Papers in Quarterly Notes are subject to external review. External reviewer for this issue was Simon Bodorkos. His assistance is appreciated.

Quarterly Notes is published to give wide circulation to results of studies in the Geological Survey of New South Wales. Papers that arise from team studies with external researchers are also welcome. Contact: john.watkins@dpi.nsw.gov.au

ISSN 0155-3410

© State of New South Wales through NSW Department of Primary Industries 2009

D.J. Pogson

Geological Survey of New South Wales, NSW Department of Primary Industries

AUTHOR

N S W D P I

Geologic al Sur vey of New South Wales

Quarterly NotesJanuary 2009 No 130

The Siluro-Devonian geological time scale: a critical review and interim revisionAbstractAgreement between biostratigraphically constrained SHRIMP U–Pb zircon isotopic dates from the Lachlan Orogen and international time scales for the Silurian and Devonian has been poor. While some of the disagreement has been resolved by the latest international Silurian and Devonian time scales, a critical review of the data points used indicates that at least one Silurian date (Laidlaw Volcanics, New South Wales, Australia) should be discarded on the grounds of inadequate biostratigraphic constraint and potential ambiguity in the interpretation of the isotopic results. The readmission of two other apparently sound dates for the late Silurian, in conjunction with a SHRIMP U–Pb zircon date of ~432 Ma for the early Wenlock Canowindra Volcanics in the Lachlan Orogen, requires significant revision of the Silurian Stage boundary ages estimated by A Geologic Time Scale 2004. Specifically, the base of the Lochkovian (Devonian) is shifted from 416 Ma to 418.1 Ma, the base of the Pridoli from 418.7 Ma to 420.0 Ma, the base of the Ludlow from 422.9 Ma to 427.0 Ma, and the base of the Wenlock from 428.2 Ma to 432.5 Ma, with the base of the Llandovery (Silurian) unchanged at 443.7 Ma.

Suitable biostratigraphically constrained, isotopic ages are scarce in the early Devonian (Lochkovian–Emsian), and the global time scale is correspondingly poorly constrained. The Turondale and Merrions formations within the Hill End Trough of the Lachlan Orogen, are characterised by good biostratigraphic control indicating middle Lochkovian and middle Pragian ages, respectively. Unfortunately, existing SHRIMP U–Pb zircon dates for these units are too imprecise (owing to problems arising from undetected heterogeneity and/or loss of radiogenic Pb in the zircon standards used for data calibration) to provide meaningful constraints on the Devonian time scale.

The mid-Silurian to early Devonian is a key period in the evolution of the Lachlan Orogen, but current international geological time scales for this period are constrained by very few data points and thus provide a poor framework for an understanding of the tectonic development of the Lachlan Orogen. Future dating in the orogen should focus on providing more robust ages for the bases of the Wenlock, Ludlow, Pridoli, Pragian and Emsian, and on confirmation of the 418.1 Ma age for the base Lochkovian via high-precision U–Pb zircon dating of suitable biostratigraphically constrained volcanic units.

January 20092

CONTENTS

Abstract 1

Introduction 2

History of time scale development 3

Applicability of isotopic dating methods for time scale use 3

Isotopic dating from the Lachlan Orogen and its relevance to the geological time scale 4

Review of isotopic dates used to constrain the Siluro-Devonian geological time scale 4

Silurian 4

Devonian 8

Conclusions and recommendations 10

Acknowledgements 11

References 11

Future papers: 16

Production co-ordination Geneve Cox, Simone Meakin general editing :

Geological editor: Richard Facer

Layout: Carey Martin

Cover photograph: Typical outcrop of the Laidlaw Volcanics at Laidlaw Trigonometric Station, with the town of Yass in the background (Photographer G. Colquhoun).

The information contained in this publication is based on knowledge and understanding at the time of writing (November 2008). However, because of advances in knowledge, users are reminded of the need to ensure that information upon which they rely is up to date and to check currency of the information with the appropriate officer of NSW Department of Primary Industries or the user’s independent adviser.

Keywords: Silurian, Devonian, Llandovery, Wenlock, Ludlow, Pridoli, Lochkovian, Pragian, Emsian, Eifelian, Givetian, Frasnian, Famennian, time scale, Lachlan Orogen, isotopic dating, graptolites, conodonts

IntroductionOver the last 15 years, a number of workers in the Lachlan Orogen of southeastern Australia (e.g. VandenBerg et al. 2000; Lyons et al. 2000; Pogson & Percival 2002, 2003) have found many points of disagreement between international geological time scales (e.g. Harland et al. 1990) and biostratigraphically constrained isotopic dates (particularly SHRIMP U–Pb zircon) obtained from the Cambrian to Devonian of the Lachlan Orogen. Some points of difference have been resolved by more recent revisions of all or parts of the international geological time scale (Gradstein et al. 2004; Kaufmann 2006), in conjunction with a better understanding of some of the problems besetting zircon standards used to calibrate earlier SHRIMP U–Pb zircon isotopic dates (e.g. Jagodzinski & Black 1999; Compston 2000a, 2000b; Black & Jagodzinski 2003). However, there remain very few isotopic dates constraining international time scale schemes for the mid-Silurian to early Devonian, and some of these are characterised by poor biostratigraphic control and/or uncertain isotopic age reliability.

This paper critically reviews the isotopic and biostratigraphic reliability of the data points currently constraining the international Siluro-Devonian time scale, and identifies conflicting or unreliable data points in the current Silurian scheme of Melchin et al. in Gradstein et al. (2004), and the current Devonian scheme of Kaufmann (2006).

It also proposes that, as an interim measure, selected biostratigraphically constrained SHRIMP U–Pb zircon isotopic dates from the Lachlan Orogen, which have been derived using homogeneous zircon standards (e.g. TEMORA: Black, Kamo, Allen et al. 2003; Black et al. 2004) and the latest analytical techniques and data reduction protocols, be used to help constrain key parts of the middle Silurian to early Devonian portion of the time scale. It is hoped that this interim revision of the Siluro-Devonian geological time scale will provide a sounder basis for relating the timing of magmatic events with the lithostratigraphic and structural development of the Lachlan Orogen. It also identifies suitable targets for future high-precision isotopic dating in the orogen, which could better constrain the Siluro-Devonian geological time scale internationally.

3Quarterly Notes

History of time scale developmentThere have been many attempts to establish an absolute geological time scale for all, or part, of the Phanerozoic. The earliest attempts were made by Holmes (1937, 1947, 1960) using isotopic age data in conjunction with other parameters derived from the rock record — including biostratigraphy, strata thickness and estimated deposition rates. A new phase of time scale development commenced with the systematic collection of a database of isotopic ages together with critical evaluation and interpretation by Harland et al. (1964).

In 1976, the International Union of Geological Sciences (IUGS) Subcommission on Geochronology recommended a new set of radioactive decay constants for the U–Th–Pb, Rb–Sr, and K–Ar systems (Steiger & Jäger 1977, 1978). This work led to the revision of pre-existing isotopic ages, and largely resolved widely observed disagreements between Rb–Sr and K–Ar isotopic dates for the same sample. Further revisions of the time scale followed utilisation of the new decay constants, together with a more rigorous selection of isotopic data and statistical methods, to try to minimise any misfit of stratigraphically inconsistent isotopic ages (e.g. Odin 1982; Harland et al. 1982, 1990).

More recent versions of the time scale have also relied heavily on geomathematical/statistical techniques in conjunction with best-fit line techniques (described in detail by Gradstein et al. 2004) for the estimation of numerical ages for stage boundaries. This is especially so in the Palaeozoic where there is a relative scarcity of reliable isotopic dates with high biostratigraphic precision.

Applicability of isotopic dating methods for time scale useEarlier versions of the geological time scale relied heavily on K–Ar and Rb–Sr isotopic dating. However, the precision of dates determined using these methods is inherently limited, especially in the Phanerozoic, and both decay series are relatively readily reset (partially or completely) by subsequent thermal events. The resulting potential ambiguity between the age of the ‘closure event’ recorded by the isotopic system (e.g. Dodson 1973) and the age of the geological event targeted by the stratigrapher limits the usefulness of K–Ar and Rb–Sr dates in constraining points on the geological time scale, particularly in the Palaeozoic.

