Upload
windanov
View
3
Download
1
Tags:
Embed Size (px)
DESCRIPTION
Stratigraphy of paralic coal bearing strata
Citation preview
Sequence stratigraphy of paralic coal-bearing strata: an overview
Michael Holz a,*, Wolfgang Kalkreuth a, Indranil Banerjee b
aInstituto de Geociencias, Univ. Federal do Rio Grande do Sul (UFRGS), Av. Bento Goncalves, 9500, 91501-970 Porto Alegre, RS, Brazilb7324-61st Avenue NW, Calgary, Alberta, Canada T3B 3W8
Abstract
Sequence stratigraphy arose in the late 1980s to fundamentally change the science of stratigraphy. Former practice of
labeling formations and erecting stratigraphic columns gave place to a dynamic genetic stratigraphic analysis, where the main
concern is about understanding the history of sedimentation and to establish models able to predict facies. Born and mainly
applied to an environment of petroleum prospecting and exploration, sequence stratigraphy has gained entrance to other
branches of sedimentary geology. The present paper gives a short introduction to sequence stratigraphic concepts and shows an
overview of its application on coal-bearing strata. Two case studies, one from the Early Permian coals of the Parana Basin,
Brazil, and one from the Lower Cretaceous coals of the Western Canada Sedimentary Basin illustrate the concepts. D 2002
Published by Elsevier Science B.V.
Keywords: Sequence stratigraphy; Coal petrology; Permian; Cretaceous; Parana Basin; Western Canada Sedimentary Basin
1. Introduction: sequence stratigraphywhat is it
about?
Sequence stratigraphy focuses on the understand-
ing of the genesis of the sedimentary strata rather than
on description and labeling, as was the case with li-
thostratigraphy, the most popular stratigraphy until
the 1980s. Insofar, it provides a descriptive and pre-
dictive framework for subdividing strata.
To understand sequence stratigraphic thinking, one
must remember that four geological variables control
sedimentation and the variation of the base level: cli-
mate, sedimentary input, tectonics and eustasy. Cli-
mate is an important factor for weathering and erosion
control and will determine the type of sediment made
available. Climate, together with the rate of tectonic
uplift, controls the rate of sedimentary influx.
However, for the sequence stratigraphic model,
tectonic movementsuplift and subsidencecom-
bined with eustatic variations, are the most important
parameters. The tectonic effect combined with eustatic
variation results in relative sea level change (Fig. 1A),
creating the so-called accommodation, which is the
ultimate space available for deposition of sediment.
Sea level ( = base level) variations leads to modifica-
tion of the accommodation space: if sea level falls,
space creation is minimum or nil; if sea level rises,
space is created in an increasing manner. Near the in-
flection point of the rising limb of a sea level curve is
the maximum of accommodation (Fig. 1B).
The rate of accommodation combined with the
sedimentation rate controls the deposition of sediment.
If the rate of creation of space is less than the sedi-
mentation rate, progradation will occur on the shelf, if
0166-5162/02/$ - see front matter D 2002 Published by Elsevier Science B.V.
PII: S0166-5162 (01 )00056 -8
* Corresponding author. Tel.: +55-51-3316-6836; fax: +55-51-
3316-7302.
E-mail address: [email protected] (M. Holz).
www.elsevier.com/locate/ijcoalgeo
International Journal of Coal Geology 48 (2002) 147179
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179148
the rate of accommodation is greater than the sedimen-
tation rate, transgression and retrogradation of sedi-
ments will take place (Fig. 2). In short, answering the
questionwhat happens to sedimentation if I vary the
base level?is what sequence stratigraphy is all about.
Basically, concepts of sequence stratigraphy (e.g.,
Wilgus et al., 1988) deal with the delineation of
chronostratigraphic surfaces that represent events of
rise or fall of relative sea level. These surfaces are
boundaries for the depositional systems tracts ( = asso-
ciation of genetically and spatially related depositional
systems) and for the depositional sequences ( =major
stratigraphic units bounded by unconformities). Every
systems tract has a well-defined stratigraphic position
within the depositional sequence and is the result of a
particular sedimentation regime, dictated by the com-
bined influence of sea level fluctuation ( = eustasy)
and basin tectonics ( = subsidence). Stratigraphers deal
with systems tracts developing during three distinct
phases of relative sea level: lowstand systems tract
( = some progradation and mostly aggradation of sedi-
ments), transgressive systems tracts (retrogradation of
sediments) and highstand systems tracts (some aggra-
dation and mostly progradation of sediments).
While the basic model (Exxon model) predicts
only three systems tracts (e.g., Wilgus et al., 1988),
several workers later recognized formation and pres-
ervation of parasequences also during the falling
phase of sea level. Among the first to draw attention
to this fact were Hunt and Tucker (1992), proposing
the concept of stranded parasequences, followed by
Plint (1996) who proposed the term falling stage
systems tract. The basic difference of the Exxon
concept and the concept of these works is that one
excludes the possibility of creation of accommodation
during sea level fall, while the latter predicts sedi-
mentation, not only in the distal setting of the basin
(the basin floor fan of the Exxon model), but also in
paralic settings.
During lowstand times, progradation and aggrada-
tion of fluvio-deltaic and shoreface sediment is char-
acteristic. During times of transgressive systems tract
development, an overall retrogradation occurs until a
maximum flooding epoch, when the basin area rea-
ches its maximum extent. At this time, almost all
sediments are trapped near the coastline so that over
most of the basin floor, only fine-grained sediments
are deposited, forming a thin layer of muddy sediment
called the condensed section. The phase of maximum
flooding is followed by times of stationary and
regressive shoreline positions, due to progradational
regime during highstand systems tract deposition.
Therefore, the events of rise and fall, and the subse-
quent conditions of sedimentation (aggradation, pro-
gradation or retrogradation), are mapped and put
together in a chronostratigraphic framework, which
is the essence of sequence stratigraphy analysis.
From the viewpoint of methodology, the sequence
stratigraphic analysis is based upon the concept of the
so-called parasequence, which is defined as a con-
formable succession of genetically related beds or
bedsets bounded by marine-flooding surfaces (Van
Wagoner et al., 1988). The stacking pattern of para-
sequence sets is an important criterion for delimiting
systems tracts, as shown in the previously mentioned
Fig. 2, where the bounding surfaces between the
sedimentary units are flooding surfaces delimiting
parasequences.
The triumphal march of sequence stratigraphic
concepts in geology since the 1980s and its popularity
relies on three main facts:
contrary to the seismic stratigraphy of the 1970s(e.g., Payton, 1977), sequence stratigraphy is
available for everyone who has stratigraphic
data, since it is applicable, not only to seismic
data, but also to outcrops and well logs; it is applicable at almost every scale, from
basinwide to flume; it is predictive, meaning that one can poten-
tially predict the occurrence of certain facies
within the sequence stratigraphic framework.
Fig. 1. (A) A cartoon showing the concept of accommodation, which is the space between the basin floor and the base level ( = approximately
the sea level). If sea level rises eustatically and/or if tectonic subsidence is active, the space available for sedimentation increases. (B) A single
eustatic sea level cycle illustrating that the inflection points F and R of the cycle correspond to the maximum negative and positive rate of
eustatic change, hence corresponding to the time of minimum and maximum creation of space (modified from a concept by Posamentier et al.,
1988).
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179 149
The latter fact is the reason for the rapid develop-
ment and worldwide interest of sequence stratigraphy:
for the first time in geological research, stratigraphers
had a predictive tool that really workssomething
like find the sequence boundary, follow it basinward,
find the lowstand fan, and you have a reservoir, told
in simple words. The new stratigraphy made fame
and fortune for a generation of oil consultants by
Fig. 2. Progradation, aggradation and retrogradation of sedimentary units ( = parasequences, see discussion ahead in the text) is a function of
accommodation space. If the rate of deposition is larger than the rate of space creation, the incoming sediments easily fill up the space available
and prograde basinwards, resulting in shoreline advance. A rate of deposition smaller than the accommodation results in retrogradation and
shoreline retreat. If both rates are equivalent, aggradation will occur, and the shoreline will stay relatively stationary (modified after Posamentier
et al., 1988).
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179150
using the sequence stratigraphic concepts to develop
new oil fields or to recover or enhance production in
oilfields close to exhaustion.
And the coal geologists?
2. Coal geologists doing sequence stratigraphy
how it began
Coal seams form in a broad spectrum of deposi-
tional systems, from alluvial fan setting to strand-
plains and subaqueous deposition. Although the
dynamics of coal accumulation in these setting have
been well understood since the beginning of the last
century, the dynamics of the allocyclic controls evi-
dent in many or almost all coal basins of the world
were not so clear to coal geologists.
Looking a few decades back, we see that in the
1960s and 1970s, the focus of coal research was on
the role of depositional environment on peat forma-
tion, and the main goals of most coal geologists were
to understand aspects such as facies studies and plant
community reconstitution (e.g., Murchison and West-
oll, 1968; Horne et al., 1978). The knowledge of ba-
sinwide transgressiveregressive cycles, and the fact
that coal seams were cyclically appearing within the
rock successions, led to attempts towards developing
a large-scale model of coal formation and distribution.
