Facies Concept

Facies ConceptLecture 3#

DefinitionFacies is a body of rock characterized by a particular combination of lithology, physical and biological structures that bestow an aspect that different from the bodies of rock above, below and laterally adjacent

The facies concept refers to the sum of characteristics of a sedimentary unit, commonly at a fairly small (cm-m) scale including:

Lithology Grain size Sedimentary structures Color Composition Biogenic content

Facies associations/assemblage :o constitute several facies that occur in combination,

and typically represent one depositional environment (note that very few individual facies are diagnostic for one specific setting)

o groups of facies genetically related to one another and which have some environmental significance (Collinson, 1969)

Facies successions: a progressive change in certain facies properties in a

specific direction (vertically or laterally).

o Facies analysis is the interpretation of strata in terms of depositional environments (or depositional systems), commonly based on a wide variety of observations

o Facies Model is a general summary of a particular depositional system, involving many individual examples from recent sediments and ancient rocks

Why do we need to understand the deposits of differing environments?

Sedimentary & geological history (small-large scale)

Economic importance -e.g. hydrocarbons, water

Economic importance -minerals and metals (e.g. gold-diamonds-gypsum-aggregates)

Prediction/management of contemporary environments

Every depositional environment puts its own distinctive imprint on thesediment, making a particular facies. Thus, a facies is a distinct kind ofrock for that area or environment.

Sandstone facies (beach/shallow marine environment) Shale facies (offshore marine environment) Limestone facies (far from sources of terrigenous input/ deep sea)

Means that two different facies found superimposed on oneanother and not separated by an unconformity, must have been deposited adjacent to each other at a given point in time

Johannes Walther (1860-1937)

Walther’s Law: ‘Facies adjacent to one another in a continuous vertical sequence also accumulated adjacent to one another laterally’

Generally, facies are distinguished by what aspect of the rock or sediment is being studied.

Lithofacies

• Facies based on petrological characters such as grain size and mineralogy

Biofacies

• Facies based on fossil content (flora & fauna)

Ichnology

• Facies based on trace fossil

o Presence of cross bedded sandstone can form during deposition in deserts, rivers, deltas, lakes, beaches and shallow marine

o In contrast, present of hermatypic corals indicate that the sediments were deposited in shallow clear and warm seawater

Ripples developed on surface of a sand body. This texture can be diagnostic of depositional environment. These ripples are diagnostic of near-shore tidal environment, but ripples also develop in fluvial (river) and aeolian (air, sandstorm) environments.

Facies = Lithofacies (core log sample)

Facies associations/assemblage, facies sequences and facies codes

Facies Association (combination few facies)

Facies Succession (progressive change)

Beach environment

Tidal environment

Facies Succession

Beach Env.

Deltaic Env.

Sedimentary Facies :

1. Non-Marine Environment:• Lacustrine• Fluvial & Alluvial Fan• Delta & Estuarine

2. Coastal and Shore face Environment:• Estuarine• Turbidite

3. Marine Environment:

• Turbidite

Lake

Braided Stream

Alluvial Fan & Fluvial System

Meandering

Delta & Estuary

Marine delta(estuary) River delta

Turbidite System(Deep sea depositional)

Questions…