PAPERS AND PROCEEDINGS OF THE ROYAL SOCIETY OF TASMANIA, VOLUME 95

THE GEOLOGY OF THE SOUTH ARM-SANDFORD AREA,TASMANIA

By

DAVID C. GREEN

Clarence High School, Hobart.

(With 4 text figures.)

ABSTRACTPermian mudstones, sandstones and limestones

are intruded hy Jurassic dolerite in the form ofgently transgressive sheets. sills and Thesub-horizontal intrusions are to theGrange Mudstone and show a pronounced devel­opment of coarse-grained dolerite near the top ofthe sill. Two Tertiary volcanie centres, one abreccia-filled vent, are situated near Tertiary faults,and folded ?Lower TerLi3xy limonitic sandstonesand clays are recorded. Tertiary tensional faultinghas tilted the strata to the west and caused therepetition of one stratigraphic horizon, the MalbinaSiltstone and Sandstone. Necks, spits and coastaldunes are related to older, higher sea levels.

INTRODUCTIONThe area mapped consists of squares 5271, 5270,

5371 and 5370, the South Arm, Howrah, Ralphs BayCanal and Clifton Beach squares which form thesouth-east part of the Hobart Sheet. Access to thearea is readily available from the main South Armroad which is sealed as far as the Clifton Beachturnoff. The lower areas are used for orchardingor sheep grazing while the uncleared higher areassupport an open sclerophyll eucalypt forest with alow scrub and bracken understory. The rainfallis a little lower than that of the Hobart area, 18-20inches per annum.

Exposure is poor, except along the coastline whereexcellent sections are available. Mapping wascarried out by walking boundaries, where this waspractical, and pinpointing the outcrop on aerialphotographs. The base map was constructed withthe aid of a slotted template layout based on alimited amount of trigonometrical control. Thecompilation should not be regarded as entirelyaccurate in the Clifton Beach square as trigon­ometrical control was not available. The geologicalboundaries were transferred from the aerial photo­graphs to the base map with a rectiplanigraph.Grid references are given as 6 figure coordinatesreferred to the State grid system. The map isincluded herewith as figure 1.

Specimens collected during field mapping arehoused in the Geology Department, University ofTasmania, and the specimen numbers referred toin this paper are those of the catalogued specimensof the collection.

17

PREVIOUS LITERATUREJohnston (888) referred to fossiliferous rocks

near Pipe Clay Lagoon but the first regional surveyof the area was that made by Nye (924). Lewis(946) mapped the area in more detail; unfortun­ately, the distribution of rock types on the mapaccompanying his text does not concur with hisrecorded field observations. Hosking and Hueber(1954) referred briefly to the Grange Mudstoneat May Point, an occurrence which had been pre­viously described by Jukes (847) and Stephens(900) .

ACKNOWLEDGMENTSThe author is indebted to the staff of the Geology

Department of the University of Tasmania for theconsiderable assistance and encouragement receivedduring field mapping and especially to Mr. M. R.Banks who kindly read the manuscript.

Acknowledgment is also made to the Directorof Education for permission to publish this report,which was prepared during the term of an Educa­tion Department Studentship.

PHYSIOGRAPHYThe physiography is controlled by tilted fault

blocks formed early in the Tertiary which strikeapproximately north and dip to the west at anglesof up to 15 0

• The faults bounding these blocksprovide planes of weakness along which latererosion has been directed.

Topographic relief is low with the exception ofthe range of hills above Rokeby, which rises to morethan 800 feet. South of Ralphs Bay Canal, MountMather (581 feet) and Mount Augustus (535 feet>are more dissected remnants of this range. Thetilted blocks of Permian sediments forming thesemountains exhibit an almost cuesta-like structure,the westward slope approximating to a dip slope.

The present morphology of the area has beendetermined principally by the post-glacial drowningof the area and subsequent lowering of the sea level.Since the last glacial phase the coastline has beenuplifted to the extent of 2 or 3 feet above the presentH.W.M. and this former shoreline has been namedthe Milford level by Davies (1959). The peninsulais made up of several islands which are joined bydeposits related to higher sea levels. The evolutionof the Ralphs Bay neck has been described by

18 THE GEOLOGY OF THE SOUTH ARM-SANDFORD AREA, TASMANIA

Davies (959) and a similar mechanism of form­ation is proposed for the neck connecting theSandford Peninsula to South Arm. Both erosionaland depositional features are present; emergedshore platforms, abandoned cliffs, raised cobble,sand and shelly beaches, beach bordering dunes,stranded sea caves and cave deposits, spits andnecks, all characteristic of a recently emerged coast­line.

The Llanherne level (Davies, 1959) forms wave­cut platforms and appears to be an erosional featureapproximately 15 feet above H.W.M. It is possiblyrelated to the last interglacial stage or a minoroscillation during the Wtirm glaciation. Earlierlevels become progressively more dissected withelevation. Lewis (946) recognised one level at5-15 feet above sea level and another at between50 and 60 feet. Other poorly developed levels arefound at 100 and 180 feet with a better developedlevel at 250 feet. Submergence during interglacialstages has usually been brief. A deposit of marinesand at Mary Ann Bay contains a Pleistocenefauna 60 feet above the present high water mark.

Cliffs up to 150 feet in height and rising from awavecut shore platform form most of the easternseaboard. Wave action has been concentrated atright angles to the coast along minor faults andprominent joints, the closely spaced bedding planesand joints of the metamorphosed Grange Mudstoneprevent extensive wave notching. The western sidesof the peninsula have mature beaches in the pro­tected bays and shore platforms developed on theMalbina Siltstone and Sandstone along exposedstretches.

Pipe Clay Lagoon at Cremorne is almost closedby ,a well developed, slightly arcuate sand-spit,post Milford in age, which has grown southwardsfrom the Milford level shoreline. Tidal scour isresponsible for the preservation of a narrow channelat the southern extremity. A shingle beach spit4 feet in height is present on the eastern side ofGellibrand Point.

The coastal dunes are the result of a plentifulsupply of sand from the beaches, the absence ofobstacles in the sea (thus allowing the wind toblow more strongly than elsewhere) and the absenceof vegetation in that part of the beach from whichthe sand is derived. The sand blown landwards fromthe frontal dune collects in dunes at right anglesto the direction of the prevailing winds and thusa series of wavelike dunes is formed, separated byrelatively flat-floored troughs. The parallel setsof coastal dunes produced in this manner areclearly demonstrated along Hope Beach, CliftonBeach, Opossum Bay and Half Moon Bay. Suchdunes are characterised by an asymmetrical cross­section and cross bedding with a steep leeward slopewhich corresponds to the angle of rest of the sands.In the case of the dunes of Hope Beach, the leewardslopes of which are fixed by vegitation, the angle ofrest is 40 0. The angle of rest for dry unfixed sandof the same particle size as that in the ridges nearHope Beach is approximately 32°.

Most of the dunes which have migrated inlandare now fossil dunes and contain an abundance ofcalcified roots and stems. These dunes are nowperched and deprived of a supply of sand. The

oldest dunes, I.e., those furthest inland and withthe highest elevation, have almost lost their dune­like character and only in artificial exposures,such as present day sand pits, can cross bedding berecognised. Where the dunes have reached adegree of stability, a bed of podsolized sandstone(dark brown in colour) is commonly developed, andmay be seen at the base of most sand excavations.The sand deposits are not worked below thisresistant layer which assists materially in thestabilisation of the dunes.

