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.)
ABSTRACT PREVIOUS LITERATURE Permian mudstones, sandstones and limestones Johnston (888) referred to fossiliferous rocks are intruded hy Jurassic dolerite in the form of near Pipe Clay Lagoon but the first regional survey gently transgressive sheets. sills and The of the area was that made by Nye (924). Lewis sub-horizontal intrusions are to the (946) mapped the area in more detail; unfortun Grange Mudstone and show a pronounced devel ately, the distribution of rock types on the map opment of coarse-grained dolerite near the top of accompanying his text does not concur with his the sill. Two Tertiary volcanie centres, one a recorded field observations. Hosking and Hueber breccia-filled vent, are situated near Tertiary faults, (1954) referred briefly to the Grange Mudstone and folded ?Lower TerLi3xy limonitic sandstones at May Point, an occurrence which had been pre and clays are recorded. Tertiary tensional faulting viously described by Jukes (847) and Stephens has tilted the strata to the west and caused the (900) . repetition of one stratigraphic horizon, the Malbina Siltstone and Sandstone. Necks, spits and coastal ACKNOWLEDGMENTS dunes are related to older, higher sea levels. The author is indebted to the staff of the Geology Department of the University of Tasmania for the INTRODUCTION considerable assistance and encouragement received during field mapping and especially to Mr. M. R. The area mapped consists of squares 5271, 5270, Banks who kindly read the manuscript. 5371 and 5370, the South Arm, Howrah, Ralphs Bay Acknowledgment is also made to the Director Canal and Clifton Beach squares which form the of Education for permission to publish this report, south-east part of the Hobart Sheet. Access to the which was prepared during the term of an Educa area is readily available from the main South Arm tion Department Studentship. road which is sealed as far as the Clifton Beach turnoff. The lower areas are used for orcharding or sheep grazing while the uncleared higher areas PHYSIOGRAPHY support an open sclerophyll eucalypt forest with a The physiography is controlled by tilted fault low scrub and bracken understory. The rainfall blocks formed early in the Tertiary which strike is a little lower than that of the Hobart area, 18-20 approximately north and dip to the west at angles inches per annum. 0 of up to 15 • The faults bounding these blocks Exposure is poor, except along the coastline where provide planes of weakness along which later excellent sections are available. Mapping was erosion has been directed. carried out by walking boundaries, where this was Topographic relief is low with the exception of practical, and pinpointing the outcrop on aerial the range of hills above Rokeby, which rises to more photographs. The base map was constructed with than 800 feet. South of Ralphs Bay Canal, Mount the aid of a slotted template layout based on a Mather (581 feet) and Mount Augustus (535 feet> limited amount of trigonometrical control. The are more dissected remnants of this range. The compilation should not be regarded as entirely tilted blocks of Permian sediments forming these accurate in the Clifton Beach square as trigon mountains exhibit an almost cuesta-like structure, ometrical control was not available. The geological the westward slope approximating to a dip slope. boundaries were transferred from the aerial photo The present morphology of the area has been graphs to the base map with a rectiplanigraph. determined principally by the post-glacial drowning Grid references are given as 6 figure coordinates of the area and subsequent lowering of the sea level. referred to the State grid system. The map is Since the last glacial phase the coastline has been included herewith as figure 1. uplifted to the extent of 2 or 3 feet above the present Specimens collected during field mapping are H.W.M. and this former shoreline has been named housed in the Geology Department, University of the Milford level by Davies (1959). The peninsula Tasmania, and the specimen numbers referred to is made up of several islands which are joined by in this paper are those of the catalogued specimens deposits related to higher sea levels. The evolution of the collection. of the Ralphs Bay neck has been described by
17 18 THE GEOLOGY OF THE SOUTH ARM-SANDFORD AREA, TASMANIA
Davies (959) and a similar mechanism of form oldest dunes, I.e., those furthest inland and with ation is proposed for the neck connecting the the highest elevation, have almost lost their dune Sandford Peninsula to South Arm. Both erosional like character and only in artificial exposures, and depositional features are present; emerged such as present day sand pits, can cross bedding be shore platforms, abandoned cliffs, raised cobble, recognised. Where the dunes have reached a sand and shelly beaches, beach bordering dunes, degree of stability, a bed of podsolized sandstone stranded sea caves and cave deposits, spits and (dark brown in colour) is commonly developed, and necks, all characteristic of a recently emerged coast may be seen at the base of most sand excavations. line. The sand deposits are not worked below this The Llanherne level (Davies, 1959) forms wave resistant layer which assists materially in the cut platforms and appears to be an erosional feature stabilisation of the dunes. approximately 15 feet above H.W.M. It is possibly Hope Beach and Clifton Beach slope towards the related to the last interglacial stage or a minor sea at angles of up to 10°, the steepness of the oscillation during the Wtirm glaciation. Earlier strand plain being due in par,t to the exposed levels become progressively more dissected with nature of the beaches. Storm waves commonly elevation. Lewis (946) recognised one level at produce "nips" in the poorly consolidated beach 5-15 feet above sea level and another at between sands. Behind the necks a shallow lagoon is formed 50 and 60 feet. Other poorly developed levels are and commonly a salt marsh is