The last 25 years have seen the development of new analytical techniques which provide high-precision U–Pb isotopic age determinations from magmatic zircon. Zircon has the advantage of being much more resistant to thermal resetting than the K-bearing minerals used for K–Ar and Rb–Sr isotopic age dating. These U–Pb zircon isotopic age dates have been mainly derived via Isotope Dilution–Thermal Ionization Mass Spectrometric analysis (ID–TIMS), but have also been obtained by the use of the Sensitive High Resolution Ion Microprobe (SHRIMP), especially in Australia. Consequently, the use of isotopic dates derived by these methods (particularly the former), where available, has generally superseded those derived by Ar–Ar, Rb–Sr and K–Ar methods for time scale use (although recent improvements in Ar–Ar techniques have improved the utility of Ar–Ar isotopic data for the purpose of constraining the time scale: Villeneuve in Gradstein et al. 2004).

The isotopic age data derived by both the ID–TIMS and SHRIMP techniques can be influenced by: the presence within the sample of an inherited zircon component (i.e. pre-dating the true igneous crystallisation age of the host rock), producing an ‘older’ apparent magmatic age; and/or post-crystallisation loss of radiogenic Pb from the zircon lattice, producing a ‘younger’ apparent magmatic age. Techniques to recognise and avoid both these problems have been developed.

Use of the SHRIMP technique in the 1990s also suffered from originally undetected micrometre-scale heterogeneities present in some of the zircon standards used (e.g. SL13: Jagodzinski & Black 1999; Compston 2000a), and uncertainty regarding the absolute 206Pb/238U calibration value to be used for other standards (e.g. QGNG: Daly et al. 1998; Black et al. 2004). These problems have largely been rectified by the identification and calibration of homogenous Phanerozoic zircon standards (e.g. TEMORA: Black, Kamo, Allen et al. 2003; Black, Kamo, Williams et al. 2003; Black et al. 2004); together with advances in instrumentation, in data reduction protocols, and in the understanding of potential sources of error in SHRIMP analyses (e.g. Black & Jagodzinski 2003, Black 2005). These advances have, in general, improved both the precision of U–Pb SHRIMP data and its consistency with ID–TIMS data obtained from the same sample. Thus, although the use of SHRIMP isotopic age data has not been favoured for time scale use in the past, it is now considered comparable to isotopic age data derived via ID–TIMS (Villeneuve in Gradstein et al. 2004).

January 20094

et al. 2000; Wilde 2002), and also utilised selected data from the published work of Wyborn et al. (1982), Tucker, Krogh et al. (1990), Tucker and McKerrow (1995), Tucker, Bradley et al. (1998). It showed that the Silurian to Devonian geological time scale was constrained by very few data points, and that some of them were considered to be of uncertain reliability. Agreement with locally derived SHRIMP U–Pb zircon isotopic ages for the middle to late Silurian was particularly poor. The revised Silurian to Devonian time scale of Pogson and Percival (2002, 2003) (Table 1) was constructed by combining locally derived biostratigraphically constrained SHRIMP U–Pb zircon isotopic dates with selected isotopic dates from the literature, using graphical methods.

The publication of A Geologic Time Scale 2004 (Gradstein et al. 2004) provided little new data for the Siluro-Devonian. Although some older data points have been abandoned, the choice of data and its treatment still leaves significant disagreement with SHRIMP-derived data from the mid-Silurian to early Devonian of the Lachlan Orogen.

Review of isotopic dates used to constrain the Siluro-Devonian geological time scale

SilurianThe time scale of Melchin et al. in Gradstein et al. (2004) for the Silurian is constrained by just seven isotopic dates. Not only are the data sparse overall, but some key isotopic dates used to constrain the middle to late Silurian are of questionable reliability, and these dates have been used to reject other data that appear to be more reliable.

This review rejects the hitherto widely accepted isotopic date of 420.7±2.2 Ma (Wyborn et al. 1982) for the Laidlaw Volcanics (New South Wales, Australia) which had been used to help constrain the base of the Ludlow by Melchin et al. in Gradstein et al. (2004). There are three reasons for this.

Firstly, the isotopic date is derived from an average of Rb–Sr whole-rock, Rb–Sr biotite and K–Ar biotite dates. These isotopic systems are potentially readily reset (partially or completely) by post-crystallisation thermal events, and the preferred date of Wyborn et al. (1982) can only be regarded as a minimum age for igneous crystallisation of the analysed sample.

Isotopic dating from the Lachlan Orogen and its relevance to the geological time scaleA significant number of new isotopic age dates (primarily via SHRIMP) was obtained during regional mapping projects undertaken in the Lachlan Orogen of southeastern Australia during the 1990s by the Geological Survey of New South Wales (GSNSW), the then Geological Survey of Victoria (now Geoscience Victoria — GSV), and the then Bureau of Mineral Resources (BMR; later the Australian Geological Survey Organisation — AGSO; now Geoscience Australia — GA). A collection of these data, which included a significant number of dates with good biostratigraphic control, together with pre-existing isotopic data, revealed a poor fit with the contemporary geological time scale (Harland et al. 1990), especially for the Palaeozoic.

The revisions by Tucker, Krogh et al. (1990), Tucker and McKerrow (1995), Young and Laurie (1996) and Tucker, Bradley et al. (1998) (Table 1) reduced some of the discrepancies between these new isotopic dates and the time scale of Harland et al. (1990). This led VandenBerg et al. (2000) to establish their own modified geological time scale for the Cambrian to Devonian of Victoria. Their modified geological time scale drew on the abovementioned revisions for the Ordovician to Devonian (Table 1) which provided the best fit to the local isotopic data, and revisions to the Cambrian were largely based on the summary of Bowring and Erwin (1998). In New South Wales at the same time, Lyons et al. (2000) also preferred to use a modified geological time scale for the Ordovician to Devonian (similar to that of VandenBerg et al. 2000; see Table 1) in the Explanatory Notes for the Forbes 1:250 000 geological map sheet. It drew heavily on Young and Laurie (1996) but incorporated the revisions of Young (1997) and Tucker, Bradley et al. (1998) around the Siluro-Devonian boundary.

Projects undertaken by GSNSW, including a workshop on the evolution of the Hill End Trough and a synthesis of the Lachlan Orogen in New South Wales, led to a review of the Ordovician to Devonian time scale for the Lachlan Orogen by Pogson and Percival (2002, 2003). That review drew on a database of biostratigraphically constrained SHRIMP U–Pb zircon isotopic ages obtained by GSNSW and BMR/GA over the previous 10 years (Pogson & Watkins 1998; Meakin & Morgan 1999; Jagodzinski & Black 1999; Lyons

5Quarterly Notes

Table 1. Comparison of recent time scales for the Ordovician to Devonian

PERIOD EPOCH STAGE Ma1 Ma2 Ma3 Ma4 Ma5 Ma6 Ma7 Ma8 Ma9 Ma10

Carboniferous Early Tournasian 363 354 362 354 354 362 359.2 360.7 360.7

Devonian

LateFamennian 367 364.5 376.5 364.5 364 376.5 374.5 376.1 376.1

Frasnian 377 369.5 382.5 369.5 370 382.5 385.3 383.7 383.7

MiddleGivetian 381 377.5 387.5 377.5 380 387.5 391.8 388.1 388.1

Eifelian 386 391 384 394 384 390 395 397.5 391.9 391.9

Early

Emsian 394 400 399.5 409.5 400 401 406 407.0 409.1 409.1

Pragian 401 412 404.5 413.5 408 410 411.2 412.3 412.3

Lochkovian 409 417 410 418 418 418 418 416.0 418.1 418.1

Silurian

LatePridoli 411 419 414 419 419 419 419.5 418.7 420.0

Ludlow 424 423 420 424 426 422.9 427.0

EarlyWenlock 430 428 425 428 428 433 428.2 432.5

Llandovery 439 443 434 441 441 441 443 443.7 443.7

Ordovician

Late

Bolindian

Eastonian

Gisbornian

443 447 450.2 450.2

454.5 454 455.8 455.8

458 459 459 459 461 460.9 460.9

Middle

Darriwilian

Yapeenian

Castlemanian

467 469 468.1 468.1

471 470 468.8 468.8

477 477 477 473 471.8 471.8

Early

Chewtonian

Bendigonian

Lancefieldian

Pre-Lancefieldian

481 474 473.6 473.6

484.5 475 476.4 476.4

510 495 490 513 490 490 489 487.2 487.2

490.2 488.3 488.3

Note: Ages in million years (Ma) apply to the base of the stage.