Hope Beach and Clifton Beach slope towards thesea at angles of up to 10°, the steepness of thestrand plain being due in par,t to the exposednature of the beaches. Storm waves commonlyproduce "nips" in the poorly consolidated beachsands. Behind the necks a shallow lagoon is formedand commonly a salt marsh is found in this position,e.g. the marshes south of Ralphs Bay Canal.

Away from the sore platforms and cliffs, only themassive pebbly sandstones of the lower MalbinaSiltstone and Sandstone outcrop at all stronglyto form low cliffs. Sandstone beds in the FerntreeFormation outcrop as benches, and talus from thesebeds covers the shaly mudstone beds. In a similarmanner, the upper mudstone member of the Mal­bina Siltstone and Sandstone is often covered withscree from the Risdon Sandstone. Inland theGrange Mudstone outcrops very poorly; one bandmore resistant than others is the zone of Thamno­pora and Cladochonus 65 feet below the top of theformation. Fine cliff exposures of Grange Mudstoneare found at Cape Deslacs, May Point and to theeast of Single Hill.

Much of the area is covered by an extensive andoften deep sandy soil and inland the dolerite iswea;thered deeply, especially on Droughty Point,where local run-off after heavy rain produces nar­row steep-sided gullies. Terracettes due to minorsoil slip on the steep sides of dolerite hills arecommon. More extensive landslips are found in thebasalt soil near Cape Contrariety. The ClarenceRivulet is the only prominent drainage channel. Itscourse has been largely determined by the easternedge of the Rokeby basalt, but the stream is now cutlargely in its own gravels. During the summer therivulet degenerates into a series of stagnant pools.The run-off in other parts of the area goes directlyto the sea or into a chain of lagoons along the low­lying central portion of the Sandford Peninsula.

STRATIGRAPHYPermian System

The Permian System in this area contains fourformations, Grange Mudstone, Malbina Siltstoneand Sandstone, Risdon Sandstone and FerntreeMudstone. A complete section is not available andneither the base of the Grange Mudstone nor thetop of the Ferntree Mudstone is exposed.

Grange Mudstone,This formation consists of at least 150 feet of

richly fossiliferous mudstone, siltstone, limestoneand dolomite, with thin lenses of sandstone andgranule conglomerate. It is the uppermost forma­tion of the Cascades Group <Banks and Hale, 1957)

DAVID C. GREEN 19

in the Hobart area, and is overlain with a slighterosional break by the basal member of the MalbinaSiltstone and Sandstone.

At the base of the measured section at CapeDeslacs (see Brill, 1956) is an impure dolomite,light green to grey in colour which contains thinbands of dark green diopside due to contact meta­morphism. Twenty feet above sea-level granuleconglomerates and pebbly siltstones predominateand a thin layer of recrystallised calcite sheets iscommonly found along the bedding planes. Thenext 16 feet are composed of coarse-grained sand­stones interbedded with impure limestones, dolom­ites and pebbly mudstones. Large spiriferids occuras rolled fragments, the rocks are poorly-sorted andcontain pebbles up to two inches in diameter.Massive, fossiliferous limestones and dolomites withmudstone intercalcations occupy the succeeding 24feet. Pebbles become increasingly common towardsthe top of the formation and the remaining 27feet are composed of pebbly siltstones, granule con­glomerates, calcerenites ,and subordinate impurelimestones and dolomites. Graded bedding is acommon feature and fewer fossils are found inthis part of the section.

A thin section cut from an arenaceous limestone68 feet above sea level at Cape Deslacs contains35% of sand grade, subangular to rounded quartzparticles of up to 4 mms. in diameter. The grainsare separated by irregular patches of recrystallisedcalcite. A few fragments of prehnite are present,but most of the matrix is made up of very finecalcite, the individual grains being so small that theslide is opaque. Grains of zircon and small lathsof andesine are also present.

North of Ralphs Bay Canal, on the western shoreof Ralphs Bay, another section of the upper partof the Grange Mudstone is exposed. This has beenmeasured in detail and may be correlated with thesection at Cape Deslacs. The base of the sectionis covered by a shingle beach in which fragmentsof mudstone containing Cladochonus are present.The bottom 30 feet of the section is composed ofalternating pebbly and rather fissile mudstones,with subordinate thin bands of bryozoal calcarenitesand spiriferid calcirudites. This is overlain by 5feet of slightly argillaceous limestone containingabundant Strophalosia jukesi, fenestellids andstenoporids. As at Cape Deslacs, the limestoneunderlies sparsely fossiliferous mudstone, withpebbles becoming increasingly common towards thebase of the Malbina Siltstone and Sandstone, somebeds having distinct graded bedding.

On the shoreline of Ralphs Bay beneath Mt.Mather a similar section is exposed and a zone ofCladochonus and Thamnopora is also present at thesouthern end of the hills behind Rokeby (317178),65 feet below the Malbina Siltstone and Sandstone,where it is associated with Schuchertella. TheCladochonus-Thamnopora zone is 68 feet below thebase of the Malbina Siltstone on Maria Island (pers.comm. M. R. Banks). Other fossils include Poly­Pora woodsi, Fenestella dispersa, Stenopora crinita,Strophalosia cj. clarkei and Strophalosia typica.

At 341103, Grange Mudstone is well exposed in aP.WD. quarry as a fossiliferous mottled limy mud­stone with dolomitic bands. As in the case with

every outcrop in this area the Grange Mudstonehas been afl'ected by the intrusion of Jurassicdolerite.

Metamorphism of the Grange Mudstone.Where the Grange Mudstone is intruded by doler­

ite, the contact metamorphic efl'ects are consider­able. Silicification commonly extends for as muchas 100 feet from the point of intrusion, and contactmetamorphism is most pronounced in the calcareousb':)ds. The rock immediately adjacent to the doleritein the case of a sill, e.g. north of Cape Deslacs iseither fissile and heavily weathered or hardened ~ndvery closely jointed. Some specimens from thecontact zone contain prisms of pale green wollaston­ite and coarsely recrystallised calcite. At MayPoint impure limestones are interbedded with mud­stones and near the margin of the transgressivedolerite the argillaceous limestone is dark green incolour and recrystallised.

Thin section 1094, a dark green hornfels collectedwithin one foot of the contact, contains 40% ofrecrystallised calcium carbonate, 10% of subangularquartz fragments, 20% of clay material and 30%of small green granules with high relief, which areprobably diopside. The matrix is an intimatemixture of recrystallised calcite and clay flakeswith a few well-rounded grains of zircon and needlesof apatite. Lamellar calcite crystals form veinsfrom 0.1 to 0.5 mms. in width. The rocks retaina clastic texture microscopically, although in thehand specimen they appear to be recrystallised.

