227

THEA NOTEGINGIN

ON THECHALK,

MINERALOGY OFWESTERN AUSTRALIA.

By DOROTHY CARROLL, B.A., B.Se., University of Western Australia.

[Received 3rd January, 1939·]

[Read in abstract 5th ,Hay, 1939.]

I. INTRODUCTION.CRETACEOUS rocks cover two areas of about 18,000 and

250 square miles respectively along or near the westcoast of Western Australia. In the larger of these areas, knownas the North West basin, the Cretaceous has a thickness ofabout 2,000 feet and consists of a lower series (Albian) of mud­stones, shales, siltstones, and cherts; and an upper (Turonian­Santonian-Campanian) of glauconitic sands, shales, and chalk.

In the smaller area, known as the Gingin District, from thetownship of Gingin (31° 21' S. II5° 54' E.) there are about 400feet of Cretaceous rocks consisting of a basal bed of greensandof variable thickness (from a few inches to 30 feet or more),followed by a 30 foot bed of Chalk, which is succeeded by theUpper Greensand, 50 to 100 feet thick. Below the LowerGreensand is a series of clays and micaceous sandstones con­taining plant remains of rather Jurassic aspect. These plantshave not yet been fully described, but indicate either an upperJurassic or Lower Cretaceous age [5J*. The name" Gingin Chalk"has been applied to this thin bed of chalk since the descriptionsof Western Australian geology by F. T. Gregory in 1861 [8J,when its Cretaceous character was recognised. The discovery ofUintacrinus [20J fixed the age of Santonian, and somewhatlater M arsupites [21J was identified.

Gingin is situated about 50 miles north of Perth at thesouthern end of this Cretaceous area. The chalk outcrops in anumber of places in and around the township, but disappearsfarther south although Cretaceous foraminifera have recentlybeen identified in borings beneath Perth [12, p. 71].

The palseontology of the Chalk has already been described[7J, but as far as the writer is aware no reference has been madeto its mineralogy, although some minerals occurring in thegreensands have been noted [15; 17]. The material describedhere is from Molecap Hill, about half a mile south-east of theGingin railway station.

II. LABORATORY PROCEDURE.Several pounds of chalk were soaked in water for some time,

and when soft were washed through a coarse sieve to remove the* For list of References see pp. 233-4.

228 D. CARROLL,

larger fossils. The fine clay was washed out of the materialpassing the sieve, the bulk of which was then reduced by panning.Treatment with hydrochloric acid followed to remove thecarbonate. The residue from this treatment was dried andsieved through a 90 mesh LM.M. sieve and the heavy mineralsconcentrated with bromoform, and thereafter mounted formicroscopic examination.

In order to obtain quantitative data for the chalk, a samplefrom about 4 feet above the base of the bed at Molecap wasgiven the following treatment. The chalk was broken, withoutrubbing, into small pieces in a mortar, dried and weighed.Carbonates were removed from the whole sample by soaking instrong RCl, and the residue washed, dried and weighed. Thisresidue had appeared to consist almost entirely of glauconite.

CaC03, estimated from loss on acid treatment ., 87.3%Insoluble residue 12.7%This insoluble residue was then mechanically analysed by a

set of Tyler standard sieves:-

Remarks.

Fragments of fossilswith some large grains

glauconiteQuart'z, shell fragments. glauconiteGlauconite, quartz. shell fragmentsGlauconite, quartz, felsparGlauconite, quartz, felspar, heavy minerals

0//0

1.440·57

1. 143. 899. 20

69· IS14. 6 1

0·490.240.12

0.06

Sieve Iopenings,

mm. 1.

3.96I--~-I

1.980·99

Mesh.

+32

+60+II5+250- 25 0

+ retained on.

The +II5 and +250 fractions from this mechanical analysiswere separated in bromoform and a very small quantity of heavyminerals, approximately, 0.2 gm., was obtained. This figurewould possibly be slightly larger if each fraction had beenseparated with bromoform, but on inspection it was seen thatthis would not have much advantage, as the heavy mineralswere more plentiful in these two grade sizes. Taking thisfigure as a conservative estimate of the heavy mineral contentof the chalk, this can be expressed as about 0.07% of the whole.The glauconite content is about 9% of the whole.