The most famous and popular attempt to explain
coal cycles was the cyclothem concept of the North
American school of stratigraphers (e.g., Weller, 1930;
Moore, 1964), staying popular until the late 1960s.
The cyclothem concept was based on the assump-
tion of a single transgressiveregressive cycle formed
by a facies framework with 10 rock units in coal-
bearing strata (Fig. 3). The position of these units
within the cyclothem was determined by the prevail-
ing state of marine regression or transgression. Dis-
crepant facies successions and contrasting deposi-
tional environments interpreted within the cycles led
to a profusion of variations from the basic model and
revealed the rigidity of the concept. For instance, the
cyclothem model positions the turning point from the
regressive to progressive ( = transgressive) cycle above
the coarsest clastic fraction of the cyclothem, without
considering if this coarse facies represents a fluvial
(i.e., regressive facies) or a tidal channel or a washover
fan (i.e., transgressive facies).
The cyclothem concept was a very rigid template.
Even the attempt to quantify and predict the facies
succession by means of Markov chain analysis (e.g.,
Duff and Walton, 1962) was not sufficient to diminish
the fact that the concept was not practical for the day-
to-day coal geologists who needed answers to ques-
tions like:
why does the coal seam occur in this particularlevel within the rock succession?
what are the roof and floor conditions of thestudied coal seam?
why does the coal seam pinch out or split in acertain direction?
how do the lithology of the splits and the roofrock vary locally or within the basin?
why are the coal properties not constant withinthe same seam and how do they vary?
Some of these questions can obviously be
answered without sequence stratigraphy, only by
control of the depositional system. However, the most
important questionshow the coals seams are posi-
tioned within the succession and how their properties
vary vertically and horizontallyare only answered if
petrographical and geochemical signatures of the coal
seams are integrated to a sequence stratigraphic
framework, as we comment later in this paper.
Analyzing the research papers on coal geology
published in the last 20 years, one can note a clear
shift from a time of depositional process-orientated
coal research (until the late 1970s, e.g., Horne et al.,
1978) to an epoch where allocyclic control of the
coaly rock record was investigated. This was at the
beginning of the 1980s, when coal researchers began
to understand that basinwide processes also play an
important role in controlling the formation and
regional distribution of coal seams.
Factors such as climate (e.g., Parrish et al., 1982),
tectonics (e.g., Fielding, 1987) and eustasy (e.g., Ryer,
1981) were investigated and integrated with coal
research and helped to clarify certain aspects of coal
accumulation and preservation, which before were
never properly understood.
The role of transgressiveregressive cycles in coal
formation, as recognized since the pioneering cyclo-
them concept, continued to attract the attention of
stratigraphers. Ryer (1981), for instance, showed that
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179 151
Fig. 3. A coal-bearing succession showing several cyclothems (Moore, 1964). Note that the eustatic control and the unconformity surfaces now
used to delimit depositional sequences are clearly indicated, but were never used properly to make coal seam correlation and basin analysis,
because that aspect was eclipsed by the strong facies-predictive aspect of the concept (discussion in the text).
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179152
the thickest and most extensive coals occur within
stacked deltaic sandstones in the vicinity of the trans-
gressive maximum of a basin. Using this concept, he
built up a predictive model for the Cretaceous coal-
bearing strata of the Western Interior of the United
States. Subsequent work (e.g., Aitken and Flint, 1995;
Flint et al., 1995) confirms the implication of increas-
ing base level for coal formation. However, Fasset
(1986) shows that the models based on the coals of
some part of the Western Interior of USA are not
applicable in other areas. The latter author presents
research results on two coal-bearing formations (Fruit-
land and Lower Menefee) in the San Juan Basin of
Colorado, New Mexico (USA). Fasset (1986) draws
attention to the fact that, although both formations
form part of a huge transgressive cycle, one developed
thick and extensive coal seams (as predicted by the
model of Ryer and others), while the other formation
has almost no thick coal beds.
This kind of apparent discrepancy or nonfunction-
ability of the coal-forming models of the 1980s was
only solved in the 1990s, when stratigraphers under-
stood that it is not the absolute amount, but the rate of
change of accommodation that is the important vari-
able, as will be discussed later.
However, base level change as a control of coal-
forming environments continued in the coal strati-
graphers mind, but the shift of the focus of research
from the depositional system scale to a basinwide
scale lasted the whole decade of the 1980s. The
change in the manner of thinking in coal geology
was neither easy nor quick. For instance, from the 16
papers in the special volume on coal-bearing strata
published by the Geological Society of London (Scott,
1987), none focuses on sequence stratigraphic princi-
ples, although concepts such as punctuated aggra-
dational cycles (Goodwin and Anderson, 1985) and
parasequences (Van Wagoner, 1985) were already
available for application, besides the entire framework
of seismic stratigraphic concepts from the late 1970s
(e.g., Payton, 1977).
Another example is the Geological Society of
America 1988 Centennial Meeting and the subse-
quent publication of papers on distribution and qual-
ity of Cretaceous coals (McCabe and Parrish, 1992).
In this publication, only in 1 paper out of 23 some
aspects on parasequences and coal formation are dis-
cussed.
This indicates that oil and gas-orientated sequence
stratigraphy evolved more quickly and spread more
readily in academic circles than the sequence stratig-
raphy applied to coal-bearing strata.
Insofar, papers on theoretical concepts (e.g., Cross,
1988, focusing on the importance of accommodation
balanced with progradational sediment input to form
thick, vertically stacked coaly sequences) and regional
key studies (e.g., Arditto, 1991, showing a sequence
stratigraphic analysis of the Late Permian coals of the
Sydney Basin, Australia) are benchmarks in the recent
history of coal geology.
3. Sequence stratigraphic models for coal-bearing
strata
3.1. Introduction
The new stratigraphy was formally presented to
coal geologists by Diessel (1992), who was the first to
make a comprehensive integration of coal formation
and preservation with the concepts of the above-des-
cribed Exxon sequence stratigraphic model. In his
renowned textbook, the author dedicates a 52-page
chapter to coal formation and sequence stratigraphy,
discussing the chemical and mineralogical signature
of regressive and transgressive coals as depicted by
sequence stratigraphic analysis, and links coal devel-
opment to the systems tracts of a depositional se-
quence (Fig. 4).
Since then, sequence stratigraphy has enabled coal
geologists to reinterpret and solve old problems by
looking at different angle and thinking in a different
manner about coal seam formation and the strati-
graphic record. A good example of this new think-
ing is that of the formation of very thick coal layers,
known from different basins and different ages world-
wide. Some coal seams have up to 90 m of total
thickness. No modern peat-forming environment can
explain such huge thickness of peat accumulation
(e.g., Shearer et al., 1994; Banerjee et al., 1996).
Investigation of base level variation and the rec-
ognition of key surfaces within the stratigraphic
framework of coal-bearing basins provide a clue to a
reasonable explanation for the formation of thick coal
layers. Studies from several authors have shown
conclusively that most thick coal beds are composed
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179 153
of several amalgamated paleo-peat bodies separated
by events of remarkable drops of water table (e.g.,
Shearer et al., 1994). Within this new view of coal
seam development, organic degradative or inorganic
partings are the stratigraphic signature of basinwide
base level falls and thus the thick coal seams may
represent amalgamation of a number of high-fre-
quency depositional sequences under the sequence
stratigraphic viewpoint (Banerjee et al., 1996).
Sequence stratigraphic approach permits reinter-
pretation of well-known coal-bearing strata, solving
some of the problems regarding coal formation and
cyclicity. For the Australian Gunnedah and Bowen
basins, for instance, the traditional deltaic model
could not satisfactorily explain the thick, laterally
continuous and low-ash coal seams. Arditto (1991)
postulated a sequence stratigraphic model for these
basins, where coastal ponding during transgression
lead to the development of the thick coal seams.
Michaelsen and Henderson (2000) recognized a cli-
matic overprint on the stratigraphic signature of the
coal-bearing succession in the north-central Bowen
Basin. There, the geometric and facies relationships
indicate that sedimentation was controlled by climatic
and sea level cycles, the prime factors of facies
stacking.
3.2. The role of accommodation in coal formation
Bohacs and Suter (1997) discussed in detail the
controls of coaly rock formation, emphasizing what
Cross (1988) modeled: the fundamental control on
coal formation and preservation is the accommodation
rate in relation to peat production. As previously
pointed out by Gastaldo et al. (1993), Aitken and Flint
(1995), and others, Bohacs and Suter (1997) showed
that the most important coal formation (in regard to
thickness and regional extent) occurs within the trans-
gressive systems tract, where creation of accommoda-
tion is large. The authors predict symmetrical pairs of
thickness-geometry attributes throughout the cycle, as
mires should respond mainly to the rate of change in
base level and not to the direction of that change. Fig. 5
summarizes the predictive model of coal thickness/
geometry as depicted by Bohacs and Suter (1997):
. During the deposition of the lowstand systems
tract, the small accommodation rate creates space that
is promptly filled vertically. Then, the mire extends
Fig. 4. Diessels (1992) diagrammatic model for the development of transgressive and regressive coal seams within a depositional sequence,
drawing attention to the fact that minor sea level drops can lead to regressive coals within the transgressive systems tract, and may result in
transgressive coals within the overall progradational highstand systems tract.