Malbina Siltstone and Sandstone.The Malbina Siltstone and Sandstone formation

conSIsts of aL least 280 feet of coarse sandstone,granule conglomerate, pebbly mudstone and silt­stone which is correlated with a type sectionmeasured by Messrs. M. R. Banks and D. E. Read(see Banks and Read this volume) during 1959near Jarvis Creek in the Claremont square. TheMalbina Siltstone and Sandstone underlies theRisdon Sandstone and overlies the Grange Mud­stone and the formation replaces the undefinedterm" Woodbridge Glacial Formation".

The basal member "A" overlies the GrangeMudstone with an erosional break or diRstem owingto current scour. This is especially marked in thebasal beds at Cape Deslacs and at 314174. Im­mediately above the base, member" A" contains5-10 feet of poorly-sorted conglomerate and pebblysandstone grading into a pebbly siltstone. Angularquartzite and granite boulders of up to 15 cms. indiameter are relatively common and the matrix isof fine sand to clay grade. The rock is thick-bedded,cream or grey in colour, tough and shows a rhythmicgraded bedding which has been described by Brill(956) .

Rock fragments are composed almost entirely ofquartzite and granite with subordinate angularfragments of slate, phyllite and quartz muscoviteschist.

In thin section 1082, 80% of angular to sub­rounded quartz grains with a mean diameter of0.45 mms. are present. Fresh plagioclase frag­ments are rare. The presence of a bent fragmentof feldspar and quartz showing undulose extinction

20 THE GEOLOGY OF THE SOUTH ARM-SANDFORD AREA, TASMANIA

indicates a metamorphic source. The matrix (12%)is composed of silt grade quartz, chlorite, limoniteand a small amount of micaceous clay gradematerial with zircon and rutile as accessory min­erals. Rock fragments, mainly quartz mica schistand micaceous quartzite, constitute 5% of the rock.Small granules and powdery aggregates of limoniteare concentrated around the edges of quartz grains.Where regrowth has occurred, the original outlineis shown as a ring of inclusions of fine ilmenite.The texture is mosaic away from the larger patchesof matrix.

Large poorly preserved casts and moulds ofspiriferids are common, IngelareUa and" Spirijer"avicula have been identified. A peculiar featureof both this and the overlying member is that thespiriferids are aligned parallel to the bedding.Pebbles of the underlying Grange Mudstone areconfined to the basal conglomerate.

At 348059 ,the basal member is exposed on top ofa small hill. Here the rock is a poorly-sorted sub­greywacke sandstone containing approximately 25%of highly angular quartzite fragments in a slightlycalcareous matrix of sand to silt grade. Similarexposures are found on the shoreline beneath Mt.Mather and north of Ralphs Bay Canal.

Graded bedding, although imperfect, is present inthis member and has been recognised in each ofthe above mentioned localities. A typical sediment­ation unit consists of a poorly-sorted granule con­glomerate at the base which grades upward withinapproximately 9 inches into a siltstone which con­tains less than 5% of angular quartz fragments.There are very few fenestellids in this memberand it is rare to find complete shells of the brachio­pods. These factors suggest that the depositionalarea was one of considerable turbulence.

The remainder of member "A" consists ofapproximately 55 feet of fossiliferous pebbly silt­stone alternating with sandstone and subordinatebands of granule conglomerate. This membercommences with a band of granule conglomeratethree feet thick resting on an irregular beddingplane. Ingelarella and" Spirijer " avicula are com­mon while Schuchertella, Stenopora crinita, Peru­vispira and crinoid stems are present. Fenestellidsand stenoporids as extended mats are almostentirely restricted to the siltstones and mudstones,although broken fragments are found in the coarserbands. Graded beds are present in this member anda small amount of load casting appears to havetaken place at the base of some of the coarserbeds.

Member" B" comprises approximately 125 feetof massive sandstone and siltstone, cream or whitein colour and poorly fossiliferous, which containfewer pebbles than the underlying member. Thismember forms most of the shoreline of the penin­sula and commences with a pebbly sandstone 3 feetthick, which is overlain by massive sandstone bedswith subordinate siltstone layers. The mean grainsize decreases towards the top of the member wherefissile and non-fissile siltstones alternate. Pebblesof quartzite, phyllite and granite as large as fourinches in diameter are scattered through thismember. The matrix is composed of clay minerals,chlorite and quartz, and is of clay grade. The

sorting is poor and lenses of sandstone occur inmany places.

The pebbles occur in irregular patches and maybest be described as "erratics '. Evidence that thepebbles have in some cases been dropped into themud on the sea floor is provided at 306155, wherean elongate pebble with the long axis verticalhas broken fine cross laminations and bent those atthe base of the pebble. Similar rocks occur at manyplaces on the peninsula, but correlation can be madeonlY on lItholOgIcal similarity unless the strati­graphic position can be ascertained.

At 313069 approximately 15 feet of pebbly sand­stone overlie a pebbly mudstone and are overlainby two feet of very pebbly coarse mudstone contain­ing large angular pebbles of quartzite and slate.This massive sandstone unit is recognised in severallocalities, but is very variable in character and inthickness. Such exposures are referred to asmember "C", but it is not clear that all suchexposures can be correlated on the evidence avail­able.

The top of the section consists of approximately75 feet of poorly exposed fissile siltstone with sub­ordinate sandstone and limestone bands. Poorexposures of this member are present near the topof Mt. Mather, and at 271185 sparsely fossiliferousmudstones are exposed which contain poorly pre­served spiriferids and pelecypods. This exposureis probably close to the top of the formation. TheTerrakea zone recorded in the Hobart area benellththe Risdon Sandstone is present at only one locality,345002, where it is poorly exposed.

There are three horizons in this formation IIIwhich pebbly sandstone like the Risdon Sandstoneoutcrop. The first is approximately 10 feet abovethe base, the second approximately 60 feet above thebase, and the third, approximately 185 feet abOVethe base. The uppermost one is easily mistakenfor the Risdon Sandstone if the outcrop is poor.

Erratics as much as four feet in width are exposedat 264165 on the Tranmere shore platform. Thisexposure is unusual in that sedimentary rocks arepresent as erratics in addition to the normal typesfrom an igneous and metamorphic terrain. Angularfragments of mudstone, siltstone and sandstonecomprise 40% of the surface area of a rock faceabout 3 feet square. The fragments have straightedges in many cases and appear to have undergonevery little transport. In the same bed, there isa concentration of quartzite and granite bouldersas much as 30 cms. in diameter, with a roundedschist fragment 25 cms. x 15 cms. and a boulderof grey granite 100 cms x 80 cms. The largest erraticis a subrounded boulder of quartz sandstone 120cms. x 80 cms., which contains mica and a smallamount of feldspar, is veined with quartz and showsfaint cross-bedding.

At the western end of Hope Beach, 267006, amassive sandstone 16 feet 6 inches in thickness over­lies fossiliferous mudstone and pebbly coarse mud­stones of at least 50 feet in thickness which appearto be similar to the basal part of the Malbina Silt­stone and Sandstone. Two bands, 3 feet 6 inchesand 14 feet 9 inches beneath the base of the sand­stone, are extremely fossiliferous and containabundant poorly preserved impressions of a spinoseproductid.