Sources of data: Ma1 Harland et al. (1990) Ma2 Tucker and McKerrow (1995) Ma3 Young and Laurie (1996) Ma4 Ordovician and Early Silurian: Tucker, Krogh et al. (1990) Late Silurian and Early Devonian: Tucker, Bradley et al. (1998) Ma5 VandenBerg et al. (2000) Ma6 Lyons et al. (2000) Ma7 Pogson and Percival (2002, 2003) Ma8 Gradstein et al. (2004) Ma9 Kaufmann (2006) Ma10 Revised Silurian–Devonian geological time scale, this paper

January 20096

Secondly, the Laidlaw Volcanics was originally thought to be biostratigraphically constrained to the early Ludlow by conodont faunas in the sedimentary rocks overlying and underlying the Laidlaw Volcanics in the type area at Yass (Link & Druce 1972). More recent reassessment of these conodont faunas in view of changes in conodont taxonomy (Simpson 1995; I.G. Percival pers. comm. 2005) indicates that they have an age range of middle–late Llandovery to early Ludlow. Thus they provide no useful biostratigraphic constraint for the Laidlaw Volcanics.

Finally, the samples analysed by Wyborn et al. (1982) came from separate fault blocks to the northwest and southwest of Canberra, which lie 40 km and 50 km distant respectively from the type area at Yass. Consequently, the biostratigraphic control in the analysed samples is even poorer than that constraining the Laidlaw Volcanics in the type area.

However, an alternative isotopic age constraint for the base of the Ludlow is provided by a 40Ar/39Ar isotopic date for a bentonite within the Middle Elton Formation (Shropshire, England) which is associated with graptolites indicative of the earliest Ludlow

nilssoni and scanicus graptolite zones (Figure 1). The original 40Ar/39Ar isotopic date for this bentonite of 423.7±1.7 Ma (Kunk et al. 1985) was revised to 426.8±1.7 Ma by Melchin et al. in Gradstein et al. (2004) as a consequence of the revision of the age of the monitor standard used, following the recent intercalibration of that monitor standard with the U–Pb system. This isotopic date points to an age of at least 427.0 Ma for the base of the Ludlow.

In addition, acceptance of the unreliable isotopic date for the base of the Ludlow provided by the Laidlaw Volcanics led Melchin et al. in Gradstein et al. (2004) to discount two other apparently sound isotopic dates.

(1) The age of the Silurian–Devonian boundary was reduced to 416 Ma by using the Laidlaw Volcanics date, thereby discounting a widely accepted U–Pb zircon ID–TIMS date of 417.6±1.0 Ma (Tucker, Bradley et al. 1998) for a bentonite within the Kalkberg Formation (New York State, USA) which is biostratigraphically constrained to the woschmidti conodont Zone marking the base of the Early Devonian (Figure 1). A recalibration of the Devonian Time Scale by Kaufmann (2006) used the Kalkberg Formation date

Typical outcrop of the Hawkins Volcanics, 14 km southwest of Yass (Photographer R. Cameron).

7Quarterly Notes

The Canowindra Volcanics (Pogson & Watkins 1998) is the best biostratigraphically constrained and isotopically dated of the volcanic units associated with the beginning of the Tabberabberan Tectonic Cycle and has a SHRIMP U–Pb zircon date of 431.7±3.1 Ma (Black 2006) (Figure 1). The unit is conformably underlain by the Gospel Oak Shale (Pickett 1982) containing graptolites referable to the griestoniensis to centrifugus graptolite zones (latest Llandovery to earliest Wenlock) (Pogson & Watkins 1998) and conformably overlain by the Avoca Valley Shale (Pickett 1982) containing graptolites referable to the late Wenlock to early Ludlow (Pogson & Watkins 1998). The Glenisla Volcanics, which is considered an equivalent of the Canowindra Volcanics, has a SHRIMP U–Pb zircon date of 432.2±2.8 Ma (Lyons et al. 2000). Another unit equivalent to the Canowindra Volcanics, the Hawkins Volcanics, contains a marine horizon near its base. Limestones from that horizon contain the conodonts K. ranuliformis and P. greenlandensis (Percival 2001), indicating an early Wenlock age, and equivalent sedimentary rocks yielded graptolites probably referable to the rigidus to lundgreni graptolite zones (Sherwin & Strusz 2002), indicating an early to middle Wenlock age. The combined faunal assemblages confirm an early Wenlock age for the felsic volcanism marking the beginning of the Tabberabberan Tectonic Cycle, and the SHRIMP U–Pb zircon dating of these units suggests an age of 432.5 Ma for the base of the Wenlock.

Melchin et al. in Gradstein et al. (2004) used isotopic dates obtained from volcanic horizons in sequences constrained to the cyphus graptolite Zone towards the base of the Llandovery, to help constrain the age of the base of the Silurian. These data points include a U–Pb zircon ID–TIMS date of 438.7±2.1 Ma (Tucker, Krogh et al. 1990; Tucker & McKerrow 1995) for a volcanic horizon in the Lower Birkhill Shales (Moffat, Scotland); and an 40Ar/39Ar date for a volcanic horizon in the Descon Formation (Esquibel Island, Alaska, USA) of 436.2±5.0 Ma (Kunk et al. 1985), revised to 439.4±5.0 Ma by Melchin et al. in Gradstein et al. (2004) (Figure 1) as a consequence of the recent revision of the age of the monitor standard. These two data points, together with a U–Pb zircon ID–TIMS date of 445.7±2.4 Ma (Tucker, Krogh et al. 1990) for a volcanic ash within the P. pacificus graptolite Zone (latest Late Ordovician) of the Upper Hartfell Shales (Moffat, Scotland), allowed Melchin et al. in Gradstein et al. (2004) to estimate an age of 443.7 Ma for the Ordovician–Silurian boundary (Table 1). This age is accepted as the current best estimate for this level.

to derive an age of 418.1 Ma for the Silurian–Devonian boundary, and this age is considered to be the current best estimate for this level (Table 1).

(2) A U–Pb zircon ID–TIMS date of 420.2±3.9 Ma (Tucker 1991 in Tucker & McKerrow 1995) for a volcanic ash from the Upper Whitcliffe Formation (Ludlow, England) which lies in the latest Ludlow (just below the probable base of the Pridoli) was rejected by Melchin on the grounds of insufficient biostratigraphic control and its large (analytical) uncertainty. However, ages of 418.1 Ma for the base of the Devonian and 427.0 Ma for the base of the Ludlow (discussed above) are compatible with the 420.0 Ma date for the base of the Pridoli suggested by the Upper Whitcliffe Formation, and this age is considered to be the current best estimate for this level (Table 1).

Melchin et al. in Gradstein et al. (2004) used a U–Pb zircon ID–TIMS date of 430.1±2.4 Ma (Tucker & McKerrow 1995) from a volcanic ash horizon in the Buttington Shales (Welshpool, Wales) to constrain the Llandovery–Wenlock boundary. This volcanic ash is reported to lie within the crenulata graptolite Zone (M.G. Bassett pers. comm. 1989 in Tucker & McKerrow 1995), which lies near the top of the Llandovery (Figure 1), and this date led Tucker and McKerrow (1995) and Melchin et al. in Gradstein et al. (2004) to propose an age of 428.2 Ma for the Llandovery–Wenlock boundary (Table 1). However, an age of 428.2 Ma for the Llandovery–Wenlock boundary is not compatible with an age of at least 427.0 Ma indicated for the Wenlock–Ludlow boundary (see discussion above). It is also not compatible with several SHRIMP U–Pb zircon dates constraining the commencement of felsic magmatism of early Wenlock age, which marks the onset of the major Tabberabberan Tectonic Cycle in the Lachlan Orogen (discussed below).

The commencement of Tabberabberan Tectonic Cycle felsic volcanism has been isotopically dated at ~432 Ma using the SHRIMP U–Pb zircon technique. The oldest sedimentary successions associated with the Tabberabberan Tectonic Cycle sometimes include carbonate rocks containing the conodont Kockelella ranuliformis (e.g. Borenore Limestone — Bischoff 1986; Hawkins Volcanics — Percival 2001) or include shales containing graptolites referable to the rigidus to lundgreni graptolite zones (e.g. Mirrabooka Formation — Sherwin 1971; Hawkins Volcanics — Sherwin & Strusz 2002) which indicate an early to middle Wenlock age.

January 20098

DevonianThe time scale for the Devonian of House and Gradstein in Gradstein et al. (2004) (Table 1) relied on a mix of isotopic dates derived by a variety of methods, some of which are of uncertain reliability and biostratigraphic constraint. Their age of 416 Ma for the Silurian–Devonian boundary follows Melchin et al. in Gradstein et al. (2004), following the decision of the latter authors to accept the mean K–Ar/Rb–Sr isotopic date for the Laidlaw Volcanics (regarded here as having poor biostratigraphic constraint and being isotopically unreliable), rather than the well constrained U–Pb zircon ID–TIMS date of 417.6±1.0 Ma (Tucker, Bradley et al. 1998) for the Kalkberg Formation, which indicates an age of 418.1 Ma for the Silurian–Devonian boundary (see discussion above).