III. THE HEAVY MINERAL ASSEMBLAGE.The heavy minerals identified, in order of abundance, are ;­

Opaque grains (ilmenite, magnetite, leucoxene, limonite, pyrite),zircon, garnet, rutile, tourmaline, epidote, staurolite, amphibole

:\!DIERALOGY OF GINGIN CHALK, W. AUSTRALIA. 229

and chlorite, kyanite, sphene, anatase, sillimanite, andalusite,zoisite, brookite, and monazite.OPAQUE MINERALS include ilmenite, magnetite. leucoxcne, limonite and

pyrite. The most abundant of these is ilmenite, followed by leu­coxene in cream-colured opaque grains with a faint silvery lustre;and limonite in reddish-brown grains. Some very small grains ofmagnetite were removed with a magnet. One or two grains of pyritewere noted in some of the mounts, and possibly some of the limonitegrains which dissolved very rapidly in acid contained a core of pyrite.

ZICRON: Excluding the opaque grains, zircon is the most abundant ofthe heavy minerals. It occurs in two principal varieties-tal purplegrains, either zoned or clear. The habit is stoutly prismatic brown todeep strong purple in colour. sometimes with inclusions, and generallyzoned, although not invariably so; usually not acicular; and (b)clear, colourless, non-zoned grains, occurring as small "chunky"worn prisms. Some of these are broken, whereas others merelyhave the edges rounded off. Both types show the result of a con­siderable amount of transport.

GARNET: Occurs in fresh angular fragments, generally colourless, but afew pink to pinkish-brown grains were also seen.

RUTILE is present in two forms-tal stout reddish-brown prismaticgrains showing evidence of a long period of abrasion; and (b) yellow,somewhat acicular prisms with unworn crystal faces. There are alsonumerous small irregular crystal aggregates and occasional geniculatetwins. Type (a) is considered to have been derived from the weather­ing of pre-existing rocks, whereas (b) would appear to be authigenicand to have been formed during the deposition of the Chalk. A moredetailed discussion of rutile will be given under anatase.

TOURMALIl\E is less plentiful than garnet and rutile. It occurs as sharp­edged prismatic grains and occasional rounded grains or brokenfragments. The prismatic grains are pleochroic pink-grey, almostcolourless, yellowish-brown, grey-mauve. The broken fragments areusually blue or grey, but occasicnal particoloured grains are seen.Rounded grains, mostly grey, are not at all plentiful.

EPIDOTE is rather abundant, zoisite scarce. Epidote is in the usual brightyellowish-green grains, somewhat worn, but freshly chipped fragmentsalso occur.

STAUROLITE occurs in two varieties-(a) deep brownish-yellow stoutgrains which are strongly pleochroic; and (b) pale yellow grains.Both types are angular, broken, and rather" chippy."

AMPHIBOLE Ali'D CHLORITE: Rather scarce, in bluish green to green.. weak," raggy grains, all somewhat chloritized. The amphibole isa metamorphic type common, as derived grains, to many WesternAustralian sediments.

SPHENE in colourless, rather thick, somewhat rounded grains; notabundant.

ANATASE is present in small tabular basal grains of pale yellow colour. Thetitanium-bearing minerals rutile, anatase, brookite, and leucoxenecomprise a series of minerals with identical chemical compositionswhich arise authigenically in sediments under certain conditions.Rutile needles (and occasionally anatase) are a common feature in cer­tain types of metamorphic rocks, e.g. epidiorites. and represent theoriginal titanium content of the pyroxenes, which cannot beincorporated in the resulting amphiboles. Rutile, anatase andbrookite therefore arise as alteration products when sedimentsare in certain environments; for example, Brammall and Harwoodrecord (4) the leaching of titanium from biotite when acted on by

23° D. CARROLL.

sour moor water. It is possible too that the reducing conditionsfavourable for the formation of glauconite from biotite would releaseTi02 from the biotite concerned. Unfortunately in the analyses given(6, p. 1359) Ti02 was not determined, but it is probable that wherethere is a loss in alumina there may also be a loss in TiOz, whichwould give rutile and anatase in irregular grains, such as are seen inthe heavy residues of the Chalk. Leucoxene has lately been proved[19J to be identical either with rutile or anatase. It is an altera­tion product of ilmenite and is a microcrystalline form of TiOz ; itmay also be a possible step in the production of larger grains ofrutile or anatase. Leucoxene is plentiful in these residues.

