Marginal Marine and Open-Shelf Environments

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.