Geology of coal-bearing Palaeogene sediments, onshore Torquay Basin, Victoria

Australian Journal of Earth Sciences (2001) 48, 657–679

INTRODUCTION

The Torquay Basin, southern Victoria, is mostly an off-shore basin, but a small part occurs onshore adjacent to theOtway Ranges. It extends for 30 km northeastwards fromAnglesea and Torquay, and merges across the ParaparapHigh with the northeastern Port Campbell Embayment ofthe Otway Basin (Figure 1). Offshore, the Torquay Basin isseparated from the Bass Basin to the east by the shallowMornington – King Island basement ridge. Geographicallyto the north the Torquay Basin is separately distinguishedfrom the Port Phillip Basin by the Nepean Peninsula,although no structural divide exists (Abele et al. 1988)(Figure 1). The Torquay Basin is bounded to the south bythe Cape Otway – King Island basement ridge that separatesit from the Otway–Sorell Basin (Figure 1). Offshore seismicdata indicate that the Torquay Basin is comprised of a cen-tral deep area with the Snail Terrace on the eastern side(Trupp et al. 1994). The full Tertiary section offshore isintersected in three wells: Nerita 1, Snail 1 and Wild Dog 1.

Over a distance of 30 km, near-continuous coastal cliffsections expose the Tertiary succession between EasternView in the south and Torquay in the north (Figures 2, 3).The cliff sections expose parts of the basal beds of the non-marine Eastern View Group in the south, the upperDemons Bluff Group succession of marginal marine clas-tics and volcanics around Anglesea, and the fully marineTorquay Group carbonates near Torquay in the north(Figure 2).

Inland from the township of Anglesea, brown-coaldeposits of the Eastern View Group are mined by opencutmethods. Up to 1 Mt of brown coal is produced annually tosupply fuel to the adjacent Alcoa coal-fired power station.The mining has exposed, over a strike length of 2 km, a 35 m-thick coal seam at the base, and up to 80 m of over-burden sandstone, siltstone, carbonaceous shale and minorcoal. These excellent exposures reveal the relationshipbetween the Eocene upper units of the Eastern ViewGroup, the overlying Boonah Formation, and the lowerunits of the Salt Creek and Anglesea Formations.

The coal-bearing strata of the Anglesea area havereceived little attention in recent times, with the mostdescriptive references being Raggatt and Crespin (1952,1955), Gloe (1971) and Abele (1979). More recent papers concentrate mostly on describing the Palaeogene toNeogene carbonate successions of the area (Abele et al.1988; Reeckmann & Partridge 1988; George & Wallace 1992;Reeckmann 1994). The relationship between the EasternView Group exposed in the Anglesea coal mine and thedeeper stratigraphy has not been published previously, andcan now be re-interpreted using recent deep aquifer drillingand age dating in the area. The mine exposures also pro-vide a record of the Boonah Formation, the Salt Creek

Geology of coal-bearing Palaeogene sediments, onshore Torquay Basin, VictoriaG. R. HOLDGATE,* T. A. G. SMITH, S. J. GALLAGHER AND M. W. WALLACE

School of Earth Sciences, University of Melbourne, Vic. 3010, Australia.

New mapping at Anglesea coal mine, and coal resource and deep groundwater drilling have provided new perspectives on the economically important Eastern View Group coal-bearing sedimentary succession in the onshore Torquay Basin. In the Anglesea Syncline, the upper 35 m-thickbrown-coal seam of the Eastern View Group is overlain by a low-angle unconformity. Units overlyingthe coal seams include high-energy, cross-cutting sand channels of the Boonah Formation and lowerenergy channel and interchannel systems of the overlying Salt Creek and Anglesea Formations. Themine section can be correlated from borehole data with the Eastern View Group and Demons BluffGroup exposed in coastal cliff sections along the adjacent Anglesea to Torquay coast. Recently drilledcoal and groundwater exploration bores provide new data on the extent of the coal measures inthe Anglesea area, and details of the underlying Tertiary succession that include typical Otway Basinunits such as the Pember Mudstone and Pebble Point Formations. The stratigraphy below the coalmeasures suggests that the Otway Ranges were not present during Palaeogene times. The rank ofthe brown coals on and around the Otway Ranges is higher than any other Tertiary coals in onshoreVictoria, and they preserve similar patterns of rank distribution to the high-rank black coals in theunderlying Lower Cretaceous Otway Group. Evidence for large overburden thicknesses is lacking,and the high ranks may have been augmented by higher than normal geothermal gradients in theEarly Tertiary. Comparisons between the observed depositional cycles, sequence–stratigraphic cycles,and worldwide coastal onlap curves suggest that the observed disconformity boundaries aresequence boundaries that provide a chronostratigraphic framework. Sequences present may includeTB4.1–4.5 in the overburden units, and TB 2.4–3.6 in the coal-bearing interval.

KEY WORDS: coal deposits, Eastern View Group, sedimentology, sequence stratigraphy, stratigraphy,Torquay Basin.

*Corresponding author: [email protected]

Formation (new name), and the lower beds of the AngleseaFormation not seen in coastal outcrops.

All coal and oil exploration, groundwater and stratigraphic drilling results have been compiled. Fromthis, the onshore basin can be structurally divided fromsouth to north into the southeast-plunging AngleseaSyncline, the east-pitching Bald Hills Anticline, the east-plunging Bells Beach Syncline and the Paraparap High (all new structural names) (Figures 1–3). Some of thesestructures lack surface expression and for that reason havenot been previously recognised. The new structures canalso be seen on the Barwon Seismic Survey Lines BA92-400,402, 403, 405, 407 and 409 (Gas and Fuel Corporation 1992).

The specific objectives of this study are to: (i) summar-ise the bore and well data, including coal lithotype and coal-quality data; (ii) present a stratigraphic correlation of theEastern View Group coal seams, and the overlying BoonahFormation, Salt Creek Formation and Anglesea Formation;(iii) establish the deeper stratigraphy at Anglesea in thecontext of regional correlations; and (iv) present asequence–stratigraphic analysis of the Eastern View andDemons Bluff Groups.

DATA SOURCES AND DEFINITIONS

The regional stratigraphic interpretations rely essentiallyon coal/groundwater bore cores, cutting logs, and oil-welldata. We carried out most of the interpretations of down-hole geophysical logs. The database is derived from severalsources.

State Electricity Commission of Victoria andGeological Survey of Victoria databases

The State Electricity Commission of Victoria and theGeological Survey of Victoria coal, stratigraphic andgroundwater bore databases are the primary sources forcoal data in Victoria and include the results for more than 200 bores in the Anglesea area. For the regional studies,selection of approximately 100 key and/or better sampledand geophysical logged bores was made. Of particular usewere the series of coal bores drilled by Western MiningCorporation under the Hootzpa Program in the 1980s(Meyer 1982a); and the deep stratigraphic and groundwaterdrilling carried out by HydroTechnology P/L in the early

658 G. R. Holdgate et al.

Figure 1 Location and structural map of the central coastal basins of Victoria including the Torquay Basin. The main basement andOtway Group outcrops and generalised structure contours on base-of-Tertiary are also shown (adapted from Abele et al. 1988).

Coal-bearing sediments, Torquay Basin, Vic. 659

Figure 2 Subcrop map of the Anglesea area, showing key borehole locations, principle structures and cross-section lines.

1990s (Stanley 1994). Two coal lithotype bores were loggedby the State Electricity Commission of Victoria for coal-to-oil conversion studies in the Anglesea coal mine (Higginset al. 1981).

Oil company well data

In the onshore Torquay Basin, Hindhaugh Creek 1 andAnglesea 1 have geophysical logs together with some coring and cuttings samples. Petroleum exploration is concentrated mainly in the offshore part of the basin where three wells are located (Nerita 1, Snail 1 and Wild Dog 1). Coal-rank determinations exist through vitrinitereflectance measurements in some wells.

