Cold-climate, fluvial to paralic coal-forming environments in the Permian Collinsville Coal Measures, Bowen Basin, Australia

International Journal of Coal Geology, 7 (1987) 365-388

365

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Cold-climate, Fluvial to Paralic C o a l - f o r m i n g E n v i r o n m e n t s in the P e r m i a n Collinsville Coal Measures, B o w e n Basin, Australia I. PETER MARTINI l and DAVID P. JOHNSON ~

~Department of Land Resource Science, University of Guelph, Guelph, Ont. NIG 2WI, Canada ~Department of Geology, James Cook University, Townsville, Qld., 4811 Australia (Received November 26, 1985; revised and accepted June 17, 1986)

ABSTRACT Martini, I.P. and Johnson, D.P., 1987. Cold-climate, fluvial to paralic coal-forming environments in the Permian Collinsvillecoal measures, Bowen Basin, Australia. Int. J. Coal Geol., 7: 365-388. The Middle Permian Collinsville Coal Measures of the northern Bowen Basin illustrate a range of cold to cold-temperate, coal-forming environments. Cold climate is indicated by Glossopteris flora in the coal measures, and by restricted marine fauna dominated by brachiopods and bryozoa in correlative marine sequences of the Back Creek Group which contains also abundant lonestones (dropstones). Sedimentation was characterised by an overall transgression, interrupted by local fluvial and coastal progradation in a shallow, epicontinental sea during a relatively quiescent tectonic period. Six sedimentary environments are represented: fluvial, fluvio-paralic, barrier-strandplain, backbarrier, tidal flat and open marine. The basal coal formed from peat of swamps of abandoned areas of gravelly braided streams, and is massive, dull, and with high ash (20%), low sulphur (1%) contents. Overlying coals developed from peats formed in fluvio-paralic and paralic environments, and thicker seams are generally brighter, with low to moderate ash (8-17%) and moderate to high total sulphur (1-6%) contents. Seams associated with fluvial influence show splits and high ash yield, while seams associated with coastal deposits show high sulphur levels (up to 21%). In contrast to reported models of coal-forming environments, no clearly defined deltaic or interdistributary bay-fill sequences were identified in the area studied. Rather, vast freshwater wetlands backed low-gradient, progradational coasts locally having bars and barriers. The barriers were not prerequisites for substantial peat accumulation, although may have locally assisted peatland development by raising the profile of coastal equilibrium.

INTRODUCTION

In contrast to Euramerican, Carboniferous coal measures which have been widely analyzed in terms of their environmental settings (Wanless, 1964; Horne and Ferm, 1978; Fielding, 1984), Permian Gondwanan coal measures, partic0166-5162/87/$03.50

© 1987 Elsevier Science Publishers B.V.

366 ularly sequences deposited in paralic environments, have received much less attention ( Diessel, 1970). Previous work has focussed on fluvial coal measures {Niyogi, 1966; Packham, 1969; Cassyhap, 1970; Galloway and Hobday, 1983; Johnson, 1984). The Collinsville Coal Measures at the northern end of the Bowen Basin in eastern Australia contain eight major and several minor seams which, judging by their surrounding sediments, have accumulated in fluvial to coastal environments (Table 1 ). The main aims of this paper are to reconstruct in some detail the depositional environments of the Collinsville Coal Measures and associated units of the Back Creek Group, and to analyse conditions of formation of the cold climate, fluvio-paralic coals. This study is based on lithofacies analysis of mine highwall exposures, cores and outcrops (Fig. 1). Petrographic examination of thin sections supplemented field data. Information about unit thicknesses, coal types and properties were obtained from the Collinsville Coal Company for the Collinsville coalfield itself. However, no sufficient information is available to extend the subsurface mapping to other parts of northern Bowen Basin, and our interpretations are based primarily on analysis of vertical sequences of lithofacies. BASIN FRAMEWORK

Tectonic setting The Bowen Basin contains Permo-Triassic sequences, which unconformably overlie lower to middle Paleozoic rocks. The hinterland was a stable craton to the west and a tectonically active, volcanic terrain to the east and north (Dickins and Malone, 1973; Staines and Koppe, 1980). Although regarded as a post-orogenic basin by Day et al. (1978), no comprehensive and acceptable model in terms of plate tectonics has yet been proposed. This is possibly either a back-arc basin formed in response to subduction east of the present Australian coastline, or alternatively a marginal cratonic downwarp unrelated to subduction. Basin subsidence and sedimentation occurred from the Permian to middle Triassic with uplift in the late Triassic (Staines and Koppe, 1980). Maximum coal ranks (medium-volatile bituminous) were achieved within only 20 Ma ( millions of years), before uplift began (Beeston, 1981 ).

Regional stratigraphy Three regional, Permian stratigraphic units were mapped by Dickins and Malone (1973) and consist of: (1) lower Permian Lizzie Creek Volcanics, primarily andesitic and intermediate to basic terrestrial volcanic flows; (2) middle Permian Back Creek Group, characterized by marine and continental deposits; and (3) upper Permian Blackwater Group, containing continental

367 TABLE 1 Stratigraphic framework for the Collinsville-GebbieCreek area Blackwater Group Back

Blenheim Formation (Scotville Member)

Murray Creek Garrick Peace

(0-1 m) (0.6-2.4m) Upper coal (0- 1 m) ~ measures

CollinsviUe Gebbie

Coal

Glendoo Sandstone Group

Measures Formation Scott Denison Potts Little Bowen Bowen Blake

{1-2.5 m) {1.3 m) (0-1 m) (0-0.5 m) (3-7.6 m) (0-12 m)

] Lower Coal

(Wall Sandstone Member )

