Classification of Lithofacies
ing interpretation, and the geologist needs to be
clear about this as the work proceeds. Eventually, as
discussed in later sections of this chapter, each resulting complete surface can be assigned an appropriate interpretation and rank, and can then be
colored or coded accordingly on the final drawing.
Before leaving the outcrop, the geologist should
be sure to record two critical items of observation:
the orientation of the outcrop and its scale. Both may
vary along the profile, in which case this should be
measured and carefully recorded. Scale should be
measured vertically to reduce possible distortions
(very little of the face can be photographed from a
point perpendicular to it in the horizontal plane). A
surveying rod may be included in one or more of the
photographs, or the size of a recognizable feature of
the outcrop may be measured and marked on the
overlay.
The final, drafted profile may need to be published as a fo ldout, because even the A4 and quartosized paper that is becoming fashionable for
geological journals, is too small to contain a large
profile illustration without severe reduction. I recommend publishing the photomosaic separately
from the line drawing interpretation, preferably with
the former positioned immediately above the latter.
The photograP-h should be marked only with the
necessary identifying information. This is analogous
to the practice now adopted for the publication of
seismic sections. An uninterpreted line is given together with the interpretation, so that the reader can
make an independent assessment of the raw data.
Alternatively, the overlay can be printed in place on
the outcrop photograph, or it can be published on its
own, without the photograph. Both methods have
the disadvantage of the loss or obscuring of information.
4.3 Classification of lithofacies
Observation and classification oflithofacies are now
standard components of the fa cies-analysis methodology for studying sedimentary rocks. Good summaries of the principles and methods are contained
in Reading (1986), Miall (1990, Chap. 4), and Walker
and james (1992). Beds are classified on the basis of
their primary depositional attributes, notably (in the
case of fluvial clastic deposits) bedding, grain size,
texture, and sedimentary structures. Biogenic structures and fo ssils may be important locally as additional descriptive attributes. Chemical sediments,
77
such as pedogenic calcretes, coal, and evaporites,
typically form minor components of most fluvial
systems, but need their own careful characterization.
As noted by Miall (1990, p. 150), the scale of an
individual lithofacies unit depends on the level of
detail incorporated in its definition. Facies may be
defined very broadly to encompass mappable stratigraphic units, or they may be defined finely to
accomodate the level of detail obtainable in the centimeter-by-centimeter logging typically carried out
on core. For the purpose of architectural-element
analysis, a relatively fine degree of descripion and
subdivision is required.
It is good research practice to approach each new
rock unit afresh, with the aim of making complete,
unbiased observations of all important lithofacies
attributes. However, sedimentological research has
demonstrated that much of the apparent variability
in sedimentary units disguises a. limited range of
basic lithofacies and biofacies types. The depositional processes which control the development of
clastic fluvial lithofacies, such as traction-current
transportation, with its accompanying fluid turbulence and its effects on beds of clastic grains, are
common to all rivers and obey the same physical
laws everywhere1 with the production of similar
suites of lithofacies. For example, hydrodynamic
structures, such as ripples and cross-bedding, are
fo rmed by the migration of ripples and dunes. Considerable experimental work has shown that in the
development of these bedforms there are consistent
empirical relationships between bedform size and
shape and a limited suite of physical parameters, of
which the most important are the depth and velocity
of the flow, and the grain-size of the sediment (e.g.,
Harms et al. 1982). Repetition of similar conditions
leads to repetition of similar depositional products,
which are then susceptible to a universal empirical
classification in the field.
Miall (1977) reviewed braided-river deposits, and
demonstrated a consistency in the lithofacies assemblages occurring in a wide range of modern and
ancient sandy and gravelly sediments. However,
many of the researchers whose work was examined
in this review had erected their own local classification, which had obscured the similarities between
the deposits and prevented the recognition of common depositional themes. Miall proposed a simple
classification, making use of a two-letter code to
facilitate quick field and laboratory identification
and documentation. Use of this lithofacies scheme
by a number of workers led to the recognition that
the scheme could be applied to all fluvial deposits,
ing interpretation, and the geologist needs to be
clear about this as the work proceeds. Eventually, as
discussed in later sections of this chapter, each resulting complete surface can be assigned an appropriate interpretation and rank, and can then be
colored or coded accordingly on the final drawing.