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179154
horizontally, forming continuous coal layers with a
dulling upwards trend (i.e., from ever wet at the base
to dry at the top), a fact historically observed in many
coal seams (e.g., Smith, 1962; Teichmuller, 1962).
Hence, the coals of this depositional phase are mod-
erately thick and continuous.
. During late lowstand and initial transgressive
systems tract deposition, the increasing accommoda-
tion rate permits the peat to accumulate to its full
capacity in place, hence the mire does not need (or
may not be able) to extend laterally, and thick but
relatively isolated, laterally discontinuous coal seams
are formed.
. In the middle transgressive systems tract, the
high accommodation rate precludes mires accumula-
tion until the space available has been filled, and only
thin, discontinuous and scattered coals are formed.
Mires are stressed and eventually inundated, and pre-
servation decreases. A few isolated peats may accumu-
late in domed mires located in areas of high rainfall.
. In the late transgressive and initial highstand
systems tract, the contrary situation occurs: first, the
Fig. 5. The coal geometry-and-thickness predictive model of Bohacs and Suter (1997). Lowstand and highstand coals are similar in geometry
and thickness since the rate of space creation of the lowstand systems tract is a mirror of that of the highstand systems tract. Compare the
illustration with Fig. 1B.
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179 155
Fig. 6. The basic sequence stratigraphic model and the occurrence and distribution of paralic coals, as depicted by Bohacs and Suter (1997).
M.Holzet
al./Intern
atio
nalJournalofCoalGeology48(2002)147179
156
Fig. 7. The concept of Galloway (1989): marine flooding surfaces are the boundaries of the genetic stratigraphic sequences. For some coal stratigraphers, coal seams are the landwards
correlative surfaces of these flooding surfaces. Note that the genetic sequence boundaries correspond to the maximum flooding surfaces of the Exxon-type depositional sequence.
M.Holzet
al./Intern
atio
nalJournalofCoalGeology48(2002)147179
157
accommodation rate permits the formation of thick
and isolated coals, then laterally more continuous coal
seams are formed.
Within a depositional sequence, normally repre-
sented as a basinwards-extended clinoform, the occur-
rence and distribution of paralic coals are clearly
predictable. Fig. 6 shows the optimal occurrence and
distribution of paralic coals within a complete depo-
sitional sequence.
The authors draw attention to the fact that, for a
given peat production rate, the occurrence of paralic
coals may vary significantly due to local rate of
change in accommodation. Lower accommodation
rates favor initiation of mires earlier in the lowstand
systems tract and later in the highstand systems tract,
while higher rates would delay or even prevent wide-
spread peat accumulation.
Local variation in sediment supply may alter com-
pletely the sedimentation regime (e.g., local progra-
dation in an overall retrogradational setting due to
fault-controlled alluvial sedimentation), constraining
the above-mentioned model. Keeping that in mind,
the Bohacs and Suters (1997) model is one of the
most advanced theoretical approaches to sequence
stratigraphic analysis of coal-bearing strata.
3.3. Genetic stratigraphy and flooding surfaces
While some coal researchers favor the concept of
unconformity-bounded depositional sequences, gener-
ated by base level falls; others prefer to work with the
genetic stratigraphic sequences of Galloway (1989).
That author, building on the concept of the depositio-
nal episode of Frazier (1974), proposed a stratigraphic
unit bounded by surfaces of maximum transgression,
enveloping what he called a genetic stratigraphic se-
quence (Fig. 7), a unit that is readily recognizable in
shallow marine and marginal settings, but hard to
recognize in nonmarine settings. Hamilton and Tadros
(1994) proposed that major, regionally extensive coal
seams can function as genetic sequence boundaries
because they have the attributes of genetic sequence
boundaries as depicted by Galloway (1989), such as
Fig. 8. The twin coal sequence stratigraphic model of Banerjee et al.
(1996): the formation of transgressive regressive coal couplets
with a basinwards split is controlled by base level variations and
leads to formation of thick and apparently homogeneous coal seams
landwards. (a) Sea level and water table rise and peat accumulation
takes place ahead of rising sea level. (b) Peat is drowned by
advancing sea. (c) Sea level is at its maximum (maximum flooding
surfaceMFS), followed by prograding land-derived sediments,
sea retreats followed by formation of regressive peat layer. (d) Sea
level continues to fall causing subaerial erosion of regressive peat
layer. (e) Sea level rises again, starting next cycle. W.T. =water
table; S.L. = sea level; TST= transgressive systems tract; HST= high-
stand systems tract.
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179158
the absence of clastic influx, which is extremely
characteristic of times of maximum flooding.
However, the concept of coals as genetic sequence
boundaries did not evolve. In a quite incisive reply to
the paper of Hamilton and Tadros (1994), Aitken
(1995) showed his reasons why coal seams are not
genetic sequence boundaries. The main argument is
similar to that discussed by Shearer et al. (1994): coal
seams are frequently not single, but multi-episodic bo-
dies, hence do not represent a single surface and can
thus not be taken as maximum transgressive surfaces.
However, this does not invalidate the usage of
flooding surfaces to study coal-bearing strata. Pashin
(2000) used flooding-surface bounded depositional
cycles to make 3D models of accommodation space.
Diessel et al. (2000a,b) also used flooding surfaces to
identify accommodation trends in coal seams, includ-
ing nonmarine flooding surfaces correlative with
marine-flooding surfaces.
It seems that sequence stratigraphers working with
coal-bearing strata repeat the methodological paradox
of their colleagues from other branches of sequence
stratigraphy: although the depositional sequence is
defined and bounded by some type of regional un-
conformity, the main conceptual tool for correlation
and study of the coal seams are the flooding surfaces.
The landwards amalgamation of several coal coup-
lets can lead to the formation of very thick coal beds,
which may contain sequence boundaries and flood-
ing surfaces.
3.4. A comprehensive model
In a geological model presented by Banerjee et al.
(1996) in their study of Lower Cretaceous coals in the
Western Canada Sedimentary Basin (Fig. 8), several
aspects of the sequence stratigraphic model of coal
deposition were dealt with:
1. Typical signatures of transgressive and regres-
sive seams based on vertical in-seam variations.
2. Interpretation of split coal seams as trans-
gressiveregressive coal couplets and high-
frequency sequences (fourth-order).
3. Progressive basinward splitting of regional
thick seams over progradational platforms in-
dicating landward amalgamation of high-fre-
quency sequences.
3.4.1. Typical coal seam signatures
Although Diessel (1992) dealt with geochemical
and organic petrological coal seam signatures in
detail, an added dimension of the new model is the
addition of palynological signatures in the vertical
profile of coal seams to distinguish between trans-
gressive and regressive seams (Fig. 9). Parallel zona-
tion of plant communities in vegetated coastal low-
lands that would be reflected in a progradational or
retrogradational vertical coal-bearing succession in
the geological record (Casagrande et al., 1974; Coates
et al., 1980) has earlier been recognized. Banerjee et
al. (1996) identified five plant communities on the
basis of the relative proportions of terrigenous pollens,
spores and aquatic cysts including dinoflagellates, and
their contrasting vertical succession define either
transgressive or regressive seams (Fig. 9).
3.4.2. Transgressiveregressive coal couplets
The interpretation of coal seam splits as trans-
gressiveregressive coal couplets marking fourth-
order sequences, presented by Banerjee et al. (1996),
is a key to the model for coal-bearing stratigraphic
sequences because it integrates the geochemical, paly-
nological and petrological signature of the coal seams
with the sedimentation regime.
According to Diessel (1992), a split coal seam
might represent a transgressiveregressive coal cou-
plet separated by a wedge of marine sediments. Flint
et al. (1995) also noted that landward amalgamation
of flooding surfaces produce split coal seams. A
progressive basinward splitting pattern of regional
coal seams found in this study is consistent with these
examples.
3.4.3. Progradational platform
The enigma of thick regional coal seams can be
solved by the approach adopted in this model. A
prograding platform advances basinward by the addi-
tion of successive wedges of coastal plain sediments.
Each of these wedges, in all probability, represents a
high-frequency (fourth-order?) sequence. Landwards,
these sequence boundaries merge, amalgamating the
sequences into thicker units. Therefore, regionally
thick coal seams might contain a number of sequence
boundaries, probably of different orders, growing over
a prograding platform through multiple sea level (or
base level) cycles.
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179 159
3.5. Coal petrology and sequence stratigraphy
Petrological and geochemical signatures of coal
seams formed in transgressive and regressive deposi-
tional settings have been studied by several authors
(e.g., Diessel, 1992, 1998, Diessel et al., 2000b;
Banerjee et al., 1996; Petersen and Andsbjerg, 1996;
Petersen et al., 1998; Holz et al., 1999; Banerjee and
Kalkreuth, in press). According to these studies, petro-
graphic parameters such as vitrinite content and type,
vitrinite reflectance, fluorescence properties, tissue
preservation and gelification indices and other coal
petrographical parameters often show significant var-
iations from seam base to top (Fig. 9) and can be re-
lated to the depositional regime (transgressive versus
regressive) under which the precursor peat accu-
mulated. The transgressive/regressive nature of coal
seams is also reflected by chemical signatures such as
hydrogen and sulphur contents (Diessel, 1992) and by
variations in palynomorph assemblages (Banerjee et
al., 1996).