In addition, House and Gradstein in Gradstein et al. (2004) also relied on two SHRIMP U–Pb zircon dates (Jagodzinski & Black 1999) from the Lachlan Orogen (New South Wales, Australia) to help constrain

the Lochkovian and Pragian. The first of these (409.9±6.6 Ma — Jagodzinski & Black 1999) is from the Limekilns area of the Hill End Trough, where the volcaniclastic Merrions Formation (Pogson & Watkins 1998) is overlain by the Limekilns Formation (Pogson & Watkins 1998). The basal Limekilns Formation contains the Pragian dacryocanarid Nowakia acuaria (Wright & Haas 1990). Further west, where the equivalent Cunningham Formation (Packham 1968) overlies the Merrions Formation, the basal beds of the Cunningham Formation contain a transported shelly fauna also suggesting a Pragian age (Packham et al. 2001). The top of the Merrions Formation marks the end of felsic volcanic/volcaniclastic deposition in the Hill End Trough and can be correlated with the top of the Riversdale Volcanics on the adjacent Capertee High (Packham et al. 2001). The top of the Riversdale Volcanics is biostratigraphically constrained to the top of the kindlei conodont Zone (Colquhoun 1995; Meakin & Morgan 1999). These data point to a late Pragian age for the top of the Merrions Formation and

Sampling the Canowindra Volcanics for SHRIMP zircon U–Pb dating, 8 km east of Canowindra (Photographer L. Black).

9Quarterly Notes

suggest a mid-Pragian age for the Merrions Formation (Packham et al. 2001) and therefore this SHRIMP date potentially constrains the Pragian portion of the Devonian time scale.

The second of these dates (413.4±6.6 Ma — Jagodzinski & Black 1999) is from the type section of the lower Turondale Formation (Packham 1968) in the Hill End Trough, where the overlying upper Turondale Formation hosts allochthonous limestones containing conodonts referable to the delta to basal pesavis conodont zones (Packham et al. 2001). In addition, a brachiopod fauna near the top of the Turondale Formation (Wright in Packham 1969) belongs to the Boucotia australis assemblage Zone (Garratt & Wright 1988), which is equivalent to the upper delta to pesavis conodont zones, thereby potentially constraining the Lochkovian portion of the Devonian time scale.

Unfortunately, despite the good biostratigraphic control, neither of these SHRIMP U–Pb zircon dates (Jagodzinski & Black 1999) is suitable for the purpose of meaningfully constraining the Early Devonian time scale, due to issues related to the precision and accuracy of the analytical results. Direct calibration of the Turondale Formation and Merrions Formation U–Pb dates using the co-mounted SL13 zircon standard yielded dates that were too young, probably owing to age heterogeneity in that standard (Jagodzinski & Black 1999; Black & Jagodzinski 2003). Consequently, precision was compromised by the necessity of indirectly recalibrating the Turondale Formation and Merrions Formation U–Pb dates against the more homogeneous QGNG standard, via the use of a third Hill End Trough sample as an internal reference, as detailed by Jagodzinski and Black (1999). Finally, the reference 206Pb/238U date for the QGNG standard used by Jagodzinski and Black (1999) to indirectly recalibrate the Turondale Formation and Merrions Formation U–Pb dates is unreported, so it is possible that the radiogenic Pb loss event responsible for the slight discordance of the reference 206Pb/238U date for QGNG relative to its reference 207Pb/206Pb date (Black, Kamo, Williams et al. 2003) was not accounted for. This combination of problems means the two SHRIMP U–Pb zircon dates of Jagodzinski & Black (1999) cannot be used to help constrain the Lochkovian–Pragian of the Devonian time scale.

Most of the remainder of the Early to Late Devonian time scale of House and Gradstein in Gradstein

et al. (2004) is constrained by only four U–Pb zircon ID–TIMS dates originally obtained by Tucker, Bradley et al. (1998). These are accepted, although a subsequent synthesis (Kaufmann 2006) identified the need for corrections to the biostratigraphic constraints originally assigned to several of the dates by Tucker, Bradley et al. (1998).

However, House and Gradstein in Gradstein et al. (2004) used SHRIMP U–Pb zircon dates from the earliest Carboniferous (Claouè-Long et al. 1993; Roberts et al. 1995) to help constrain their Devonian–Carboniferous boundary. Unfortunately, these SHRIMP U–Pb zircon dates used the SL13 zircon standard, which is now known to display age heterogeneity (see above). House and Gradstein in Gradstein et al. (2004) attempted to compensate for this by applying a proportional percentage correction to these dates, but Black and Jagodzinski (1999) argued that the variability in the potential error inherent in SL13-calibrated SHRIMP U–Pb zircon dates calibrated to the SL13 zircon standard does not allow this sort of correction. Consequently, the SL13-calibrated SHRIMP U–Pb zircon dates used to constrain the Devonian–Carboniferous boundary (Claouè-Long et al. 1993; Roberts et al. 1995) must be considered suspect.

The time scale for the Devonian of House and Gradstein in Gradstein et al. (2004) has been superseded by the more recent revision of Kaufmann (2006) (Table 1). The Kaufmann (2006) Devonian time scale utilises a larger number of well-constrained data points than that of House and Gradstein in Gradstein et al. (2004), and is entirely based on ID–TIMS 207Pb/206Pb zircon dating of volcanic ash horizons, most of which are well-constrained within the detailed Devonian conodont biostratigraphy.

The only real weakness in the Kaufmann (2006) Devonian time scale lies with the Lochkovian–Pragian, where there are no U–Pb zircon ID–TIMS dates other than that from the Kalkberg Formation in New York State, USA (417.6±1.0 Ma — Tucker, Bradley et al. 1998; uncertainty expanded to 3.0 Ma by Kaufmann 2006) lying within the woschmidti conodont Zone (Figure 1) at the base of the Lochkovian (and the Devonian). The Esopus Formation isotopic date from the Appalachian Basin, New York State, USA (408.3±1.9 Ma — Tucker et al. 1998; uncertainty expanded to 3.9 Ma by Kaufmann 2006) (Figure 1), which is used to constrain the base of the Emsian, is also not as well-constrained biostratigraphically as claimed by Kaufmann (2006).

January 200910

Specifically, there is no rigorous faunal control on the maximum age of the unit, which only contains an endemic brachiopod fauna of quite uncertain age. The only real biostratigraphic constraint for the Esopus Formation isotopic date comes from the overlying Carlisle Center Member (Ver Straeten 2007), which contains conodonts equivalent to the lower excavatus conodont Zone (Kaufmann 2006).

Thus, although there are no other biostratigraphically constrained and reliable isotopic dates available for the boundaries of the Lochkovian and Pragian, the estimated ages for the bases of the Lochkovian, Pragian and Emsian of Kaufmann (2006) (Table 1) are preferred over those of House and Gradstein in Gradstein et al. (2004). The larger number of well-constrained, high-precision U–Pb zircon ID–TIMS dates presented by Kaufmann (2006) for the remainder of the Devonian also makes his estimates for the bases of the Eifelian, Givetian, Frasnian, Famennian and Tournaisian (i.e. the Devonian–Carboniferous boundary; Table 1) preferable to those of House and Gradstein in Gradstein et al. (2004).

Conclusions and recommendationsThe middle to late Silurian and early Devonian is a key period in the evolution of the Lachlan Orogen, but the current international geological time scales for these periods (Melchin et al. in Gradstein et al. 2004; Kaufmann 2006) are constrained by very few data points. As an interim measure, a revised geological time scale for the Siluro-Devonian is presented in Table 1. It is clear that additional high precision, biostratigraphically constrained isotopic dates are required. However, in many cases, the geological and biostratigraphic framework of the Lachlan Orogen has the potential to provide them, and recommendations for future isotopic dating are noted below.

In summary, three revisions to the Silurian time scale (relative to the time scale of Melchin et al. in Gradstein et al. (2004)) are proposed.

(1) The rejection of the isotopic date from the Laidlaw Volcanics, New South Wales, Australia (Wyborn et al. 1982), used as the primary constraint for the early Ludlow, on the grounds of its uncertain reliability and poor biostratigraphic constraint.

(2) The primary reliance on and/or re-inclusion of three previously discounted but apparently sound dates, one (from the Kalkberg Formation, New York State, USA)

constraining the earliest Devonian (i.e. as a minimum age for the latest Pridoli; Tucker, Bradley et al. 1998), the second (from the Upper Whitcliffe Formation, Ludlow, England) constraining the latest Ludlow (Tucker 1991 in Tucker & McKerrow 1995), and the third (from the Middle Elton Formation, Shropshire, England) constraining the earliest Ludlow (Kunk et al. 1985, revised by Melchin et al. in Gradstein et al. 2004).

