Late Ordovician biostratigraphy of the northern RockleyâGulgong Volcanic Belt. Ian G. Percival. ABSTRACT. Previously published age determinations for ...
NEW SOUTH WALES
No 108 February 1999
Late Ordovician biostratigraphy of the northern Rockley–Gulgong Volcanic Belt Ian G. Percival ABSTRACT Previously published age determinations for Ordovician formations in the Sofala–Mudgee– Gulgong–Dunedoo region have, with rare exceptions (a record of late Darriwilian conodonts, and identifications of Bolindian graptolites), supported deposition of these strata largely in the Gisbornian. Revision of a conodont fauna from the Sofala Volcanics has led to recognition of diagnostic late Eastonian species, in agreement with the age of associated corals. Similar conodont and coral faunas from the Tucklan Formation, and from allochthonous limestones within the Burranah Formation, suggest the main episode of Late Ordovician sedimentation in the northern Rockley–Gulgong Volcanic Belt was younger than previously believed. There is a high degree of similarity between these faunas and those from limestones in the basal Malongulli Formation of the Molong Volcanic Belt to the west.
Keywords: Late Ordovician, conodonts, corals, Rockley–Gulgong Volcanic Belt, Sofala Volcanics, Tucklan Formation, Burranah Formation
INTRODUCTION The first indication of the presence of Late Ordovician strata in the northern Rockley–Gulgong Volcanic Belt (figure 1) was provided by the occurrence of poorly preserved graptolites in the lower or middle part of the Sofala Volcanics. These were identified by Packham (1968, 1969) as being comparable with Glyptograptus teretiusculus, which suggested an age at least as old as Gisbornian (early Late Ordovician). Pickett (1978) confirmed his age determination with recognition of conodonts (discussed below), the alga Vermiporella cf V. canadensis, and tabulate coral Plasmoporella sp in limestones within the Sofala Volcanics, supposedly from very near to the top of the formation. A similar macrofossil assemblage was obtained from limestones in the upper part of the Cudgegong Volcanics (now equated with the Sofala Volcanics) by Pickett (1982). The stratigraphic context of this sample was discussed by Pemberton (1989). The underlying ‘Lue beds’, now Adaminaby Group (Colquhoun et al 1997), which represent a deepwater turbidite facies, include chert from which Stewart and Fergusson (1995) identified the conodont Pygodus serra in thin sections. They concluded that the age of this horizon was most likely late Darriwilian (Da3-Da4), or latest Middle Ordovician. Remapping of the Dubbo 1:250 000 map sheet (Morgan et al 1999) by the Geological Survey of New South Wales and the Australian Geological Survey Organisation has resulted in the revision of
ages of some strata previously mapped as Devonian on the first edition of the map (Offenberg et al 1971). Allochthonous limestones in the Burranah Formation yielded Plasmoporella and Vermiporella, the former indicative of the Late Ordovician (Pickett 1995). This paper revises the previous dating of the upper Sofala Volcanics, now recognised as including late Eastonian (middle Late Ordovician) clasts, and documents a conodont fauna of Eastonian age recovered from the Burranah Formation and the Tucklan Formation. A biostratigraphic synthesis incorporating these results is presented (figure 2), with suggested correlations to strata on the Molong Volcanic Belt to the west.
REVISION OF CONODONT FAUNA FROM THE SOFALA VOLCANICS The original determinations of conodonts from a sample of the Sofala Volcanics (Pickett 1978) were made prior to the advent of multielement terminology. Of the elements he illustrated, the form-species Eobelodina occidentalis (Pickett 1978, cover photograph 1 and 2) is (as Pickett surmised) the eobelodiniform element attributable to the same Belodina apparatus to which the grandiform element (Pickett 1978, cover photograph 7 and 8) belongs. Panderodus compressus, illustrated by Pickett (1978, cover photograph 5 and 6) remains a valid species. Of greatest biostratigraphic significance is the occurrence of the late Eastonian index Taoqupognathus tumidus (Pickett 1978, cover photograph fig 4, referred to as gen. unident; see
also figure 3.5 herein), which Zhen and Webby (1995) recognised as suggesting a younger age than was interpreted by Pickett (1978).
CAINOZOIC Alluvium Basalt
Four samples in the Geological Survey Micropalaeontological Collection (C012, C014, C015 and C033) contain residues from the same location (Surface Hill, 6 km southwest of Sofala; Figure 1b) as Pickett’s (1978) illustrated conodonts. All samples were taken from allochthonous limestone blocks within an agglomerate. The last of these residues was prepared about a year after Pickett’s (1978) paper appeared, and contains a more diverse and relatively abundant suite of conodont elements, identified and illustrated below (figure 3).
TRIASSIC Undifferentiated sedimentary rocks 32° 05’
PERMIAN Undifferentiated sedimentary rocks C1459
CARBONIFEROUS Intrusions DEVONIAN Undifferentiated sedimentary rocks Mafic intrusions 32° 10’
SILURIAN Undifferentiated sedimentary rocks ORDOVICIAN
Conodont sample C033
(figure 3.1-3.3, 3.6-3.10)
GR 751958E 6332329N Sofala 1:50 000 map
Sofala Volcanics Town, Village 32° 15’
Roads Limestone clast C1500
Fossil locality (in limestone clast)
Fig 1b 0
5 km 32° 20’
This fauna is unquestionably late Eastonian (Ea3) in age, as indicated by the occurrence of Taoqupognathus tumidus, first described from (and apparently otherwise restricted to) the lower Malongulli Formation in the Cliefden Caves area, south of Orange, by Trotter and Webby (1995). Most of the other genera in sample C033 are also present in the Malongulli Formation.
Wattle Flat 33° 10’
C1500 C1421 C1420
149° 50’ 149° 30’
Conodonta Belodina confluens Sweet, 1979 Belodina sp F Trotter & Webby, 1995 Panderodus sp ?Phragmodus sp Plectodina sp ?Plectodina sp Scabbardella sp Taoqupognathus tumidus Trotter & Webby, 1995 Yaoxianognathus sp
Pickett (1978) based his Gisbornian age determination on identification of Belodina monitorensis Ethington & Schumacher 1969 and correlations with the North American succession in which this species occurs. The element illustrated by Pickett as B. monitorensis has the stout appearance of that species, with a strong costa paralleling the Figure 1. Locality map showing sample sites and distribution of Late Ordovician rock units on the northern Rockley– Gulgong Volcanic Belt, based on Raymond et al (1998) and Morgan et al (1999).
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MOLONG VOLCANIC BELT
ROCKLEY – GULGONG VOLCANIC BELT
Cliefden Caves Limestone Subgroup
Gi2 Gi1 Da4
Tucklan Formation (and allochthonous limestone in Burranah formation)
Yuranigh Limestone Member
Wahringa Limestone Member
g c Adaminaby Group (formerly ‘Lue Beds’) ?
Figure 2. Late Ordovician stratigraphic framework, northern Rockley–Gulgong Volcanic Belt, with suggested correlations to formations on the Molong Volcanic Belt. Note that the left-hand stratigraphic column is composite; the Fairbridge Volcanics is overlain in the northern MVB by the Reedy Creek Limestone (correlative of the Cliefden Caves Limestone Subgroup) and the Cheesemans Creek Formation (Malongulli Formation equivalent), but palaeontological similarity is greater between the Malongulli Formation and strata on the Rockley–Gulgong Volcanic Belt.
anterolateral margin. However, as Zhen and Webby (1995) found with the younger species B. confluens, the local range does not entirely agree with that in North America. Trotter and Webby (1995) discussed and illustrated a species from the lower Malongulli Formation (late Eastonian) which they informally referred to B. sp F, while noting its similarity to the eobelodiniform element of B. monitorensis. The same species occurs in the Sofala Volcanics, as well as in the Burranah Formation samples discussed below. This reassessment of the conodont fauna agrees with age implications of the presence of the coral Plasmoporella at the same horizon in the upper Sofala Volcanics. This coral is representative of Webby’s (1969) coral/stromatoporoid Fauna III, also of late Eastonian age. Hence the Sofala Volcanics (at the Surface Hill locality, at least) appears not to extend into the latest stage (Bolindian) of the Late Ordovician, unless the age of the matrix enclosing the allochthonous limestone clasts is substantially younger than the clasts themselves. This is a possibility, given
that poorly preserved late Bolindian (Bo3) graptolites have been identified by A.H.M. VandenBerg (in Rickards et al 1998) from within the Sofala Volcanics at a locality (GR 756500E 6373600N, Broombee 1:25 000 map) near Windamere Dam, southeast of Mudgee. Unfortunately, the stratigraphic level of this graptolite assemblage is unknown.
NEW MICROFAUNAS FROM THE BURRANAH FORMATION Three samples from the Burranah Formation in the Mudgee region, details of which are given below, produced low yields of conodont elements which had been subject to varying degrees of geothermal heating. However, the presence of age-diagnostic species enabled useful conclusions to be reached. Sample C1420 is from a limestone occurring as allochthonous blocks in volcaniclastic conglomerate. Hence (as with the Sofala Volcanics sample) the age of deposition of the matrix may be either contemporaneous with the age of the conodont fauna, or somewhat younger. The stratigraphic
context of the other samples is not known.
Conodont Sample C1420 (figure 3.20-3.22) GR 739220E 6395930N Mudgee 1:50 000 map Conodonta Belodina confluens Sweet, 1979 Belodina hillae Savage, 1990 Belodina cf B. monitorensis Ethington & Schumacher, 1969 Drepanoistodus sp cf D. suberectus (Branson & Mehl, 1933) Oistodus sp cf O. venustus Stauffer, 1935 Panderodus sp Most species identified in sample C1420 are long-ranging. However, Belodina hillae is only known from the early Eastonian Cliefden Caves Limestone Subgroup in the Molong Volcanic Belt. One very large fragment, referred to Belodina cf B. monitorensis due to its robust appearance, could also be closely related to B. sp F of Trotter and Webby (1995) from the late Eastonian Malongulli Formation.
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18 21 17 20
23 25 26
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Conodont Sample C1421 (figure 3.15-3.17, 3.19) GR 739260E 6397100N Mudgee 1:50 000 map Conodonta Belodina confluens Sweet, 1979 Belodina ?hillae Savage, 1990 Belodina cf B. monitorensis Ethington & Schumacher, 1969 Belodina sp E Trotter & Webby, 1995 Panderodus sp Yaoxianognathus sp nov None of the conodont elements recovered from the above sample is indicative of more than a generalised Late Ordovician age. Belodina cf B. monitorensis could well be conspecific with B. sp E, described from the late Eastonian Malongulli Formation (Trotter & Webby 1995). The fauna is very similar to that from sample C1420, but has been subject to much higher temperatures and greater deformation.
Conodont Sample C1500 (figure 3.11-3.14, 3.18) GR 736270E 6397140N Mudgee 1:50 000 map Conodonta Belodina sp cf B. monitorensis Ethington & Schumacher, 1969 Drepanoistodus sp Panderodus gracilis (Branson & Mehl, 1933) Taoqupognathus sp ?Yaoxianognathus sp
The high Conodont Alteration Index (CAI greater than 5, indicated by the black colour of elements in C1500) is most like the assemblage in C1421, and
suggests a comparable thermal history for these two samples. The presence of Taoqupognathus (in particular the diagnostic Sc3 element, although incomplete) indicates that the age of sample C1500 is Eastonian, based on the occurrence of three species of this genus on the Molong Volcanic Belt (Savage 1990; Trotter & Webby 1995; Zhen & Webby 1995).