found in this position, found at 100 and 180 feet with a better developed e.g. the marshes south of Ralphs Bay Canal. level at 250 feet. Submergence during interglacial Away from the sore platforms and cliffs, only the stages has usually been brief. A deposit of marine massive pebbly sandstones of the lower Malbina sand at Mary Ann Bay contains a Pleistocene Siltstone and Sandstone outcrop at all strongly fauna 60 feet above the present high water mark. to form low cliffs. Sandstone beds in the Ferntree Cliffs up to 150 feet in height and rising from a Formation outcrop as benches, and talus from these wavecut shore platform form most of the eastern beds covers the shaly mudstone beds. In a similar seaboard. Wave action has been concentrated at manner, the upper mudstone member of the Mal right angles to the coast along minor faults and bina Siltstone and Sandstone is often covered with prominent joints, the closely spaced bedding planes scree from the Risdon Sandstone. Inland the and joints of the metamorphosed Grange Mudstone Grange Mudstone outcrops very poorly; one band prevent extensive wave notching. The western sides more resistant than others is the zone of Thamno of the peninsula have mature beaches in the pro pora and Cladochonus 65 feet below the top of the tected bays and shore platforms developed on the formation. Fine cliff exposures of Grange Mudstone Malbina Siltstone and Sandstone along exposed are found at Cape Deslacs, May Point and to the stretches. east of Single Hill. Pipe Clay Lagoon at Cremorne is almost closed Much of the area is covered by an extensive and by ,a well developed, slightly arcuate sand-spit, often deep sandy soil and inland the dolerite is post Milford in age, which has grown southwards wea;thered deeply, especially on Droughty Point, from the Milford level shoreline. Tidal scour is where local run-off after heavy rain produces nar responsible for the preservation of a narrow channel row steep-sided gullies. Terracettes due to minor at the southern extremity. A shingle beach spit soil slip on the steep sides of dolerite hills are 4 feet in height is present on the eastern side of common. More extensive landslips are found in the Gellibrand Point. basalt soil near Cape Contrariety. The Clarence The coastal dunes are the result of a plentiful Rivulet is the only prominent drainage channel. Its supply of sand from the beaches, the absence of course has been largely determined by the eastern obstacles in the sea (thus allowing the wind to edge of the Rokeby basalt, but the stream is now cut blow more strongly than elsewhere) and the absence largely in its own gravels. During the summer the of vegetation in that part of the beach from which rivulet degenerates into a series of stagnant pools. the sand is derived. The sand blown landwards from The run-off in other parts of the area goes directly the frontal dune collects in dunes at right angles to the sea or into a chain of lagoons along the low to the direction of the prevailing winds and thus lying central portion of the Sandford Peninsula. a series of wavelike dunes is formed, separated by relatively flat-floored troughs. The parallel sets of coastal dunes produced in this manner are STRATIGRAPHY clearly demonstrated along Hope Beach, Clifton Permian System Beach, Opossum Bay and Half Moon Bay. Such The Permian System in this area contains four dunes are characterised by an asymmetrical cross formations, Grange Mudstone, Malbina Siltstone section and cross bedding with a steep leeward slope and Sandstone, Risdon Sandstone and Ferntree which corresponds to the angle of rest of the sands. Mudstone. A complete section is not available and In the case of the dunes of Hope Beach, the leeward neither the base of the Grange Mudstone nor the slopes of which are fixed by vegitation, the angle of top of the Ferntree Mudstone is exposed. rest is 40 0. The angle of rest for dry unfixed sand of the same particle size as that in the ridges near Grange Mudstone, Hope Beach is approximately 32°. This formation consists of at least 150 feet of Most of the dunes which have migrated inland richly fossiliferous mudstone, siltstone, limestone are now fossil dunes and contain an abundance of and dolomite, with thin lenses of sandstone and calcified roots and stems. These dunes are now granule conglomerate. It is the uppermost forma perched and deprived of a supply of sand. The tion of the Cascades Group in the Hobart area, and is overlain with a slight every outcrop in this area the Grange Mudstone erosional break by the basal member of the Malbina has been afl'ected by the intrusion of Jurassic Siltstone and Sandstone. dolerite. At the base of the measured section at Cape Metamorphism of the Grange Mudstone. Deslacs (see Brill, 1956) is an impure dolomite, light green to grey in colour which contains thin Where the Grange Mudstone is intruded by doler bands of dark green diopside due to contact meta ite, the contact metamorphic efl'ects are consider morphism. Twenty feet above sea-level granule able. Silicification commonly extends for as much conglomerates and pebbly siltstones predominate as 100 feet from the point of intrusion, and contact and a thin layer of recrystallised calcite sheets is metamorphism is most pronounced in the calcareous commonly found along the bedding planes. The b':)ds. The rock immediately adjacent to the dolerite next 16 feet are composed of coarse-grained sand in the case of a sill, e.g. north of Cape Deslacs is stones interbedded with impure limestones, dolom either fissile and heavily weathered or hardened ~nd ites and pebbly mudstones. Large spiriferids occur very closely jointed. Some specimens from the as rolled fragments, the rocks are poorly-sorted and contact zone contain prisms of pale green wollaston contain pebbles up to two inches in diameter. ite and coarsely recrystallised calcite. At May Massive, fossiliferous limestones and dolomites with Point impure limestones are interbedded with mud mudstone intercalcations occupy the succeeding 24 stones