Sedimentary Facies

Morgan Schaller

Previous Work

Majority of the semester has been largely descriptive, identifying:– Lithology– Sedimentary fabric and texture– Sedimentary structures

Now we’ll go more into interpretation-– Sediments are products of sedimentary

processes– Snapshot of the environment of deposition– Geology starts with the result, asks “what was

the experiment?”

Facies Defined:

In Geology, “facies” is often misused and misunderstood… There are 2 “camps”:– Observational facies

• I was taught that observations were “sub-facies”

– Facies Interpretation• Interpretation or inference of the depositional environment

based on observations

AGI (1984) Definition:– The aspect, appearance, and characteristics of a rock

unit, usually reflecting the conditions of its origin; especially as differentiating it from adjacent or associated units this is observational!

Facies Defined:

• For this class and lab facies will be defined as…– A description of characteristic physical, chemical and

biological properties of a sedimentary unit, which may be used to interpret depositional environments

• Variability in preservation of ^^ in the sedimentary record

• Main criteria in facies definition are:– Lithology, sedimentary structures, paleontology, and

sediment body geometry

Q: What are you looking at? How can you describe it in the framework of its depositional environment?

~10cm

A: Facies Associations• Depositional environments are generally

composed of multiple subenvironments– Ex. Delta

• Thus, we can expect that facies will vary throughout the environment to reflect the transition between subenvironments

• We must assume that adjacent facies represent adjacent environments

• Conversely, deep lake facies don’t form naturally next to offshore marine facies

The Nile River Delta:

Delta Components and Facies:

How are these related?

• The association of certain facies near or adjacent to each other are characteristic off distinct depositional environments

• Based on observations from modern environments of deposition and parallels to past environments

• By understanding these associations (modern), we can generate facies models

Facies Models• General summary given to a depositional system

• 2-D Vertical successions of sediment that represent facies relationships expected in the geologic record

• 3-D Block Diagrams

• 4-D Models detailing sedimentary processes (inc. T)

• Takeaway Message: For any sedimentary package, you can examine the facies and their relationships to each other, apply a facies model and assign a depositional environment

TIM

E

CENSOREDDepositional Environment??

TIM

E

Regressive Sequence

WALTHER’S LAW:• Only those environments which are laterally

adjacent may be superimposed conformably one on top of another… (this will be on your next quiz)

• In other words, facies may be diachronous and in a given region will shift with time in response to the different factors that control deposition

• Thus, facies boundaries shift so that deposits of adjacent environments are represented vertically

• Fluvial Environment• Meandering River

– Cross bedded sandstones– Coarse gravel deposits– Broad sheets of silty shale

with root casts and plant debris

– Isolated Shale bodies– Beds of peat and coal

• Facies models help solve the puzzle of depositional environment

What facies would you expect from this depositional environment?

Facies Changes:• Future Oil Pigs, listen up:• Transgression (Beach shifts landward)• Regression (Beach shifts seaward)• Heavily dependent on sediment supply,

subsidence and changing sea level• Transgressive and Regressive systems can be

identified by observing facies and lithological changes in rock/sediment

• Transgressive – sediments fine upwards, represent deeper water environments

• Regressive- sediments coarsen upwards, represents shallowing effect

Transgressive System:

Facies Successions:

• Coarsening Upward:– Indicates increase in transporting power

• Fining Upward– Indicates decrease in transporting power

• Larger scale than ‘Graded bedding…’

Facies Distribution:

• What factors control the nature and distribution of facies?

• Big Three: Erosion, Transportation and Deposition

Gamma Logs:• Proxy indicators of facies

changes• Gamma ray logs are a

measure of downhole radioactivity, for our purposes, 40K (feldspars, etc.) and organics

• Mud = high gamma emitters• Clean sands = low gamma

emitters• Glauconite = indicator of

marine conditions, on shelf• Carbonaceous = organic

matter (this is *often* where the oil lives ;)

Cheat sheet:

~10cm

BONUS:

Bonus #1

Bonus #2