NEW MICROFAUNAS FROM THE TUCKLAN FORMATION Three samples from the Tucklan Formation south of the Dunedoo area were analysed for microfossils. Of these, two contained biostratigraphically useful faunas. The aspect of both the conodont assemblages and the associated brachiopod fauna is strongly reminiscent of the Late Ordovician (late Eastonian to early Bolindian) Malongulli Formation of the Molong Volcanic Belt.
Conodont Sample C1459 (figure 3.23-3.29) GR 724681E 6445350N Dunedoo 1:50 000 map Conodonta Belodina confluens Sweet, 1979 Drepanoistodus ?suberectus (Branson & Mehl, 1933) Panderodus sp Taoqupognathus sp Yaoxianognathus cf Y. wrighti Savage, 1990 ? Yaoxianognathus sp Brachiopoda acrotretide pedicle valve (cf ?Hisingerella) ?Orbiculoidea sp
new genus of ?craniopsid The most diagnostic conodont in the above fauna is Taoqupognathus, but unfortunately the sole specimen is embedded in a flake of insoluble matrix which obscures the critical morphological distinction between the older T. philipi (Ea1 age), T. blandus (Ea2 age) and the younger T. tumidus (Ea3 age). Although Yaoxianognathus wrighti is restricted to Ea1 and Ea2 strata on the Molong Volcanic Belt, the genus is now known to range into Ea3. The presence of the new ?craniopsid genus is especially interesting. This undescribed brachiopod was previously known only from the lower part of the Malongulli Formation and its equivalents along the Molong Volcanic Belt (Percival, unpublished data). The age range indicated for sample C1459 is therefore most likely late Eastonian (Ea3).
Conodont Sample C1462 (figure 3.30-3.31) GR 730300E 6425100N Goolma 1:50 000 map Conodonta Taoqupognathus sp Panderodus sp The conodont yield from this sample was meagre and poorly preserved, but fortunately included a single element of Taoqupognathus. Either an early Eastonian (Ea2) or late Eastonian (Ea3) age is indicated for sample C1462, based on T. blandus studied by Zhen and Webby (1995) from the Cliefden Caves Limestone Subgroup of the southern Molong Volcanic Belt, and T. tumidus described by Trotter and Webby (1995) from the overlying Malongulli Formation.
Figure 3. Representative Late Ordovician conodonts from the Sofala Volcanics (3.1-3.10), Burranah Formation (3.11-3.22) and Tucklan Formation (3.23-3.31). 3.1 Taoqupognathus tumidus, Sc3 element, x90; 3.2 Taoqupognathus tumidus, ?Sc element, x60; 3.3 Yaoxianognathus sp, Pa element, x90; 3.4 ?Phragmodus sp, x110; 3.5 Taoqupognathus tumidus, P element, x90; 3.6 Belodina sp F Trotter & Webby 1995, compressiform element, x 50; 3.7 Belodina confluens, compressiform element, x 50; 3.8 Belodina confluens, eobelodiniform element, x90; 3.9 ?Plectodina sp, x90; 3.10 Plectodina sp, Sc element, x110. 3.4-3.5 from conodont sample C0015; all others from conodont sample C0033. 3.11 Belodina cf B. monitorensis, compressiform element, x60; 3.12 Belodina cf B. monitorensis, x90; 3.13 Belodina cf B. monitorensis, eobelodiniform element, x90; 3.14 Panderodus gracilis, x60; 3.15 Yaoxianognathus sp nov, x90; 3.16 Belodina sp E Trotter & Webby 1995, compressiform element, x90; 3.17 Belodina cf B. monitorensis, ?compressiform element, x60; 3.18 ?Yaoxianognathus sp, x45; 3.19 Belodina ?hillae, x60; 3.20 Belodina hillae, compressiform element x60; 3.21 Belodina hillae, grandiform element, x90; 3.22 Belodina cf B. monitorensis, x90. 3.11-3.14 and 3.18 from conodont sample C1500; 3.15-3.17 and 3.19 from conodont sample C1421; 3.20-3.22 from conodont sample C1420. 3.23 ?Yaoxianognathus sp, P element, x60; 3.24 Yaoxianognathus cf Y. wrighti, x60; 3.25 Drepanoistodus ?suberectus, x60; 3.26 Taoqupognathus sp, x80; 3.27 Yaoxianognathus cf Y. wrighti, P element, x60; 3.28 Belodina confluens, grandiform element?, x40; 3.29 Belodina confluens, x60; 3.30 Panderodus sp, x45; 3.31 Taoqupognathus sp, Sc element, x100. Last two specimens from C1462; 3.23-3.29 from C1459.
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Conodont Sample C1467 GR 730800E 6425200N Goolma 1:50 000 map Conodonta 2 indeterminate coniform elements The more complete conodont element in the above sample is laterally compressed and has a deep continuous furrow down its inner side, with a
posteriorly extended heel. There is insufficient material to be certain of the identification — it could either be a species of Panderodus such as P. panderi, or alternatively is referable to Protopanderodus. Both genera range through the Middle and Late Ordovician.
CORAL FAUNAS FROM THE SOFALA VOLCANICS, BURRANAH FORMATION AND TUCKLAN FORMATION A well-preserved example of Plasmoporella from the Sofala Volcanics (same locality as conodont sample C033, listed above) was illustrated by Pickett (1978). Compared to the Plasmoporella illustrated from the Burranah Formation (Pickett 1995), the Sofala Volcanics example has slightly larger cystose dissepiments, but otherwise appears to be identical.
Burranah Formation (identifications by John Pickett) GR 739150E 6396160N Mudgee 1:50 000 map Coelenterata Heliolites sp A ?Nyctopora sp Plasmoporella sp Algae Vermiporella sp
Tucklan Formation 4.2
One locality in the Tucklan Formation, from the same site and horizon as conodont sample C1459 (described previously), also yielded a coral fauna, listed below, and illustrated in figure 4. GR 724681E 6445350N Dunedoo 1:50 000 map Coelenterata Heliolites sp B ?Plasmoporella sp indeterminate ?heliolitid indeterminate solitary rugosan
4.3 Figure 4. Late Ordovician tabulate corals from the Tucklan Formation. 4.1 Heliolites sp B, transverse section, x3.8; 4.2 ?Plasmoporella sp, transverse section, x4.1; 4.3 ?Plasmoporella sp, longitudinal section, x3.8. All from conodont locality C1459.
Whereas in typical Plasmoporella the tabulae are distinctly convex, those in the specimens questionably referred to the genus from the Tucklan Formation are only very gently domed to flat or gently concave; occasionally the tabulae are axially depressed. In this respect the species is similar to Propora. The tabularia are narrower and more widely spaced than in Plasmoporella from the Sofala Volcanics and the Burranah Formation.
There are also differences in the Heliolites in the Burranah Formation, distinguishing it from that in the Tucklan Formation. The latter, informally referred to Heliolites sp B, has more crowded tabulae, and closer transverse diaphragms crossing the tubuli, than does H. sp A from the Burranah Formation (although only a single longitudinal section is available of this form). Despite slight differences noted in the coral species, there is a general similarity in faunas (and floras) from the three formations. The presence of Plasmoporella suggests correlation with Fauna II (Ea2 age) or Fauna III (Ea3) of the Molong Volcanic Belt coral/stromatoporoid biozonation. Heliolites appeared late in Fauna I (Ea1), then diversified and became prominent in Fauna II (Webby & Kruse 1984). Hence a mid-Eastonian age is suggested by the coral fauna examined from the Sofala Volcanics, Burranah Formation and Tucklan Formation.
SYNTHESIS — REVISED LATE ORDOVICIAN BIOSTRATIGRAPHY OF THE NORTHERN ROCKLEY– GULGONG VOLCANIC BELT With reassessment of the age of the Cliefden Caves Limestone Subgroup to Eastonian rather than Gisbornian (Zhen & Webby 1995), similarities in coral and conodont faunas from the northern Rockley–Gulgong Volcanic Belt imply a comparable revision. There was widespread sedimentation in middle to late Eastonian time over much of the area from Sofala to Dunedoo, including the Tucklan Formation to the northwest, the Burranah Formation in the vicinity of Gulgong–Mudgee, and the Sofala Volcanics to the south (figure 1). It is likely that these formations are lateral equivalents, at least in part (figure 2). The unfossiliferous Coomber Formation in the Botobolar–Lue district east of Mudgee has been correlated with the Sofala Volcanics on the basis of similar radiometric signatures (Fergusson & Colquhoun 1996) and may also be of comparable age. Based on the presence of a faunal association directly comparable with that in limestone clasts in the basal Malongulli Formation (Molong Volcanic Belt), which were initially deposited on the slopes of volcanic islands below the shelf edge, limestones in the Tucklan Formation were also probably deposited in relatively deep water. This Quarterly Notes 108, 1999
faunal similarity between deeper water deposits on the Molong Volcanic Belt and Rockley–Gulgong Volcanic Belt supports the contention of Fergusson and Colquhoun (1996) that the source area for the Late Ordovician sediments of the Sofala Volcanics and Coomber Formation (and presumably their correlatives) lay to the west — either the Molong Volcanic Belt itself, or intervening volcanic islands (now covered or eroded). Limestones in the Burranah Formation and Sofala Volcanics are definitely allochthonous, and hence the apparent ages are maxima. The older age limit of the Sofala Volcanics remains poorly constrained by the record of possible late Darriwilian to Gisbornian graptolites (Packham 1968). The occurrence of Pygodus serra in cherts of the “Lue beds” (Stewart & Fergusson 1995), now Adaminaby Group (Colquhoun et al 1997), provides the sole wellestablished age (late Darriwilian) for the underlying strata (figure 2). Dating of the remainder of the Sofala Volcanics is based on proven late Eastonian conodont and coral faunas (discussed herein), and a record of poorly preserved graptolites of probable late (but not latest) Bolindian age (Rickards et al 1998). Although Colquhoun et al (1997) claimed that this latter sample was taken from well below the formation top (thus allowing the possibility that the Sofala Volcanics spanned the Ordovician–Silurian boundary), structural complications and discontinuity of outcrop prevent the level of the graptolitic horizon from being precisely known. There is as yet no conclusive evidence that deposition on the Rockley–Gulgong Volcanic Belt continued into the Early Silurian.
ACKNOWLEDGMENTS I thank John Pickett, Barry Webby and Dick Glen for their review of this paper, and Richard Facer for his precision in editing it. Gary Dargan processed the conodont samples and prepared the thin sections. Peter Cockle photographed the conodonts; Yongyi Zhen assisted with their identification. Digital preparation of the conodont illustrations was done by David Barnes and Dora Lum. Samples, other than those collected by John Pickett, were obtained by members of the Geological Survey of New South Wales Dubbo 1:250 000 mapping team (John Watkins, Simone Meakin, Gary
Colquhoun). This is a contribution to IGCP Project No. 410: The Great Ordovician Biodiversification Event.
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Cudgegong area. Palaeontological Report 82/10. Geological Survey of New South Wales, Report GS1982/197 (unpublished) PICKETT J.W. 1995. Fossils make a prospective link. Minfo, New South Wales Mining and Exploration Quarterly No. 49, 53. RAYMOND O.L., POGSON D.J., et al [14 others] 1998. Bathurst 1:250 000 Geological Sheet SI/55-8. (second edition). Geological Survey of New South Wales, Sydney/Australian Geological Survey Organisation, Canberra. RICKARDS R.B., WRIGHT A.J. & PEMBERTON J.W. 1998. Graptolite evidence for the ages of the Sofala Volcanics and Willow Glen Formation, northern Capertee High, N.S.W. Alcheringa 22, 223-230. SAVAGE N.M. 1990. Conodonts of Caradocian (Late Ordovician) age from the Cliefden Caves Limestone, southeastern Australia. Journal of Paleontology 64, 821-831.
