ‘‘classic’’ characteristics of deltas
1) Display seaward dipping surfaces called clinoforms
- caused as a result of building out into deeper water, can be used to represent time
2) Form coarsening-upward successions
- because coarse bedload is deposited rapidly at the mouth bar
- finer suspended load is carried further offshore
Describe a classic delta coarsening-up succession
- prodelta muds
- mouth bar (delta front) heterolith and sandstone
- channel-based sandstones of the channels feeding the delta
Describe an example of a ‘‘pure’’ delta shallowing up succession
Frewens Sandstone, Cretaceous, Wyoming
from top to bottom of the exposure
* cross bedded ripple cross laminated or massive mouthbar sandstones
* Heterolith: laminated mud and ripple cross laminated sandstone: hyperpycnal flows, or expansion of homopycnal plume (floods) and suspension setting
* Offshore laminated mudstones transported by hypopycnal plumes
