1. sedimentary facies

1. Sedimentary facies

2. Facies associations

3. Sedimentary structures

4. Biotic assemblages

Sedimentary depositional Environments

Sedimentary deposits have a set of associated conditions.

To recognize a depositional environments We can use a combination of:

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Sandy river channel

Floodplain

Modern depositional Environments

Meanders

Floodplain

Niger Delta, Nigeria

Suface of 70.000 Km2


Indus Delta, Pakistan


Aeolian dunes, Namibia desert

Beach dunes, California


Perito Moreno Glacier, Argentina

Crusoe Glacier, Swiss

Glacier Front

Cirque glaciers


Lagoon

Bora Bora, French Polynesia

Patch Reef

Great barrier Reef, Australia

Lagoon

Channel


Lagoon

Patch ReefTurbidity flow

Turbidity flow


La Playa. Death Valley, California

Uyuni salt lake. Bolivia

Sabkha (coastal salt flat), Abu Dhabi, UAE


Defining a sedimentary rock we will be able to recognize the palaeoenvironment of formation, learning from the modern

depositional environments.

“Present is key to understand the past”

Charles Lyell, 1830

Uniformitarianism Theory

Modern and

ancient sand

dunes

“Present is key to understand the past” By examining the characteristics of various environments on Earth today we can interpret

the environments in which ancient sediments were deposited.

Modern and

ancient coral reef

Modern and ancient

evaporitic deposits

…..not only on our Planet

Layers of sedimentary rocks on Mars suggest that it may have hosted numerous lakes and shallow seas.

We can use a combination of:

1. Sedimentary facies

2. Facies associations

3. Sedimentary structures

4. Biotic assemblages


How can we recognize a depositional environments looking at a sedimentary rock?

Depositional Environment

Sedimentary Facies

To define a sedimentary facies we include information about:

Grain sizes and typesSedimentary structures ColorBiota

-Used to interpret rock record through identification of ancient depositional environments-The term is used at different scales (basin, outcrop, sample, thin section)

The sum of the characteristics of a sedimentary unit resulting from some particular set of physical, chemical and biological parameters that work to produce a unit with specific

textural, structural, and compositional properties (after Boggs, 2001)

Not all the aspects of the rock are necessarily indicated in the facies name and different characteristics can be emphasized, depending on

what must be highlighted to better characterize the rock

The full range of the characteristics of a rock are given in the facies description

that would form part of any study of sedimentary rocks(facies analysis).

Sedimentary Facies

Sedimentary Facies

Example:

cross-bedded quartzose brownish medium sandstone

•cross-bedded brownish sandstone

•brownish medium sandstone

•cross-bedded quartzose sandstone

•quartzose medium sandstone

Or…if we wish to give more importance to specific features:

Sedimentary Facies

• Lithofacies: the description is confined to the physical and chemical characteristics of a rock.

• Biofacies: the description is focused on the fauna and flora content of a rock.

•Ichnofacies: the description is focused on the presence of the trace fossils (record of biological activity) in the rock.

A single rock unit may be described in terms of:

Example:

A rock unit might have as lithofacies a “grey bioclastic packstone”, a biofacies of “echinoid and crinoids” and with a ‘Thalassinoides’ ichnofacies.

The sum of these and other characteristics would constitute the sedimentary facies, first fundamental feature to individuate a depositional palaeoenvironment.

A sedimentary facies association refers to two or more facies which belong to the same depositional palaeo-/environment and which in turn are determined

by the same combination of processes which occur(ed) there (Collinson, 1969; Reading & Levell, 1996).

Sedimentary Structures

Wave Ripples: e.g. shoreface environment

Fenestral fabric: tidal carbonate environment

Mud cracks: sub-aerial environment

Cross-bedding: e.g.eolian environment

Biogenic features

5 μm

Rudists (molluscs)

S. dinarica (Green algae)

Bioturbation

Calcareous nannoplankton

Sedimentary Structures, biota and depositional environments

Larger foraminiferaPlank. foraminifera

Immenhauser, 2005

Walther’s law of faciesin a conformable succession the only facies that can occur together in

vertical succession are those that are observed side by side in nature

i.e. a vertical change of facies implies a corresponding lateral shift of

facies within a relatively conformable succession of genetically related

strata Johannes Walther (1860–1937)

Lateral and Vertical facies succession

Continental

Fluvial

•Alluvial

•Braided system

•Meandering system

Desert

Lacustrine

Glacial

Marginal-marine

Delta

•Delta plain

•Delta front

•Prodelta

Beach/barrier island

Estuarine/lagoonal

Tidal flat

Marine

Neritic

•Continental shelf

•Organic reef

Oceanic

•Continental slope

•Deep ocean floor

Depositional environments

Modern Environments• Plan view, some cross-sections

• Geomorphology

• Sedimentological process of erosion, transport and deposition

• Climatic setting (rainfall, T range, …)

Fossil record

• Cross sections, rare plan views (3D seismic)

• Sedimentary structures to reconstructs sedimentological process

1. Fluvial

a. Alluvial fan

b. Braided stream

c. Meandering stream

2. Desert

3. Lacustrine

4. Glacial

5. Delta

a. Plain

b. Front

c. Prodelta

Depositional environments-continental

The fluvial environment

Fluvial Environment

Three main geomorphological zones

• Erosional zone

• Transfer zone

• Depositional zone

Are not always all present!