and near the margin of the transgressive feet. Pebbles become increasingly common towards dolerite the argillaceous limestone is dark green in the top of the formation and the remaining 27 colour and recrystallised. feet are composed of pebbly siltstones, granule con Thin section 1094, a dark green hornfels collected glomerates, calcerenites ,and subordinate impure within one foot of the contact, contains 40% of limestones and dolomites. Graded bedding is a recrystallised calcium carbonate, 10% of subangular common feature and fewer fossils are found in quartz fragments, 20% of clay material and 30% this part of the section. of small green granules with high relief, which are A thin section cut from an arenaceous limestone probably diopside. The matrix is an intimate 68 feet above sea level at Cape Deslacs contains mixture of recrystallised calcite and clay flakes 35% of sand grade, subangular to rounded quartz with a few well-rounded grains of zircon and needles particles of up to 4 mms. in diameter. The grains of apatite. Lamellar calcite crystals form veins are separated by irregular patches of recrystallised from 0.1 to 0.5 mms. in width. The rocks retain calcite. A few fragments of prehnite are present, a clastic texture microscopically, although in the but most of the matrix is made up of very fine hand specimen they appear to be recrystallised. calcite, the individual grains being so small that the slide is opaque. Grains of zircon and small laths Malbina Siltstone and Sandstone. of andesine are also present. The Malbina Siltstone and Sandstone formation conSIsts of aL least 280 feet of coarse sandstone, North of Ralphs Bay Canal, on the western shore granule conglomerate, pebbly mudstone and silt of Ralphs Bay, another section of the upper part stone which is correlated with a type section of the Grange Mudstone is exposed. This has been measured by Messrs. M. R. Banks and D. E. Read measured in detail and may be correlated with the (see Banks and Read this volume) during 1959 section at Cape Deslacs. The base of the section near Jarvis Creek in the Claremont square. The is covered by a shingle beach in which fragments Malbina Siltstone and Sandstone underlies the of mudstone containing Cladochonus are present. Risdon Sandstone and overlies the Grange Mud The bottom 30 feet of the section is composed of stone and the formation replaces the undefined alternating pebbly and rather fissile mudstones, term" Woodbridge Glacial Formation". with subordinate thin bands of bryozoal calcarenites and spiriferid calcirudites. This is overlain by 5 The basal member "A" overlies the Grange feet of slightly argillaceous limestone containing Mudstone with an erosional break or diRstem owing abundant Strophalosia jukesi, fenestellids and to current scour. This is especially marked in the stenoporids. As at Cape Deslacs, the limestone basal beds at Cape Deslacs and at 314174. Im underlies sparsely fossiliferous mudstone, with mediately above the base, member" A" contains pebbles becoming increasingly common towards the 5-10 feet of poorly-sorted conglomerate and pebbly base of the Malbina Siltstone and Sandstone, some sandstone grading into a pebbly siltstone. Angular beds having distinct graded bedding. quartzite and granite boulders of up to 15 cms. in On the shoreline of Ralphs Bay beneath Mt. diameter are relatively common and the matrix is Mather a similar section is exposed and a zone of of fine sand to clay grade. The rock is thick-bedded, Cladochonus and Thamnopora is also present at the cream or grey in colour, tough and shows a rhythmic graded bedding which has been described by Brill southern end of the hills behind Rokeby (317178), (956) . 65 feet below the Malbina Siltstone and Sandstone, where it is associated with Schuchertella. The Rock fragments are composed almost entirely of Cladochonus-Thamnopora zone is 68 feet below the quartzite and granite with subordinate angular base of the Malbina Siltstone on Maria Island (pers. fragments of slate, phyllite and quartz muscovite comm. M. R. Banks). Other fossils include Poly schist. Pora woodsi, Fenestella dispersa, Stenopora crinita, In thin section 1082, 80% of angular to sub Strophalosia cj. clarkei and Strophalosia typica. rounded quartz grains with a mean diameter of At 341103, Grange Mudstone is well exposed in a 0.45 mms. are present. Fresh plagioclase frag P.WD. quarry as a fossiliferous mottled limy mud ments are rare. The presence of a bent fragment stone with dolomitic bands. As in the case with of feldspar and quartz showing undulose extinction 20 THE GEOLOGY OF THE SOUTH ARM-SANDFORD AREA, TASMANIA indicates a metamorphic source. The matrix (12%) sorting is poor and lenses of sandstone occur in is composed of silt grade quartz, chlorite, limonite many places. and a small amount of micaceous clay grade The pebbles occur in irregular patches and may material with zircon and rutile as accessory min best be described as "erratics '. Evidence that the erals. Rock fragments, mainly quartz mica schist pebbles have in some cases been dropped into the and micaceous quartzite, constitute 5% of the rock. mud on the sea floor is provided at 306155, where Small granules and powdery aggregates of limonite an elongate pebble with the long axis vertical are concentrated around the edges of quartz grains. has broken fine cross laminations and bent those at Where regrowth has occurred, the original outline the base of the pebble. Similar rocks occur at many is shown as a ring of inclusions of fine ilmenite. places on the peninsula, but correlation can be made The texture is mosaic away from the larger patches onlY on lItholOgIcal similarity unless the strati of matrix. graphic position can be ascertained. Large poorly preserved casts and moulds of At 313069 approximately 15 feet of pebbly sand spiriferids are common, IngelareUa and" Spirijer" stone overlie a pebbly mudstone and are overlain avicula have been identified. A peculiar feature by two feet of very pebbly coarse mudstone contain of both this