Lithology definitions

The coals are all soft to hard brown coals, with in situ mois-ture content between 44% and 48%. Most coals would be classified in the American ASTM classification system(ASTM 1979) as falling within the Lignite B range. In themore deeply buried areas near the coast and offshore, thisrank may increase.

GENERAL STRATIGRAPHY

The Tertiary stratigraphy of the Torquay Basin comprisesthe largely non-marine Paleocene to Eocene Eastern ViewGroup unconformably overlying the Lower Cretaceous

Otway Group (Abele et al. 1988). The Eastern View Groupin turn is overlain by marginal marine and volcanic sedi-ments of the Eocene to Oligocene Demons Bluff Group,and Oligocene to Miocene Torquay Group (Figure 4). In theadjacent Otway Basin, the Wangerrip Group exhibits similarities to the Eastern View Group, but lacks theyounger coaly beds. The Demons Bluff Group is equivalentin age to the Nirranda Group, and the Torquay Group is a time and facies equivalent to the Heytesbury Group(Abele et al. 1988).

Raggatt and Crespin (1952, 1955) first defined theEastern View Group as the Eastern View Coal Measureswith a Boonah Sandstone interval at the top. These unitswere redefined by Abele (1968) as the Eastern ViewFormation. The formation was given group status byTrupp et al. (1994). The Eastern View Group comprises alargely non-marine to paralic succession of brown coal,shale and sandstone that is over 500 m thick at the core ofthe Anglesea Syncline (Figure 3). The formation thins to thenorth and northwest over the Paraparap and BellarineHighs (Figures 1–3). Offshore in Nerita 1 well, the group(Eastern View Coal Measures in the well completion report;Shell Development 1967) is 683 m thick. It is overlain by a141 m-thick Boonah Formation that is separated from theEastern View Group on seismic by an angular uncon-formity. In turn, the Boonah Formation is overlain by 277 m of Demons Bluff Group (Shell Development 1967).Seismic mapping throughout the offshore Torquay Basinby Trupp et al. (1994) reveals a major regional unconformity


Figure 3 Base-of-Tertiary structure contour map and coal isopach thickness map of the Anglesea area. Principle structures are alsoshown.

between the Eastern View and Demons Bluff Groups with the Boonah Formation forming the basal unit of theDemons Bluff Group.

Raggatt and Crespin (1952, 1955) first defined theDemons Bluff Formation and subdivided it into theAnglesea Siltstone, Addiscott Greywacke and AngahookMembers. Later the first two were redefined by Singleton(1973) as the Lower and Upper Members of the AngleseaSand. Reeckmann (1979) gave the Angahook Member for-mation status. The Demons Bluff Group was introduced by Trupp et al. (1994).

New mapping by the authors in Alcoa’s Anglesea coalmine has identified a thick sandstone interval separatedfrom the mined coal seams below by a low-angle uncon-formity. This sandstone unit is referred to in this paper asthe Boonah Formation. Similar to the offshore area, theBoonah Formation is considered a basal member of theDemons Bluff Group. We introduce a new unit, the SaltCreek Formation, to describe middle (coaly) units of theAnglesea coal mine overburden not seen in outcrops on the coast. Using mine exposures, borehole data and byremeasuring the coastal outcrops, we have redefined theAnglesea Formation to include the upper beds in theAnglesea mine overburden. We also restore the AddiscottFormation in the coastal outcrops. The new terminologyis shown in Figure 4, described below, and is formallydefined in Appendix 1.

STRUCTURE AND STRATIGRAPHY OF THEANGLESEA AREA

Most of the significant coal seams of the Eastern ViewGroup occur in the upper parts of the group. In theAnglesea area, the coal seams are folded by the AngleseaSyncline, the axis of which plunges south below theAnglesea township into the offshore Torquay Basin(Figures 2, 3, 5). The mining of the coal seams occurs north-west of the township in the updip direction, where theseams are shallowest (Figure 5). The uppermost and thick-est A coal seam or A Group (George 1962) is up to 36 m thickand locally splits near the base into the A1 and A2 seams.The A Group coal seam has the lowest moisture content(44–48%) of any economic brown-coal deposit in Victoria.Based on palynological dating (Christophel et al. 1987), theA Group coal seam lies within the Lower Nothofagiditesasperus spore-pollen Zone. Unconformably overlying the A1coal seam is the Boonah Formation, and/or carbonaceousclastic sediments belonging to the Middle N. asperus ZoneSalt Creek Formation (Figure 5).

Below the A Group coal seam, is a 20 m-thick coarse sand-stone and minor claystone interval. It has been dated innearby groundwater bores within the Proteacidites asper-opolus spore-pollen Zone (Stanley 1994). Beneath this unitfor 60–70 m are a series of 2–15 m-thick B Group coal seamsinterbedded and laterally equivalent to shale and sandstone.


Figure 4 Stratigraphy of the Torquay Basin (from Abele et al. 1988) and proposed interpreted sequence stratigraphy. Paly’n, palynologyzones; Up.M.d, Upper Malvacipollis diversus Zone; Mid.M.d, Middle M. diversus Zone; Lw.M.d, Lower M. diversus Zone; Up.L.bal., UpperLygistepollenites balmei Zone; LwrL.balmei, Lower L. balmei Zone; Fm, formation; Mbr, member.


This interval is dated in nearby groundwater bores withinthe Malvacipollis diversus spore-pollen Zone (Stanley 1994).George (1962) subdivided the B Group coal seams into theB1, B2 and B3 seams. These seams were mined in the 1950son the western flanks of the Anglesea Syncline in two RocheBrothers’ opencuts, but are not exposed today (Figure 5).

The oldest C Group coal seams are separated from the B Group coal seams by a 10–15 m interval of fine sandstoneand silty claystone that appear, on bore log cross-sections,to separate the two groups at a local disconformity (Holdgate1996). The C Group coal seams include a 20 m-thick seam atthe top (the C1), which is best developed on the western flankof the Anglesea Syncline. In turn, C Group coals overlie aseries of interbedded thin coal (<5 m thick), sandstone andshale over a stratigraphic interval of 60 m (Figures 5, 6).Fossils in nearby groundwater bores suggest that the C Group seams lie within the Lower Malvacipollis diversusspore-pollen Zone (Stanley 1994).

Below the C Group coal seams a 100 m-thick interval ofinterbedded sandstone and shale overlies a 250 m succes-sion of thick mudstone with basal coarse sandstone, overOtway Group arkosic sandstone. Palynological dating innearby groundwater bores suggests that the strata belowthe C Group seam interval belongs to the Lygistepollenitesbalmei Zone (Stanley 1994) (Figure 6).

To the north of the mine, the Anglesea Syncline mergesagainst the prominent east-plunging Bald Hills Anticline.This anticline is interpreted to outcrop at the coast atJarosite Headland, where it is expressed as a low-angle fold in beds of the Demons Bluff Group (Figures 2, 3).The Otway Group was intersected near here at 135 m inWRC-121768 bore, overlain by 55 m of the Eastern ViewGroup (mostly comprising the B Group seams and inter-seam sediments; Figure 6). Minor uneconomic coal seamsoccur north of the Bald Hills Anticline. In the adjacent Bells Beach Syncline, at bore WRC-119348, only the B and C Group coal seams are present towards the top of a200 m-thick section of the Eastern View Group (Figure 6).Northwest of the Bells Beach Syncline the Eastern ViewGroup appears to thin onto the Paraparap High whereyounger units, such as the Torquay Group, unconformablyoverlie the Otway Group (Figure 4).

In the offshore Torquay Basin, the thickness of theEastern View Group in three wells varies from 30 m in Snail1 on the Snail Terrace to 683 m in Nerita 1 in the basin centre. The formation includes meander-belt channel anddistributary mouth bar sandstone interbedded with shale that constitutes up to 70% of the formation.Increasing marine influence towards the top of the groupis interpreted by Trupp et al. (1994).

