Sequence stratigraphic analysis and the origins of Tertiary brown coal lithotypes, Latrobe Valley, Gippsland Basin, Australia

International Journal of

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International Journal of Coal Geology 28 (1995) 249-275

Sequence stratigraphic analysis and the origins of Tertiary brown coal lithotypes, Latrobe Valley, Gippsland Basin, Australia G.R. Holdgate a,., A.P. Kershaw b, I.R.K. Sluiter c "Department of Earth Sciences, Monash University, Clayton, Vic. 3168, Australia b Department of Geography and Environmental Science, Monash University, Clayton, Vic. 3268, Australia c Department of Conservation and Natural Resources, 253 Eleventh St., Mildura, Vic. 3500, Australia

Received 28 October 1994; accepted 15 March 1995

Abstract

The methods of sequence analysis have been applied to the onshore Gippsland Basin and in particular to the Latrobe Valley Group coal measures which include up to five coal seams each exceeding 100 m in thickness. The methods appear to provide new depositional concepts to the evolution of these seams, and the development of coal lithotypes. In the eastern half of the Latrobe Valley evidence for marine transgressions into the coal measures are recorded in most of the interseam sediment splits by the presence of contained foraminifera and dinoflagellates. To the west (inland) these splits pinch out into continuous coal. However, they can be followed westwards as enhanced organic sulphur levels along sharply defined boundaries between light coal lithotypes below and dark coal lithotypes above. The dark lithotype immediately overlying each of these boundaries contains the highest sulphur value and warmer climate pollen assemblages (Sluiter et al., 1995, this volume). Colorimeter and lithotype logging strongly supports an upwards lightening cyclicity to coal colour at 12-20 m intervals through the approx. 100 m thick seams, with cycle boundaries defined at sharp planar to undulating surfaces. The lightening upward lithotype cycles together with their unique boundary conditions (i.e. enhanced organic sulphur levels, warm climatic indicators and laterally equivalent marine clay splits) are interpreted as parasequences and parasequence boundaries respectively. Each major coal seam can comprise up to five parasequences and is interpreted to represent deposition during an outbuilding high stand systems tract at one of several maximum periods of Tertiary coastal onlap. The top of each major seam shows evidence of truncation (erosion?) on a regional scale and these surfaces are interpreted to represent the sequence boundaries. The major seams are usually conformably underlain by marine clays and extensive aquifer sands, being deposits of the late transgressive systems tracts. The low stands and early parts of the transgressive systems tracts appear not to be represented in the Latrobe Valley due to its (more) basin margin location, but are probably present down-dip in the equivalent marine facies of the Seaspray Group. * Corresponding author. 0166-5162/95/$09.50 © 1995 Elsevier Science B,V. All rights reserved SSDIO 166-5 162(95)00020-8

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Stratigraphic correlation of the sequence boundaries identified in the coal measures to the adjacent, internationally dated marine Seaspray Group, provides a basis for chronostratigraphic correlation of the coal successions to the coastal onlap charts of Haq et al. (Exon Mesozoic-Cenozoic chronostratigraphic chart, version January 1988, and August 1989). From this dating it appears that each major seam is confined to high stands of third order eustatic cycles. It therefore follows that the lithotype cycles (parasequences) that comprise each seam are related to fourth order eustatic cycles. By analogy all the coal cycles may have developed under subtropical conditions as ombrogenous forested peat swamps in a similar manner to the Holocene, though tropical, swamps of Indonesia.

1. Introduction and geological setting Tertiary age brown coals known as the Latrobe Valley Group (Abele et al., 1988) comprise a significant part of the sedimentary succession in the onshore part of the Gippsland Basin in Victoria, Australia (Fig. lc). The Middle to Late Eocene successions, known as the Traralgon Formation, are the most widespread, covering most of the onshore basin. They are overlain by the Oligo-Miocene marine Seaspray Group in the eastern half of the EASTERN

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sea-level as interpreted for the Tertiary period (e.g. Vail et al., 1977; Haq et al., 1987) impacted on the peat swamps by flooding associated with coastal onlap during rise in relative sea-level when the coastal sand barriers were overstepped, or as exposure and subaerial erosion of the peat surfaces during lowering of relative sea-level. The permanency of the Balook Formation barrier system over long periods of time has been a major contributing factor to the preservation of these peat swamps despite the sea-level fluctuations. Structural control on peat accumulation is not apparent, as the coal seams show evidence of drape rather than faulting over pre-Tertiary basement blocks (Abele et al., 1988). Movements on the faults mainly took place in the post-coal forming Pliocene period, when uplift and erosion stripped many hundreds of metres of coal off the southern and western sides of the basin. However, some of the structural highs today seem to also have been relatively positive features during the Tertiary as some interseam sediments tend to thin onto the highs. In general, the onshore basin is perceived to have experienced slow but steady subsidence through time. The Latrobe Valley brown coal, when dry, shows prominent banding typically on a scale of 1-3 m intervals. The banding is referred to five main lithotypes (George, 1982) and reflects: 1. the dry coal colour varying from dark (black) to pale (yellow-brown) and, 2. the degree of shrinkage and gelification. The bands range from dark, highly gelified lithotypes with prominent shrinkage cracks to ungelified light to pale lithotypes with no shrinkage cracks. Contacts between differing lithotypes range from gradational to sharp planar to undulose. The most clear cut boundaries occur where thick dark coal lithotypes of regional extent overlie light-pale coals. Lithotypes within thick coal seams (circa 100 m thick) tend to show an overall vertical lightening upward succession (Mackay et al., 1985). In addition, each seam in total is comprised of a series of lightening-upward successions varying between 10 and 20 m in thickness. Earlier palaeobotanical research tended to support a relationship between lithotype and depositional environment (Blackburn and Sluiter, 1994). The pale and light lithotypes contain plant species characteristic of open water conditions. The medium light to medium dark lithotypes include species indicative of swamp forests, and the dark lithotypes are thought to reflect emergent peat surfaces with stunted forest heaths (Luly et al., 1980; Sluiter and Kershaw, 1982; Kershaw and Sluiter, 1982; Kershaw et al., 1991). However, Mackay et al. (1985) and Anderson and Mackay (1990), using statistical (Markov chain) analysis and biochemical-petrological data, developed an alternative model suggesting that aerobic degradation is responsible for lithotype, whereby the light-coloured coal represented the products of the most severe degradation. Both concepts put forward for the Latrobe Valley seams require either a decrease or increase in rates of basin subsidence as the controlling factor to lithotype succession. More recently, the recognition of marine dinoflagellates and foraminifera in the clay sediments between the major coal seams suggests a eustatically-controlled origin for coal formation (Holdgate and Sluiter, 1991; Holdgate, 1992a, b); this control can also be linked to lithotype succession. A re-evaluation of the brown coal palaeoflora in relation to lithotype (Sluiter et al., 1995, this volume) demonstrates that lithotype variation can be accommodated by the eustatic model, which considers marine interseam influence important in the development of regionally correlatable darker lithotypes.