Measures Tiverton Formation

Lizzie Creek Formation deposits (Table 1; Fig. 1). Age control is sparse. Levingston (1981) reported K-Ar dates of 264 and 274 Ma for the Lizzie Creek Volcanics, that is early Permian, and all sedimentary units contain elements of the Glossopteris - Gang a m o p t e r i s flora indicating a Permian age. The Back Creek Group has been divided into the Tiverton, Gebbie, and Blenheim Formations (following the usage of Staines and Koppe, 1980). The Tiverton Formation conformably overlies Lizzie Creek Volcanics along the eastern margin where it is reported up to 550 m thick further south (Dickins and Malone, 1973), but thins northward and is absent at Collinsville (Fig. 1 ). The Gebbie Formation transitionally overlies the Tiverton Formation, and along the eastern margin, contains a prominent, basal quartzose sandstone, the Wall Sandstone Member (Fig. 2). At the northern end of the Basin the Gebbie Formation passes laterally into the Collinsville Coal Measures, which consist of lower and upper coal measures separated by a marine tongue, the Glendoo Sandstone Member (Webb and Crapp, 1960). The Blenheim Formation conformably or disconformably overlies the Gebbie Formation and the Collinsville Coal Measures, commonly with a basal conglomerate. Towards the base there is a widespread unit up to 30 m thick and composed of calcareous siltstone/sandstone with abundant productid brachiopods, known as the Scottville Member (Fig. 2; Runnegar and McClung, 1973).

368

0 L

CRETACEOUS-~] TRIASSIC-

10 km --

Acid and minor basic plutonics Rewan Formation Blackwater group .~ Blenheim Formation o Gebbie Formation O

PERMIAN

Collinsville coal measures

m

(J

,~ Tiverton Formation Lizzie Creek volcanics

CARBONIFEROUS_ DEVONIAN

O

Acid and minor basic plutonics Section Coal mine Fault

Fig. 1. Location and geological map of the northern Bowen Basin, after Webb and Crapp (1960) and Dickins and Malone (1973).

Dickins and Malone (1973) summarized the fauna of the Lizzie Creek Volcanics and the Back Creek Group in terms of four faunal zones (I-IV). These zones were considered to persist basinwide, and were used as a time-correlative scheme in regional compilations (Dickins and Malone, 1968, 1973). However, McLung (1981) considered the faunas to be facies related, so the time connotation may be invalid. Certainly, the top of the Back Creek Group (base of

369

C341

EXMOOR

GEBBIE CREEK

COLLINSVILLE C473

: ] - - __ SCOTTVILLE •

MEMBER

lO0+m

BLENHEIM FORMATION L ~'~--

C O A L SEAM:

MURRAY

_~ ~-

GARRICK

~"

PEACE

~ p----~__ ~" s . s . ~

-

~

GLENOOO S.8,

DE%OT~ I~

LITTLE B. ~"~='~ CONWAY

E~ ~

S,S.

~..

BLANE - - ~ 1

GEBBIE

~ ' ~~ ; .~

FORMATION

F ~

~

~

\

~

~

•

J S J

2°[ 0

WALL

.l. . . . . . . 5

_l 10 km

-.

SANDSTONE MEMBER

~

Fig. 2. Cross section from Collinsville to Exmoor along the eastern flank of the northern Bowen Basin. For grain size scale represented by beds see Fig. 3.

the Blackwater Group) has been shown to be diachronous by Milligan (1971) and Koppe (1978). Paleoclimate

The Permian climate was cool to cool-temperate. Paleozoic glaciers persisted in parts of Australia until the late Permian ( Crowell and Frakes, 1971 ). Paleomagnetic data indicate the Permian pole lay just south of Australia, and thus there was probably a strong, north-south climatic gradient across the continent ( Irving, 1964). Other evidence for cold climate includes lack of warmwater marine faunas such as corals and fusulinids (Dickins, 1978), and the widespread occurrence of lonestones in Australian Permian marine sequences (Crowell and Frakes, 1971; Dickins and Malone, 1973). Permian floras have low diversity, perhaps also evidence for cold climate (Rigby, 1971 ).

370 C-341 I 100 MURRAY

o _J

J ____ GARRICK

I2

< rr

< fL TIDAL FLATS & CHANNELS

.__

PEACE 150

i

BARRIER

Peace

I Glendoo

~00

LEGEND

Wavy Bedding

~ "J .-LL

~

TIDAL

FLATS

SCOTT ~ GENISON

I~

m~

~

%Z

S!

BAR

o .--

BAR

~

l CHANNEL LL .Jc:~ fl

)~)> Herringbone Cross-Beds Plane Beds

~

Scour With Cross-Beds Cut and Fill

BRAIDEDMEANDERING

CONWAY

Hummocky Cross-Stratification

=

~- T

BOWEN~~

Flaser Bedding Ripple Cross-Lamination Cross-Beds

H

POTTS ~ ± ~ LITTLE BOWEN 250

Z -<_J ~{:1. STRAND t'~ -PLAIN ~. Z

~'C ,Do

Pyrite (abundant) Lonestones Concretions

.J~ < W £3

BLAKE • 300

BRAIDED

~;

Burrows and Bioturbations

~

Fossils ~G--= present;~"-(~'-= abundant)

( ~ = rare; SS = c o m m o n ; SSS = abundant)

,.,,r

~--r tLm

Plant Fragments Rootlets Coal

i

c~

×

Igneous intrusions

Fig. 3. Core log and environmental interpretation of the well C341 drilled in the Collinsville Coal Measures. The coal seams are named. SEDIMENTARY SEQUENCES

The Collinsville Coal Measures pass laterally into a much thicker section at Gebbie Creek (Fig. 2). Within the sections logged, six distinct sedimentary sequences have been identified and interpreted as forming in the following settings: fluvial, fluvio-paralic, barrier-strandplain, back-barrier, tidal flats, and open marine. Fluvial

Fluvial sequences contain organized pebble conglomerates and medium to coarse, quartzose lithic sandstone, such as at the base of the Collinsville Coal Measures ( Figs. 3, 4 ). The conglomerates have subrounded to subangular pebbles, primarily of volcanics and quartzites, and a coarse sandy matrix. Locally

371

Fig. 4. Sand and gravelly sand overlying the Blake seam at Collinsville. Hammer for scale.