MORGAN E.J., CAMERON R.G., COLQUHOUN G.P., MEAKIN N.S., RAYMOND O.L., SCOTT M.M., WATKINS J.J., BARRON L.M., HENDERSON G.A.M., KRYNEN J.P., POGSON D.J., WARREN A.Y.E., WYBORN D., YOO E.K., GLEN R.A. & JAGODZINSKI E.A. 1999 in prep. Dubbo 1:250 000 Geological Sheet SI/55-4. (second edition). Geological Survey of New South Wales, Sydney/Australian Geological Survey Organisation, Canberra.
STEWART I.R. & FERGUSSON C.L. 1995. Ordovician conodonts from the Lue Beds, Mudgee and Sunlight Creek Formation, Goulburn, New South Wales. p.164 In Jell P.A. ed. APC 94: Papers from the First Australian Palaeontological Convention. Australasian Association of Palaeontologists, Memoir 18.
PACKHAM G.H. 1968. The lower and middle Palaeozoic stratigraphy and sedimentary tectonics of the Sofala–Hill End–Euchareena region, N.S.W. Linnean Society of New South Wales, Proceedings 93, 111-163.
TROTTER J.A. & WEBBY B.D. 1995. Upper Ordovician conodonts from the Malongulli Formation, Cliefden Caves area, central New South Wales. AGSO Journal of Australian Geology & Geophysics 15 (for 1994), 475-499.
PACKHAM G.H. ed. 1969. The geology of New South Wales. Geological Society of Australia, Journal 16, 1-654.
WEBBY B.D. 1969. Ordovician stromatoporoids from New South Wales. Palaeontology 12, 637-662.
PEMBERTON J.W. 1989. The OrdovicianSilurian stratigraphy of the Cudgegong-Mudgee district, New South Wales. Linnean Society of New South Wales, Proceedings 111, 169-200.
WEBBY B.D. & KRUSE P.D. 1984. The earliest heliolitines: a diverse fauna from the Ordovician of New South Wales. Palaeontographica Americana 54, 164-168.
PICKETT J. 1978. Further evidence for the age of the Sofala Volcanics. Geological Survey of New South Wales, Quarterly Notes 31, 1-4. PICKETT J.W. 1982. Preliminary report on conodont samples from the
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Ordovician stratigraphy of the northern Molong Volcanic Belt: new facts and figures Ian G. Percival, Elisabeth J. Morgan and Martin M. Scott ABSTRACT The oldest fossiliferous strata in the northern Molong Volcanic Belt of central New South Wales, overlying the volcanogenic Mitchell Formation, are limestones and graptolitic beds in the Hensleigh Siltstone of early Bendigonian (Early Ordovician) age. A significant time-break spanning approximately 15-20 Ma separates the Hensleigh Siltstone from the overlying Fairbridge Volcanics, which includes in its lower part allochthonous limestones with reworked Early Ordovician conodonts and Darriwilian (Middle Ordovician) brachiopods. Autochthonous carbonate units newly-recognised at two levels within the Fairbridge Volcanics include the older Wahringa Limestone Member of late Darriwilian to early Gisbornian age, and the Yuranigh Limestone Member of late Gisbornian age. These successions predate the main phase of Late Ordovician carbonate deposition, represented in the northern Molong Volcanic Belt by the Reedy Creek Limestone, of early Eastonian age. Allochthonous and autochthonous limestones in the Oakdale Formation, deposited in deeper water to the east of the volcanic belt, yield conodonts which span an age range equivalent to the Wahringa, Yuranigh and Reedy Creek limestones. Similar offshore facies, including probable autochthonous limestones in both the Oakdale Formation and the correlative Sourges Shale, are present on the western flank of the Molong Volcanic Belt, but only Eastonian ages have been recorded for these strata. The northern Molong Volcanic Belt can be shown to have had an Ordovician volcanogenic and sedimentological history closely comparable with that of the Junee–Narromine Volcanic Belt to the west.
Keywords: Ordovician, conodonts, graptolites, brachiopods, corals, Molong Volcanic Belt, Hensleigh Siltstone, Wahringa Limestone Member, Yuranigh Limestone Member, Reedy Creek Limestone, Oakdale Formation, Sourges Shale.
The Molong Volcanic Belt (MVB) is a prominent meridionally-aligned tectonic feature of early Palaeozoic age within the Lachlan Orogen of central New South Wales. It extends through the western area of the region covered by the Bathurst and Dubbo 1:250 000 map sheets, which were recently remapped by the Geological Survey of New South Wales and the Australian Geological Survey Organisation (AGSO) (Raymond et al 1998; Morgan et al 1999). In the course of this work, Geological Survey of New South Wales geologists [Scott and Morgan] recognised new fossiliferous strata in the northern sector of the MVB, between Orange and Wellington (figure 1). Palaeontological investigations [Percival] on these strata have resulted in significant advances to knowledge of how Ordovician sequences correlate across the region. This paper presents definition of a new stratigraphic unit (Wahringa Limestone Member), lists fossils identified from new stratigraphic horizons and localities, and discusses implications of the biostratigraphic discoveries for revised understanding of the geological history of the MVB. Description of the fauna will be published elsewhere.
Previous models of the development of the Lachlan Orogen in central New South Wales (eg, Webby 1976) commenced with an Early to Middle Ordovician interval of basinal deposition and intervening arc-related volcanism. The volcanic islands thus formed became emergent early in the Late Ordovician. Around the flanks of the islands, shallow water carbonates developed along the MVB and Parkes Platform (now identified as the Junee– Narromine Volcanic Belt, or JNVB — Glen et al 1998). These limestones were succeeded by Late Ordovician deep water clastic and interspersed volcanic units, prior to a final episode of more continuous volcanism which persisted nearly to the end of the Ordovician. The presence of Bendigonian (Early Ordovician) graptolites, and deep-water trilobites of probable Darriwilian (late Middle Ordovician) age (Packham 1969), had been reported in the basinal successions, but it is important to note that in those syntheses, no in situ carbonate horizons of pre-Late Ordovician age were recognised in the MVB. In the JNVB to the west, however, Pickett (1985) identified Middle Ordovician conodonts from a level
within the Goonumbla Volcanics below the Late Ordovician Billabong Creek Limestone Member. That was the first indication that submarine volcanoes had built up to shallow water depths in central New South Wales during the Middle Ordovician. The present research has disclosed a much more complex geological history than previously envisaged, particularly for the northern MVB, with multiple limestones having been deposited in the Early, Middle and Late Ordovician. This has enabled accurate definition of a major stratigraphic break between the first (Early Ordovician, Lancefieldian or older) and second (Darriwilian– Gisbornian) phases, of the three episodes of volcanicity that characterise the Ordovician history of central New South Wales. Palaeontological data were collected from four main areas: Bakers Swamp (25 km south of Wellington), where the stratigraphic sequence comprises the Mitchell Formation, Hensleigh Siltstone, Fairbridge Volcanics and Oakdale Formation (figure 2); the area south of Molong, including the Printhie property, where the Fairbridge Volcanics are overlain by the Reedy Creek Limestone and Cheesemans Creek Formation (figure 3); the Narrawa property, 15 km Quarterly Notes 108, 1999
"Wahringa" C1427, C1465, C1553
Undifferentiated Devonian Undifferentiated Silurian
late Middle Late to early Late Ordovician Ordovician
REFERENCE Oakdale Formation
limestone in Oakdale Formation
Syncline, with plunge Anticline
Fairbridge Volcanics allochthonous limestones Wahringa Limestone Member
Dip, and strike
Fossil locality Thrust fault Road
Hensleigh Siltstone Type section Mitchell Formation 23087
Figure 2. Geological map of the “Wahringa” area, 26-28 km south of Wellington (mapping by Ian Percival and Martin Scott) (for location see pages 14-15).
Quarterly Notes 108, 1999 9
west of Wellington, where fossiliferous Oakdale Formation is exposed (figure 4); and the area immediately east of Cumnock where the Sourges Shale is fault-bounded (figure 5). Additional samples from the Oakdale Formation were collected from several localities between Orange and Wellington (shown on figure 1).
STRATIGRAPHIC SUCCESSION Mitchell Formation The Mitchell Formation, formerly the ‘Mitchell Grit’ of Kemezys (1959) and ‘Mitchell Breccia’ of Wolf et al (1968), represents the oldest rocks in the MVB. To the east its outcrop abuts the Neurea Fault, while the top of the formation is conformable with the Hensleigh Siltstone (figure 6). The lithology and geochemistry of the Mitchell Formation is very similar to that of the Fairbridge Volcanics of the Cabonne Group and, without identification of the intervening Hensleigh Siltstone, the two formations
would be difficult to differentiate. It is possible that the Mitchell Formation is more widespread but has been mapped elsewhere as Cabonne Group. The formation crops out only in the Bakers Swamp area, 25 km south of Wellington (figure 2). The type section for the Mitchell Formation is modified from that of Kemezys (1959) to exclude what is now the overlying Hensleigh Siltstone to the west. The revised section now extends from the Neurea Fault (at GR 679750E 6369250N, Cumnock 1:50 000 map), west to the Mitchell Highway (GR 679000 6369300) and further west along a tributary of Neurea/Bakers Swamp Creek to GR 678500 6369900. This section includes a syncline-anticline pair, with an estimated minimum thickness for the Mitchell Formation of 1 100 m.
sorted, massive to crudely bedded and graded, latitic conglomerate is the most common lithology. The conglomerate includes rounded to subangular clasts up to 20 cm in diameter. The poorly sorted and massive nature of the conglomerate, together with the dominance of latite clasts, suggests the material was derived as a downslope mass flow from a volcanic centre. Interbedded sandstone units probably represent turbidite deposits. No fossils have been found in the Mitchell Formation but it must be earliest Ordovician (Lancefieldian) or older, as early Bendigonian graptolites occur within the overlying Hensleigh Siltstone (Kemezys 1959; Packham 1969).
The Mitchell Formation comprises latitic volcaniclastic conglomerate and sandstone with minor primary volcanic (latite lava and monzodiorite intrusions) and polymict sedimentary rocks. Poorly
The Hensleigh Siltstone (Wolf et al 1968) conformably overlies the Mitchell Formation (cf figure 7) and similarly crops out only in the Bakers Swamp area. The boundary between the
Middle to Late Devonian
Labechia banksi Labechiella regularis
WAHRINGA LIMESTONE MEMBER C1463
early Gisbornian Gi1 - late Darriwilian Da4
Belodina compressa Periodon grandis Sowerbyites sp nov
Belodina cf B. monitorensis Periodon cf P. aculeata
Appalachignathus, Pygodus, Phragmodus C1456
Periodon aculeata, Phragmodus flexuosus
Maclurites, cf Calathium abundant lithistid sponges
Girvanella, Reutterodus, Drepanoistodus first plectambonitoid brachiopods
(no older than Darriwilian)
HENSLEIGH SILTSTONE C1481
Pendograptus, Didymograptus asperus, Dichograptus maccoyi Bergstroemognathus extensus, Juanognathus variabilis, Jumudontus, Paracordylodus gracilis, Reutterodus, Scolopodus, Prioniodus, Acodus, Walliserodus australis
MITCHELL FORMATION 23122
Figure 6. Diagrammatic stratigraphic column for the Ordovician succession west of the Neurea Fault at “Wahringa”, showing occurrences of significant fossils.