KYANITE, ANDALUSITE, SILLIMANITE are present in small amounts only,and these grains have no unusual features.

BROOKITE is to be expected when authigenic rutile and anatase are present.After careful search of the residues a number of typical, very small,grains of brookite were found. These are pale yellow in colour andhave the rather unusual optical properties of brookite.

MONAZITE is present as rounded pale green grains; rather scarce. Thepresence of monazite indicates derivation from granites or gneisses.It is a frequent constituent of river sands near Perth.

IV. MINERALS OF THE .. LIGHT FRACTION."The light fraction makes up the bulk of the material left

from the acid treatment. The main constituents of the variousgrades of material in this fraction were given with the mechanicalanalysis. The minerals occurring here are glauconite, quartz,and felspar.GLAUCONITE, the principal constituent, is abundant in bright brownish­

green grains, many somewhat rounded, while others resemble thealtered biotite grains described by Galliher [6, P. p. lIOJ. Pig­mentary glauconite also occurs in patches on quartz and felspargrains, and appears to have the same relationship to these grains assome of the clay minerals of the beidellite-nontronite group have tograins of quartz and felspar in soils. This film of glauconite may havebeen formed by the breakdown of soft glauconite [6, p. 1356], or itmay indicate a gel-like condition of the glauconitic constituents. Theorigin of some of these glauconite grains can be directly traced tobiotite flakes, which have been altered by the action of sea-water.

QUARTZ is in clear, angular to sub-angular and rounded grains with fewinclusions. The rounded are subordinate to the angular grains.

FELSPAR is in fairly large clear, angular grains. In places the grainsare altered to kaolin or sericite. The felspar is orthoclase; noplagioclase or microline was noticed in the mounts.

OTHER MATERIAL: In the finer portions of the Chalk which were washedout in the preliminary treatment it was noted that none of the veryminute grains were amorphous. Prisms of calcite, derived fromthe disintegration of the larger fossils, were plentiful.

V. ORIGIN AND SIGNIFICANCE OF THE DETRITALMINERALS.

The assemblage of heavy minerals, including as it does suchspecies as zircon, staurolite, kyanite, andalusite, garnet and tour­maline is definitely continental and terrigenous in origin. It

MINERALOGY OF GIl'iGI:\" CHALK, W. Al"STRALlA. 2::>1-'

represents the very fine material derived from the wearing downof igneous and metamorphic complexes. Hence, from tilederived minerals, coupled with the evidence afforded by th e,authigenic minerals (glauconite and the titaniferous group) ;,.picture of the conditions during the deposition of the chalk embe formed.

Below the chalk is a bed of greensand ill which glauconiteis the principal constituent. The conditions suitable for gla u­conite genesis han been studied by Galliher ~6;, who found thatglauconite is produced by the action of sea-water on biotite in ~'.

reducing environment. Still water, slow sedimentation, and anabundant supply of biotite are required for glauconite to form.The depth may be as shallow as 5 to 10 fathoms, but the maxi­mum amount of glauconite is obtained from a depth of about50 fathoms. Rapid accumulation of clays brought from the landbv strong rivers is not conducive to glauconite formation [10,p. 236]. According to Galliher, glauconite forms in the zonewhere only verv fine sand and mud are being deposited, butwater shallower than this is indicated by the coarse grains ofgreensands at Gingin (up to 2 mm.). At the top of the lowe,greensand there is a phosphatic pebble bed I to 2 feet thiek inwhich small pebbles of quartz (some as much as 12 mm. long) arcof frequent occurrence. The presence of the phosphatic and thequartz pebbles indicate strong currents [9, iii., p. 373]. Glau­conite is a prominent constituent of the chalk, and therefore itis to be supposed that the environment of chalk deposition wasstill suitable for glauconite formation.

