physical geology - coastal processes

Physical Geology lecture outline - Coastal Processes

Coastlines as interfaces between the terrestrial and the marine: very dynamic locations.

What are processes that move sediment, shape coastal landforms, and determine the geologic evolution of coastlines?

waves:
- what determines wave size?
- the importance of wave base.
- prevailing wind directions and longshore drift.
storms:
- deeper reach through increased wave base.
- come in a range of sizes.
- return flow of storm surge also transports sediment deep offshore to create distinctive layer - a storm bed. This often represents a permanent sediment loss from shoreline system.
- can consider storm energy levels.
tides.
river sedimentation ('deltaic' forces): supplier of sediment.
uplift vs. subsidence (tectonism).
mass wasting: wave undercutting, often accelerated during storms.
biologic (esp. reefs, mangrove swamps).
offshore currents.
ice.
eolian processes: wind blown sands, coppice dunes.
eustatic changes.
humanity.

There is a great diversity of coastlines. We will look at just three types:

deltas.
barrier island complexes.
emergent shorelines.

Deltaic complexes:

Image of a delta building out into a fjord in Svalbard. Since this fjord was occupied by ice some 15,000 years ago or less, we know this delta has built out in that time period. Note the active portion to the left where transport and deposition is occuring via a braided stream. The oldest portion of the delta surface is the darker surface, where tundra vegetation has had a chance to establish itself. The braided stream has wandered back and forth over the delta surface with time. Note also the thin narrow beach ridge that has trapped small ponds behind it. What process is it that forms the smooth beach ridge, and why doesn't the delta have a birds-foot shape? Remember that this is in a more sheltered fjord.
definition of a delta: body of sediment that forms where a river meets a lake or the sea.
how can deltas be classified?
cross section view and progradation and coarsening upwards sequence.
evolution of in map view, lobe switching and abandonment.
subsidence and mass wasting as contributing processes.
the Atchafalaya and the Mississippi delta dynamics.
stream table example.

Barrier Island complexes:

common with low profile sandy slopes, e.g. associated with coastal plains.
ridge fundamentally caused by wave action piling up sediment and vegetation working to stabilize it.
better known examples: Padre Island, Galveston, Hilton Head - common along the Gulf Coast and southern half of east coast.
air photo mosaic images of barrier island complexes SE of Savannah Georgia taken from USGS (terraserver site).
air photo mosaic form USGS (terraserver site) of Gulf coast barrier island SW of Houston.
false color satellite image of thin barrier island complexes along North Carolina coast.
geomorphic components of the system: offshore sand bars, shoreface, dunes, spillover fans, tidal channels, lagoonal area.
character of sediments in different portions?
dynamics of change? barrier island migrations and regressions and transgressions.
storm deposits:
- storms transport sediment back into the lagoonal area via spillover fans.
- storms transport debris semi-permanently offshore.
- brings up question of what is more important to the shape and character of the Barrier Island - day to day events and dynamics (e.g. tidal events) or rare events such as the 100 year hurricane. Could compare, for example, the volume of sediment moved by each during the time span under consideration.
what will cause barrier island dynamics to change and migrations to occur? sediment budget, storm energy, tectonic and eustatic changes.

Emergent coastlines:

some associated features - sea cliffs, sea stacks, sea caves, sea arches.
importance of wave cut terrace formation and mass wasting.
examples: sections of California's coast line, large parts of the Arctic due to rebound from glacial retreat and unloading.
why discrete 'steps' seen?

This is a photo of some sea cliffs that surround Bear Island (Bjørnøya) on the Barents Shelf. The upper flat surface was due to erosion by an ice sheet. Since then waves have cut back into the bedrock to create these sea cliffs. The bays form along material that is easier to erode while the headlands are more resistant.

In this photo, also from Bear Island, the surface zone outlines a shallow and relatively flat area that is being cut by constant wave action - i.e. a wave cut terrace in the process of being formed. Note the sea cliffs in the back, with some talus at their base. Here the process of undercutting and mass wasting are important.

In this photo from Bear Island a sea arch has formed as the waves have cut through a rock fin of dipping strata with softer shales beneath harder sandstones. This is one of a number of erosional features such as sea stacks and sea caves, that form along erosional and emergent coastlines. The rebound of the crust after the glaciers melted away makes many coastlines in the Arctic area emergent.

This is a photo from a mountain top looking down on curved beach ridges in Svalbard. There are literally scores of beach ridges here, marking the gradual rise of the land out of the sea. Each beach ridge is made up of coarse gravel, and it is possible to fine driftwood and whalebones in some of them. The uplift is due to glacial rebound and associated unloading. An active surf zone can be seen on the far shore and the process continues, although probably at a decreasing rate.

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