seq strat of paralic coal bearing strata an overview nov2010

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).


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


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).


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


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.


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.


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

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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.


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.


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).


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).


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.


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


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


164

Fig. 12. Depositional dip orientated correlation section, for location, see Fig. 10.

M.Holzet

al./Intern

atio


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.


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


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


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


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).


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


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


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


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


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


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


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.

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