GEOL 440 Sedimentology and stratigraphy: processes, environments and deposits

Lecture 10: Sedimentary Environments and facies: an introduction; Controls on deposition,

preservation and accumulation

…….two more key texts….. CD…..rivers, deserts, estuaries, deltas, shorelines, shelves, deep sea

So, why do we need to understand the deposits of differing environments?

sedimentary & geological history (small-large scale)

economic importance - e.g. hydrocarbons, water

economic importance -minerals and metals (e.g. gold-diamonds-gypsum-aggregates)

prediction/management of contemporary environments

AGI Image Bank

..when we have limited

data…….

AGI Image Bank

How do we interpret/aid prediction of subsurface??

How do interpret the inherently 3D from 2d information??

What do we need to quantify in order to characterise the deposits of differing environments?

grain size & typesedimentary structures - type, size, distribution larger-scale attributes - e.g. sandbody sizepalaeocurrentsbody/trace fossilsvertical and lateral variations (time & space)

Sedimentary Facies….….the essential sedimentary & palaeontological attributes of a

sedimentary environment….examine an environment - ancient and/or moderncharacterise key attributes - vertical and lateralestablish a ‘facies model’ - a key summaryof the environment

One key tool-the vertical profile or log(tho’ never neglect lateral variability &

characteristics)

height

grain size

cl si f m c gravelsand

Sedy structures

Tracefossils

CLASTIC CARBONATE

height

grain typemu wa pa gr

Body fossils

Trace fossils

Diagenetic features; vugs

Defining a Facies

Facies: A Sum of Attributes

Boggs, p.242

Johannes Walther (1860-1937)

Walther’s Law: ‘Facies adjacent to one another in a continuous vertical sequence also

accumulated adjacent to one another laterally’

Facies Successions

Facies Succession – a progressive change in certain facies properties in a specific direction (vertically or laterally).

* Removes the environmental ambiguity of individual facies.

Facies Model

A general summary of a given depositional system., e.g. deltas

Deriving a Facies Model

Facies Analysis Flow Chart

Example

Controls … Autocyclic and Allocyclic

Mike Blum, 2007

….for rivers…..

from today make sure you can…….

Reading:•Boggs, Part IV•Harold Reading (Ed): Chapter 2 (Controls on the sedimentary rock record)•Walker, Roger. Chapter 1, Facies Models revisited (in: Posamentier and Walker)

•Define a facies and facies association•Appreciate general approach to defining a facies•Think of the data you’d need to define a facies

Now…think of the advantages AND disadvantages of facies modelling…what are the possible pitfalls?