Fig. 5. Visualization of the inferred environment of sedimentation of the coal measures at the time of the Blake Seam. Vertical line pattern indicates eroded peat, superimposed on which there is a gradation of grassy-like and arboreal vegetation. Braided streams show flow (arrow), small sand subaqueous dunes and larger sandy and gravelly longitudinal linguoid bars with some foresetted forms.

372 there are concentrations of coalified wood. The conglomerates grade upward and interfinger with poorly sorted, coarse to very coarse, pebbly sandstones. The fluvial sequence below the Blake Seam is characterized by thin (0.3-0.7 m) units of plane-bedded conglomerate and pebbly sandstone. The part above the Blake Seam is more sandy and contains repeated units with medium to large-scale cross-bedding and planar beds, separated by thin layers of fine sandstone and siltstone. Upward-fining cycles, 1-2 m thick, show a transition from massive to plane beds at the base to cross-beds and ripple cross-lamination at the top. Erosional contacts occur, and cut and fills range from small intrastratal scours to channels several metres deep and tens of metres wide. Rare Glossopteris leaves occur in fine sediments, and bioturbation by roots is developed locally. The environment of deposition is interpreted as fluvial with wide, shallow, low-sinuosity braided channels below the Blake seam and braided-meandering streams above it. The lower gravelly portion probably formed as sheet or longitudinal bars; whereas the upper part suggests development of transverse bars, and local point bars (Fig. 5 )

Fluvio-paralic Fluvio-paralic sequences comprise complex interstratification of medium to coarse, lithic sandstone intervals, laminated sandy silty sandstones, and shales. Thin conglomeratic layers occur. The Bowen-Potts interval displays best the characteristics of this sequence (Fig. 3). In some places, thin lenses of laminated, non-bioturbated shales and siltstones lie directly on top of the Bowen Seam. These shales are cut in part by channel coarse sandstones containing penecontemporaneous slump structures. Channel sandstones locally incise the coal, particularly in the eastern area of the coalfield. In other parts of the open cuts, and in holes C341 and C473, the Bowen Seam is overlain by laterally continuous, horizontally bedded, fine silty sandstones, locally rich in pyrite (Fig. 6). Planar and wavy laminations are common. Bioturbation is present in some layers, and increases in amount downdip where the sediments also become more shaley. Thin discontinuous coals occur and may reach 0.5 m in thickness (Little Bowen seam). This lower interval is overlain by medium to coarse sandstones with scattered pebble layers, which show erosional surfaces, large-scale trough cross-beds, and inclined accretionary surfaes (2-3 m thick). Lag deposits of shale pebbles and carbonaceous fragments occur throughout. In northern, open cut exposures, the top interval of the fluvio-paralic sequence is a sandy siltstone containing lenticular sandstone interbeds. In downdip, southeastern parts of the coalfield, an upward-coarsening, well-sorted, quartz sandstone sequence (Potts sandstone) is developed.

373

Fig. 6. Flat-lying shale and sandy beds on top of the Bowen seam at Collinsville. Clumps of grass at the top for scale.

The sequence in the Bowen-Potts interval, and similar sequences just below the Bowen Seam and in some of the Garrick-Murray interval, were probably formed in river-dominated coasts (perhaps in a lower deltaic plain), periodically reworked by tidal, brackish waters. Unbioturbated shales indicate small, temporary freshwater lakes formed over the peatland, but no deep bay existed. Elsewhere, the horizontally bedded sandstones formed as fluvial splays, perhaps affected by tidal waters. As the fluvial channel belts migrated onto this area, pre-existing deposits were deeply incised. The channel sandstones were deposited in slightly meandering, coastal streams. To the north, the sandstones are overlain by floodplain deposits, while to the south they are overlain by quartz sandstones formed in a coastal barrier setting, such as for the Potts sandstone. The environment is here termed fluvio-paralic rather than lower deltaic plain to emphasize that most of the sediment influx was quickly redistributed along the coast by waves and tides. River channels are still recognizable, but no clearly defined distributary mouth bar, nor bioturbated, fine, prodelta or bay units were found.

374

Barrier-strandplain Barrier-strandplain sequences are thick (12-20 m), generally upward-coarsening, composite sandstone bodies occurring in both the Collinsville Coal Measures and the Gebbie Formation (Figs. 3, 7). Some sequences are known locally as the Potts and Peace sandstones, and the Wall Sandstone Member. The main parts of these sequences are fine to very coarse, quartz sandstone, and some sections contain thin pebble layers, micaceous partings and thin, laminated, silty, carbonaceous units. Sedimentary structures are dominantly udulose plane beds to low-angle cross-beds. Sinuous ripple marks and crosslaminations are present but not common. Low and high-angle accretionary surfaces, and herringbone cross -beds occur near the top of the Wall Sandstone Member (Fig. 8A). Single, vertical burrows occur at several horizons, but shelly fossils are rare. Rootlets may be present, especially at the top of some sandstone units. The principal sandy units of the Gebbie Formation are underlain by mudstone and siltier, unfossiliferous, bioturbated, sublithic to lithic sandstones, which locally preserve ripple cross-laminations with minor flasers. The Glendoo Sandstone Member is a complex unit. Its uppermost part is a winnowed, quartzose sandstone (Peace Sandstone) overlying a thick (20 m) interval of fine to medium, quartz sandstones, locally bioturbated and fossiliferous, generally with massive to plane beds, and local cut and fills. The middle part of the Glendoo Sandstone Member is generally a thick (7-10 m) unit of fine to medium sandstone which shows recurring cycles (0.5-1.0 m thick) characterized by a basal, thin, very coarse sandstone or pebble layer ( one clast thick), overlain by well-sorted fine sandstone showing horizontal or very lowangle, plane beds. Very few shells are present. The lowermost part of the Glendoo Sandstone Member is characterized by fine sandstone, showing pervasive ripple cross-lamination with local flaser and irregular wavy bedding. These sequences are interpreted as shoaling upward, prograding sandy shorelines. Some developed complete barriers and back-barrier differentiation (Wall Sandstone Member), while others maintained strandplain conditions (Glendoo Sandstone Member). The lower part of most prograding sections was deposited in a silty environment, where bioturbation is common, but shelly faunas were either absent or have been removed by physical or chemical processes. Some units of the Gebbie Formation are typical sequences which show progradation from the open shelf through the shoreface to the barrier ( Fig. 7, level 260 m). Other units (Fig. 7, level 300 m) develop over semi-protected back-barrier, tidally influenced deposits. Some of the sandstone bodies do not evidence emergence, perhaps being never shallower than the upper shoreface zone. Others show well-sorted, massive sandstone caps, possibly aeolian deposits (Potts and Peace sandstones), overlain by coals.

