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Newly Recognised Eocene Sediments in the Beaufort River Palaeochannel

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Newly Recognised Eocene Sediments in the Beaufort River Palaeochannel by J. D. Waterhouse, D. P. Commander, C. Prangley, and J. Backhouse This paper records the occurrence of these sediments and discusses their geological relationships. The An Eocene palaeochannel has recognition of Eocene palaeo- channels in the southwestern Yilgarn Craton is important for the identification of low-salinity groundwater resources, which may occur where palaeochannels are not coincident with present saline drainages, such as at Dardadine The palaeochannel sediment San near Darkan, to the north of the Beaufort River (Wharton, P. H., 1990, written comm.). Craton reduced the gradient for s The sediments yield a rich spore Late Eocene age, with no evidenc Previous work Sediments described from many palaeochannels elsewhere in the western part of the Yilgarn Craton (Laws, 1993; Salama et al., 1993) have not been dated due to lack of suitable material. Eocene palaeo- channel sediments in the western Yilgarn Craton were previously KEYWORDS: Yi known from Darkin Swamp, 65 km east-southeast of Perth (Hill and Merrifield, 1993). The nearest definitely Eocene palaeochannels are just north of the Stirling Ranges (Appleyard, in prep.), adjacent to the Bremer Basin. An Eocene palaeochannel has been few localities on or near drainage In 1992, R. J. George collected identified for the first time on the divides (Wilde and Backhouse, 1977; samples of cuttings from an western margin of the Yilgarn Hill and Merrifield, 1993). Recent exploration drillhole described by Craton. Previously, Eocene drilling has confirmed the presence Muggeridge (1981) near Duranillin. sediments were known from the of Eocene sediments in the newly These were determined to be Eocene eastern and southern Yilgarn Craton defined palaeochannel (which in age and their geology and (Balme and Churchill, 1959; Hos, drained to the west) of the westerly geomorphological position 1975; Hocking and Cockbain, 1991; flowing Beaufort Palaeoriver. It is suggested that they were part of a Kern and Commander, 1993), but located to the west of the Meckering palaeochannel system, as indicated the age of sediments in Line (Mulcahy, 1973), where on the geological map of Western palaeodrainages that drained Tertiary palaeodrainages have not Australia (Myers and Hocking, towards the west was uncertain. In previously been recognized (van de 1988). the Blackwood River catchment Graaff et al., 1977; Morgan, 1993) (Fig. I), and to the north, probable and where only one Eocene age has In February 1994, groundwater Eocene plant fossils in arenaceous been reported (Hill and Merrifield, exploration drilling programs were sediments have been identified at a 1993). carried out to investigate both the 82 I \ 118" Geological and \ Meckering Line -+, geomorphological setting The study area is located within the southwestern portion of the Yilgarn 33" Craton, where the bedrock mainly consists of Archaean granitic rocks. The area lies within the Darling Plateau, in the 'Narrogin semi- stripped etch plain' subdivision of Finkl and Churchward (1973). The highest points in the landscape are Archaean rocks at an elevation of about 360 m. \ Stirling Ranges A number of post-Permian sedimentary units which overlie bedrock in the southwest Yilgarn Craton, including Bremer Basin , 50km , sediments, have been described by \ Finkl and Fairbridge (1979) and Backhouse and Wilson (1989). Those 35" Figure 1. Blackwood for which firm dates have been JP2 10 11 River catchment established are Cretaceous or Tertiary in age. Isolated remnants of the Kojonup alaeochannel near Lake palaeovalley it is inferred to be the Sandstone (which is of assumed, but 'owerrinning, and sediments in the former course of the Beaufort River. unconfirmed, Eocene age, and oscabel area where the occurrence The palaeochannel has been fluvial in origin) are preserved on f low-salinity groundwater was recognized over a length of some drainage divides at the southern nown from a 48 m-deep artesian 60 km,'with a markedlv different marpin of the Blackwood catchment orehole. The two areas are now orientation to the modgrn drainage (WiiJde and Backhouse, 1977) at an ?cognizedto be part of the same patterns in the Towerrinning area. elevation of about 320 m. These alaeochannel system (Fig. 2), and The sediments it contains are Eocene exposures are located within 20- y the location and size of the in age. 30 km of the study area. Figure 2. Location of the Beaufort Palaeochannel 83 Figure 3. Geological cross section across the Beaufort Palaeochannel The Beaufort Palaeochannel, where places, the overlying clay extends environments within the recognized over its length, occurs over a wider area to the flanks of the palaeovalley. Later, more