Quarterly Notes 108, 1999 10
(Conodont Alteration Index, CAI, of 1.5-2) of Early Ordovician age, which indicates approximate contemporaneity with the graptolitic siltstone succession.
CHEESEMANS CREEK FORMATION
Taoqupognathus blandus Periodon grandis Besselodus, Spinodus
Conodont Sample C1481
coral – stromatoporoid Fauna II (Eastonian Ea2)
allochthonous limestone in Hensleigh Siltstone GR 678450E 6369700N, Cumnock 1:50 000 map
REEDY CREEK LIMESTONE
Belodina confluens Yaoxianognathus wrighti Chirognathus cliefdenensis
C1377, C1437 YURANIGH LIMESTONE MEMBER
Belodina confluens Scabbardella, Staufferella Sowerbyella, Dinorthis Ishimia, gastropds
coral – stromatoporoid Fauna I (Eastonian Ea1)
pre - Brachiopod Fauna A (Gisbornian Gi2)
Figure 7. Diagrammatic stratigraphic column for the Ordovician succession at “Printhie” and along strike, showing occurrences of significant fossils.
Hensleigh Siltstone and the overlying Fairbridge Volcanics is now recognised as a low angular unconformity, although locally the contact may be faulted, eg at GR 678200E 6369800N (Cumnock 1:50 000 map). The type section of the Hensleigh Siltstone is along a tributary of Neurea/Bakers Swamp Creek and partly on Neurea/Bakers Swamp Creek itself, from GR 678500E 6369900N to GR 678310E 6370150N (Cumnock 1:50 000 map). The Hensleigh Siltstone is approximately 310 m thick. The Hensleigh Siltstone consists of buff to dark grey, laminated siltstone and shale, minor volcaniclastic sandstone and, at the base of the formation, laminated calcareous siltstone. Volcaniclastic detritus is common in the basal units, presumably being derived from the underlying Mitchell Formation. Allochthonous limestone blocks and scattered volcanic clasts also occur locally within the Hensleigh Siltstone. Limestone blocks
at GR 678600E 6370100N and GR 678450E 6369700N consist of partly bioturbated or burrowed, grey wackestones to packstones bearing sponge spicules and rare trilobite moulds. Siltstone in the upper part of the formation contains a narrow, graptolite-rich band yielding early Bendigonian forms, discussed below. Laminated calcareous siltstones at the base of the Hensleigh Siltstone at GR 678825E 6370300N (Cumnock 1:50 000 map) are believed to represent undisturbed, in situ deeper water sediments, and are quite distinct lithologically from allochthonous limestone pods emplaced slightly higherstratigraphically in the Hensleigh Siltstone. Both the autochthonous laminated siltstones (sample C1536), and the allochthonous limestones (sample C1481), yielded a similar, exceptionally rich, diverse, and very well-preserved conodont fauna
Conodonta (figure 8.1-8.2, 8.4-8.12) Acodus deltatus Lindström Bergstroemognathus extensus (Graves & Ellison) Drepanodus arcuatus Pander ?Drepanodus sp Drepanoistodus sp Juanognathus variabilis Serpagli Jumudontus cf brevis Nicoll Oistodus sp Paracordylodus gracilis Lindström Paroistodus sp Prioniodus sp Protopanderodus sp Reutterodus cf andinus Serpagli Scandodus sp Scolopodus rex Lindström or S. cf multicostatus Barnes & Tuke Stolodus sp Tropodus ?sweeti (Serpagli) “Walliserodus” australis Serpagli Within Australia, the Hensleigh assemblage most closely resembles that described by McTavish (1973) from the Emanuel Formation of the Canning Basin. Nicoll (in Shergold et al 1995) subdivided the Emanuel Formation into five conodont assemblages, from A (oldest) to E (youngest), spanning the equivalent of late Lancefieldian (La3) to middle Bendigonian (Be2).Bergstroemognathus extensus appears in assemblage B, together with Jumudontus brevis and “Acodus” deltatus, and Paracordylodus gracilis first appears in assemblage D. This early Bendigonian age accords precisely with that indicated by a graptolite fauna, including Didymograptus asperus Harris and Thomas, Dichograptus maccoyi Harris and Thomas, and Pendograptus sp, from overlying beds in the upper part of the Hensleigh Siltstone.
Fairbridge Volcanics The Fairbridge Volcanics (Adrian 1971) crops out in three meridional tracts in the northern MVB (figure 1): a western
Quarterly Notes 108, 1999 11
6 7 8 11 12 13
22 18 17 21 24 23
Quarterly Notes 108, 1999 12
belt extending 45 km along strike from north of Borenore to west of Cundumbul; a northern band of 5 km strike length at Bakers Swamp; and a largely fault-bounded eastern belt approximately 7 km northwest of Orange (not discussed in this paper). The Fairbridge Volcanics in the western belt, where the name originated, is faulted at its base and, to the north, also at the top. North and south of Molong the formation is conformably overlain by the Reedy Creek Limestone and, where that formation is absent, the Cheesemans Creek Formation. Two autochthonous carbonate-dominated units, the Wahringa Limestone Member (formally defined below) in the Bakers Swamp area, and the Yuranigh Limestone Member (Scott & Pogson, in Pogson & Watkins 1998) south of Molong, are now recognised as units within the Fairbridge Volcanics. During remapping of the Bathurst 1:250 000 geological map sheet the Fairbridge Volcanics was redefined to include Stevens’ (1950) ‘Cargo Volcanics’ and was placed in the newly introduced Kenilworth Group (Pogson & Watkins 1998). However, additional information, reported below, has led to a revision of this assignment.
The Fairbridge Volcanics at Bakers Swamp, formerly the ‘Blathery Creek Volcanics’ of Kemezys (1959) and Wolf et al (1968), overlie the Hensleigh Siltstone with slight angular discordance. This suggests a significant but previously unrecognised Middle Ordovician break in deposition on the MVB. The top of the Fairbridge Volcanics at Bakers Swamp is faulted against the Oakdale Formation, while to the west the Ordovician sequence is
unconformably overlain by the Late Devonian Catombal Group.
formation of 1 500 m to 2 100 m east of Molong.
Several allochthonous limestone pods, including one described as an algal bioherm by Wolf et al (1968), are now known from the lower part of the Fairbridge Volcanics. These limestones are very significant as they yielded the first biostratigraphic data from this formation. Based on the Darriwilian age indicated by the limestone fauna, the maximum age of the Fairbridge Volcanics is much younger than that proposed by earlier workers. This major revision to the stratigraphic position of the Fairbridge Volcanics confirms its inclusion within the Middle to Late Ordovician Cabonne Group, as it correlates in age with other units of that group, including the Weemalla Formation and Blayney Volcanics.
Together with basaltic andesitic to latitic lavas and intrusions which are characteristic of the Fairbridge Volcanics, massive to bedded limestone breccia and conglomerate, calcareous volcaniclastic sandstone and allochthonous limestone blocks are common in the formation in the Bakers Swamp area. Breccia and conglomerate units consist of unsorted, chaotically mixed limestone clasts, including blocks up to 5 m in diameter, as well as fossil and volcaniclastic detritus. Calcarenite beds are fine to coarse grained, locally graded and polymict, consisting of angular to rounded fossil, lithic and crystal fragments in a carbonate mud. Allochthonous limestone blocks and clasts are massive to bedded, grey, fine to coarse grained packstone grading to grainstone and wackestone. Grains are subangular to rounded and consist mostly of fossil debris, as well as oncolites, ooids and minor lithic and crystal fragments. The carbonate material is generally clean, well-rounded and even-grained, resembling beach sand.
A type area for the Fairbridge Volcanics has been nominated at Bakers Swamp (Morgan, Scott & Percival, in Meakin & Morgan in prep 1999) in three composite sections: from the top of the Hensleigh Siltstone to the base of the Wahringa Limestone Member along Bakers Swamp Creek (from GR 678320E 6370150N to GR 677820E 6370750N, Cumnock 1:50 000 map); through the Wahringa Limestone Member from GR 678700E 6372000N to GR 678600E 6372300N; and from the top of the Wahringa Limestone Member to the faulted top of the formation from GR 679320E 6372800N to 677970E 6375480N. The Fairbridge Volcanics is estimated to be approximately 2 750 m thick in the type area, although this may be a low estimate given that the top of the formation is faulted. Adrian (1971) gave an approximate thickness for the
The “algal bioherm” from which Wolf et al (1968) described Girvanella, is undoubtedly of allochthonous origin. The limited outcrop, restricted to a creek gully at GR 678020E 6370300N (Cumnock 1:50 000 map), consists of a limestone breccia which coarsens upwards over 3 m stratigraphically from rounded pebble-sized clasts at the base to metre-sized blocks with prominent calcimicrobial stromatolites (figure 9.1). This outcrop is the oldest datable sediment within the lower part of the Fairbridge Volcanics. A limited
Figure 8. Selected conodonts typical of Ordovician strata on the northern Molong Volcanic Belt, including the Hensleigh Siltstone (8.1-8.12), allochthonous limestones in the Fairbridge Volcanics (8.13, 8.14), Wahringa Limestone Member (8.15-8.20), Yuranigh Limestone member (8.21), allochthonous limestones in the Oakdale Formation (8.22-8.23), and autochthonous bedded limestone in the Oakdale Formation (8.24). 8.1 Bergstroemognathus extensus Pa element, x35; 8.2 Bergstroemognathus extensus Sd element, x60; 8.3 Bergstroemognathus extensus Sa element, x55; 8.4 Acodus deltatus, x 45; 8.5 Prioniodus sp, x 65; 8.6 Reutterodus cf R. andinus, cone-like element x50; 8.7 Paracordylodus gracilis, x120; 8.8 Stolodus sp, x80; 8.9 Scolopodus ?rex, x55; 8.10 Tropodus ?sweeti, x45; 8.11 Juanognathus variabilis, x70; 8.12 Drepanodus arcuatus, x50; all from conodont sample C1481, except 8.3 from C1536 (same horizon). 8.13 Reutterodus sp, bipennate element, x105; from conodont sample C1427. 8.14 Phragmodus flexuosus, S element, x75; from conodont sample C1463. 8.15 Phragmodus flexuosus, S element, x60; 8.16 Periodon aculeata, M element, x95; 8.17 Drepanodus sp, x110; 8.18 Pygodus serra, x70; 8.19 Ansella sp, x120; 8.20 Belodina compressa, compressiform element, x60; 8.15-8.18 from conodont sample C1450; 8.19-8.20 from conodont sample C1458. 8.21 Belodina confluens, compressiform element, x50; from conodont sample C1437. 8.22 Appalachignathus delicatulus, Pa element, x60; 8.23 Appalachignathus delicatulus, Sa element, x60; both from conodont sample C1425. 8.24 Belodina confluens, compressiform element, x80; from conodont sample C1388.
Quarterly Notes 108, 1999 13
Figure 4 Wellington
Well ingt on
City or town
Railway Fault, accurate
Fossil locality Road Molong
Dip and strike
Yuranigh Limestone Member Wahringa Limestone member
Reedy Creek Limestone
Cheesemans Creek Formation
limestone member latite member
Mesozoic volcanic and sedimentary units
Quarterly Notes 108, 1999
e3 ur g Fi
Figure 5. Geological map of the area immediately east of Cumnock (compilation by Elisabeth Morgan).