375

jj_I~!u_ ~' s~l~ oo SHELF

~m

SHELLRANK

LL o-

I~ lOOm -JLUZT~ 455---

r

I~

SHELF

o

4O0--

350--

3o0--

MASSFLOW ~z~BAR C IS TIDALFLAT m MARSH BARRIER RHOREFACE e~- H~ S z~ ~ i LAGOON MARSH BARRIER SHOREFACE ~_

~- ~

nr

N<(

SHELF

~, If SHOREFACE I11

=~" ~

SHELLBANK

~)

150

s~

MARSH&COASTAL RIDGES SHORE

100~ J,~ I~

CHANNEL (TfDAL?)

~,

BACK .....

t~

ER

5 0 ~ I~

BARRIER

Meters

Fig. 7. Vertical section and environmental interpretation of the Gebbie Formation at Gebbie Creek. For legend see Fig. 3.

376

A

Fig. 8. Features of the Gebbie :. A. Herringbone cross-bed at the roof of the Wall Sandstone Member. B. Wavy bedding in tidal flat deposits, locally burrowed.

Back barrier

Typically, the back-barrier sequence is thinly to medium bedded shale with lenticular sandstone. The best development is just above the Wall Sandstone Member, where the section can be divided into two parts (Fig. 7). The lower part consists of medium sublithic sandstone and minor shale at the base, grading to shale with minor lenticular sandstone at the top. The sandstones are generally massive or show low-angle cross-beds, and cut-and-fills near the top. The shales are generally not bioturbated, and grade vertically into coaly shales and dirty coals. The upper part consists of alternating, thin- to medium-bedded, fine, moderately sorted sandstones. Some beds are intensely bioturbated, others retain plane laminations and traces of ripple cross-laminations. While the sandstones are mainly horizontally bedded, some are scoured and others pass laterally into large, steeply dipping foresets. This sequence is interpreted as the back-barrier deposits of the Wall Sandstone Member. Its lower part ranges from sandy washover deposits with tidal channels at the base, to muddy lagoonal deposits at the top. The high carbonaceous content, the thin coaly layers and lack of bioturbation of the shales may

377 indicate brackish to freshwater conditions. Emergence is recorded by the coal at the top of the lower part. The upper part is interpreted as coastal flats dissected by channels, possibly with tidal influence. The bioturbation indicates the re-establishment of marine conditions, probably associated with reducing influence of a submerging barrier to seaward.

Tidal fiat Wavy bedding characterizes the tidal flat sequences, and consists of rippled, fine to very fine sandstone, alterna~i,-in, dark, draping laminae of organic-rich mudstone on a 1-2 c vy bedded intervals alternate with medium to thick beds of fine ~ing ripple cross-lamination, some with flasers. Beds of medil ldstone, showing low-angle cross-beds and channellized scour dier tidal flat sequences are found in the middle and upper C Measures, where there are thick (1-3 m) rippled sandstones ~ e in some layers, and irregular wavy bedding in others (Fig. i flat-topped ripple marks were observed just above the Scol ,est of Collinsville. Some intervals, such as above the Garric wide (20 m), shallow ( 2 m) scour channels, filled in part by 1 layers, and in part with c~ enerally uncommon and wavy bedded and rippled sediments I consists primarily of isolated, sub-t~ 2. filled with sand. Tidal flat sequences recur at seve~ :: ~",~ linsville Coal Measures and the Gebbie Formation, but are "" lrd the top. The tidally influenced shores extended for consi, vith fresher water conditions present in only certain area~ ~annel fill and lack of bioturbation suggest freshwater drai the lower part of the Garrick-Murray interval in hole C34~ rd and basinward the sequences show more contiuous sect: ~ flaser bedding, and more common bioturbation, as show from Collinsville to Gebbie Creek (Fig. 2). Tidal origin is indicated by the rhyt hologies, especially wavy and flaser bedding, and by the la. r i p p l e marks. The relatively minor amount of bioturbatio ., urackish water environment or to locally intense reworkin ,ediments by waves and tides. Occurrence of ripple cross-laminated sandstone sheets without flaser bedding, records temporary sandflat development due perhaps to changes in sediment supply or to higher energy situations (Figs. 9A,B ). Similar wavy bedded facies have been described from deltaic bay fills (Coleman and Prior, 1982).

Open marine Open marine sequences contain body fossils, intense bioturbation, and burrows lined with dark organic muddy material. The sediments are fine to medium

378

A-SCOTT S E A M Muddy .~

bandy ~

Bar ~

Fen & Marsh ~ ..j~.,,/"~,~_(

~

.....

.

......... i .,,,. " .,,,.~

" ....

~

~

Tidal Flat

/.Y

0 I

B GAR.,OKSEAM

I

/

I

~

Approximate Scale

Fen & Marsh .,El,

eat

I

J

500 m

Peat .,I . . . .

k,.