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The aims of this paper are therefore, to describe the evidence for marine influence in the coal seams and its relationship to lithotype cycles. A model is proposed which places seam development in the context of sequence analysis, the methods of which are described by Van Wagoner et al. (1990). At the same time the model provides a more refined chronostratigraphic framework for seam dating and a basis for determining peat accumulation rates.

2. Evidence for relative sea-level change in the Latrobe Valley coal measures 2. I. Facies distribution

A generalised stratigraphic bore-hole log cross-section across the eastern half of the Latrobe Valley between Loy Yang open cut and Sale township is shown in Fig. 3. It demonstrates the intertongueing of coals with interseam sands and clays as occurs throughout the eastern half of the Latrobe Valley, and depicts their facies relations to the open marine Seaspray Group. All the interseam splits (i.e. sediment partings between the major seams) extend across the whole Latrobe Valley as far west as Morwell and Yallourn (not shown on Fig. 3) and are traditionally recognised as the major coal seam boundaries. Sand aquifers are associated with all the major interseam splits and occur between the coal seams of Yallourn-Morwell 1A (Y-M1A), Morwell 1A-Morwell 1B (M1A-M1B) and Morwell 1B-Morwell 2 (M1B-M2) coal seams. Most of these sand aquifers are westerly tongues from the Balook Formation which have therefore transgressed inland to Loy Yang for over 30 kin. Conformably above each interseam sand is a coal seam approximately circa 100 m thick which progrades eastward in a stepwise fashion as a series of subseams. The progradations are considered to represent the outbuilding of the coally facies at times of relative stabilised sea-levels (the high stand systems tract), whilst the transgressive sands at the base of each major seam represent periods of relative sea-level rise (the late transgressive system tract). The clay subseam splits between each subseam are considered to represent short duration flooding events during the high stand periods. Up to five subseams branch from each major seam, and are mappable units across the eastern half of the Latrobe Valley. They are designated by letter in reverse stratigraphic order, e.g. the M1B seam is subdivided from the top into M1Ba, M1Bb, M1Bc, M1Bd and M1Be subseams which are shown on Fig. 3 and Fig. 6. The subseam boundaries can be located within the continuous coal of the major seams by lithotype cycles, organic sulphur and warm climate pollen species (detailed later). 2.2. Fossil evidence

Brackish to marine microfossils have been found associated with all interseam and most subseam clays and sand partings, and the species are described in detail by Holdgate and Sluiter ( 1991 ) and Holdgate (1995). There are now some eighteen stratigraphically distinct marine fossil bearing clay splits in'the Morwell and Traralgon Formations, principally occurring in a distinctively bioturbated silt lithofacies. Time constraints has meant the

G.R. Holdgate et al. /International Journal of Coal Geology28 (1995) 249-275

255

Yalloum Formation microfossils have yet to be examined although the characteristic bio.. turbated silty facies are present. The microfossils include a single species of arenaceous foraminifera (Ammodiscus sp.) and some 20 taxa of marine algae cysts (dinoflagellates). Both types of microfossil occur in abundance in the marine limestones and marls of the Seaspray Group. Quantitative pollen analysis through the Morwell Coal Seams at Loy Yang and Flynn Fields (Sluiter, 1984) has shown that the relative abundance of warmer climate species such as Myrtaceae pollen increases dramatically over a number of beds at intervals of between 4 and 12 m. These are shown graphically on bore-hole logs for these two coalfields in Fig. 6. The major Myrtaceae peaks occur at about 12 m intervals. These beds were referred to as 'interseam influence zones' by Holdgate and Sluiter (1991) as they were found to be stratigraphically equivalent to clay splits separating subseams down-dip. They are interpreted to represent periods of warming accompanied by eustatic sea-level rise and relative coastal onlap. The most intensively studied are the eight 'interseam influence zones' of the M1B seam and are designated from 1 to 8 (in ascending stratigraphic order) on Fig. 3. Two 'interseam influence zones' (nos. 8 and 1) are correlatives to the major interseam splits above and below the M1B seam respectively. The remaining six (2 to 7) correspond stratigraphically to subseam clay splits closer to the marine boundary and would better be referred to as 'subseam influence zones'. Blackburn and Sluiter (1994) reduced the six to four by merging several closely related Myrtaceae peaks so that numbers 4 and 7 on Fig. 3 are no longer applied. This has enabled a better correspondence between 'subseam influence zones' and lithotype cycles. From the evidence of fossil environments, scale of bed thickness, and stratigraphic superposition, the subseam splits and their correlative interseam influence zones in the coal seams represent parasequences and parasequence boundaries, respectively. They developed as outbuilding peat deposits during a succession of high stand systems tract periods. The major interseam sediments at the base of each seam also show high Myrtaceae peaks, but are more analogous to the late part of the transgressive systems tracts and/or the condensed section. They always precede the outbuilding phase of the coally high stands, and include the major aquifer sand horizons. 2.3. Erosion and compaction phenomena

Erosional events sometimes accompanied by differential compaction have been located by the mapping of coal faces, lithotype boundaries and interseam sediments both within the open cuts, and regionally from subsurface data. They appear to represent hiatuses in peat accumulation when exposure and varying degrees of sub-aerial erosion of the peat surface occurred. The erosional events can be expressed either as major surfaces of regional significance which occur along the major seam boundaries, and which are mappable by stratigraphic correlations of coal exploration bores, or as minor erosional and/or compaction events and phenomena within the coal seams themselves which have more localised significance. The latter can be mapped as lithotype boundaries (i.e. subseam boundaries) in the open cuts and on lithotype bore log cross-sections (Figs. 4-6). The regional erosional surfaces are less easily discerned at the open cut scale, even though some are present in the face exposures. They are mainly identified from regional bore log

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G.R. Holdgate et al. / International Journal of Coal Geology 28 (1995) 249-275

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cross-sections (Holdgate, 1995). An example of downcut beneath the Yallourn aquifer interseam can be seen at Rosedale East and West (Fig. 6) which has resulted in the removal of most of the M 1Aa subseam. Similar erosive surfaces are evident at the top of the M 1B coals (e.g. at Morwell Open Cut, Fig. 6), and the top of the M2 coal. At this scale, the major erosional events are considered to represent sequence boundaries. Minor erosional events sometimes accompanied by differential compaction phenomena occur as coal intraseam boundaries and can be mapped as boundaries separating different coal lithotypes (two examples are illustrated on Fig. 4 and Fig. 5). These erosional and compactional events are seen in the open cut faces as sharply defined, planar to undulating contacts, with a surface relief of up to 1 m. They are always developed on the top of lighter . su~sE.

