PAPEHS AND PH,OCEEDTNGS OF THE ROYAL SOC£ETY OF TASMANIA. VOLUME 100 CONSIDERATIONS ON THE EM PLACEMENT OF THE JURASSIC DOLERITES OF TASMANIA
By
F. L. SUTHERLAND (Queen Victoria Museurn, Launceston, Tasmania)
(-with 4 text -(igures.)
ABSTRACT on the margins of the downwarps and on small The emplacement of the Jurassic dolerites of intervening highs. On the upper flanks of the Tasmania is considered in terms of the tectonic main highs, feeders were mainly plug and dyke structure of Tasmania at the time of intrusion. At like plugs. Where basement highs exceeded heights this time Tasmania consisted of a Permo-Triassic of about 4,000 feet above the basement level in the sedimentary cover draped in gentle downwarps over downwarps, dolerite feeders were lacking. an uneven basement of folded Precambrian-Palaeo At the time of the dolerite intrusions Tasmania zoic rocks, with uplifted highs in the west, north was probably situated below the Antarctic Circle, east, and east. just east of the Ross Sea, and comparisons are made with the similar Berrar dolerites of Characteristics required for the recognition of Antarctica. Differences in the emplacement of the dolerite feeders in Tasmania, particularly cones, dolerite appear to be due to structural differences are discussed and the structures are examined in between the two regions at the time of emplacement. the light of these. In form, the cones are generally Only limited comparisons can be made between asymmetrical structures. In many cases there is Jurassic trends associated with the dolerites of the insufficient evidence present to determine whether two regions, but maxima in the trends of Ta,s they represent rootless conical sheets emplaced manian dykes were apparently similarly orientated under a fiotational mechanism, or whether they to those of Antarctica. represent conical feeders. Possible modes of emplacement are considered. Some of the cones INTRODUCTION appear to be secondary structures developed in the sedimentary cover at about the base of the Triassic Tasmania is .extensively intruded by tholeiitic sequence, possibly from cupola-like structures on dolerite (for map see Spry 1962) in the form of underlying sill sheets. dykes, sills, plugs and cones (Carey 1958a; Spry 1962J. The intrusion is Middle Jurassic (167 The intruding dolerite was controlled by, and million years; McDougall 1961). The pattern of also caused, extensive Jurassic faulting. The intrusion is complex, the dolerite spreading out recognisable tectonic pattern is shown. The fault through sub-horizontal Permo-Triassic strata from ing in places suggests the development of horst feeders rising through the folded Precambrian like and graben-like structures in the downwarps Palaeozoic basement. Secondary feeders rise from between the main pre-existing highs. Some of the the dolerite spreading out from the primary major Tertiary faults of the State were originally feeders. The intrusion was associated with exten lines of Jurassic movements, and there is a sug sive normal faulting. Little is known concerning gestion that the Tertiary Midlands, Cressy, Tamar, the factors controlling the sites of intrusion and and possibly the Derwent and Oyster Bay trough the emplacement of the dolerite, and no overall structures were inherited, in part, from Jurassic tectonic pattern has yet been recognised. This structures. It is considered east-north-easterly paper represents a preliminary attempt in this and north-easterly tensional forces were important direction. in producing the fault pattern and that it was con trolled to a certain extent by the margins of the RECOGNITION OF PRIMARY FEEDER§ main pre-existing highs and structural trends in Prima.ry feeders such as dykes, plugs, and the basement. The greatest fracturing took place possrbly cones can be recognised where the Permo in the central trough of the main downwarp Triassic cover has been stripped by erosion, expos between the highs of western and eastern Tasmania. ing the folded basement. Elsewhere other methods Dolerite magma probably rose here from feeder· are necessary to demonstrate downward extension dykes and its emplacement was controlled by com of dolerite structures into the basement. Thus pensation surfaces, giving rise to transgressive sills, McNeil (1965) has suggested that the dolerite cap meeting in numerous large dykes, with considerable ping of St. Patrick's Head represent,s a. plug rather rafting of sedimentary blocks. Away from the main than a sill remnant, on the grounds of the differen downwarp, sill sheets were more extensively tiation of the dolerite. Conversely, McDougall and developed and in the broad scheme their emplace Stott i1961l show from gravity measurements that ment apparently followed compensation paths. the dyke at Red Hill is unlil{ely to continue in depth Cone structures were emplaced, apparently mostly to the basement.