The phenomenon of reduced vitrinite reflectance at
seam base and seam top has been observed in many
brackish and marine-influenced coal seams (Diessel,
1992, Diessel et al., 2000b; Banerjee et al., 1995) and
has been attributed to alkine sea water percolating into
the upper and basal parts of the precursor peats,
neutralizing the organic acids, which in turn promotes
an increase in activity of anaerobic bacteria (Diessel,
1998). Waste products of these bacteria are believed to
be incorporated in the vitrinite molecular structure,
increasing the hydrogen content and, consequently,
reducing the vitrinite reflectance.
Increase of sulphur values in upper and basal parts
of brackish and marine-influenced coals is related to
the availability of sulfate in marine water, which,
when penetrating peat layers, is used by sulfate
reducing bacteria to produce H2S, which reacts with
Fe to form pyrite (FeS2).
Gelification Index (GI) and Tissue Preservation
Index (TPI), introduced by Diessel (1986), have been
used widely in coal petrographic studies to assess
Fig. 9. Petrographic, chemical (A) and palynological (B) signatures of transgressive and regressive coal seams (from Diessel, 1992; Banerjee et
al., 1996).
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179160
depositional environment and coal facies. The GI ratio
contrasts macerals of vitrinite and inertinite groups
that have undergone gelification with those that have
not (GI Index = vitrinite +macrinite/fusinite + semifu-
sinite + inertodetrinite). As such, the Gelification In-
dex is considered to represent a measure of relative
humidity during early peat formation, with high
values indicating relatively high water tables and
low values the opposite. The TPI ratio contrasts ma-
cerals of vitrinite and inertinite groups exhibiting ori-
ginal botanical cell structures with those where no
botanical cell structure is visible (TPI = telinite + col-
lotelinite + fusinite + semifusinite/collodetrinite + vi-
trodetrinite +macrinite + inertodetrinite). As such, the
TPI ratio is considered to reflect the precursor material
(woody over herbaceous), but also defines the degree
of degradation.
4. Examples of application
The following two examples of the integration of
sequence stratigraphic concepts and coal character-
ization come from Permian coal-bearing strata of the
Parana Basin, Brazil (Holz and Kalkreuth, in press)
and from the Cretaceous of the Western Canada
Sedimentary Basin (Banerjee and Kalkreuth, in press).
4.1. Example 1: sequence stratigraphy and coal
petrology applied to the early Permian coal-bearing
Rio Bonito Formation, Parana Basin, Brazil
4.1.1. Geographical and geological characterization
of the study area
The southern region of Brazil (Fig. 10A), compris-
ing Parana, Santa Catarina and Rio Grande do Sul
states, has been known for its abundant and econom-
ically important coal seams since the beginning of the
1900s (e.g. White, 1908). These coal occurrences are
historically assigned to the Rio Bonito Formation, a
fluvial to marine sandstone and shale-prone lithostrati-
graphic unit of Early Permian age, approximately
deposited between 262 and 258 Ma (Artinskian/Kun-
gurian, using the time scale of Harland et al., 1989).
The coal seams have characteristics that are indicative
of an origin in limno-telmatic moors, where pterido-
phytic arborescent and herbaceous plant material
accumulated after some transport, promoting hypau-
tochthonic coal seams, rich in inertinite (e.g., Correa
da Silva, 1991). Coals in Rio Grande do Sul were
deposited in a back-barrier depositional setting, an
interpretation based on regional sequence stratigraphic
analysis (e.g., Holz, 1998) and tissue preservation and
gelification indices derived from maceral analysis
(e.g., Alves and Ade, 1996).
Here, the results presented by Holz and Kalkreuth
(in press) are summarized, focusing on conditions of
coal formation in Early Permian time. The study
investigates petrographical and geochemical charac-
ters of coal seams formed in transgressive and regres-
sive depositional settings, by comparison between a
fourth-order stratigraphic framework and the vertical
variation of parameters such as vitrinite content,
Gelification Index and Tissue Preservation Index.
The study area is part of a tectonic unit in south-
western Gondwana known as the Parana Basin, a
large intracratonic basin (e.g., Milani et al., 1994).
This basin is located at the central-eastern part of the
South American Platform (Fig. 10A). The fill of the
basin is divided by Milani et al. (1994) into six
second-order depositional sequences (Ordovician
Silurian to Late Cretaceous). Our study interval,
focusing on the coal-bearing Rio Bonito Formation,
is located at the base of the third sequence of Milani et
al. (1994), namely the Carboniferous/Early Triassic
Sequence, which forms the thickest sedimentary
sequence of the basin (2800-m thick at depocenter).
The base of the Carboniferous/Early Triassic sequence
occurs only in the depocenters of the basin, specifi-
cally in Santa Catarina and Parana states. During the
Late Carboniferous and Early Permian, strata onlap-
ped the marginal areas of the basin, as in Rio Grande
do Sul, where the oldest rocks of this depositional
sequence have a Sakmarian to Artinskian age. At that
time, the study area was located approximately 41south (Smith et al., 1981). In that location, during
summer in the southern hemisphere, a low pressure
cell over Central Africa and the contrasting high-
pressure center over the Panthaslassa ocean created
an atmospheric gradient that was responsible for west-
to-east summer winds, bringing humidity to the east-
ern margin of the Parana Basin (Holz, 1998). There-
fore, during the Artinskian and beginning of the
Kungurian, which is the time of formation of the main
coal seems, the climate was very cold and ever-wet
(e.g., Patzkowsky et al., 1991).
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179 161
Fig. 10. (A) Location map of southern Brazil and the Parana Basin. (B) Detailed map of the Candiota area, showing location of the correlation
section and borehole control for the study area.
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179162
Geographically, the study area is located in the
southwestern region of Rio Grande do Sul state (Fig.
10A), and covers about 2000 km2, including Brazils
most important coal deposit, the Candiota Coal Field,
developed in the 1970s by the Brazilian Agency Cia.
de Pesquisas de Recursos Minerais (CPRM). The well
logs and cores from this exploration program and data
obtained from outcrop locations constitute the base for
the stratigraphic and petrographic analysis of the cur-
rent study.
4.1.2. Facies and depositional systems of the coal-
bearing succession
An overview on the general stratigraphy of the
coal-bearing succession is given in Fig. 11A, which
shows the entire Early Permian (Sakmarian to Kun-
gurian/Ufimian) interval in southernmost Brazil. This
interval comprises the lithostratigraphic units Itarare,
Rio Bonito, Palermo and basal Irati, and records a
second-order transgressive cycle that began at the time
of deposition of the topmost Itarare unit and has its
maximum flooding surface within the Palermo For-
mation (e.g., Milani et al., 1994; Holz, 1999). This
second-order cycle is punctuated by important third-
order base level falls, with generation of several third-
order depositional sequences. The two coal-bearing
intervals of the Rio Bonito Formation are linked to
third-order sequence 2 and the base of third sequence
3 (Fig. 11B). In Rio Grande do Sul state, most of the
coals occur within the transgressive systems tract of
sequence 2, as detailed by Holz (1998) and Holz et al.
(2000), where the reader also finds a detailed facies
framework that permits the recognition of four main
depositional systemsalluvial fan, delta, lagoonal
estuary and barrier/shoreface. According to these
studies, the coals are linked to swamps and marshes
in a lagoonal estuary setting.
4.1.3. Sequence stratigraphy of the studied interval
In order to establish the sequence stratigraphic
framework of the coal-bearing interval in the Candiota
area, we used a data set acquired from 56 well logs
(gamma ray and resistivity logs), core description
from 14 boreholes and 6 outcrop sections. Fig. 12
shows a dip-orientated correlation section and Fig. 13
highlights a representative well log to illustrate facies
distribution and stratigraphy of the studied interval.
The regional correlation of the above-mentioned
lithofacies within the different depositional systems
led to a high-resolution third-order sequence strati-
graphic framework. The sequence boundaries, para-
sequence limits, systems tracts and major flooding
surfaces for the third-order sequences S2 and S3 are
shown in Fig. 11B.
Sequence boundaries SB1 (between the crystalline
basement and the Permian succession) and SB2 (flu-
vial sediments overlying marine shales and sand-
stones) are easily recognizable throughout the study
area and delimit third order sequence 1, where no coal
seams occur. Sequence boundary 3 (SB3) has a differ-
ent signature reflecting differential subsidence: some
areas clearly experienced temporary regression and
basinward shift of facies, while in others, the trans-
gression rapidly reworked the regressive sediments
and left only a thin veneer of pebbly sandstone, the
typical signature of a transgressive surface coinciding
with a sequence boundary. The total coastal encroach-
ment during the transgressive movement recorded by
sequence 2 reached about 70 km (Holz, 1998).
Within depositional sequence 2, seven parasequen-
ces are recognized (Fig. 11B), two forming the low-
stand systems tract of the sequence, four forming the
transgressive systems tract and one parasequence
forming the highstand systems tract.
Depositional sequence 3 is topped by boundary SB
4 (Fig. 11A). As this sequence has only a few coal
layers at its base ( = the lowstand systems tract LST3),
the stratigraphic overview of Fig. 11B shows only the
basal portion of this sequence.
