Records of the Western AlIstralian MlIsellm Supplement No. 58: 145-162 (2000). Silurian to basal Devonian conodonts from the Broken River Crossing, northern Australia Andrew Simpson Research Associate, Centre for Ecostratigraphy and Palaeobiology, Macquarie University, NSW, Australia 2109 Abstract - Stratigraphy and conodont biostratigraphy of the Graveyard Creek Group, particularly the Jack Formation, near the Broken River Crossing, northwest of Charters Towers, are described. A minor lenticular limestone within pelitic lithologies near the Crossing, yielded a late Llandovery Cs fauna, consistent with evidence from graptolites. Diagnostic conodonts were obtained from two limestone units in the overlying Jack Formation. The lowest 72 m of the lower of these units is referred to the Ludlow (but not latest Ludlow) ploeckensis and siluricus zones, and correlates broadly with the interval Silverdale Formation to lower Hume Limestone of the Yass Basin, southeastern Australia. The upper unit yielded IcriodllS wosclzmidti Izesperius from two localities, which indicates a latest Pridoli-early Lochkov age and provides the possibility of recognising the Silurian-Devonian boundary within the sequence. INTRODUCTION and Talent (1989). Strata younger than the Graveyard The Graveyard Creek Group crops out in Creek Group have been described by Wyatt and Jell northern Queensland in the Broken River Province (1980). Models for the geological evolution of the (White 1959; Arnold and Henderson 1976) west of Broken River and Hodgkinson Provinces have been Townsville. The outcrops investigated in this study presented by Arnold and Fawkner (1980) and (Figures 1 and 2) are adjacent to the Wandovale­ Withnall et al. (1988). Pandanus Creek road where it crosses the Broken Previous work on the Graveyard Creek Group River approximately 230 km northwest of Charters has focused mainly on broad scale regional Towers. mapping, based largely on air-photo interpretation. Pioneer investigations of the geology of the The only prior age data come from a few rare Broken River area have been chronicled by White graptolite occurrences (middle to late Llandovery; (1965) and Jell (1967). White (1959) defined the Thomas 1960), and from spot sampling of rugose Graveyard Creek Formation and differentiated, in and tabulate coral faunas (Hill in White 1959, 1965). parts of the formation, two members: the Jack This latter work provided field parties with Limestone Member at the top of the sequence in the generalised ages for the numerous Silurian and south, and a basal unit, the Crooked Creek Devonian carbonates of the region but did not allow Conglomerate Member. White (1961, 1962, 1965) for age determinations and correlations of high produced the first synthesis of the geological history resolution. The only previous study of Silurian of the region. Since White's time there has been conodonts from the region was incidental to a study increased awareness of a major time break between of the Devonian faunas of the Broken River his Jack Limestone Member and underlying strata Formation by Telford (1975). Of the succession of equated, though with some uncertainty Qell pers. eight faunas discriminated by Telford (1975), only comm.), with the Graveyard Creek Formation the oldest, from two localities in the Jack Formation, developed typically in the Pandanus Creek area 35 was referred to the Pridoli (latest Silurian). More km to the north. recent work (Simpson 1999) documents Early Amold and Henderson (1976) re-interpreted the Silurian conodonts from allochthonous carbonates pre-Devonian stratigraphy of the region, including in the Graveyard Creek Group. the area about the Broken River Crossing. The There are three major studies of Australian Geological Survey of Queensland completed field Silurian conodont faunas to date. They are the late work in the Clarke River 1:250,000 sheet area in 1987 Silurian faunas of the Yass Basin (Link and Druce and accorded the Graveyard Creek Formation group 1972), the early and middle Silurian faunas from status and the Jack Limestone Member fom1ation midwestem New South Wales (Bischoff 1986) and status (Withnall et al. 1988). Additional data on the the early to late Silurian faunas of the Limestone Silurian of the region have been presented by Jell Creek region (Simpson and Talent 1995). 146 A. Simpson GRAVEYARD CREEK SUBPROVINCE IDIIlllJ Ba sol t I~'I Bundoek Creek Group t:·::·:;.:·1 Broken River Group . / I Broken River I Area ~ Undifferentiated } Graveyard Creek i_..., D Crooked Creek Conglomerate Group I 1':'<:':·1 Judea Formation 1._.