Journal of Earth Science, Vol. 32, No. 3, p. 512–523, June 2021 ISSN 1674-487X Printed in China https://doi.org/10.1007/s12583-021-1310-9 Summary of East Gondwanan Conodont Data through the Ireviken Event at Boree Creek Andrew Simpson *1, David Mathieson2, Jiri Frýda3, Barbora Frýdová3 1. Archives and Collections, University Library, Macquarie University, Sydney NSW 2109, Australia 2. Department of Biological Sciences, Macquarie University, Sydney NSW 2109, Australia 3. Department of Environmental Geosciences, Czech University of Life Sciences Prague, 16500 Prague, Czech Republic Andrew Simpson: https://orcid.org/0000-0001-6584-6451 ABSTRACT: The Ireviken Event was the first Middle Paleozoic event consisting of synchronised faunal, isotopic and facies change to be recognised. An analysis of the conodont faunas throughout the Boree Creek/Borenore Limestone succession in the central western region of the Tasman fold belt of New South Wales (Australia) revealing all five conodont zones that comprise the event is presented. While some zonal boundaries are precise, allowing direct comparison of stratigraphic intervals on other paleo-continents, some can only be approximated. Conodont data from pre-Ireviken Event strata, in contrast, only permit the identification of a broad Telychian chronology. The identification of Wenlock post-Ireviken Event conodont zones is incomplete due to lithological variability, namely the presence of tuffaceous beds near the top of the formation and an unconformity between the Boree Creek and overlying Borenore Lime- stone. The Boree Creek Formation contains the only example of the Ireviken Event discovered to date from the Tasman fold belt of eastern Gondwanaland. KEY WORDS: conodonts, extinction, Ireviken Event, Silurian, Boree Creek, eastern Gondwanaland. 0 INTRODUCTION 2007; Munnecke et al., 2003) and the Avalonian part of Laurussia The Early Silurian Ireviken Event is one of the most pro- (e.g., Loydell and Frýda, 2007). There is one study from tropical found intervals of species turnover during the Paleozoic that Gondwana (New South Wales; Talent et al., 1993) and only two has been identified to date. Jeppsson (1990, 1984) first devel- records from higher latitudes of peri-Gondwana (Frýda et al., oped an oceanic model for the Silurian based on observed pat- 2015) and Gondwana (Vecoli et al., 2009; Wenzel, 1997). The terns of change in conodont faunas through time. He proposed only available geochemical measurements from eastern Gond- 13 two types of stable and four types of unstable oceanic states, wana are 5 elevated δ Ccarb values reported by Talent et al. (1993) the former referred to as an “episode” and the latter, an “event”. from the Boree Creek Formation, a unit of limestones and vol- He (Jeppsson, 1998) postulated strong causal links between the caniclastic sediments from the Middle Paleozoic Waugoola Group, characteristics of an event and the preceding and subsequent in the Lachlan fold belt of central western New South Wales, Aus- episodes. Events exhibited an internal architecture of datum tralia. The only closely sampled conodont data from eastern planes and step-wise sequential extinction of conodont taxa. Gondwana through this time interval also comes from the Boree Although the model was derived to explain the patterns of Creek Limestone (Molloy, 2006; Cockle, 1999; Bischoff, 1986). chronology of conodont taxa in the intensely sampled Gotland succession, geographic terms were applied to event nomencla- 1 GEOLOGICAL SETTING ture so as not to limit the effectiveness of these terms with The stratigraphy of the Boree Creek Formation has been non-specialists (Jeppsson, 1998, p. 240). considered in several papers, principally by Walker (1959), The Ireviken Event is associated with a prominent early Sherwin (1971), Talent et al. (2003a, 1993, 1975), Pickett Sheinwoodian carbonate carbon isotope anomaly that reaches up (1982), Bischoff (1986), Holloway and Lane (1998), Jell and to +5 δ13C (Munnecke et al., 2003). This perturbation of carbon Talent (1989), Cockle (1999), and Valentine et al. (2003). Tal- cycle has been reported from different parts of Laurussia, includ- ent et al. (1975) and Pickett (1982) have covered discussion on ing its Laurentian part (e.g., Brand et al., 2006; Saltzman, 2001), the relationship of the Boree Creek Formation to other units. the Baltoscandian Basin (e.g., Racki et al., 2012; Kaljo et al., The Boree Creek Formation (Sherwin, 1971) is a Silurian unit of limestones and volcaniclastic sediments forming part of *Corresponding author: [email protected] the Middle Paleozoic Waugoola Group (Pogson and Watkins, © China University of Geosciences (Wuhan) and Springer-Verlag 1998; Jenkins, 1978) in the Lachlan fold belt of central western GmbH Germany, Part