DA VID C. GREEN 21

The basal unit of the massive sandstone is 1foot in thickness, is graded and has an irregularbottom contact with the underlying fossiliferousmudstone and contains more than 80% of angularpebbles of both quartzite and fenestellid mudstoneas much as 15 ems. long. The matrix is of sandgrade, the rock is poorly sorted and best describedas a petromict paraconglomerate (Pettijohn, 1957).This unit is succeeded by a massive, poorly-sortedpebbly sandstone, which contains pebbly layers atintervals of approximately one foot and, at the base,large subangular pebbles of quartzite and granite.The massive sandstone is overlain by a pebbly silt­stone containing abundant small calcareous tubesabout 2 mms. in diameter. It is thought that thismember is the basal part of member" C " and thatthis member varies considerably in both pebblecontent and thickness.

Irregular markings at least 35 ems. in lengthand 2 ems. in diameter occur on the undersurfaceof several beds of fine sandstone above the massivesandstone at 267006 and on at least one horizonbelow it. The best example is found on a blockwhich has fallen from the cliffs, and on the beddingface more than two hundred separate markings,both straight and curved, are present. The mostprobable explanation is that worm trails left ina soft mud were later filled by fine sandstone. Themarkings are apparently continuous with the sand­stone of the overlying bed and show no structureswhich allow identification of the parent organism.

A dark grey, coquinoid limestone is exposed ap­proximately 160 feet above the base of the formationat 313193. Half the fossil fragments are of productidbrachiopods while bryozoans, foraminifera, pelecy­pods and productid spines are present. The shellfragments are resorbed in some cases and are setin a matrix of recrystallised sparry calcite combinedwith smaller angu:ar grains of silt to clay grade.Accessory minerals include plagioclase, ilmenite,muscovite and chlorite.

The Malbina Siltstone and Sandstone is a se­quence of interbedded siltstone, pebbly siltstone,sandstone, granule conglomerate and rare lime­stone. Many of the sandstones are lithic greywackes(Pettijohn 1957), subgreywackes or protoquartzitesas the labile fraction varies from less than 3%to as much as 30%. Rock fragments are morecommon than feldspars, but the presence of a fewfresh feldspar phenoclasts suggests a degree ofmineralogical immaturity. The finer-grained silt­stones, often relatively well-laminated, contain lessthan 3% of phenoclasts which are rarely of greaterthan sand dimensions; quantitatively, these occupyover half the section and must be considered as thedominant rock type.

Almost all the large pebbles found in the MalbinaSiltstone and Sandstone have come from a terrainof metamorphic and acid igneous rocks. The frag­ments have suffered little fiuviatile transport andthe relief of the source area must have been suchthat river transport was at a minimum.

The depositional area at this time was probablythat of a shelf covered by silt. Currents appear tohave had little effect and conditions were favourablefor the growth of bryozoans. Deposition appearsto have taken place below wave base, as the matrix

of many of the rocks consists of material from boththe silt and clay grades and is not well-sorted.

When sufficient gravel had accumulated close tothe shoreline, where relatively robust spiriferidswere common, a slight tectonic disturbance or evena climatic change would be sufficient to spread thr:gravels ovec the sea floor in the form of a " slurry'or tubidity current. Periodic disturbances of thi,nature WOUld be competent to produce the gradeo.bedding of member" A". A small amount of post­depositional loadcastingtook place before lithi­fication.

A change in sedimentation conditions is shownby unit " B .. and conditions were unfavourable forthe development of marine life. The large erraticsoften seen in otherwise well-sorted siltstone areconcentrated in patches and the most plausibleexplanation of this observation involves rafting,probably by ice. These deposits have none of thesecondary characteristics of large scale turbiditesequence such as abundant slump structures.

The incidence of erratics decreases upwards;member" C .. represents a temporary return to thedeposition of a pebbly sandstone but silt depositioncontinued under conditions increasingly favourableto life towards the top of the sequence. At thisstage the relief of the land surface had become lowerand the supply of anguIar fragments had dimin­ished, but periodic occurrences of Risdon-type sand­stone through the Ferntree Mudstone suggest thatstability was not completely attained.

Risdon SandstoneThe Risdon Sandstone outcrops strongly between

the Malbina Siltstone and Sandstone, which under­lies it, and the overlying Ferntree Mudstone. Theformation is approximately 15 feet thick, and isbest exposed as a bench at the summit of Mt.Mather, 317148, and at 313182.

The basal member is a conglomerate, approxim­ately 1 foot thick and poorly exposed. This isoverlain by a thick-bedded pebbly sandstone whichis poorly-sorted and weathers to a dark brown togreY-black colour. The number of pebbles decreasestowards the top of the bed and at the top, theformation is well-sorted sandstone. A thin-section(1056) from the centre of the formation contains10% of angular quartz and quartzite pebbles witha small amount of twinned plagioclase and micro­cline. The matrix consists almost entirely of quartzand small rock fragments in the 0.5-1 mm. sizegrade. A few quartz fragments show unduloseextinction. Some quartz grains show suturedboundaries, forming a patchy mosaic texture. Th"matrix is cemented by a small amount (1 %) offerruginous clay.

Sutured boundaries in a relatively clean sand­stone are probably due to the absence of inter­stitial clay with its cushioning effect during dia­genesis. There is no suggestion that thermal meta­morphic recrystallisation has taken place.

Ferntree MudstoneThis formation is conformably underlain by the

Risdon Sandstone and underlies the Triassic rocks.The only large outcrop of Ferntree Mudstone is

22 THE GEOLOGY OF THE SOUTH ARM-SANDFORD AREA, TASMANIA

at 313184 while Mt. Mather is capped by approx­imately 10 feet of mudstone overlying the RisdonSandstone.

Lenses of poorly-sorted sandstone approximatelyfive feet in thickness are present 20 feet and 100feet above the base. Poorly-sorted siltstone bandscont,ain up to 25% of angular quartz fragmentsof coarse-silt grade. The marix of the thin­bedded siltstone beds is made up of indeterminaterock fragments, cloudy feldspar and fine quartzparticles of clay grade with small amounts ofbiotite and muscovite.

The sandstone bands are of fine to medium sandgrade, poorly-sorted and pale cream in colour.Angular pebbles of quartzite and slate greater than2 mms. diameter form 3-5% of these bands. Thematrix is dominantly quartz with a small amountof feldspar and rock fragments, subangular in shapeand of medium sand grade size. Only the lower 170feet of this formation is exposed in the hills behindRokeby, the section being terminated by a steepintrusive dolerite contact.

TRIASSIC SYSTEMA quartzose sandstone outcrops on the north­

east tip of Betsey Island which is correlated onlithological grounds with the Knocklofty Formationof the Hobart area. Approximately 60 feet of clean,well-sorted sandstone are exposed; the bottom 25feet are bedded at intervals of between three inchesand one foot, extensively cross-bedded, and containa carbonaceous pebbly band nine inches in thick­ness. Thirty-five feet of friable thick-bedded sand­stone occur above this unit, medium yellow orangein colour on the weathered surface.

A thin-section (1055) contains quartz fragmentsof high sphericity ranging in size from 0.6 mms. to0.025 mms. with a mean diameter of 0.3 mms. and1% of indeterminate, sericitised rock fragments.The rocks dip to the west at an angle of 30° and neara fault contact against the dolerite a considerabledrag dip is visible.

