FACIES

1
FACIES
By OTHMAN M.HAKAMI
SUPERVISED Dr.RUSHDI J. TAJ
2
FACIES CONSTRUCTION
Facies definition
Facies relationships
3
Facies definition
A facies is a body of rock with specified
characteristics. Where sedimentary rocks can be
handled at outcrop or from boreholes, it is
defined on the basis of colour, bedding,
composition, texture, fossils and sedimentary
structures. A biofacies is one for which prime
consideration is given to the biological content.
If fossils are absent or of little consequence
and emphasis is on the physical and chemical
characteristics of the rock, then the term
lithofacies is appropriate. Apart from these new
uses of the word facies, it has also been used in
different senses in the past (1) in the strictly
observational sense of a rock product, e.g.
'sandstone facies' (2) in a genetic sense for
the products of a process by which a rock is
thought to have formed, e.g. 'turbidite facies'
for the products of turbidity currents (3) in an
environmental sense for the environment in which
a rock or suite of mixed rocks is thought to have
formed, e.g. 'fluvial facies' or 'shallow marine
facies' and (4) as a tectofacies, e.g.
'post-orogenic facies' or molasse facies'.
4
Facies relationships
Introduction
Contacts
Cycles
Associations and sequences
5
Introduction
Individual facies vary in interpretative value. A
rootlet bed and coal seam, for example, indicates
that the depositional surface was very close to
or above water level. A current-rippled sandstone
implies that deposition took place in the lower
part of the lower flow regime from a current that
flowed in a particular direction. However, it
indicates little about depth, salinity or
environment. The importance of facies
relationships has long been recognized, at least
since Walther's Law of Facies (1894) which states
that 'The various deposits of the same facies
area and, similarly, the sum of the rocks of
different facies areas were formed beside each
other in space, but in a crustal profile we see
them lying on top of each other ... it is a basic
statement of far-reaching significance that only
those facies and facies areas can be
superimposed, without a break, that can be
observed beside each other at the present time'
(translation from Blatt, Middle-ton and Murray,
1972, pp. 187-8).
6
Contacts
Walther's warning has often been ignored by
geologists who have failed to describe the type
of contact between facies. The three main types
of contact are gradational, sharp and erosive,
though sometimes one needs to differentiate those
that are abruptly gradational, where a transition
occurs over a few centimetres. Some contacts show
extensive boring, burrowing, penecontemporaneous
deformation or diagenesis of the underlying
sediments so that the adjacent facies have become
mixed or even inverted.
7
Cycles
The idea that patterns of facies repeated each
other, or the concept of cyclic sedimentation,
has been one of the most fruitful in sedimentary
geology. It enabled geologists to bring order out
of apparent chaos, and to describe concisely a
thick pile of complexly interbedded sedimentary
rocks. The concept of cyclic sedimentation has,
however, been criticized on two grounds. Firstly,
the establishment of cycles is too often
subjective and pleas have been made for a more
rigorous analysis of the sequences (Duff and
Walton, 1962). Secondly, regardless of the
sophistication of the techniques used to
establish a cycle, significant facts essential to
sedimentologi-cal interpretation are omitted,
usually due to concentration on selected features
or the desire to establish an 'ideal' cycle.
Since the use of cycles is based on the idea
that there is a regularity to sedimentary
sequences and that sedimentation is a normal
steady process apparently random events are
commonly neglected, although, in some
environments, they may dominate sedimentation.
8
Associations and sequences
A valuable result of the cyclic concept was to
focus attention on the relationship of facies to
each other, that is on facies which tend to occur
together (associations) and on sedimentary
sequences. It demonstrated the advantage of
interpreting a facies by reference to its
neighbours. Facies associations are groups of
facies that occur together and are considered to
be genetically or environmentally related. For
example, thick-bedded turbidites may be
interbedded with conglomerates, slumps and
mudstone while thin-bedded turbidites are
interbedded with mudstone alone. Each grouping
would then be identified as a distinct
association. A facies sequence is a series of
facies which pass gradually from one into the
other. The sequence may be bounded at top and
bottom by a sharp or erosive junction, or by a
hiatus in deposition indicated by a rootlet bed,
hardground "or early diagenesis. A sequence may
occur only once, or it may be repeated (i.e.
cyclic).