of Springer Nature 2021 New South Wales, Australia. This suite of rocks is interpreted as representing a marginal platform to deep water sequence Manuscript received October 10, 2020. (Downes et al., 2013) formed through the deposition of strata Manuscript accepted May 13, 2021. from the accretion of the intra-oceanic Macquarie arc to the Simpson, A., Mathieson, D., Frýda, J., et al., 2021. Summary of East Gondwanan Conodont Data through the Ireviken Event at Boree Creek. Journal of Earth Science, 32(3): 512–523. https://doi.org/10.1007/s12583-021-1310-9. http://en.earth-science.net Summary of East Gondwanan Conodont Data through the Ireviken Event at Boree Creek 513 Gondwana Plate (Glenn et al., 2007). thickness of up to 600 m (Pickett, 1982) that is believed to be the The Boree Creek Formation (Figs. 1 and 2) unconformably lateral equivalent of the Mirrabooka Formation and the Molong overlies volcanics and volcaniclastics of the Cheesemans Creek Limestone (Pickett, 1982; Talent et al., 1975). Formation (Sherwin, 1971) dated as Late Ordovician on grapto- Three informal units have previously been identified lite evidence (Percival and Glen, 2007; Percival et al., 2001). The within the Boree Creek Formation (Sherwin, 1971) in ascend- formation is unconformably overlain by the Borenore Limestone, ing order: (1) the lowest, limestone unit A, essentially equiva- a bedded to massive and brecciated unit of carbonates with a lent to the Rosyth Limestone of Walker (1959), consists of Figure 1. Regional geological map of the Boree Creek area (after Cockle, 1999 and Molloy and Simpson, 2012) showing the location of Kalinga Gully and the stratigraphic sequence sampled (Fig. 3). 514 Andrew Simpson, David Mathieson, Jiri Frýda and Barbora Frýdová Figure 2. Detailed geological map of the Kalinga Gully area studied (after Molloy and Simpson, 2012). thinly bedded nodular limestones rich in fossils including small position as Bischoff’s (1986) B Section. This work identified a brachiopods and corals topped by a reddish coarse grained major isotopic excursion corresponding to Jeppsson’s (1998, limestone (Cockle, 1999); (2) tuffaceous trilobite bed; (3) lime- 1997a, b, 1990) Datum 2 of the Ireviken Event. stone unit B, a thin to moderately thickly bedded, partially In Kalinga Gully, six separate lithological units of the Boree dolomotised interval of limestones (Bischoff, 1986). Creek Formation can be discriminated (Fig. 3). The lowermost This three-fold subdivision (Sherwin, 1971) was largely unit is a thinly bedded argillaceous limestone. The contact with adopted by subsequent authors such as Pickett (1982), Bischoff the underlying Cheesemans Creek volcanics is obscured by soil (1986), Holloway and Lane (1998) and Cockle (1999). These cover, but the unit was estimated to be approximately 40 m thick three units are easy to discriminate in the designated type sec- (Cockle, 1999), it is highly fossiliferous with pyritised fo- tion at Cheesemans Creek, however, it has been noted by a raminifers, gastropods, ostracods and brachiopods (Valentine et number of authors that lateral facies variation and interfinger- al., 2003). Algae, stromatoporoid fragments and silicified corals ing lithologies can make the identification of unit boundaries have also been reported from this interval (Valentine et al., 2003). problematic (Valentine et al., 2003; Pickett, 1982). Overlying this is a massive coarse grained red 5 m thick lime- The section studied in this investigation is the easternmost stone unit. Molloy and Simpson (2012) reported subtle differ- exposure of the Boree Creek Formation outcropping on both ences in lithology within the red limestone unit. Valentine et al. sides of Kalinga Gully, approximately 1 km from Borenore (2003) indicated that the red colour is the result of fine grained Caves (Fig. 2). This is the same section studied by Bischoff haematite iron particles and stylolites. These two units combined (1986, Section B) and also the subject of a study of lingulifor- comprise the equivalent of Sherwin’s (1971) limestone unit A mean brachiopods through the Ireviken Event (Valentine et al., (Fig. 3). Overlying the red limestone is a 5 m thick unit of 2003: BM Section). The section was resampled by Molloy fine-grained lensoidal grey limestone considered to be the basal (2006: BM Section) in a quest for better chronologic resolution portion of Sherwin’s tuffaceous trilobite bed (Fig. 3), this unit is through the event using conodonts, and for partial correlation of not present in the type section of the Boree Creek Formation. these results with intervals on other continental blocks (e.g., Valentine et al. (2003) reported that this unit is topped with a thin Molloy and Simpson, 2012). As part of an earlier investigation coarse-grained