At 322194 is a small outcrop of a well-sortedquartz sandstone under a thick cover of Recentsands. Triassic sandstones outcrop just to the northof the Ralphs Bay Canal square along the road toSeven Mile Beach, and the outcrop at 322194 isprobably the southward continuation of this down­faulted wedge of Triassic rocks.

JURASSIC SYSTEMDolerite

In several places on the Sandford Peninsula thetops of dolerite sills or gently shelving sheets ofpresumed Jurassic age (Hills and Carey, 1949) areexposed. In almost all exposures the dolerite isintrustive into the Grange Mudstone within 100feet of the base of the Malbina Siltstone and Sand­stone. Typical examples are Cape Deslacs, SingleHill, Roches Beach and Cape Contrariety. The peg­matitic zone is particularly well exposed and peg­matitic veins are common in the upper zone.

In hand specimen the fresh dolerite is even­grained, blue-grey in CDlour and of medium grainsize. At an intrusive contac~ the dolerite became

dark-grey to black in colour and fine-grained.The pegmatitic veins exposed in cliff sections mayeither be concordant with the top of the sill, asthin transgressive "dykes", or irregular schlierenof coarse-grained dolerite in relatively fine-grainedmaterial. The concordant veins are remarkablyconstant in thickness for several hundred yards withan average width of two feet, but thin rapidly atthe extremities. The contact between the pegmatiteveins and the normal dolerite is typically sharp atthe base and a thin band of finer grain size andlight colour may occur at the border of the vein.

The dolerite sills of this area differ from thesills hitherto described (Carey, S.W. <Edit.) DoleriteSymposium, July, 1958) in that the upper zone isless than 20 feet in thickness. A similar occurrenceat 191017 (Kingborough Sheet) north of PiersonsPoint has been found by Mr. H. A. Bartlett.

The composition of the dolerite at a chilledmargin is comparable to the Mount Wellington sill,containing 48% pyroxene, 45% plagioclase, 5%mesostasis and 2% iron ore. At the contact thetexture is fine-grained, intergranular and porphy­ritic. Lathlike microphenocrysts of orthopyroxene,plagioclase and some clinopyroxene are commonand slide 1000 contains a phenocryst of enstatite0.7 mms. by 0.3 mms. in size. The groundmass isvery fine-grained and consists of a small amountof quartz with small subhedral laths of twinnedplagioclase, anhedral pyroxene and scattered gran­ules of iron ore.

Away from the upper contact both the plagioclaseand pyroxene increase rapidly in size and thetexture becomes sub-ophitic. Some orthopyroxeneremains but it is strongly corroded and is often rim­med by augite. In slide 1014 a phenocryst of second­ary hypersthene is present. Pleichroic hornblendeoccurs as small prisms containing inclusions andoften surrounds clinopyroxenes. Biotite and ironore are scattered through the mesostasis and otheraccessory minerals include calcite, apatite, stilbiteand prehnite.

The composition of the rocks in the pegmatiticzone is extremely variable and veins of pegmatiticmaterial are found within 20 feet of the top of thesill north of Cape Deslacs. The pegmatitic zoneproper extends well below this, as coarse-grainedpegmatitic dolerite is exposed along the shorelinebelow Calvert's Hill.

In general, a high proportion of mesostasis ischaracteristic <13-37%), and in the slides countedthe amount of plagioclase (34-51 %) exceeds theamount of pyroxene <17-38%), while iron ore(0.6-5.6%) appears to be concentrated only in somespecimens. The texture is hypautomorphic to auto­morphic granular with an intersertal groundmass.Laths of zoned plagioclase (Ab30-Ab,,) of up to 5mms. in length occur in a groundmass of low reliefbroken by rods, octahedra and anhedra of iron ore.Heavily chloritised primary orthopyroxene ac­counts for less than 2% of the pyroxene of slide1003, 30 feet below the upper contact.

Both pigeonite and augite are present, uniaxialpigeonite is comparatively common and bladedpyroxenes as much as 11 cms. in length can befound at 386094. Some of the clinopyroxene is

DAVID C. GREEN

altered to pleochroic chlorite. The large auto-morphic plagioclase laths are of zoned labradoritebut some of the smaller subhedral laths are moresodic in composition (Ab,o-Ab7o). The mesostasis inthe pegmatitic rocks is dominantly of two types,types 2 and 3 of Spry (1958), and contains quartz,orthoclase and some subhedral plagioclase whichhas a refractive index indicative of albite. The mostcommon type of mesostasis is a coarse-grainedquartz-orthoclase graphic intergrowth.

On the road to Clifton Beach at 343102 a plugof dark-coloured, fine-grained dolerite outcrops.This rock contains approximately 8% of primaryenstatite which is rimmed by simply-twinned clino­pyroxenes Euhedral radiating laths of plagioclase(30%) are intergrown in an ophitic texture. Themesostasis consists of anhedral orthoclase withsmall needles of quartz and apatite and irregularfragments of iron ore which are closely associatedwith both the altered borders of the primary ortho­pyroxene and the mesostasis which accounts for28% of the rock, an anomalously high value.

A slide (024) cut from a loose block on top ofBetsey Island shows the considerable increase iniron in the end products of the magma. The textureis intergranular and the rock contains plagioclase(50%), pyrozene (20%), mesostasis (20%) and ironore (10 %). The plagioclase is zoned from Ab,o toAb,o and occurs as subhedral, interpenetrating lathsup to 1.25 mms. in length. Pyroxenes, both pigeoniteand augite, of prismatic habit average 10 mms. inlength and are commonly altered to a red ironoxide along cleavage cracks and rimmed by smallflakes of green hornblende. The pale brown colourof the unaltered augite suggests that it may be aferroaugite. The mesostasis is predominantlyanhedral quartz and orthoclase, containing manysmall skeletal crystals of iron ore. In a small patchon the slide the mesostasis is composed of chloriteand iron ore in equal proportions.

At the junctions of the pegmatitic lenses and thehost rock a thin white line may indicate that thehigh concentration of water and potash in theresidual has been responsible for deuteric alteration.No feeder dykes for the pegmatitic lenses were seen.The residual material forming the mesostasis isoften well crystallised. The occurrence of peg­matitic segregation so close to the top of a sillsuggests the magma was at least partly liquid andit seems likely that the magma was still in the formof a crystal" mush" in order to allow stratiformsegregations to develop.

TERTIARY SYSTEMIt is probable that Tertiary sediments occupy

much of the Sandford Peninsula, although theyare now overlain by a cover of Quaternary sands.Pipe Clay Lagoon Beds

On the southern shore of the entrance to PipeClay Lagoon, sandstones and clays are gently foldedand interbedded with bands of limonite. The sand­stone is friable and is light brown to chocolatebrown in colour. Some cross-bedding is present. Insection, the limonitic bands show a complexly cren­ulated structure. There is some resemblance to con­cretionary liesegang rings, but it seems more prob­able that intrastratal flowage and interal readjust­men along the clay layers has been responsible forthe strucure. The crenulations become less pro­nounced close to the margin of the bed and theuniform thickness precludes large scale slumping.

