BMR J. Aust. Geol. Geophys, 1, 171-174 171

The Bullara Limestone, a new rock-stratigraphic unit from the Carnarvon Basin, Western Australia

George C. H. Chaproniere

A new rock unit, the Bullara Limestone, is proposed for a Late Oligocene bioclastic limestone, which is probably restricted to the subsurface of Rough Range. The Bullara Limestone is a lateral equivalent of the lower part of the Mandu Calcarenite, and contains a Tertiary lower e stage larger foraminiferal fauna and a Zone N.3 planktic fauna.

Condon et al. (1955, 1956) formalized the rock-strati•graphic nomenclature that had been previously used informally by Clapp (1925), Raggatt (1936), Singleton (1941) and Condon (1954), for the North West Cape part of the Carnarvon Basin, and created several new units. McWhae et at. (1958), Condon (1968) and Quilty (1974) have presented summaries of the rock stratigraphy for this part of the Carnarvon Basin. based on the earlier published work. The scheme of Condon et at. (1955), as modified by Condon (1968), has been retained for this note. However, because of additional subsurface information resulting from a number of petroleum exploration wells drilled by West Australian Petroleum Pty Ltd. (W APET), an additional name is required for a bioclastic limestone in the subsurface of Rough Range. The purpose of this note is to formally define this unit. Figure 1 gives the relevant locality information, and Figure 2 illustrates the stratigraphic rela•tionships for the area discussed.

Bullara Limestone (new name)

The name Bullara Limestone is proposed for a massive, brown to grey, poorly to well cemented, often recrystallized bioclastic calcarenite that unconformably underlies the Trealla Limestone and disconformably overlies the Giralia Calcarenite lateral equivalent, in the Rough Range area. It differs from the Mandu Calcarenite (Condon et al .. 1955) in being coarser grained, containing less mud and a different fauna, and by being brownish.

The type section is designated as the interval between 220 and 306 m in Rough Range South No. 1 Well (tat. 22°37'-17.5"S, long. 113°57'37.6"E) (Fig. 1). The type thickness is 86 m. Figure 3 gives the geophysical logs and other in•formation for the type section. Representative samples from cores 4 and 7 to 11 have been placed in the collections of the Department of Geology, University of Western Australia (registered numbers UWA53673, to UWA53678, and UW A 70600). The name is taken from the 'Bullara Homestead', 12 km southeast of the type section (Fig. O. Approval for this new name has been given by the Strati•graphic Nomenclature Committee of the Geological Society of Australia.

The top of the Bullara Limestone is nearly wholly recry•stallized, but the rocks become progressively less altered down the sequence. The interval about 274 m is made up of lithologies containing poorly sorted, coarse-grained, bio•clastic material (algal nodules, larger foraminiferids and rare molluscan fragments) in a matrix containing very little or no mud (see Chaproniere, 1975, fig. 8F-H). Below 274 m the lithologies are generally finer grained and the larger foraminiferids and algal nodules are absent.

The Bullara Limestone is probably restricted to the sub•surface of Rough Range: its only surface exposure may be a recrystallized limestone exposed in a valley on the east side

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of Rough Range, referred to the Tulki Limestone by Condon et al. (1955), and to the Mandu Calcarenite by Condon (1968). The unit has been recognized in three wells north of the type section: the thinnest sequence is seen between 119 and 156 m in Exmouth No.1 Well (37 m) and the thickest is 134 min Learmonth No.2 Well (between 215 and 349 m); in Exmouth No. 2 Well it is 83 m thick (between 167 and 250 m).

On the basis of the fauna, the limestone can be subdivided into two parts. The larger foraminiferal fauna from the upper part comprises Lepidocyclina (Eulepidina) ephippioides. L. (Nephrolepidina) sumatrensis. Hetero•stegina borneensis. Operculina complanata and Gypsina howchini. a fauna which forms the Lepidocyclina (Eule•pidina) ephippioides-Heterostegina borneensis (= LF.2 in Fig. 3) association of Chaproniere ( 975); smaller benthic foraminiferids are common, but subordinate to the

172 GEORGE C. H. CHAPRONIERE

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larger forms, and planktic forms are extremely rare. Gypsina howchini. Operculina complanata and very rare Lacazinella sp. cf. L. wichmanni are the only larger fora•miniferids found in the lower part; this assemblage forms the Operculina complanata - Gypsina howchini (= LF.1 in Fig. 3) association of Chaproniere (1975); smaller benthic forms dominate the faunas from this part, and planktics have not been encountered. Fragments of articulated coralline algae are important constitutents at all levels.

