1.12: 13. Siliclastic Shorelines and Estuaries
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The vast majority of ancient sedimentary rocks accumulated in near-coastal depositional environments. These environments have sediment supplied by rivers and the sediment gets deposited just below sea level due to the rapid decrease in flow speed as rivers enter the oceans. The overall geometry and characteristics of shore lines are generally determined by 5 factors:
- Sediment supply (high=constructional, low=erosional);
- Wave energy (high=erosional, low=constructional);
- Tidal range (changes shape of the shoreline, but can be either erosional or constructional);
- Climate (changes sediment composition due to weathering and carbonate/evaporite formation, can be either erosional or constructional).
- Tectonics and geomorphology
Shorelines erode when the supply of sediment to the shoreline is less than the transport of sediment away from the shoreline. For example, coastlines with strong waves and storms but little sediment input from rivers tend to erode. Also, coastlines with steep topography and a narrow continental shelf tend to be erosional because much of the sediment gets transported offshore. These shorelines can develop wave-cut platforms.
Erosional shorelines are preserved as unconformities in the rock record.
Shorelines accumulate sediment when the supply of sediment is greater than the rate of transport of sediment away from the shoreline. Deltas are examples of prograding constructional shorelines because the sediment supply is so large that they build outward from the coast. Constructional shorelines also form with lower sedimentation rates. They just need enough sediment to accumulate material to go into the rock record. Estuaries are flooded river valleys that accumulate sediment and can be parts of constructional shorelines even though the rate of sediment accumulation in them might be low.
Beaches, Barrier Islands, and Lagoons
Well developed beaches tend to form in areas with high sediment supply and high wave energy. They are constructional features. Tidal range and climate can vary and will affect the geometry of a beach, but beaches can form in a full range of conditions.
Wave action is critical to beaches. Waves come in off the ocean, and start to feel the shore, especially on the upper shore face. Wave ripples form here, as well as other cross stratification from storms and tidal channels. In shallower water, the sand and rocks are washed back and forth almost continuously which produces well rounded and well sorted grains. The grain sizes present depend on the wave energy and can range from cobbles to fine sand. In the zone where waves break, the erosional force is particularly strong due to high flow speeds and abundant sediment suspended in the water. The eroded sediment is washed up onto the foreshore and deposited as the water drains back into the ocean. If the waves are breaking very close to the beach or the beach front is very steep, the sand gets washed back into the ocean and there is net erosion. The deposition of sand leaves stratification parallel to the slope on the beach. The angle of sand on a beach is always significantly less than the maximum angle of repose because of the constant wave action. As wave strength or sediment supply vary, you can get cross stratification. This is often disrupted by burrowing organisms such as clams and crabs, but can be preserved.
Over time, sand builds up at the far reach of the waves forming a high called a berm. Storm surges wash over the berm and deposit sediment on the landward dipping surface. They can also erode the berm. When a beach develops offshore of exposed land, it creates a barrier island. These islands are very dynamic, shifting in location, growing, and eroding as storms reshape them.
The area behind the berm can either be land or water. If the beach transitions directly into land, the overall depositional environment is called a strand plain (I don't know why). If the area behind the berm is a lagoon or estuary, the berm forms a barrier island (or peninsula) that protects the area from the full force of marine processes, i.e. waves and storms.
Wind commonly reworks this sand to form coastal eolian dunes. These dunes are similar to those in arid environments, but due to the proximity of the ocean, they are commonly more moist, and vegetation is common. The vegetation stabilizes the dunes reducing their migration as well as disrupts stratification in them.
Tidal Channels: Areas of strong flow like river channels, scour, lag, dune migration (usually only one direction). migration avulsion, sand bars; mud drapes during slack flow, different fauna
Mud flats flooded at high tide and exposed at low tide, fine seds brought in w/ rising tide deposited as the tide turns. tidal channels develop w/ ebb flow, vegetation between them traps sed. High sed rates, very nutrient rich, lots of bioturbation.
Lagoons: Lagoons are areas of quiet water and generally sites of mud deposition (either carbonate or siliciclastic). Coarser sediments are washed in during storms and are commonly wave rippled. Tidal channels commonly cut into lagoonal deposits as the location of concentrated flow migrates back and forth. Lagoons have extremely variable water chemistry depending on climate. If the climate is humid, they commonly contain a mix of fresh and salt water (brackish). If the climate is dry and a river does not flow into it, they tend to be evaporitic and hypersaline. The types of organisms living in the lagoon is a good indicator of local climate.
Tidal range is very important for estuaries. If the tidal range is low, there is a stable shore line and little mixing of fresh and salt water. Most of the estuary is fresh water, like the Everglades. If the tidal range is high, the ebb and flood of the tide feeds way into the river mouths and tidal channels and mud flats develop.
Sea level change
Sea level can change due to subsidence of the shoreline, which can be tectonic or due to compaction. As sea level changes, the location of these different facies shift in space. As sea level rises relative to the land, they shift one way, and vice versa.
Estuaries: Estuaries are river mouths that have been drowned by relative sea level rise and contain a mix of fresh and salt water. Both marine and fluvial processes influence them. With long term global sea level rise, many river mouths were inundated with sea water and became estuaries which are very important ecological sites. We had more than geologically typical until we started filling them in.