'"oo,..;

,..;oo,..;

oo;:;

<ll;.,

o

~

....o

23

THE GEOLOGY OF THE SOUTH ARM-SANDFORD AREA, TASMANIA

of the basalt bedsxenoliths. The steep

of "Y'I"1'Yl~"'O'Y'

The basalt fragments and small flows show well-

26 THE GEOLOGY OF THE SOUTH ARM-SANDFORD AREA, TASMANIA

Analyst:-W. St. C. Manson (from Edwards, 1950)Aurousseau, 1926 (from Spry, 1955)W. St. C. Manson (from Edwards, 1950)

The iron ratio (Wager, 1956-25.5) is low, sug­gesting that differentiation has not reached anadvanced stage, although post-fractional oxidationwill reduce the ratio to some extent. Since analkaline residual liquid is characteristic of silica-

differs from the otherwise similar Blinking BillyPoint basanite (Spry, 1955) in that it contains nonormative nepheline.

Since 70-80% of the olivine in the Rokeby basalthas been altered to iddingsite, it is appreciablyoxidised and the high state of oxidation is shownby the presence of haematite in the norm. Theassociation of olivine phenocrysts in a groundmassof pyroxene, iron ore and a feldspar base is sug­gestive of a limburgite, a conclusion already drawnby Edwards (1950). A comparison may be madewith the Hawaiian Alkaline Series of MacDonald(949) and Wager (956), in which basalts oligo­clase are a middle stage differentiate.

ZrO

deficient alkalic rocks, the mineralogical compos­ition of the base depends upon the amount ofsilica present. Thus the apparent difference be­tween the Rokeby basalt and the Blinking BillyPoint basanite may be due to a small difference inthe amount of silica present in the end products ofcrystallisation.

Small patches of scoriaceous basalt which aresimilar to the Rokeby basalt occur at 323115, 335115,322105 and 333106. They are probably eroded rem­nants of a nearby volcanic centre.

Pleistocene SeriesMuch of the area is covered by Pleistocene gravels,

sands and clays, some of which are terrestrial andshoreline deposits, while others are marine depositsand represent transgressive phases. Recent sandsoften obscure these deposits and it is difficult todefine a line of demarkation. In some cases roundedpebbles of basalt occur in the gravels.

The Rokeby Rivulet has cut into a poorly-beddedseries of impure clays and gravels and approxim­ately one mile to the east, at the head of Ralphs Bay,are small cliffs of poorly-bedded gravels and sands.Although these gravels and sands are poorly con­solidated, they stand in vertical cliffs of approx­imately 20 feet, and dip to the east at approximately3 0

• The bounders and pebbles, derived mainly fromPermian sediments, are angular, of low to moder­ate sphericity and show little evidence of fiuviatiletransport. The lower part of this exposure isbetter sorted and throughout the matrix is of siltto sand grade. Lenses of sandstone of limited lateralextent are common.

SimHar gravels are exposed in a stream channelcrossed by the Roches Beach Road, and along theRifle Range Road where a bore passes through82 feet of gravels before reaching solid rock. Cliffsections on the South Arm Peninsula show asimilar thickness of gravels, sands and clays,which Nye (1924) has correlated with the gravelsof the Sandford Peninsula.

At several points around the shoreline, con­solidated cross-bedded sandstones occur. These arebest exposed at the southern extremity of DroughtyPoint 264135 where the sandstone is slightly iron­stained, well-sorted and shows festoon currentbedding and scouring of the upper parts of eachbed. The bottom of each bed is commonly markedby a layer of pebbles or small broken shells. Thesedeposits represent old storm beaches and have anorigin similar to those forming the beaches ofthe present day. Similar deposit are found belowthe Recent sand dunes of Opossum and Half MoonBays.Mary Ann Bay Sandstone.

The Mary Ann Bay Sandstone is defined as thatformation of friable, fossiliferous marine sand­stone, approximately 40 feet thick and probablyPleistocene in age, which is overlain by Recentsands and rests unconformably on an irregularerosion surface of dolerite. The type exposure isin the cliffs of 246093, Mary Ann Bay.

This is a poorly consolidated friable sandstone,perched 60 feet above sea-level containing a fewlarge pectinacean shells and a gastropod fauna very

nil

8.56

9.10

3.99

1.02

1.53

1.l0

tr.

tr.

3.50

1.00

0.08

45.10

12.86

5.66

7.10

100.60

Limburgite,WoodendVictoria

nil

9.93

2.84

4.71

8.26

5.51

3.16

1.73

1.64

2.10

1.65

0.16

0.16

0.09

45.59

12.48

BlinkingBilly

Pt. Basanite

13.90

7.78

34.58

3.50

6.73

5.80

99.60 100.06

TABLE II

Norm.

or

ab

an

Rokebybasalt

{ ~:fs

hy {enfs

7.01

7.33 di

0.52

4.11

1.35

99.98

MgO

FeO 4.16

SiO. 46.64

Al.03 13.22

Fe.03 9.81

CaO

S03 nil H.O 2.66

H.O 2.14 rfo 5.7401 ~

CO. tr. lfa

TiO. 2.50 mt 6.73

P.O. 1.00 hm 5.12

MnO 0.19 ilm 4.71

Cl tr. ap 2.35

DAVID C. GREEN 27

M-O-Z60 M=O'19~ M=0279 M=o.368

% O,"'O3~' Q,""'0-524 Q,=0-361 Qr=0-865

.0 oro:::.. .. 03"'o-~,:: Q3=O;:"~ Qf"0'~~~

s",·q·l} Sc-"~1l 50=1·88

S.,= '36 Sk=,'Ql s,~'"

5.=0'19s..,=\·03

'0

MARYANN BAY SANDSTONE PRESENT DA'r BEACH SAND HOPE BEACH SAND FROM TOP OF BEACH DUNES

HOPE BEACH

SHELLY SAND FROM RALPHS BAY

~ 60

M=O-'S7

OFo-.nQ3=0:;;,5"",,'-30

5K=' '0"

M=O"~9

Q,=0'96

03=O'',;~,

5"""" .35..,..='·07

M =0'16 1

0,=0-,';Q3=O''.:",.

5,,=.·n5.1"""-0'

MECHANICALANALYSES

OF

QUATE RNARY

DEPOSITS

SOUTH ARM- SANDFORD AREA

CAND NEAR SANDFORD CHURCH BASE OF SHQREO\JNE oPOSSUM SA" OLD DUNE DEPOSIT 200ABOVE PIPE

CLAY LAGOON

Figure 3.

similar to that found on the present shoreline,although no determination of genera could he made.A mechanical analysis show that it is well-sorted(So = 1.36) and that there is a slight predominanceof a coarse admixture. Most of the sand grainsfall in the size range 0.5-0.125 mms. and they aresubangular to rounded. The fossil fragments arebroken but not abraded to any extent and formmost of the coarse admixture.

Recent Series.Almost the entire area with the exception of

Hayes Hills, the hills to the east of Rokeby andSingle Hill, is covered by Quaternary alluvium. Adeep sandy soil covers all that area which liesbelow an altitude of approximately 100 feet. Muchof the sand, being swept inland by the prevailingwinds, is derived from old and present day shore­lines. Most of the dunes are now stabilised byvegetation, with the exception of those near theexposed ocean beaches on the south coast. Marshr:eposits have formed on the protected side ofnarrow necks and the saline swampy areas suppJrta samphire vegetation.