BROWN COAL LITHOTYPES AND COAL QUALITY

Currently, Alcoa mines coal to supply the 150 MW stationnext to the opencut. The power produced is used for

aluminium smelting at Point Henry near Geelong. Over 33 Mt of A Group coals have been mined since opencutdevelopment commenced in the 1960s and, at currentextraction rates, enough coal reserves remain for a further11 years. Coals of the B and C Group seams were onceextracted by Roche Brothers from two pits west of the present mine for local Geelong power stations. Minor coalproduction at the Eastern View type section in Stony Creekwas also obtained from the thin (<4 m) coal seams in the1870s (Thomas & Baragwanath 1950). Investigations intodeepening the present Alcoa mine to extract coal from theunderlying B and C Group seams have been carried out byCampbell (1970) and Gloe (1971), but technical difficultieswere presented by the intervening aquifer-pressured sands.

The A Group coal seam has the lowest in situ moisturecontent (44–48%) and the highest rank/highest net wet specific energy (13.2 MJ/kg) of any brown coal extractedin Victoria. The seam has a relatively high sulfur contentof 3.8% (dry basis). The vertical distribution of sulfur isshown at 1 m intervals in Figure 7 in the old part of themine. Peaks of sulfur up to 5% occur at the top and base ofthe seam, with other high peaks being associated with darkto medium dark coal lithotypes. The lighter lithotypesappear to have the lowest sulfur content at approximately3.2%. The ash content of the A Group coal seam is 4–6% drybasis, with most of the ash comprising (in decreasing order of abundance) Al2O3, MgO, CaO, Fe2O3, SiO2, andtraces of Na2O, K2O and TiO2 (Higgins et al. 1981; Gloe &Holdgate 1991). Coal maceral analysis indicates a high vitrinite content (>95%), low liptinite (3%) and minor inertinite (0.5%) (Higgins et al. 1981).

One special lithotype bore (Angahook 82) revealed thatthe seam comprises various bands of brown-coal lithotypes,each band being 1–2 m thick (Higgins et al. 1981). The band-ing chiefly comprises medium light (51.5%) to medium dark(29.6%) coal lithotypes, with the remainder being dark(10.8%) and light (8.1%) lithotypes. The A seam contains upto five lightening-upward lithotype cycles approximatelyevery 6–8 m (Figure 8), with the prominent clay split in thewestern side of the mine occurring near, or at the top of,the second cycle (Figure 9). Holdgate and Clarke (2000) com-pared the lightening-upward lithotype cycles at Angleseato other brown-coal lithotype cycles at Bacchus Marsh,Altona and Latrobe Valley. They concluded that the cyclesare comparable but thinner, probably because of thegreater compaction (higher coal rank) at Anglesea, and thatthe cycling itself was a depositional product of watertablechanges.

The A seam lithotypes were mapped across the minearea in five transects for this project, and the coal cyclesand other boundaries positioned with regard to a datumalong the coal-seam floor (Figure 9). Samples were alsotaken for colorimeter readings on powdered dry coal usinga Hunterlab Colorimeter at the Herman Research Labora-tories, Melbourne. This instrument measures three colourcomponents (lightness, redness and yellowness) givingnumerical values that can be calibrated to standard litho-types. These data, in most cases, confirmed the field inter-pretations.

Five lightening-upward cycles are apparent for the twoeastern transects where the full coal thickness is preserved.These cycles correspond to the five cycles in Angahook-82


Figure 5 Three east–west cross-sections across the Anglesea coalmine area, showing the syncline form of the major coal seamgroups, and the principle unconformities (adapted from Gloe1971). See Figure 2 for location.


Figure 6 Southwest–northeast regional cross-section through the Tertiary deposits of the Anglesea area, showing stratigraphic cor-relations, geophysical logs, palynological ages, sequence boundaries and structures. The baseline datum is placed at the B1 coal seam.See Figure 2 for location. Fm, formation.

bore (Figure 8). The three western transects show erosionto the upper two cycles at the A Group coal seam/BoonahFormation unconformity. The most western transect alsopreserves evidence that cycle 2 is largely replaced by theinterseam split that divides the seam into A1 and A2 subseams. Occurrences of quartz sand were noted in coalat this general stratigraphic level across the mine area, anda comparable interseam split is known from drill data tothe east of the mine area. Thus, a marked ‘interseam influence’ prevailed through the coal swamp during thetime of cycle 2. The occurrence of resin (tree sap) and leaflitter horizons are indicated (Figure 9), although it is notclear whether these features have regional stratigraphicsignificance.

The underlying B and C Group seams were cored in litho-type bore Jan Juc-60, the results of which are shown inFigure 8 (derived from Higgins et al. 1981). To the authors’

knowledge, no other lithotype data exist for these oldergroups of seams. The data suggest lightening-upward coallithotype cycles are frequent and occur over similar thick-nesses to those in the A Group coal seam. However, thereare more frequent interseam sediment splits that generally occur at or around cycle boundaries. The thick-est continuous coal seam occurs in the C Group, where twocycles stack together to form 22 m of continuous coal. In general, the proportions for each lithotype component aresimilar to the A Group coal seam, with a small increase inrelative abundance of lighter lithotypes (from 8.1 to 13.4%).Gloe (1971) described coal analysis data for six bores in theB and C Group seams. Weight-averaged moisture content is45.3% with a variance of 4.6%. Ash content averages 6.4%(dry basis) (variance of 3.6%). Three to four percent of thisash comprises mainly calcium and magnesium sulfates withmedium to low iron oxide content. Soda and chlorine rarelyexceed 0.1% of the coal. The rest of the ash is adventitioussand and clay (SiO2 and Al2O3). Sulfur content in the coalssystematically decreases with depth; ranging over 3% for theB1 and B2 seams, and averaging 2.1% for B3 seam and 1.2%for the C seam. Coal maceral analysis for B and C Groupseams suggests a high vitrinite content (>95%), low liptinite(3%) and minor inertinite (0.5%) (Higgins et al. 1981).

STRATIGRAPHY AND SEDIMENTOLOGY OF THEOVERBURDEN AT ANGLESEA MINE

Boonah Formation

Unconformably overlying the A Group coal seam in theAnglesea coal mine, and in nearby bores, are clean sand-stone with minor carbonaceous shale. Down dip the sandstones increase from 10 m to 70 m in thickness.These sediments belong to the Boonah Formation (Holdgate1996; Smith 1998). The basal coarse channel gravels and sandstones overlie the A Group seam at an erosionalangular unconformity (Figures 10–13, 14a). The unit isthickest in the Anglesea Syncline, it thins to the northeastand southwest outside the synclinal area. The unconformitysurface defines the upper boundary of the Eastern ViewGroup. Due to this unconformity, the Boonah Formation is placed in the Demons Bluff Group rather than theEastern View Group. The formation lies within the N. asperus palynological Zone, but it is uncertain whetherit belongs to the Lower or Middle N. asperus Zone. Availabledating offshore suggests it belongs to the Middle N. asperusZone (Figure 4).

In the Anglesea coal mine, the Boonah Formation is upto 32 m thick. It comprises cross-cutting, coarse-grainedsand-channel units in the upper and lower 10 m, withinterbedded shale and carbonaceous (leafy) siltstone occurring in the middle 10–12 m (Figure 11). The basalcoarse facies is exposed mainly in the central part of theAnglesea Syncline (Traverses T1, T6, T8; Figure 11) andonlap the top of the A coal seam to the west (Figure 14a).Cross-cutting, fining-upward channel units range between1 m and 3 m thick and extend laterally from 30 m to 50 m.Finer grained overbank deposits are poorly represented,but where preserved, consist of planar-laminated carbon-aceous siltstone (Figure 14b).


Figure 7 Coal lithotypes and detailed sulfur profiles for the AGroup coal seam in the old part of the Anglesea coal mine. db,dry basis; Dk, dark; Md, medium dark; Ml, medium light; L, light.