and the overlying member is that the ing large angular pebbles of quartzite and slate. spiriferids are aligned parallel to the bedding. This massive sandstone unit is recognised in several Pebbles of the underlying Grange Mudstone are localities, but is very variable in character and in confined to the basal conglomerate. thickness. Such exposures are referred to as At 348059 ,the basal member is exposed on top of member "C", but it is not clear that all such a small hill. Here the rock is a poorly-sorted sub exposures can be correlated on the evidence avail greywacke sandstone containing approximately 25% able. of highly angular quartzite fragments in a slightly The top of the section consists of approximately calcareous matrix of sand to silt grade. Similar 75 feet of poorly exposed fissile siltstone with sub exposures are found on the shoreline beneath Mt. ordinate sandstone and limestone bands. Poor Mather and north of Ralphs Bay Canal. exposures of this member are present near the top of Mt. Mather, and at 271185 sparsely fossiliferous Graded bedding, although imperfect, is present in mudstones are exposed which contain poorly pre this member and has been recognised in each of served spiriferids and pelecypods. This exposure the above mentioned localities. A typical sediment is probably close to the top of the formation. The ation unit consists of a poorly-sorted granule con Terrakea zone recorded in the Hobart area benellth glomerate at the base which grades upward within the Risdon Sandstone is present at only one locality, approximately 9 inches into a siltstone which con 345002, where it is poorly exposed. tains less than 5% of angular quartz fragments. There are three horizons in this formation III There are very few fenestellids in this member which pebbly sandstone like the Risdon Sandstone and it is rare to find complete shells of the brachio outcrop. The first is approximately 10 feet above pods. These factors suggest that the depositional the base, the second approximately 60 feet above the area was one of considerable turbulence. base, and the third, approximately 185 feet abOVe The remainder of member "A" consists of the base. The uppermost one is easily mistaken approximately 55 feet of fossiliferous pebbly silt for the Risdon Sandstone if the outcrop is poor. stone alternating with sandstone and subordinate Erratics as much as four feet in width are exposed bands of granule conglomerate. This member at 264165 on the Tranmere shore platform. This commences with a band of granule conglomerate exposure is unusual in that sedimentary rocks are three feet thick resting on an irregular bedding present as erratics in addition to the normal types plane. Ingelarella and" Spirijer " avicula are com from an igneous and metamorphic terrain. Angular mon while Schuchertella, Stenopora crinita, Peru fragments of mudstone, siltstone and sandstone vispira and crinoid stems are present. Fenestellids comprise 40% of the surface area of a rock face and stenoporids as extended mats are almost about 3 feet square. The fragments have straight entirely restricted to the siltstones and mudstones, edges in many cases and appear to have undergone although broken fragments are found in the coarser very little transport. In the same bed, there is bands. Graded beds are present in this member and a concentration of quartzite and granite boulders a small amount of load casting appears to have as much as 30 cms. in diameter, with a rounded taken place at the base of some of the coarser schist fragment 25 cms. x 15 cms. and a boulder beds. of grey granite 100 cms x 80 cms. The largest erratic Member" B" comprises approximately 125 feet is a subrounded boulder of quartz sandstone 120 of massive sandstone and siltstone, cream or white cms. x 80 cms., which contains mica and a small in colour and poorly fossiliferous, which contain amount of feldspar, is veined with quartz and shows fewer pebbles than the underlying member. This faint cross-bedding. member forms most of the shoreline of the penin At the western end of Hope Beach, 267006, a sula and commences with a pebbly sandstone 3 feet massive sandstone 16 feet 6 inches in thickness over thick, which is overlain by massive sandstone beds lies fossiliferous mudstone and pebbly coarse mud with subordinate siltstone layers. The mean grain stones of at least 50 feet in thickness which appear size decreases towards the top of the member where to be similar to the basal part of the Malbina Silt fissile and non-fissile siltstones alternate. Pebbles stone and Sandstone. Two bands, 3 feet 6 inches of quartzite, phyllite and granite as large as four and 14 feet 9 inches beneath the base of the sand inches in diameter are scattered through this stone, are extremely fossiliferous and contain member. The matrix is composed of clay minerals, abundant poorly preserved impressions of a spinose chlorite and quartz, and is of clay grade. The productid. DAVID C. GREEN 21 The basal unit of the massive sandstone is 1 of many of the rocks consists of material from both foot in thickness, is graded and has an irregular the silt and clay grades and is not well-sorted. bottom contact with the underlying fossiliferous When sufficient gravel had accumulated close to mudstone and contains more than 80% of angular the shoreline, where relatively robust spiriferids pebbles of both quartzite and fenestellid mudstone were common, a slight tectonic disturbance or even as much as 15 ems. long. The matrix is of sand a climatic change would be sufficient to spread thr: grade, the rock is poorly sorted and best described gravels ovec the sea floor in the form of a " slurry' as a petromict paraconglomerate (Pettijohn, 1957). or tubidity current. Periodic disturbances of thi, This unit is succeeded by a massive, poorly-sorted nature WOUld be competent to produce the gradeo. pebbly sandstone, which contains pebbly layers at bedding of member" A". A small amount of post intervals of approximately one foot and, at the base, depositional loadcastingtook