facies term

FaciesFacies And And FaciesFacies Analysis Analysis

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Content

1. Introduction- Definition and History of facies

2. Types of facies- Lithofacies, ichnofacies, biofacies, seismic facies.

3. Facies sequence and facies association

4. Facies tools

5. Sedimentary log

6. Facie model 7. Facies analysis and environment

8. Applications and limitations of facies

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Definition and History of Facies

Facies is the aspect, appearance, and characteristics of a rock unit, usually reflecting the conditions of its origin; especially as differentiating it from adjacent or associated units.

The word facies was first introduced by a Swiss geologist, Amanz Gressly in 1838, as part of his contribution to the foundation to mordern stratigraphy.

Facies analysis refers to the interpretation of rocks and sediments for the purpose of reconstructing the processes that were responsible for the original deposition

Walther`s law of facies succession

In 1894, a German geologist in the person of Johannes walther proposed that Facies that occur in conformable vertical successions of strata also occur in laterally adjacent environments. Thus, facies boundaries may shift so that the deposits of an adjacent environment may lie directly atop those of a laterally related environment

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Types of facies

Lithofacies: These facies are characterized by sedimentary attributes, such as grain-size, sedimentary structures, bedding, color

Biofacies: These facies are Characterized by fossil content (body fossils ) such as shell and tooth.

Seismic facies: These facies are Characterized by seismic reflection amplitude and continuity. (Interpreted in terms of large-scale lithologic characteristics).

Ichnofacies: The assemblage of trace fossils like foot prints and burrow traces.

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LithofaciesLithofacies Biofacies

iichnofacies Seismicfacies

Facies Sequence and Facies Association

Facies association that reflect a particular depositional environment is said to be formed when a group of different individual facies caused by different processes all occur in the same environment. These facies all share the same environment.

Facies sequence or facies succession is a facies association in which facies occur in a particular order. They occur when there is a repetition of a series of processes as a response to regular changes in conditions.

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Facies b and c form

a facies sequence

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Facies Tools

Facies tools are devices from which data associated with facies analysis are acquired. They include :

Outcrops

Core samples

Wire line log

Seismic section

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FACIES TOOLS

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AN OUTCROP

A WIRE LINE LOG

A CORE SAMPLE

A Seismic section

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Sedimentary Log

Sedimentary log also referred to as litholog is a graphical representation of a series of beds in a sediment or sedimentary rock.

Litho logs consist of the following sections: Vertical and horizontal scale section

Lithology section

Facies name and facies code section

Process interpretation section

Environment interpretation section

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A sedimentary log

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Facies model

Facies model is the general summary given to a depositional system. It`s usually an interpretative device erected to explain a facie assemblage.

Facies models could take the following form:

2D VERTICAL SUCCESSION DIAGRAM 3D BLOCK DIAGRAM 4D MODEL(SIMPLY 3D ADDED WITH PREVAILING PROCESSES)

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3D BARRIER ISLAND BLOCK DIAGRAM

2D VERTICAL SUCCESSION DIAGRAM FOR A DELTA ENVIRONMENT

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4d facies model (adds understanding of process)

Facies analysis and environment

Deltaic facies and environment:

Delta can be classified based on energy into:

river dorminated delta wave dorminated delta tide dorminated deltaDeltas are generally characterized by top set bed, fore set bed,

and bottom set bed

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River dorminated delta

These are deltas in which river activities are prevalent at the delta , with the effects of wind and tides being minor at the delta front. Examples include : Mississippi delta

River dominated deltas are usually characterized by :

distributary channel interdistributary bay

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)

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River dominated: birds-foot delta(Mississippi-type)p

prodelta

Delta front

Delta plain

Mississippi delta

Prograding mouth bar succession Clean sand, low- & high-angle 3D strat.

… representing beach & dune deposits Well-sorted sand, planar & ripple x-lam.,

tangential x-bedding (bar crest); poorly sorted sand/silt, cut & fill,

small-scale trough x-bedding (bar back);

large channel fills near top (distributary channel) Interbedded mud/sand, wave & current ripple x-lam.,

parallel/lenticular lam., bioturbation, contorted bedding, possible cut & fill

… representing distal mouth bar deposit

Finely laminated mud, bioturbation, marine fauna, contorted bedding

… representing prodelta deposit

Silty/sandy slump sheet … representing redeposited

(distal) mouth bar sediment Homogenous mud, finely laminated or bioturbated, marine fauna

… representing shelf deposit

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Distributary channel fill facies

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Ripple-laminated, fine-grained sand- & siltstone,

plant-rich & with palaeosol-coal units

… representing filling of shallow channel after avulsion Trough & planar x-bedded sandstone

with unidirectional palaeocurrents

… representing migrating dunes/bars in channel

Massive, locally x-stratified sandstone with internal

erosion surfaces & some soft-sediment deformation

… representing main channel subject to flooding Basal coarse lag deposit above erosional surface

… representing channel avulsion & erosion

into delta front progradation succession

Wave dorminated delta

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These deltas are dominated by wave action at the delta front .

Examples inlcude : Nile delta

A wave-dominated delta formed where wave activity reworks the sediment brought to the delta front to form coastal sand bars and extensive mouth-bar deposits.

Wave dorminated delta

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Wave-dominated delta front succession

Medium-grained sand, plane-parallel lam. & low-

angle x-strat., passing into sand/mud with plant roots

... representing beach to non-marine deposit

Fine- to medium-grained, cross-bedded sand

... representing wave-modified mouth

bar deposit on upper shoreface Fine-grained, bioturbated sand with

HCS & plane-parallel lamination

... representing wave-swept lower shoreface deposit

CU mud to sandy mud to sand with pervasive

bioturbation, storm beds & hummocky x-strat.

... representing regressive shelf deposit

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Tide dominated deltas

These deltas are dominated by tidal activities

Coastlines with high tidal ranges experience onshore and offshore tidal currents that move both bed load and suspended load. A delta building out into a region with strong tides will be modified into a pattern that is different to both river- and wave-dominated deltas

Examples include: Gnages delta,

These deltas are characterized by : tidal flat tidal sand bar tidal channel

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Alternating sand-silt-mud, x-lam., “flaser-linsen”

bedding, small channel fills, brackish-water fauna

... representing tidal flat facies on delta plain

Erosive, FU sand with herringbone x-strat., mud

drapes, tidal bundles, ripple x-lamination ... representing tidal

channel on delta plain

Bidirectional x-bedded sand, mud drapes, tidal

bundles, reactivation surfaces, scours/channel fills

... representing migrating tidal current ridges &

inter-ridge channel fills on delta front

Bioturbated mud passing into interbedded

mud-silt-sand, marine fauna

... representing open shelf to prodelta succession

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Tidal flat and channel in Ord river delta

Why is Facies analysis essential to a sedimentologist?

Advantages and Application:- 1. simplifies and standardizes observations-permits rapid logging of field sections and drill core. 2.Facies analysis is useful in hydrocarbon exploration. 3. it is also useful in paleogeography. 4. it also finds its importance in paleoecological reconstruction. 5.Facies analysis is also useful in groundwater exploration

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Disadvantages include:

1) Discourages observations of the rare or unusual-may lead to inappropriate simplifications

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