Present day beach sands (1113) contain as muchas 25% of subrounded shell fragments up to 4 mms.in size. Quartz particles range from 0.20 mm. to1.5 mms. in grain-size, a few are subangular but themajority are subrounded. Almost all shell materialfalls in the 2 mms.-0.5 mms. size range. Sand fromthe top of the beach dunes (114) contains quartzgrains (90%), shell materials (7%), small rockfragments (3%) and some heavy minerals. Thissample shows a decrease of 0.12 mms. in mediumgrain-size compared with that at the sea shore,is slightly better sorted and almost all the quartzgrains are subrounded at least, showing the effectof some wind transport. The quartz particles areof moderate to high sphericity and the heavy min­erals include zircon and ilmenite with a few smallgrains of rutile, magnetite, ? zoisite and sphene.No micas were found in this sample, another factorwhich indicates aeolian transport.

The inland dunes, which have suffered extensivewind transport, are characterised by excellent sort­ing (So = 1.2-1.3), a slight predominance of thefi:.~er admixture (Sk = 1.7) and the modal class is0.25-0.125 mms. These deposits consist almost

GEOLOGY

Black sandy soil24" Light brown sandy soil

Grey sandAlternation grey black sand

area.

entirely of quartz, subrounded to well-rounded inand few heavy minerals. Carbonaceous

-rO'lr'Y'ln"iY\,c< of are also present.distinct difference exists between the dune

on an coast and those on theof and lagoons. Here the

CielPoEnts the areis not as

a beach dune So . A mechanieal CAi.i..i.CAi.i..YO.i.O

at 309025 has bimodal

DAVID C. GREEN 29

A steeply inclined intrustive contact is visible at?~1l97, the Malbina Siltstone and Sandstone is nothighly baked, but the dolerite becomes fined-grainedand well-jointed in the direction of the contact.This contact then swings to the north and acrossthe Rokeby Road at 268196 where the intrusionbecomes a thin dyke. The northern boundary ofthis intrusion is less well defined and the intrusionappears to have rafted up a block of mudstone, asthe mudstone in the quarry at 267197 is closelyjointed and strongly thermally metamorphosed.

The dolerite beneath Mt. Mather is a gentlyshelving sheet; on the shoreline the contact dipsat a low angle under the Grange Mudstone. At MayPoint, 353162. the Grange Mudstone is underlain bydolef'ite which rises in a series of steps to a con­formable contact at the top of the intrusion.

Clarence Hills are composed of dolerite whichintrudes Permian sediments and dips steeply to thewest. The contact is clearly exposed only at aheight of 680 feet at 311187 where it is a normalintrusive contact. Nearer sea level the steep dip ofthe sediments suggests that they have been draggedthe sense of the movement being eastern side up­thrown. This is in the opposite direction to thatwhich would be expected if the dolerite has causedthe drag dip, and -later fault movement along theline of the contact is a possible solution. The in­trusive contact is cut by several Tertiary faults,the dolerite-sediment contact being clearly dis­placed at 310194.

The sill north of Cape Deslacs is concordant withthe Grange Mudstone and dips gently to the south­east, while the dolerite forming Calverts Hill alsoappears to be concordant with the sediments to thenorth of a fault contact, as the well developed sub­horizontal joints have the same dip as that of thesediments.

On the Cape Contrariety Hors,t an intrusiveboundary has a steeper contact and dips underGrange Mudstone at 347051. The northern end ofthis contact (351055) is markedly transgressiveand may be a Jurassic fault boundary. A thintongue of dolerite intrudes the Grange Mudstonenearby.

Joint Patterns.A critical survey of joint patterns in the Malbina

Siltstone and Sandstone was undertaken in anattempt to relate the well-developed jointing tothe regional structure. The orientation of over700 joints was determined in the field and plottedon a rose diagram (Fig. 4aJ.

The most important are the near vertic'al jointsstriking at 250° (Set II), 295° (Set I) and 335° (SetIII), and the same basic joint pattern is recognisedover the entire area although the relative import­ance of individual sets varies from bed to bed.There is little evidence of horizontal displacementalong the joints and joint planes cross pebbles with­out displacing them except for a tensional move­ment of approximately a millimetre in some jointsof set III.

Differences in lithology are responsible for var­iations in the minor maxima but Set I are unde­fleeted in passing through several beds. The jointsof Set I are generally straight, have a maximum

deviation of approximately 10° from the meanat anyone locality and often occur in pairs about9 inches apart. They have the characters of smallshear joints. In exceptional cases this set mayform a zig-zag pattern for a small distance.

A less well developed set of shear joints whichare similar to Set I trend towards 250° (Set II)but show a maximum deviation of 15° from themean. The prominent set of joints which followsthe strike of the Tertiary faults (Set III) showsa considerable deviation from the mean and passesinto feathery joints at the extremities. The surfacesof the joint planes are rough and these joints areparallel or sub-parallel to the strike of the strata.

Although the joints of Set I commonly truncatethe others it has not been possible to determine ageneralised sequence for the whole area. It isassumed that many of the joints are closely con­temporaneous. In some cases Set I and Set IIappear to be related, possibly as a conjugate pairof shear joints. At least some of the joints mustbe attributed to the intrusion of Jurassic dolerite,bu no distinction can be made.

Approximately 150 joints were measured duringfield mapping from the exposures of dolerite ex­amined, the most important being vertical jointswhich trike at 230 0

, 10" and 320 0 and sub-horizontaljoints. The near vertical joints tend to dip to theeast at high angles.

The joints are irregular and of limited lateralextent. However, near intrusive contacts they arecommonly parallel to the contact. Near such con­tacts the joints are often filled with silica, calciteand zeolites and occur as closely spaced plates.

T<here appears to be no correlation between Ter­tiary fault trends and the joints in the dolerite,but the joint pattern is rather similar to thatproduced for the Permian sediments, with one im­portant addition, that of the joints striking at230°.

Geological HistoryThe Grange Mudstone (Upper Artinskian) was

deposited on a stable shelf and intercalations oflimestone and dolomite occur frequently. Towardsthe top of the section clastic material becomes morefrequent and calcarenites and granule conglomer­ates are interbedded with the mudstone. A slighterosional break was followed by the deposition ofthe basal member of the Malbina Siltstone andSandstone as a small-scale turbidite deposit on amildly unstable shelf. The adjacent land mass wasrising either isostatically due to the melting ofan ice cap, or due to epeirogenic movements. Ice­bergs dotted the surface of the sea and depositedtheir load as erratics. The depositional area becamemore stable as member "C" was deposited andtowards the top of this formation conditions becamemore favourable for marine life.

The Risdon Sandstone has formerly been des­cribed as a near-shore deposit, but it is possible thatit marks a period of instability rather than a drasticchange in sea-level. The basin of accumulation ap­pears to have become more shallow and during thedeposition of the Ferntree Mudstone may havebeen virtually deltaic. The adjacent land surface at

30 THE GEOLOGY OF THE SOUTH ARM-SANDFORD AREA, TASMANIA

N N10

10

88

66

44

MALBINA SILTSTONE ANDSANDSTONE

JURASSIC DOLERITE

JOINT FREOUENCY DIAGRAMSa.