Every parasequence begins with a flooding event
and turns progressively progradational. Therefore,
during initial times of parasequence development,
the associated peat-forming environments are strongly
transgressive. Towards the top of each parasequence,
the coals were formed in a progressively more regres-
sive depositional environment, since the sedimenta-
tion is prograding toward the basin.
The parasequences mapped in the study area have
a variable thickness (312 m) and the boundaries are
marked by fine-grained sandstone with a wave-domi-
nated or wave-influenced origin (hummocky cross-
bedding or wavy and lenticular bedding), passing
upwards to more current-originated facies (fine to
coarse-grained sandstones with trough and planar
cross bedding), capped by coal layers. Within our
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179 163
Fig. 11. (A) Stratigraphic overview of the coal-bearing Early Permian succession. The dotted rectangle indicates the stratigraphy of the study area (from Holz et al., 2000). (B)
Detailed sequence stratigraphy of the study area. The most important coals occur within the transgressive systems tract of sequence S2, within the parasequences PS4 to PS8.
M.Holzet
al./Intern
atio
nalJournalofCoalGeology48(2002)147179
164
Fig. 12. Depositional dip orientated correlation section, for location, see Fig. 10.
M.Holzet
al./Intern
atio
nalJournalofCoalGeology48(2002)147179
165
Fig. 13. Description and interpretation of a representative well log (HV-60), showing the succession of depositional systems from deltaic to
shoreface and offshore settings. For location, see Fig. 10.
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179166
stratigraphic framework, as abridged in Fig. 11B, the
bases of parasequences PS 2 and PS 9 are erosional
transgressive surfaces, as indicated by the occurrence
of Glossifungites ichnofossils (base of PS2) and an
intraclastic veneer composed of nodules (chert?),
shell fragments and muddy rip-up clasts (base of
PS 9).
The coals are rare in the lowstand systems tracts of
both sequences 2 and 3. The relationship of systems
tracts and coal geometry and thickness predicted by
Bohacs and Suter (1997) is not observed, probably
because the lowstand systems tract of sequence 2 is
strongly progradational in the beginning, due to
tectonic reactivation of source areas that is observed
not only regionally, but even on a basinwide scale
(e.g., Milani et al., 1994). Few coals were formed in
the deltaic environments of that systems tract due to a
low rate of accommodation combined with high
clastic input. The presence of the erosional trans-
gressive surface is an indication that the late lowstand
systems tract and its thicker coals might not have been
preserved, hence up to 20 m of strata are missing
because of the erosional transgressive surface, as
shown by correlation of the parasequences (see sec-
tion in Fig. 12).
For the transgressive systems tract of sequence 2,
some of the geometric relationships between systems
tracts and coal layers, as predicted by the Bohacs and
Suters (1997) model, have been observed. As
depicted by the model (Fig. 5), the thickest coal seams
occur within the transgressive systems tract of
sequence 2, with cumulative thickness up to 12 m.
The initial transgressive systems tract (parasequences
2 to 4, Fig. 14) has thick but relatively isolated coal
layers, including the most important of the Candiota
mining area (coal seams Candiota InferiorCCI
and Candiota SuperiorCCS, Fig. 14). In the late
transgressive systems tract (parasequence 5, Fig. 14),
the coals are thinner and somewhat scattered.
The difference in coal thickness and continuity
between the late and the early transgressive systems
tract is explained by the fact that in the late trans-
gressive systems tract, the high accommodation rate
precluded peat accumulation within mires until the
space available was filled, and only thin and scattered
coals were formed; whereas in the early transgressive
systems tract, the accommodation rate permitted the
formation of thick and less scattered coals, because
the peat accumulation kept pace with the increasing
accommodation.
The highstand systems tract of sequence 2, as well
as the lowstand systems tract of sequence 3, are thin.
Both systems tracts have only a few coal layers (Fig.
14), which are relatively thin (0.1 to 0.5 m) and
confined to the extreme northnortheastern part of
the study area. This is due to reduced accommodation
space and to the fact that the systems tract is domi-
nated by marine facies (lower shoreface).
4.1.4. Coal seam characteristics in the study area
For the present study, 17 coal seams were analyzed
from a shallow coal exploration borehole of the
Candiota Coalfield (SGQ-26, for location see Fig.
10B) representing the entire coal-bearing strata of
the Rio Bonito Formation as defined in Fig. 11A,B
(parasequences 3 through 8). By the time the major
coal seams were formed, the paleo-shoreline was
located approximately 40 km southsouthwestwards.
The coal seams were sampled as (a) full seam
channel samples and (b) as seam subsections (for the
thicker seams in 30-cm intervals each) to study in-
seam petrographic variations.
4.1.4.1. Petrographic characteristics of full seam
channel samples. Coal distribution in borehole
SGQ-26, along with petrographic characteristics and
sequence stratigraphic interpretation (limits of para-
sequences, third-order sequence boundary and sys-
tems tracts) is shown in Fig. 14. According to
sequence stratigraphic interpretation, the top seams
(seam S3, S4, S5, S6 and S7) form part of third-order
sequence 3 (parasequences 7 and 8 in Fig. 14). Third-
order sequence 2 has thin coals developed at the top
(parasequences 4 and 5, seams S8 and S9) and in
parasequence 3 (seams I4 and I5) at the base of the
coal-bearing interval. Maximum coal development
occurs in parasequence 4 with the Candiota Superior
(CCS) and Candiota Inferior (CCI) seams.
The petrographic composition of the coal seams is
shown in terms of organic matter types (content of
vitrinite, liptinite and inertinite groups) and mineral
matter content (Fig. 14). There appears to be an
overall trend to decreasing vitrinite content from the
base of the coal-bearing interval of third-order
sequence 2 to the top (parasequences 3 to 4), paral-
leled by an overall increase in inertinite content. This
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179 167
Fig. 14. Sequence stratigraphic interpretation of the coal-bearing strata in borehole SGQ-26, Candiota Coalfield and coal petrographic characteristics of enclosed coal seams. Maceral
groups and mineral matter in vol.%. GI =Gelification Index; TPI = Tissue Preservation Index (for explanation, see text); SB3 = lower limit of third-order sequence 3. Legend: see Fig.
13.
M.Holzet
al./Intern
atio
nalJournalofCoalGeology48(2002)147179
168
trend is shifted in parasequence 4, where the thick
seams have somewhat higher vitrinite contents, asso-
ciated with a decrease in liptinite and inertinite mac-
erals. The sharp increase in inertinite macerals in
parasequence 4 is caused by a high contribution of
fusinite in BL seam (55 vol.%) and by a combination
of high fusinite, semifusinite and inertodetrinite con-
tents in S9 seam (total 79 vol.%). In coal seams above
parasequence 4, a return to higher vitrinite and lower
inertinite content is indicated (Fig. 14).
Mineral matter contents (volume basis) are highly
variable, ranging from 4 to 30 vol.%. In regard to
sequence stratigraphic position of the coal seams,
there does not appear to be a relationship between
mineral matter content and stratigraphic position,
although in parasequences where multiple seams have
been analyzed (parasequences 3, 4 and 8), subtle trends
of increasing and decreasing mineral mater contents
can be noted.
Vitrinite reflectance values follow roughly the
trend shown by the inertinite contribution (parase-
quences 3 to 4), indicating that slightly increased
reflectance values are associated with higher inertinite
content and vice versa (Fig. 14). In the top part of the
studied interval (parasequences 5 to 8), the relation-
ships between vitrinite reflectance and petrographic
and/or stratigraphic position are not well defined.
When applying the GI-TPI concept (Diessel, 1986)
to Candiota coals, it is apparent that the GI values
roughly parallel the vitrinite contents determined in
the samples, suggesting successively drier conditions
during peat accumulation (from basal coal seams in
parasequence 3 to top of 4). For the same interval,
there is also a trend to higher TPI values, suggesting
higher input of woody material and preservation, in
particular in the fusinite-rich seams BL and S9 of
parasequence 4. The trend to relatively high TPI va-
lues actually continuous into parasequences 5, 7 and 8
(Fig. 14), with the exception of S5 and S6 seams,
where greater amounts of structureless collotelinite ac-
counts for lower TPI values.
4.1.4.2. Petrographic characteristics of seam sub-
sections. Petrographic characteristics of seam sub-
sections are discussed for seams developed in
parasequences 4 and 5 (Fig. 15), which comprise
seams L1, CCI and CCS at the base and seams BL
and S9 at the top of parasequence 4.
The first coal to develop in parasequence 4 is a
0.30-cm-thick seam (L1), characterized by low vitri-
nite content that increase upward (Fig. 15). Liptinite
contents, mainly in form of sporinite, are 15 and 19
vol.%, respectively. The remainder is made up of
inertinite macerals, mainly fusinite and inertodetrinite.
The overlying thick coals (CCI and CCS) show a
significant increase in vitrinite macerals, with both
seams indicating a very similar pattern in terms of in-
seam maceral distribution, namely highest vitrinite
contents at seam base, followed by decreasing vitrinite
contents towards the seam center and increasing
values towards the seam top, except in the uppermost
sample.
