https://orcid.org/0000-0002-9390-3801
Figures & data
Figure 1. (a) Location of eastern Australia in the Terra Australis Orogen in a portion of East Gondwana (after Cawood, Citation2005) and the depositional system (blue) prior to the Silurian deformation along the paleo-Pacific margin of eastern Australia. (b) Schematic reconstruction after the convergence between East and West Gondwana between ca 490 and 460 Ma and the location of major features associated with evolution of the Gondwana super-fan system that produced the deposition along the paleo-Pacific margin (modified after Squire et al., Citation2006). The super-fan system was a result of high rates of erosion in the Transgondwanan Supermountains, which were established from ca 650 and 515 Ma. The present-day areal extent of the Bengal and Nicobar fan systems (Fergusson & Coney, Citation1992) adjacent to the Sumatra subduction zone is shown for scale. (c) Early Paleozoic evolution of southeastern Australia (after Moresi et al., Citation2014). This outlines the development of an orocline in response to the presence of VanDieland and the retreat of a subduction zone. (d) Structural zones of southeastern Lachlan Orogen (after VandenBerg et al., Citation2000).
Figure 2. Ordovician to Carboniferous stratigraphic column and link to tectonic events for the southeast Lachlan Orogen. The Pinnak Sandstone ranges from Early to Middle Ordovician and extends into Bendoc Group and Late Ordovician at Mallacoota (VandenBerg & Stewart, Citation1992). The period 460–445 Ma corresponds to the age range or stages of graptolite evolution (Gi = Gisbornian; Ea = Eastonian; Bo = Bolindian) identified in sediments deposited in the Late Ordovician and overlaps with the Benambran orogenic events identified in central Victoria (Wilson et al., Citation2020) and the D1 events described in this paper. The Devonian stratigraphy is after Young (Citation2007), where the Merimbula Group is composed of the Twofold Bay Formation (dark blue), Bellbird Creek Formation (broken horizontal lines) and Worange Point Formation (pink). The middle Carboniferous mega-kinging is the event identified by Powell (Citation1984).
Figure 3. Portion of the deep seismic reflections acquired in 2018 across the Kuark and Mallacoota zones and boundary with the Deddick Zone along the McLauchlan Creek Fault Zone (modified from data and interpretation in Resources Victoria, Citation2024, March 4) and also showing locations of the major faults mentioned in the text. The location of the seismic line is shown in . (a) Migrated seismic section along the western portion of line 2 and the total data acquired along line 3 (186 A-SL3 2D PSTM). PSTM = PreStack Time Migration. Display shows the vertical scale equal to the horizontal, assuming a crustal velocity of 6000 ms−1, TWT = two-way time. (b) Interpreted cross-section (modified after Resources Victoria, Citation2024, March 4). Blue (Oada) represent Ordovician sequences; mauve (Ox) = possible Macquarie igneous province; green (Cxv) = Cambrian mafic igneous rocks, the base of which must be faults; Cxv stipple = possible Cambrian–Ordovician; magenta = early Silurian – middle Devonian plutons; yellow = Devonian basin-fill (Dme) and volcaniclastic rocks. Legend data modified from Resources Victoria, Citation2024, March 4. The thick imbricated package of Cambrian and Ordovician units related to the accretionary prism abut the MacLauchlan Creek Fault Zone and are juxtaposed against units in the west related to the Macquarie Arc. The inferred subduction zone remnant may be a portion of the Tabberabberan or younger subduction zone.
Table 1. Structural history and deformation style, focussing on Mallacoota with references to other regional examples.