Fig. 9. Visualization of the inferred environments of the Scott (A) and Garrick (B) coal seams. Dots=sand; short dashes=mud; wavy lines=ripples; black layers=mud drapes; inclined lines = ripple cross-laminations.

lithic sandstones to sandy siltstones, generally poorly sorted, due in part to homogenization by bioturbation. Thin layers of pebble conglomerate, commonly only one clast thick, recur throughout the sequence. Lonestones (pebbles and boulders) are also present. Four distinct facies can be readily recognized. Facies 1. Intensely bioturbated intervals with lonestones, but rare fossils occur

in the middle Gebbie Formation and the lower Blenheim Formation. In the Gebbie Formation, bioturbated unfossiliferous beds are associated with the shoreface part of upward-coarsening sections ascribed to prograding barriers. In certain layers of the Blenheim Formation, skeletal material is preserved in concretions but is absent in the rest of the bed. Absence of body fossils may be due to adverse environmental conditions such as high turbidity or lower salinity, or to removal during sediment redistribution or dissolution during diagenesis. Facies 1 is interpreted as deposition on a shallow shelf with terrigenous influx. Facies 2. A composite, well-sorted, fine to very fine, sublithic sandstone unit

379

(1.7 m thick) occurs in the Gebbie Formation (Fig. 7, level 200). The lower 20 cm shows undulose plane beds with a few vertical burrows extending down from the upper surface. The middle part has low-angle cross-beds and hummocks up to 0.3 m thick. The upper 0.2 m is like the lower part but without burrows. This sandstone unit is contained within intensely bioturbated units with fossiliferous layers and traces of primary laminations. It is interpreted as the result of deposition by at least two main storm events on the shoreface, and is similar to storm deposits with hummocky cross-stratification reported by Dott and Bourgeois (1982) and Moore and Hocking (1983). Facies 3. Bioturbated fossiliferous units occur in both the Gebbie and Blenheim

Formations, and generally alternate with unfossiliferous, bioturbated, siltier sediments. The Gebbie Formation is characterized by bivalve and spiriferid brachiopod faunas, and the Blenheim Formation by productid brachiopods, with minor bryozoa, bivalves, gastropods and a species of solitary rugose coral ( McLung, 1981 ). The dominance of the bivalves and the lower faunal diversity of the Gebbie Formation may indicate more rapid sedimentation of coarser sediments in a shallower marine environment. Several coquinoid horizons occur in the Blenheim Formation, and one, the Scottville Member, is up to 30 m thick. This interval is dominated by productids, with a poorly sorted, silty sandy matrix, and bedding on a 0.1-0.3 m scale. Some beds contain exclusively concave-downward pedicle valves or flat-lying brachial valves, while others have randomly oriented valves. Some shells have 1 cm thick encrusting stenoporid bryozoans, indicating periods of no sediment influx or reworking. Articulated shells are very rare, and spines are normally broken from the valves, indicating local reworking. Lonestones of granitic, volcanic and sedimentary rocks are present. The Scottville Member and associated coquinoid units extend throughout the northern Bowen Basin, overlain and underlain by rarely fossiliferous, bioturbated, silty fine sandstones ( Facies 1 ). The coquinoid units are interpreted as open shelf, shell banks formed under low terrigenous influx, perhaps during the maximum transgressive phase of the Blenheim Formation. The banks developed an interfingering relationship with storm reworked, bioturbated, less fossiliferous, shelf sands. Facies 4. A matrix-supported conglomerate facies, 3-4 m thick, with individual

conglomerate units to 2.5 m thick, occurs at the base of the Blenheim Formation ( Fig. 7, level 350). The conglomerate units thin upward and are interbedded with increasingly finer and bioturbated sandstones, with iron-stained concretions at the conglomerate-sandstone contacts. The conglomerates are poorly sorted, and composed of granule to boulder size, in a coarse to medium sandy matrix with scattered shells. Clast shape is related to size and composition; coarser, acid volcanic boulders are angular, while basic volcanic and quartzite pebbles are subrounded. The thicker conglomeratic units show amal-

380 gamated beds, some with a thin, basal, clast-supported pebble conglomerate overlain by matrix-supported pebble and boulder conglomerate. Clasts, particularly in the lower layers, are mainly flat-lying, whereas most of the larger clasts in the upper layers are randomly oriented. This facies may have been deposited by subaqueous mass flows, redistributing material accumulated on shoals by ice rafting or derived from fan-deltas prograding westward from the active tectonic belt. It is important to note that the coarser clasts which may have been ice-rafted into these and other marine units cannot have been derived from the rivers feeding the Collinsville Coal Measures, because such clasts are absent from those fluvial deposits. The sources of the ice-rafted clasts were coastal cliffs and/or the fan-deltas themselves. Alternatively, facies 4 may represent residual shoreline gravel deposits left during the basal Blenheim transgression. COAL-FORMINGENVIRONMENTS The six major sedimentary sequences described earlier indicate these Gondwanan coals were formed in a wide range of environments, ranging from continental fluvial peatlands to paralic systems.

Fluvial The Blake Seam is the typical coal deposited in the fluvial sequence of the Collinsville Coal Measures. The seam develops either abruptly over sandy and gravelly braided stream deposits, or gradationally over carbonaceous mudstones and extends for at least 18 km along strike. It is capped by coarse braided stream deposits. The coal contains moderately high ash indicating a minerotrophic peatland with fine sediment transported into the wetland either during floods or by wind deflation from seasonally dry channel areas ( Table 2 ). The dull nature may reflect a treeless peatland, or high rates of plant decomposition due either to relatively dry peatland conditions or to high nutrient status of the peatland waters.

Fluvio-paralic plain The fluvio-paralic peat-forming environment is represented by the Bowen seam and the thinner associated coals, such as the Conway and Little Bowen Seams. The thin fluvio-paralic seams are generally dirty coals, and only during the deposition of the Bowen Seam did the peatland reach a stage free of terrigenous influx, as evidenced by the relatively lower ash levels and the indication of a raised peatland. Sulphur levels in the fluvio-paralic seams are consistently higher than for the fluvial Blake Seam. Although these levels may not reflect higher sulphur contents for the original peat, nevertheless they reflect an

~Fo~

~ ~

~

~ o o ~

~

J

b~

~

o

~

~

~

~.

. . . . . .

_

~ o

~.~

b~

~

~

~

~ ~

~

~

~ o o ~

~

~

¢0 ¢o

2

F

F

.<

=

¢D

¢D

~8

Oo

382 increasingly brackish environment of deposition for the total interval. Consistently lower sulphur values for "washed coal" (F1.50 sulphur) than for the whole sample (total sulphur) indicate much of the sulphur was introduced by circulating waters from the surrounding sediments and deposited as pyrite in crevices (Table 2).