A return to low vitrinite contents is indicated for
the coal seams developed in the top of parasequence 4
(BL, S9). The three subsections from seam BL indi-
cate successively lower vitrinite contents from seam
base to seam top, with relatively high liptinite con-
tents (1727 vol.%), mainly in form of sporinite. The
remainder are inertinite macerals, predominantly in
the form of fusinite (22 to 53 vol.%). The trend of low
vitrinite and high inertinite content continuous
towards the top of parasequence 6, where seam S9
is characterized by a very low vitrinite content (6
vol.%) and high inertinite content (55 vol.%).
Vitrinite reflectances show a distinct pattern in the
three major seams (BL, CCS, CCI) developed in
parasequence 4, namely a trend to higher values in
the central part of the seam, with decreasing values
towards seam base and top. As discussed earlier, this
may reflect influence of brackish or marine water at
seam base and top during accumulation of the pre-
cursor peat.
TPI values in seams CCI and CCS show a simi-
lar trend, with little variation in the basal part of the
seam (Fig. 15), followed by a maximum in the up-
per part of the seam and lower values at the very
top. At the level of the BL seam (parasequence 4),
the very high fusinite contents are the reason for the
high TPI values reaching 8.0 in the top part of the
seam (Fig. 15).
The GI values in seams CCI and CCS run essen-
tially parallel to the trend shown for vitrinite contents,
with peak values at seam base, followed by reduced
values in the central parts of the seams and a return to
higher values in the top part except for the uppermost
seam subsection. At the level of seam BL, GI values
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179 169
are extremely low as a response to the high inertinite
content of that seam.
4.1.4.3. Petrography of full seams and sequence
stratigraphic framework. Comparison of petro-
graphic coal characteristics, as discussed above, with
the sequence stratigraphic framework of the enclosing
strata shows that coal characteristics are, to a large
part, controlled by depositional setting. During the
initial phase of the TST (Fig. 14), peat accumulation
took place associated with relatively high water tables,
favorable for the preservation of organic matter in the
form of vitrinite (seams I4 and I5). From there on is a
trend to successively drier conditions up section, as
depicted by decreasing vitrinite and increasing iner-
tinite contents (seams I3 to L2).
The upper part of the initial TST is characterized
by formation of a thin seam at the base (L1) followed
by two thick seams (CCI, CCS) in parasequence 5.
The overall petrographic features suggest relatively
stable conditions during peat formation for the CCI
and CCS seams, in which plant growth and preserva-
tion was in equilibrium with basin subsidence, and
probably records an epoch of sea level stillstand and
Fig. 15. Petrographic characteristics for coal seam subsections in parasequences 5 and 6, borehole SGQ-26, Candiota Coalfield. Maceral groups
and mineral matter in vol.%. GI =Gelification Index; TPI = Tissue Preservation Index (for explanation, see text); PS = parasequence (from Holz
and Kalkreuth, in press).
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179170
mostly aggradational sedimentation. Insofar, within
the overall transgressive trend as recorded by the
TST, the main Candiota seams CCS and CCI were
formed during a phase of relative stillstand.
The petrographic features of BL and S9 seams,
within the late TST, strongly suggest a regressive
setting of the seams (Fig. 14), as indicated by the
high inertinite content (fusinite and semifusinite
account for 4449 vol.%). It has been suggested
(Diessel, 1992) that coals of this type may have been
formed as back-barrier coals in a regressive phase of
an overall transgressive period. The sequence strati-
graphic analysis of the BL and S9 seam interval in-
dicate in contrast a predominantly transgressive phase
during peat formation, with the unlikelihood of a
larger regression. In that case, the high amounts of
fusinite macerals would have had their origin in forest
fires at or near the mire margins followed by trans-
portation into the mire by wind and/or water (hypau-
tochthonous to allochthonous origin). However, the
absence of greater amounts of inertodetrinite (genet-
ically linked to fusinite and mechanically degraded by
transportation processes) in the BL and S9 seam
supports the interpretation of in situ origin of the inert
material.
The petrographic composition of the single coal
seam (S8) developed in PS 5 indicates a return to
more moist conditions in the topmost late TST.
The variations observed in vitrinite and inertinite
contents for seams developed in PS 3 to 5 are also
reflected in vitrinite reflectance values and GI and TPI
values (Fig. 14), all suggesting in general a drying up-
ward trend in the upper part of third-order sequence 2.
Coal seams developed in the LST of sequence 3
(parasequences 7 and 8 in Fig. 14) show relatively
high vitrinite contents at the top (PS 9) and at base and
top of the coal-bearing interval of PS 8, with some-
what drier conditions in seam S4 (39 vol.% fusinite).
Although these LST coal seams were deposited in a
prograding depositional environment, as opposed to
the retrograding Initial and Late TST, petrographic
characteristics are similar and at this point do not
allow pinpointing of individual coal seams as belong-
ing to specific system tracts.
4.1.4.4. Petrography of seam subsections and se-
quence stratigraphic framework. The in-seam char-
acteristics for CCI, CCS and BL seams (Fig. 15) show
petrographic characteristics reported elsewhere for
transgressiveregressive coal seams (Diessel, 1992;
Banerjee et al., 1995; Banerjee and Kalkreuth, in
press).
The CCI and CCS seams of the Candiota area have
strikingly similar petrographic characteristics to the
marine/brackish-influenced Greta seam of the Sydney
Basin, Australia (Diessel, 1992). Similar features
include highest vitrinite content at seam base (Fig.
15), and decreasing vitrinite contents towards seam
center, after which vitrinite contents return to higher
values up seam. The very top of the seam is charac-
terized by a significant decrease in vitrinite content.
The similarity of petrographic characteristics in CCI
and CCS seams suggest that the precursor mires were
experiencing similar wet to dry cycles during their
lifetime (Fig. 15).
Influence of brackish/marine conditions during
early and late peat formation are also reflected by
vitrinite reflectance and GI and TPI values (Fig. 15).
In transgressive seams, vitrinite reflectance is typi-
cally highest in the center of the seam and lower
towards seam base and seam top. The lower vitrinite
reflectances in those parts of the seam affected by
brackish/marine water have been explained by the
incorporation of degraded algal and/or bacterial waste
material into the vitrinite and increased bacterial
degradation (Diessel, 1992), causing a suppression
in reflectance of the associated vitrinite. The GI values
also suggest a transgressive setting of the two seams
(Fig. 15), with GI values highest at seam base and
seam top (except for the uppermost sample) and drier
conditions during accumulation of the central part of
the peat. The changes observed for TPI values are not
that striking, however, both seams show a slight trend
to lower TPI values at seam top, a fact related to
deposition of increasing amounts of detrital macerals
prior to drowning of the mire.
The three subsections of BL seam (PS 4) suggest
successively drier conditions from seam base to seam
top (Fig. 15) during a regressive phase as indicated by
very high inertinite contents, dominated by fusinite
and semifusinite macerals (3658 vol.%). This high
content of structured inertinite is reflected in the high
TPI values (2.48) and very low GI values (0.11
0.58). The depositional model suggested for accumu-
lation of the precursor peat is that of a back-barrier
mire where the organic matter occasionally was
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179 171
exposed to oxidation processes during periods of low
water tables. Alternatively, the fusinite may have had
its origin from forest fires at mire margins or nearby
areas, although larger inertodetrinite contents, consid-
ered to indicate hypautochthonous/allochthonous ori-
gin of organic matter in coal seams, are absent in the
seam subsections of BL and S9 seams.
4.2. Example 2: Lower Cretaceous Mannville coals in
Alberta, Canada
An integrated case study of the stratigraphy, sed-
imentology, geochemistry, organic petrology and
palynology of coal and coal-bearing strata of the
Lower Cretaceous Mannville Group over a 9000
km2 area in the subsurface of south-central Alberta,
Canada, led to the construction of a sequence strati-
graphic model of a thick paralic coal. The database of
this study consists of geophysical logs (550 wells),
examination of cored intervals (a total of 1500 m), and
petrological, geochemical and palynological analyses
of samples of coal and associated rocks (Banerjee et
al., 1996; Banerjee and Kalkreuth, in press).
The study area is located in south-central Alberta
and forms part of the Western Canada Sedimentary
Basin (Fig. 16). The regional stratigraphy of the
Mannville Group suggests shoreline sedimentation
in a prograding barrier coastline. A carbonate interval
containing three limestone beds (Fig. 16) located close
to a second-order maximum flooding surface (MFS)
divides the lower Mannville transgressive strata from
the prograding, coal-bearing upper Mannville strata
(Fig. 17). Towards the top, a large number of incised
valley-fills interrupt regional coal seams, indicating
increasing fluvial influence. Regionally, five major
coal seams ( > 2 m thick) can be traced for more than
100 km (Fig. 17).
The unconformity-bounded Mannville Group in
Alberta is a 100300 m thick, siliciclastic depositional
sequence of a second-order based on the hierarchical
system suggested by Embry (1995), consisting of six
third-order sequences (Fig. 17). Coal seams are con-
fined to the highstand systems tract and the thickest
and the most extensive seam (the Medicine River or
the MR seam) lies above the maximum flooding sur-
face of this second-order sequence (Fig. 17). Most coal
seams overlie third-order sequence boundaries that
generally coincide with transgressive surfaces.
