Marginal Marine and Open-Shelf Marginal Marine and Open-Shelf EnvironmentsEnvironments

A delta forms where a river meets the A delta forms where a river meets the sea.sea.

DeltaDelta

• 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.

DeltaDelta

• 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).

DeltaDelta

• 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.

LagoonsLagoons

• 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.

LagoonsLagoons

• 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.

LagoonsLagoons

• 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.

Open-Shelf Deposits Include TempestitesOpen-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.

FossilsFossils

• 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.

ReefsReefs

• 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.

ReefsReefs

• 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.

ReefsReefs

• 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.

ReefsReefs

• Look at Figure 6-12 to see the development of a coral atoll in the Pacific.

• Charles Darwin explanation is still accepted today.