Figure 4. Location of the structures immediately south of Mallacoota township (adapted from Wilson et al., Citation1982). (a) Location map of coastal exposures between Bastion Point and Bruces Creek. (b) A composite section in the region of Geology Point. The eastern portion is dominated by upward facing sandstones with intercalated shale containing south plunging F2 folds. The western portion is dominated by an F1 fold in laminated chert and mudstone, which is faulted and folded by F2 folds with steep axial surfaces (S2). The junction between the two sequences is faulted, and an undeformed porphyry intrusion is located in the northeastern chert outcrop. (c) A section adjacent to the car park at Quarry Beach in a laminated chert-pelagic mudstone succession with a landward or west F1 fold-vergence. (d) Refolded F2 folds in the turbidite sequence with an excellent crenulation cleavage associated with F3 folds. (e) Map of coastal exposures south of Bruces Creek and two cross-sections (A–A′ and B–B′) illustrating a slump unit in the turbidites folded by the south-plunging F2 folds. SL, sea-level. (f) Paleocurrent directions restored to pre-folding orientations in this section of coastal exposures. All stereonet plots of structural data in this and subsequent figures are equal-area lower-hemisphere projections.
Figure 5. Simplified geological map (after Mallacoota 1:250 000 Sheet SJ55-8) showing the location of seismic line 3. (a) Map and cross-sections (b and c) highlighting main rock units and structural trends identified in the Kuark and Mallacoota zones. PCF, Pleasant Creek Fault and together with Combienbar Fault is the location of the inferred boundary between the Kuark (in west) and Mallacoota zones (in east).
Table 2. Characteristics and interpretation of the principal lithofacies from Mallacoota.
Figure 6. Representative photos showing syn-sedimentary and D1 deformation features. (a, b) Ball-and-pillow structures that pre-date deformation at Secret Beach. Scale = 25 cm ruler and is the same in both images. The arrow in (b) indicates a quartz-filled bedding-parallel fault. (c) Syn-sedimentary sandstone dyke (D) in black shale unit north of Shipwreck Creek. (d) Folded early quartz veins, indicated by white arrows parallel to the S2 cleavage. (e) Folded and boudinaged early quartz veins (indicated by white lines I and II) overprinted by later sub-planar extensional veins. (f) Reactivated low-angle normal faults marking shearing in a chert sequence. Hammer scale = 34 cm.
Figure 7. Folding and faulting in chert sequence north of Geology Point. (a, b) South facing section in the chert sequence containing isoclinal F1 folds many of which are refolded by F2 with a north–south-trending sub-vertical S2 axial surface. This chert sequence is thrust eastwards over a package of sandstones. Components of the fault zone include secondary faults, boudins of sandstone and bedding-parallel lenses of massive quartz (<20 cm wide). (c) North facing section illustrating boudinaged chert layers or wedges between several steep-east-dipping quartz-filled faults (F–F) that have a combination of both reverse and normal senses of movement. (d) Tight south-plunging F1 anticline in interbedded chert and pelagic mudstone and wedge of chert between two reverse faults. Scale = 25 cm ruler.
Figure 8. Map showing the spatial variation of fold geometry and non-cylindricity in a dominantly sandstone sequence at Secret Beach. The southwestern outcrops are only accessible at low tide and northeast outcrops are islands. The collapsed tight to isoclinal abutting hinges of the major F2 folds are most likely formed by migration of shale and bedding-parallel thrusts, which induced greater variation of hinge plunge and is accompanied by a localised development of an S3 crenulation cleavage. The limb parallel faults comprise high-angle thrust faults infilled with brecciated quartz accompanied by marked changes in younging directions and are overprinted by oblique sub-vertical quartz-filled strike-slip faults. The stereonets illustrate the variation of the F2 and F3 fold plunges. Contour intervals for bedding (S0) are 0.5, 3, 5 and >10% per 1% area.
Figure 9. Structures observed in the vicinity of Secret Beach. (a) Elongate boudin and F1 folds in quartz-filled early north–south-trending fault reactivated during the D2 folding event. (b) Early D1 quartz veins folded (curved white lines) and brecciated by quartz-filled fracture sets and a related S3 crenulation in the shale unit. (c) Tight F1 fold in turbidites with the sandstone unit containing rip-up clasts of shale. (d) F1 folds offset by low-displacement sinistral faults. Scale = 25 cm ruler.