The following geochemical and petrographical
properties of coal were used to determine transgres-
sive or regressive origin of the coal: sulphur content
(S); hydrogen index (HI); maceral content and derived
Tissue Preservation Index (TPI), Gelification Index
(GI) and vitrinite/inertinite ratio (V/I); and vitrinite
reflectance.
4.2.1. Glauconitic coal seamtransgressive signa-
tures
According to the coal depositional model, the
glauconitic coal seam (or the precursor mire, in the
strict sense) was formed in a transgressive environ-
ment. The stratigraphic evidence is provided by the
fact that the seam is overlain by marine shales.
The coal seam was sampled in seven consecutive
column samples and was analyzed petrographically by
image analysis for gross petrographic composition
(maceral groups and mineral matter) and reflectance
variations from seam base to seam top. Additionally,
conventional maceral analysis was carried out to
determine type of macerals and to calculate facies-
critical ratios such as TPI and GI indices.
The seam shows elevated sulphur and hydrogen
indices at seam top (Fig. 18) related to the marine roof
rock caused by the enrichment in lipid-rich degraded
components and availability of sulphate in marine
water. Sulphur and hydrogen indices are also elevated
at the seam base, suggesting a brackish influence
during early seam formation.
Tissue Preservation Indices (TPI) range from 0.5 to
2.8 and show a definite trend to lower values towards
the seam top (Fig. 18). The trend indicated by the
range in Gelification Indices (GI) follows essentially
that of the overall vitrinite content, suggesting wetter
conditions in the mire at the seam base and top (Fig.
18). Vitrinite reflectances show a trend to lower values
at seam top and base, consistent with previously
described characteristics of a transgressive seam (Die-
ssel, 1992).
4.2.2. Medicine river coal seam, upper leafregres-
sive signatures
From this seam, 13 spot samples were collected to
study the in-seam variations from the base to the top
of the seam (Fig. 19).
The regressive nature of the seam is indicated by a
number of coal petrographic parameters: a general
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179172
Fig. 16. Location of the study area within Alberta, Canada and generalized lithostratigraphic northsouth cross-section of the Mannville Group investigated in this study (modified
from Banerjee and Kalkreuth, in press).
M.Holzet
al./Intern
atio
nalJournalofCoalGeology48(2002)147179
173
Fig. 17. A sequence stratigraphic interpretation showing the hierarchy of sequences within the Mannville Group. SBI is a first-order sequence boundary and SBII a second-order one.
Coal seam splits define fourth-order sequences boundaries marked a, b, c. These are arranged within subhorizontal third-order sequences as stacked progradational wedges marked 1
to 6. Most coal seams overlie third-order sequence boundaries or drape over incised-valley fills. Larger sandstone bodies have been interpreted in terms of barrier islands, flood tidal
deltas and incised-valley fills. For location of section AAV, see Fig. 14 (modified from Banerjee and Kalkreuth, in press).
M.Holzet
al./Intern
atio
nalJournalofCoalGeology48(2002)147179
174
Fig. 18. Profile of coal properties in transgressive glauconite seam. Note high vitrinite/inertinite ratios at seam base and top. Seam base and top are also characterized by slightly lower
vitrinite reflectance, increased sulphur contents and hydrogen indices (modified from Banerjee et al., 1995).
M.Holzet
al./Intern
atio
nalJournalofCoalGeology48(2002)147179
175
Fig. 19. Petrographic profile of the Upper Medicine River coal seam formed in a regressive setting. Note in top part of the seam very low tissue preservation indices as a result of high
vitrodetrinite and inertodetrinite contents, low vitrinite/inertinite ratios and corresponding low gelification indices and a trend to slightly elevated vitrinite reflectance values (modified
from Banerjee and Kalkreuth, in press).
M.Holzet
al./Intern
atio
nalJournalofCoalGeology48(2002)147179
176
upward decrease in vitrinite content (Fig. 19) and a
predominance of detrital macerals (inertodetrinite and
vitrodetrinite), > 60 vol.% in all samples. As a con-
sequence, TPI is typically low throughout the seam
except at the very base. GI decreases and vitrinite
reflectance increases towards the top (Fig. 19).
The transgressiveregressive nature of the coal
seams is also reflected in the palynomorph assembla-
ges (see also Fig. 9). In the Lower Cretaceous (Mann-
ville) coal seams of the WCSB, the palynological
analysis led to the reconstruction of five plant com-
munities. This was done on the basis of terrigenous pol-
lens and spores and aquatic cysts, including dinofla-
gellates (Banerjee et al., 1995). In a transgressive seam,
the vertical succession of palynomorphs shows increas-
ing marine influence and decreasing tree cover, from
forested swamps at the base to salt marsh at the top. In
a regressive seam, a reverse trend is found (Fig. 9).
A common feature of these coals is the basinward
progressive splitting pattern demonstrated by the re-
gional coal seams as recognized elsewhere (Coates et
al., 1980; Fielding, 1987). Each of the individual splits
seems to represent a transgressiveregressive twin
coal couplet (Banerjee et al., 1996; Diessel, 1992).
5. Conclusion
The models and examples discussed in this paper
conclusively show that coal geologists must consider
that the new stratigraphy is a powerful tool, not
only for the complete understanding of coal formation
and preservation in the different sedimentary environ-
ments, but also because it permits prediction of coal
seam thickness, continuity and quality. Sequence
stratigraphy may be used to understand and to explain
variations of coal parameters; but high-resolution
analysis of coal parameters may also be a helpful tool
to the sequence stratigrapher. With a high-resolution
sequence stratigraphic framework of a coal basin
followed by detailed petrographic analyses of the coal
seams, one may predict coal quality and provide
guidelines to optimal exploitation.
Coal geologists have to deal with a large amount of
variables. Aside from the factors controlling sedimen-
tation (climate, eustasy and tectonics), coal formation
also depends on the type of flora, peat accumulation
rates and groundwater table, i.e., aquifer hydrology.
This results in the development of conceptually differ-
ent models of sequence stratigraphy and the interpre-
tation of sedimentary regimes and coal characteristics,
as discussed in this paper.
Concerning the case studies, we conclude that coal
geology and coal petrology interpretations benefit
from sequence stratigraphic analysis in spite of the
different geological settings (e.g., foreland basin in
Canada versus intracratonic basin in Brazil) and the
differences in the tectonic and eustatic signature of the
sedimentation. The concept of parasequences and
systems tracts as the building blocks of a depositional
sequence is an important aid for the analysis and the
understanding of coal genesis. In the Brazilian case
study, the eustatic signature is stronger than the tec-
tonic one. The parasequences are commonly topped
by coal seams, and there are almost no coal splits or
amalgamation controlled by tectonics, as in the Cana-
dian example.
Acknowledgements
M. Holz and W. Kalkreuth acknowledge the
Brazilian National Research Agency (CNPq) for
research support (grants 352887/96-6 and 300971/
97-4RN). FAPERGS (97/1537.9) and the Brazilian
Ministry of Science and Technology (PADCT/
FAURGS/FINEP 87.98.0749.00) are acknowledged
for providing research grants to carry out the coal
characterization in the context of this study. We
acknowledge Dr. P. Michaelsen (James Cook Uni-
versity, Australia) and Dr. M. Gibling (Dalhousie
University, Canada) for their constructive comments
on the manuscript. The revised manuscript was also
critically read by Dr. C. Scherer (UFRGS, Brazil). Dr.
M. Silva (UFRGS) was contracted to carry out the
maceral analyses on the Parana Basin coals and M.
Kern (UFRGS) is thanked for technical help to pre-
pare many of the figures for publication. CPRM (Cia.
de Pesquisas de Recursos Minerais) and CRM (Cia.
Rio-Grandense de Minerac ao) are thanked for pro-viding access to sample material and well cores.
References
Aitken, J.F., 1995. Utility of coal seams as genetic stratigraphic
sequence boundaries in non-marine basins: an example from
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179 177
the Gunnedah Basin, Australia: discussion. AAPG Bulletin 79,
11791181.
Aitken, J.F., Flint, S.S., 1995. The application of high-resolution
sequence stratigraphy to fluvial systems: a case study from the
Upper Carboniferous Breathitt Group, eastern Kentucky, USA.
Sedimentology 42, 330.
Alves, R.G., Ade, M.V.B., 1996. Sequence stratigraphy and organic
petrography applied to the study of Candiota Coalfield, RS,
South Brazil. International Journal of Coal Geology 30, 231
248.
Arditto, P.A., 1991. A sequence stratigraphic analysis of the Late
Permian succession in the Southern coalfield, Sydney Basin,
New South Wales. Australian Journal of Earth Science 38,
125137.
Banerjee, I., Kalkreuth, W., in press. Sedimentology, sequence strat-
igraphy, palynology, organic petrology and geochemistry of
Mannville coals in south-central Alberta. Geological Survey of
Canada, Bulletin.
Banerjee, I., Kalkreuth, W., Davies, E., 1995. Sequence stratigraphy
of coal with examples from the Mannville Group in Central
Alberta. Proceedings of the Oil and Gas Forum 95, Energy
from Sediments, Geological Survey of Canada, Open File
3058, pp. 151157.
Banerjee, J., Kalkreuth, W., Davies, E., 1996. Coal seam splits and
transgressive regressive coal couplets: a key to stratigraphy of
high-frequency sequences. Geology 24, 10011004.