Figure 4. Geological map of the “Narrawa” area, 15 km west of Wellington (mapping by Alice Warren).
Figure 3. Geological map of the area south-east of Molong (compilation by Martin Scott).
Figure 1. Simplified geological map of the northern Molong Volcanic Belt, based on Raymond et al (1998) and Morgan et al in prep (1999), showing locations of areas considered in greater detail in figures 2, 3, 4, and 5. For clarity, faults are not shown.
C1394 1 km
ge an Or
Quarterly Notes 108, 1999
Allochthonous limestones within boulder and cobble size volcanic conglomerates at a comparable level, or perhaps slightly higher than the “bioherm”, in the Fairbridge Volcanics yielded elements of the Middle to early Late Ordovician genera Pygodus and Appalachignathus (indicative of a probable late Darriwilian age), together with Phragmodus and Panderodus. This composite sample (C1463; GR 679150E 6371900N, Cumnock 1:50 000 map) was taken from several small limestone clasts at the one isolated stratigraphic level (in some cases lying adjacent to each other), though differing markedly in appearance. Preservational differences were also apparent in the conodont fauna, which appears to include reworked specimens. Wide variation in the CAI of the elements, from 1.5 to approximately 5, confirms the heterogeneity of the clasts.
Wahringa Limestone Member of the Fairbridge Volcanics
Figure 9. Selected macrofossils from limestones in the Fairbridge Volcanics on the northern Molong Volcanic Belt. 9.1 Stromatolite, showing partially silicified calcimicrobial laminae comprised of Girvanella sp, x0.3; from allochthonous limestone at GR 678020E 6370300N (Cumnock 1:50 000 map). 9.2 natural cross-section through gastropod Maclurites sp, x0.9; 9.3 longitudinal cross-section through nautiloid ?Michelinoceras sp, x1.7; 9.4 unidentified crinoid calyx, x1.9; 9.5 natural transverse section, and 9.6 natural longitudinal section, of lithistid sponge referable to ?Archaeoscyphia sp (identification by John Pickett), both x0.6; 9.2-9.6 from lower part of Wahringa Limestone Member in its type section.
fauna recovered from residues of the basal limestone clasts (sample C1553) lacked conodonts, but included incomplete specimens of a brachiopod referred to ?Anoptambonites. Its presence means that the age of the basal clasts can be reasonably reliably estimated as no older than Darriwilian. No earlier occurrences of this genus, or the family to which it belongs, are known world-wide (Cocks & Rong 1989). Conodonts recovered from the upper part of this outcrop (samples C1427,
C1465) include one specimen (figure 8.13) of the distinctive bipennate ramiform element (together with numerous coniform elements) of the Early Ordovician conodont Reutterodus (closely comparable with Serpagli 1974, plate 28, fig 5). Presumably the limestone comprising the “bioherm” was derived from the underlying Hensleigh Siltstone succession, which also includes limestones with Reutterodus (sample C1481), and was incorporated into the Fairbridge Volcanics during erosion and redeposition.
The Wahringa Limestone Member [first recognised by Scott during remapping of the Dubbo 1:250 000 geological map area (Morgan et al in prep 1999)], is here formally defined. Although the outcrop of this unit was initially plotted as occurring within the ‘Cargo Andesite’ in a map compilation by Byrnes (in Pickett 1982, fig 17; and also in Lishmund et al 1986, fig 28), its significance was not then recognised and it has not been previously named nor studied in detail. The member is an autochthonous limestone lens conformable within volcaniclastic rocks in the lower part of the Fairbridge Volcanics in the Bakers Swamp area (figures 2, 6). The name is derived from the Wahringa property on which the member crops out, the Wahringa homestead being located at GR 680150E 6370750N, Cumnock 1:50 000 map. The Wahringa Limestone Member is exposed adjacent to Bakers Swamp Creek from GR 677650E 6370350N northeast to GR 679350E 6372800N, a strike length of approximately 400 m. The northeastern-most margin is not exposed, while to the southwest the member is overlain unconformably by the Late Devonian Catombal Group. The type section of the Wahringa Limestone Member is just north of a bend in Bakers Swamp Creek, and extends from GR 678700E 6372000N Quarterly Notes 108, 1999
to GR 678600E 6372300N, Cumnock 1:50 000 map. The limestone has been measured in this section at 88 m in thickness. The Wahringa Limestone Member consists of medium to thinly bedded fossiliferous limestone, largely grainstone, with lesser packstone and wackestone, as well as calcareous sandstone, breccia and siltstone. Calcimicrobes are abundant throughout the limestone, especially at the base where many of the sedimentary grains are oncolites, indicative of shallow, well-circulated marine conditions. The unit consists of three parts: lower beds rich in oncolites, ooids and volcaniclastic detritus; a middle part that is muddy, thinly bedded and rich in brachiopods; and an upper section that is more massive. The most common facies in the lower part of the Wahringa Limestone Member is a brown to grey, fine to coarse grained, poorly to moderately well-sorted, oncolitic skeletal grainstone, locally grading to packstone. Fossil grains are subangular to rounded, frequently microbial-coated and include echinoderms, gastropods, trilobites, brachiopods, sponge spicules, Solenopora and dasycladalean algae. Large oncolites with wellpreserved Girvanella filaments, welldefined cyanobacterial strands and ooids are also abundant. Other distinctive fossils at this level include the large gastropod Maclurites, and a receptaculitid similar to Calathium. Minor detrital mineral fragments include angular quartz, plagioclase, K-feldspar, olivine, magnetite, epidote, apatite, zircon, garnet, mica and ?fuchsite. Rare porphyritic rhyolite, andesite and chert clasts are also present. Unsorted, medium to coarse grained, bedded, skeletal, volcanolithic calcareous sandstone and breccia also occur in the lower part of the Wahringa Limestone Member. These rocks consist of well-rounded carbonate grains, including oncolites, oolites, echinoderm ossicles, and sparse mollusc and brachiopod shells. Also present are angular to subrounded lithic fragments, including limestone (wackestone and packstone), calcareous sandstone, porphyritic quenched trachyte, andesite and latite, vitric felsic tuff, chert, and rare ?granite, metaquartzite, quartz-veined metasiltstone and microdiorite. Detrital
angular minerals include quartz, plagioclase, ?fuchsite, Ti-magnetite, unidentified mafic minerals, epidote and garnet (Larry Barron pers comm 1997). The matrix consists of micrite. Some units are graded and exhibit relict burrow structures. Rare fine to coarse grained, latitic volcaniclastic sandstone lacking organic material also occurs at the base of the member. The presence of ooids and the abundance of algae in the basal Wahringa Limestone Member indicate shallow, well-circulated marine conditions. The middle part of the Wahringa Limestone Member is characterised by a brown to brownish grey, fine to coarse grained, skeletal grainstone, dominated by remains of echinoderms, brachiopods, dasycladalean algae, molluscs, trilobites and ostracodes. The unit is well-sorted, commonly laminated and interbedded, with silty layers, and grades between packstone and wackestone. Beds may exhibit burrows. The grainstone includes wellrounded fossil fragments, some with thin micritic rinds or microbial coatings; sponge spicules; minor mineral grains including quartz, feldspar, Ti-magnetite, mafic grains and ?fuchsite; scattered oncolites of Girvanella; and rare ooids. Rare, rounded lithic clasts include porphyritic and microlitic latite and chert. The matrix consists of microcrystalline micrite. The most common lithology in the upper part of the Wahringa Limestone Member is brownish grey to light brown, fine to coarse grained, skeletal, oncolitic grainstone and lesser packstone to wackestone with no internal lamination. Grains are wellwashed and include ostracodes, dasycladalean algae, rare ooids (some with micritic rinds) and oncolites of Girvanella. Echinoderm and mollusc fragments, as well as rare latite clasts, are also present. Sparse detrital mineral fragments include quartz, magnetite, plagioclase and mafic minerals. Irregular voids may be infilled with spar and laminated ?marine cement. Minor brownish grey, calcareous, volcaniclastic sandstone and conglomerate — rich in shelly, lithic and crystal detritus — also occur at the top of the member. The beds are poorly to moderately sorted and are locally graded. Shelly debris includes trilobites and ostracodes, with fewer echinoderm ossicles and rare stictoporoid bryozoans. Lithic clasts are
angular to rounded and include fine grained wackestone, skeletal packstone, porphyritic latite, porphyritic and microlitic basaltic andesite and microdiorite. Minor detrital quartz, plagioclase, augite and hornblende fragments are also present.
Biostratigraphy The diversity, relative abundance, and good preservation of the conodonts in sample C1450 permits an accurate age determination for the basal Wahringa Limestone Member. Periodon aculeata, which ranges through the upper Middle Ordovician into the basal Late Ordovician (Darriwilian Da2 to Gisbornian Gi1), is the most common species in the assemblage. Co-occurrence with Phragmodus flexuosus restricts the possible age of this interval to Da3-Gi1. The presence of Pygodus serra also supports a late Darriwilian age.
Conodont Sample C1450 basal Wahringa Limestone Member GR 678650E 6372000N Cumnock 1:50 000 map Conodonta (figure 8.15-8.18) Drepanoistodus sp Erraticodon sp Panderodus sp Periodon aculeata Hadding Phragmodus flexuosus Moskalenko Prioniodus (Baltoniodus) sp Protopanderodus cf P. liripipus Kennedy, Barnes & Uyeno Pygodus serra (Hadding) Gastropoda (figure 9.2) Maclurites sp Nautiloidea (figure 9.3) Michelinoceras sp Brachiopoda ?Multispinula sp indeterminate lingulate fragments acrotretide Trilobita pliomerinid cephalon and pygidium Ostracoda (unidentified) Echinodermata (figure 9.4) unidentified crinoid calyx Porifera (figures 9.5, 9.6) several genera of lithistid sponges Algae cf Calathium sp undetermined dasycladales
Quarterly Notes 108, 1999 17
Sample C1456, from slightly higher in the Wahringa Limestone Member (figure 6), is most likely latest Middle Ordovician (Da4) or earliest Late Ordovician (Gi1). Evidence for the age derives from quite close similarities to the conodont fauna described from the Pratt Ferry Limestone of Alabama by Sweet and Bergström (1962). That formation is restricted in time to the upper Pygodus anserinus Zone, according to Ross et al (1982). The co-occurrence of early species of Belodina with Periodon aculeata (or P. cf aculeata) in both the Pratt Ferry and Wahringa limestones suggests contemporaneity. Conodonts from sample C1450 (figure 6) also include P. aculeata but lack Belodina, and several other age-significant taxa found in C1450 do not persist into C1456. The fish scale found in the latter is especially interesting, as there are no previous records of vertebrates (excluding conodonts) from the Ordovician of the MVB. Given the rarity of Ordovician fish fossils, it is doubtful if it could be identified to an age-diagnostic form.