(3) The highlighting of U–Pb SHRIMP zircon evidence for an eruptive age of ~432 Ma for the early Wenlock Canowindra Volcanics (Black 2006) and its equivalents in the Lachlan Orogen used herein to constrain the base of the Wenlock.

Relative to Melchin et al. in Gradstein et al. (2004), this review moves the end of the Silurian from 416 Ma to 418.1 Ma (following Kaufmann 2006), the base of the Pridoli from 418.7 Ma to 420.0 Ma, the base of the Ludlow from 422.9 Ma to 427.0 Ma, and the base of the Wenlock from 428.2 Ma to 432.5 Ma.

Future isotopic dating in the middle Silurian of the Lachlan Orogen should focus on the establishment of a robust age for the base of the Wenlock, by precise U–Pb zircon dating (via SHRIMP and/or ID–TIMS) of the Canowindra Volcanics and other biostratigraphically constrained equivalents (e.g. Dripstone Formation). Similarly, precise U–Pb zircon dating (via SHRIMP and/or ID–TIMS) of biostratigraphically constrained late Silurian volcanic units could potentially provide additional constraint for the Ludlow (e.g. Bells Creek Volcanics, Mullions Range Volcanics) and Pridoli (e.g. Meloola Volcanics).

The Devonian time scale of Kaufmann (2006) is accepted as currently the best available. However, the bases of the Emsian and Pragian respectively are relatively poorly constrained, as isotopic data are sparse and (for the Emsian) do not have good biostratigraphic control.

Future dating involving precise U–Pb zircon dating (via SHRIMP and/or ID–TIMS) of biostratigraphically constrained Early Devonian volcanic units from the Lachlan Orogen could provide isotopic age constraint for the Pragian (e.g. Merrions Formation, Riversdale Volcanics) and Lochkovian (e.g. Bulls Camp Volcanics, Turondale Formation, Cuga Burga Volcanics). Precise dating of these units has the potential to significantly refine the estimated ages of the bases of the Emsian and Pragian, as well as corroborating the existing constraint on the base of the Lochkovian provided by the Kalkberg Formation date of Tucker, Bailey et al. (1998).

Ma Period Epoch Stage Graptolite zones Conodont zones

CA

RB.

Early Tournasian

International Standard

Recognised in Australia

International Standard

Recognised in Australia

360 360.7

duplicata L (lower)

sulcata

DEV

ON

IAN

Late

Famennian

U

praesulcata L

U

expansa (middle)M

L

postera U

L

trachytera

marginifera

(upper most)

U

370 L

rhomboideaU

L

crepida M

L

triangularisU

376.1

M

L

linguaformis

rhenana

U

Frasnian

L

380 jamieaeU

hassi L

punctata

transitans

383.7falsiovalis

norrisi

Middle

Givetian

disparilishermanni

U

varcus M

L

388.1 hemiansatusensensis

kockelianus

390Eifelian

australis

costatus

391.9partitus

patulus

patulus

Early

Emsian

serotinus serotinus/pseudoserotinus

400

inversus inversus

nothoperbonus

excavatus

Unothoperbonus/

gronbergi/perbonus

M

L

409.1pacificus kitabicus dehiscens

410 yukonensispireneae pireneae

Pragian thomasi

kindlei kindlei

fanicus

thomasisulcatus sulcatus

412.3 falcarius

pesavis

Lochkovian

hercynicuspesavis

SILU

RIA

N

praehercynicus deltadelta

eurekaensis

uniformis uniformis

eurakaensis

418.1woschmidti woschmidti

Pridoli

420.0

bouceki–transgrediens transgrediensdetortus

eosteinhornensisbouceki

420

Late

lochkovensis– branikensis

parultimusremscheidensis

interval zone

Ludlow

parultimus–ultimusspineus crispa crispaformosus

formosus

bohemicus tenuis– kozlowskii

snajdrikozlowskiipraecornutus–cornutus

interval zone

leintwardinensis– linearis lientwardinensis siluricus siluricus

ploeckensis ploeckensis

scanicusscanicusnot zoned

427.0nilssoni

progenitornilssoni stauros

Wenlock

ludensis ludensisbohemica

praedeubeli–deubeli praedeubeli–deubeli

parvus–nassa parvus–nassa

430

Early

lundgreni sagittalundgreni

perneri–rigidustestis amsdeni

riccartonensis riccartonensis rhenanamurchisoni ranuliformis

432.5 centrifugus centrifugus ranuliformis

lapworthi–insectus amorphognathoides

spiralis

amorphognathoides

spiralis

cellonicrenulata–griestoniensis

cellonicrispus

crenulata–griestoniensis

turriculatus

crispus

guerichiturriculatus

tenuis–staurognathoides

staurognathoides

sedgwickiiguerichi

convolutussedgwickii

argenteusconvolutus

Llandoverypectinatus–

triangulatus

leptotheca

pseudopesavis

magnus

440cyphus

triangulatus

kentuckyensisvesiculosus

combinatus

acuminatus acuminatus

ascensus ascensus nathani

upper lower

Um

443.7 440 430 420 410 400 390 380 370 360 350

360.3 ± 2.6 (overlapping age) Hasselbachtal beds 79 and 70, Germany

363.3 ± 2.4 Nordegg Tuff, Canada363.4 ± 3.8 Bailey Rock Rhyolite, Piskahegan Group, Canada

363.8 ± 4.2 Tuff, Carrow Fm., Piskahegan Group, Canada

377.2 ± 3.7 Steinbruch Schmidt, Germany

381.1 ± 3.3 Belpre Ash, Chattanooga Shale, USA

390.0 ± 2.5 Tioga Ash bed B, USA

391.4 ± 3.8 Tioga Middle Coarse Zone, USA

392.2 ± 3.5 “Hercules I”, Wettledorf, Germany

overlapping age at 390.05 ± 2.45

426.8 ± 1.7 Middle Elton Formation, England

417.6 ± 3.0 Kalkberg Formation, USA

408.3 ± 3.9 Esopus Formation, USA

407.7 ± 2.7 Hunsruck Slate,Germany

431.7 ± 3.1 Canowindra Volcanics, NSW, Australia

430.1 ± 2.4 Buttington Shales, Wales)(

420.2 ± 3.9 Upper Whitecliffe Formation, England

438.7 ± 2.1 Lower Birkhill Shales, Scotland

439.4 ± 5.0 Descon Formation, Alaska, USA

440 430 420 410 400 390 380 370 360 350

The rectangles associated with each isotopic age calibration point show the 2σ error in the horizontal dimension and the biostratigraphic range constraint in the vertical dimension.

Figure 1. A revised time scale for the Silurian to Devonian showing stage boundaries, calibration points and biostratigraphic zonesG

eological Survey of New

South Wales

•

••

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•••

•

•

•

•

•

••

•

•

•

••

•

•

Qua

rter

ly N

otes

No

130:

The

Silu

ro-D

evon

ian

geol

ogica

l tim

e sca

le: a

criti

cal r

evie

w an

d in

terim

revi

sion

Figu

re 1

. (ov

erle

af)

A re

vise

d tim

e sc

ale

for t

he S

iluria

n to

Dev

onia

n sh

owin

g st

age

boun

darie

s, ca

libra

tion

poin

ts a

nd

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tratig

raph

ic zo

nes.

Sour

ces

Grap

tolit

e zo

nes a

re fr

om S

trusz

(200

7) a

nd K

oren

et a

l. (20

07) w

ith so

me

amen

dmen

ts fo

r Aus

tralia

by

L.

She

rwin

(per

s. co

mm

. 200

8).

Cono

dont

zone

s are

from

Stru

sz (2

007)

, Maw

son

(1995

) and

Kau

fman

n (2

006)

.

Deta

ils o

f the

tim

e sc

ale

calib

ratio

n po

ints

can

be fo

und

in: K

unk

et a

l. (19

85),

Rode

n et

al. (

1990

), Tu

cker

, Kr

ogh

et a

l. (19

90),

Tuck

er (1

991)

, Tuc

ker a

nd M

cKer

row

(199

5), F

anni

ng (1

997)

, Tuc

ker,

Brad

ley

et a

l. (19

98),

Mea

kin

and

Mor

gan

(1999

), Ly

ons e

t al. (

2000

), Ri

char

ds e

t al. (

2002

), M

elch

in e

t al. i

n Gr

adst

ein

et a

l. (20

04),

Trap

p et

al. (

2004

), Ka

ufm

ann,

Trap

p an

d M

ezge

r (20

04),

Kauf

man

n, Tr

app

et a

l. (20

05),

Kauf

man

n (2

006)

an

d Bl

ack

(200

6).

