The Todd River Dolomite in the northeastern Amadeus Basin is composed of six facies described in detail by Kennard (1991). Three siliciclastic-carbonate units are overlain by high-energy reef shoals, low-energy shelf deposits with patch reefs, and stromato-litic mudrocks. Six archaeocyath taxa and a radiocyath were described by Kruse (in Kruse and West 1980) as predominantly from the reef-shoal facies at Ross River. Most are restricted to the Amadeus and Georgina basins, but Beltanacyathus sp. at the base of the reef-shoal facies, an indeterminate trilobite, and the brachiopod Edreja aff. dis-tincta (Laurie and Shergold 1985; Laurie 1986) higher in the section indicate that both the upper tenuis and tardus zones may be represented. Rare archaeocyaths in micro-bial bioherms in the underlying barrier bar facies have not been described.
In their sequence stratigraphic study of the Amadeus Basin, Lindsay et al. (1993) concluded that the barrier-bar, reef, and stromatolitic mudflat facies were deposited in transgressive and highstand systems tracts. In the Arrowie and Stansbury basins the tenuis and tardus zones occur in these systems tracts in sequence —C 1.1, confirming that the same sequence is represented in all three basins. Subaerial exposure and dissolution at the top of the Todd River Dolomite (Kennard 1991) are complex and may be related not only to the Flinders Unconformity but also to lowstand at the top of sequence -G1.3. The long hiatus in figure 6.5 between the Todd River Dolomite and overlying units reflects these lowstand events.
The disconformably overlying Chandler Formation is considered by Lindsay et al. (1993) to be of Botoman age. Like Shergold (1995), we favor an Ordian-early Tem-pletonian age because that is the age of fossils in the laterally equivalent Chandler Formation limestone and the lower Giles Creek Dolomite ("Giles Creek Dolomite" in figure 6.5). The Chandler Formation is composed primarily of halite with a medial unit of fetid limestone devoid of fossils. Bradshaw (1991) envisages a deep desiccated basin with two stages of drawdown and an intervening flooding event. It is overlain by the late Templetonian-Floran Tempe Formation, Hugh River Shale, or Giles Creek Dolomite. Major changes in coastline configuration wrought by late Botoman tectonic activity in the Arrowie and Stansbury basins may also have resulted in epeirogenic uplift of the Amadeus Basin region (Chandler Movement; Oaks et al. 1991).
The Chandler Formation salt may result from alternating lowstand and transgression in sequences -C2.1 to -C2.3, a time of global fall in sea level (Toyonian regression of Rowland and Gangloff 1988). If the salt indeed marks this Early Cambrian regression, an age discrepancy arises because of the Ordian-Early Templetonian fossils, which seemingly correlate with the South Australian sequence -S3.1 (cf. figures 6.2
and 6.5). Most of the Chandler Formation halite (225-470 m thick) might, however, be appreciably older than the thin (10 m) fossiliferous carbonate beds that occur in upper levels. Alternatively, correlation of the salt with the Toyonian regression is untenable, and a basal Middle Cambrian epoch of desiccation may be invoked.
The archaeocyath fossil record in the Georgina Basin is sparse. Kruse (in Kruse and West 1980) described four archaeocyaths and a radiocyath from the Errarra Formation in the Dulcie Syncline. Correlation with the tardus zone is favored, but the reported co-occurrence of Dailyatia ajax and Yochelcionella in drillhole Tobermory 12 (Laurie 1986) suggests a younger, mid-Botoman age at that locality. Dailyatia ajax is now known to be long-ranging in the Stansbury Basin. Further studies of brachio-pods, mollusks, and small shelly fossils are warranted in the Georgina and Amadeus Basins.
MIDDLE CAMBRIAN TRILOBITE SEQUENCE BIOSTRATIGRAPHY Background
There remains a fundamental dilemma in Australia as to exactly what is to be regarded as Early Cambrian and what Middle Cambrian. The correlation of the South Australian basins with those of central and northern Australia (Amadeus and Georgina in particular) is fraught with interpretative difficulty due principally to a dearth of South Australian trilobites. In this paper, we base our definition of the Early-Middle Cambrian boundary on the suggestion of Jell (1983), who regarded the first occurrence of the eodiscid genus Pagetia to define Middle Cambrian time, following the last occurrence of Pagetides. Although Pagetia occurs widely in central and northern Australian basins, it has been recorded only recently in South Australia, in the Stansbury Basin. There, Ushatinskaya et al. (1995) have recovered 10 specimens of Pagetia sp. from the shallow marine Coobowie Limestone in Port Julia 1A corehole on Yorke Peninsula. This suggests that the epoch boundary should be sought in the Stansbury Basin between sequences -£2.3 and -£3.1. The Moonan Formation, which underlies the Coo-bowie Limestone, consists of transgressive black shale followed by highstand siltstone and sandstone, necessitating the addition of a new sequence, -£2.3, where previously only one was considered (Gravestock 1995). Stratigraphically beneath are the Stans-bury Limestone, Corrodgery Formation and Ramsay Limestone (Daily 1990) (see figure 6.2).
By correlation, the Wirrealpa Limestone is Early Cambrian from the occurrence of Redlichia guizhouensis Zhou (Lu et al. 1974) in association with archaeocyaths and a radiocyath (Kruse 1991), and brachiopods, mollusks, and small shelly fossils from both the Ramsay and Wirrealpa limestones (Brock and Cooper 1993). Shales and red beds of the succeeding Moodlatana Formation, containing Onaraspis rubra at lower levels, may be Early or Middle Cambrian. The base of the Middle Cambrian in central and northern Australia has been taken traditionally at the beginning of the Ordian stage (sensu Opik 1967b), in which species of Onaraspis, Redlichia, Xystridura, and Pagetia all occur, but archaeocyaths are absent. Because only two Middle Cambrian trilobite taxa are currently known in South Australia, the ensuing text is restricted to central and northern Australian basins, specifically the Amadeus and Georgina basins.
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