The middle 10–12 m of the Boonah Formation consistspredominantly of finer grained shale and siltstone lensesinterbedded with fine-grained channel sandstone. Thelenses often form complexes of vertically overlappingshale lenses 1–5 m thick and 30 m wide, separated by finechannel sandstone units (Figures 13, 14c). The shale andsiltstone lenses are interpreted to represent finer grainedsediments deposited by lower energy channels and braidedsystems within meander loops and oxbow lakes. They maycontain abundant plant debris and leaf litter. The shalelenses are usually trough-shaped and infill channel sands.The fine-grained sandstone units are laminated and partially cross-bedded. They are usually carbonaceousand contain abundant plant leaf matter. This facies uncon-formably overlies the A coal seam in the old part of themine, where its lowest few centimetres are ferruginisedquartz gravel deposits (Figure 14a).

The upper 10 m of the Boonah Formation is composedof fine- to coarse-grained sandstone similar in appearanceto the basal unit. It is seen mainly towards the axis of theAnglesea Syncline (Figures 11, 13), and is progressively truncated to the west by the overlying erosional discon-formity. Cross-cutting, fining-upward channel units rangebetween 1 m and 3 m in thickness and 30–50 m in width(Figure 13).

From the facies of the mine exposures, the BoonahFormation is interpreted to preserve fluvial-deposited,high-energy channel sandstones, that up-section becamemore braided and meandering, associated with an increasein lacustrine deposits. There was no marine influencedetected in the finer grained units as indicated by theabsence of dinoflagellates in palynology samples.

Regionally, the Boonah Formation can be identified by its generally clean (low gamma) log profile to the north-east across the Bald Hills Anticline and can be correlatedacross the Bells Beach Syncline, where it appears to thinout on its northern flank (Figure 6). In the offshore TorquayBasin, Boonah Formation sandstones (up to 141 m thick)were considered to be hydrocarbon reservoir targets in thethree offshore wells. Volcaniclastics are present in the for-mation in Wild Dog 1 (Trupp et al. 1994).

Salt Creek Formation (new name)

The Salt Creek Formation (new name; Appendix 1) discon-formably overlies the Boonah Formation in the Angleseacoal mine (Figures 10–13, 14d). The disconformity can betraced along the highwall of the current mine for over 1 km.Upper units of the Boonah Formation are locally eroded outalong the line of the disconformity, which is demarcated at the base by a series of stratigraphically correlatable channel features. The formation comprises micaceouskaolinitic sandstone and carbonaceous mudstone 20–25 mthick. This unit includes a localised 1 m-thick coal seam afew metres above the base, and a more regional 2 m-thickcoal seam approximately 10 m above the base that can be correlated across the current mine highwall. The measuredsections (T1, T8; Figure 11) are most representative of thisformation. The lowest 10 m comprises interbedded carbon-aceous sandstone and thin coal seams. The sandstones aremore poorly sorted and finer grained than in the BoonahFormation. Other than for the basal channels, most beds are


Figure 8 Coal lithotype logs for the Anglesea coal mine: compo-site from two bores—Angahook-82 and Jan Juc-60 (Higgins et al.1981). Palynological zones and suggested sequence chrono-stratigraphy (Haq et al. 1988) are also shown. Figure modifiedfrom Holdgate and Clarke (2000).

arranged in more sheet-like configurations and/or havevery low-angle channel forms than the steeper cross-cuttingchannel forms of the Boonah Formation. The carbonaceousshale units of the Salt Creek Formation contain abundantwell-preserved plant macrofossils that are interpreted torepresent deposition within meander loops of a floodplainsuccession. The basal shales form a series of stratigraphi-cally linked infill channels that cut down into the BoonahFormation. Spores and pollens of the Middle N. asperusZone, together with excellently preserved plant macrofossilshave been obtained by Christophel et al. (1987) from thesebeds. The thin coal seams in the Salt Creek Formation are high in ash content, include siltstone interbeds, andcomprise mainly dark to medium dark lithotypes. Well-burrowed (Callianassa?) silty sandstone beds, 1–2 m thick,immediately overlie the upper coal seam and were sampledat the Burrows and T1 (sample 1.79) traverses (Figure 11).Dinoflagellates (Spiniferites sp.) were found in both samplesand suggest that a short-duration marine transgressionoccurred onto coastal peat swamp facies below. The upper10–15 m of the Salt Creek Formation comprises sheet-likesandstone with small-scale, low-angled channel features,some with fining-upward profiles, and some laminated andcross-bedded units. As for the lower beds of this formation,the sandstone is finer grained, poorly sorted with a highersiltstone and shale matrix, and of more sheet-like geometrycompared to the underlying Boonah Formation.

The Salt Creek Formation is interpreted to represent a complex of low-energy fluvial channel and braided sheet-like sands deposited on a lower coastal plain, which periodically became higher energy channel-like withincreased downcutting into earlier deposits. A major disconformity is produced by this downcutting at the baseof the formation. Short-duration marine flooding events(preserved as burrowed silty sandstone containing marinedinoflagellates) followed the infilling of the downcut surface. The remaining facies preserve low-energy fluvialchannel and braided sheet-like sandstone and shale-filledmeander loops, and the localised development of thin coalswamps. The coaly nature of this formation justifies thata distinction be made between it and the overlying AngleseaFormation.

Anglesea Formation

The upper highwall succession in the Anglesea coal mine comprises up to 20 m-thick red to purple carbonaceous and pyritic siltstone to fine sandstone, referred to in the pre-sent study as the lower beds of the Anglesea Formation (seeAppendix 1 for revised definition) (Figure 13). The upperbeds of this formation are exposed in coastal cliff sections,where it has been referred to as the ‘lower AngleseaMember’ (Reeckmann & Partridge 1988; Smith 1998). At theAnglesea coal mine, this formation appears to overlie the


Figure 9 Correlation of A Group coal seam brown-coal lithotype logs across the Anglesea coal mine. Baseline datum placed along theA Group coal-seam floor.

Salt Creek Formation on an erosional contact that can belinked across the mine face as a series of stratigraphicallyconnected sandstone channels. The disconformity contactis a 0.1 m shale layer between cross-bedded micaceous finequartz sandstone of the top Salt Creek Formation below,and the homogenous micaceous fine quartz sandstone ofthe overlying Anglesea Formation. Other more localisedlow-angle channel forms occur at the bases of overlyingunits, such as at samples 1.68 and 1.69 (T1; Figure 11), wherepervasive limonite cements form ferricrete layers in thecoarser grained channel bases. Red limonite stainingappears to increase rapidly over the top 5 m of the mem-ber, and is associated with weathering from the presentground level. Interbedded carbonaceous shale and lamin-ated siltstone units immediately overlie the basal channellayers, and form a 5–7 m-thick unit along the mine face. Thebasal silty shale of this unit is pyritic and contains dino-flagellates Spiniferites, Cleistosphaeridium and Cyclone-phelium sp. (sample 8.4 in T8; Figure 11) indicative of ashort-duration marine transgression.

The Anglesea Formation in the Anglesea coal mine isinterpreted to represent a complex of low-energy fluvialchannel and braided sheet-like sands deposited on a lowercoastal plain, which periodically became higher energychannel-like with increased downcutting into earlierdeposits. At least one short-duration marine flooding event(preserved as pyritic shale containing marine dino-flagellates) followed the infilling of the basal downcut surface. The remaining facies preserve low-energy fluvialchannel and braided sheet-like sandstone and shale-filledmeander loops.