Paralic Coal seams derived from peat of paralic environments are contained within sequences which display either abundant wavy and flaser bedding and bioturbation indicating tidally influenced, low-lying shores; or upward-coarsening, winnowed sandy sequences interpreted as barriers. A particular seam may be associated with either of these sequences in different areas. Examples of the former are the Denison, Scott, Garrick and Murray Seams, and of the latter, the Ports and Peace Seams in holes C341 and C473 (Figs. 2, 3). Thin coals occur within coaly shales and thin sandstones which overly the Wall Sandstone Member in the Gebbie Creek section, and are interpreted as back-barrier deposits. Coal type is similar to the Bowen Seam and much brighter than the Blake Seam. Sulphur levels in paralic seams are generally high, particularly in the Murray Seam which is immediately overlain by marine sediments. Sulphur content of Garrick Seam coal is consistent through the seam, whereas sulphate, pyritic and elemental sulphur contents increased towards the seam top closer to the overlying marine sediments (Table 2; Smith and Batts, 1974). This indicates that most of the sulphur is introduced by circulating marine waters after deposition of the peat. An important corollary is that the peat itself accumulated in freshwater, albeit in a coastal environment. ENVIRONMENTALANALYSIS

Lithostratigraphic model Tentative lithostratigraphic correlations for the northern Bowen Basin were proposed by Dickins and Malone (1973) and McClung (1981) on the basis of transgressive marine intervals identified by the presence of body fossils, increased intensity ofbioturbation and characteristic shoreline deposits. Gross correlation of the fluvial-dominated Collinsville Coal Measures with the paralic to shallow marine Gebbie Formation is reasonably well accepted (Dickins and Malone, 1973; Staines and Koppe, 1980; McClung, 1981 ). The next logical step is to extend this concept of lateral and vertical facies relationships to the offshore marine units of part of the Blenheim Formation. The Blenheim Formation records a relatively large transgression over the whole region. It was in turn followed by a major regression in the upper Permian during which the fluvial coal measures of the Blackwater Group were deposited (Jensen, 1975).

383 COLLINSVILLE

GEBBIE

.,~T~. P

--

. " _ .---....~.~ .--=..... . = ..

~OO~N S ~ E

COA'_

MEASURES

-- P ~ _ _ ~ : . : -

---

CREEK

j--~'~l,.~:

"

.

.

.

.

.

"" ' : . . ~ ,~--~__'~_~--~,,,"

"

. "

,J

C/Z

~ 1

'

. i

'

•

<

<

~

~

" . . ~ " ~ - L .

" .

'-.Ss

>

"

-'.

" .'

.'~'~"

"-

~ t, . , . ~ S A N D Y S H O R E ,~* ~ " ~ j (nor, ccal.bearing) F L

~__ F L U V I A L - P A R A L I C . Icoal bearing)

~'~ro~-~'~< | 0

I

i

I 30 k m

".

"

'. kJ

L~

A p p r o x i m a t e sca4e

Fig. 10. Diagrammatic representation of facies relationships, Back Creek Group, Northern Bowen Basin, indicating facies equivalence of previously named stratigraphic units. An overall transgressive situation is punctuated by periodic coastal progradation. Maximum transgression is marked by the coquinites of the Scottville Member which were deposited on an open shelf beyond the reach of normal terrigenous influx.

The time equivalence of the correlations is suggested by the diachronous contact between the Blenheim Formation and the Blackwater Group, and does not conflict with different faunas within each lithofacies, each fossil assemblage indicating paleoecology rather then time (McClung, 1981 ). The generalized stratigraphic relation that emerges for the Back Creek Group is an overall transgression, periodically halted and reversed by prograding shorelines ( Figs. 2, 10). The interplay of semi-stable to mild tectonism, a slow relative rise of sea-level, and a continuous, relatively large amount of riverine sediment input generated a complex system of transitional environments.

Variations in peat forming environments Comparison of the cores and mine exposures in the Collinsville coalfield with the outcrop section at Gebbie Creek allows interpretation of lateral and vertical variations in the paleoenvironments and associated peatlands (Figs. 2, 10, 11 ). The lower Collinsville Coal Measures show an upward change from fluvial to fluvio-paralic to paralic settings. The substantial fluvial influx evident in the sections below the Bowen Seam may be the main source for the

384

f f

f' I,

,k '1"

f

~-~:--:;, . .

4-

~.~__-_

/

"::... !( -.

•

~ // / /

..~'~ iio i/ ..,~'lb.d"~ / / /'

/

-,..,,~

.?

/.v

!.L~ ~//

Io /"

,

!!-::-..! ........ Approximate Scale

0 L

5 km I

Fig. 11. Visualization of idealized paleoenvironments existing during the deposition of the Back Creek Group in northern Bowen Basin. Note that not all environments were always present at the location shown. The coast has both sandy and muddy tidal flats with sand barriers more developed to the southeast, perhaps due to steeper offshore gradients allowing more wave and current reworking. The vast peatlands range from brackish marshes at the coast to freshwater fens, which become more tree-covered inland. Ice rafting (ice floes indicated in the diagram) may have played an important role in the offshore deposition of parts of the basin.