Figure 10. Trend surface maps in the coastal outcrops near Shipwreck Creek and representative structural data. Inset map shows location. (a) Shows trend in bedding in the folded sediments north (a, b) and south (c) of Shipwreck Creek. The narrow chert units in (c) are all associated with bedding-parallel faults. Contour intervals in (a, c) are 0.5, 3, 5, 7 and >10% per 1% area. Contour intervals in (b) are 1.5, 7 and >10% per 1% area. The location of is also shown in (c).
Figure 11. Representative structures from south of Shipwreck Creek. (a) Limb wedge thrust slices and radial quartz veins on the eastern limb of an F2 fold. The sandstone sequence is thrust over the anticlinal structure, outlined in white in shale unit. Scale = 25 cm ruler. (b) Steep west-dipping thrust between sandstone sequence in the east and a shale-rich sequence with prominent upright axial-planar S2 cleavage. Hammer scale = 34 cm. (c) Early bedding-parallel quartz veins (D1) folded by south-plunging F2 fold with S2 orientation indicated by white line. The fold is crosscut by later northeast–southwest-trending quartz veins. (d) South-southeast-plunging upright F2 folds in alternating beds of sandstone and siltstone warped by F3 folds and crosscut by later east- and west-dipping faults. Y = Younging direction in sandstones from southern area in . Hammer scale = 34 cm. (e) Quartz accumulations in a refolded early fault, between a sandstone and a shale unit showing oblique-slip quartz-rich slickenfibres. Hammer scale = 34 cm.
Figure 12. Fault geometries and structural data from typical sites immediately south of Shipwreck Creek. (a–c) Fault outcrop patterns in localities identified in (adapted from Schapper, Citation1991). Many of the bed-parallel D1 faults have either been folded, reactivated or crosscut by later faults. (d–f) Equal-area, lower-hemisphere stereoplots of D1 faults and lineations from the area displayed in , with early faults having steep-plunging lineations (black dots) and reverse west-over-east movement senses. Folded and reactivated D1 faults have both shallow plunging dextral (red dot) and sinistral (green dot) movement senses. (g) Rose diagram showing average trends of D1 faults identified in area shown in . (h) D3 faults showing dominant northerly trends and dextral movement senses. (i) Late D4 faults with both sinistral and reverse movement senses and a dominant east–west trend. The geometrical data have been replotted from measurements tabulated in Schapper (Citation1991, appendix 3).
Figure 13. Maps of coastal outcrops north of Seal Creek and representative structural data. The positions of cross-sections 1–5 illustrated in are shown and inset shows location of the detailed maps. Chert units occur as isolated lenses or boudins bounded by D1 bedding-parallel faults between the turbidites and are folded by F2. Contour intervals are 0.5, 3, 5, 9 and >12% per 1% area.
Figure 14. Schematic block diagram and cross-sections looking north showing the sequences and section lines identified in . (a) The chert unit in section 2 is part of an extended anticline that is crosscut by a large sinistral fault in the north. Further south in the cherts, reactivated D1 bedding-parallel faults bound blocks with different strain histories. (b) Sections illustrating structural relationships north of Seal Creek to Fold Beach.
Figure 15. Representative structures in the vicinity of Fold Beach. (a) Cross-section of F2 anticline and syncline in thick sandstone with thin intercalated shale beds. Ruler scale = 25 cm. (b) Asymmetric F2 folds in a shale-dominated sequence with thin intercalated sandstone. (c) Enlargement of the area near the scale in (b). Collapsed F2 fold with thickening of shale in hinge and limb thrust in sandstone bed. (d) Longitudinal section through cylindrical F2 fold (width of photo 30 m) with extensional quartz-filled extensional veins sub-perpendicular to fold axis. White box = location of (e), which is enlargement, showing rectangular network of extensional veins parallel and perpendicular to the F2 fold axis. Superimposed on these are later irregular veins (X) associated with off-setting faults. (f) Sinistral offset of 70 cm of bedding and S2 fabrics along a weathered fault zone. The quartz veins are splays of hydrothermally filled sheared joints related to the fault zone.