place before lithi large subangular pebbles of quartzite and granite. fication. The massive sandstone is overlain by a pebbly silt stone containing abundant small calcareous tubes A change in sedimentation conditions is shown about 2 mms. in diameter. It is thought that this by unit "B .. and conditions were unfavourable for member is the basal part of member" C " and that the development of marine life. The large erratics this member varies considerably in both pebble often seen in otherwise well-sorted siltstone are content and thickness. concentrated in patches and the most plausible explanation of this observation involves rafting, Irregular markings at least 35 ems. in length probably by ice. These deposits have none of the and 2 ems. in diameter occur on the undersurface secondary characteristics of large scale turbidite of several beds of fine sandstone above the massive sequence such as abundant slump structures. sandstone at 267006 and on at least one horizon below it. The best example is found on a block The incidence of erratics decreases upwards; which has fallen from the cliffs, and on the bedding member" C .. represents a temporary return to the face more than two hundred separate markings, deposition of a pebbly sandstone but silt deposition both straight and curved, are present. The most continued under conditions increasingly favourable probable explanation is that worm trails left in to life towards the top of the sequence. At this a soft mud were later filled by fine sandstone. The stage the relief of the land surface had become lower markings are apparently continuous with the sand and the supply of anguIar fragments had dimin stone of the overlying bed and show no structures ished, but periodic occurrences of Risdon-type sand which allow identification of the parent organism. stone through the Ferntree Mudstone suggest that A dark grey, coquinoid limestone is exposed ap stability was not completely attained. proximately 160 feet above the base of the formation Risdon Sandstone at 313193. Half the fossil fragments are of productid brachiopods while bryozoans, foraminifera, pelecy The Risdon Sandstone outcrops strongly between pods and productid spines are present. The shell the Malbina Siltstone and Sandstone, which under fragments are resorbed in some cases and are set lies it, and the overlying Ferntree Mudstone. The in a matrix of recrystallised sparry calcite combined formation is approximately 15 feet thick, and is with smaller angu:ar grains of silt to clay grade. best exposed as a bench at the summit of Mt. Accessory minerals include plagioclase, ilmenite, Mather, 317148, and at 313182. muscovite and chlorite. The basal member is a conglomerate, approxim The Malbina Siltstone and Sandstone is a se ately 1 foot thick and poorly exposed. This is quence of interbedded siltstone, pebbly siltstone, overlain by a thick-bedded pebbly sandstone which sandstone, granule conglomerate and rare lime is poorly-sorted and weathers to a dark brown to stone. Many of the sandstones are lithic greywackes greY-black colour. The number of pebbles decreases (Pettijohn 1957), subgreywackes or protoquartzites towards the top of the bed and at the top, the as the labile fraction varies from less than 3% formation is well-sorted sandstone. A thin-section to as much as 30%. Rock fragments are more (1056) from the centre of the formation contains common than feldspars, but the presence of a few 10% of angular quartz and quartzite pebbles with fresh feldspar phenoclasts suggests a degree of a small amount of twinned plagioclase and micro mineralogical immaturity. The finer-grained silt cline. The matrix consists almost entirely of quartz stones, often relatively well-laminated, contain less and small rock fragments in the 0.5-1 mm. size than 3% of phenoclasts which are rarely of greater grade. A few quartz fragments show undulose than sand dimensions; quantitatively, these occupy extinction. Some quartz grains show sutured over half the section and must be considered as the boundaries, forming a patchy mosaic texture. Th" dominant rock type. matrix is cemented by a small amount (1 %) of Almost all the large pebbles found in the Malbina ferruginous clay. Siltstone and Sandstone have come from a terrain Sutured boundaries in a relatively clean sand of metamorphic and acid igneous rocks. The frag stone are probably due to the absence of inter ments have suffered little fiuviatile transport and stitial clay with its cushioning effect during dia the relief of the source area must have been such genesis. There is no suggestion that thermal meta that river transport was at a minimum. morphic recrystallisation has taken place. The depositional area at this time was probably that of a shelf covered by silt. Currents appear to Ferntree Mudstone have had little effect and conditions were favourable This formation is conformably underlain by the for the growth of bryozoans. Deposition appears Risdon Sandstone and underlies the Triassic rocks. to have taken place below wave base, as the matrix 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 approx dark-grey to black in colour and fine-grained. imately 10 feet of mudstone overlying the Risdon The pegmatitic veins exposed in cliff sections may Sandstone. either be concordant with the top of the sill, as Lenses of poorly-sorted sandstone approximately thin transgressive "dykes", or irregular schlieren five feet in thickness are present 20 feet and 100 of coarse-grained dolerite in relatively fine-grained feet above the base. Poorly-sorted siltstone bands material. The concordant veins are remarkably cont,ain up to 25% of angular quartz fragments constant in thickness for several hundred yards with of coarse-silt grade. The marix of the thin an average width of two feet, but thin rapidly at bedded siltstone beds is made up of indeterminate the extremities. The contact between the pegmatite rock fragments, cloudy feldspar and fine quartz veins and the normal dolerite is typically sharp at particles of clay grade