Cenozoic tectonics and landform evolution of the coast and adjacent highlands of southeast New South Wales

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<ul><li><p>Australian Journal of Earth Sciences (2000) 47, 245257</p><p>INTRODUCTION</p><p>The landform evolution of southeastern Australia has long been the subject of much controversy. Many of thecontroversial issues were highlighted in the recent paperby Ollier and Pain (1994) on the geomorphology andtectonics of south eastern Australia, the discussions byBishop (1996), Nott (1996a), Li et al. (1996), and the reply by Ollier and Pain (1996). One of the more importantsources of controversy is the relationship between the coast and inland plateaux around 1000 m above sea-levelnear the Great Dividethe divide between coastal andinland drainage. Remnants of old land surfaces occur bothon the inland plateaux and along the coast. They have deepweathering profiles and are in places overlain by Early to mid-Cenozoic sediments and basalt. The sediments and basalt may also be deeply weathered, and sandy andgravelly sediments are commonly converted to silcrete orferricrete.</p><p>There are at present two main schools of thought on therelationships between the old land surfaces of the coast andthe highlands. Some of the more recent authors believe thatthe differences of elevation are essentially erosional, andthat there has been no significant tectonic displacementbetween the old land surfaces of the highlands and the coastsince they were formed.</p><p>Young and McDougall (1982) concluded that there havebeen no geomorphically significant earth movements in theUlladulla area since the Permian. Nott et al. (1991) con-cluded that the coastal lowland near Merimbula is mid-Cenozoic in age and therefore, uplift of the highlands musthave occurred by that time. Nott and Purvis (1995) acceptedthe conclusions of Young and McDougall and maintainedthat there has been no significant tectonic displacement inthe Mt Dromedary area since the Early Cretaceous. Nott(1996a) and Nott and Purvis (1996) restated many of theearlier arguments of Young and McDougall against relativedownwarp of the coast. Spry et al. (1999) found evidence ofpossible neotectonics in the area between Brooman andMoruya, but also concluded that their observations add fur-ther weight to claims that the coastal lowlands in southeastNew South Wales are at least mid-Oligocene in age. Kohnand Bishop (1999) agreed with this conclusion.</p><p>The main alternative to the idea of tectonic stability asfar back as perhaps the Late Palaeozoic is that the coastalzone (and also the continental shelf) has been downwarpedrelative to the Great Divide since the rifting and separationof the Lord Howe Rise from the eastern margin of Australiain the Late Cretaceous. Ollier (1982) expressed this idea interms of relative upwarping of the Great Divide. In adiscussion of Young and McDougalls paper (Brown 1983),I argued that they had not convincingly ruled out the</p><p>Cenozoic tectonics and landform evolution of the coast and adjacent highlands of southeast New South WalesM. C. BROWN</p><p>51 Debenham Street, Mawson, ACT 2607, Australia.</p><p>This paper has been written firstly to dispel a widely held notion that the structure and stratigraphyof Permian and Triassic sedimentary rocks of the southern Sydney Basin precludes post-Permiantectonic lowering of the coastal zone of southeast New South Wales with respect to the highlands;and secondly to present evidence of Cenozoic tectonic lowering of the coastal zone by gentle tiltingand, locally, by faulting. The apparent horizontal attitude of Triassic sandstone in an eastwest sectioninland from Nowra precludes later faulting or steep monoclinal folding in this immediate area, butdoes not preclude geomorphically significant tilting or gentle warping; and it places no constraintson the tectonic and geomorphic history in areas south of the sandstone outcrop. The form of anOligocene erosion surface around Ulladulla, and the distribution of older rock units below this surfaceare also not incompatible with significant Cenozoic tectonism. Near Ulladulla, Mogo, Moruya, andMerimbula, mid-Cenozoic non-marine sediments and basalts in northsouth-trending palaeovalleysdip gently seaward at angles around 1about the same as the eastward slope of the offshoreseismic basement and also similar to the average slope between the coastal zone and inlandplateaux; indicating that the coastal zone and continental shelf have been downwarped relativeto the inland plateaux in these areas. The area between Brooman, on the Clyde River, and the coastbetween Termeil and Batemans Bay, has been lowered by Cenozoic faulting. The steep slope onthe west side of the Clyde River is a dissected 200300 m fault scarp, and there are two other east-facing fault scarps between the Clyde River and the coast. Fault blocks between the Clyde Riverand the coast are gently backtilted to the west. Further south, there may be more faults and tilt blocksbetween Batemans Bay and Moruya. Tectonic lowering of the coastal zone and continental shelfappears to be mostly post-mid-Cenozoic. This suggests that marine shelf sediments are likely to benot older than Miocene. There could also be older Cenozoic non-marine sediments and perhapsbasalts in palaeovalleys at the base of the section.