133 134 EMPLACEMENT OF .JURASSIC DOLERfTES OF TASMANIA
Carey ( 1958a) demonstrated a technique for CONE STRUCTURES IN TASMANIA locating possible feeders by means of isostrats. Dyke-like feeders are indicated by parallel closely (Figures 1 and 2 l spaced, successively falling isostrats; plugs and Several cone-like structures have been mapped cones by concentric isostrats falling successively in Tasmania to date and two further possible cases inwards. Isostrat patterns similar to those of a.re proposed here. These structures are examined cones, however, can result from alternative struc in the light of the characteristics listed above. tures. Further, even if a conical structure is demonstrated it may not represent a feeder, but 1. Great Lake Structure (Carey, 1958a; Jones .. merely a basin-shaped sheet emplaced under the 1953; Haigh, 1963; McDougall, 1964). This struc :ftotational mechanism outlined by Bradley (1965). ture shows characteristics 1, 2 (in part), :3, 5, 8 Characteristics required for the recognition of cone and 9. Only small patches of roof rocks remain. like feeders in Tasmania are set out below. If these belong to the Permian System as suggested 1. The dolerite is demonstrably in situ. Dolerite by McDougall (1964) then they also demonstrate ranges in Tasmania are commonly skirted by talus characteristic 4. McKellar 0957) in the Weston deposits resulLing from Cainozoic glacial, peri Greek area notes a tendency towards an arrange glacial, and gravitational action and these, in ment of faults radial from Great Lake. The faults cases, have been mapped as dolerite outcrops are possibly Jurassic in origin (characteristic 6). (s.ee McNeil, 1965; Sutherland, 1965; Jennings, There is thus considerable evidence for a conical 1963, pp. 91-92). structure at Great Lake. The large thickness of the dolerite here (Jones, 1963) suggests it is a 2. The dolerite body is annular in plan with centre of intrusion but this has not yet been inwardly dipping sides and rises from a basal proved conclusively. feeder channel. Annular outcrop may be obscured by the rugged dissection of the Tasmanian land 2. Huon Structure (Carey, 1958al : This struc scape and later Tertiary faulting; while the inward ture shows characteristic 1, 2 On pa.rt), 3, 4, 5, 6 dip of the sides may be obscured by lack of exposure, and 7. Further detailed mapping and geophysical or only become apparent by compensating for surveying is being carried out here by the Geology Tertiary epeirogenic tilting. In many cases the Department, University of Tasmania. These root zone will not be exposed and the presence of investigations tend to confirm the presence of a a feeder channel, or otherwise, cannot be con cone-like feeder (D. E. Leaman and I Naqvi, pers. firmed by direct observation. commJ. 3. Isostrats and structure contours are concentric 3. Esperance structure (Carey 1958a): A small in plan and fail successively inwards towards the conical feeder was proposed here on the basis of centre of the cone. Isostrats, unlike structure characteristics 3 and 5. Further investigations contours, are not greatly affected by the factors are required to establish this. mentioned under 2. This is discussed in detail by 4. Billop Structure (Carey 1958al : The proposal Carey (1958a). of a small intrusive cone at this locality is based on 4. The roof rocks within the cone are uplifted characteristics 2 and 3. Further detailed investi dilationally with respect to the outside rocks. gation is again required. 5. The conical structure may be expressed physio 5. Golden Valley Structure (Carey, 1958a; Suther graphkally as a basin where exhumed by erosion of land, 1965) : Carey suggested the presence of a the softer sedimentary core. small conical feeder here on the basis of charac 6. The intrusion of the cone may be accompanied teristics 2, 3 and 6. Further investigation by by concomitant radial, peripheral and tangential Sutherland indicated the need for more detailed faulting, particularly in the uplifted roof rocks. examination of the structure in regard to charac Strong updragging and shattering due to forcible teristic 1. intrusion of the cone may also occur. 