The actual conditions of deposition of chalk are not well­known, although numerous papers have been written on thissubject. These have been summarised by Boswell [1J, and theconsensus of opinion seems to favour chemical precipitation ofCaC03 in a fairly warm sea. The surface waters of many partsof the ocean are saturated with CaCO], so that calcareous mud,similar to that found around coral islands, could be deposited.Recent work by Black [2J has shown, however, that aragonite andnot calcite is precipitated around coral islands. It is possiblethat calcite may be precipitated from colder waters. Tarr [18J,speculating on the depth of water in which chalk might havebeen deposited chemically, places it at less than 400~5()()

fathoms, and sees no reason whv it should not be less than eofathoms. The amount of quartz and other detrital mineralsin the Gingin chalk indicates a lesser depth than 400 fathoms,for the British chalk is much purer, some horizons having chalkof 98 to 99% CaCO], whereas at Gingin CaCO] varies from 75 t o89% [14].

Evidence of depth given by the fossils indicates on thewhole a moderate depth. Recently Oakley [11J has discussed the

PROC. GEOL. Assoc., VOL. L., PART 2, 1939. 15

232 D. CARROLL,

~...,-.

GEOLOGICAL MI\P0_PAR T 0 F S au T H - W £ S T

AUSTRALIA.

[alter Ctar",e]

LEGEND

K,4/NOZOIC c=:I "<.CR~ r s c e o o e C==:JJURASSIC L- JPR E - s.::,.""'/,,,;.. C~-~TJeli M 8 R;":~TE'''' rlNClJa E;-:;--ti

~~,:_----------

~

,------

,I +, .; +

"J+ .. .j. +-

: + ...I,+- -+- + .j..

,;. ..... <\.

/' .. .j. ..

\ + +

, , ... ... .. .j.. 12'*... + iJ~ ~ '\ +

FIG. 14.-GEOLOGICAL SKETCH-MAP OF PART OF SOUTH-WEST AUSTRALIA,

INDICATING THE POSITION OF GIl\GIN.

evidence of depth afforded by the sponge fauna, and concludesthat about 160 fathoms is indicated. Boswell [1J makes thefollowing statement: "The evidence of the echinoids and lamelli­branchs, and particularly of the gastropods, suggests a moderatedepth of about 200 fathoms." It is generally agreed that clearwater conditions are necessary.

Estimates of the time chalk takes to form differ, but basedon the rate of disintegration of radioactive minerals, an average

MDlERALOGY OF GIXGIX CHALK, W. A1JSTRALIA. 233

rate of deposition would be I foot in 10,000 years [1J, so that theGingin Chalk took about 300,000 years to form, and during thistime there was no very marked change in environment.

The distributive province that provided the scanty heavyminerals in the Gingin Chalk is not known with certainty.The proximity to sources of staurolite, andalusite, kyanite, silli­manite, zircon, and garnet in the Chittering Valley [13; 16]might lead one to suggest that the distributive province was tothe east (see map, Fig. 14). But when consideration is given tothe depth of water in which the chalk was laid down, it is probablethat the metasediments and gneisses of the Chittering Valleyand those still farther to the east were themselves beneath thesea. The presence of distinct varieties of purple zircon both inthe chalk and in gneisses suggests that the detritals in the chalkwere derived from the weathering of gneiss; on the other hand,the fact that purple zircon is not an uncommon variety in thefew specimens of gneisses examined for accessory minerals inthis part of Western Australia is significant. Until more detailedfield-work has clarified the relationship between the Cretaceousand]urassic and the underlying Pre-Cambrian formations, specu­lation concerning provenance cannot safely extend beyond thefact that this suite of heavy minerals was derived from a Pre­Cambrian complex of gneisses and schists. This complex mayhave been situated much farther to the east, or, conceivably,to the west, where all traces of it have been lost by down­faulting such as is known to have occurred in the Naturaliste­Leeuwin area in the extreme south-west of the State.