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G.R. Holdgate et al. / International Journal of Coal Geology 28 (1995) 249-275

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lithotype coal, and the hollows are filled in with very dark coal lithotypes often showing lamination. These grade upwards into 2-8 m thick highly gelified dark coal lithotypes of regionally mappable extent. Large pieces of wood, minor traces of sand and marcasite can occur on some boundaries. Differential compaction phenomena in the underlying lighter lithotypes is seen as faint banding which tends to follow the undulating contact, and by small scale faulting from below which appears to terminate at the contact. All the erosional-compactional surfaces overlain by dark lithotype bands can be correlated in lithotype logged bores throughout the different Latrobe Valley coal fields, and constitute valuable stratigraphic markers. A cross-section derived from the lithotype bore data of Allardice et al. (1978); and Kiss et al. (1984), through the Morwell, Loy Yang, Flynn, and the two Rosedale Fields is shown on Fig. 6. Some erosional losses or compaction of coal at these boundaries can be demonstrated between the various fields (e.g. compare the M 1Bb subseam between Loy Yang and Flynn). However, in most instances this would generally amount to less than a few metres. Similar more localised erosional-compactional effects within single field areas can also be seen where more than one lithotype bore log is available. These surfaces are designated by their thickness above the individual seam base, and the two examples (the #64 m and the #78 m surfaces above seam base) shown in Fig. 4 and Fig. 5 refer to the M1B seam exposed at Loy Yang Open Cut. The strongly undulating boundary of the #64 m surface is a localised phenomena, at Morwell Open Cut the same surface is planar. One exception to the generally minor degree of erosion between subseams occurs in the Yallourn Coal Seam. This exception is illustrated on Fig. 7 which shows a detailed correlation for three lithotype and colorimeter logged bore holes for the Yallourn, Yallourn East and Loy Yang Open Cuts. It demonstrates that below the base of the Y2 subseam, significant losses of coal occur to the top of the Y3 subseam, with its complete removal in the case of the Yallourn East Open Cut. It was also noted that the significant coal losses appear to occur immediately below a regionally correlatable dark lithotype band. In this example the erosion is significantly more pronounced and regional than any other like surfaces, and may indicate that a major erosion surface (sequence boundary) is present within this otherwise continuous coal seam. Down-dip and in an easterly direction the minor erosional-compactional surfaces become the base of the subseam splits as shown on Fig. 3. The top and base of each subseam usually comprises a light and dark lithotype coal respectively, so that the base of the dark lithotype coal directly overlies the clay splits. From this relationship, the subseam splits are tied stratigraphically to events in the coal seams such as lithotype change and 'interseam influence zones' (Holdgate and Sluiter, 1991 ). As these surfaces in the down-dip direction are considered to be parasequence boundaries, it follows that the parasequence boundaries updip manifest themselves in the coal seams as minor erosional surfaces at the top of the various subseams. They are also associated with significant changes in lithotype, and occur at the top of lithotype cycles (see section on lithotypes).

2.4. Sulphur distribution Most sulphur in Latrobe Valley coals is in the organic form. Pyritic sulphur is comparatively rare (Kiss et al., 1985). Regionally, organic sulphur increases in an easterly direction

G.R. Holdgate et al. / International Journal of Coal Geology 28 (1995) 249-275

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from an average (dry mineral and inorganic free basis) of < 0.3% at Morwell and Yallourn, to 0.3% at Loy Yang, to 0.5% at West Rosedale, to 2.0% at East Rosedale. The increase in sulphur towards marine environments has been noted in other coal basins (e.g. Suggate, 1959; Home et al., 1978; and Diessel, 1992). Kiss et al. (1985) considered it probable that in the Latrobe Valley coals the sulphates were derived from marine transgressions which formed the source for the in-situ production of reduced sulphur species H2S, S +, and S, and their subsequent incorporation into the reactive peat matrix. Where thick Latrobe Valley coals have been sampled for sulphur every 0.5-1.0 m, or simply as the more common composite bulk sampling over 6.0 m intervals, sulphur is found to vary considerably over quite short intervals (Kiss et al., 1985). Organic sulphur content is also found to be independent of other coal-ash variations such as silica, aluminium and iron. Detailed sulphur sampling in coal at 0.5 m intervals at Yallourn, Morwell, Loy Yang and Flynn Fields (unpublished SECV bore analyses) and by the senior author in the Rosedale Field (unpublished data), has led to a regional correlation of sulphur trends within the main coal seams. Parts of this correlation for the upper beds of the M 1B coal seam, and lower beds of the M1A coal seam are illustrated together with lithotypes and other regional data on Fig. 6. The highest sulphur values at Loy Yang (up to 2.77%) occur in coals at the stratigraphic level of the M1A-M1B interseam split even though the two coal seams have merged in the area of the sampled bore. To the west at Morwell, the same stratigraphic level has a sulphur content of 0.55%, which is 0.30% higher than for the rest of the coal seams. At the Flynn Field east of Loy Yang the sulphur in a 6.0 m composite sample across this stratigraphic level averages around 0.7%, but at the boundary would probably to be considerably higher if sampled on a 1.0 m basis. Further east again in the Rosedale Field, coal at the same stratigraphic level (here defined by an interseam sediment) contains up to 4.34% sulphur. Coals near the major seam boundaries tend to have the higher sulphur values, and typically show a lateral easterly increase in sulphur toward the interpreted marine boundary. Coals are always high in sulphur where they are overlain by the burrowed facies interseam sediments containing marine fossils indicators. Three other high sulphur peaks (Fig. 6) at Loy Yang, with values of up to 1.42%, occur in the M 1B/M 1A coals around the minor erosional-compactional surfaces, both in the dark lithotypes above the surface and in the light lithotypes below the surface. Results of detailed sampling for sulphur around the #64 m and #78 m surfaces at Loy Yang are shown on Fig. 4 and Fig. 5, respectively. Higher sulphur values occur in the lithotypes immediately at the top of or below the surface. The highest sulphur value occur in the laminated dark lithotype layers above the surface. The sulphur decreases upwards through the remainder of the dark lithotype to become the background value of 0.3% about 2.5 m above the surface. The lighter coals beneath the surface contain sulphur levels which grade down to background values of 0.3% at around 2-3 m below the surface. Laterally eastward these stratigraphic levels show increased sulphur concentration toward the marine Seaspray Group, similar to trends observed near the major interseam boundaries. Sulphur enrichment of coal near the interseam levels and near the minor erosion-compaction surfaces appears to be the result of incursion of marine waters following transgression over the exposed peat surface. An SEM analysis of sulphur in coal immediately above and below the #64 m surface at the Loy Yang Open Cut has allowed for the determination of the sulphur forms (Corcoran, 1992). The nature of the sulphur was concluded to be

