1. Marginal Marine and Open-Shelf Environments
A delta forms where a river meets the sea.

2. Delta
When a river empties into a lake, it will drop its load of sediment into a fanlike pattern.
This depositional body is called a delta.
Sand is lost first, followed by silt, and finally clay.
The delta structure includes: delta-plain, delta-front, and pro-delta deposits.
Delta-plain beds consist of sand and silt.
When a river slows down, sand builds at the bottom, causing the channel to break into smaller channels.
Distributary channels are formed.

3. Delta
Delta front beds slope seaward from the delta plain, usually lying in waters too deep to be agitated by surface waves.
Pro-delta beds consist of clay.
Fresh water is less dense than seawater that clay is carried far from the distributary channels.
Fresh water floats on top of seawater and carries the clay.
A delta pro-grades, or grows seaward, the coarse deposits build over the finer-grained pro-delta beds (Walther’s Law).

4. Delta
The Mississippi River delta projects far into the sea. It is called a river-dominated delta.
The growing portion, active lobe, is the site of the functioning distributary channels.
The Mississippi delta is shrinking rapidly.
Levees and dams have limited deposits and the removal of groundwater.
Louisiana coast loses 40 sq. miles per year and the salt water drowns wetlands.

5. Lagoons
Barrier islands – long stretches of shoreline are fringed by barrier islands, composed largely of clean sand piled up by waves.
Most barrier islands get their sand from the marine realm.
They are built up as waves and the shallow currents that flow along the coast, longshore currents, winnow sediments and sweep sand parallel to the shore.

6. Lagoons
Lagoons lie behind long, barrier islands, such as the Texas coast.
Lagoons trap fine-grained sediment and usually have muddy sands.
A barrier island and the lagoon behind it form a barrier island-lagoon complex.
Tidal flats – formed of sand – whose surfaces are exposed and flooded as the tide ebbs and flows.

7. Lagoons Freshwater gets trapped in lagoons so it looks brackish.
Freshwater runoff determines the salinity.
Laguna Madre, TX, is an example of a typical lagoon.
Excludes many forms of marine life.
Large numbers of segmented worms.
It can pro-grade over a broad belt of shoreline.

8. Open-Shelf Deposits Include Tempestites
On open shelves where tides produce strong currents and sand is abundant, currents may pile the sand into large ridges.
Where waves are stronger, waves will flatten out the bottom, and the sand will spread into sheets.
Storms will produce tempestites, which are sandy beds usually a few centimeters thick.
These deposits are graded as having formed as sand settled before silt or mud.

9. Fossils
Ancient sediments deposited within barrier island-lagoon complexes often yield fossils.
Lingula, a living fossil genius today tolerates near shore environments of brackish water with variable salinity.
In finer-grain sediments deposited off-shore is a fossil community that includes types of brachiopods and trilobites.

10. Reefs Carbonate sedimentation usually prevails.
Usually formed by organisms that secrete calcium carbonate, organic reefs form their own depositional record - as bodies of limestone.
Grow in shallow waters.
Basic framework:
Skeletons of organisms (corals)
The framework is strengthened by cementing organisms that encrust the surface of the reef.

11. Reefs Skeletons are trapped in porous material.
Many ancient buried reefs serve as traps for petroleum.
They alter patterns of sedimentation.
Leeward side (side nearest land) – there is often a calm lagoon.
Reef flat – horizontal upper surface that stands close to sea level.
A pile of rubble called talus, often extends seaward from the living surface.

12. Reefs Reefs grow rapidly in the manner of a pro-grading delta.
Isolated patch reefs are often found in lagoons behind elongate reefs.
Elongate reefs that face the open sea have lagoons behind them known as barrier reefs.
Reefs that grow right along the coastline without a lagoon behind them are known as fringing reefs.
Atolls – circular or horseshoe-shaped structures known as atolls.
Atolls usually form on volcanic islands.

13. Reefs
Look at Figure 6-12 to see the development of a coral atoll in the Pacific.
Charles Darwin explanation is still accepted today.