Conodont Sample C1456 lower part of Wahringa Limestone Member GR 678675E 6372025N Cumnock 1:50 000 map Conodonta Belodina cf B. monitorensis Ethington & Schumacher ‘Belodina’ alabamensis Sweet & Bergström Drepanoistodus sp Panderodus sp Periodon cf P. aculeata Hadding ?Walliserodus sp Scolecodont Gastropod steinkerns Trilobite fragments Lingulate brachiopod fragments Echinoderm spines, with articulation boss at base Fish scale, ornamented with small smooth elongate tubercles A further conodont sample (C1458), taken about midway stratigraphically within the Wahringa Limestone Member, has an entirely Gisbornian aspect, including Belodina compressa and a younger species of Periodon, P. grandis. Co-occurrence of these species indicates, according to Sweet
(1988), an age spanning the Phragmodus undatus – Plectodina tenuis zones of the North American Midcontinent Fauna. Belodina compressa in North America ranges through the early Late Ordovician Amorphognathus tvaerensis Zone of the North Atlantic Fauna (equivalent to the early Gisbornian to early Eastonian interval in the Australian succession). Zhen and Webby (1995) reviewed previous records of B. compressa in Australia, and concluded all were better placed in B. confluens. However, the abundant material available in the Wahringa Limestone Member allows an unequivocal identification of B. compressa, indicating that this unit is older than any of the other East Australian Late Ordovician limestones bearing B. confluens.
Conodont Sample C1458 middle part of Wahringa Limestone Member GR 678700E 6372050N Cumnock 1:50 000 map Conodonta (figure 8.19-8.20) ?Ansella sp Belodina compressa (Branson & Mehl) Besselodus or Dapsilodus sp Drepanoistodus sp Panderodus unicostatus (Branson & Mehl) Panderodus ?panderi (Stauffer) Periodon grandis (Ethington) Protopanderodus sp ?Pseudobelodina sp ?Scabbardella sp ?Yaoxianognathus sp Brachiopoda ?Leptellina sp Sowerbyites sp nov indeterminate lingulates, all fragmentary Gastropoda unidentifiable steinkerns (internal moulds) Ostracode Echinoderm spines, with articulation boss at base (cf C1456)
DISCUSSION The Wahringa Limestone Member conodont (and macrofossil) fauna of sample C1458 bears no close resemblance with faunal assemblages found in the Cliefden Caves Limestone
Subgroup, or the equivalent Reedy Creek Limestone, of early Eastonian age. Hence the C1458 assemblage is more likely to have an older (Gisbornian) age. There is a marked difference between the conodont fauna of that level, and the stratigraphically older sample C1450 (basal Wahringa Limestone Member), which is demonstrably late Darriwilian. While C1450 and C1458 differ in facies, having been deposited in different water depths and turbulence conditions, this is probably not the cause of the faunal discrepancy. It seems probable that the 88 m thick Wahringa Limestone Member spans at least two zones, from late Darriwilian (Da4) to early Gisbornian (Gi1). Evidence supporting correlation of the Wahringa Limestone Member with the Tasmanian succession (Banks & Burrett 1980, table 2) is given by stromatoporoids in the middle part of the section, identified by Barry Webby (pers comm 1996) as similar to those he described from the Cashions Creek Limestone and overlying Lichenaria unit in Tasmania (Webby 1979). These levels contain representatives of Faunal Assemblages OT10 and OT11, which have been correlated with Da3-Da4 and Gi1 intervals.
Yuranigh Limestone Member of the Fairbridge Volcanics The Yuranigh Limestone Member is an autochthonous limestone up to 180 m thick that lies conformably within the upper part of the Fairbridge Volcanics, approximately 1 300 m below the base of the overlying Reedy Creek Limestone (figure 7). The limestone, on Printhie property, 3 km south of Molong was named [by Scott] during remapping of the Bathurst 1:250 000 geological map area (Raymond et al 1998), and was defined by Scott and Pogson (in Pogson & Watkins 1998, pp 26-27). The type section extends from GR 675300E 6332800N to GR 674950E 6332800N, Molong 1:50 000 map (figure 3). The Yuranigh Limestone Member consists of bioturbated limestone, calcareous siltstone and lithic sandstone. Basal beds are characterised by coarse calcarenite grading into lithic sandstone and volcaniclastic pebble conglomerate, incorporating detritus probably derived from underlying volcanic strata. Macrofossils from the lower part of the Yuranigh Quarterly Notes 108, 1999
Limestone Member are dominated by gastropods and brachiopods. New brachiopod species constitute a distinct pre-Brachiopod Fauna A (Percival 1992) assemblage, although no definitive age is indicated. Associated microfossils are largely ostracodes, and few conodonts have been recovered. Upper beds of the Yuranigh Limestone Member are marly carbonates of wackestone to packstone lithology. Conodonts are sparsely represented at this horizon (samples C1377, C1437) byelements attributable to four genera (Pogson & Watkins 1998, appendix 1). Of these, Staufferella is widespread in Middle to Late Ordovician strata in North America, and also occurs in the Late Ordovician Fork Lagoons Beds of central Queensland (Palmieri 1978), but has not previously been recorded from New South Wales. The element of Staufferella in sample C1377 is closely comparable with that of a species from the Late Ordovician Galena Formation of Minnesota (Ethington 1959). Another species identified in the Yuranigh Limestone Member, Belodina confluens (figure 8.21), has been shown to be much longer ranging in eastern Australia than in the North American succession (Zhen & Webby 1995), with occurrences low in the Fossil Hill Limestone at Cliefden Caves broadly correlated with the Phragmodus undatus Zone of the North American Midcontinent Fauna. This Zone is of latest Gisbornian to early Eastonian age in Australia (Webby 1995; Zhen & Webby 1995). There is a disparity in faunal content between the Yuranigh Limestone Member and the succeeding Reedy Creek Limestone (and its correlatives, including the Fossil Hill Limestone). Also, there is mutual exclusivity to fossil assemblages in the older Wahringa Limestone Member. Hence it is reasonable to infer that the Yuranigh Limestone Member occupies the interval between those two levels, and is of late Gisbornian (Gi2) age.
Reedy Creek Limestone The Reedy Creek Limestone (Ross 1961, after Pritchard (1955) and Adrian (1956)) crops out for over 24 km along strike north and south of Molong (figure 3). The formation was revised during remapping of the Bathurst 1:250 000 geological map area (Raymond et al 1998) and included in
the new Barrajin Group (Morgan, Scott & Pogson in Pogson & Watkins 1998). The limestone overlies the Fairbridge Volcanics conformably or with slight disconformity (Adrian 1971). North of Molong, the formation is conformably overlain by the Cheesemans Creek Formation, while to the south of Molong these Late Ordovician volcaniclastic rocks interfinger with the limestone. Maximum thickness of the unit is about 850 m (Morgan, Scott & Pogson in Pogson & Watkins 1998). During the remapping of the Molong 1:100 000 map area, extensive fossil collection enabled identification of many taxa not previously recorded from the Reedy Creek Limestone. In particular, a rich conodont fauna allows precise correlation with other Late Ordovician limestones on the MVB along strike to the south. Macrofossils from the Reedy Creek Limestone are, in general, quite distinct from those in the underlying Yuranigh Limestone Member. However, occurrence of the conodont species Belodina confluens in both units implies that a substantial time gap does not separate the top of the Fairbridge Volcanics and the overlying Late Ordovician limestones, at least in this area of the MVB. The highly fossiliferous thinly-bedded lower section of the Reedy Creek Limestone has been dated as Late Ordovician (Eastonian) on the basis of macrofossils, such as corals and stromatoporoids, indicative of Fauna I in the local biostratigraphic scheme of Webby (1969). That section is overlain by an upper massive relatively poorly fossiliferous limestone of previously indeterminate age. There is usually no development of a thin-bedded conspicuously fossiliferous facies above the massive limestone, such as in the Cliefden Caves Limestone Subgroup and the Regans Creek Limestone, along strike to the south, and the Bowan Park Limestone on the western side of the MVB. A Fauna II age conodont assemblage has now been recovered from a thinbedded limestone in the Reedy Creek Limestone, rich in shelly macrofossils with a distinctive lithology reminiscent of the Trilobite Hill Limestone Member of the Cliefden Caves area. Conodonts were abundant and diverse in this sample (C1394), with diagnostic species, such as Periodon grandis and Taoqupognathus blandus, well
represented. These are characteristic of the “upper assemblage” of the Cliefden Caves Limestone Subgroup succession (Zhen & Webby 1995). The range of the nautiloids Cliefdenoceras gregarium, previously known only from the Fossil Hill Limestone (Stait et al 1985), and Trocholites costatum, of Fauna I age, now can be extended into Fauna II as a result of their occurrence with these conodonts. Unfortunately, because sample C1394 is from an isolated outcrop of the Reedy Creek Limestone, a precise level within the formation cannot be established. It is significant in demonstrating that the full section of the Late Ordovician carbonate succession that developed on the eastern flank of the MVB is also shown to exist in the region north of Molong, though considerably diminished in thickness from that of the correlative limestone formations to the south.
Conodont sample C1394 upper Reedy Creek Limestone GR 679048E 6323222N Molong 1:50 000 map Conodonta Ansella sp nov Aphelognathus packhami Savage Aphelognathus webbyi Savage Aphelognathus sp Belodina confluens Sweet Belodina ?hillae Savage Besselodus sp Chirognathus cliefdenensis Zhen & Webby Chirognathus sp Drepanoistodus suberectus (Branson & Mehl) Panderodus gracilis (Branson & Mehl) Periodon grandis (Ethington) Phragmodus ?undatus Branson & Mehl Plectodina tenuis (Branson & Mehl) ?Pseudobelodina sp Pseudooneotodus mitratus (Moskalenko) ?Serraculodus sp Spinodus sp Taoqupognathus blandus An ?Yaoxianognathus tunguskaensis (Moskalenko) Yaoxianognathus sp Porifera anthaspidellid sponge hexaxon spicule
Quarterly Notes 108, 1999 19
Coelenterata indeterminate favositid Bryozoa Orbignyella sp indeterminate stictoporoid Brachiopoda fragmentary small lingulides Schizotreta sp ?Hisingerella sp Sowerbyites isotes Percival Wiradjuriella halis Percival Nautiloidea Cliefdenoceras gregarium Stait, Webby & Percival Trocholites costatum Stait, Webby & Percival Trilobita harpid fringe Echinodermata indeterminate ossicles Problematica byroniids Algae dasycladales, most likely Vermiporella Isolated limestone blocks near Eurimbla (north of Molong) also contain the Fauna II stromatoporoid Ecclimadictyon (Webby 1969). Farrell (in Talent 1995) listed brachiopods from that area, including Doleroides and Sowerbyella, which appear identical to species of Brachiopod Fauna B (= Fauna II) age from the Cliefden Caves Limestone Subgroup and the Bowan Park Limestone. The Eurimbla blocks may be allochthonous (Farrell 1996), or may have been tectonically displaced, but their most likely source is the Reedy Creek Limestone which is the closest in situ carbonate unit of Fauna II age.
