Depositional Depositional Environments, Environments, Facies, Facies Facies, Facies Models and Models and

PaleogeograpyPaleogeograpyGeologic History in Three Geologic History in Three

DimensionsDimensions

Depositional Depositional EnvironmentsEnvironments

Sediments accumulate in some environment of Sediments accumulate in some environment of deposition or deposition or depositional environmentsdepositional environments

These areas receive net depositionThese areas receive net deposition Erosion may occur, but deposition dominatesErosion may occur, but deposition dominates Features of these depositional environments are Features of these depositional environments are

preserved in the rock recordpreserved in the rock record Examples:Examples:

Sediment textureSediment texture Sedimentary structures (formed by processes in the Sedimentary structures (formed by processes in the

environment)environment) Fossils of organisms that lived in the environmentFossils of organisms that lived in the environment

Ancient environments can be reconstructed from the clues that are preserved in the sedimentary rocks

Depositional Depositional EnvironmentsEnvironments

FaciesFacies All the properties of a body of rock that allow us All the properties of a body of rock that allow us

to differentiate it from those above, below or to differentiate it from those above, below or laterally adjacent to itlaterally adjacent to it

Properties includeProperties include Lithology – rock type, including color, etc.Lithology – rock type, including color, etc. Composition – mineral content Composition – mineral content Texture – grain size, sorting, roundnessTexture – grain size, sorting, roundness Sedimentary structuresSedimentary structures FossilsFossils

Facies means aspect – same Latin root as “face” Facies means aspect – same Latin root as “face” Overall appearance of a rock bodyOverall appearance of a rock body Facies are the products of depositional Facies are the products of depositional

environmentsenvironments Examples:Examples:

Planar laminated fine quartz arenite faciesPlanar laminated fine quartz arenite facies Bioturbated, poorly sorted muddy skeletal limestone faciesBioturbated, poorly sorted muddy skeletal limestone facies Cross-stratified arkosic conglomerate faciesCross-stratified arkosic conglomerate facies Stromatoporoid-tabulate coral reef faciesStromatoporoid-tabulate coral reef facies

Facies ModelFacies Model

An idealized description of a faciesAn idealized description of a facies Constructed from modern Constructed from modern

environments and ancient rocksenvironments and ancient rocks Serves as a Serves as a

Norm for comparisonNorm for comparison Framework for observationFramework for observation Predictor of patternsPredictor of patterns

Facies PatternsFacies Patterns

Groups of facies commonly show patternsGroups of facies commonly show patterns ProximalProximal Facies (near the source) tend Facies (near the source) tend

to be coarse grainedto be coarse grained DistalDistal Facies (far from source) tend to be Facies (far from source) tend to be

finer grainedfiner grained This pattern is displayed upstream and This pattern is displayed upstream and

down in rivers and onshore to offshore in down in rivers and onshore to offshore in coastal areascoastal areas

Facies are arranged according to Facies are arranged according to distribution of depositional environmentsdistribution of depositional environments

Facies MigrationFacies Migration

Facies migrate through space and timeFacies migrate through space and time Migration is in response to Migration is in response to

environmental factorsenvironmental factors Sediment supplySediment supply Sea level changeSea level change SubsidenceSubsidence

Facies become stacked during Facies become stacked during migrationmigration

A single facies is likely to be different A single facies is likely to be different ages in different locationsages in different locations

Walther’s LawWalther’s Lawof the Correlation of Faciesof the Correlation of Facies

Only works where there are no unconformitiesOnly works where there are no unconformities Only facies that were laterally adjacent during Only facies that were laterally adjacent during

deposition (result of laterally adjacent deposition (result of laterally adjacent environments) can be stacked verticallyenvironments) can be stacked vertically

Vertical arrangement of facies gives us Vertical arrangement of facies gives us information oninformation on Distribution of environmentsDistribution of environments How environments migrated through space and timeHow environments migrated through space and time

Used as a basis to build Used as a basis to build facies mapsfacies maps or or paleogeographic mapspaleogeographic maps

Accurate time Accurate time correlationcorrelation of facies is essential of facies is essential Time lines provide framework for correlationTime lines provide framework for correlation

Bio-eventsBio-events Volcanic ashesVolcanic ashes Other thin, unique lithologies or marker bedsOther thin, unique lithologies or marker beds

Representing Facies:Representing Facies:Stratigraphic SectionsStratigraphic Sections

DeltasDeltas

Marginal Marine DepositionMarginal Marine Deposition

Ganges Delta, IndiaGanges Delta, India

Mississippi Delta Mississippi Delta (partial), USA(partial), USA

Mississippi Delta (partial), Mississippi Delta (partial), USAUSA

Drowning!Drowning!

Niger Delta 3D ModelNiger Delta 3D Model

Modern Delta Modern Delta SubenvironmentsSubenvironments