6. Zeehan Structure (Spry, 1958; Blissett, 1962; 7. Rocks within and around the apex of the cone Bradley, 1965) : This shows characteristics 1, 2 may show greater baking effects than is normal (in part), 4 and 8. Characteristic 3 is not with sedimentary-dolerite intrusive contacts. This demonstrable as the Permo-Triassic cover has been is due to the concentrated heating here. Pyrite in mostly removed by erosion, but is in agreement with the original sediments may be converted to pyrr the stratigraphic level of the intrusion of the hotite (Carey 1958a). dolerite to the south-west at Firewood Siding. 8. Differentiation zones of the dolerite will Spry notes that the Permian Tillite outcropping parallel the flare of the cone, passing from the around the cone is more indurated than is normal upper zone along the inward margin, through the but whether this indicates characteristic 7 is not "pegmatitic" and central zones, to the lower zone certain. There is, as yet, insufficient evidence to along the outer margin. This has been demon determine whether the structure represents a strated by Spry (1958), and Sutherland 0964). conical feeder (Carey, 1958a), or a rootless conical There wi.ll be a corresponding variation in density sill sheet ------·--~~~~-----··-~~~-~-·--·----~--~ --·------·------~-·---··------~~------· LEGEND TERTIARY l!IASALT w. E. ------~------~--·--~- 'i.l. !.. •• JURASSIC DOLERITE 4 TRIASSIC SYSTEM r::-_] PERMIAN SYSTEM s.w. FOI_DE D BA SEM E. NT FAULT 136 EMPLAC~~MBjNT OF JURASSIC DOLEHI'TES OF TASMAN lA LEGEND Q- DOLERITE CONE DCLEPIT F[:f i)£R IN aASLMEN...,. f PROi\AlH_[ JL'RASSIC FAUL_T POSSIBLE JURASSiC FAULT U ~ D ~3,500-- 3,500 STflUCTURAL FORM Llrlf: ON BASE OF ~1,000 --- 1,ooo r=oo CAS CADES G R () U P APr>ROXIIvlATF_ PRECA"':BRIAN BASEMENT H!Gi• OZOi (. GRANITE HIGH ;I PRECAMBRIAN RUCTURAl J/ PALAE0Z01 r-~.. . ~ :·. ··. . { N 0 \;} • ~ ' oo ,~ REO:t;:L,:Y Mi~::~r~'M- REGIO~.!AL SU ~lVE.YS EJ PHOi"' 1Nf!:;IH>j;I£TATION .__ j l FIG. 2,--Map showing- Jurassic tectonic features, including distribution of kno\vn dolerite feeders and cone structures, faulting_, and structural highs. 1. Great Lake Structure; 2. Huon Structure; 3. Esperance Strudure; 4, Billop Strueture; ti, Golden Valley Structure; B. Zee-han Structure; 7. Bloomfield Structure; 8. Collins vale Structure; fJ. Patersonia Structure; 10. Cheyne River Strllcture; 11. Mangalore Structure; 12. Little Denison River Structure; _1S. Tiers Fault Line; 14. North-West Bay-Cascades-Dromedary Fault Line; 15. ~Pyennan Geanticl.ine; 16. Roeky Cape Geanticline; _17. Collinsvale Basin Structul'e at convergencp of si.ll bodies (arrows). J F, L SUTHERLAND 137 138 EMPLACEMENT OF JUHASSIC DOLERrms OF TASMANIA 8. Collinsvale Structure (Sutherland, 1964) : McDougall as a possible extension of the Tertiary This structure shows most of the characteristics Bagdad Fault, but he suggested it was probably an of a conical feeder, namely 1, 2 Cln part), 3, 4, 5, intrusive contact due to the presence of sandstones 6 and 8. Limited investigation also showed the metamorphosed to quartzite. McDougall's map structure to be compatible with characteristic 9. however, shows little apparent differential dis In regard to characteristic 2 the north-eastern placement at the western end of the structure. outer margin of the cone S.E. of Abbottsfield is Check. mapping by the writer proved the small area. shown on other mapping (see Banks, 1965) as 11 between the basalt plug and the dolerite margin vertical contact displaced slightly southwards by a about one mile north-west of Goat to be Tertiary downthrow to the north-east. This Ferntree Mudstone and not Triassic as implies a slight outward dip of the dolerite margin. shown by McDougall. The break in rock Careful examination at this place by the writer, is probably due to a fault, concomitant with however, showed the margin to slope slightly dolerite intrusion and down-throwing to the north inward and meet the Tertiary fault a few yards east. This was apparently the line of ascent of