STRATIGRAPHY AND SEDIMENTOLOGY OF THEANGLESEA COASTAL SECTIONS

In coastal outcrops and bores, the Anglesea Formation comprises carbonaceous pyritic shaly siltstone, fine sandstone and silty shale with burrows and arenaceousforaminifers, suggesting deposition in a restricted shallow-marine environment. More detailed descriptions and sedimentology of the coastal outcrops are described inRaggatt and Crespin (1952, 1955), Abele (1979), Reeckmann(1979, 1994), Abele et al. (1988), Reeckmann and Partridge(1988), George and Wallace (1992) and Webb (1995). Figure 15illustrates the location and sedimentology of the measuredsections between Aireys Inlet and Bells Headland as re-measured by Smith (1998). Reeckmann and Partridge (1988)described a regional erosion surface between their upperand lower members of the Anglesea Formation, with theupper member occurring within the Upper N. asperus Zoneand the lower member within the Middle N. asperus Zone.Based on nearby well control, it is considered likely thattheir lower member equivalents are present in the Angleseacoal mine (the Anglesea Formation; discussed earlier), butthe upper member is not exposed in the mine highwall sections. Because their lower member is not the ‘lowest’beds of the Anglesea Formation, it is recommended thattheir lower member be referred to as the upper beds of theAnglesea Formation. Their upper member renamed theAddiscot Formation in the present study; this was origin-ally described as the Addiscott Member by Raggatt and

Crespin (1952 p. 117, 1955 pp. 91, 117), with the type sectionat Addiscot Beach (southwest side of Bells Headland)(Figure 15 between 6 m and 18 m). Correlation of the sectionbetween Addiscott Beach and Anglesea Main Beach (Figure15) shows that the Anglesea Formation and AddiscotFormation can be separated by a widespread disconformityand downcut surface as recognised by Reeckmann andPartridge (1988) between their upper and lower membersof the Anglesea Formation. Although the originally namedAddiscott Member has been considered a weathering phe-nomena (Singleton 1973), we consider that a distinct unitexists between the ‘typical’ Anglesea and Angahook litholo-gies, which validates the original observations and termi-nology of Raggatt and Crespin (1952, 1955). We thereforerecommend raising the Addiscot Member to formation sta-tus within the Demons Bluff Formation (Appendix 1). ItsUpper N. asperus Zone age also places it in the EarlyOligocene in contrast to the Middle N. asperus Zone age(Late Eocene) for the Anglesea Formation (A. D. Partridgepers. comm. 2001).

The Addiscot Formation is overlain by the AngahookFormation, which consists of shallow marine to coastal plaindeposits of sandstone, conglomerate, shale, basalt and pyroclastics outcropping in cliff sections between Fairhavenand Bells Headland (Reeckmann & Partridge 1988). The type section at Soapy Rocks is 18 m thick (George & Wallace1992); at Urquharts Bluff the unit is up to 37 m thick (Abele et al. 1988). The basalt of Angahook Formation (overlain by Point Addis Limestone) at Eagle Rock stack (Aireys Inlet) was dated by Abele and Page (1974) at 27.0–26.5 Ma, i.e. Late Oligocene (time-scale of Haq et al.1988) (Figure 15).

DISCUSSION

Inter-regional correlations and Otway Rangesuplift

A thick succession of mudstone and basal sandstone thatunderlies coal measures of the Eastern View Group in theAnglesea Syncline is similar in age, lithology and log character to the Pember Mudstone and Pebble PointFormation in the adjacent Otway Basin (as indicated inFigure 6). Stanley (1994) has also remarked on this similarity. The M. diversus Zone ages obtained for the overlying B and C Group coal seams and associated sediments are similar (yet more coaly) to the upper part of the Dilwyn Formation in the Otway Basin. Coal seams of these zones are also common in the Bass andGippsland Basins (Brown 1975a,b; Partridge 1976; Baillie &Bacon 1989; Baillie 1992). They preserve similar coarseningupward cyclic units on gamma-log profiles, a characteristicof the deltaic Dilwyn Formation in the Otway Basin(Holdgate 1981; Arditto 1995). However, the A Group coalseam is not known to be present in the Dilwyn Formationof the eastern Otway Basin, but its age equivalent probably


Figure 10 Detail bore log cross-section of the western limb of theAnglesea Syncline at the Anglesea coal mine, showing the prin-ciple lithostratigraphic units and unconformities. See Figure 2for location. Fm, formation.



occurs in the more southern (and younger) progrades to theDilwyn Formation of the offshore Port Campbell Embay-ment (Arditto 1995). The Kingston brown-coal deposit at thewestern end of the Otway Basin in South Australia has asimilar Lower N. asperus Zone age and is thought to be acontemporaneous deposit (Harris 1980; Wood 1981; Meyer1982b; Holdgate & Clarke 2000).

These relationships suggest: (i) stratigraphic continu-ity existed between the Otway and Torquay Basins in theearly part of the Palaeogene; (ii) that the basins were thesame; and (iii) that the Otway Ranges were not a structuraldivide at this time (i.e. there existed a single sedimentarybasin between Kingston in the west and the Mornington –King Island High in the east). Given the continuity of coalseam ages within the Bass Basin, it is probable that therewas also some stratigraphic continuity between theTorquay and Bass Basins. We suggest that interpretationsof Otway Ranges’ uplift (based on the fission track historyof the Lower Cretaceous Otway Group) primarily refer tomid-Cretaceous compression and uplift of a proto-Otway


Figure 11 Detailed lithostratigraphic logs of measured sectionsacross the Anglesea coal mine, showing the proposed formationboundaries. See Figure 12 for lithological legend. Numbers 1.42etc. are sample localities.

Figure 12 Lithological key forcross-sections in Figures 11 and 15.

Figure 13 (a) Composite photograph of the coal and highwall succession at the Anglesea coal mine and interpreted formation bound-aries and lithostratigraphic units. (b) Channel structures and disconformity between the Boonah Formation and the overlying SaltCreek Formation at section T8 (see Figure 11). (c) Channel structures, coaly units and disconformity between the Boonah Formationand overlying Salt Creek Formation at the central part of the mine. Note 4.5 m-high dump trucks (circled) for scale.

Ranges (Cooper & Hill 1997). Subsequent (Late Cretaceous?)peneplanation of the proto-Otway Ranges and removal ofcompression in the Early Tertiary, allowed reconnection ofthe Otway and Torquay Basins into a single basin. The connection is preserved in an inlier of Palaeogene (LowerL. balmei Zone) coal seams and interbedded sediments at Benwerrin (Thomas & Baragwanath 1950; V. Archer pers.comm. 1986) that could be a facies variation to the PemberMudstone or Pebble Point Formations. The Benwerrininlier occurs at a present-day topographic height of 300 mon top of the Otway Ranges (Figure 1). The Benwerrinbrown-coal seam has the lowest moisture content for anyTertiary brown coal in Victoria (average 30%; Thomas &Baragwanath 1950). It has a relatively high reflectance (Rv max.) of 0.40%, which is thought to indicate Tertiaryburial of 1.0–1.5 km (Cooper & Hill 1997). In addition, twoother coal deposits of M. diversus Zone age occur againstthe uplifted northwestern flanks of the Otway Ranges atWensleydale and Deans Marsh (Thomas & Baragwanath1950; Knight 1975; V. Archer pers. comm. 1986; Morgan 1992)(Figure 1). The Wensleydale coal seam is up to 40 m thickand is folded into a tight syncline surrounded by OtwayGroup sedimentary rocks. It has a similar age to theAnglesea B and C Group coal seams, but it is of lower rank, containing, on average, a moisture content of 50–55%.

This suggests lesser burial depths for this seam than atAnglesea.

It is concluded that the Eastern View Group of theTorquay Basin and the Wangerrip Group of the OtwayBasin are basically the same, implying that uplift of the present Otway Ranges largely post-dates the Early Eocene.Possibly the start of this uplift is preserved at Anglesea asthe unconformity between the A Group Seam and the over-lying Boonah Formation (Figures 4, 6), with most upliftoccurring in the later Tertiary (Cooper & Hill 1997).