G.R. Holdgate et al. / International Journal of Coal Geology 28 (1995) 249-275

261

entirely organic, and from different scanned areas was fairly evenly distributed. Sulphur in the dark lithotype was found to be the dominant element with minor Ca and C1; no Fe was detected. Sulphur in the light lithotype below the erosion boundary was secondary to C1 but still significant; some Ca and Si were detected but no Fe. Tests for boron which is often enhanced by marine environments, proved equivocal, as the clay mineral content was difficult to estimate (Diessel, pers comm.). The presence of laminae, enhanced sulphur values, and traces of sand grains is taken to indicate a possible brackish origin for the laminated dark lithotypes. They are also found to be the most highly gelified coals (see Fig. 6). The high sulphur values in the ungelified lighter lithotype coals underneath the erosion-compaction surfaces is thought to have been introduced by the downward diffusion of brackish-marine waters and sulphur fixation by bacteria within the lighter lithotype peat matrix. All sulphur highs associated with erosional and compactional boundaries are traceable laterally into high sulphur coal immediately underlying and overlying seam and subseam splits and the sulphur content always increases in percent towards the interpreted marine edge. Using sulphur values alone the M1B coal seam is divisible into 5 subseam interval,; designated from a to e (Fig. 6); and the M1A into 3 subseams (a to c). The Yallourn and M2 coals seams are similarly subdivisible.

3. The origin of Latrobe Valley brown coal lithotypes and sequence analysis 3.1. Previous theories

The origin of brown coal lithotypes both in the Latrobe Valley and in brown coals in other coal basins throughout the World remains an intensely debated subject. Most theories envisage the lithotype variations to be a product of water level fluctuations in the original peat swamp, which in turn controlled the plant communities living at the time (see Teichmtiller, 1989). Current argument resides about whether the dark or light lithotypes represent the more water-logged conditions of deposition, with most palynologists and palaeontologists favouring the light lithotype, and the petrologists favouring the dark lithotype (Anderson and Mackay, 1990). The earlier depositional models came from studies by Teichmiiller (1958) and Stach et al. (1982) on German brown coals and their comparison to the Florida Everglades as a modern peat analog. A similar form to these models was proposed for the Latrobe Valley coal seams from palynological evidence by Luly et al. (1980), Sluiter and Kershaw (1982), Kershaw and Sluiter (1982), Kershaw et al. ( 1991 ); and from plant macrofossil evidence by Blackburn ( 1980), Blackburn ( 1985 ). Interpretations of biochemical data by Verheyen et al. (1984) and Finotella and Johns (1986) also tended to support these views. However studies in Germany by Hagemann and Hollerbach (1980), Winkler (1986) and Hagemann and Wolf (1987) suggest that other peat swamp environmental factors such as pH and ell, which control the biogeochemical transformation of the organic matter after deposition, are more important to lithotype than the original plant material. These authors now favour greater aerobic decomposition for the lighter coals, and more intensive anaerobic decom-

262

G.R. Holdgate et al. / International Journal of Coal Geology 28 (1995) 249-275

position for the darker lithotypes that is, the darker lithotypes are more waterlogged, and the lighter lithotypes are more weathered. Anderson and Mackay (1990) reviewed the evidence for origin of lithotypes in the Latrobe Valley coals, and from petrological and biochemical data, favour a similar model to that of Hagemann and Wolf (1987). They also invoke a cyclic depositional model which suggests either progressive swallowing of the basin with coal infilling resulted in a lightening upward lithotype trend, or the development of ombrogenous peats which become increasingly raised, more oxygenated, and therefore 'lighter' with time. Modern examples of such raised peats are cited from coastal tropical peats in Indonesia (Anderson, 1964; Anderson and Muller, 1975; and Esterle et al., 1992). To date, none of the papers on the Latrobe Valley have considered in their models the following: 1. the stratigraphic correlation of lithotypes across the Latrobe Valley 2. the cyclic nature of coal lithotypes in relation to their regional distribution 3. the effects of erosional downcutting 4. the relationship between lithotype cycles and marine cycles such as interseam influence, marine fossils, and sulphur content. 3.2. Relative abundance and regional distribution o f brown coal lithotypes

Holdgate (1992b) correlated all the lithotypes between the main Latrobe Valley coal fields. Fig. 6 depicts part of this lithotype correlation for the upper M1B seam and the lower part of the M 1A seam and emphasises the regional correlation of lithotypes across the whole Latrobe Valley. Further, it was noted that a major correlation basis for lithotypes could be obtained by mapping dark regionally correlatable lithotype bands, which can be up to 10 m thick. These were found to always overlie, with sharp discordance, light or pale lithotypes below (Fig. 4 and Fig. 5 and Fig. 6), and that some losses and/or compaction of the lighter coals may occur below these boundaries. The coal losses and/or compaction below these surfaces can create serious correlation difficulties; this may have happened in previous attempts at correlation. Vertical upward cycling of lithotypes going from dark to light is also a feature in the Latrobe Valley coals, and these cycles occur on average every 12 to 18 m. This is particularly noticeable on the colorimeter logs, and described in detail by Mackay et al. (1985), who examined this cycling trend at Morwell using Markov chain analysis. The regional nature of these cycles and their relationships to the erosion-compaction surfaces were shown by Holdgate (1992b), whereby at the top of each lightening up cycle occurs an erosionalcompaction event immediately overlain by either dark coals of the next cycle, or by interseam sediments. Each cycle can be ascribed to the defined subseams, and, although some secondary cycles and even trend reversals may occur within each subseam, the overall lightening upward cycle prevails, the top of which is defined by a sharp bounding surface. The relative abundance of lithotypes can be severely affected by the degree of erosional downcut into the upper lighter layers of the underlying coal cycle or subseam. This effect is important when considering any statistical analysis of lithotype abundance for the region. To avoid this problem and compare stratigraphically correlatable portions of each subseam cycle, Fig. 8 was constructed to depict the cumulative lithotype abundance for each subseam.