the to the south of the contact on the downthrown Tertiary basalt plug, and its presence brings Fern- side. It is perhaps also worthwhile to record a tree Mudstone within the dolerite against further example of characteristic 4, just to the Triassic rocks on the outside. In with south of the mapped area" Here Triassic sand the concomitant faulting there is strong updragging, stones and Permian l",ernt1~ee Mudstone are exposed shattering, and thermal metamorphism noted by underneath the dolerite by a branch of the McDougall along the intrusive contact at Strathalie Mountain River and are diiationally downthrown Hill ( cha.racteristics 6 and 7 l . of the order of 1,000 feet with respect to the rocks A noticeable feature of the Mangalore structure within the cone. is the much thickness of the dolerite to the 9. Patersonia Structure (Longman, et al., 1964) : west and than to the south and east FORM OF THE CONE STRUCTURES emplacement of tonguecJike wedges of dolerite The cone-like structures of Tasmania arc from the magma rising along the feeder fault. generally asymmetrical. Most are elongate in Bradley 0965, fig. 5) illustrates the stress con plan. They commonly 11ave a north-westerly ditions at the apex of such a wedge and shows that orientation, but some have a tendency to an east there will be a tendency to tensional fra.cturing of west orientation (Figure 2J. The Collinsvale the roof rocks in a plane inclined to that of the structure at first sight is elongate north-mtsterly wedge. Thus, with a circular tongue-like wedge but iLs throat east of Glen Lusk has a north there will be a tendency to form. a conical fracture westerly orientatiorL In sections one side Gf the around its margin. Bradley also notes (Zoe. cit., cone is generally steeper than the other and is p. :j6) that in the early stages of intrusion, when commonly almost vertical (Figure 1) . such a wedge is fairly small in area, the magmatic may be raised considerably by the load of These structures present some difficulty in regard over-hanging roof. Such conditions would to interpretation of the mode of emplacement. As favour cone-lik.e extension of the dolerite wedge, has been shown, in many instances. there is m"vU. '------·------FIG. 3.---Dia.grammatie representation of suecessive e·mplacement of dole-rite (black) intruding a]ong a fault, and giving the typical asymmetrical section as shown by sonw Tasmanian cone structures. 181 ANTA RCT! CA I DYKE TREND FIG. 4."-·-Trends of .Jurassic dolerite dykes in Victoria Land, Antarctica (from Gunn and Warren 1962} con1pared with those of Tasmania (Banks 1958). Tasmania is restored to its probable location in the Jurassic. F. L. SUTHl<~RLAND 141 TECTONICS OF THE DOLERITE IN'I'RUSIONS The cone-like structures are situated mostly . A knowledge of the structure of Tasmania prior along the margins of the main uplifted highs of w the intrusion of the dolerite is important in pre-dolerite Tasmania or on smaller highs between understanding the control of its emplacement. these. In some cases these eorresnond to Pre Banks (1962J has suggested epi-Permian uplifting cambrian basement highs. Thus the little Denison, in eastern, north-eastern and western Tasmania. Cheyne River, and Great Lalw Structures are The evidence he gives for this is further supported located along the eastern margin of the Tyennan by the presence of Permian rocks in a conglomerate Geanticline, the Zeehan structure along the eastern bed in the upper beds of the Triassic System (Nye, margin of the Rocky Cape Geanticline and the 1921) . Banks Some, at least, of the major Tertiary faults of the The fault pattern in southern Tasmania is very State were originally lines of Jurassic movement. complex. There appears to be a graben structure This has already been suggested for the Tiers bounded to the west by the North West Bay Pault (Carey, 1958al. There is also evidence that Cascades-Dromedary Fault and occupied by a com the North-West Bay-Cascades-Dromedary Pault plicated horst-like structure extending from Storm line is a pre-dolerite feature. The grounds are: Bay to Grass Tree Hill. Whether the Tertiary (ll the fault line appears to be interrupted and Derwent graben was initially a Jurassic structure displaced by the intrusive dolerite of the Ridgeway is not clear. Some of the Jurassic faulting Mt. Nelson's area; (2) the dolerite