9
In clastic environments, two important kinds of
sequence are those in which the grain size
coarsens upwards from a sharp or erosive base
(coarsening-upwards sequence) and those in which
the grain size becomes finer upwards to a sharp
or erosive top (fining-upwards sequence).
Sequences may reflect (1) sedimentological
controls such as local changes in the environment
brought about by the progradation of a delta to
give a coarsening-upwards sequence or by the
lateral migration of a meandering river to give a
fining-upwards point bar sequence (2) external
controls such as sea-level oscillations, climatic
change or tectonic movements in the source area
or in the basin leading to changes in the now
power of the transporting water or in the grain
size of the sediment available. Some carbonate
rocks lend themselves to the same treatment as
clastic sediments because sedimentary structures
and textures are clearly visible. However, facies
sequences are modelled less on grain-size changes
related to physical processes and more on a whole
spectrum of biological, chemical and physical
factors related to specific environments (see
Sect. 10.2.2 and 10.5). In addition many
carbonate rocks are so modified by diagenesis
that facies can only be distinguished by
petrographic study and by the definition of
micro-facies.
10
FACTORS CONTROLLING THE NATURE AND DISTRIBUTION
OF FACIES
Sedimentary processes
Introduction
Climate
Sediment supply
Sea-level changes
Tectonics
Water chemistry
Biological activity
Volcanism
11
Introduction
Although for many purposes a facies analysis that
results in an environmental and
palaeogeographical reconstruction may sullice,
those geologists who are concerned with the
dynamics of ihe earth, past climates and the
evolution of life need also to consider the
underlying controls which govern the formation of
facies and iheir vertical and lateral
distribution. Facies distribution and changes in
distribution are dependent on a number of
interrelated controls. 1 Sedimentary processes 2
Sediment supply 3 Climate 4 Tectonics 5 Sea-level
changes 6 Biological activity 7 Water
chemistry 8 Volcanism
12
Sedimentary processes
Processes intrinsic to sedimentation in a given
environment may themselves be responsible for
facies distribution and facies change. By the
very nature of the sedimentary environment these
changes are inevitable, though their exact timing
is usually governed by an unusual event such as
an exceptionally violent flood, storm or seismic
shock. This 'triggering' mechanism must be
distinguished from the fundamental cause which
may be delta progradation, river aggradation or
sediment instability. Differential compaction of
varied underlying sediments and subsurface
sediment movement such as that associated with
salt domes and growth faults leads to
differential subsidence (Sect. 6.8). This affects
the overlying sediment in the same way as
vertical tectonic movements.
13
Sediment supply
The availability of sediment is one control on
the thickness of depositional facies it may also
govern water depth and environment. Sediment is
supplied from two sources (1) extrabasinal,
which is mainly terrigenous. The type is governed
by geology, topography, climate and tectonics.
(2) Intrabasinal, which is mainly biochemical md
derived from chemical precipitation, plant or
animal growth, the erosion of material
previously deposited within the basin, or from
sediment extruded upwards from below as sand or
mud volcanoes. The type is governed by climate,
water composition, tectonism and sea-level
changes. Within any depositional environment the
effect of sediment supply depends on its
availability, on subsidence and sea-level change
(see also Chapters 7 and 10). Two situations may
be envisaged.
14
(1) Transgressive when subsidence and rise of
sea level are more important than the supply of
terrigenous sediment, the environment is starved
of sediment. This results in reworking, erosion
and diagenesis of deposits, transgression and
deepening of the environment, together with an
increase in chemically and biologically formed
sediments. (2) Regressive when subsidence and
rise of sea level arc less important than
terrigenous sediment supply, progradation and an
increase in the proportion of continental facies
result. In the construction of models different
constraints apply according to whether (lie
supply of sediment is abundant and available to
fill any depositional void or is intermittent so
that certain areas are temporarily starved.
