sedimentary environment.pdf

8/22/2019 Sedimentary environment.pdf

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CAUSE EFFECT

Process

Physical

Chemical

Biological

Sedimentary

Environment

Erosional

Non-depositional

Depositional Sedimentary

Facies

Geometry

Lithology

Sedimentarystructures

Paleocurrents

Fossils

The relationship between sedimentary environments and sedimentary facies

CAUSE EFFECT

Process

Physical

Chemical

Biological

Sedimentary

Environment

Erosional

Non-depositional

Depositional Sedimentary

Facies

Geometry

Lithology

Sedimentarystructures

Paleocurrents

Fossils

The relationship between sedimentary environments and sedimentary facies

Terrestrial/Continental

Alluvial fan

FluvialLacustrine

Eolian

Glacial

Braided

Meandering

Transitional

(Shorelines)

Lobate (deltas)

Linear (beaches)

Lagoons

Marine

Reef

Shelf

Submarine channel andfanPelagic

Classification of Major Depositional Environments

Sedimentary Environments• A part of the earth’s surface, physically, chemically, and

biologically distinct from adjacent terrain.

• defined by, fauna and flora, geology, geomorphology,

climate, weather, temperature, and if sub-aqueous, the

depth, salinity, and current system of the water.

• could be a site of erosion, non-deposition, or deposition.

• Erosional/non-depositional environments

preserved in the rock

record as unconformities.

Sedimentary Facies• A mass of sedimentary rock which can be defined

and distinguished from other by its geometry,

lithology, sedimentary structures, paleocurrent

pattern, and fossils (Selley 1970).

TERRESTRIAL ENVIRONMENTS

Alluvial Fan Environment• What is an alluvial fan?

• a fan-shaped, terrestrial deposit

• Where is it found?

• At the transition of mountains and plains

• where there is tectonic movement which

maintains high relief between mountains and

plains

• How is it formed?

• mountains shed sediment off their flanks,

streams carry it away as alluvium.

• stream carries lots of alluvial sediment

easily when its slope is steep and energy is

abundant.

• As stream valley emerges into a basin

(plain), slope and energy decrease

deposition of sediments in an alluvial fan

• Deposits

• lots of coarse particles

• usually sandstone and conglomerate

• poor sorting

Fluvial (River) Environment

Rivers/streams are elongated bodies of water that flow through

channels

Three ways streams transport sediments:

• in solution (dissolved load)• in suspension (suspended load)

• along the bottom of the channel (bedload)

Type and amount of material in suspension is

controlled by:

• water velocity

• settling velocity of each grain which depend

on:

• size

• shape

• specific gravity

bedload

• composed of coarser particles - cannot becarried by suspension

• bedload particles move along by:

• rolling

• sliding

• Saltation

Ability of streams to carry sediments is described by:

• capacity - maximum load of sediment that a

stream can transport

• competence - measure of the maximum size

of particles it is capable of transporting

Two Types of Channels:

1. Meandering

• consist of a single channel and thalweg (deepest poin

in a channel cross section)

• low gradient(slope) and high sinuosity

(curved/sinuous shape in map view)

• sediments deposited at the inner sides of meanders

(channel bends)

• associated with vegetated areas under a humid

climate

• deposition of sediments takes place in the channel, on

the levees and in the basins.



• gravel and coarse sand are normally found on the

channel floor (`lag deposits').

• finer sand settles along the inner bends of the river,

on so-called `point bars'.

• Associated with oxbow lakes

Evolution of an Oxbow lake (1) On the inside of the loop, the river travels more

slowly leading to deposition of silt.

(2) Meanwhile water on the outside edges tends to

flow faster, which erodes the banks making the

meander even wider.

(3) Over time the loop of the meander widens until

the neck vanishes altogether.

(4) Then the meander is removed from the river's

current and the horseshoe shaped oxbow lake is

formed.

Without a current to move the water along, sediment

builds up along the banks and fills in the lake.

2. Braided

• have a single channel of low sinuosity (almost

straight) and high slope, with multiple `thalwegs' and

bars.

• high sediment load

• During times of maximum discharge, the channel is

completely inundated

• In times of low discharge, multiple thalwegs and barsreappear within the channel

• occur in areas with a highly irregular water

regime, and abundant sediment supply

• Deposits contain alternating areas (lenses)

of coarse gravel and sand

Lacustrine Environment• From the word “lacuna”, meaning lake

• lake - landlocked body of standing, non-marine water

• formed in depressions or basins with internal

drainage or limited flow

• Geometry - circular or elongate in plan view;

lenticular (lens-like) in cross section

• Low energy laminated fine sediments

Eolian Environment• eolian (from Aeolus, Greek god of winds) - describes

activity of deposits of winds

• deserts (arid environment)

• wind

Point bar

deposits



• a turbulent stream of air

• like water, it has the ability to erode,

transport and deposit

• two properties:

• low density - limits competence

• unrestricted flow - enables spread

over wide areas

• lack of rain allows more effective wind

work

• sand transport

• Creep - large particles are rolled to the

surface after coming into contact with

saltating particles.