Figure 4.b.

this time was of low relief and the climate wasbecoming warmer as erratics are less frequent.Poorly-sorted sandstone bands, which occur atintervals show that periodically the area was slightlyunstable.

The Ferntree Mudstone is separated from thebasal Triassic grits by a possible disconformity. Thequartz sandstones of the Triassic were probablydeposited in lakes in a hot climates. The intrusionof Jurassic dolerite probably brought deposition toa close. The Permian and Triassic sediments wererafted in a sea of dolerite and the country rockswere thermally metamorphosed and silicified. Theintrusion of the dolerite left the area with aconsiderable relief and it is likely that extensiveerosion took place during the remainder of theMesozoic, as it is probably that the topography of

the early Tertiary landscape differed little fromthat of today. The Lower Tertiary conglomerates.sandstones, and clays were deposited at this point,probably in a series of lakes and lagoons.

The Lower Tertiary landscape was strongly dis­turbed by a series of northerly trending tensionalfaults. Explosive volcanic activity took place atcentres which are situated near major Tertiaryfaults and the basalts flooded down the valleys leftas a result of the Tertiary faulting. The basaltsare deeply dissected. During the Pleistocene, ex­tensive sand and gravel deposits accumulated overmuch of the area. Successive drops in sea-level haveremoved some of this material, but much of thearea has been covered by Recent sand, which hasbeen distributed by wind from both the present dayand older shore dunes.

10 8810 88 9

79 80 4 87 10

79 8574 78 10 6

78

Section Along North End

11 60

10 60 609 60 608 59 60 07 59 4 59 76 59 0 59 4

59

4 56 573 54 56

90

Section begins approximately 5' above

Supplementary section at Cape Deslacs.

(To cover missing interval)

90

88

Unit

74 6

6

34 THE GEOLOGY OF THE SOUTH ARM-SANDFORD AREA, TASMANIA

Unit ft. ins. ft. ins. Unit ft. ins. ft. ins.

27 10 - 31 0

50 2 - 53 0

31 0 35

3 13 0 - 15 5

pebbly granule conglomerate-ir­regular bottom contact-matrix ofsand grade-pebbles up to 4"­

abundant moulds of spiriferidswhich are aligned--graded.

coarse-fine pebbly mudstone-fewfossils-infrequent pebbles up to2"-one or two brachiopods at16' 7".

pebbly coarse sandstone-few largepebbles of quartZlite and phylliteup to l'-many spiriferids~p'Oor

preservation.fine sandstone---granule conglomer­

ate at base.basal bed of Malbina Siltstone and

Sandstone--matrix is s'and grade-­pebbles low sphericity, subangular,quartzite, slate and mudstone up to1'-a few fenestellids at top­poorly preserved. moulds of spiri­ferids aligned parallel to bedding­base is a calcirudite-many pebblesof underlying Grange Mudstone.

calcirudite-only of limited lateralextent.

mudstone.caleirudite-granule conglomerate-­

lens.partly covered by boulders, limy

mudstone-many pebbles-ealcar­enite in p-art.

richly fossiliferous mudstone--fene..stellids.

calcirudite, similar to that at GapeDeslaes.

fissile mudstone.sparsely fossiliferous mudstone.coarse mudstone--fossiliferous.fosSiil coquina and calcarenite.fenestellid mudstone-a few pebbles

-appears to be graded-almost acalcarenite at base.

fossiliferous mudstone-faintly sili­cified contains alate spiriferid.

calcirudite-calcarenite, rolled f-ossilfragments.

mudstone, few pebbles or fossils.richly fossiliferous unit, almost a

coquina, contains Fenestella s,pp.,Polypora, Ingelarella, spiriferide,productids, pectens and a smallgastropod-preservation poor-afew quartzite boulders up to fourinches in diameter.

thinly bedded limy mudstone­weathered green to yellow incolour at contact with dolerite.Gently shelving /intrusion, ratherirregular contact but contacteffects not pronounced.

Dolerite

3 3

2 10

2 6'1 10

6 10

6

5 115 64 64 14, 0

20 720 6

10 5

13 6

12 9

?

2 10

2

11

10o

o 0 - 0

5 64 114 14 03 3

10 610 5

6 10

12 9

10 9

2

5

4

32

5 19 6 - 22

4 15 5 - 19 6

12

11

109876

1413

Grange MudStone.

15 10 7 10 9

massively bedded sandstone-a fewfossils (spiriferids) -few largepebbles up to 4", 1 or 2 of granite.

sandstone-few pebbles.mudstone.sandstone.unfossiliferous mudstone.slightly pebbly sandstone.mudstone.sandstone.pebbly sandstone-massively bedded.sandstone-a few pebbles.unfossiliferous mudstone.pebbly conglO'lllerate.fossiliferous coarse mudstone.granule conglomerate.fossiliferous mudstone---spiriferids,

stenoporids and fenestellids.pebbly conglomerate.sandstone-few pebbles-well-sorted.unfossiliferous mudstone.sparsely pebbly sandstone.fenesteIlid mudstone.sparsely pebbly and fossiliferous

mudstone few Stenopm-a and Fene­stella at top.

massively bedded resistant verypebbly granule conglomerate,matrix coarse sand, pebbles up to6" few fossils.

fine mudstone.coarse p'ebbly mudstone-pebbly

base.mudstone.pebbly granule conglomerate.CD/a.rse fossiliferous mudstone--bed­. ding "bout I' thick-spiriferids in

layers, also fenesteIlids, pebbly atbase.

ooarse pebbly fossiliferous mud­stone--conta;ins an alate SPIrl­ferid and Fenestella in finer bands.

fine fenesteIlid mudstone (also Steno­pora spp).

pebbly fine granule conglomerate­angular pebbles.

fossiliferous mudstone--fossils ofCaCO~J fenestellids, stenoporids~spiriferids, IngelaJl'ellar-lngelarellaalways associated with the coarserbands.

pebbly fine granule conglomeratewith sandstone matrix-fossils inlayers-g.p~cimen of IngelareUa insandstone at top of bed-possiblygraded.

mudstone~ occasional pebbles.granule conglomerate.fossiliferous sandstone--contains

pectens~ bryozoans, spiriferidsand an ingelarellid which is morebulbous than the more commonspecies-preservation very poor.

pebbly mudstone-poorly exposed.covered by sand and soil-no

structural break.

41 &

27 102:7 02;6 11

36 7

36

50 249 0

47 246 745 7

80 07& 075 &74 674 071 971 069, 067 064 864 554 363 10&3 7

&2 8&2: 4(ll &&0 059 058 1

85 0

,2J5 623 6

23 622 6

3& 7

3& 0

35

27 026 1125 6

49 04,7 2,

46 74" 741 &

76 075 674 674 071 971 069 3m 064 864 564 363 1063 762 8

62 461 660 059 058 153 0

80 0

76

15

21

11

1098

2019

181716

14

13

12

43

,4241403938373635333231302928

27262524'2()22