The fluvial environment

• Rivers are main arteries of sediment transport of clastic detritus and supply almost all clastic deposits to oceans (exc. glaciers, wind, coastal erosion)

• Rivers also act as depositional systems

• Best know depositional environments because it is easy to study modern river processes and relate them to the stratigraphic record

Hay, 1998

Global sediments transfer into the oceans

From catchment area to depositional environments

Some definitions

• FLUVIAL: anything associated with rivers

• CHANNELS: depressions or scours in land surface which contain the flow in a river system

• FLOODS occur when water is supplied into a river at a higher rate than can be carried within the channel

• OVERBANK or FLOODPLAIN: area of land between or beyond channels which receives water only when the river is in flood

• ALLUVIAL: more general term for land surface processes that involve the flow of water (e.g. alluvial fans)

• ALLUVIAL PLAIN: low-relief continental area where sediment is accumulating, which may include the floodplains of individual rivers.

Catchment area (drainage basin)

Area that supplies water to a river system through superficial and groundwater run-off, and soils (acting as a sponge).

Main controls on water supply•Size of the drainage basin•Climate

River discharge (D)

A: cross-sectional area occupied by the flow (m2 or ft2).V: average flow velocity (m/s or ft/s).

D=AVD = (1.2 x 59) x 1.5D = 106.2 ft3/s

Normal conditions Floods conditions

D = (3.1 x 82) x 3.1D = 788.02 ft3/s

The volume of water flowing in a river in a time period

Perennial fluvial systems

Ephemeral fluvial systems

•Large drainage basin

•Regular rainfall

•Poor to moderate discharge variation

•Small drainage basin

•Seasonal rainfall

•High discharge variation

Fluvial Deposits

Flow within a river channel

The river segments of downstream erosion and/or deposition within a channel are given by the position of the thalweg

Due to friction with the bottom of the channel, the banks and air above, the highest power of the flow is in the deepest part of the channel.

The streamline of the deepest part of the channel is called thalweg.

Types of rivers

Uncommon

floodplain

floodplain

floodplain

floodplain

floodplain

Lateral bars

Mid-channel bars

From the point of view of sedimentary geology,

meandering and braided rivers are the most

important because these are the types that are

most frequently recognized in the geological record

Types of rivers

Stream Load

Sediment in water moves as suspended load or bed load

Bedload rivers = BraidedRivers with a high proportion of sediment carried by rolling and saltation along the channel floor

are referred to as Bedload rivers.

The braided form is given by the sand or gravel that get deposited along the channel as bars and split the flow in two directions

Fluvial –Braided Stream

Relatively steep slope, rivers have

several channels separated by banks

Yield: sand (normal), gravel (flood)

Gravel river bars morphology is

modified during flooding events

Morphology of a Braided stream

Scoured channel-base

Morphology of a Braided stream

The bars in a channel vary in shape and size:

•Longitudinal : elongate along the axis of the channel.•Transverse: wider than they are long.•Linguoid: with their apex pointing downstream.

Bars may consist of sand, gravel or a mixture of both ranges of clast size (compound bars).

Bar migrationBars are not stationary for a prolonged period, they migrate.

Mid-channel gravel bars

Mid-channel sand bars

Cross-bedded bar deposit

Braided stream cross-bedding

Depositional architecture of a braided river

Lateral migration of the channels leads to abandonment of bars and creates a channel-fill succession

Right to left migration of a braided stream

Braided river

Mixed load rivers = Meandering

Rivers sediment carried as both bed- and suspended load

“Gooseneck”, Utah Alaska

Fluvial – Meandering Stream

Present in continental plains

One single, sinuous channel

Meanders due to a perturbation of uniform flow within

a channel (e.g., sediments, talus)

Clay deposition: more stable river bars (less erodible)

Erosion and deposition in meanders

The bank of the channel close to thalweg has faster flowing waters leading to erosion, and consequent

deposition on the opposite side

Morphology of a meandering river

Fluvial – Meandering Stream

Meandering Rivers: Point Bars

Meandering channels migrate, leaving sandy deposits at inside bends

Point bars of the inside bends of a meandering river

Sedimentary structures in point-bars

• Lateral migration, grading vertically from coarser (faster flow) to finer-grained (floodplain deposits)

• Large scale cross-beds at base (dunes), and smaller sets at the top (ripples)

• If lateral migration is not continuous (series of steps), periods of non- deposition and slower flow may be characterized by mud-draping (lateral accretion surfaces)

Lateral accretion surfaces

Large low-angle cross stratification, perpendicular to the flow!

Lateral accretion of a point bar in a channel

Floodplain deposits

Levèe: repeated deposition of sand close to the channel edge forming a bank which is higher than the level of the floodplain.

When the levée breaks, water mixed with sediment is carried out onto the floodplain to form a crevasse splay, a low cone of sediment formed by water flowing through the breach in the bank and out onto the floodplain.

Upward-coarsening of the sediments

Fine sandSilt, clay,

soil

Thin ,fine grained depositsDessication structuresSoils formation

Floodplain deposits

Fan shapeDm thicknessNormal gradingInterbedded with floodplaindeposits

Depositional architecture of a meandering river

Point of avulsion

Oxbow lake

Oxbow lake former by cut-off of a meander loop

Braided and Meandering Rivers

Meandering Streams

• Single channel• High sinuosity• Mostly fine grained• Mixed load• very low depositional gradient (1/100)°

Braided Streams

• Multiple Channels• Low sinuosity• Mostly coarse grained• Bed load• low depositional gradient (<0.5°)

Fluvial – Meandering vs. Braided Stream

Ancient Fluvial Deposits:Lithofacies

• coarser sediments in channels and finer on floodplains

• channel areas: all grain sizes, with distinctive spatial organization

• Floodplains: mixture of flood events and soils

• Relative proportions of lithofacies may help to characterize different fluvial environments

A mud-filled channel

Stacked sandstone filled channels

Floodplain deposits of sandstone and mudstone

Huesca Channel, Spain (Oligo-Miocene)

Modern Rivers Brazil

Ancient Rivers, Arizona

1. sedimentary facies

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