• Saltation - bouncing and jumping movement

of grains. Involves bedload.

• Suspension - occurs when fine dust and dirt

are lifted into the wind. Involves suspended

load.

Wind Erosion• needs chemical and mechanical weathering to act

effectively

• two types of wind erosion:

• Abrasion

• Sandblasting• shaping of solid rock

surfaces by constant

impact of grains by wind.

• deflation

• erosion of ground

when dry, loose particles of

dust and salt are lifted and

blown away

Deserts• concentrated in two regions:

• subtropics

• middle-latitudes

• areas where rainfall is less than 250 mm (10 in.)/year

or where evaporation exceeds precipitation.

• Rainfall in deserts may vary from 0.2 cm./yr. to about

40 cm./yr. Rainfalls of 5-20 cm./yr. are common.

• Temperature extremes can vary from –60 degrees F.

in Mongolian deserts to 137 degrees F. in the SaharaDesert. Temperatures in excess of 180 degrees may

occur in sands exposed to full solar radiation.

• Great daily extremes can occur.

• Due to lack of vegetation, wind velocities are high.

Causes of Deserts

• caused by high mountains causing available moisture

to condense and precipitate on their higher parts,

reducing moisture available for lowlands in the lee of

mountains.

• Direct blocking of moisture may also occur.

Wind Deposits

• deflation lag deposits - Coarsest clasts (desertpavement)

• loess - Unconsolidated, unstratified aggregation of

small, angular mineral fragments, usually buff in

color. Generally believed to be wind-deposited.

• dunes - Sand dunes form when there is (1) a ready

supply of sand, (2) a steady wind, and (3) some kind

of obstacle such as vegetation, rocks, or fences, to

trap some of the sand. Sand dunes form when moving

air slows down on the downwind side of an obstacle.

Dunes may cover large areas and reach heights up to

500m.Ventifacts - sha ed b the wind



Types of Sand Dunes:• Barchan dunes - crescent-shaped dunes. They

form in areas where there is a hard ground surface, a

moderate supply of sand, and a constant wind

direction.

• Transverse dunes- large fields of dunes that

resemble sand ripples on a large scale. Consist of

ridges of sand with a steep face in the downwind

side, form in areas where there is abundant supply of

sand and a constant wind direction.

• Linear dunes - long straight dunes that form in

areas with a limited sand supply and converging wind

directions.

• Parabolic dunes - are "U" shaped dunes with an

open end facing upwind. Form in areas with

abundant vegetation and constant wind. Most

common in coastal areas.

• Star dunes - dunes with variable arms and slip face

directions. Form in areas with abundant sand supply

and variable wind direction.

Glacial Environment

• Glaciers• permanent (on a human time scale) body of ice that

shows evidence of downward movement due to

gravitational pull.

• form at or above snowline

• Snowline - where ice can be created and

remain all year round

• The snowline, at present, lies at sea level in

polar latitudes and rises up to 6000 m in

tropical areas.

• formed by recrystallization of snow due to pressure

of overlying compacted snow.

• Recrystallized snow has decreased air and

increased grain size and density forming

solid blocks of ice.

Types of Glaciers

• Alpine/Mountain Glaciers

– Relatively small glaciers at higher

elevations in mountainous regions.

• Ice Sheets: (Continental glaciers):

– the largest types of glaciers on Earth.

– cover large areas of the land including

mountain areas.

– Modern ice sheets cover Greenland and

Antarctica.

• Ice Shelves:

– are sheets of ice floating on water and

attached to land.

– usually occupy coastal embayments.

• Glacial Erosion • Plucking -particle detachment by moving glacial ice

• Abrasion - debris in basal ice grinds into the bedrock

and produce:

– Glacial striations - long parallel scratchesand grooves that are produced by rocks

embedded in the ice scraping against

the rock underlying the glacier.

– Glacial polish - rock that has a smooth

surface produced as a result of fine

grained material embedded in the glacier

acting like sandpaper on the

underlying surface.

Landforms produced by mountain glaciers:

Cirques - bowl shaped depressions that occur at the heads of

mountain glaciers

Glacial Valleys - Valleys that once contained glacial ice

become eroded into a "U" shape in cross section.