G.R. Holdgate et al. / International Journal of Coal Geology 28 (1995) 249-275

COAL SUBSEAM

YALLOURN LOY MORWELL YANG d i

Y2

~:::::ii::::::

FLYNN a

263

WEST ROSEDALE EAST ROSEDALE I I

?

i

?

?

?

?

Y3 Y4

-

~

M 1Aa

~ ?

M lab

,~

~

....-....:.:.:.:.:.!!~!'!'i!'i"~ ~~

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MIBd

M18e

--__-N O N M A RINE

MARINE

? = no data

II DARK ~ B

MEDIUMDARK~MEDIUM L I G H T ~ LIGHT/PALE

Fig. 8. West to east distribution of lithotypes for each correlatable subseam in the Latrobe Valley Coal Measures (cumulative %).

As a result a more meaningful comparison of lithotype abundance can be made within subseams throughout the Latrobe Valley coal fields. It should be noted that data for subseams Y lc at Yallourn and M1Aa at Rosedale are missing due to erosional downcut by overlying boundaries, and M 1Be at Morwell is missing due to lateral facies change of coal into clastic sediments. However the following trends are apparent:

264

G.R. Holdgate et al. / International Journal of Coal Geology 28 (1995) 249-275

1. For most of the subseams the medium-light lithotype makes up the most abundant percentage, followed by medium-dark, light/pale, and dark in decreasing order of abundance. 2. The light/pale percentage is best represented in some of the subseams (e.g. M1Ac and M 1Bd), but this may be partly a function of the relatively less erosion of these particular subseam cycles in total. 3. From west to east, for most coal subseam cycles, the relative abundance of the mediumdark and dark lithotypes increases, whereas the medium-light and light/pale lithotypes decrease (e.g. M l Bb subseam). This suggests that increasing marine influence to the east favours the development of darker coals by greater bacterial degradation in lowerlying, less acidic, coastal areas. In contrast, the lighter coals are more abundant in the west, being more removed from marine influence, and occurring in places where the peat swamps were likely to be more elevated relative to sea-level, more acidic, and to have experienced greater weathering. These facts tend to lend support to the ideas of Hagemann and Wolf (1987) and Anderson and Mackay (1990). The percentage of Myrtaceae pollen in M1B coal at Loy Yang and Flynn Fields is plotted on Fig. 6. High abundance of this pollen species is considered by Sluiter (1984) to indicate warmer climatic conditions. The higher peaks correlate to the 'interseam influence zones' of Holdgate and Sluiter ( 1991 ). The correlation of the peaks against lithotype indicates that in all but one case they occur in the dark lithotypes and in particular the regional dark lithotypes occurring at the base of each subseam. As warmer climates tend to promote relatively higher sea-levels, the regional darks may have formed under more waterlogged anaerobic conditions. Their higher sulphur content, as discussed previously, would also suggest that their basal beds accumulated in brackish water conditions, resulting from the incursion of saline waters into the peat swamps. 3.3. A summary and sequence model, for origin of Latrobe Valley brown coal lithotypes

For most seams, lithotypes cycle by lightening upwards through every 12-18 m of coal. These cycles form a basis for seam subdivision into subseams. An idealised three cycle (three subseams) coal seam and the perceived relationships these subseam cycles have to the sequence analysis terminology is portrayed on Fig. 9. Each subseam (lithotype cycle) is thought to represent one parasequence. Each major (approx. 100 m thick) coal seam is comprised of three or more parasequences. Each major seam grades laterally into fluviolacustrine clastics to the west, and marine Balook Formation sands and Seaspray Group carbonates to the east. At the top of each parasequence cycle, light lithotype coals are overlain by a planar to undulose surface below which may occur small scale erosion and/or compaction phenomena. Down-dip this surface underlies subseam clay splits containing marine microfossils. These are stratigraphically correlative surfaces and are interpreted to represent the parasequence boundaries. The commencement of each parasequence cycle begins with a thick strongly gelified dark lithotype of regionally correlatable extent, which usually contains enhanced organic sulphur and abundant pollen of warmer climatic affinities. The regional dark lithotypes, at least in their lower parts, are lateral facies equivalents to the marine derived subseam elastic

G.R. Holdgate et al. / International Journal of Coal Geology 28 (1995) 249-275

265

sediment splits to the east, and overlie the distal ends of these splits back in the main peat swamp areas. The clay splits and their correlative high sulphur coals are thought to represent a downshifl in facies and are analogous to the small scale marine flood events that accompany renewed parasequence deposition. From the criteria of vertical thickness, vertical facies succession, interseam influence and organic (marine derived) sulphur, each lightening upward cycle and its down-dip correlative subseam (with clay split), represents a parasequence. The evidence suggests that recognising parasequence boundaries in near continuous thick coally sequences can be augmented by knowledge of the vertical distribution of lithotypes and organic sulphur through the coal seams, and that in thick coals, events of parasequence cyclicity may be present. The regional dark lithotypes are most likely derived from peat deposited in the mos~ waterlogged parts of the peat swamp, and the high-sulphur intervals in their lower part,; were deposited in a brackish water environment. They formed in the earliest part of the; parasequence cycle simultaneously with the marine clay splits to the east. They then grade up through medium-dark, medium-light, to light and pale coal lithotypes over 12-18 m thick cycles. The upper coal beds of the lightening upward cycle represent the final phase', of peat accumulation for the parasequence, and locally exhibit evidence for erosion (either marine and/or subaerial agents), and exposure (weathering), accompanied locally by differential compaction. The lightening upward cycle is most likely a product brought about by either of the,, following two scenarios: (i) gradual upward and outward (prograding) of ombrogenous peats resulting in lithotype change through increased drying out or weathering due to elevation above ground water tables (i.e. the Anderson and Mackay, 1990 model) or (ii) SEDIMENT TYPE & ENVIRONMENT i: marinesands

COAL LITHOTYPE

[]pale

m

~

Bdark

bloturbatedmuds&silts

~

fluvia/-/a.,custrine

light

~medJum I[oht

Yinnar LANDWARD

Morwell

mediumdark

I~mln=t~PIHmrl~

Loy Yang

Sale

Rosedale

l

lOm

SEAWARD

o 10kin

Fig. 9. Idealised sequence across the Latrobe Valley showing 3 coal lithotype cycles (parasequences) comprising one major coal seam (and interseam) and its sequence boundaries.

