Geology & Geophysics of the Koonenberry Belt, Far Western New
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Geology & geophysics of the Koonenberry Belt, far western New South Wales, and eastern Australian correlates: Part II Delamerian Fold-Thrust Belts in eastern Australia Nicholas G. Direen Clv ic-1-0(0-s B.Sc (Hons) (UTas) A thesis submitted in fulfilment of the requirements of the degree of Doctor of Philosophy at the University of Tasmania, Hobart, Australia February 1999 198 PART 2: DELAMERIAN FOLD-THRUST BELTS IN EASTERN AUSTRALIA Chapter 9: Fleurieu "Arc", South Australia 9.1 Adelaide Fold-Thrust Belt, Delamerian type area. Possible correlations with NSW 9.2 Truro Volcanics: structure, stratigraphy, tectonic affinity. Review. 9.3 Petrology & geochemistry 9.4 Relationship to Koonenberty Belt (MA V) 9.5 Kanmantoo Group: stratigraphy, structure, tectonic affinity. Review. 9.6 Sedimentology 9.7 Overall structural and magmatic style of the Adelaide Fold Belt. 199 Chapter 9: Fleurieu "Arc", South Australia 9.1 Adelaide Fold-Thrust Belt, Delamerian type area. Possible correlations with the Koonenberry Fold-Thrust Belt. Part 1 of this study has shown that the Koonenberry Belt-Bancannia Trough region of far north-western New South Wales is a polydeformed fold and thrust belt which underwent major compressive tectonism during the Late Cambrian and again during the Silurian. The earlier of these two deformations has been correlated with the Delamerian Orogeny in South Australia, following the conclusions of Mills (1992) that these deformations are part of larger orogenic events. The Delamerian Orogeny is further correlated with the Tyennan Orogeny of Tasmania (Turner et al., 1998) and Ross Orogeny in Antarctica (Flottmann et al., 1993). The Delamerian Orogeny was originally defined by Thompson (1969) as a Late Cambrian to Early Ordovician deformation, with several discrete "movements". Recent work in the Fleurieu Peninsula and southern Flinders Ranges has challenged many of the entrenched ideas about the structural development (Jenkins, 1990; Jenkins & Sandiford, 1992; Flottmann et al., 1994) and timing (Haines & Flottmann, 1998a) of the Delamerian Orogeny. Haines & Flottmann (1998a) reviewed earlier chronological and geological evidence for the onset and duration of the Delamerian Orogeny. They interpreted the earliest indicator of deformation as the deposition of a foreland-basin red-bed package (the Billy Creek and Minlaton Formations) unconformably overlying the Hawker Group. The latter is considered to be an equivalent of the enigmatic Kanmantoo Group (see 9.5 below). SHRIMP zircon U-Pb dates from a tuff horizon within the red-bed package indicate an age of deposition of 522.8 ± 1.8 Ma. (ibid.). The earliest assumed syntectonic intrusive recorded is a deformed granite from Vivonne Bay, Kangaroo Island, Rb-Sr dated at 523 ± 6 Ma (Preiss, 1995), but a more widely accepted date for the onset of deformation is the 516 ± 4 Ma SHRIMP U-Pb zircon date from the Rathjen Gneiss (Preiss, 1995). The Rathjen Gneiss is interpreted by most workers as a syn-tectonic granite, although recent evidence suggests that the gneiss protolith may have been a pre- or syn-tectonic sill complex or ignimbrite (A Burtt, pers. comm. 1998; Foden et al., unpublished data). The oldest, unarguable syn- tectonic granite is the Willoughby Granite from Kangaroo Island, SHRIMP U-Pb zircon dated at 508 ± 7 Ma (Haines & Flottmann, 1998a). This accords with SHRIMP dates 200 from syn-tectonic mafic dykes averaging 510 ± 2 Ma (Chen & Liu, 1996). The youngest SHRIMP date for a deformed granite is 487 ± 2 Ma from the Summerfield Granodiorite (Sandiford et al., 1992). Thus the Delamerian Orogeny may have commenced as far back as 525 Ma, and was definitely in progress from 508 to 487 Ma. This overlaps with the 497.5 Ma to 492.5 Ma timeframe for Delamerian deformation in the Koonenberry Fold Belt (see Part 1). In addition to apparently synchronous timing of deformation, there seems to be broad tectonostratigraphic equivalence between packages in both belts. Previous workers have proposed lithostratigraphic correlations between the Mt Arrowsmith Volcanics and Truro Volcanics (Crawford et al., 1997), the Kara beds and the Normanville Group, and the Teltawongee Group and the Kanmantoo Group (Mills, 1992). The comparative simplified stratigraphic columns and the proposed correlations for both fold-belts are shown in Figure 9.1.1. The studies below examine the characteristics and