</p><p>KEY WORDS: Cenozoic, continental shelf, landforms, Oligocene, New South Wales, tectonics</p></li><li><p>246 M. C. Brown</p><p>possibility of relative coastal downwarp of 12. Ollier andPain (1994) restated Olliers (1982) ideas in terms of coastaldownwarp rather than relative uplift of the highlands. Ina discussion (Brown 1996) of Nott and Purvis (1995) Iconcluded that evidence from the topography of the MtDromedary area and the structure of the Cretaceous Mt Dromedary intrusion was consistent with gentle east-erly downwarping of the coastal zone relative to the inlandplateau. Orr (1996) stated also that the coast near MtDromedary may have been downwarped relative to theinland plateau. Bishop and Goldrick (1999) envisaged thepossibility of relative downwarp of parts of the coastalzone but seemed to support a minimal role for tectonics inthe geomorphic evolution of the rest of eastern Australia.</p><p>There have also been suggestions that faulting mayaccount largely for the differences in elevation between oldland surfaces of the highlands and the coastal zone(Andrews 1911; Webb &amp; OSullivan 1996). I agree with Youngand McDougall (1982, 1983) that the structure of thePermian sedimentary rocks rules out the possibility ofsignificant later faulting in the Ulladulla area. However,further south, there is good evidence, discussed later, thatfaulting has caused significant net lowering of an areabetween the Clyde River near Brooman and the coastbetween Termeil and Batemans Bay.</p><p>I believe that the inland plateaux and the presentcoastal lowlands were parts of a continuous low-relief landsurface from the Late Cretaceous to the mid-Cenozoic. Thissurface was probably exhumed from beneath a former thickcover of Early Cretaceous volcanic and volcaniclasticrocks (Brown 1996).The surface was at times differentiallyeroded to form valleys up to several hundred metres deep,which in many cases were later largely infilled withCenozoic basalt and/or sediments. Downwarping of thecoastal zone, accompanied by faulting in places, accountsessentially for the present mean differences in elevationbetween the plateaux and the coastal zone. The tectonic low-ering of the coastal zone possibly commenced in the LateCretaceous or Early Cenozoic, but much of it occurred afterthe mid-Cenozoic sedimentation and basalt flows of thecoastal zone.</p><p>Since my 1983 attempt to refute the conclusions ofYoung and McDougall (1982) asserting a minimal role fortectonics in the Ulladulla area, none of the publicationsasserting an important role for tectonics in the geomorphicevolution of this and adjacent areas have pointed out whatI consider to be fundamental flaws in the arguments forlong-term tectonic stabilityparticularly those based onthe bedrock Permian and Triassic geology. This has led tothe uncritical acceptance of these arguments by some later</p><p>Figure 1 Cenozoic faulting and tilting insoutheast New South Wales. Downthrowsides of faults with well-defined scarps,and monoclines, are marked; approxi-mate vertical Cenozoic displacements(in hundreds of metres) are shown bynumbers. B.R.F., Barneys Range Fault;B.W.F., Berridale Wrench Fault; M.F.,Murrumbidgee Fault; L.G.F., LakeGeorge Fault; S.F., Shoalhaven Fault;C.R.F., Clyde River Fault; C.F., CockwhyFault; Mg.F., Murramarang Fault; P.P.F.,Point Perpendicular Fault. Informationis from published geological maps, Sharp(1994) and the author.</p></li><li><p>Cenozoic tectonics and landforms, NSW 247</p><p>authors, the unwarranted extrapolation of Young andMcDougalls findings into areas hundreds of kilometresaway from Ulladulla, and a lack of attention to details ofthe structure and palaeogeography of Cenozoic basalts andsedimentsthe key to a better understanding of the geo-morphic and tectonic history. If these arguments had beenrefuted earlier, then Nott and Purvis (1996) would have beenless justified in stating To this day the avoidance of thefield evidence supporting an entirely erosional origin of thecoastal lowlands between Ulladulla and Nowra still remainsthe crux of the issue.</p><p>While agreeing with the above statement as it stands Ialso believe that the previous interpretations of this fieldevidence, which assert a minimal role for Cenozoic tec-tonics, are based on fallacious arguments and inaccurateand incomplete observations, and that the field evidenceclearly indicates a substantial role for Cenozoic tectonics.This includes evidence from the latest careful study of mid-Cenozoic basalt and sediments in the BroomanMoruyaarea by Spry et al. (1999), evidence downplayed by theauthors and ignored by Kohn and Bishop (1999) in theircomments on the study.