of the Red Hill parallels the trends of the main Tertiary faults, Hickman Hill-Picket Hill body appears to rise from some of which, then, may represent movements the North-West Bay Fault, east of which it is along old Jurassie lines. Similarly there is also either absent or at a considerably lower strati the possibility that the Terttary Oyster Bay graben level (McDougall and Stott, 1961); (3) at is, in part, ,Jurassic in origin. the outer margin of the Collinsvale cone There is some suggestion that the pattern of structure is intrusive into the lower part of faulting was controlled to a certain extent by the the Ferntree Mudstone on the western side of the margins of the main pre-existing basement highs fault. but on the eastern side is intrusive into beds and their associated structural trends. The com of the Triassic System, suggesting a pre--intrusive n'-'""",(j fra.cturing in the southern part of the State downthrow to the north-east; and (4) at the north possibly partly due to superposition of differing western end of the Dromedary Fault, the Drome trends in the basement as can be seen to the west dary sill body is instrusive into F'erntree Mudstone where the Palaeozoic trends oppose those of the on the western side of the fault, but on the opposite Precambrian. In the northern part of the State side the same sill body intrudes at least 650 feet these trends generaJly coincide and appear to be above the base of the Triassic GVIcDougall, 1959), matched by the Jurassic trends. again suggesting a pre-intrusive downthrow to the north-east. Bradley (1965) has suggested that The Jurassic tectonic pattern is as yet too incom where dolerite bodies intrude across young faults plete to give a coherent picture of the stress they upstep on the downthrown side by an amount environment but it is suggested north-eas,t to east about seven times that of the throw of the fault. tension was important. Tensional At Chigwell the eastern limb of the Collinsvale direction is compatible with the structure is near vertical and is intruded a.long the of many of the dolerite cone structures Cascades F'ault, upstepping from low down in the and orientations of of the meg;a,dykes. Permian succession on the upthrown side to a con It is also noticeable that the margins of siderable height in the Triassic succession on the the highs and the trends in the base- downthrown side. Applyi.ng dictum, this rocks are co~incident with each other, and a pre-dolerite throw of 350 feet. are normal to these tensional directions, as in a lesser throw is implied this northern Tasmania, there tends to be a corn)SJJ>Ontd- fault line at the north-western end of the paralielism in the faulting dary Pault, and is of the order of 100 feet. appears less than elsewhere. The faulting in places of horst-like and EMPLACEMENT OJF THE DOI,ERITE down warps between the main pre-existing highs. The following of the emplacement of It also that the Tertiary Midlands, and the Tasmanian is presented, arising from are, in part, an inheritance from t;he discussion the pre- dolerite the nature and of In the Tamar Valley Permo-Triassic strata show the dolerite cones and feeders, and the associated regional dips from both sides in towards the faulting. Tamar Hi.ver, which t.o occupy the ;~xis Prior to the emplacement of the dolerite, and as of a downwarp (see ooMNk-'.!~M-''b result of epi-Permian Tasmania consisted of Green, 1959: 1965; and Longman, of a cover of strata draped in 1964!. This downwarp does not continue much between highs in the east, north-east, south of the Tamar River where westerly and between lesser highs. continue to Poatina. There has been under Jurassic forces, and but this structure north-east to east-north .. easterly tension, be explained the in the between the uplifted may be a with of and long dolerite is structures. has on the west side of the Tamar River and out that it is in such downwarps that the to the north-east. ,Just south of Black- fractures first under tension, rather than the has observed steep in the highs. Dolerite magma rose up along these the Triassic(?) strata at the intrusive feeder fractures. Vi/here the margins and struc Similar is also observed at Long tural trends of the basement highs coincided and Point where Green records a dip of 68° to the were normal to the main tensional forces there was north-east. This