with small amounts of the base and a thin band of finer grain size and biotite and muscovite. light colour may occur at the border of the vein. The sandstone bands are of fine to medium sand The dolerite sills of this area differ from the grade, poorly-sorted and pale cream in colour. sills hitherto described (Carey, S.W. are now overlain by a cover of Quaternary sands. ,..; o o Pipe Clay Lagoon Beds ,..; On the southern shore of the entrance to Pipe Clay Lagoon, sandstones and clays are gently folded and interbedded with bands of limonite. The sand o o stone is friable and is light brown to chocolate ;:; brown in colour. Some cross-bedding is present. In section, the limonitic bands show a complexly cren ulated structure. There is some resemblance to con cretionary liesegang rings, but it seems more prob able that intrastratal flowage and interal readjust differs from the otherwise similar Blinking Billy deficient alkalic rocks, the mineralogical compos Point basanite (Spry, 1955) in that it contains no ition of the base depends upon the amount of normative nepheline. silica present. Thus the apparent difference be Since 70-80% of the olivine in the Rokeby basalt tween the Rokeby basalt and the Blinking Billy has been altered to iddingsite, it is appreciably Point basanite may be due to a small difference in oxidised and the high state of oxidation is shown the amount of silica present in the end products of by the presence of haematite in the norm. The crystallisation. association of olivine phenocrysts in a groundmass Small patches of scoriaceous basalt which are of pyroxene, iron ore and a feldspar base is sug similar to the Rokeby basalt occur at 323115, 335115, gestive of a limburgite, a conclusion already drawn 322105 and 333106. They are probably eroded rem by Edwards (1950). A comparison may be made nants of a nearby volcanic centre. with the Hawaiian Alkaline Series of MacDonald (949) and Wager (956), in which basalts oligo Pleistocene Series clase are a middle stage differentiate. Much of the area is covered by Pleistocene gravels, sands and clays, some of which are terrestrial and shoreline deposits, while others are marine deposits TABLE II and represent transgressive phases. Recent sands often obscure these deposits and it is difficult to Blinking Limburgite, define a line of demarkation. In some cases rounded Rokeby Billy Woodend pebbles of basalt occur in the gravels. basalt Pt. Basanite Victoria The Rokeby Rivulet has cut into a poorly-bedded series of impure clays and gravels and approxim SiO. 46.64 Norm. 45.59 45.10 ately one mile to the east, at the head of Ralphs Bay, are small cliffs of poorly-bedded gravels and sands. Al.03 13.22 or 7.78 12.48 12.86 Although these gravels and sands are poorly con solidated, they stand in vertical cliffs of approx Fe.03 9.81 ab 34.58 9.93 5.66 imately 20 feet, and dip to the east at approximately 0 3 • The bounders and pebbles, derived mainly from FeO 4.16 an 3.50 2.84 7.10 Permian sediments, are angular, of low to moder ate sphericity and show little evidence of fiuviatile MgO 7.01 6.73 4.71 8.56 transport. The lower part of this exposure is better sorted and throughout the matrix is of silt CaO 7.33 di ~: 5.80 8.26 9.10 to sand grade. Lenses of sandstone of limited lateral { extent are common. 4.11 fs 5.51 3.99 SimHar gravels are exposed in a stream channel 1.35 13.90 3.16 1.02 crossed by the Roches Beach Road, and along the hy {en Rifle Range Road where a bore passes through 0.52 fs 1.73 1.53 82 feet of gravels before reaching solid rock. Cliff sections on the South Arm Peninsula show a H.O 2.14 rfo 5.74 1.64 1.l0 similar thickness of gravels, sands and clays, 01 ~ which Nye (1924) has correlated with the gravels CO. tr. lfa nil tr. of the Sandford Peninsula. At several points around the shoreline, con TiO. 2.50 mt 6.73 2.10 3.50 solidated cross-bedded sandstones occur. These are best exposed at the southern extremity of Droughty P.O. 1.00 hm 5.12 1.65 1.00 Point 264135 where the sandstone is slightly iron stained, well-sorted and shows festoon current MnO 0.19 ilm 4.71 0.16 0.08 bedding and scouring of the upper parts of each bed. The bottom of each bed is commonly marked Cl tr. ap 2.35 0.16 tr. by a layer of pebbles or small broken shells. These deposits represent old storm beaches and have an S03 nil H.O 2.66 0.09 nil origin similar to those forming the beaches of the present day. Similar deposit are found below ZrO the Recent sand dunes of Opossum and Half Moon Bays. 99.98 99.60 100.06 100.60 Mary Ann Bay Sandstone. The Mary Ann Bay Sandstone is defined as that Analyst:- formation of friable, fossiliferous marine sand W. St. C. Manson (from Edwards, 1950) stone, approximately 40 feet thick and probably Aurousseau, 1926 (from Spry, 1955) Pleistocene in age, which is overlain by Recent W. St. C. Manson (from Edwards, 1950) sands and rests unconformably on an irregular The iron ratio (Wager, 1956-25.5) is low, sug erosion surface of dolerite. The type exposure is gesting that differentiation has not reached an in the cliffs of 246093, Mary Ann Bay. advanced stage, although post-fractional oxidation This is a poorly consolidated friable sandstone, will reduce the ratio to some extent. Since an perched 60 feet above sea-level containing a few alkaline residual liquid is characteristic of silica- large pectinacean shells and a gastropod fauna very 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 03"'o-~,:: Q3=O;:"~ Qf"0'~~~ %.0 oro:::.. .. Sc-"~1l 50=1·88 s",·q·l} S.,= '36 Sk=,'Ql s,~'" s..,=\·03 5.=0'19 '0 MARYANN BAY SANDSTONE PRESENT DA'r BEACH SAND HOPE BEACH SAND FROM TOP OF BEACH DUNES SHELLY SAND FROM RALPHS BAY HOPE BEACH MECHANICAL ANALYSES M=O-'S7 M=O"~9 M =0'16 1 OFo-.n Q,=0'96 0,=0-,'; ~ 60 03=O'',;~, Q3=0:;;, Q3=O''.:",. OF 5"",,'-30 5,,=.·n 5"""" .3 5K=' '0" 5..,..