385 extensive coastal deposits of the Wall Sandstone. Accordingly the fluvial and fluvio-paralic peatlands of the Collinsville area ( Blake, Bowen and associated seams) may be represented by the thin coals and carbonaceous shales which overlie the Wall Sandstone. This sandstone acted as a true barrier, at least for the lower part of the interval. Reduced supply of coarse sediments, continued subsidence and infilling of the back-barrier area led to gradual decline in the influence of the barrier, and to development of a broad coastal plain dissected by small channels (Fig. 7). A comparable increase in marine influence in the Collinsville area may be manifested by the development of the Potts sandstone. Fluvial influence continued during deposition of the interval from the Potts Seam to the base of the Glendoo Sandstone Member. Fluvial channels and splays are present in the Potts-Denison interval, but decrease in importance in the Denison-Scott split. The overbank deposits are locally bioturbated, contain ripple cross-laminae, and perhaps are reworked by tides. The general setting is paralic with local river influx (Fig. 11 ). To the southeast, in the Gebbie Creek section, brackish coastal deposits developed, with temporary and local emergence forming rooted, thin peats. Although thin bars were formed as indicated by well-sorted sandy units, the overall setting was of a wide, muddy shoreline which developed laterally and downdrift from the vast wetlands at Collinsville. No major barrier was formed at this time. The upper Collinsville Coal Measures and equivalent Gebbie Creek section represent paralic to fluvial environments with wide, sandy shores. Tidal processes were important, with fluvial channels present in northern areas, and barriers developed locally at Gebbie Creek (Fig. 7). Wide but locally discontinuous peatlands developed to the north, and were probably equivalent to coastal wetlands in the Gebbie Creek area, where thin carbonaceous units accumulated, partly of allochthonous origin ( Fig. 11 ). Part of the paralic coal-forming setting may be reasonably well compared with the transgressive "estuarine washover barrier-marsh system" described by Kraft et al. (1979) in Delaware Bay, or the wetland of the regressive coasts of southern Hudson Bay (Martini et al., 1980; Martini, 1981a,b; 1982). In both cases, there is no significant back-barrier lagoon, and relatively small bars and barriers form at the shoreline, with wide, flat areas to landward. Barriers are not determinant factors for peat formation. Even where significant barrierlagoon complexes occur the back-barrier flats are of limited extent, prone to terrigenous influx and subject to alternating wet and dry conditions. The result is a marsh environment where only dirty and thin peats accumulate ( Godfrey et al., 1979). Similar ancient examples are reported, for instance, by Horne and Ferm (1978), and occur in the Gebbie Creek section above the Wall Sandstone Member. Important conditions for peat accumulation are sustained precipitation and impeded drainage to maintain a water table near the ground surface. The breadth and flatness of the coastal plain is sufficient to restrict the drainage,

386 and protect the peatland from erosion and from terrigenous influx. The first environment landward from the coast where substantial, clean peats can accumulate is the freshwater fen. Development of thick peat requires a delicate equilibrium between subsidence and rate of organic sedimentation, and low or no clastic sediment input. SUMMARYAND CONCLUSIONS The geological boundary of the middle Permian rocks around the margin of the northern Bowen Basin marks the erosional remnant of a more extensive depositional basin. The depositional basin bordered the northern end of an epicontinental sea, which may have been partially open to an ocean further east. Sediments were derived mainly from the north, transported by large, southward flowing rivers, and redistributed longshore by tides, waves and icerafting, for instance along the eastern margin of the Basin to form the Wall Sandstone. Seasonal ice cover may have occurred, certainly ice-floes existed and redistributed large amounts of sediment. The numerous lonestones in the open marine units were derived in part from fan-delta (or outwash aprons) or coastal cliffs, or they were ice-rafted from southern, colder regions of the Basin, perhaps locally glaciated. Apparently, nearshore deposits do not contain ice-rafted material. This may be due to two causes. Firstly, very shallow or barred coasts may have impeded landward transport of large, gravel-bearing ice floes. Secondly, shore-ice may only have transported sandy sediments which were reworked by waves and tides, thus destroying any diagnostic scour and rafting features. Nevertheless, cold to cold-temperate climatic conditions are indicated by fossil and palaeomagnetic evidence. The basal coals of the Collinsville Coal Measures formed from peats developed initially on wide areas abandoned by gravelly, braided streams, and later in peatlands associated with low-sinuosity meandering streams. Higher coals developed from paralic peats, formed primarily on river-influenced coasts. Several features observed in the Collinsville Coal Measures are similar to those observed in other coal sequences. However, the association of coals with gravelly braided streams is seldom reported in the literature. Furthermore, the fluvio-paralic (or deltaic) sequences of the Collinsville Coal Measures differ from those of river-dominated deltas, which have been proposed as coal measure analogues, and instead were formed on a prograding shoreline. Substantial peatlands developed on wide, flat coastal plains, with or without sandy barriers. Barriers may assist peatland development by raising the profile of equilibrium of the coastal zone, and by providing some protection from storms, but they are not a necessary prerequisite. Although fluvio-paralic and paralic coals have higher sulphur levels than fluvial coals, most of this sulphur was introduced by circulating marine and

387 diagenetic w a t e r s after deposition, a n d the p e a t a c c u m u l a t e d in v a s t f r e s h w a t e r p e a t l a n d s , possibly fens or raised bogs. ACKNOWLEDGEMENTS We t h a n k the Collinsville Coal C o m p a n y P t y Ltd., a n d B r u c e R o b e r t s o n in particular, for g e n e r o u s help in t h e field a n d access to c o m p a n y i n f o r m a t i o n . J a m e s Cook U n i v e r s i t y p r o v i d e d office a n d l a b o r a t o r y facilities. F u n d s for field work of the senior a u t h o r were p r o v i d e d by t h e N a t i o n a l a n d E n g i n e e r i n g R e s e a r c h Council of C a n a d a .