Bohacs, K., Suter, J., 1997. Sequence stratigraphic distribution of
coaly rocks: fundamental controls and paralic examples. AAPG
Bulletin 81, 16121639.
Casagrande, D., Cohen, A., Given, P., Spackman, W., 1974. The
comparative study of the Okefenokee swamp and the Ever-
gladesMangrove swamp marsh complex of Southern Florida.
Geological Society of America Annual Meeting Field Trip.
Pennsylvania State University Coal Research Section, Univer-
sity Park, 265 pp.
Coates, E., Groat, C., Hart, G., 1980. Subsurface Wilcox lignite in
west-central Louisiana. Gulf Coast Association of Geological
Societies, Transactions 30, 309332.
Correa da Silva, Z.C., 1991. The formation of coal deposits in South
Brazil. Gondwana 7th Proc., Instituto de Geociencias, USP, Sao
Paulo, pp. 233252.
Cross, T.A., 1988. Controls on coal distribution in transgressive
regressive cycles, Upper Cretaceous, Western Interior, USA. In:
Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier,
H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea-Level Changes:
An Integrated Approach, vol. 42. Society of Economic Paleon-
tologists and Mineralogists, Tulsa, OK, pp. 371380, Special
Publication.
Diessel, C.F.K., 1986. The correlation between coal facies and dep-
ositional environments. Advances in the Study of the Sydney
Basin, Proc. 20th Symposium. The University of Newcastle,
Australia, pp. 1922.
Diessel, C.F.K., 1992. Coal-Bearing Depositional Systems. Spring-
er, Berlin, 721 pp.
Diessel, C.F.K., 1998. Sequence stratigraphy applied to coal seams:
two case histories. In: Shanley, K.W., McCabe, P.J. (Eds.), Rel-
ative Role of Eustasy, Climate and Tectonism in Continental
Rocks, vol. 59. Society of Economic Paleontologists and Min-
eralogists, Tulsa, OK, pp. 151173, Special Publication.
Diessel, C.F.K., Boyd, R., Wadsworth, J., Chalmers, G., 2000a. The
identification of accommodation trends in coal seams. AAPG
Annual Meeting Abstracts (CD-ROM).
Diessel, C.F.K., Boyd, R., Wadsdworth, J., Leckie, D., Chalmers,
G., 2000b. On balanced and unbalanced accommodation/peat
accumulation ratios in the Cretaceous coals from Gates Forma-
tion, Western Canada, and their sequence-stratigraphic signifi-
cance. International Journal of Coal Geology 43, 143186.
Duff, P., Walton, E.K., 1962. Statistical basis for cyclothems: a
quantitative study of the sedimentary succession in the East
Pennine coal field. Sedimentology 1, 235255.
Embry, A.F., 1995. Sequence boundaries and sequence hierarchies:
problems and proposals. In: Steel, R.J., et al. (Eds.), Sequence
Stratigraphy on the Northwest European margin. NPF, vol. 5.
Norwegian Petroleum Society/Elsevier, New York, pp. 111,
Special Publications.
Fasset, J.E., 1986. The non-transferability of a Cretaceous coal
model in the San Juan Basin of New Mexico and Colorado.
Geological Society of America, Special Paper 210, 155171.
Fielding, C.R., 1987. Coal depositional models for deltaic and allu-
vial plain sequences. Geology 5, 661664.
Flint, S., Aitken, J., Hampson, G., 1995. The application of se-
quence stratigraphy to coal-bearing coastal plain successions:
implications for the U.K. coal measures. In: Whateley, M.,
Spears, D. (Eds.), European Coal Geology, vol. 82. Geological
Society, London, pp. 116, Special Publications.
Frazier, D.E., 1974. Depositional episodes: their relationship to the
Quaternary stratigraphic framework in the northeastern portion
of the Gulf Basin. University of Austin Bureau of Economic
Geology Geological Circular 74-1, 28 pp.
Galloway, W., 1989. Genetic stratigraphic sequences in basin anal-
ysis: architecture and genesis of flooding surfaces bounded dep-
ositional units. AAPG Bulletin 73, 125142.
Gastaldo, R.A., Demko, T.M., Liu, Y., 1993. Application of se-
quence and genetic stratigraphic concepts to carboniferous
coal-bearing strata: an example from the Black Warrior Basin,
USA. Geologische Rundschau 82, 212226.
Goodwin, P.W., Anderson, E.J., 1985. Punctuated aggradational
cycles: an general hypothesis of episodic stratigraphic accumu-
lation. Journal of Geology 93, 515533.
Hamilton, D.S., Tadros, N.Z., 1994. Utility of coal seams as genetic
stratigraphic sequence boundaries in non-marine basins: an ex-
ample from the Gunnedah Basin, Australia. AAPG Bulletin 78,
267286.
Harland, W.B., Armstrong, R.L., Cox, A.V., Craig, L.E., Smith,
A.G., Smith, D.G., 1989. A Geologic Time Scale. Cambridge
Univ. Press, Cambridge.
Holz, M., 1998. The Eo-Permian coal seams of the Parana Basin in
southernmost Brazil: an analysis of the depositional conditions
using sequence stratigraphic concepts. International Journal of
Coal Geology 36 (12), 141163.
Holz, M., 1999. Early Permian sequence stratigraphy and the palae-
ophysiographic evolution of the Parana Basin in southernmost
Brazil. Journal of African Earth Science 29 (1), 5161.
Holz, M., Kalkreuth, W., in press. Sequence stratigraphy and coal
M. Holz et al. / International Journal of Coal Geology 48 (2002) 147179178
petrology applied to the Early Permian coal-bearing Rio Bonito
Formation, Parana Basin, Brazil. In: Pashin, J., Gastaldo, R.
(Eds.), AAPG Special Publication on Coal Geology.
Holz, M., Ade, M.B., Kalkreuth, W., 1999. Coal seam petrology
analyzed within a sequence stratigraphy framework: preliminary
results from the Early Permian Rio Bonito Formation of south-
ernmost Brazilian Gondwanaland. In: Lemos de Souza, M.J.,
Marques, M.M., Fernandes, J.P. (Eds.), 2nd Symposium on
Gondwana Coals, Porto, 1998. Memoria da Faculdade de Cien-
cias do Porto, Departamento de Geologia, vol. 5, pp. 3749.
Holz, M., Vieira, P.E., Kalkreuth, W., 2000. The Early Permian
coal-bearing succession of the Parana Basin in southernmost
Brazil: depositional model and sequence stratigraphy. Revista
Brasileira de Geociencias 30, 420422.
Horne, J.C., Ferm, J.C., Caruccio, F.T., Bagarz, B.P., 1978. Depo-
sitional models in coal exploration and mine planning in Appa-
lachian Region. American Association of Petroleum Geologists
Bulletin 62 (12), 23792411.
Hunt, D., Tucker, M.E., 1992. Stranded parasequences and the
forced regressive wedge systems tract: deposition during base-
level fall. Sedimentary Geology 81, 19.
McCabe, P.J., Parrish, J.T., 1992. Controls on the distribution and
quality of Cretaceous coals. GSA Special Paper 267, pp. 116.
Michaelsen, P., Henderson, R., 2000. Facies relationships and cy-
clicity of high-latitude, Late Permian coal measures, Bowen
Basin, Australia. International Journal of Coal Geology 44 (1),
1948.
Milani, E.J., Franca, A.B., Schneider, R.L., 1994. Bacia do Parana.In: Feijo, F.J. (Ed.), Cartas estratigraficas das bacias sedimen-
tares brasileiras. Boletim de Geociencias da Petrobras, Rio de
Janeiro.
Moore, R.C., 1964. Paleoecological aspects of Kansas Pennsylva-
nian and Permian cyclothems. In: Merriam, D.F. (Ed.), Sympo-
sium on Cyclic Sedimentation, vol. 169. Kansas Geological
Survey Bulletin, Lawrence, KS, pp. 287380.
Murchison, D.G., Westoll, T.S. (Eds.), 1968. Coal and Coal-Bearing
Strata. Oliver and Boyd, Edinburgh, 307 pp.
Parrish, J.T., Ziegler, A.M., Scotese, C.R., 1982. Rainfall patterns
and the distribution of coals and evaporites in the Mesozoic and
Cenozoic. Palaeogeography, Palaeoclimatology, Palaeoecology
40, 67101.
Patzkowsky, M.E., Smith, L.H., Markwick, P.J., Engberts, C.J.,
Gyllenhaal, E.D., 1991. Application of the FuijitaZiegler pa-
leoclimate model: early Permian and late Cretaceous examples.
Palaeogeography, Palaeoclimatology, Palaeoecology 86, 6785.
Pashin, J.C., 2000. Using flooding surfaces in coal-bearing strata to
model accommodation space: example from the Black Warrior
foreland basin, Alabama. American Association of Petroleum
Geologists Annual Meeting Abstract (CD-ROM).
Payton, C.P. (Ed.), 1977. Seismic StratigraphyApplications to
Hydrocarbon Exploration, vol. 26. American Association of
Petroleum Geologists Memoir, Tulsa, OK, 516 pp.
Petersen, H.I., Andsbjerg, J., 1996. Organic facies development with