='·07 5.1"""-0' 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, Present day beach sands (1113) contain as much although no determination of genera could he made. as 25% of subrounded shell fragments up to 4 mms. A mechanical analysis show that it is well-sorted in size. Quartz particles range from 0.20 mm. to (So = 1.36) and that there is a slight predominance 1.5 mms. in grain-size, a few are subangular but the of a coarse admixture. Most of the sand grains majority are subrounded. Almost all shell material fall in the size range 0.5-0.125 mms. and they are falls in the 2 mms.-0.5 mms. size range. Sand from subangular to rounded. The fossil fragments are the top of the beach dunes (114) contains quartz broken but not abraded to any extent and form grains (90%), shell materials (7%), small rock most of the coarse admixture. fragments (3%) and some heavy minerals. This sample shows a decrease of 0.12 mms. in medium Recent Series. grain-size compared with that at the sea shore, Almost the entire area with the exception of is slightly better sorted and almost all the quartz Hayes Hills, the hills to the east of Rokeby and grains are subrounded at least, showing the effect Single Hill, is covered by Quaternary alluvium. A of some wind transport. The quartz particles are deep sandy soil covers all that area which lies of moderate to high sphericity and the heavy min below an altitude of approximately 100 feet. Much erals include zircon and ilmenite with a few small of the sand, being swept inland by the prevailing grains of rutile, magnetite, ? zoisite and sphene. winds, is derived from old and present day shore No micas were found in this sample, another factor lines. Most of the dunes are now stabilised by which indicates aeolian transport. vegetation, with the exception of those near the The inland dunes, which have suffered extensive exposed ocean beaches on the south coast. Marsh wind transport, are characterised by excellent sort r:eposits have formed on the protected side of ing (So = 1.2-1.3), a slight predominance of the narrow necks and the saline swampy areas suppJrt fi:.~er admixture (Sk = 1.7) and the modal class is a samphire vegetation. 0.25-0.125 mms. These deposits consist almost GEOLOGY entirely of quartz, subrounded to well-rounded in and 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 the of and lagoons. Here the CielPoEnts the are is not as a beach dune So .A mechanieal CAi.i..i.CAi.i..YO.i.O at 309025 has bimodal Black sandy soil 24" Light brown sandy soil Grey sand Alternation grey black sand area. DAVID C. GREEN 29 A steeply inclined intrustive contact is visible at deviation of approximately 10° from the mean ?~1l97, the Malbina Siltstone and Sandstone is not at anyone locality and often occur in pairs about highly baked, but the dolerite becomes fined-grained 9 inches apart. They have the characters of small and well-jointed in the direction of the contact. shear joints. In exceptional cases this set may This contact then swings to the north and across form a zig-zag pattern for a small distance. the Rokeby Road at 268196 where the intrusion becomes a thin dyke. The northern boundary of A less well developed set of shear joints which this intrusion is less well defined and the intrusion are similar to Set I trend towards 250° (Set II) appears to have rafted up a block of mudstone, as but show a maximum deviation of 15° from the the mudstone in the quarry at 267197 is closely mean. The prominent set of joints which follows jointed and strongly thermally metamorphosed. the strike of the Tertiary faults (Set III) shows a considerable deviation from the mean and passes The dolerite beneath Mt. Mather is a gently into feathery joints at the extremities. The surfaces shelving sheet; on the shoreline the contact dips of the joint planes are rough and these joints are at a low angle under the Grange Mudstone. At May parallel or sub-parallel to the strike of the strata. Point, 353162. the Grange Mudstone is underlain by dolef'ite which rises in a series of steps to a con Although the joints of Set I commonly truncate formable contact at the top of the intrusion. the others it has not been possible to determine a generalised sequence for the whole area. It is Clarence Hills are composed of dolerite which assumed that many of the joints are closely con intrudes Permian sediments and dips steeply to the temporaneous. In some cases Set I and Set II west. The contact is clearly exposed only at a appear to be related, possibly as a conjugate pair height of 680 feet at 311187 where it is a normal of shear joints. At least some of the joints must intrusive contact. Nearer sea level the steep dip of be attributed to the intrusion of Jurassic dolerite, the sediments suggests that they have been dragged bu no distinction can be made. the sense of the movement being eastern side up thrown. This is in the opposite direction to that Approximately 150 joints were measured during which would be expected if the dolerite has caused field mapping from the exposures of dolerite ex amined, the most important being vertical joints the drag dip, and -later fault movement along the 0 0 line of the contact is a possible solution. The in which trike at 230 , 10" and 320 and sub-horizontal trusive contact is cut by several Tertiary faults, joints. The near vertical joints tend to dip to the the dolerite-sediment contact being clearly dis east at high angles. placed at 310194. The joints are irregular and of limited lateral The sill north of Cape Deslacs is concordant with extent. However, near intrusive contacts they are the Grange Mudstone and dips gently to the south commonly parallel to the contact. Near such con east, while the dolerite forming Calverts Hill also tacts the joints are often filled with silica, calcite appears to be concordant with the sediments to the and zeolites and occur as closely spaced plates. north of a fault contact, as the well developed sub T N N 10 10 8 8 6 6 4 4 JURASSIC DOLERITE MALBINA SILTSTONE AND SANDSTONE JOINT FREOUENCY DIAGRAMS a. b. Figure 4. this time was of low relief and the climate was the early Tertiary landscape differed little from becoming warmer as erratics are less frequent. that of today. The Lower Tertiary