266

G.R. Holdgate et al. / International Journal of Coal Geology 28 (1995) 249-275

that cyclical climate change and reduction of effective precipitation influenced the changes of lithotype. Whichever model or combination is invoked, gradual cessation of peat development could have been caused by elevation related to doming or relative change in sealevel, coupled with starvation of water-nutrient supplies and increased acidities. Post-cessation oxidation of the starved peat surface, coupled with renewed marine transgression, would cause the commencement of a new cycle. Compaction and localised erosion following marine flooding may also provide a mechanism for the minor ( 1-2 m) erosional effects seen immediately beneath parasequence boundaries, as occur in Fig. 4 and Fig. 5. The subseam cycles of peat outbuilding (progradation) that comprise each major seam occur within high stand systems tract periods. Late transgressive system tracts are represented by the regional interseam clastic sediments at the base of each major seam. The early transgressive and low stand systems tracts appear not to be represented in the Latrobe Valley, but are probably found down-dip in the Seaspray Group.

4. Dating the Latrobe Valley coal sequences Following the identification of the interpreted sequence boundaries and parasequences in the Latrobe Valley coal measures, an attempt can now be made to correlate these events to the World chronostratigraphic coastal onlap charts derived by Haq et al. (1988). Each major coal seam in the Latrobe Valley is bounded by sequence boundaries of regional extent. These boundaries have been extended into the equivalent marine facies of the Seaspray Group in a series of regional bore and well log cross-sections (Hoidgate, 1995) where foraminiferal dating provides a basis for international correlation. Palynological dating of the coal seams and the marine sediments by Partridge (1971) add considerable substance to the regional correlations. A diagrammatic summary of the proposed correlations along with all the palaeontological time range constraints is shown in Fig. 10, and a stratigraphic correlation of the onshore Gippsland Basin formations and sequences to the standard Australian and International biostratigraphies and sequence chronostratigraphies are shown on Fig. l 1. Each major coal sequence takes its name from the major coal seam within, and each sequence is over 100 m in thickness. Two mainly non-coally sequences (the Upper Nothofagidites asperus and the Honeysuckle Hill Sequences) are also included as they provide important additional biostratigraphic constraints. The following sequences in stratigraphic order are described along with the main biostratigraphic ranges and their estimated sequence correlations. 4.1. The Honeysuckle Hill Sequence

This sequence is largely non-coal bearing and comprises mainly fluvial derived gravels, sands and clays. Palynological dating places this interval mainly within the Proteacidites asperopollis spore-pollen Zone which has a time range of between 48.0 and 50.5 m yr (Partridge, 1988 ). No specific sequence dating is applied to this unit as it may include older beds of the underlying Malvacipollis diversus spore-pollen Zone. Basaltic lavas of the

G.R. Holdgate et al. I International Journalof Coal Geology 28 (1995) 249-275

26'7

Carrajung Volcanics occur at the base of the Honeysuckle Hill Sequence, and are radiometrically dated at 55 m yr (Wellman, 1974). 4.2. The Traralgon 2 Sequence

The Traralgon 2 (T2) sequence is comprised of up to 100 m of T2 coals interbedded with lesser clays and sands. Down-dip and in the offshore parts of the Gippsland Basin the coals split and intertongue with barrier sands and marine clays. Dating of the coally sequence (Partridge, 1971; Whitelaw, 1983; and Sun Feng, 1991 ) confines it to the Lower Nothofagidites asperus spore-pollen Zone which ranges between 39.5 and 48.0 m yr (Partridge, 1988). Additional constraints to this age range are provided by the presence of marine dinoflagellates in intraseam clay splits consistent with the Deflandrea heterophylcta dineflagellate Zone. This zone has a range between 39.5 and 41.2 m yr (Partridge, 1988). At least two sequences occur within this time range to which the T2 Sequence could belong ( Haq et al., 1988). We propose that the sequence ending at 40.5 m yr is the correct correlation and that this s e q u e n c e boundary is w e l l e x p r e s s e d by erosion o f the upper beds of the T2

coal seam. The base of the T2 Sequence is represented by marine clays developed imme-W

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G.R. Holdgate et al. / international Journal of Coal Geology 28 (1995) 249-275

268

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Fig. l 1. Table showing proposed stratigraphic position for the major coal seams and equivalent marine sequences for the onshore Gippsland Basin, tied to the international sequence chronology of Haq et al. (1988).

diately above the Proteacidites asperopollus spore pollen Zone age coal horizon of the Honeysuckle Hill Sequence below. These clays are taken to represent a late transgressive system tract maximum and flooding surface dated at around 41.2 m yr. Hence most of the T2 Sequence was formed during the subsequent high stand period. The above dating gives the T2 Sequence and T2 coal seam a time range of 0.7 m yr.

4.3. The Traralgon 1 Sequence The Traralgon 1 (T1) sequence is up to 100 m thick, comprising mainly coal in the west which grades laterally into sands and clays towards the interpreted shoreline. It is dated within the Middle Nothofagidites asperus spore-pollen Zone (Partridge, 1971; Whitelaw, 1983; Sun Feng, 1991), which has a range of between 36.5 and 39.5 m yr (Partridge, 1988). The T1 sequence commences above a prominent regional erosion surface interpreted to be the 40.5 m yr sequence boundary. Above this surface, fluvially derived sands and

G.R. Holdgateet al. / International Journal of Coal Geology28 (1995)249-275

269

gravels infill erosional hollows, and in turn, are overlain by a widespread, transgressive marine clay. Coals commence immediately above this clay and are considered to represent the high stand systems tract. They can form a continuous 100 m seam in the updip direction, but split into subseams towards the present day coast where they grade into barrier sands. The age dates constrain the T1 coals to around a maximum flooding surface date of 37.5 rn yr and a sequence boundary at the top dated at 37.0 m yr (Haq et al., 1988). This gives a depositional period of 0.5 m yr for the high stand coals of the T1 Sequence. 4.4. The Upper Nothofagidites asperus Sequence