Figure 16. Maps of coastal outcrops south of Seal Creek to Little Rame Head and representative structural data. The inset map shows location. Contour intervals in (a) are 0.5, 2, 5, 7 and >10% per 1% area. Contour intervals in (b and c) are 1, 3, 5 and >10% per 1% area.
Figure 17. Maps of coastal outcrops south of Little Rame Head and representative structural data. The inset map shows location of (a–c). (c) Trends in bedding south of Shipwreck Creek. Contour intervals in (a, b) are 0.5, 2, 5, 7 and >10% per 1% area. Contour intervals in (c) are 0.5, 2, 7 and >10% per 1% area. (d) Cross-section between X–X′–Xʺ in coastal outcrops illustrated in subarea (c).
Figure 18. Maps between the Benedore River and coastal outcrops of the Sandpatch Point Granite with representative structural data. The inset map shows the location. (a–c) Trend surface maps and distribution of folded sediments with bedding and fold orientation data. Contour intervals in (a) are 0.2, 2, 5, 7 and 10% per 1% area. Contour intervals in (b and c) are 0.5, 2, 5 and 10% per 1% area.
Figure 19. Images showing structures south of the Benadore River. (a) Variations in F2 hinge geometry near Black Cocky Creek with a collapsed syncline sitting on top of the anticline in the foreground. Ruler scale is 25 cm. (b) Tight F2 fold in a laminated shale unit wedged between boudinaged sandstones. (c) East-verging F2 folds in interbedded shales and siltstones. (d) Pervasive quartz-filled tension gashes and en échelon vein arrays normal to F2 fold axes at the south end of Benadore Beach. (e) Early D1 bed-parallel fault with evidence of reactivation and folding of early quartz veins. (f) Reactivated dextral northeast-trending fault with cataclastic material and southerly plunging quartz fibres (white line). This fault obliquely crosscuts bedding that strikes 160°. Scale = 25 cm ruler.
Figure 20. Schematic block diagram illustrating relationships between structural features in the Ordovician sedimentary rocks and the Late Devonian mafic dykes outcropping at the Little Rame Head ().
Figure 21. Fold form surface map and distribution of major faults northwards from the Little Rame Head. The inset shows the location. (a) Map-scale outcrop pattern of F2 folds with a Z-shaped vergence (b) The map-scale pattern of the more prominent faults. (c–e) Rose diagrams from the north, central and southern areas, highlighting the sense of shear and strike-direction associated with the major faults. These indicate there was a near-east–west-trending compression event postdating the regional folding event producing the dominant northwest-trending sinistral faults.
Figure 22. Fold form surface map and major fault pattern from the Benedore River to the Sandpatch Point Granite. The inset shows the location. (a) Outcrop-scale Z-shaped folds that plunge south and are progressively rotated to a north–south orientation, which parallels the contact with the Sandpatch Point Granite. Note the difference in fold style to that in . (b) Map-scale pattern of the more prominent faults. (c) Rose diagram highlighting the sense of shear and strike-direction associated with the major faults. The calculated paleostress was a near-east–west-trending compression postdating the regional folding event.
Figure 23. Representative orientation of foliations, folds, faults and paleostress in areas north of Seal Creek and south of the Benedore River. Superimposed on the equal area lower hemisphere projections are rose diagrams from and . (a) Data from south of Shipwreck Creek, which is typical of areas dominated by D2 structures northwards to Mallacoota. The paleostress related to the north-northeast-trending foliation is oblique to that calculated from the later faults. The folds generally plunge south-southwest, parallel to the flow plane and extension direction defined by the foliation. (b) Data from south of Benedore River where the foliation and F2 folds plunge north–south, and the two east–west paleostress directions coincide, related to the foliation development and the overprinting later faults.
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