Before leaving the outcrop, the geologist should
be sure to record two critical items of observation:
the orientation of the outcrop and its scale. Both may
vary along the profile, in which case this should be
measured and carefully recorded. Scale should be
measured vertically to reduce possible distortions
(very little of the face can be photographed from a
point perpendicular to it in the horizontal plane). A
surveying rod may be included in one or more of the
photographs, or the size of a recognizable feature of
the outcrop may be measured and marked on the
overlay.
The final, drafted profile may need to be published as a fo ldout, because even the A4 and quartosized paper that is becoming fashionable for
geological journals, is too small to contain a large
profile illustration without severe reduction. I recommend publishing the photomosaic separately
from the line drawing interpretation, preferably with
the former positioned immediately above the latter.
The photograP-h should be marked only with the
necessary identifying information. This is analogous
to the practice now adopted for the publication of
seismic sections. An uninterpreted line is given together with the interpretation, so that the reader can
make an independent assessment of the raw data.
Alternatively, the overlay can be printed in place on
the outcrop photograph, or it can be published on its
own, without the photograph. Both methods have
the disadvantage of the loss or obscuring of information.
4.3 Classification of lithofacies
Observation and classification oflithofacies are now
standard components of the fa cies-analysis methodology for studying sedimentary rocks. Good summaries of the principles and methods are contained
in Reading (1986), Miall (1990, Chap. 4), and Walker
and james (1992). Beds are classified on the basis of
their primary depositional attributes, notably (in the
case of fluvial clastic deposits) bedding, grain size,
texture, and sedimentary structures. Biogenic structures and fo ssils may be important locally as additional descriptive attributes. Chemical sediments,
77
such as pedogenic calcretes, coal, and evaporites,
typically form minor components of most fluvial
systems, but need their own careful characterization.
As noted by Miall (1990, p. 150), the scale of an
individual lithofacies unit depends on the level of
detail incorporated in its definition. Facies may be
defined very broadly to encompass mappable stratigraphic units, or they may be defined finely to
accomodate the level of detail obtainable in the centimeter-by-centimeter logging typically carried out
on core. For the purpose of architectural-element
analysis, a relatively fine degree of descripion and
subdivision is required.
It is good research practice to approach each new
rock unit afresh, with the aim of making complete,
unbiased observations of all important lithofacies
attributes. However, sedimentological research has
demonstrated that much of the apparent variability
in sedimentary units disguises a. limited range of
basic lithofacies and biofacies types. The depositional processes which control the development of
clastic fluvial lithofacies, such as traction-current
transportation, with its accompanying fluid turbulence and its effects on beds of clastic grains, are
common to all rivers and obey the same physical
laws everywhere1 with the production of similar
suites of lithofacies. For example, hydrodynamic
structures, such as ripples and cross-bedding, are
fo rmed by the migration of ripples and dunes. Considerable experimental work has shown that in the
development of these bedforms there are consistent
empirical relationships between bedform size and
shape and a limited suite of physical parameters, of
which the most important are the depth and velocity
of the flow, and the grain-size of the sediment (e.g.,
Harms et al. 1982). Repetition of similar conditions
leads to repetition of similar depositional products,
which are then susceptible to a universal empirical
classification in the field.
Miall (1977) reviewed braided-river deposits, and
demonstrated a consistency in the lithofacies assemblages occurring in a wide range of modern and
ancient sandy and gravelly sediments. However,
many of the researchers whose work was examined
in this review had erected their own local classification, which had obscured the similarities between
the deposits and prevented the recognition of common depositional themes. Miall proposed a simple
classification, making use of a two-letter code to
facilitate quick field and laboratory identification
and documentation. Use of this lithofacies scheme
by a number of workers led to the recognition that
the scheme could be applied to all fluvial deposits,