Oakdale Formation The Oakdale Formation (Strusz 1960; Oakdale Group of Vandyke & Byrnes 1976) is the most extensive Ordovician unit in the MVB (figure 1). The formation occurs in two broad tracts, the eastern belt extending from Lucknow in the south to Golan in the north, a distance of over 130 km, while the western belt crops out in several small fault slices between Bournewood and Ponto localities, west of Wellington. The Oakdale Formation was significantly revised during remapping of the Bathurst and Dubbo 1:250 000 geological map areas (Raymond et al 1998; Morgan et al in prep 1999) and
the term was extended to include the Angullong Tuff (Packham 1968) northwest and southeast of Orange; undifferentiated strata (Offenberg et al 1971) north of Wellington; and undifferentiated rocks previously considered Silurian in age (Bradley, in Pickett 1982) west and southwest of Wellington. The formation has been included in the Late Ordovician Cabonne Group (Pogson & Watkins 1998; Meakin & Morgan in prep 1999). The base of the Oakdale Formation in the eastern belt is almost always faulted, an exception being to the east of Lucknow where the formation overlies the Byng Volcanics with apparent conformity. In turn it is unconformably overlain by rocks of the late Early to Late Silurian Mumbil Group and locally the Early Silurian Waugoola Group. The base of the Oakdale Formation is not exposed west of Wellington. In this area the formation is conformably overlain by the Late Ordovician to Early Silurian Kabadah Formation north of the Bournewood locality, while near Ponto the formation is unconformably overlain by rocks of the Silurian Cudal Group. The Oakdale Formation is dominated by cherty and volcaniclastic siltstone and sandstone turbidite sequences, volcaniclastic mass flow conglomerate and minor primary volcanic rocks — as well as limestone and limestone breccia. Units within the Oakdale Formation are laterally discontinuous and no complete section of the formation has been found. The turbidite sequences are well-bedded to laminated, commonly graded and are crystal- and lithic-rich. Siltstone beds locally include graptolite-rich bands. Well-bedded, dark coloured, autochthonous limestones occur on Barham Winchester property, 10 km northeast of Molong. The conglomerate horizons are massive to coarsely bedded, clast- to matrix-supported, poorly sorted and can commonly be traced for over 2 km along strike. They generally have a high magnetic susceptibility (8 000 x 10-5 SI) and exhibit a high potassium response on airborne radiometric images. Clasts range from millimetres to tens of centimetres in diameter and are dominantly andesitic to trachytic in origin, with lesser sandstone, siltstone, chert and limestone. Limestone blocks up to 10 m in diameter occur
sporadically within conglomerate beds, probably representing material that slumped from a carbonate shelf on the margins of the marine basin. The depositional environment interpreted for the Oakdale Formation is generally deep-water, on the shelf-edge or further offshore. Conodonts recovered during this study from several allochthonous limestone clasts in the Oakdale Formation are of late Darriwilian to early Gisbornian age, whereas those extracted from autochthonous limestone are Eastonian, consistent with the age indicated by graptolites from the enclosing siltstones. Strusz (1960, 1961) reported Late Ordovician (Eastonian) graptolites from the Oakdale Formation in the Wellington 1:100 000 map area, between Neurea and Dripstone. From nearby outcrops Webby (1973, 1974) described the trilobites Shumardia and ?Geragnostus (now reidentified by J. Laurie (pers comm 1990) as Dividuagnostus), and suggested that these came from an older, possibly Gisbornian, section (Webby et al 1981). Vandyke and Byrnes (1976) listed additional Late Ordovician graptolites from the Oakdale Formation, and also noted the occurrence in associated limestones of the tabulate coral Coccoseris speleana Hill. The latter is indicative of coral/stromatoporoid Fauna I of the local biostratigraphic scheme of Webby (1969), and Webby and Kruse (1984). Sherwin (1994a) identified three graptolite species generally indicative of a late Eastonian age from the Oakdale Formation, and Sherwin (1994b) recorded brachiopods and graptolites, some of the latter suggesting an age straddling the Eastonian–Bolindian boundary. Late Ordovician conodonts had previously been recovered from the Oakdale Formation north of Wellington, by Pickett (1972), who reported eight form-species. During the current mapping program, two distinct faunas have been identified. Allochthonous limestone blocks at several widely scattered localities yielded, inter alia, the shallow water conodont Appalachignathus delicatulus, type and only species of this distinctive genus, which ranges through the upper Pygodus serra – Pygodus anserinus – lower Amorphognathus tvaerensis zones of the North Atlantic faunal succession (Bergström et al 1974).
Quarterly Notes 108, 1999 20
This range spans the North American late Whiterockian-early Mohawkian series, which Webby (1995) equated with the late Darriwilian to early Gisbornian interval.
Conodont Sample C1490 llochthonous limestone in Oakdale Formation GR 684000E 6361550N Cumnock 1:50 000 map Conodonta Ansella sp Appalachignathus delicatulus Bergström, Carnes, Ethington, Votaw & Wigley ?Bryantodina sp ?Icriodella sp Panderodus or Protopanderodus sp Plectodina aculeata Hadding Gastropoda Brachiopoda lingulate fragments Co-occurrence of Appalachignathus and Plectodina aculeata in sample C1490 implies correlation with the P. aculeata and lower Erismodus quadridactylas zones of the basal Mohawkian in the North American Midcontinent Fauna (cf Sweet 1988, chart 1), approximately equivalent to the upper half of the Gisbornian 1 stage in Australian usage (Nicoll & Webby 1996). In Tasmania, Banks and Burrett (1980) recorded Appalachignathus sp from their assemblages OT10 and OT11, correlated with the late Darriwilian (Da3-Da4). The abundance of this genus in widely separated localities (C1414, C1425 and C1490) along the northern MVB suggests derivation from shallow-water carbonates of this age (Wahringa Limestone Member and equivalent units, now eroded or covered). Those units presumably accumulated on the crest or flanks of the MVB, prior to their redeposition as allochthonous blocks in deeper-water younger clastic beds of the Oakdale Formation.
property, 10 km northeast of Molong) yielded Belodina confluens and ?Yaoxianognathus tunguskaensis, indicative of the “lower assemblage” (early Eastonian, Ea1) of Zhen and Webby (1995).
Conodont Sample C1432 autochthonous limestone in the Oakdale Formation GR 681476E 6371233N Cumnock 1:50 000 map Conodonta Panderodus sp Paroistodus sp Periodon sp Plectodina sp Protopanderodus sp Yaoxianognathus cf Y. sesquipedalis (Nowlan & McCracken) Ostracoda 4 genera Bryozoa several genera, mainly trepostomes, and a phylloporinid Brachiopoda lingulates (abundant but fragmentary) 2 articulate genera (1 is possibly Christiania) Trilobita remopleuridid odontopleurid Porifera lithistid sponge Late Eastonian graptolites confirm the age given by the shelly fauna in sample C1432 (listed above), which strongly resembles the assemblage obtained from limestone breccias in the basal Malongulli Formation (Ea3) near Cliefden Caves. Conodonts are not abundant in the sample, but those that occur appear identical (or nearly so) with specimens illustrated from that horizon by Trotter and Webby (1995).
Eastonian faunas — western belt Conodont Sample C1491 Oakdale Formation
Eastonian faunas — eastern belt
GR 669273E 6401756N Wellington 1:50 000 map
The second distinctive conodont auna recovered from the Oakdale Formation occurs in bedded, apparently in situ limestones, probably deposited in moderately deep water. Sample C1388 (from Barham Winchester
Conodonta Belodina confluens Sweet Drepanoistodus sp Panderodus sp Periodon grandis (Ethington)
?Plectodina sp Protopanderodus sp cf P. liripipus Kennedy, Barnes & Uyeno Brachiopoda new orthiid, transitional between Orthinae and Productorthinae undescribed new genus of ?craniopsid Multispinula sp acrotretide discinide lingulate Gastropoda indeterminate opercula Conulariida 2 fragments Coelenterata Quepora cf Halysites praecedens Webby & Semeniuk Porifera indeterminate cylindrical or hemispherical sponge Bryozoa ?Homotrypa stictoporoid Trilobita ?new encrinurid genus of sub-family Cybelinae indeterminate proetiid genus Problematica byroniid Only Eastonian ages have been obtained for the Oakdale Formation and equivalent offshore strata flanking the west side of the northern MVB. Many components of the diverse assemblage in sample C1491 (listed above), particularly the undescribed ?craniopsid brachiopod and Multispinula, occur also in the lower Malongulli Formation (late Eastonian) near Cliefden Caves. Comparable similarities are noted with the Tucklan Formation south of Dunedoo, which also contains an Eastonian conodont fauna (Percival 1998). The late Eastonian age is supported by early halysitid corals, which make their first appearance in the New South Wales succession in Fauna III. Webby and Semeniuk (1969) described Quepora calamus from the upper Cargo Creek Limestone (late Eastonian Fauna III), and Halysites praecedens from a slightly younger horizon in the Canomodine Limestone and the upper part of the Clearview Limestone Member of the Ballingoole Limestone
Quarterly Notes 108, 1999 21
at Bowan Park (also Fauna III). The species from the Oakdale Formation is morphologically a Quepora (lacking microcorallites and septa), but displays the growth habit of H. praecedens, with open chains not surrounding lacunae.
Sourges Shale The Sourges Shale (Offenberg et al 1971; Bradley, in Pickett 1982) crops out in a fault- bounded, complexly folded meridional belt east and northeast of Cumnock (figure 5). The formation has been substantially redefined by Morgan (in Meakin & Morgan in prep 1999; Morgan et al in prep 1999) because its age is significantly older than thought by earlier workers (Maggs 1963; Offenberg et al 1971; Bradley, in Pickett 1982). The eastern margin of the Sourges Shale lies just west of the boundary shown by Bradley (in Pickett 1982) and does not include his Late Silurian graptolitic black cherts. The type section for the Sourges Shale extends along Sourges Creek southeast of Brookvale homestead from just west of the limestone member at GR 665140E 6357200N to GR 666550E 6357310N (Cumnock 1:50 000 map), where the shale is faulted against the Hanover Formation. Thickness of the formation is uncertain due to complex internal folding and faulting. The Sourges Shale comprises pale grey to buff, well-bedded to laminated shale, siltstone and fine sandstone, rare latitic volcanic rocks, and a prominent but discontinuous limestone member. The latter is wellbedded to massive, and is abundantly fossiliferous. Two samples from the limestone member yielded diverse Late Ordovician (Eastonian) faunas.
Conodont Sample C1519 limestone member of Sourges Shale GR 665280E 6354080N Cumnock 1:50 000 map Conodonta Belodina confluens Sweet Belodina cf B. hillae Savage Panderodus sp ?Paroistodus sp Phragmodus sp Serraculodus sp Taoqupognathus philipi Savage or T. blandus An Yaoxianognathus wrighti Savage
?Yaoxianognathus tunguskaensis (Moskalenko) distinctive but unknown “fishhook” element Coelenterata–Tabulata Bagjolia sp ?Fletcheria stipulosa Webby Heliolites sp Tetradium cf T. tenue Webby & Semeniuk Tetradium ?cribriforme (Etheridge) Brachiopoda Eodinobolus sp Protozyga definitiva Percival Zygospira carinata Percival Gastropoda ?Murchisonia sp Lophospira sp Trochonema sp Holopea sp The macrofauna in sample C1519 (listed above) matches precisely the assemblage in the Gerybong Limestone Member of the Daylesford Limestone in the Bowan Park Limestone Subgroup. The tabulate corals, particularly Tetradium cf tenue and ?Fletcheria stipulosa, closely resemble species previously described solely from the Gerybong Limestone Member. It is also at this level that Zygospira carinata first appears, in Brachiopod Fauna AB (Percival 1992). The depositional environment of the Gerybong Limestone Member has been interpreted (Percival 1995) as ranging from low intertidal (Benthic Assemblage 2) through shallow subtidal (BA 3). This accords well with evidence for the depth of deposition of the limestone member in the Sourges Shale, as the Eodinobolus valves present are all transported from their preferred BA 1-2 habitat and stacked in the outcrop as if by storm or current activity. The age indicated is early Eastonian (Ea1), equivalent to Webby’s (1969) coral/ stromatoporoid Fauna I.
