Transcript Facies, Alluvial Fans, Braided Streams

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
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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
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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
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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 pointbar 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
DELTA
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
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
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
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
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Fluvial
alluvial fan, braided stream, meandering stream
Desert
dunes, playa lakes, salinas
Lacustrine
Glacial
Deltaic
delta plain, delta front, prodelta
• Marginal- Marine
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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
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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
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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 type
If 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 section
Once 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
STRATIGRAPHIC LOG