15
Climate
Facies are affected mainly by temperature and
rainfall, though wind levels may he locally
important. Temperature indicators include
evaporites, palaeosols, vegetation, tillites,
some oolites and commonly the fauna. Rainfall
indicators include vegetation, palaeosols,
evaporites, dune-bedded aeolian sandstones, clay
mineral provinces and fluvial and lake
morphology. Climate is also a measure
ofpalaeolatitude and an environmental
interpretation needs to lit the known
palaeolatitude. However, one must remember that
(1) climates are affected by oceanographic
currents and the proximity, size and orientation
of land masses similar temperatures are found
today at latitudes which differ as much as 20" on
either side of the Atlantic (2) world climatic
distribution may have been very different in the
past e.g. during the Jurassic it was more equable
than today (for discussion see papers in Berger
and Crowell, 1980).
16
Tectonics
Tectonics affects sedimentation by governing the
broad distribution of highlands and basins and
the geographical framework of sediment supply,
climate and environment. It also causes local
facies changes most spectacularly seen across
fault lines
17
Sea-level changes
Sea-level changes may be either local or
world-wide (eustatic) and affect sedimentary
facies in many different ways. Local sea-level
changes are the result of sediment input,
sediment loading of ihe crust, vertical tectonic
movement, tilting ofcrustal blocks, and isostatic
depression and rebound, as well as eustatic rises
and falls. Global or eustatic sea-level changes
can be brought about by changes either in the
volume of oceanic waters or in the volume of the
ocean basins, chiefly effected by tectonic
mechanisms (Donovan and Jones, 1979). A number of
tectonic mechanisms may affect the volume of the
ocean basins (Donovan and Jones, 1979 Pitman and
Golovchenko, 1983). (1) Changes in the volume of
mid-ocean ridges may he caused by subduction of
existing ridges, by creation of new ones or by
changes in spreading rates an increase in
spreading rate increases the volume a decrease
in spreading rate decreases the volume. (2)
Continental collision reduces the area of
continent, increases that of the ocean and sea
level drops in consequence. (3) Influx of
sediment to oceans from llie land may raise sea
level though this effect is normally reduced by
isostatic depression beneath the sedimentary
wedge. (4) Mid-plate thermally induced uplift of
oceanic floor may also decrease the volume of the
oceans and cause eustatic rise (Schlanger,
Jenkyns and Premoli-Silva, 1981).
18
First and second-order global cycles of relative
change of sea level during Phanerozoic time (from
Vail, Mitchum and Thompson III, 1977).
19
Second- and third-order global cycles of relative
change of sea level during Cenozoic time (from
Vail, Mitchum and Thompson III,1977).
20
Biological activity
The building of coral, bryozoan, algal and other
reefs (Chapter 10) and the development of thick
plant accumulations are the principal
constructive elements in organic sedimentation.
Animals and tree roots, by inhibiting current
flow and erosion, may trap sediment. On land,
plant cover assists in soil development and
moderates the erosive effects of rainfall,
run-off and wind. Micro-organisms such as
foraminifera, radiolaria, algae and diatoms which
commonly live in near-surface waters may provide
a constant rain of pelagic sediment in oceans and
lakes. Bacteria are particularly important in
soil formation, as weathering agents in the
oxidation and reduction of iron and as reducers
of sulphate. Organisms are closely associated
with chemical precipitates. They have a strong
effect on the pH and Eh of sediment pore waters.
Plant roots disturb the soil and concentrate
solutions around them to form concretions. Since
organisms have evolved through geological time,
the type, amount and sites of biological activity
have continually changed. An understanding of the
contemporary biosphere is necessary if ancient
and modern facies or ancient facies from
different systems are to be compared.
21
Water chemistry
The salinity and composition of sea and lake
waters varies from place to place and over
geological time. Water chemistry governs the
formation of carbonates and other chemical and
biochemical sediments. Variations in lempenilure
and salinity are largely the result of climatic
zonation and fluctuation. Oceanic circulation,
resulting in upwelling of nutrient-rich waters,
is responsible for local accumulation of some
oozes, phosphates and diatomites.
22
Volcanism
Volcanic activity provides a local, intrabasinal
source of sediment and of ions in solution.
Leaching of hot pillow lavas by sea water,
formation of clay minerals by chemical exchange
with sea water anil associated hydrothermal
discharge of metal-rich fluids have an important
eflect upon sedimentation in pelagic seas . In
lakes there may be a close connection between the
composition of the volcanic source and lake
precipitation
23
The End