</p><p>In this paper the arguments for long-term tectonicstability and their shortcomings will be discussed, concen-trating particularly on the area of the southern SydneyBasin around Bendalong and Ulladulla. Then evidence forCenozoic coastal downwarping and faulting from thecoastal zone between Bendalong and the Victorian borderwill be considered, concentrating on direct evidence of</p><p>Cenozoic tectonism from the structure, sedimentology,and pre-depositional topography of the mid-Cenozoic sedi-ments and basalts of the coastal zone (evidence which hashitherto been largely ignored). Localities mentioned in thepaper are shown on Figures 1, 2 and 5. Topographic infor-mation is taken from 1:250 000, 1:100 000, and 1:25 000 topo-graphic maps published by the Australian and New SouthWales governments. Geological information, unless other-wise noted, is taken mostly from published 1:250 000 and1:100 000 geological maps of the Australian GeologicalSurvey Organisation and the Geological Survey of NewSouth Wales.</p><p>CASE AGAINST TECTONIC LOWERING OF THECOASTAL ZONE</p><p>Structure of the Permian and Triassic rocks of thesouthern Sydney Basin</p><p>The case for long-term tectonic stability is based largely onobservations of Young (1977) on the gently dipping Triassicand Permian sedimentary rocks of the southern SydneyBasin. Young noted that the top of the Triassic HawkesburySandstone, between Tallong and Nowra, near the coastabout 45 km to the east, has a near constant elevationbetween 600 and 650 m, while the top of the NowraSandstone in the underlying Permian sequence falls from600 m to near sea-level over the same distance (an average</p><p>Figure 2 Aspects of the geology andlandforms of the UlladullaBendalongarea. Palaeozoic bedrock, mostly gentlyeast-dipping Permian sedimentary rocks,is shown blank; Oligocene fluvial sedi-ments are stippled; Oligocene basaltsshown by v symbols. Topography isshown by 100 m and 500 m topographiccontours and selected spot elevations inmetres. Structure contours (heavy lines)are drawn on the unconformity at thebase of the Oligocene sequence. Awayfrom the immediate outcrop of theOligocene basalt and sediments, struc-ture contours are speculative and aredrawn so that the contoured surface isjust above the highest bedrock outcrops.Lines A and B are the lines of section inFigures 6 and 3 respectively. Informationis from published topographic and geo-logical maps, Faul (1984), Johnson (1985)and the author.</p></li><li><p>248 M. C. Brown</p><p>dip of 0.76). From this he concluded that there had beenno significant deformation in the area since the end of thePermian. This conclusion was repeated in the paper byYoung and McDougall (1982) on the Ulladulla area about50 km further south and well south of the outcrop limit ofthe Hawkesbury Sandstone, in which they stated that theevidence is unequivocal. In their reply to my discussionof their paper (Brown 1983), they retreated somewhat fromthis and state that they cleave to the doctrine of Williamof Occam and prefer not to multiply entities unnecessarily(Young &amp; McDougall 1983). However, the idea of tectonicstability since the Permian has been accepted withoutreservation by Nott et al. (1991), Nott and Purvis (1995, 1996)and Nott (1996a), and extrapolated as far south as theMerimbula area, about 200 km south of the outcrop of boththe Permian and Triassic rocks.</p><p>Bishop and Goldrick (1999) noted that the TriassicHawkesbury Sandstone has been gently warped, implyingsome post-Permian tectonism, but agreed with Young thatthere could not have been any Late Mesozoic or Cenozoicdownwarp of the coastal zone in the outcrop area of thesandstone.</p><p>The structure of the rocks of the southern Sydney Basinplaces some constraints on the geological and geomorphichistory. In particular the lack of significant faults or steepmonoclinal warps rules out the possibility of such dis-placements in the Mesozoic or Cenozoic in this immediatearea.</p><p>However the structure of Sydney Basin rocks places noconstraints on the possibility of gentle regional tilting orwarping in the Cenozoic. If this has occurred then it mustof course be reflected in the dips of Sydney Basin strata.For example, if the area had been gently tilted 1 east inthe Cenozoic, then any pre-existing easterly dips must havebeen increased by 1 and any westerly dips reduced by thesame amount; and an observation that some of the Permianor Triassic rocks are now horizontal would imply that theyformerly had a westerly dip of 1. Thus the widely heldnotion that the apparent horizontal attitude of Triassicsandstone in an eastwest section rules out the possibilityof a gentle seaward Cenozoic tilt is not valid unless theMesozoic tectonic history clearly demonstrates Mesozoictectonic stability or otherwise shows that the sandstonecould not have had a gentle westerly dip component at thestart of the Cenozoic.</p><p>Brown (1983) argued along the above lines to show thatYoung and McDougall (1982) had not disproved the possi-bility of gentle Cenozoic seaward tilting in the Ulladullaarea. In reply Young and McDougall (1983) supplied no evi-dence for Mesozoic tectonic stability or any other clearstatement about Mesozoic tectonic history, and stated:Browns suggestion that a Permian sequence that oncedipped to the east and north-east could subsequently bewarped...</p></li></ul>


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