Fluvial Systems - meandering streams and lacustrine Flashcards
(35 cards)
What sediment size dominates meandering streams?
Mud
What are some characteristics of a meandering stream?
- single channel
- less sediment load and less variable discharge
- max velocity & erosion at outer meander, and deposition at inner meander
What is a flood plain?
Flat plain near a fluvial system, gets periodically flooded.
A meandering stream is occasionally/constantly flowing.
constantly
What is an oxbow lake?
As the meandering stream gets more and more curve-y, the water constantly wants to go straight, so eventually, the very curved part gets cut off.
Meandering streams have less/more energy than a braided stream.
less
What are some common sediments you would see in an oxbow lake?
They have a lot of clay and are organic-rich.
What is a crevasse splay?
It is like an alluvial fan in shape, but happens when a natural levee breaks when the meandering stream floods. Consist of fine-very fine grained sandstones with ripples, trough crossbedding, parallel lamination and scour and fill structures. Upper part full of roots and burrows.
One of the most characteristic products of a meandering river is the _______________ sequence, which forms at the inner bank of a meander.
point-bar
At the edge of the meandering stream bank, where the bed is just high enough to begin to slow the current, gravel and coarse sand are deposited, forming a thin, discontinuous ___________ lag.
channel
What are natural levees?
Wall of sand and mud that forms a high bank on each side of a river, formed when a flood recedes.
What are the layers in a Bouma sequence?
The classic sequence has a scoured base overlain by a massive graded bed (unit A) which represents the coarsest material to settle of suspension as the turbidity current slowed. Over is plane lamination (B) which is believed to represent high-flow-regime plane beds. C displays ripples and wave lamination, indicative of lower flow regime. D is laminated silt and E is laminated mud that settled out of suspension during the waning of the turbidity current. In some cases E is topped by laminated hemipelagic mud that settled from suspension in the episodes between turbidity currents.
Study this slide summarizing meandering stream deposits
What sediment size are lake deposits dominated by?
mud
What is a very strong defining feature of lake deposits?
Laminated mud, thin bedding
How does the water in a lake mix?
wind, especially at the upper part
if it’s shallow, there will be a mixing zone. however, the bottom is commonly anoxic (oxygen deprived)
Does the vertical deposit of a former lake area have coarsening or fining up?
Coarsen-up as the lake dries.
In the attached image, the rootlets and desiccation cracks signify edges of the lake; climbing ripples, parallel-laminated sand/mud-stone and wave ripples are at shore line; hummocky, burrows and graded beds are close to shore. The bottom: homogeneous (ie. laminated) mudstones are at the basin
How can you tell the difference between lake and ocean deposit?
Scale.
Lakes are significantly smaller. Also, lake deposits tend to be thin whereas oceans are thick.
What is black shale?
Organic rich shale
If you see a lake deposit with slightly green sediment, what would that tell you?
It is a little anoxic.
A deltatic deposit looks like an alluvial fan, but it is ______ from the source and it is ______ dominated.
far, mud
______ form where rivers carry more sediment into the sea than marine erosion can carry away.
Deltas
T/F: Deltas are more common on active, subducting continental margins
False, they seldom do bc there is no stable shallow shelf on which sediments can accumulate
What are topset beds in a Gilbert-type delta?
Topset beds are the nearly flat, upper layers of a Gilbert delta, formed in shallow water by river deposits. They consist of coarse to medium sediment (like sand and gravel) and often show horizontal bedding.