tectonic affinities of the proposed correlative packages, and also compares the overall structural and magmatic style of the Adelaide Fold-Thrust Belt with respect to the Koonenberry Fold Belt. 9.2 Truro Volcanics: structure, stratigraphy, tectonic affinity. Review. In 1959 regional mapping by geologists of the South Australian Geological Survey in the Truro area, northern Mt Lofty Ranges (Figure 9.2.1), resulted in the description of some poorly exposed mafic to intermediate volcanics at Dutton. The exposures included altered andesitic and basaltic lavas, tuffs, agglomerates, and interbedded limestones. Subsequent investigators (Forbes & Daily, 1972) described the stratigraphy at the Dutton type section, naming them the Truro Volcanics. In the type section, the Truro Volcanics disconformably overlie calcareous rocks correlated with the Mt Terrible Formation of the Normanville Group on Fleurieu Peninsula, and are intimately associated with marbles and siltstones correlated with the Fork Tree Limestone of the Normanville Group. This association implies a subaqueous environment of eruption for these volcanics. Later investigations in the region described new outcrops of tuffaceous andesitic volcanics interbedded with laminated shales at Accommodation Hill, Sedan Hill and Red Creek (Gatehouse et al., 1991a,b) and elsewhere (Cobb & Farrand, 1984) (Figure 9.5.1). The enclosing sedimentary sequence for these other occurrences has been correlated with the Heatherdale Shale (Gatehouse et al., 1991a), the formation above the Fork Tree Limestone on Fleurieu Peninsula (Abele & McGowran, 1959). 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VVVVVVVVVVVV 0000000 00000000000000000000000000 ' 00 ' SSSSS' VVVV ... 6L % SSESSSSSSS AAAAAAAAAA ,.. 000000 1 " ill - 00000 ■ . 00 0000000 . , .... 000000000000000000000 ... -•' VVVV 11 00 ,.. 0*•** e - - 0000000 . • I. 0000 . _ ■ 000000 -- •••• 0000000000000 000000000000000 IIII 71 ^ VVVVVVVVVV ,.. 0 - SSESS 0000 . , 0 N "" ‘ I-I- 0 , * 11 VVV " r 0 ,.. 0000 - — 0000000 CX:CO A 1 0000000 \ . , - N 1'1 " frill VVVV SS ...• / 00 /N 0000 r \ ■ 1 00 1 00 \ 1 0 0 . fill Figure 9.2.1 Locality Map. Truro • cbco • Mildura cco 4_rt • Peebinga ADELAIDE e 41/4- c:is Fleurieu Peninsula MELBOURNE • King a Island Smithton Trough 0 500 km raLaLs 201 mafic volcanics at nearby Mame River (Mills, 1973) were found to be rift tholeiites similar to those found in the Smithton Trough in northwestern Tasmania (Van der Stelt, 1990; and see Chapter 11). The first geochemical analysis of the Truro Volcanics was undertaken by Van der SteIt (1990). This indicated that the Truro Volcanics were transitional tholeiitic to alkaline andesites, trachyandesites, basalts and trachybasalts. Further analyses were undertaken by Gatehouse et al. (1991b), confirming these affinities for samples drilled in Mt Rufus-1, a stratigraphic drillhole in the Karinya Syncline. Mafic volcanics and interbedded carbonates intersected in stratigraphic drillhole Peebinga-1, some 180 km to the east-southeast (Figure 9.2.1) were correlated with the Truro Volcanics (Rankin et al., 1991) on the basis of similar geochemistry and an association with shale and limestone. The mafic-intermediate lavas in the Truro Volc,anics have never been directly dated. Assumed, indirect evidence for their age comes from Early Cambrian archaeocyathan ages for the Fork Tree Limestone and ?middle Early Cambrian trilobites from the Heatherdale Shale, all on Fleurieu Peninsula (Jago et al., 1984). Cooper et al. (1992) dated a felsic tuff layer from the Heatherdale Shale from the Fleurieu Peninsula using the single crystal SHRIMP zircon U-Pb method. They reported an age of 526 ± 4 Ma, which is early-mid Botomian (Early Cambrian) (Shergold, 1995), agreeing with fossil evidence. It is unclear how this tuff came to be correlated with the Truro Volcanics, as no felsic units have been previously reported from the sequence, and Cooper et al. (1992) made no such correlation. This error may have arisen as a result of an oversimplified stratigraphic column in Gatehouse et al. (1992). Veevers et al. (1997) paper also assumes this inaccurate correlation. On the basis of their similar distinctive immobile trace element and isotopic geochemistry, Crawford et al. (1997) postulated a correlation between the Truro Volcanics and the Mount Arrowsmith Volcanics. This gives rise to a conflict in apparent ages of the two units: 587 ± 8 Ma for the MAV (Crawford et al., 1997), 526 ±4 Ma for the Truro Volcanics.