suggests intrusion along a Jurassic a corresponding parallelism in the faulting. Where fault line downthrowing to the north-east. If this the margins and/ or strudural trends were opposed interpretation is valid then it would appear that the to each other and to the main tensional forces the Tertiary Tamar trough structure may also be partly faulting, and consequently dolerite intrusion, was l:lherited from Jurassic times. more complex. F. L. SUTHERLAND 143 The greatest fracturing probably occurred in the of the asymmetrical cones with one of the limbs central trough of the main downwarp between the emplaced along a fault could represent a fault western and eastern upHfted highs. Dolerite would scarp with locally raised rims. However, it is ascend mainly through long feeder dykes. Com considered that some, and possibly many, of the pensation surfaces (Bradley, 1965) beneath this cones represent dolerite feeders. Assuming this, downwarp would occur as steepening arches with the apparent distribution of the cones can be numerous vertical jumps across young faults. Thus, accounted for as follows, The reduced fracturing dolerite sills rising from feeders and following in the highs compared to that in the downwarped compensation paths would tend to be transgressive. areas resulted in greater resistance to dolerite Splitting of sills would also occur, due to alternative intrusion. In these areas the dolerite would tend paths of intrusion being available as a result of to intrude by conical fracturing where there was divorces between compensation and bedding sur sufficient intrusive pressure to overcome resistance. faces. Thus, rafted blocks would be a .feature. This would be mainly in the thinner parts of the Upstepping across faults would be common giving main highs, Le., the marginal edges, and under rise to numerous large dyke bodies Cmega.dykes). neath the narrow and smaller intervening ridges All this seems to correspond well with the dolerite and highs. The dolerite would exploit potentially outcrop patterns in the Midlands and in parts of favourable sites and weakness in these areas. south~east Tasmania, Possible modes of intrusion of such feeders have In the areas away from the downwarps been discussed previously. Some of the cones are there was tensional fracturing. Old Pre- emplaced in part along faults. Some are asso cambrian-Palaeozoic structures probably played a ciated with structural highs (in cases with part greater part in guiding the ascent of the dolerite emplacement along faults as welD and show con as has been noted at Golden Valley comitant radial and concentric fracturing. It has 1965) and near Takone CR. D. Gee, been pointed out that these highs would offer the pers, In the broad view, compensation greatest potentia.I for relief of upward stresses by surfaces in the Permo-Triassic cover in these areas means of such fracturing (Sutherland, 1964). would tend to be correspondingly smoother and Again, if the structural highs were expressed form flattening troughs, bottoming on the base physiographically, compensation surfaces would ment in areas of maximum uplift. Thus, in Tas also favour cone-like emplacement. The respec mania the sill sheets tend to become correspond tive roles of these and other factors in controlling ingly more extensive away from the central trough cone emplacement is difficult to determine. Thus, of J;he main between the western and the Huon cone is emplaced in a domal structure in eastern horizons proved more the intruded sediments. This dome favourable others for of sills. control of the site of intrusion by a pre These notably at about levcel of the existing structural htgh, or itself have resulted Gr.ange-Malbina Formations in the Permian suc from the intrusion of the dolerite. Even if the cession and ahove the Ross Sandstone in the initial of the cone was not controlled Triassic (see 1949; Carey, compensation surfaces tha,t would exist I951la). extensive dolerite sheet that caps it is probable that compensation influences the CentrsJ Plateau, Derwent and came into play. The Scott Arch (Carey, l958a) Huon areas conform an expression of this, with emplace behaviour with regional com- along a compensation surface com showing exceptions peripheral low surrounding the feeder centres and variations in the On the upper flanks of the main surfaces, dolerite