Tertiary coal rank trends of the Otway area anduplift history

In low-rank brown coals, moisture content can be used asa coal rank indicator (Edwards 1948; Stach et al. 1982;


Figure 14 Details of highwall sedimentary structures at the Anglesea coal mine. (a) Disconformable contact between A Group coalseam and Boonah Formation at section T3 (see Figure 11). (b) Truncation of laminated carbonaceous siltstone by coarse sandstone inthe Boonah Formation. (c) Detail of the contact between the Salt Creek Formation and the Boonah Formation. (d) Stacked shale lenses(meander loops) in the upper part of the Boonah Formation. The batter wall is 15 m high and the coal seam of the Salt Creek Formationis exposed at the top.

Figure 15 Detailed lithostratigraphic logs of measured sectionsalong the coast between Aireys Inlet and Bells Headland. SeeFigure 2 for location of sections. Proposed formation boundariesare shown (this paper) together with palynological zones fromReeckmann and Partridge (1988). Coastline profile shows foldingof the principle units, as viewed from offshore. See Figure 12 forlithological legend. Mid, middle; Up., upper; P. tuber, Proteaciditestuberculatus.


George & Mackay 1991). For brown coals of the Otway areathe following rank order can be made: Benwerrin (30%)> Anglesea (44%) > Wensleydale/Deans Marsh (53%). Thisgeographical relationship for rank order appears similarto isoreflectance contours in the underlying Otway Group(Cooper & Hill 1997), where the highest Lower Cretaceousblack-coal vitrinite reflectances occur along the axis ofthe Otway Ranges to Barrabool Hills (which includesBenwerrin). Vitrinite reflectances decrease to the southeastinto the Anglesea area (Figure 1) and northwest towards theWensleydale area (Cooper & Hill 1997). Cooper and Hill’s(1997) estimated original burial depths for the Benwerrincoal of 1.0–1.5 km is based on a reflectance of Ro 0.4%, inwhich a coal moisture content of 30% occurs. However, thisappears excessive in view of the known stratigraphynearby.

The maximum present-day overburden to the AngleseaA Group coal seam is 180 m at the coast, but an unknownamount has been stripped from the various unconformitiesabove. As a guide, similar moisture contents occur inEocene T2 coals at Longford (Gippsland Basin), where burial depths of 500 m occur and where the overlyingTertiary section is more or less complete (Holdgate et al.2000). The maximum known preserved Tertiary thick-nesses along the Anglesea to Torquay coast is at a present650 m (Figure 3), but an additional 150 m of Middle to UpperMiocene, present offshore, may have been removed from theonshore sections (Dickinson et al. 2001). In the north-eastern Otway Basin in the Barwon Downs Graben,immediately adjacent to the Otway Ranges, the maximumrecorded Tertiary thickness is 400 m, although some unconformities may occur. We suggest that the burialdepths for Anglesea coal (with 44% moisture content)could have been ~500–600 m, with a maximum of 800 m,using the known sections (and the estimates of stripping).The Wensleydale/Deans Marsh coals have the lowest rank(53% moisture), similar to Latrobe Valley coal moistures inthe Traralgon Syncline where burial depths are <300 m.

Because a trend of decreasing Tertiary thickness takesplace towards the Otway Ranges from both sides, it seemsunlikely that Benwerrin coal on the crest of the ranges wasburied to 1.0–1.5 km. We consider that an alternativeexplanation for the high axial Tertiary and CretaceousOtway Ranges coal ranks is required. Apart from burialdepth, other known factors that increase brown-coal rankin Victoria are compression by folding (tectonic effects),relative abundance of lithotypes (petrographic effects),and thermal–volcanic (heating) effects (Edwards 1945,1948). Dealing with each of these factors in turn, the following points can be made.

(1) The compressional effects are probably similar forBenwerrin, Anglesea and Wensleydale deposits, as allthree are tightly folded into synclines. Therefore, localisedfolding and compression is unlikely to have caused the rankvariations.

(2) In the Latrobe Valley, brown-coal lithotypes can varyin moisture content up to 5% between dark and light/paletypes. Therefore, the relative abundance of lithotypes canalter total coal seam moisture content (George & Mackay1991). However, this variability decreases with increasingcoal rank, and moisture content for Anglesea lithotypesvaries by less than 3% (Higgins et al. 1981). Although data

on lithotype moistures are not available for Benwerrin orWensleydale coal, they may be similar to Anglesea. Hence,we believe lithotype variability is probably not a significantfactor to brown-coal rank in the Otway Ranges area.

(3) Thermal effects that can increase coal rank varybetween contact metamorphism from nearby extrusiveand/or intrusive volcanic sources, to unnaturally highergeothermal gradients (e.g. where shallow underlying basement occurs or where regional gradients are high due to volcanic sources). New magnetic and gravity inter-pretations suggest that the Otway Ranges are atypical for onshore Victorian basins because they are underlain by Neoproterozoic sedimentary basement rocks of theTasmania plate (Simons & Moore 2000). These Neopro-terozoic basement rocks are thought to have a higher heatflow than the Palaeozoic Victorian basement rocks thatunderlie the rest of the onshore basins, although the reason for this is not evident (D. H. Moore pers. comm. 2000).

(4) Lower Cretaceous coal seams along the crest of theOtway Ranges have the highest levels of maturation forshallow depths then anywhere else in the Otway Basin(Struckmeyer 1988) and this maturation appears to be thesame in the Paleocene brown coals. Extremely high vitrinite reflectances compared with present temperaturegradients (the gradthermal model of Kantsler et al. 1978)occur along the axis of the Otway Ranges and ParaparapHigh (e.g. Olangolah 1 vitrinite reflectances (Rm) rangefrom 1.14% near the surface to 5.93% at 2.2 km; and Hind-haugh Creek 1 ranges from 1.05% near the surface to 6.00%at 2 km). These high levels of coal rank developed early inthe Cretaceous and would have persisted for some time during the mid-Cretaceous. Peak temperatures probablyexceeded 200°C and geothermal gradients were of the orderof 100°C/km (Struckmeyer 1988). Petrological data indi-cating a high geothermal gradient were recorded duringredrilling of the Hindhaugh Creek 1 well (Figure 1), inclu-ding the presence of hydrothermal quartz veinlets and natural coke, suggestive of temperatures greater than500°C (Pyecroft & Millheim 1970; Meyer 1983).

(5) The high temperature gradients were not induced by burial depth alone, but must have been strongly aug-mented by a local heating source. Such a source mayinclude dyke swarms in Cretaceous rocks (Edwards 1934)to the common Tertiary Older Volcanics plugs, sills andflows in the northeastern Otway Basin and Torquay Basin.Records of volcanic intrusions include basalts in UpperCretaceous sediments in the Seaview 1 well near PortCampbell (Tickell et al. 1992). Intrusions in the Pebble PointFormation occur at Mt Murray near Colac (Tickell et al.1992) and were referred to as the Yaugher Volcanics byLakey and Leonard (1982). Dates of 59 Ma have beenrecorded on basalt in the Straun 2 bore near Colac(McKenzie et al. 1984), and various ages of 30–26 Ma arerecorded for sills and flows in the Gellibrand province(Abele & Page 1974; McKenzie et al. 1984; Price et al. 1988),which is also near the Otway Ranges area.

Therefore, based on the evidence presented, we tenta-tively suggest that relatively high geothermal gradientsexisted in the area of the Otway Ranges during the Early Cretaceous and that these high gradients were still present during the Early Tertiary when the browncoals formed.


Sequence stratigraphy of the Palaeogene succession at Anglesea

Sequence–stratigraphic analysis is best applied to near-shore environments where alternations of marine and non-marine conditions occur. In Gippsland Basin brown-coal measures, similar to Anglesea coal, the identificationof marine interseam transgressions into the coal measuresenables the application of sequence–stratigraphic methods(Holdgate & Sluiter 1991; Holdgate et al. 1995; Holdgate &Clarke 2000). The presence of erosional disconformities andother unconformities can also be diagnostic of sequenceboundaries and correlated to the Haq et al. (1988) coastalonlap curves (Figures 4, 6).