11Quarterly Notes

Colquhoun G.P. 1995. Early Devonian conodont faunas from the Capertee High, NE Lachlan Fold Belt, southeastern Australia. Courier Forschungsinstitut Senckenberg 185, 347–369.

Compston W. 2000a. Interpretations of SHRIMP and isotope dilution zircon ages for the geological time-scale: I. the early Ordovician and late Cambrian. Mineralogical Magazine 64, 43–57.

Compston W. 2000b. Interpretations of SHRIMP and isotope dilution zircon ages for the geological time-scale: II. Silurian to Devonian. Mineralogical Magazine 64, 1127–1146.

Daly S.J., Fanning C.M. & Fairclough M.C. 1998. Tectonic evolution and exploration potential of the Gawler Craton, South Australia. AGSO Journal of Geology and Geophysics 17, 145–168.

Dodson M.H. 1973. Closure temperature in cooling geochronological and petrological systems. Contributions to Mineralogy and Petrology 40, 259–274.

Fanning C.M. 1997. SHRIMP zircon U–Pb age dates from the Dubbo 1:250 000 map sheet — a report by Precise Radiometric Isotope Services. Geological Survey of New South Wales, File GS1997/590.

Garratt M.B. & Wright A.J. 1988. Late Silurian to Early Devonian biostratigraphy of southeastern Australia. In: McMillan N.J., Embry A.F & Glass D.J. eds. Devonian of the World. pp. 647–662. Canadian Society of Petroleum Geologists, Memoir 14.

Gradstein F.M., Ogg J.G. & Smith A.G. (eds) 2004. A Geologic Time Scale 2004. Cambridge University Press, Cambridge.

Harland W.B., Armstrong R.L., Cox A.V., Craig L.E., Smith A.G. & Smith D.G. 1990. A Geologic Time Scale 1989. Cambridge University Press, Cambridge.

Harland W.B., Cox A.V., Llewellyn P.G., Pickton C.A.G., Smith A.G. & Walters R. 1982. A Geologic Time Scale. Cambridge University Press, Cambridge.

Harland W.B., Smith A.G. & Wilcock B. (eds) 1964. The Phanerozoic time-scale. (A symposium dedicated to Professor Arthur Holmes). Geological Society of London, Quarterly Journal Supplement 120s.

Holmes A. 1937. The age of the Earth. (Revised edition). Nelson, London.

Holmes A. 1947. The construction of a geological time-scale. Transactions of the Geological Society of Glasgow 21, 117–152.

AcknowledgementsHelpful comment and advice were provided by Lawrence Sherwin on graptolite zones and by Ian Percival on conodont zones. The manuscript was greatly improved by the critical and helpful reviews of Simon Bodorkos, Ian Percival and Richard Facer.

ReferencesBischoff G.C.O. 1986. Early and Middle Silurian conodonts from Midwestern New South Wales. Courier Forschungsinstitut Senckenberg 89, 1–337.

Black L.P. 2005. The use of multiple reference samples for the monitoring of ion microprobe performance during zircon 207Pb/206Pb age determinations. Geostandards and Geoanalytical Research 29, 169–182.

Black L.P. 2006. SHRIMP U–Pb ages obtained during 2005/06 for the NSW Geological Survey Projects. Geological Survey of New South Wales, File GS2006/821.

Black L.P. & Jagodzinski E.A. 2003. Importance of establishing sources of uncertainty for the derivation of reliable SHRIMP ages. Australian Journal of Earth Sciences 50, 503–512.

Black L.P., Kamo S.L., Allen C.M., Aleinikoff J.N., Davis D.W., Korsch R.J. & Foudoulis C. 2003. TEMORA 1: a new zircon standard for Phanerozoic U–Pb geochronology. Chemical Geology 200, 155–170.

Black L.P., Kamo S.L., Allen C.M., Davis D.W., Aleinikoff J.N., Valley J.W., Mundil R., Campbell I.H., Korsch R.J., Williams I.S. & Foudoulis C. 2004. Improved 206Pb/238U microprobe geochronology by the monitoring of a trace-element-related matrix effect; SHRIMP, ID–TIMS, ELA–ICP–MS and oxygen isotope documentation for a series of zircon standards. Chemical Geology 205, 115–140.

Black L.P., Kamo S.L., Williams I.S., Mundil R., Davis D.W., Korsch R.J. & Foudoulis C. 2003. The application of SHRIMP to Phanerozoic geochronology; a critical appraisal of four zircon standards. Chemical Geology 200, 171–188.

Bowring S.A. & Erwin D.H. 1998. A new look at evolutionary rates in deep time: uniting paleontology and high-precision geochronology. GSA Today 8, 1–8.

Claouè-Long J.C., Jones P.J. & Roberts J. 1993. The age of the Devonian–Carboniferous boundary. Annales de la Societie Geologique de Belgique 115, 531–549.

January 200912

Holmes A. 1960. A revised geological time-scale. Transactions of the Edinburgh Geological Society 17, 183–216.

Jagodzinski E.A. & Black L.P. 1999. U–Pb dating of silicic lavas, sills and syneruptive resedimented volcaniclastic deposits of the Lower Devonian Crudine Group, Hill End Trough, New South Wales. Australian Journal of Earth Sciences 46, 749–764.

Kaufmann B. 2006. Calibrating the Devonian time scale: a synthesis of U–Pb ID–TIMS ages and conodont stratigraphy. Earth-Science Reviews 76, 175–190.

Kaufmann B., Trapp E. & Mezger K. 2004. The numerical age of the Upper Frasnian (Upper Devonian) Kellwasser Horizons: a new U–Pb zircon date from Steinbruch Schmidt (Kellerwald, Germany). Journal of Geology 112, 495–501.

KAufmAnn B., TrApp e., mezGer K. & WeDDiGe K. 2005. Two new Emsian (Early Devonian) U–Pb zircon ages from volcanic rocks of the Rhenish Massif (Germany): implications for the Devonian time scale. Journal of the Geological Society of London 162, 363–371.

Koren T.N., Kim A.I. & Walliser O.H. 2007. Contribution to the biostratigraphy around the Lochkovian–Pragian boundary in central Asia (graptolites, tentaculites, conodonts). Senckenbergiana Lethaea 87, 187–219.

Kunk M.J., Sutter J., Obradovich J.D. & Lanphere M.A. 1985. Age of biostratigraphic horizons within the Ordovician and Silurian systems. Geological Society of London, Memoirs 10, 89–92.

Link A.G. & Druce E.C. 1972. Ludlovian and Gedinnian conodont stratigraphy of the Yass Basin, New South Wales. Bureau of Mineral Resources, Geology and Geophysics, Bulletin 134.

Lyons P., Raymond O.L. & Duggan M.B. (compiling editors) 2000. Forbes 1:250 000 Geological Sheet SI/55-, 2nd edition. Explanatory Notes. Australian Geological Survey Organisation, Record 2000/20.

Mawson R. 1995. Early Devonian polygnathid conodont lineages with special reference to Australia. In: Mawson R. and Talent J. eds. Contributions to the First Australian Conodont Symposium (AUSCOS1).pp. 389–398. Courier Forschungsinstitut Senckenberg 182.

Meakin N.S. & Morgan E.J. (compilers) 1999. Dubbo 1:250 000 Geological Sheet SI/55-4, 2nd edition. Explanatory Notes. Geological Survey of New South Wales, Sydney.

Odin G.S. 1982. Numerical dating in stratigraphy, Volumes 1 and 2. Wiley-Interscience, Chichester, England.

Packham G.H. 1968. The lower and middle Palaeozoic stratigraphy and sedimentary tectonics of the Sofala–Hill End–Euchareena region, N.S.W. Proceedings of the Linnean Society of New South Wales 93, 111–163.

Packham G.H. (ed.) 1969. The geology of New South Wales. Journal of the Geological Society of Australia 16, 1–654.

Packham G.H., Percival I.G., Rickards R.B. & Wright A.J. 2001. Late Silurian and Early Devonian biostratigraphy in the Hill End Trough and the Limekilns area, New South Wales. Alcheringa 25, 251–261.

Percival I.G. 2001. Palaeontological determinations from the Boorowa 1:100 000 sheet, between Cowra and Yass. Geological Survey of New South Wales, Palaeontological Report 2001/01, Geological Survey of New South Wales, Report GS2001/532.

Pickett J.W. 1982. The Silurian System in New South Wales. Geological Survey of New South Wales, Bulletin 29.