shows a general dolerite feeders intruded as plugs and transgression as it extends westward across the plugs, Here the intrusive pressure of the flank of the main pre-dolerite high in was appa.r.ently insufficient to overcome the greater w.,o,.,,.,., Tasmania. the dolerite descends resistance to intrude as cone-like structures. from the Triassic into the Permian Dolerite from these and from the succession at about between the 2,000 and cone feeders on lower flanks foot structural form lines on t;he base of with the dolerite rising Permian Cascades (Banks, !9G2). It from the more fractured descends further and on basement rocks at At Collinsvale t;hel"e is some about the 3,000 to 3,500 foot structural form lines. cone was intruded slightly later Dolerite cones were apparently, from the surrounding areas. Dolerite sills ri.se mainly the toward the cone from the north-west, west, and either on lower south-east. These sills on this snot or on lesser intervening ridges highs. to have up the before them complete for this cannot be given at the Gollinsvale structure (see Figure 2). present due uncertainties as to thA mode of On the north-east side of this basin structure small emplacement of many of these structures. If slivers of Permian rocks near Abbotsfield Csee Banks, they mostly represent rootless emplacements of 1965), in association with the Jurassic Glen Lusk dolerite along conical compensation surfaces, then and Hull Faults (Sutherland, 1964) outli11o a small they express local domes and ridges in the pre down faulted bloclc This block occurs where the dolerite physiography (if in the Permo-Triassic strata would remain unlifted by the wedging action cover), or depressions (if lying within the folded of the approaching sill bodies, 'The action of such basement). Such physiographic features, in cases, wedging is illustrated by Bradley 0965, figure 4). probably were structurally controlled. Thus some These faults and the adjacent basin structure have 144 EMPLACEMENT OF JURASSIC DOLERBTES OF TASMANIA controlled the Collinsvale intrusion and thus pre relief of Tasmania at the time of dolerite intrusion. sumably antedate it. The Collinsvale structure is Compensation surfaces would be much flatter and considered to have resulted from dolerite ma;rma more continuous, resulting ln the much greater intruding up along the Cascades Fault at this spot, extent of sill emplacement. The lack in contrast. or from beneath the anticlinal ridge that flanks between downwarp fracturing on one hand, and the Fault just to the west. redueed fracturing in highs on the other .hand, Secondary feeder cones probably developed on would result in the lack of the Midlands type underlying sill bodies, as exemplified by the Bloom intrusion and the cone-like intrusions respectively. field structure and possibly the Red Hill structure. The development of secondary cone sheet structures These were apparently derived from laecolithic at higher levels in Tasmania seems related to the structures on the sills. Their emplacement may presence of epi-Permian warping in the sedimentary have been controlled by the nature of the Permo cove1·. In areas where the basement relief became Triassic boundary and/or by conical compensation excessive in Tasmania, dolerite was not intruded. surfaces that would exist complementary to the This may explain the scarcity of dolerite intrusions laccolithic doming. on the mainland of Eastern Australia, which was more strongly affected by the Hunter-Bowen Where the basement in the main highs exceeded Orogeny (Osborne 1950) than Tasmania, heights of about 4,000 feet above its level in the downwarped areas, dolerite, apparently, was not Trends associated with Jurassic faulting and intruded through the basement to any extent, and dolerite intrusion in Tasmania have been analysed there is a lack of feeders in these areas. This, by Banks (1958) but lack of information regarding possibly, may be related to the lesser tendency to those of Antarctica prevents a complete com tensional fracturing in these areas, to insufficient parison. However trends of dykes in Victoria Land intrusive pressure of dolerite to allow penetration, are reported by Gunn and Warren 0962). These are mostly north-east-northerly to easterly and and to lowering of ~sogeotherms beneath the thicker parts of the basement highs. in places, as in