The presence of Globorotalia (Turborotalia) kugleri. without Globigerinoides quadrilobatus primordius (in only one sample), is indicative of Zone N.3 of Blow (1969). The presence of Heterostegina bomeensis. which becomes extinct within Zone N.3 (Clarke & Blow, 1969, fig. 1), permits correlation with the Tertiary lower e stage (Late Oligocene). This correlation is supported by the low mean values for parameter A (A = 39.31 ± 3.18 percent) for Lepidocyclina (Nephrolepidina) sumatrensis. that are similar to those obtained by van der Vlerk & Postuma (1967) for populations of Lepidocyclina (Nephrolepidina) associated with a Zone N.3 planktic fauna from Indonesia. The presence of Lacazinella sp. cf. L. wichmanni in the lower part of the Bullara Limestone suggests a correlation with either the Tertiary b or c stages (Adallls, 1970). There is no good evidence, however, for a large stratigraphic break

within the Bullara Limestone, nor is there any major change in the smaller benthic foraminiferal fauna. Moreover, the absence of typical Eocene forms within the limestone, other than probably reworked forms together with the evidence presented above, suggests that the lower part of the Bullara Limestone is of similar age to the upper part; that is Tertiary lower e. The presence of Lacazinella sp. cf. L. wichmanni can be explained either by reworking from the underlying Eocene rocks, which is the preferred ex•planation, or by its ranging to higher stratigraphic levels than previously recorded. The age of the Bullara Limestone is latest Oligocene.

The Bullara Limestone correlates with the lower part of the Mandu Calcarenite in the subsurface of Cape Range, and is its shallow-water lateral equivalent.

Acknowledgements

The writer thanks the management of W APET for supplying samples of cores and ditch-cuttings from Exmouth No.1, Exmouth No.2, Learmonth No.2 and Rough Range South No.1 Wells, and copies of geophysical logs for those wells; and Dr D. J. Belford, Dr P. J. Coleman and Mr S. Shafik for reading the manuscript.

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174 GEORGE C. H. CHAPRONIERE

References ADAMS, C. G., 1970-A reconsideration of the East Indian Letter

Oassification of the Tertiary. Bulletin of the British Museum, Natural History, Geology, 19, 87-137.

BLOW, W. H., 1969-Late Middle Eocene to Recent planktonic foraminiferal biostratigraphy. Proceedings of the 1st Inter•national Conference on Planktonic Microfossils, Geneva, I, 199-421.

CHAPRONIERE, G. C. H., 1975-Palaeoecology of Oligo-Miocene larger Foraminiferida from Australia. Alcheringa, 1,31-52.

CLAPP, F. G., 1925-A few observations on the geology and geo•graphy of the North West and Desert Basins of Western Aus•tralia. Proceedings of the Linnean Society of New South Wales, SO, 47-66.

CLARKE, W. J., & BLOW, W. H., 1969-The inter-relationships of some Late Eocene, Oligocene and Miocene larger foraminifera and planktonic biostratigraphic indices. Proceedings of the 1st International Conference on Planktonic Microfossils, Geneva, II, 82-96.

CONDON, M. A., 19S4-Progress report on the stratigraphy and structure of the Carnarvon Basin, Western Australia. Bureau of Mineral Resources, Australia-Report 15,1-163.

CONDON, M. A., 1968--The geology of the Carnarvon Basin, Wes•tern Australia, part 3: Post-Palaeozoic stratigraphy; structure; economic geology. Bureau of Mineral Resources, Aus•tralia-Bulletin 77, 1-68.

CONDON, M. A., JOHNSTONE, D. PEPRY, W. J., & CRESPIN, I., 1955-The Cape Range Structure, Western Australia. Bureau of Mineral Resources, Australia-Bulletin, 21, 1-82.

CONDON, M. A., JOHNSTONE, D., PRICHARD, C. E., & JOHNSTONE, M. H., 19S6-The Giralia and Marrilla Anticlines, North-West Divi•sion, Western Australia. Bureau of Mineral Resources, Aus•tralia-Bulletin, 25,1-86.

MCWHAE, J. R. H., PLAYFORD, P. E., LINDNER, A. W., GLENISTER, B. F., & BALME, B. E., 1958--The stratigraphy of Western Australia. Journal of the Geological Society of Australia, 4,1-161.

QUILTY, P. G., 1974--Tertiary Stratigraphy of Western Australia. Journal of the Geological Society of Australia, 21,301-318.

RAGGATT, H. G., 193~Geology of the North West Basin, Western Australia. Journal of the Royal Society of New South Wales, 70,100-174.

SINGLETON, F. A., 1941-The Tertiary Stratigraphy of Western Australia. Proceedings of the Royal Society of Victoria, 53, 1-125.

VLERK, I. M. van der, & PoSTUMA, J. A., 1967-Qligo-Miocene lepidocyclinas and Planktonic foraminifera from East Java and Madura, Indonesia. Proceedings Koninklijke Nederlandse Aka•demie van Wetenschappen, series B, 70,391-398.