In conclusion, I wish to thank Professor E. de C. Clarke andDr. C. Teichert for reading through this paper and suggestingseveral alterations and improvements.

REFERENCES.I. BOSWELL, 1'. G. H. 1<)33-34. A Piece of Chalk. Science Progress,

2X.

2. BLACK, M. 1<)33. The precipitation of Calcium Carbonate on theGreat Bahama Bank. Ceol .•Mag; lxx., p. 455.

3. BRAMMALL, A. 1<)28. Dartmoor Detritals : A Study in Provenance.Proc . Geol . "-15S0C., xxxix., p. "27.

4. ----- and H. F. HARWOOD. 1l)23-·~5. Tho Occurrence ofRutile, Brookite and Anatase on Dartmoor. Min, M ag., x x.,p. 20.

5. CLAI<KE, E. DE C. I<l38. Geology of Middle and West .Austru.lia.l l andbuch der rcgionalen Ceologie, A kad, Fcrlags. Leipzig.

G. GALLIlIER, E. \V. 1935. Glauconite genesis. Dull. Geol . Soc.A mer. 41), p. 135 r.

7. GLAl'ERT, L. 11)1<l. The Geological Age and the Organic Remainsuf the Gingin "Chalk." Hull. Grot, Survcy.; ~Ii A ust., No. 36,p. 115·

234 MINERALOGY OF GINGIX CHALK, W. AUSTRALIA.

::>. GREGORY, F. T. li-l6I. On the Geology of a Part of WesternAustralia. Quart. [ourn, Geol, Soc., xvi., p. 475.

{J. JUKES-BROWK. A . .I. and 'V. HILL. I<)03, 1{J04· The CretaceousRocks of Britain, vol , ii. and iii. Meni, Gcol, SU1'1'ey UnitedKingdom.

10. MURRAY, J. and A. F. h'El'o:ARD. IS'JI. Report of the ChallengerExpedition, Deep Sea Deposits.

I I. OAKLEY, K. P. 1937. Cretaceous Sponges; some biological andgeological considerations. PrOG. Geol . A ssoc., xlviii., p. 330.

12. PARR, W, ]. 1936-37. Upper Eocene Foraminifera from deepborings in King's Park, Perth, Western Australia. [our. RoyalSoc., Tr. A ust .. xxiv., p. 71.

13· PRIDER, R. T. 1933-34· The Geology and Physiography of theJimperding Area. jour. Royal Soc., lV. .·lust., XX., p. 1.

1+. SIMPSON, E. S. 19J0. Analyses of 'Western Australian Rocks,Meteorites and Natural Waters, Hull. ('7, Geol . SUIT., Tr. Aust..p. S(,.

IS. 1920. On Gearksutite at Gingin, Western Australia.lVIin. Mag., xix., p. 23.

16. 1925-26. Contributions to the Mineralogy of WesternAustralia, Series 1. jour. Royal Soc., TV. A'ust., xii., p. 62.

17. J93('-37. Contributions to the Mineralogy of WesternAustralia. Series X. jour. Royal Soc., TI'. .·lust., xxiii., p. 27.

If. TARR, \V. :\. I(}Z5. Is the Chalk a Chemical Deposit? Gcol,Mog., lxii., p. 252.

19. TYLER, S. A. and I{. V.'. :\],'l.RSDEN. 1()3S. The Nature of Leu­coxene. J our. Sed. Petrol. 8, 1\:0. 2, p. 55.

20. WITHERS, T. H. 1924-25. The Occurrence of U iniacrinus III

Australia. Jour. Royal Soc., W. Aust., xi., p. IS.21. 1925-26. The crinoid Mnrsu pites and a new cirripede

from the Upper Cretaceous of Western Australia. Jour. Roy.Soc., TV. A ust., xii., p. "7.

PROC. GEOL. Assoc., VOL. L. (1939). PLATE 19.

PROFESSOR CHARLES LAPWORTH.

[To face p. 2.14.