Pogson D.J. & Percival I.G. 2002. Hill End Trough time: integrating biostratigraphic zonation and isotopic age dating. In: Glen R.A. (compiler). 2002. Workshop on the Hill End Trough, 12–13 September 2002, Abstract Volume. pp. 31–35. Geological Survey of New South Wales, Report GS2002/447.

Pogson D.J. & Percival I.G. 2003. Hill End Trough time: integrating biostratigraphic zonation and isotopic age dating. In: Vassallo J.J. & Glen R.A. eds. 2003. Evolution of the Hill End Trough: Hill End Trough geoscience database. (CD-ROM). pp. 21–25. Geological Survey of New South Wales, Report GS2003/291.

Pogson D.J. & Watkins J.J. 1998. Bathurst 1:250 000 Geological Sheet SI/55-8, 2nd edition. Explanatory Notes. Geological Survey of New South Wales, Sydney.

Richards B.C., Ross G.M. & Utting J. 2002. U–Pb geochronology, lithology and biostratigraphy of tuff in the upper Famennian to Tournasian Exshaw Formation: evidence of a mid-Paleozoic magmatic arc on the northwestern margin of North America. Canadian Society of Petroleum Geologists, Memoir 19, 158–207.

Roberts J., Claouè-Long J.C., Jones P.J. & Foster C.B. 1995. SHRIMP zircon age control of Gondwanan sequences in Late Carboniferous

13Quarterly Notes

Tucker R.D., Krogh T.E., Ross R.J. & Williams S.H. 1990. Time-scale calibration by high precision U–Pb zircon dating of interstratified volcanic ashes in the Ordovician and Lower Silurian stratotypes of Britain. Earth and Planetary Science Letters 100, 51–58.

Tucker R.D. & McKerrow W.S. 1995. Early Paleozoic chronology: a review in light of new U–Pb zircon ages from Newfoundland and Britain. Canadian Journal of Earth Sciences 32, 368–379.

VandenBerg A.H.M., Willman C.E., Maher S., Simons B.A., Cayley R.A., Taylor D.H., Morand V.J., Moore D.H. & Radojkovic A. 2000. The Tasman Fold Belt System in Victoria. Geological Survey of Victoria, Special Publication.

Ver Straeten C.A. 2007. Basinwide stratigraphic synthesis and sequence stratigraphy, upper Pragian, Emsian and Eifelian stages (Lower to Middle Devonian), Appalachian Basin. In: Becker R.T. & Kirchgasser W.T. eds. Devonian events and correlations. pp. 39–81. Geological Society of London, Special Publication 278.

Wilde S.A. 2002. SHRIMP U–Pb dating of four samples from the Goulburn 1:250 000 map sheet, NSW. Geological Survey of New South Wales, File GS2009/0158.

Wright A.J. 1994. The shelly fossils from the Cummingham Formation. Geological Survey of New South Wales, File GS1994/208.

Wright A.J. & Haas W. 1990. A new Early Devonian spinose phacopid trilobite from Limekilns, New South Wales: morphology, affinities, taphonomy and palaeoenvironment. Australian Museum, Records 42, 137–147.

Wyborn D., Owen M., Compston W. & McDougall I. 1982. The Laidlaw Volcanics: a Late Silurian point on the geological time scale. Earth and Planetary Science Letters 59, 90–100.

Young G.C. 1997. Preliminary report on new Devonian fossil occurrences on the Parkes and Grenfell 1:100 000 sheets, central New South Wales. Australian Geological Survey Organisation Professional Opinion 1997/001.

Young G.C. & Laurie J.R. (eds.) 1996. An Australian Phanerozoic timescale. Oxford University Press, Melbourne.

Australia. In: Dunay R.E. & Hailwood E.A. eds. Non-biostratigraphical methods of dating and correlation. pp. 145–174. Geological Society of London, Special Publications 89.

Roden M.K., Parrish R.R. & Miller D.S. 1990. The absolute age of the Eifelian Tioga ash bed, Pennsylvania. Journal of Geology 98, 282–285.

Sherwin L. 1971. Stratigraphy of the Cheesemans Creek district, New South Wales. Geological Survey of New South Wales, Records 13, 83–89.

Sherwin L. & Strusz D.L. 2002. Dating the Mundoonen Sandstone. First International Palaeontological Congress (IPC2002), Sydney. Geological Society of Australia, Abstracts 68, 273–274.

Simpson A.J. 1995. Silurian conodont biostratigraphy in Australia: a review and critique. In: Mawson R. and Talent J. eds. Contributions to the First Australian Conodont Symposium (AUSCOS1). pp. 325–345. Courier Forschungsinstitut Senckenberg 182.

Steiger R.H. & Jäger E. 1977. Subcommission on Geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36, 359–362.

Steiger R.H. & Jäger E. 1978. Subcommission on Geochronology: convention on the use of decay constants in geochronology and cosmochronology. In: Cohee G.V., Glaessner M.F. & Hedberg H.D. eds. Contributions to the geologic time scale. pp. 67–71. The American Association of Petroleum Geologists, Studies in Geology 6.

Strusz D. 2007. The Silurian timescale — an Australian perspective. Memoirs of the Association of Australasian Palaeontologists 34, 157–171.

Trapp E., Kaufmann B., Mezger K., Korn D. & Weyer D. 2004. Numerical calibration of the Devonian–Carboniferous boundary: two new U–Pb isotope dilution–thermal ionization mass spectrometry single-zircon ages from Hasselbachtal (Sauerland, Germany). Geology 32, 857–860.

Tucker R.D. 1991. Ordovician and Silurian stratotypes of Britain. In: Thermochronology: applications to tectonics, petrology and stratigraphy. Geological Society of America short course notes. pp. 57–58. United States Geological Survey, Open-file Report 91-565.

Tucker R.D., Bradley D.C., Ver Straeten C.A., Harris A.G., Ebert J.R. & McCutcheon S.R. 1998. New U–Pb zircon ages and the duration and division of Devonian time. Earth and Planetary Science Letters 158, 175–186.

January 200914

State-wide geophysical coverage is available as digital data (4 DVD set) and in hardcopy as �at or folded maps at 1:1 500 000 scale.

Total Magnetic Intensity Isostatically Corrected Bouguer Gravity

Statewide geophysicalmaps and data sets

Ternary Radioelement SRTM DEM: Shuttle Radar Topography Mission Digital Elevation Model. Data supplied by the US Geological Survey, EROS Data Center, Sioux Falls, SD, USA.

Pricing: Statewide grids available as DVD set $110 Grid and located data requests supplied on CD/DVD at $110 per project area

Hardcopy state-wide images $11 each

Prices are in Australian dollars and include 10% GST; overseas orders are GST freeAvailable through the NSW Government Online Shop at www.shop.nsw.gov.auor email geoscience.products@dpi.nsw.gov.au to obtain products directly from DPI.

NEW

15Quarterly Notes

Quaternary geological mapping depicts the spatial distribution of both surface landforms and subsurface sedimentary materials, i.e. morphostratigraphic units. The maps produced are of higher resolution than most conventional geology maps. An innovative multi-layered data structure permits the simultaneous mapping of surface and up to two subsurface units at a given location.

ORDERINGThe nine NSW coastal Quaternary maps for the north coast region of NSW are available in both folded and � at double-sided hard copy format. Each map costs $11 including GST. Please specify folded or � at when ordering. Orders can be be made via an order form, or by phone, email or fax.The book and DVD ROM package costs $33 including GST.For web orders see SHOP NSW - the NSW government online store at www.shop.nsw.gov.au

Nine hard copy maps covering the Tweed Heads, Lismore, Grafton, Co� s Harbour, Kempsey,Port Macquarie, Taree, Forster,and Nelson Bay areas.

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Geological Survey of New South Wales

now availablenewrelease mapseriesNSW coastal Quaternary geology

NSW DPI

In 2005, the Geological Survey of NSW released high-resolution (1:25 000 or better) digital mapping data of the coastal Quaternary deposits of New South Wales (excluding the Greater Sydney region) as part of its contribution to the Comprehensive Coastal Assessment. A series of 1:100 000 and 1:25 000 hard copy map products has now been produced from the digital data for the nine areas of the North Coast. The maps are double-sided and show on one side a 1:100 000 overview of the area’s Quaternary geology. On the reverse side there are two to four maps of key areas of the region at 1:25 000. A well illustrated set of explanatory notes has now also been published to accompany the maps. The map data represents a vast improvement on hitherto available geological mapping in the coastal lowland areas of New South Wales, due to its greater degree of di� erentiation of depositional units, and a unique methodology that has enabled the simultaneous mapping of surface and shallow sub-surface sedimentary deposits.The Quaternary geology was combined with existing 1:250 000 bedrock mapping, and the GIS-based map product was complemented by linked databases, such as the locations of past mineral sands mining, mining and quarrying activity, � eld sampling data and sediment characteristics. Information contained in the mapping can assist with land-use planning and natural resource management issues, for example, through conversion to predictive maps of geological hazards, land-use capability, or location of extractive resources. A series of four hard copy maps at 1:100 000 and 1:25 000 for the South Coast region is expected to be available in 2009.