In coastal sections Reeckmann (1994) described threesequences in the ‘Anglesea Sand’ that are now given for-mation status as the Addiscot, Anglesea and Salt CreekFormations. The sequence boundaries were placed at 36 Maand 33 Ma, and the top of the formation was correlated tothe Haq et al. (1988) 30 Ma sequence boundary. However,some controversy remains in the dating of the overlyingAngahook Formation. In outcrops at Aireys Inlet and Soapy Rocks, the Early Oligocene Upper N. asperus andProteacidites tuberculatus Zones have been obtained ininterbedded sediments and volcanics (the AngahookFormation) (Reeckmann & Partridge 1988). In contrast, theradiometric age for the Angahook Formation basalt atAireys Inlet is 27.0–26.5 Ma (Abele & Page 1974), placing it in the Late Oligocene, although Bourton (1988) has suggested that the basalt may in part be an intrusion intothe Point Addis Limestone. However, palynological andnannofossil dates that are closer to the basalt date occur insamples from the Angahook Member at Point Addis andBells Headland. These include a P. tuberculatus Zone age(Reeckman & Partridge 1988), late P22–P21 nannofossildates at Bells Headland (Shafik 1988) and foraminifers in overlying Jan Juc Marl no younger then P21b/P22boundary (Li et al. 1999).

A reconciliation of the coastal cliff sequence strati-graphy (Reeckmann 1994) to that of the Anglesea coal minearea is shown in Figure 4 and is described below.

(1) The Angahook Formation probably represents pyro-clastic sediment deposition partly contemporaneous withthe age of the basalt (i.e. ca 27 Ma). The oldest age for theformation based on foraminifers would, therefore, be early Late Oligocene, with a basal sequence boundary at 30 Ma. The Upper N. asperus Zone dating for the inter-bedded pyroclastics and sediments at Aireys Inlet,Urquhart Bluff and Soapy Rocks may, therefore, includereworked sediment and spores derived from the underlyingAddiscot Formation, rather than in situ material.

(2) The Addiscot Formation should be correlated withsequences TA4.4 and/or TA4.5, which is consistent with therange of the Upper N. asperus Zone. Therefore, we considera lower sequence boundary age of 36 Ma the more likelybase for the Addiscot Formation instead of the 33 Masequence boundary proposed by Reeckmann (1994).

(3) The base of the Anglesea Formation would logicallybest correlate with the TA4.3 sequence cycle with a basalsequence boundary age of 37 Ma. This is consistent withthe palynology age of Middle N. asperus Zone. In the upperpart of the highwall at the Anglesea coal mine, the Anglesea

Formation overlies the Salt Creek Formation with a disconformity or sequence boundary. The disconformity or sequence boundary can be traced across the opencutarea, with relief of up to 5 m, and cuts out underlying bedsof Salt Creek Formation. Approximately 10 m above the disconformity, a 1–2 m-thick bed of pyritic shale containsdinoflagellates of marine origin. We interpret this bed torepresent a maximum flooding surface separating under-lying units (transgressive systems tract?) from overlyingunits (highstand systems tract?).

(4) The Salt Creek Formation does not outcrop in the coastal exposures, where borehole data at Angleseatownship suggest that the top of the member occursapproximately 80 m below sea-level (Smith 1998). In theAnglesea coal mine the Salt Creek Formation discon-formably overlies the Boonah Formation and is traceableover the 2 km length of the opencut. Because it cuts outunderlying beds of the Boonah Formation, we interpret thisas another sequence boundary. Approximately 10 m abovethe disconformity, a carbonaceous shale and thin coal seaminterval is overlain by 1–2 m of well-burrowed siltstone containing dinoflagellates of marine origin. We interpretthis to represent a maximum flooding surface, separatingunderlying units (transgressive systems tract?) from over-lying units (highstand systems tract?). The base of the SaltCreek Formation could correlate with the Haq et al. (1988)38.0 Ma sequence boundary, representing deposition withinsequence cycle TA4.2. This agrees with Middle N. asperusZone palynology evidence from clay lenses near the baseof the formation (Christophel et al. 1987).

(5) The age of the base of the Boonah Formation is difficult to determine as it is diachronous in the mine area and, in the Torquay Basin area, tectonism is indicatedby an angular unconformity. Its component high-energychannel systems probably represent deposition in a high-stand systems tract, as no transgressive facies or maximumflood surfaces are recognised. Without additional palyn-ology evidence, we consider that the Boonah Formationcould correlate to part of the Haq et al. (1988) SequenceCycle TA4.1 (39.4–38.0 Ma).

(6) The A Group coal seam occurs in the Lower N. asperus palynological Zone. Correlation to the Haq et al. (1988) coastal chronostratigraphic charts indicatesthat the zone includes five sequences with boundaries at46.5 Ma, 45.5 Ma, 42.5 Ma, 40.5 Ma and 39.5 Ma. Christophelet al. (1987) concluded that the A Group coal seam occursin the upper part of the zonal range based on the paly-nomorph assemblage. We therefore consider that the coalmay lie within part of the TA 3.5 and/or TA 3.6 sequencecycles. Considering the angular unconformity describedearlier, we would prefer the older TA3.5 age (42.5–40.5 Ma),as interpreted by Holdgate and Clarke (2000).

(7) The sandstone interval underlying the A Group coal seam in nearby groundwater bores lies within the P. asperopolus Zone (Stanley 1994). The zonal range includesseveral Haq et al. (1988) sequence cycles. Preference hasbeen given to the TA2.9 cycle based on the Otway Basinwork of Harris (1985).

(8) In nearby groundwater bores the B1 coals andassociated sediments are in the Upper Malvicipollis diversus Zone, whereas B2 and B3 seams and associated sediments are in the Middle M. diversus Zone (Stanley 1994).



They probably relate to the TB 2.5 to TB 2.7 (53.0–50.5 Ma)sequences.

(9) Coal maceral compositional differences occurbetween the Anglesea A to B1, and B2 to C Group seams(Higgins et al. 1981). Similar coal compositional changes areseen throughout the Bass, Gippsland and Otway Basins atthis time (Smith 1981, 1986; Holdgate et al. 1986). They mayrelate to the implied sea-level changes associated with thechanges from warm-temperate to near-tropical conditionsin the Paleocene – Early Eocene to climatic cooling in theMiddle Eocene (Smith 1986).

(10) An erosional surface along the top of the C1 coalseam produces lateral discontinuities and is probably asequence boundary. It demarcates a dominantly coalyinterval from non-coaly sediments below. From limitedpalynological data, the C Group coals occur towards thebase of the Lower M. diversus Zone (Stanley 1994), possiblythe TB 2.4 (54.2–53 Ma) sequence.

CONCLUSIONS

New mapping of highwall exposures at the Anglesea coalmine show a stratigraphic section throughout the upper 35 m-thick coal seam of the Eastern View Group, overlainwith a low-angle unconformity by the Boonah Formation,Salt Creek Formation and the lower stratigraphic units ofthe Anglesea Formation. The three formations can be identified on the basis of lithology and are separated by disconformities. The total exposed section amounts to more than 90 m vertically and 2 km along strike in theAnglesea coal mine.

The mine section can be correlated from borehole datawith previously described Eastern View and Demons BluffGroups exposed in coastal cliff sections along the adjacentAnglesea to Torquay coast. Drilling of a number of newbores for recent coal and groundwater exploration providednew data on the extent of the coal measures in the Angleseaarea, and the underlying Tertiary successions. This explor-ation has now defined several new southeast- and east-dipping structural features, including the AngleseaSyncline, Bald Hills Anticline, Bells Beach Syncline and thenortheast-trending Paraparap High.