__........... .../ • Gray Creek Complex CAMel CREEK SUBPROVINCE o Clarke River Group D Quartz intermediate flysch 1~:j';'J Everets Creek Volcanics • ',«" Carriers Wen limestone Ih(~] Quartz rich flysch 10, 20, 30, 40 50 I I Kilometres Figure 1 Geology of the Broken River Province, north Queensland, Australia, after Arnold and Henderson (1976) and Arnold and Fawkner (1980). Reconnaissance sampling, yielding some age­ monotonous, thickly bedded and massive diagnostic conodonts, was undertaken prior to this limestones. A small number of limestone clasts were project by a number of field workers. A full record collected from calcirudites and acid-leached of faunas recovered from this earlier sampling was independently to determine if there might be not available and the precise stratigraphic position detectable divergence in age between them and the of individual samples was unclear (see explanation associated, presumably indigenous matrix. No such of Figure 3). disharmony was proved. A total of 93 samples of The area in the vicinity of the Broken River limestone and other carbonates was collected from Crossing was subsequently mapped and three the Bullock Creek-Broken River Crossing area. stratigraphic sections were measured; two in the Basal samples of each section are designated 00; Bullock Creek valley and a third along the axis of sample numbers increase up-section. Specimens the major syncline close to the road crossing (Figure were photographed with the Jeol scanning electron 2). Another short section on the eastern limb of the microscope housed in the School of Biological syncline was measured by J.A. Talent and Sciences at Macquarie University. colleagues in 1983. These sections were sampled at 2-5 m intervals in lithologies such as thin, STRATIGRAPHY interbedded limestone and siltstone from which rich coral faunas have been documented (Munson Graveyard Creek Group 1987), but at 5-30 m intervals in the lithologically This group consists primarily of shales, siltstones, Broken River Silurian-basal Devonian conodonts 147 '"0- o'" o o o 05 10 ...---.J.- Fau It I: :ITertiary Basalt --v-- Unconlormity t----l Section Upper Limestone Unit] JACK FORMATION Spot Sample Lower limestone Unit G Graptolite Locality Siltstones POLEY COW ~ Judea Beds Greywacke /S iIts tone Interbedded FORMATION o Troiemites ',. Conglomerates Australian Mopping 7843000 Grid Coordinates Figure 2 Geology of the Broken River Crossing area showing locations of sections through the Jack Formation. and greywackes with conglomeratic and minor et al. (1995) have argued that the two represent the carbonate lenses. Typical sequences occur abqut 35 same sedimentary tract. Simpson (1999) has argued km north of the Broken River in the watershed of that only one name, the Quintan Formation be Gray Creek and its tributaries. Withnall et al. (1988) retained for this sedimentary tract. defined the Poley Cow and Quintan formations In the vicinity of Broken River Crossing, the within the Graveyard Creek Group. The latter crops underlying clastic unit (Poley Cow Formation sensu out in the north of the Graveyard Creek Withnall et a1. 1988; Quinton Formation sensu Subprovince and the former underlies the shelf Simpson 1999) consists almost entirely of carbonates of the Jack Formation in the south. Sloan conglomerates, greywackes and pelites. The 148 A. Simpson conglomerates vary greatly in lithology (Amold and BIOSTRATIGRAPHY Henderson 1976). Graptolites of middle to late Llandovery age (White and Stewart 1959; Thomas Conodonts from the underlying clastic unit 1960; Jell and Talent 1989) have been recovered BR12-08, a small isolated limestone lens within from pelitic rocks near the Broken River Crossing. the pelitic lithologies yielded elements of One small isolated limestone body from within Distomodus staurognathoides and a number of siltstones was sampled and yielded conodonts of fragments of ?Astropentagnathus irregularis comparable age (Sample BR12-08). Apart from (Mostler). Mostler (1967) considered mapping and sampling of such rare carbonates, this Astropentagnathus as characteristic of the early investigation has not focused on this unit. celloni Zone, whereas Aldridge (1972, 1975) indicated a range equivalent to the late celloni Zone Jack Formation (British Cs division). Extensive outcrops of the Jack Formation occur in In Australia, Astropentagnathus irregularis is the Jack Hills region and about the Broken River known from the Burly Jack Sandstone Member of Crossing. In the latter area they form the core of a the Glendalough Formation and the Liscombe Pools broad southwest-plunging syncline on which Limestone of New South Wales, which have been secondary and tertiary folding has been developed correlated with the griestoniensis and crispus as well as complications from extensive minor graptolite Biozones (Bischoff 1986: 161-162). There faulting (Figure 2). Two units within the Jack is no discordance in age between the limestone lens Formation