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‘An audit of geoscience information for the Greater Sydney region’ by D.J. Och

‘Petrology and geochemistry of samples from drillholes in the Louth–Bourke area’ by N.M. Vickery

‘A revised Triassic stratigraphy of the Lorne Basin, NSW’ by W. Pratt

‘Review of Cambrian and Ordovician stratigraphy in NSW’ by I.G. Percival, R.A. Glen & C. Quinn

‘A reappraisal of the Wisemans Arm Formation in the Halls Creek District’ by R.E. Brown

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02/

09

N S W D P I Quarterly Notes

Geological Sur vey of New South Wales

January 2006 No 119

The Willyama Supergroup in the Nardoo and Mount

Woowoolahra Inliers

ABSTRACT

The Nardoo and Mount Woowoolahra Inliers are 100 km north and 90 km north-northwest, respectively, of Broken Hill, in far

western New South Wales. Both inliers contain rocks which belong to the Palaeoproterozoic Willyama Supergroup. Geological

mapping of the inliers shows that they both contain graphitic to non-graphitic psammitic, psammopelitic and pelitic

metasediments. The Nardoo Inlier contains a significant amount of pegmatite and intermixed pegmatite and granitoid while

Mundi Mundi type granite is common in the Mount Woowoolahra Inlier. Structural fabrics and metamorphic mineral assemblages

within the inliers are consistent with those observed in the nearby Broken Hill and Euriowie Blocks. The metasediments are

considered to form part of the Paragon Group. However, it is not clear if they can be correlated with other occurrences of the

Paragon Group in the Broken Hill and Euriowie Blocks, or whether they are a separate (stratigraphically higher) part of the Paragon

Group. The recent finding that the Paragon Group in the Broken Hill Block is a time equivalent of stratigraphic units which host

significant Pb–Zn deposits in the Mount Isa Inlier in Queensland, increases the prospectivity of the Paragon Group. Understanding

its distribution and characteristics has, hence, become important.

Keywords: Nardoo Inlier, Mount Woowoolahra Inlier, Paragon Group, Willyama Supergroup, Broken Hill, lead–zinc–silver

mineralisation potential.

INTRODUCTION

The Nardoo and Mount Woowoolahra Inliers are 100 km north

and 90 km north-northwest, respectively, of Broken Hill, in far

western New South Wales (Figure 1). Both inliers contain rocks

belonging to the Palaeoproterozoic (Statherian) (Plumb 1992)

Willyama Supergroup and are surrounded by Adelaidean cover

rocks.

The geological mapping of Cooper et al (1975) has been,

until now, the best available for showing the distribution of

Willyama Supergroup exposure in the areas of the Nardoo and

Mount Woowoolahra Inliers. Their mapping, however, did not

differentiate the constituent rocks within the inliers. The aim

of this paper is to describe the rock types which occur within

both inliers. The work is based on mapping carried out in 1997,

as part of the Broken Hill Mapping Project undertaken by the

Geological Survey of New South Wales.

Both inliers were mapped at 1:25 000 scale using enlargements

of originally 1:60 000 scale Land and Property Information

black and white aerial photographs, which had been taken in

1965. The geological mapping is lithological in nature, being

consistent with the style adopted by Stevens and Willis (1983)

for mapping the Willyama Supergroup rocks of the Broken Hill

Block and Euriowie Block. The mapping of Cooper et al (1975)

was used as a guide. All coordinates in this paper refer to AMG

Zone 54, AGD 66 and all azimuths are with respect to true

north.

Gary Burton

Geological Survey of New South Wales,

NSW Department of Primary Industries

gary.burton@dpi.nsw.gov.au

Papers in Quarterly Notes are subject to external review.

External reviewer for this issue was Ian Hone. His assistance

is appreciated.

Quarterly Notes is published to give wide circulation to

results of studies in the Geological Survey of New South

Wales. Papers are also welcome that arise from team

studies with external researchers.

Contact: john.watkins@dpi.nsw.gov.au

ISSN 0155-3410

AUTHOR

N S W D P I

Quarterly NotesGeologic al Sur vey of New South Wales

January 2007 No 123 ABSTRACTThree Cainozoic intraplate volcanic suites in the Bingara to Inverell area, northeastern New South Wales, have been discriminated

on the basis of di�ering geophysical responses and contrasting K–Ar ages. Major isotopic and chemical characteristics can also be

used to distinguish the three suites. These newly de�ned suites are the Middle Eocene–Early Oligocene Maybole Volcanic Suite; the

Late Oligocene–Early Miocene Delungra Volcanic Suite; and the Middle Miocene Langari Hill Volcanic Suite. Four basaltic volcanic

units within the Delungra Volcanic Suite have also been distinguished: Mount Russell Volcanics; Derra Derra Volcanics; Inverell

Volcanics; and Bingara Volcanics. The Maybole Volcanic Suite is dominated by ma�c volcanic rocks of alkaline a�nity. These rocks

include hawaiite, transitional basalt, basanite and rare phonolite (not included in this study). Volcanogenic and non-volcanogenic

sedimentary units are minor but signi�cant components, hosting world-class concentrations of sapphires in the Inverell–Glen Innes

region. The Maybole Volcanic Suite occupies the eastern portion of the study area, forming ridges that outline the radial drainage

pattern of the deeply eroded Eocene–Oligocene Maybole shield volcano. The Delungra Volcanic Suite is geochemically diverse

and consists of alkaline members (Inverell and Bingara Volcanics) and tholeiitic members (Mount Russell and Derra Derra Volcanics).

These are dominated by ma�c lava �ows with minor inter�ow sedimentary horizons. The Delungra Volcanic Suite forms broad

elevated plains and prominent plugs in the central and western portions of the study area. Diamond occurrences in the Bingara

district are spatially associated with the Bingara and Derra Derra Volcanics. The Langari Hill Volcanic Suite consists of a ma�c

tholeiitic lava �ow that is spatially restricted to a prominent east–west ridge east of Inverell overlying the Maybole Volcanic Suite.

The Langari Hill Volcanic Suite is signi�cantly younger than the Maybole and Delungra Volcanic suites and represents the youngest

recognised volcanic episode in the Bingara–Inverell area.

KEYWORDS: sapphire, mapping, geophysics, geochronology, geochemistry, Cainozoic, intraplate volcanism, Maybole Volcanic Suite,

Delungra Volcanic Suite, Langari Hill Volcanic SuiteINTRODUCTIONA revised geological interpretation has been established for

selected Cainozoic igneous rocks from the Central Province

(McDougall & Wilkinson 1967) of the New England Orogen

(Figure 1). The study area lies within the Northern Tablelands

of New South Wales and encompasses the towns of Bingara,

Delungra, Inverell and Warialda. Cainozoic igneous rocks from

this area have been subdivided into three newly de�ned

volcanic suites — utilising recently acquired petrographic,

geochronological, geophysical and geochemical data.

The study area incorporates part of the world-class Glen

Innes–Inverell sapphire province, which attained a maximum

70% to 80% of the world’s sapphire production during the

1970s and 1980s (Coenraads 1990; McEvilly et al. 2004). Other

studies have demonstrated a spatial and genetic association

between sapphires and the eastern alkaline volcanic rocks

(Barron 1987; Pecover & Coenraads 1989; Sutherland et al. 1993;

Oakes et al. 1996). Similarly, a spatial relationship has been

N M Vickery1M W Dawson1W J Sivell2K R Malloch1W J Dunlap3

1 Geological Survey of New South Wales, NSW Department of Primary Industries

2 Department of Earth Sciences, University of New England, Armidale3 Research School of Earth Sciences,

Australian National University, Canberra

Papers in Quarterly Notes are subject to external review.

External reviewers for this issue were Morrie Duggan and

Larry Barron. Their assistance is appreciated.

Quarterly Notes is published to give wide circulation to

results of studies in the Geological Survey of New South

Wales. Papers are also welcome that arise from team

studies with external researchers. Contact: john.watkins@dpi.nsw.gov.auISSN 0155-3410

AUTHORS

Cainozoic igneous rocks in the Bingara to Inverell area,

northeastern New South Wales

© State of New South Wales through NSW Department of Primary Industries 2007