Arêtes - If two adjacent valleys are filled with glacial ice, the

ridges between the valleys can be carved into a sharp knife-

edge ridge, called an arête.

Horns - Where three or more cirques are carved out of a

mountain, they can produce a sharp peak called a horn.

Hanging Valleys - A valley that has greater elevation than the

valley to which it is tributary.

Fjords - narrow inlets along seacoasts once occupied by a

fjord glacier.

Barchan

Transverse

Longitudinal

Parabolic

StarStar



Glacial Deposits • Since glaciers are solid they can transport all sizes of

sediment, from huge house-sized boulders to fine-grained material.

• Glacial Drift – general term for glacial deposits

• Till - nonsorted glacial drift deposited directly from

melting ice. A till that has undergone diagenesis and

has turned into a rock is called a tillite.

• Moraines – linear deposits of till produced by the

movement or retreat of glaciers

• Glacial Marine drift • Unsorted chaotic deposits of

sediments/rocks on seafloor or lakebeds

brought by melted glaciers.

• Large single rock bodies at the floor of a

water body is called a dropstone.

TRANSITIONAL ENVIRONMENTS Transitional environments are those at or near the transition

between the land and the sea.

Delta• prograding depositional bodies that form at the point

where a river drains into a lake or sea.

Parts of a Delta:

• delta plain - composed of meandering flood plains,

swamps, and beach complex.

• delta front - steeper part.

• prodelta - broadly sloping that grades into the open

shelf.

Factors Affecting Delta Formation and Facies:• water and sediment yield of the river

• differences in river/sea water densities, buoyancy,

salinity

• shelf slope and topography

• wave and tidal energy acting on the coast

• along shore winds and currents,

• tectonics (subsidence) of the receiving basin.

Types of Deltas• River-dominated

– large sediment volume

– lobate shape = moderate sediment supply

– elongated when sediment supply is large

• Tide-dominated

– linear features parallel to tidal flow andperpendicular to shore.

• Wave-dominated delta

– smoothly arcuate; wave action reworks

sediment.

– much sandier than the other types of delta.

Delta sediments• Sand, mud, sometimes gravel

• Decrease in grain size as you move away from land

• General coarsening upward due to progradation

Lagoon• shallow salt water body separated from the deeper

sea by a shallow or exposed sandbank, coral reef, orsimilar feature

• quiet waters allow fine silt and clays to settle out of

suspension, forming sequence of mudstone and shale

• overgrown with vegetation forming salt marshes,

coal, and peat swamps, or algal mats.

• some cases, evaporites are formed.

Beach• shore of a body of water formed and washed by

waves and tides.

• usually covered by sandy or pebbly material

• usually well sorted sand and pebbles, accompanied

by mud, cobbles, boulders, smooth rocks and shell

fragments.

• Wave action

– Longshore current • a current that moves parallel to a

shore

• formed from the momentum of

breaking waves that approach shore

obliquely.

– Longshore drift

• the movement of sediment along a

beach by swash and backwash of



waves that approach the shore

obliquely.

• Spit • long ridge of sand deposited by longshore

current and drift

• attached to a land at upstream end.

• Tombolo • a sand or gravel bar that connects an island

with the mainland or another island.

MARINE ENVIRONMENT• Shallow marine

– Reefs

– Continental shelf

• Deep marine

– Submarine canyons and fans

– Pelagic

Reefs• wave-resistant, mound-like structures usually made

of fossiliferous carbonates (coral reefs) and/or sand

• build up on continental shelves

• Fringing Reefs– coral reef that is directly attached or borders

the shore of an island or continent.

• Barrier Reefs

– a long narrow coral reef roughly parallel to

the shore

– separated from it at some distance by a

lagoon.

• Atoll

– continuous or broken circle of coral reef and

low coral islands surrounding a central

lagoon.

Evolution of reefs

Continental Shelf • continuous with the coastal plain sequences of the

continents

• part of the continental margin that is between the

shoreline and the continental slope (~200m)

• carbonates, sand and mud.

• fossils are mostly marine invertebratesCarbonate compensation depth

• CCD

• Depth where the rate of dissolution and

precipitation of carbonates is equal

• Below this depth very little or no carbonates

• Continental Slope• between the continental shelf and continental shelf

and continental rise (oceanic trench)

• Continental Rise• between continental slope and abyssal plain

• gentle incline and generally smooth topography

• may bear submarine canyons and fans

• TURBIDITES

• Deposited by sediment-laden currents flowing

down submarine canyons and fans

• Sand and mud

• Abyssal Plain• Pelagic open ocean

• flat region of the ocean floor

• covered with pelagic mud with fine sand layers

from nearby turbidites

• fine-grained limestones (micrite), chert

sedimentary environment.pdf

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