This sequence is poorly represented in the Latrobe Valley, and comprises mainly sands in the near-coastal areas of the basin. The sequence has an age range of 36.0 to 36.5 m yr (Partridge, 1988). At Loy Yang this sequence probably comprises the upper coal beds of the Traralgon Formation (Partridge, pers comm.), and as such provides age constraints for the sequence in the Latrobe Valley. 4.5. The Morwell 2 Sequence

The Morwell 2 (M2) sequence comprises three main subseams in the Latrobe Valley known as the M2A, M2B and M2C. In the north-western area of the Latrobe Valley the three seams combine to form over 150 m of continuous coal. At Morwell and Loy Yang the M2B and M2C are interbedded with a number of clay and sand aquifers. At least three marine clay/sand units can be recognised of which the thickest one occurs near the base of the sequence defined by the sequence boundary erosion surface developed on top of the Upper Nothofagidites asperus spore-pollen Zone age coals. Eastward in the down-dip directions the sequence thins, and is constrained by age dating to a thin series of marine dolomitic marls and clays of the 'Lower' Lakes Entrance Formation (Fig. 10). The M2 Sequence is dated by palynology as being of the Lower Proteacidites tuberculatus spore-pollen Zone (Partridge, 1971; Sun Feng, 1991 ) which, in the offshore Gippsland Basin, has a range of between 29.0 and 35.0 m yr (Stover and Partridge, 1973; Abele et al., 1988). The presence of Faunal Unit 4 (Carter, 1963) foraminifera in the 'Lower' Lakes Entrance Formation further constrains the M2 sequence to between 30.0 and 33.0 m yr (Abele et al., 1988). Within this time range occurs one of two major periods of coastal onlap in the Lower Oligocene (Haq et al., 1988). The authors consider the main part of the coal succession represents the sequence high stand dated between about 30.0 and 32.0 m yr. This gives a depositional period of 2 m yr for the M2 Sequence. 4.6. The Morwell 1B Sequence

The Morwell 1B (M1B) sequence comprises up to 120 m of continuous coal which down-dip grades into Balook Formation sands and marine marls of the 'Upper' Lakes Entrance Formation. The coals are dated within the Middle Proteacidites tuberculatus spore-pollen Zone (Partridge, 1971 ). Foraminiferal dating places the earliest marine sediments beneath M 1B coals at the beginning of the Miocene (i.e. Faunal Unit 6 Foraminiferal Zone; Abele pers comm.). This transgression occurs immediately above the sequence

270

G.R. Holdgate et al. / International Journal of Coal Geology 28 (1995) 249-275

boundary developed on top of the M2 sequence below. The Faunal Unit 6 Zone has a time range given between 21.0 and 25.5 m yr (Abele et al., 1988), and marine beds of this age appear to be stratigraphically equivalent to all the M1B coally sequence. The older Faunal Unit 5 Zone marine beds onlap the 30.0 m yr sequence boundary surface and represent a marine depositional period in the Upper Oligocene not present in the Latrobe Valley. Hence the above dating constrains the M 1B coals of the sequence to the maximum period of lower Miocene coastal onlap (Haq et al., 1988). This covers the high stand time interval dated at about 24.8 m yr and the following sequence boundary dated at 22.0 m yr. The range gives a depositional period of 2.8 m yr for the M1B Sequence. 4. 7. The Morwell 1A Sequence

The Morwell 1A (M1A) sequence in the Latrobe Valley has a similar facies distribution and thickness to the M1B Sequence. At Loy Yang and Morwell, the two seams join to form over 180 m of continuous coal. Usually the two seams are separated by a thin marine clay bed. The M1A coal can be up to 100 m thick in the main coal fields, but over large areas of the central Latrobe Valley, the seam splits into interbedded clays, coals and lesser sands. The top of the M I A sequence is defined by a major sequence boundary which, on the structural highs, cuts out large sections of the upper beds of the M 1A sequence. Palynological dating constrains the M I A coals to within the Upper Proteacidites tuberculatus sporepollen Zone (Partridge, 1971 ) although some of the upper beds better preserved in the synclines range up into the Triporopollenites bellus Zone (Sluiter, 1984). In the down-dip direction, the equivalents to the M1A sequence are the Faunal Units 7 to 8 Zone marine limestones of the Gippsland Limestone Formation. The age range given by Abele et al. (1988) for Faunal Units 7-8 Zones is from 16.2 to 21.0 m yr. As the M I A coals appear to represent only one high stand systems tract within this time range, the authors have favoured the correlation with the high stand dated by Haq et al. (1988) at about 18.5 m yr. Hence the sequence boundary at the top is that dated at 17.5 m yr giving a I million year depositional time period for M l A coal. 4.8. The Yallourn Sequence

The Yallourn Sequence comprises up to 110 m of coal in areas near the western end of the Latrobe Valley. To the east, the coal grades laterally into Balook Formation sands and then into marls of the Wuk Wuk Marl Member of the Gippsland Limestone Formation. The seam and its correlative clay and sand facies are dated by palynology as being within the Lower Triporopollenites bellus spore-pollen Zone (Partridge, 1971 ), which is constrained to a range in the offshore Gippsland Basin of between 13.8 and 18.0 m yr (Partridge, 1988). However, in the onshore part of the basin the range is given as between 13.8 and 16.0 m yr (Abele et al., 1988). Some further work is required to resolve the age discrepancy (Partridge pers comm.). Down-dip the equivalent Wuk Wuk Marl facies is confined to the Faunal Unit 9 foraminifera Zone (Mallet, 1977). This zone has an age range of between 15.3 and 16.2 m yr (Abele et al., 1988). These ages therefore constrain the Yallourn coals to between the 16.5 m yr and 15.5 m yr sequence boundaries (Haq et al., 1988). If all the coals formed during one high stand, this gives a depositional period of 0.5 m yr for the Yallourn Coal

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Table 1 Sequence ages, ranges and interpreted rates of accumulation for Latrobe Valley brown coal Sequence

Age range (m yr)

Yallourn MIA M1B M2 TI T2 Gippsland peats (Kershaw et al., 1993) Indonesian peats (Anderson, 1964)

15.5-16.0 17.5-18.5 22.0-24.8 30.0-32.0 37.0-37.5 40.5-41.2

Total years

0.5myr 1.0myr 2.8myr 2.0myr 0.5myr 0.7myr

Accumulation brown coal (m/yr)

Rates peat (m/yr)

4500 10000 23300 13300 5000 7000

1800 4000 9300 5300 2000 2800

1760 yr

2700

4270 yr

362

seams. However, if the seam contains a sequence boundary between the Y2 and Y3 subseams as discussed previously (and shown on Fig. 7), than this range may need to be revised with further study.