REFERENCES Beeston, J., 1981. Coal rank in the Bowen Basin. Geol. Surv. Queensld. unpubl, record series 1981/48, 31 pp. Cassyhap, S.M., 1970. Sedimentary cycles and environment of deposition of the Barakar coal measures of Lower Gondwana, India. J. Sediment. Petrol., 40: 1302-1317. Coleman, J.M. and Prior, D.B., 1982. Deltaic environments of deposition. In: P.A. Scholle and D. Spearing (Editors), Sandstone Depositional Environments. Am. Assoc. Pet. Geol., Tulsa, OK, pp. 139-178. Crowell, J.C. and Frakes, L.A., 1971. Late Palaeozoic glaciation of Australia. J. Geol. Soc. Aust., 17: 115-156. Day, R.R., Murray, C.G. and Whittaker, W.C.L., 1978. The eastern part of the Tasman Orogenic Zone. Tectonophysics, 48: 327-364. Dickins, J.M., 1978. Climate of the Permian in Australia: the invertebrate faunas. Palaeogeogr. Palaeoclimatol., Palaeoecol., 23: 33-46. Dickins, J.M. and Malone, E.J., 1968. Regional subdivision of the Back Creek Group (Middle Bowen Beds), Bowen Basin, Queensland. Queensl. Govt. Min. J., 69: 292-301. Dickins, J.M. and Malone, E.J., 1973. Geology of the Bowen Basin, Queensland. Aust. Bur. Min. Res., Geol. Geophys., Bull. 130, 107 pp. Diessel, F.K., 1970. Paralic coal seam formation. Inst. Fuel Conference, Brisbane, pp. 14.1 14.22. Dott, R.H. Jr. and Burgeois, J., 1980. Hummocky stratification: significance of its variable bedding sequences. Geol. Soc. Am. Bull., 93: 663-680. Fielding, G.R., 1984. Upper deltaplain lacustrine and fluviolacustrine facies from Westphalian of the Durham coalfield, NE England. Sedimentology, 31: 547-567. Galloway, W.E. and Hobday, D.K., 1983. Terrigenous Clastic Depositional Systems. Springer, New York, NY, 423 pp. Godfrey, P.J., Leatherman, S.P. and Zaremba, R., 1979. A geobotanical approach to classification of barrier beach systems. In: S.P. Leatherman (Editor), Barrier Islands. Academic Press, New York, NY, pp. 99-126. Horne, J.C. and Ferm, J.C., 1978. Carboniferous depositional environments; Eastern Kentucky and southern West Virginia. Dept. Geol., Univ. South Carolina, Guidebook, 151 pp. Horne, J.C., Ferm, J.C., Caruccio, F.T. and Baganz, B.P., 1978. Depositional models in coal exploration and mine planning in Appalachian Region. Bull., Am. Assoc. Pet. Geol., 62: 2379-2411. Irvinng, E., 1964. Paleomagnnetism and its Application to Geological and Geophysical Problems. J. Wiley and Sons Co., New York, NY, 399 pp. Jensen, A.R., 1975. Permo-Triassic stratigraphy and sedimentation in the Bowen Basin, Queensland. Aust. Bur. Min. Res., Geol. Geophys., Bull. No. 154.

388 Johnson, D.P., 1984. Development of Permian fluvial coal measures, Australia. In: R.M. Flores (Editor), Sedimentology of coal and coal-bearing sequences. Spec. Publ. Int. Assoc. Sedimentol., 7: 149-162. Koppe, W.H., 1978. Review of the stratigraphy of the upper part of the Permian Succession in the Northern BowenBasin. Queensl. Govt. Min. J., 79: 35-45. Kraft, J.C., Allen, E.A., Belknap, D.F., John, C.J. and Maurmeyer, E.M., 1979. Processes and morphologic evolution of an estuarine and coastal barrier system. In: S.P. Leatherman (Editor), Barrier Islands. Academic Press, New York, NY, pp. 149-183. Levingston, K.R., 1981. Geological evolution and economic geology of the Burdekin River region, Queensland. Aust. Bur. Min. Res., Geol. Geophys., Bull., 208, 48 pp. Martini, I.P., 1981a. Morphology and sediments of the emergent Ontario coast of James Bay, Canada. Geogr. Annaler Ser. A, 63: 81-94. Martini, I.P., 1981b. Ice effect on erosion and sedimentation on the Ontario shores of James Bay, Canada. Z. Geomorph., 25: 1-15. Martini, I.P., 1982. Characteristics and evolution of the recent Ontario coast of Hudson Bay. Le Naturaliste Canadien, 109: 415-429. Martini, I.P., Cowell, D.W. and Wickware, G.M., 1980. Geomorphology of Southern James Bay: A low energy, emergent coast. In: S.B. McCann (Editor), Coastline of Canada. Geol. Surv. Can., Pap., 80-10: 293-301. McLung, G., 1981. Review of the stratigraphy of the Permian Back Creek Group in the Bowen Basin, Queensland. Publ. Geol. Surv. Queensland., 371, Palaeont. Paper 44. Milligan, E.N., 1971. Geology and correlation in the German Creek Coal Measures. In: A. Davis (Editor), Proceedings of the 2nd Bowen Basin Symposium. Publ. Geol. Surv. Queensl., 62: 77-85. Moore, P.S. and Hocking, R.M., 1983. Significance of hummocky cross-stratification in the Permian of the Canarvon Basin, Western Australia. J. Geol. Soc. Aust., 30:323-331. Niyogi, D., 1966. Lower Gondwana sedimentation in the Saharjuni coalfield, Bihar, India. J. Sediment. Petrol., 36: 960-972. Packham, G.H. (Editor), 1969. The geology of New South Wales. J. Geol. Soc. Aust., v.16. Rigby, J.F., 1971. Some paleobotanical observations concerning the Bowen Basin. In: A. Davis (Editor), Proceedings of the 2nd Bowen Basin Symposium. Publ. Geol. Surv. Queensld., 62: 21-29. Runnegar, B. and McClung, G., 1973. New names for marine Permian rock units in the Northern Bowen Basin, Queensland. Queensl. Govt. Min. J., Dec.: 441-440. Smith, J.N. and Batts, B.D., 1974. The distribution and isotopic composition of sulphur in coal. Geochim. Cosmochim. Acta, 38: 121-133. Staines, H.R.E. and Koppe, W.H., 1980. The geology of the north Bowen Basin. In: R.A. Henderson and P.J. Stephenson (Editors), The Geology and Geophysics of Northeastern Australia. Geol. Soc. Aust., Brisbane, pp. 279-298. Wanles, H.R., 1964. Local and regional factors in Pennsylvania cyclic sedimentation. Kansas Geol. Surv. Bull., 169: 593-606. Wanless, H.R., Tubb, J.B., Gednetz, D.E. and Weiner, J.L., 1965. Mapping sedimentary environments of Pennsylvanian cycles. Geol. Soc. Am. Bull., 74: 437-486. Webb, E.A. and Crapp, C.E., 1960. The geology of the Collinsville Coal Measures. Proc. Australas. Inst. Min. Metal., 193: 23-88.