Tasmania, the emplacement of the dykes was guided by weaknesses in the basement COMPARISON WITH THE DOLERITES OF rocks. The trends show some differences with the ANTARCTICA trends of maxima. for dykes at 348-351 o, go, 25°, and 38-41 o in Tasmania given by Banks (Zoe. cit.). Early Mesozoic palaeogeographic reconstructions However, if Tasmania is restored to its probable show Tasmania as part of southern Pangaea or position in Jurassic times, adjacent to the Ross Gondwanaland continents at the time of the dolerite Sea, then the Antarctican and Tasmanian dyke intrusions (Wegener, 1924; du Toit, 1937; King, trends show closely similar orientations (Figure 4). 1958; Carey, 1958, &c.). The Tasmanian dolerites chemically closely match the Ferrar Dolerites of Antarctica (McDougall, 1962) and reconstruction ACKNOWLEDGMENTS of the southern land mass by Carey (1958) places The author wishes to thank especially Mr. M. R. Tasmania south of the Antarctic Circle, just east Banks, Geology Department, University of Tas of the Ross Sea. This position accords with the mania, for helpful criticism and discussion, and for dating of 167 million years for the Ta,smanian reading the manuscript. dolerites (McDougall, 1961) in relation to datings of Ferrar dolerites which are 150-158 million years at latitudes south of Victoria Valley, 159-163 REFERENCES ANDFJRSON, E. M., 1942.-The Dynamics of Faulting and Dyke, million years at the latitude of Victoria Valley, Formatio-n with Applieations to Britain. Oliver and Boyd, and 175 million years at the latitude of the north Edinlrargh. coast of Victoria Land (Webb and Warren, 1965). ANAND ALWAR, lVI. A., 1960.-Geology and Structures of the Middle Denvent Valley. Pap. Roy. Soc. TastJt., 94, pp .. It is thus of interest to compare the emplacement 13-24. of the Tasmanian dolerites with those of Antarctica. BANKS, M. R., 1958.~-A Comparison of JurasRic and Ter-tiary Trends in 'Ta,smania; pp. 231-264 in Dolerite Symposiu:m, The forms of the Antarctican intrusions have been Univ. Tasm. described by Gunn and Warren 0962), Grindley ------·-, 1962.---Permian in "The Geology of Tasmania"~ ( 196·3) and others. Sills were emplaced from dyl~e J. Geol. So". Aust., 9 ( 2). ------·------, 1965.--Geo~ogical Map of Hobart. Dep. -.Min. and plug feeders as in Tasmania, but were more Tasm. extensive and the sill horizons more numerous. BLISSETT, A. H., 1962.--Explanatory :Report. One-M.ile Geo- No cone structures have yet been described. The logical Map Series--Zeehan. Dep. Nlin. 1,as1n. BnADLEY, J ., 1965.--Intrusion of Major Dolerite Sills. T1'(CnR. difference in mode of emplacement appears to be Ro'J. Soc. N.Z. 3 (4), pp. 27-55. related to structural differences in the two areas at CAREY, S. W., 1958.-A Tectonic Approach to the time of dolerite intrusion. In Antarctica a Drift; pp. 177-355 in Continental Drift .C:nm"'"·';,,., · U niv. Tasrn. cover of essentially horizontal beds of the Beacon -----~~------, 1958a. -The Tsostrat, a New Technique f(n~ Group of the Beaeon~:~field District. Rcc. near et aL, 1965.--Pi-;.wrs Rivel' 1 inch sencs. NYB, P. B._. 1921.----The UndergTound o£ the :Midlands. Undm·o. VVat. Resour. -----. 1922.-The lJndergrounrl of the ~rericho-Riehmond-Bridgewater- Wat. Uesout'. Pap. Tasnt., 2. --·-·--·-, 1924.--T'he Underground \.VH.ter Resources of the of ~.! Richrnond-Bridgewate1·-Sandford District. Under{). VVat. Univ. Resour. Pap. Tasrn., Tasn1. ------, 1924-a..--'T'he Sub-basaltic Tin Deposits of the ,JENNINGS, L B., 19ti3.~Explanatory Report. One-l\'lile Geol H.ingaroon1a Valley. lhtll. G-eol. SuT'V. 1'a,s-rn., :15. logi·:2al Map Series---JYiiddlesex. Dep. Mi--n. 'l'nstn. OSBOH.NE, G. D., 1950.-The Structural Evolution of the Hunter JONES, B. P., 1963.--A Gravity Survey of the Great Lake Th'lanning-Myall Province. Mmn. Roy. Soc. N.S.lV., 1. Area, T'as. linpttb. 1'hcsi.s Uni'v. T1Mnt. SP.RY, A. H., J958.--Some Observations on the Juras8ic Dolel'ite KING, IJ., 1958.- --The Origin and Significance of the G-reat of the Eureka C:one Sheet, near Zeeh.a.n, Tasn1ania; pp. Sub-Oceanic Ridges; PlL £3-102 in Continental Drift 93-129 in Dolerite Swrnposium.• Univ. Tasm. Symposiunt. U ni v. TasnL --~------·-·, 1962.-Igneous Activity in u The Geology of LEWIS, A. N ., 1946.--Geolo.gy of the, Hobart District. Mercury, 'rasinania ''. J. Geol. Soc. Aust., 9, (2). Hobart. SliTHI R.S.-11.