5. Depositionai rates for Latrobe Valley coal sequences If the above ages for the Latrobe Valley coal sequences realistically reflect total depositional time then some maximum rates of peat accumulation can be estimated. (It is probable that the accumulation are higher, considering that different lithotypes may vary in their depositional rates. In addition these rates do not account for peat removed at the intraseam erosion surfaces). A summary of the Latrobe Valley sequence ages and their derived accumulation rates together with modern peat accumulation rates from Gippsland and Indonesia is given in Table 1. A factor of 2.5:1 peat to brown coal was used which is not dissimilar to the conversions quoted by Hager et al. ( 1981 ) for thick German brown coals. Previous estimates for Latrobe Valley coal deposition have mostly assumed continuous deposition over the full time period represented by the seam palynological zonal ranges, and this has given rates in the order of I m/14,300 yr. (Partridge, 1971; Sluiter, 1984). The peat accumulation rates on Table 1 now range from a high of I m/1800 years (Yallourn Seam) to a low of 1 m/ 9300 years (M 1B seam), with an average of about 1 m/4200 years for the six seams. These rates are more in line with modern Holocene peat accumulation rates for Gippsland, where peats are forming at a rate of 1 m/2700 years (Kershaw et al., 1993), but still remain considerably slower than for the Holocene tropical examples from Indonesia where rates of 1 m/362 yr are recorded (Anderson, 1964). When considering the warmer subtropical climate envelope proposed for the plant communities that existed in the Latrobe Valley coals at the time of peat accumulation (Sluiter et al., 1995), it seems realistic to expect depositional rates to be higher than for today's Gippsland peats (Kershaw et al., 1993), but less than the quoted rates for Indonesian peats (Anderson, 1964). As deposition is still in progress within the short time span represented

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in the Holocene peats, than episodic accumulation cannot be factored in. Therefore the averaged accumulation rates could be meaningless over the time frame proposed for the Latrobe Valley coal seams (particularly if the parasequence-lithotype successions are driven by Milankovitch-type cycling). The lack of evidence for significant relative sea-level fall within each of the major periods of coal seam accumulation (other than the significant but localised erosion surface between the Y2 and Y3 subseams of the Yallourn Seam), and the widespread correlations for the lithotype cycles, suggests that each seam developed during a relatively stabilised eustatic period of high sea-levels and warmer climates. The fourth order cycling (parasequence-lithotype cycles) is therefore, most likely to represent perturbations during these periods of high sea-level and may, therefore, be Milankovitch-type cycles, with duration's of between 26 kyr and 100 kyr.

6. Conclusions

Sequence analysis of the Latrobe Valley coal measures has allowed for a more empirical approach to determining the origins and regional distribution of the brown coal lithotypes. This has been aided by the identification of marine flooding surfaces both within the coal seams, as defined by enhanced organic sulphur levels and associated warm-climate pollen species, and as correlative marine fossil-bearing clay splits near the major marine facies boundaries. Correlation of the coal seams and lithotypes along regional cross-sections has allowed identification of major and minor erosional surfaces or compactional phenomena. The major erosion surfaces between the major coal seams equate to sequence boundaries when relative sea-level fall exposed the Latrobe Valley peat surfaces to subaerial oxidation, erosion and downcut. The minor erosion and compactional surfaces, and their correlative sharp planar surfaces equate to parasequence boundaries. These latter events preceded and/or accompanied smaller scale marine flooding events indicated by enhanced organic sulphur in coal around the surfaces. The marine flooding events are interpreted to represent short duration events superimposed on the overall out-building of the coal seams within high stand systems tracts. The parasequences can be defined within each seam as a series of lightening upward lithotype cycles. As such, they appear to indicate that the lithotype cycling is largely controlled by a combination of doming of the peat surface with some climatic control, and subsequent relative sea-level change. The darker lithotypes at the base of each cycle may include in their lowest layers lateral equivalents to the marine clays splits. They are the most highly gelified coals and from their stratigraphic position represent a more waterlogged environment. The rest of the cycle comprises a 12-18 m thick upward lightening succession of coal lithotypes trending from medium-dark through to medium-light and then to light and pale lithotypes. The lightening-upward trend reflects the upward-and outward-building (progradation) of peat, analogous to the ombrogenous peat deposits of Indonesia (e.g. Anderson, 1964). The lightening upwards of lithotypes is most likely to be a result of nutrient starvation and increased acidities during upward building of the peat, accompanied by climatic changes. Up to five successive parasequence-lithotype cycles make up each circa 100 m coal seam

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and these cycles can be traced regionally throughout the Latrobe Valley. Cessation of each cycle was followed by localised oxidation. Erosion and differential compaction accompanied renewed marine transgression at the beginning of the next cycle. Evidence for sequence boundaries at the end of each major coal forming period is represented by significant subaerial wastage which created erosional downcut surfaces of regional extent. These sequence boundaries, in most cases, are immediately overlain by transgressive marine sediments which can extend across the whole Latrobe Valley. The main sand aquifers between the coal seams were formed at this time and are interpreted to represent depositional periods of the late transgressive systems tracts, whereas the succeeding high stand systems tracts are mainly represented by coal punctuated near the marine margins by thin marine clay splits. Dating by stratigraphic correlation to the contemporaneous marine limestones and marls of the Seaspray Group places the coal forming periods at positions on the Haq et al. ( 1988 ) coastal onlap charts at peak periods of Tertiary high sea-levels. Additionally, all the Latrobe Valley coals appear to have formed only at periods of climatic optima when subtropical conditions prevailed in the Gippsland Basin area. Intervening low stand periods are not represented in the coal. This indicates that the depositional record for the Latrobe Valley Group is extremely episodic, and using the ranges given on Fig. 11, represents less than one third of the Late Eocene to mid Miocene time period. Without allowances being made for erosion, episodic deposition, and compaction, the peat accumulation rates may have varied between 1 m/1800 yr and 1 m/9300 yr with evidence tending to favour the faster rate. This approach to coal depositional processes and lithotype origins provides some recognition criteria for parasequence and sequence boundaries in largely non-marine, thick coal settings. It also provides an example that may be applicable to other Tertiary brown coal deposits elsewhere in the World.

Acknowledgements We thank Gary Swinton for redrafting the figures to legible standards, and Jane Shearer for her valuable comments on the manuscript. We acknowledge much of the basic data to a plethora of largely unpublished reports from the former State Electricity Commission of Victoria.

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