depositional environments and sedimentary facies

Depositional Environments and Sedimentary Facies

Depositional Environments• Clastic vs. Carbonate Depositional Systems

• Short and Long Depositional Systems

Facies 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)

- Generally interpretive- Not specific to a rock, but to a predicted set of characteristics

associated with some depositional environment- Sometimes used differently

- E.g., To describe a rock unit in an outcrop - sandstone facies

FACIES

“The nature of the material deposited anywhere will be determined by the physical, chemical or biological processes which have occurred during the formation, transport and deposition of sediment. Those processes also define the environment of deposition” – Nichols, 2001.

FACIES

• Lithofacies– Based on lithology

• Biofacies– Based on fossil assemblage

• Ichnofacies– Based on trace fossil assemblage

Facies Types

• Lithofacies– characteristics of a rock which are the products of PHYSICAL and

CHEMICAL processes– coarsening upward, fining upward

• Biofacies– observations are based on fauna and flora present– Ichnofacies focuses on trace fossils– Examples

• gray limestone rock = lithofacies• echinoid and crinoids = biofacies• burrows and tracks = ichnofacies

Environment of Deposition

• Sedimentological analyses allow us to determine the environment in which a sequence of sediments or rocks accumulated– Texture– Composition/ Classification– Petrographic analyses (e.g., evaluate diagenesis)– Identify sedimentary structures– Compare results to modern environments

• Also simulate conditions in laboratory experiments (Hjülstom’s curve)

Depositional Environments and Sedimentary Facies

Depositional Environments• Areas of the Earth’s surface where distinct processes generate specific geological

(sedimentary) products– Physical

Biological

– Chemical

Sedimentary (litho) Facies and (litho) Facies Analysis

• Sedimentary (litho) facies– Lithostratigraphic Units (time independent)– Defined by sum total of (relevant) rock properties– Reflects processes during genesis and may include

• Lithology• Sedimentary Structures• Fossils• Bedding style and geometry (on various scales)• Paleo-sediment transport indicators

– e.g.: hummocky cross stratified, Zoophycus burrowed, fine- to medium-grained, sandstone

Paleocurrents and Paleocurrent Indicator Analysis

• Paleocurrent indicators are oriented sedimentary structures interpreted to have been deposited by ancient flows – Cross-beds slip faces, – pebble imbrication, – parting lineation, – tool marks and groove casts, and – ripple crest orientation

Unidirectional Paleocurrent Indicators

Cobble Imbrication

Bidirectional Paleocurrent Indicators

Rose Diagram

Facies Analysis, Facies Associations, Facies Sequences, and Facies Models

• Reoccurring associations of sedimentary facies– (x-bdd channel fill sst with root mottled, mud cracked silt and clay stone see point-

bar model)• It is possible to more precisely

determine the sum total of processes active at the site of deposition and interpret “depositional environment”

• Facies sequences are recurring (in the geological record) facies associations which occur in a particular order due to the inherent temporal changes in depositional conditions in particular depositional environments

Distinctive and Common Sedimentary Facies Associations

• Vertical successions principally identified by lithology, associations and vertical arrangement of sedimentary structures– indicative of particular sedimentary depositional environments


• Vertical successions principally identified by lithology, associations and vertical arrangement of sedimentary structures– indicative of particular

sedimentary depositional environments


• Vertical successions principally identified by lithology, associations and vertical arrangement of sedimentary structures– indicative of particular sedimentary depositional environments

Facies Analysis, Facies Associations, Facies Sequences, and Facies Models

• Facies Models are a general summary of a given depositional environment or depositional system– Lithostratigraphic unit representing depositional processes and geographic location

• The apparent existence of order in Nature suggest that there are (and have been through geological time) a limited number of different and recognizable depositional systems

• These depositional systems are identified through the use of

Facies Models

Facies Analysis and Walther’s Law• “It is a basic statement of far reaching significance that only

those facies and facies areas can be super imposed primarily that

can be observed beside each other at the present time”

Facies Analysis and Walther’s Law• Gradational (vertical) transitions from one facies to another indicate original

adjacency and genetic relationship during formation.

• Sharp/erosional (vertical) contacts between facies provides NO evidence of contemporaneous genetic relationship of depositional environments

Depositional Systems Analysis

• Depositional Systems: (lithostratigraphic units)– Three dimensional

assemblages of lithofacies, which are interpreted to be genetically linked by process and environment

Environments of Deposition• Continental

– Fluvial alluvial fan, braided stream, meandering stream– Desert dunes, playa lakes, salinas– Lacustrine– Glacial– Deltaic delta plain, delta front, prodelta

• Marginal- Marine– Beach/ barrier island– Estuary/ lagoon– Tidal Flat– Neritic continental shelf, organic reef

• Marine– Pelagic continental slope and rise, deep- ocean floor

Environment of Deposition

• Modern Analogue– Key to interpretting transport history of sediments and

rocks

• Process/ Response Model– Based on idea that a “particular set of environmental

conditions operating at a particular intensity will produce a sedimentary deposit with a unique set of properties that will identify if as the product of a particular environment” Boggs, 2001

• Unique solution is overly optimistic! • Very useful nonetheless!

Process/ Response Model

• Examples– Process Element

• Ocean Basin– Deep water; low current velocity; settling dominates

– Response Element• Laminated mudstone with abundant microfossils

• As geologists, often have to work backwords – Obtain process from response

• leads to set of predictions – what types of sedimentary structures/ associations will be deposited in

different environments– sedimentary structures/ associations aregenerally not unique to a single

environment

• Loose definition for facies

Facies Analysis

• The interpretation of strata in terms of depositional environments

• How?– Recognize sedimentary environments– Interpret rocks based on facies

• Because there are no unique solutions can’t simply analyze one outcrop and understand it!

– Rely heavily on facies associations– Vertical– Horizontal

– Can use idealized model to assist in analysis

Facies Models

• How facies models are created– NICHOLS FLOW CHART (FIG 5.1)

• assign all beds in a succession to facies

• look at patterns of distribution of facies

• develop facies association and interpret in terms of depositional environment

– e.g., fining upwards is associated with turbidite

– there are OTHER environments that have fining up!

– need multiple criteria!

– some of the criteria

– look at associations of facies within basin

Facies Models

• CAVEATS!

– function as GUIDES for INTERPRETATION!• can’t be SO inclusive to include everything

• also may find that not everything is included– e.g., Bouma sequence!

How to perform a Facies Analysis

• Procedure– 1. Measure the section

– 2. Develop detailed graphic sedimentary log• use standard symbols

– 3. Evaluate associated section• Quantitatively or qualitatively

– 4. Compare to facies models

– 5. Assess regional associations

How to make a stratigraphic column • Look at the rocks

This may seem obvious, but it is important before you begin to get a general sense of the rocks in front of you. It will make it easier to make a description of the rocks later if you get a good overview first.

• Look for changes in rock typeIf the rocks in one part of the outcrop appear very different, then it is possible that they should be their own "unit". Dividing the rock outcrop into units is based on changes either in fossils, color, rock type, and other factors, or all of these factors combined.

• Measure the sectionOnce you have determined different units, you may then measure the thicknesses of these units. (Geologists are generally not particularly interested in the length of the rocks, only the widths.)

• Start describing!Now it's time to make specific observations about the rock you are looking at. What fossils do you see? What is the color? What kind of rock is it (sandstone, shale, limestone, or something else)?

STRATIGRAPHIC LOG

depositional environments and sedimentary facies

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