Facies Models

Geol 370: Sedimentology and Stratigraphy
Topic 9: Facies Models

1. Sedimentary Facies (Lithofacies)
A lithofacies is a body of sediment/rock distinguished by a specified
combination of characteristics (composition, texture, sedimentary
structures) related to a specific set of physical, biological, and chemical
processes.

2. Aspects of Lithofacies
• Composition
• Texture (grain size, sorting, rounding)
• Sedimentary structures
• Bedding (thickness, geometry, nature of contacts)
• Fossil content (types, abundance)
Example: Well-sorted, coarse- to fine-grained, trough cross-bedded
quartz arenite that fines upward as cross-bed sets decrease in thickness.

3. Sedimentary Facies &
Depositional Environments
The unique characteristics of a lithofacies represent deposition under the
very specific conditions of a single environment. For example, the facies
of the previous slide suggests flood stage deposition on a point bar of a
meandering stream.

4. Separation of data (facies) and
interpretation (environment)
In science, it is critical to distinguish between data (what’s there - lithofacies) and
interpretations (how you think it got there – depositional environment). There is not
complete agreement upon use of the term facies. Many use it in conjuction with the
interpreted environment (e.g. point bar facies). I consider this as a misuse of the term.
Data
Interpretation

5. Lithofacies Codes
Some attempts have been made to devise
abbreviations for commonly found
lithofacies. These typically employ a
capitol letter for the major clast size (i.e.
G for gravel, S for sand, and F for fine-
grained) followed by lower case letters
denoting the major structure (i.e. t for
trough cross-bedding and p for planar
cross-bedding).

6. Facies Associations/Assemblages
Facies association: a collection of multiple, genetically-related facies
formed within a single depositional system.
Example: Non-stratified
gravel overlain by well-
sorted coarse- to fine-
grained, trough cross-
bedded sand that fines
upward as cross-bed sets
decrease in thickness
overlain by ripple-bedded
sandstone. Succession
grades laterally to
laminated mudstone that
contains thin lenses of
sandstone.

7. Common Facies Associations
Some facies associations are found repetitively in the stratigraphic record and, therefore, have
informally been given names, such as the “Point bar succession” or the “Bouma (turbidite)
sequence.”
Point bar succession Bouma sequence

8. Facies Associations (Architectural
Elements) of Miall
Miall formalized facies associations for fluvial systems. These are used
widely (but not universally) with modification.

9. Facies Associations and Depositional
Systems
A depositional system consists of genetically-related, contemporaneous
depositional environments. Vertical changes in facies associations can
reflect either lateral migration of environments within a system or
fluctuation in base-level.

10. Facies Models/Architecture
Facies models are based on facies associations and are designed to show
the three-dimensional relationships (architecture) between individual
facies (architectural elements) for a depositional system. Models can be
taylored to a specific stratigraphic unit or can be generalized to show an
“average” of characteristics for a “typical” depositional system.
Generalized
model for a
“typical”
meandering
stream system

11. Subsurface Facies Models
Subsurface facies models have the same components as those based on
outcrop but, typically, with less detail; although, recent advances in 3-D
seismic is narrowing that gap.
Fluvial model based on well logs Fluvial model based on 3-D seismic

12. Architecture Element Analysis
Architectural element analysis consists of making photomosaics of
outcrops and mapping individual elements (facies).

13. Systems Tracts
A systems tract consists of contemporaneous depositional systems.
Vertical changes in systems tracts reflects changes in sea-level.

14. Vertical Facies Successions
Facies successions occur on three scales: 1) facies assemblage associated
with a depositional system, 2) larger-scale stacking of adjacent systems
within a systems tract; and 3) long-term basin fill successions.
Idealized meandering
stream vertical profile
Idealized deltaic
succession
Stacked Cretaceous systems
tracts in the Book Cliffs

15. Walther’s Law
WaltherWalther’s Law (1894) states that facies found superimposed on one ’s Law (1894) states that facies found superimposed on one
another and not separated by an unconformity, must have been another and not separated by an unconformity, must have been
deposited adjacent to each other at a given point in timedeposited adjacent to each other at a given point in time
Floodplain
Levee
Point Bar
Photo by W. W. Little

16. Walther’s Law & Systems Tracts
Most systems tracts are preserved in the stratigraphic record as
progradational successions.
Prograding beach Prograding tidal flat

18. Transgressive/Regresssive Cycles
(Sequences)
Walther’s Law was developed to explain vertical changes in facies associated with
changes in sea-level. Sea-level cycles were once drawn as symmetrical wedges
showing smooth transitions and equal preservation of both transgressive and regressive
deposits. We now term these cycles sequences and interpret them as pulses of
progradational deposits that step landward (retrogradational), vertically (aggradational),
or basinward (progradational). Each step is a parasequence.
Old T/R cycle
Parasequence
sets