The stratigraphy below the coal measures is similar tothe Otway Basin and suggests that the intervening OtwayRanges were not present during the Paleocene and EarlyEocene. The similarities suggest that the Wangerrip Groupof the Otway Basin and the Eastern View Group of theTorquay Basin are laterally equivalent, and that theyshare common formation units, such as the Pebble PointFormation, Pember Mudstone and Dilwyn Formation. Thelack of major coaly units in the Otway Basin may be aresult of Middle Eocene erosional truncation rather thanmajor facies differences.

The brown coals of Anglesea are a useful economicresource for power generation. Their rank is higher thanany other Tertiary coals in Victoria, and they tend to showsimilar patterns of rank distribution to the underlyingOtway Group, suggesting that higher than normal geo-thermal gradients extended into the early part of theTertiary in this area. Some Tertiary uplift in the adjacentOtway Ranges probably caused the unconformity betweenthe coal measures and the Boonah Formation in the Middle

to Late Eocene, possibly at ca 40–39 Ma, but most upliftoccurred sometime later, probably in the Late Miocene oryounger.

Comparisons can be made between Anglesea depo-sitional cycles, sequence–stratigraphic cycles, and coastalonlap curves. These suggest that some of the observed disconformity boundaries, such as the base of the SaltCreek Formation, the base of the Anglesea Formation and the base of the Addiscot Formation, are sequenceboundaries. At least two marine incursions occurred as indicated by the presence of dinoflagellates, which probably represent maximum flooding surfaces in the Salt Creek and Anglesea Formations.

ACKNOWLEDGEMENTS

We acknowledge the help, support and access provided byChris Rowlands and Tony Cust of Alcoa’s Anglesea coalmine during numerous mapping visits. We also acknow-ledge the help from David Stanley of SKM GroundwaterConsultants for provision of deep groundwater bore dataand age data. The coal analysis data are open-file data collected by Alcoa, Western Mining Corporation and theState Electricity Commission of Victoria. Additional coalanalyses for colorimeter, moisture, ash and sulfur were carried out at the Herman Research Laboratories. Grain-size analysis was performed at the Department of Geog-raphy (University of Melbourne). Gordon Holm (School ofEarth Sciences) kindly provided assistance with the prepa-ration of thin sections. This research forms part of an ARC-funded study into the evolution and development of coastalbasins in southeastern Australia. We thank P. W. Baillie and A. D. Partridge for their helpful comments on the manuscript.

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Received 11 December 2000; accepted 22 April 2001


APPENDIX 1: STRATIGRAPHIC NOMENCLATURE

The following definitions and descriptions apply to the newand redefined formations of the Demons Bluff Groupdescribed in this paper.

Salt Creek Formation (new name)

Type section Anglesea coal mine, Anglesea. AMG252500E, 5746260N, southern batters, southwest corner,traverse T1 (Figure 11; RL +3.0 to +31.0 m a.s.l.).Designation The second lowest formation of the DemonsBluff Group, the lowest being the Boonah Formation.Thickness 20–25 m.Description Micaceous kaolinitic sandstone and carbon-aceous mudstone. The unit includes a localised 1 m-thickcoal seam a few metres above the base, and a more regional2 m-thick coal seam approximately 10 m above the base thatcan be correlated across the current mine highwall.Boundary contacts Lower boundary defined by a discon-formity and channel cut into the underlying BoonahFormation. Upper boundary defined by a disconformity andchannel filled with the overlying Anglesea Formation.Age Late Eocene, Middle Nothofagidites asperus Zone(Christophel et al. 1987).

Anglesea Formation (redefined; previously‘Anglesea Siltstone’)

Type section The original type section for the AngleseaSiltstone was defined as being in the Demons Bluff coastalcliffs shortly northeast of the mouth of the Anglesea River;Raggatt and Crespin’s (1955) Section 1. This is illustratedin Figure 15 approximately at Black Rocks, AMG 258600E,5746250N. A more complete section, however, occurs atJarosite Headland, AMG 261200E, 5748200N; also Angleseacoal mine, Anglesea, AMG 252500E, 5746260N, southern batters, southwest corner, traverse T1 (Figure 11; RL +31.0 to +50 m a.s.l.).Previous names In coastal outcrops and bores, theAnglesea Formation was first defined by Raggatt and

Crespin (1952, 1955) as the Anglesea Siltstone. Later theAnglesea Siltstone was redefined and combined with theAddiscott Greywacke (a unit above) as the Anglesea Sand(Singleton 1973) or Anglesea Member (Abele 1979; Abele et al. 1988) with lower and upper members. Reeckmann and Partridge (1988) referred to the Anglesea Sand as a formation with a lower and upper member separated by a disconformity. Based on nearby well control, it is considered likely that their lower member equivalents are present in the Anglesea coal mine (the AngleseaFormation, discussed earlier), but the upper member is notexposed in the mine highwall sections. Because their lowermember is not the ‘lowest’ beds of the Anglesea Formation,it is recommended in the present study that their lowermember be referred to as the upper beds of the AngleseaFormation. It is also recommended that their upper mem-ber be renamed the Addiscot Formation as originallydescribed as the Addiscott Member by Raggatt and Crespin(1952 p. 117; 1955 pp. 91, 117).Designation The third lowest formation of the DemonsBluff Group.Thickness >20 m; approximate total thickness could beup to 50 m.Description The upper beds of this formation are exposedin coastal cliff sections where it has been referred to pre-viously as the lower Anglesea Member (Reeckmann &Partridge 1988; Reeckmann 1994; Smith 1998) (Figure 15:between 0 m and 6 m on Addiscot Beach section; and between0 m and 25 m on Jarosite Headland section). In the Angleseacoal mine (Figure 11) the lower beds of this formation areup to 20 m thick. They comprise red to purple carbonaceousand pyritic siltstone to fine-grained sandstone. An inter-vening unexposed section up to 20 m is estimated from boredata between the coastal section and the mine section.Boundary contacts Lower boundary defined by a discon-formity and channel cut into the underlying Salt CreekFormation. Upper boundary defined by a disconformity andchannel cut filled by the overlying Addiscot Formation asseen at Addiscot Beach and Jarosite Headland.


Age Late Eocene, Middle Nothofagidites asperus Zone(Reeckmann & Partridge 1988).

Addiscot Formation (redefined; previously‘Addiscot Greywacke’)

Type section The original type section for the AddiscotGreywacke was at the eastern end of Addiscot Beach, i.e.just southwest of Bells Headland [Raggatt & Crespin’s (1955)Section 5]. The type section could include any of the exposures between Addiscot Beach and Jarosite Headland,AMG 261900E, 5748000N to 261200E, 5748200N. This localityhas a good continuous vertical exposure of more than 50 m and the basal disconformable contact with theAnglesea Formation and upper contact with the AngahookFormation can be seen.Previous names The unit was first referred to as the‘Addiscot Greywacke’ by Raggatt and Crespin (1952, 1955).It was later changed to the ‘upper Anglesea Sand’ (Singleton

1973). The disconformable boundaries were described byReeckmann and Partridge (1988), Reeckmann (1994) andSmith (1998).Designation The second to top formation of the DemonsBluff Group, the upper formation being the AngahookFormation.Thickness ~25 m.Description Carbonaceous pyritic clayey siltstone,fine-grained sandstone and silty shale with burrows andarenaceous foraminifers (Figure 15: between 6 m and 18 m on Addiscot Beach section; and between 25 m and 50 m onJarosite Headland section).Boundary contacts Lower boundary defined by a discon-formity and channel cut into the underlying AngleseaFormation. Upper boundary defined by a disconformityoverlain by volcaniclastic sediments of the AngahookFormation as seen at Jarosite Headland.Age Late Eocene to Early Oligocene, Upper Nothofagiditesasperus Zone (Reeckmann & Partridge 1988).

Documents

Geology of coal-bearing Palaeogene sediments, onshore Torquay Basin, Victoria