uniform low in the landscape within a palaeovalley (Figs 3 and 4). White lacustrine conditions may have discontinuous valley. In places, the clay, presumably weathered, can be prevailed throughout the system, modern drainages cross this seen close to the surface along parts with subsequent erosion of some of palaeovalley, making its continuity of the course of the palaeochannel in the areas where lacustrine clay was less obvious; however, its course can excavated farm dams. Its character- deposited. Alternatively, a system of be traced once these overprinting istically massive appearance separate lakes may have developed features have been recognized. In distinguishes it from exposures of along the palaeovalley as gradients comparison with the known deeply weathered basement rock, in decreased, in a similar way to the outcrops of Kojonup Sandstone, the which some remnant igneous or chains of lakes presently occupying valley floor where the Beaufort metamorphic textures are preserved. the palaeodrainage east of the Palaeochannel is present ranges in Meckering Line (Fig. 1). elevation from 226260 m (Fig. 3), At most locations the sequence with the base of the sediments as includes a wide range of On Towerrinning Transect C much as 6670 m lower. The interbedded sediments without a (Fig. 2), the lacustrine clay is well palaeochannel sediments are thus consistent upper clay unit. The developed and overlain by a second consistently 100-150 m lower than multiple layers of differing fluvial sand sequence with an the nearest known outcrop of sedimentary materials show little overlying less well-developed clay. Kojonup Sandstone. apparent correlation from borehole The deeper sands, in the earlier- to borehole, which typically are formed fluvial sequence, are spaced at 406500 m intervals along subrounded to rounded, typical of the transects. This is interpreted to those elsewhere in the Sfratigraphy reflect deposition by a meandering palaeochannel, and are believed to The sediments in the palaeochannel river system with an associated have been transported along it. can be divided into two units. The wide range of depositional However, the shallower sand is lower unit consists of interbedded sands, clayey sands, and clays, which are all carbonaceous in Catchment places. The upper unit, where Kojonup divide present, is dominantly composed of Sandstone clay and silt, with lesser amounts of + sand. The lower unit is commonly dark brown and grey, and is Beaufort oxidized in places to yellow and red, Palaeochannel whereas the upper unit is typically outcrop of pale grey to white. 250 + + +granitic + The sediments in the lower portion IaiG: + + basement + of the palaeochannel occupy a +++i+ Figure 4. 'Sand, minor+ + + + + + + relatively narrow channel that is 200 + :. + Relationship of +clay + i + + + + + + 200-500 m wide. They overlie Tertiay units in and weathered Archaean bedrock. In mAHD around the Beaufort JDW7 10.11.94 Palaeochannel 84 remarkably angular and appears to The palaeochannel trends west- sedimentation along the drainage to have been derived locally. The wards from Towerrinning, suggest- cease, with lacustrine conditions upper fluviatile / lacustrine ing that the Beaufort Palaeoriver developing instead. Subsequent sequence, with its angular sands, discharged across the Darling Scarp tilting to the south (Cope, 1975) has may reflect local rather than in the vicinity either of the present resulted in drainage diversion and regional conditions along the Collie River, which occupies a deep capture, and the modern Blackwood palaeodrainage. valley through the Darling Scarp, or River has cut through Archaean the Preston River, which contains a bedrock south of Lake Towerrinning substantial thickness of sediments to behead the ancestral Beaufort that are as yet undated River. The palaeochannel west of Palynology (Commander, 1993). Towerrinning has also been cut by Carbonaceous material from depths Darlinup and Haddleton Creeks, The relationship of the palaeo- of 12 to 42.5 m, representing almost both of which are southerly flowing channel sediments with the more the entire palaeochannel sequence, tributaries of the Blackwood River. elevated Kojonup Sandstone is contained rich and diverse spore intriguing. If the Kojonup Sandstone The palynomorph assemblages and pollen assemblages. The represents an earlier deposit on a indicate that Eocene marine assemblages are dominated by landscape that was subsequently influences do not appear to extend Haloragacidites harrisii, Nothofagidites rapidly dissected by uplift into the Blackwood catchment, and spp., bisaccate gymnosperm pollen, associated with continental breakup are restricted to the area south of the and a diverse range of proteaceous along the margin of the craton, Ravensthorpe Ramp (Hocking and pollen. Marine indicators have not substantial evidence of sediment-
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