Conodont Sample C1547 limestone member of Sourges Shale GR 665300E 6357360N Cumnock 1:50 000 map Conodonta Aphelognathus packhami Savage Belodina confluens Sweet Belodina hillae Savage Belodina sp
Panderodus gracilis (Branson & Mehl) Panderodus panderi (Stauffer) Phragmodus undatus Branson & Mehl Plectodina ?tenuis (Branson & Mehl) Pseudooneotodus mitratus (Moskalenko) Taoqupognathus philipi Savage Yaoxianognathus wrighti Savage ?Yaoxianognathus tunguskaensis (Moskalenko) Coelenterata ?Palaeophyllum sp ?Plasmoporella sp Algae Girvanella sp The age of sample C1547 (above) is most likely middle Eastonian (probably Ea2-Ea3), based on the occurrence of Phragmodus undatus — which has previously only been rarely recorded from Fauna II, Ea2 age strata (Zhen & Webby 1995) and is only common in Fauna III (Ea3) (Trotter & Webby 1995). This age determination is supported by the presence of corals which, although sparse and poorly preserved, appear to be genera which only occur in Faunas II and III. However, the conodont fauna also contains several elements which are restricted elsewhere to early Eastonian (Ea1) strata. Examples include Aphelognathus packhami, which appears to be almost entirely confined (with one exception) to Fauna I in the lower part (Fossil Hill Limestone) of the Cliefden Caves Limestone Subgroup (Savage 1990; Zhen & Webby 1995). Confirmatory evidence is provided by co-occurrence of species such as Yaoxianognathus wrighti and potentially Taoqupognathus philipi (although the diagnostic Sc3 element of the latter was not recovered, and the species may equally well be identified as T. blandus, of Fauna II age).
REGIONAL TECTONIC SYNTHESIS Our developing understanding of the geological evolution of central New South Wales during the Ordovician is based on detailed mapping combined with thorough palaeontological analysis, which has confirmed a previously recognised consistency between the stratigraphic histories of the central JNVB and northern MVB (Percival et al 1997). The palaeontological data presented herein also define the
Quarterly Notes 108, 1999 22
presence of a Middle to Late Ordovician volcanic hiatus (Glen et al 1998) across this region of the Lachlan Orogen. The oldest dated formation in the JNVB is the Yarrimbah Chert Member of the Nelungaloo Volcanics, determined by Sherwin (1979) as late Lancefieldian to possibly early Bendigonian in age. On the northern MVB, the Hensleigh Siltstone contains early Bendigonian graptolites and conodonts, and overlies the volcaniclastic Mitchell Formation. Above the graptolite horizons in both volcanic belts, there are no units that can be positively dated until limestones with Pygodus conodont assemblages of late Darriwilian (Da3Da4) age. This level appears to be of regional significance, representing a widespread cessation of shallow water sedimentation in the volcanic belts — throughout the Lachlan Orogen outcrop in central New South Wales no fossils have been reliably dated as between middle Bendigonian (the minimum age of the Hensleigh Siltstone) and late Darriwilian (Percival et al 1997). Conodont faunas documented by Nicoll (1980), Pickett (1985), Stewart and Glen (1986), Pickett (1992), Fowler and Iwata (1995), Iwata et al (1995) and Stewart and Fergusson (1995) are all no older than Da3 (figure 10). The maximum extent of the stratigraphic break between the Early Ordovician and late Middle Ordovician volcanic episodes could therefore encompass all of the Chewtonian, Castlemainian and Yapeenian stages. The basal Fairbridge and time-equivalent Goonumbla Volcanics probably commenced accumulation during the early or middle Darriwilian, and persisted (with intermittent development of relatively thin, shallow-water carbonates around isolated emergent volcanic islands) into Late Ordovician (late Gisbornian) time. More widespread limestone deposition then characterised the latest Gisbornian to early Eastonian history of the MVB and JNVB. Given the similarity now recognised between their sedimentary and volcanic histories during much of the Ordovician (figure 10), it is probable that these areas did not start to become distinct entities until late in the period, around late Eastonian time at the earliest. Although the record of Ordovician sedimentation in the Rockley-Gulgong Volcanic Belt to
the east is comparatively incomplete, deep water late Eastonian faunas in that belt are identical with those of the MVB (Percival 1999). This lends further support to the arc-rifting model of Glen et al (1998). Whereas the thrust of this latter paper was directed towards documentation of a volcanic hiatus in the Lachlan Orogen, our work has concentrated on recognition and definition of significant sedimentological breaks in the depositional history of this region. The two streams of research are related by the dependence of fossiliferous shallow-water sediments on emergent (or nearly so) volcanic islands as loci of formation; their presence in turn allows age-dating of the volcanic phases.
SUMMARY In the Molong Volcanic Belt succession of central New South Wales, the oldest fossiliferous strata currently recognised are found in the Hensleigh Siltstone, where Early Ordovician conodonts occur in limestones underlying beds containing graptolites of early Bendigonian age. These strata are most similar in age to the late Lancefieldian– early Bendigonian Yarrimbah Chert Member in the Junee–Narromine Volcanic Belt, and presumably shared a comparable deep-water age depositional environment. A major time break of regional significance, throughout the central New South Wales region of the Lachlan Orogen, spans the Chewtonian, Castlemainian and Yapeenian stages. It separates a earliest Ordovician phase of volcanism and volcaniclastic deposition (Mitchell Formation) and subsequent deep water sedimentation (Hensleigh Siltstone), from an episode of Middle to early Late Ordovician volcanism (represented in the study area by the Fairbridge Volcanics). The Fairbridge Volcanics contain evidence at several levels of allochthonous limestones derived from local shallow-water carbonate deposits. These algal-rich limestones also include brachiopods and conodonts indicative of an age no older than Darriwilian (late Middle Ordovician). Autochthonous limestones are present at two distinct stratigraphic levels in the Fairbridge Volcanics. The older of these,
the Wahringa Limestone Member, has a late Darriwilian–early Gisbornian (Da4-Gi1) age which is well established by the co-occurrence of Periodon aculeata, Phragmodus flexuosus and Pygodus sp in the basal beds of the member. Belodina compressa is also abundant in the middle and upper part of the Wahringa Limestone Member, but this conodont species is absent from the younger Yuranigh Limestone Member of the upper Fairbridge Volcanics. Any time gap separating deposition of the Yuranigh Limestone Member (most likely of late Gisbornian, Gi2 age) and the overlying Reedy Creek Limestone was of relatively limited duration, as the long-ranging conodont Belodina confluens (which in North American successions succeeds the occurrence of B. compressa) is prominent at both levels. The brachiopod fauna of the Yuranigh Limestone Member differs only at species level from those of the Reedy Creek and equivalent Eastonian limestones. The Reedy Creek Limestone contains a typical Fauna I (of early Eastonian, Ea1) age macrofossil and microfossil assemblage in its lower, thinly bedded section. Conodonts identified in this study demonstrate that the upper part of the formation extends into Fauna II (Ea2 age). The Oakdale Formation appears to have been deposited from ?Gisbornian to Eastonian or Bolindian time on the eastern flank of the MVB, although these sediments enclose allochthonous limestones of late ?Darriwilian to Eastonian age which were presumably sourced from the Wahringa and Yuranigh Limestone Members (or their equivalents) and the Reedy Creek Limestone, deposited on the crest of the MVB to the west. Ages obtained for the Oakdale Formation, and the correlative Sourges Shale, on the western flank of the MVB are restricted to the Eastonian.
ACKNOWLEDGMENTS Barry Webby, Gordon Packham, John Pickett and Dick Glen are thanked for their incisive reviews of this paper. Gary Dargan assisted with thin section preparation. SEM illustrations of the conodonts were taken by Yongyi Zhen, who also checked their identifications. Macrofossil photographs were prepared by David Barnes. Margaret McLaren
Quarterly Notes 108, 1999 23
NORTHERN MOLONG VOLCANIC BELT
JUNEENARROMINE VOLC. BELT
ROCKLEYGULGONG VOLCANIC BELT
Da4 Da3 Da2 Da1
WAHRINGA LST MBR
* TUCKLAN and BURRANAH *FORMATION
ADAMINABY GROUP and TRIANGLE GROUP
* * * *
* * * *
REEDY CREEK LST
Gunningbland Shale Mbr
? CK * MANS FM
Ya2 Ya1 Ca4 Ca3 Ca2 Ca1 Ch2
Be2 Be1 La3
YARRIMBAH CHERT MEMBER
La2 La1.5 La1
Figure 10. Revised correlations through the Ordovician for selected regions across the Lachlan Orogen, from west (Cobar Trough) to east (Monaro Trough, and accreted Narooma Terrane). Note the extent of the stratigraphic gap from the late Bendigonian and Chewtonian, to the early Darriwilian. Abbreviations in left-hand column refer to the standard subdivisions of the Victorian graptolite-based biostratigraphic scheme. Asterisks mark points of biostratigraphic control. References for individual regions — Cobar Trough: Iwata et al (1995), Stewart &Glen (1986); Junee–Narromine Volcanic Belt: Percival et al (1997), Pickett (1985), Sherwin (1979); Northern Molong Volcanic Belt: this paper; Rockley–Gulgong Volcanic Belt: Fowler & Iwata (1995), Percival (1999), Stewart and Ferguson (1995); Monaro Trough: Jenkins (1982), Nicoll (1980); Narooma Terrane: Stewart & Glen (1991).
Quarterly Notes 108, 1999 24
and Cheryl Hormann skilfully drafted the maps and diagrams. Richard Facer improved the final presentation with his attention to detail. This is a contribution to IGCP Project No 410: The Great Ordovician Biodiversification Event.
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Note. Figure 8 in the Molong paper of this issue was prepared in the "usual" way before digital manipulation. Figure 3 in the Gulgong paper (and figures 3 and 4 in the paper by I.G. Percival in Quarterly Notes No 107) were prepared directly from digital data recorded by the SEM. Readers may be interested in the comparison.
Quarterly Notes 108, 1999 27
Quarterly Notes of the Geological Survey of New South Wales No 108 GUEST EDITOR:
Richard A. Facer GEOLOGICAL EDITOR:
Ross Stewart MANAGER PUBLISHING & MARKETING:
Peter Walker CARTOGRAPHY:
CONTENTS Late Ordovician biostratigraphy of the northern Rockley–Gulgong Volcanic Belt
Margaret McLaren Cheryl Hormann
Ordovician stratigraphy of the northern Molong Volcanic Belt: new facts and figures ............................................ 8
I.G. Percival. E.J. Morgan & M.M. Scott
Val Grant PUBLISHING ASSISTANT:
NEXT ISSUE Development of DIGS® Digital Imaging GS (Geological Survey) System G. Brookes, J. Mong, J. Xie, C. Bembrick, N. Aadil, G. Kouzmina, B. Shi & S. Barry
Issued February 1999 ISSN 0155-3410 Published under the authority of the Minister for Mineral Resources 1999
© New South Wales Department of Mineral Resources 1999. Short quotations from the text of this publication and copies of maps, figures, tables, etc (excluding any subject to pre-existing copyright) may be used in scientific articles, exploration reports and similar works provided that the source is acknowledged and subject to the proviso that any excerpt used, especially in a company prospectus, Stock Exchange report or similar, must be strictly fair and balanced. Other than for the purposes of research or study the whole work may not be reproduced without the permission in writing of the Department.
Quarterly Notes 108, 1999 28