conglomerates. Poorly-sorted sandstone bands, which occur at sandstones, and clays were deposited at this point, intervals show that periodically the area was slightly probably in a series of lakes and lagoons. unstable. The Lower Tertiary landscape was strongly dis The Ferntree Mudstone is separated from the turbed by a series of northerly trending tensional basal Triassic grits by a possible disconformity. The faults. Explosive volcanic activity took place at quartz sandstones of the Triassic were probably centres which are situated near major Tertiary deposited in lakes in a hot climates. The intrusion faults and the basalts flooded down the valleys left of Jurassic dolerite probably brought deposition to as a result of the Tertiary faulting. The basalts a close. The Permian and Triassic sediments were are deeply dissected. During the Pleistocene, ex rafted in a sea of dolerite and the country rocks tensive sand and gravel deposits accumulated over were thermally metamorphosed and silicified. The much of the area. Successive drops in sea-level have intrusion of the dolerite left the area with a removed some of this material, but much of the considerable relief and it is likely that extensive area has been covered by Recent sand, which has erosion took place during the remainder of the been distributed by wind from both the present day Mesozoic, as it is probably that the topography of and older shore dunes. 10 88 10 88 9 79 80 4 87 10 79 85 74 78 10 6 78 Section Along North End 11 60 10 60 60 9 60 60 Section begins approximately 5' above 8 59 60 0 Supplementary section at Cape Deslacs. 7 59 4 59 7 6 59 0 59 4 (To cover missing interval) 90 59 88 90 4 56 57 3 54 56 Unit 7 4 6 6 34 THE GEOLOGY OF THE SOUTH ARM-SANDFORD AREA, TASMANIA Unit ft. ins. ft. ins. Unit ft. ins. ft. ins. 43 80 0 85 0 massively bedded sandstone-a few 5 19 6 - 22 pebbly granule conglomerate-ir fossils (spiriferids)-few large regular bottom contact-matrix of pebbles up to 4", 1 or 2 of granite. sand grade-pebbles up to 4" ,42 76 0 80 0 sandstone-few pebbles. abundant moulds of spiriferids 41 75 6 7& 0 mudstone. which are aligned--graded. 40 74 6 75 & sandstone. 4 15 5 - 19 6 coarse-fine pebbly mudstone-few 39 74 0 74 6 unfossiliferous mudstone. fossils-infrequent pebbles up to 38 71 9 74 0 slightly pebbly sandstone. 2"-one or two brachiopods at 37 71 0 71 9 mudstone. 16' 7". 36 69 3 71 0 sandstone. 3 13 0 - 15 5 pebbly coarse sandstone-few large 35 m 0 69, 0 pebbly sandstone-massively bedded. pebbles of quartZlite and phyllite 33 64 8 67 0 sandstone-a few pebbles. up to l'-many spiriferids~p'Oor 32 64 5 64 8 unfossiliferous mudstone. preservation. 31 64 3 64 5 pebbly conglO'lllerate. 2 12 9 13 6 fine sandstone---granule conglomer 30 63 10 54 3 fossiliferous coarse mudstone. ate at base. 29 63 7 63 10 granule conglomerate. 10 9 12 9 basal bed of Malbina Siltstone and 28 62 8 &3 7 fossiliferous mudstone---spiriferids, Sandstone--matrix is s'and grade- stenoporids and fenestellids. pebbles low sphericity, subangular, 27 62 4 &2 8 pebbly conglomerate. quartzite, slate and mudstone up to 26 61 6 &2: 4 sandstone-few pebbles-well-sorted. 1'-a few fenestellids at top 25 60 0 (ll & unfossiliferous mudstone. poorly preserved. moulds of spiri 24 59 0 &0 0 sparsely pebbly sandstone. ferids aligned parallel to bedding '2() 58 1 59 0 fenesteIlid mudstone. base is a calcirudite-many pebbles 22 53 0 58 1 sparsely pebbly and fossiliferous of underlying Grange Mudstone. mudstone few Stenopm-a and Fene stella at top. 21 50 2 - 53 0 massively bedded resistant very pebbly granule conglomerate, Grange MudStone. matrix coarse sand, pebbles up to 15 10 7 10 9 calcirudite-only of limited lateral 6" few fossils. extent. 20 49 0 50 2 fine mudstone. 14 10 6 20 7 mudstone. 19 4,7 2, 49 0 coarse p'ebbly mudstone-pebbly 13 10 5 20 6 caleirudite-granule conglomerate- base. lens. 18 46 7 47 2 mudstone. 6 10 10 5 partly covered by boulders, limy 17 4" 7 46 7 pebbly granule conglomerate. mudstone-many pebbles-ealcar 16 41 & 45 7 CD/a.rse fossiliferous mudstone--bed enite in p-art. . ding "bout I' thick-spiriferids in 12 6 10 richly fossiliferous mudstone--fene.. layers, also fenesteIlids, pebbly at stellids. base. 11 11 6 calcirudite, similar to that at Gape 15 3& 7 41 & ooarse pebbly fossiliferous mud Deslaes. stone--conta;ins an alate SPIrl 10 5 6 5 11 fissile mudstone. ferid and Fenestella in finer bands. 9 4 11 5 6 sparsely fossiliferous mudstone. 14 3& 0 36 7 fine fenesteIlid mudstone (also Steno 8 4 1 4 6 coarse mudstone--fossiliferous. pora spp). 7 4 0 4 1 fosSiil coquina and calcarenite. 13 35 36 pebbly fine granule conglomerate 6 3 3 4, 0 fenestellid mudstone-a few pebbles angular pebbles. -appears to be graded-almost a 12 31 0 35 fossiliferous mudstone--fossils of calcarenite at base. CaCO~J fenestellids, stenoporids~ 5 2 10 3 3 fossiliferous mudstone-faintly sili spiriferids, IngelaJl'ellar-lngelarella cified contains alate spiriferid. always associated with the coarser 4 2 2 10 calcirudite-calcarenite, rolled f-ossil bands. fragments. 11 27 10 - 31 0 pebbly fine granule conglomerate 3 10 2 6 mudstone, few pebbles or fossils. with sandstone matrix-fossils in 2 o '1 10 richly fossiliferous unit, almost a layers-g.p~cimen of IngelareUa in coquina, contains Fenestella s,pp., sandstone at top of bed-possibly Polypora, Ingelarella, spiriferide, graded. productids, pectens and a small 10 27 0 27 10 mudstone~ occasional pebbles. gastropod-preservation poor-a 9 26 11 2:7 0 granule conglomerate. few quartzite boulders up to four 8 25 6 2;6 11 fossiliferous sandstone--contains inches in diameter. pectens~ bryozoans, spiriferids o 0 - 0 thinly bedded limy mudstone and an ingelarellid which is more weathered green to yellow in bulbous than the more common colour at contact with dolerite. species-preservation very poor. Gently shelving /intrusion, rather 7 23 6 ,2J5 6 pebbly mudstone-poorly exposed. irregular contact but contact 6 22 6 23 6 covered by sand and soil-no effects not pronounced. structural break. ? Dolerite