View - School of Geosciences

More documents

Recommendations

Info

828 Discussion and Reply diorite is indicative of a significant unconformity between the Hervey Group and the volcanics rather than a regional gentle angular relationship. Southern Cowra Trough The Moura Formation, a ?Pridoli turbiditic unit, in the western Cowra Trough has been intruded by the Eugowra Suite with zircon SHRIMP ages of 393.0 ± 2.5 and 394.2 ± 2.1 Ma (Lyons et al. 2000). The zircon ages are close to the Tucker et al. (1998) Emsian–Eifelian boundary of 394 Ma and substantially younger than the Silurian– Devonian boundary placed at 418 Ma. The Hervey Group overlies the Eugowra Suite granites unconformably (Figure 1). The Moura Formation has been correlated lithologically by Lyons et al. (2000) with the Pridoli Gooligal Group 15 km to the east. The Gooligal Group is overlain by formations of the Gregra Group, which establishes that deposition in the Cowra Trough continued until at least into early Pragian time and the top could be as young as late Emsian (Meakin & Morgan 1999). The Silurian formations of the western part of the trough are overlain unconformably by the bimodal Dulladerry and Warrumba Volcanics in what has been called the Dulladerry Rift. The Warrumba Volcanics unconformably overlie the Young Granodiorite and Silurian Volcanics at the southern end of the Dulladerry Rift. SHRIMP zircon ages of 384.4 ± 3.4 and 382.7 ± 2.8 Ma (Lyons et al. 2000) confirm the late Givetian to earliest Frasnian age for the Warrumba Volcanics on the Tucker et al. (1998) Devonian time-scale suggested by the fish and plant remains. Powell et al. (1980) have documented a discordance of 3–30 between the Dulladerry Volcanics and the Hervey Group and a much greater discordance of 27–97 with the Moura Formation (?Pridoli). The relationships indicate that there were a number of orogenic events following the early Pragian (or later) end of deposition in the Cowra Trough. Deformation of the trough in Early Devonian time preceded the emplacement of the Eugowra Suite granites around the Emsian–Eifelian boundary. Upper Givetian to basal Famennian volcanics were deposited unconformably over the deformed Cowra Trough succession. The Eugowra granites were exposed either prior to or after the Dulladerry–Worrumba Volcanics, and both the volcanics and the granite were overlain unconformably by the Hervey Group fluvial deposits. Final deformation was in Carboniferous time. Allowing for the uncertainties in both the time-scale and zircon ages, the Eugowra Suite granites could be Eifelian and younger than the Hatchery Creek Conglomerate. Currowong and Tullamore Synclines Clear evidence of mid-Devonian tectonism is recorded 60 km west of the Eugowra Granite where the Tullamore Syncline, containing the Hervey Group, trends a few degrees east of north (Figure 1). Unconformable beneath the Hervey Group on its southeastern flank trending a little west of north is the Currowong syncline containing Lower Devonian rocks of the Trundle Group (Lyons et al. 2000). The difference in strike between the two synclines is ~10. The Currowong Syncline contains the Carrawandool Volcanics dated at 403.2 ± 2.1 Ma (Emsian) that are intruded by two granite bodies: the Dalrida Granite, dated by SHRIMP at 395.4 ± 1.7 Ma and the Bundaburrah Granodiorite, which is unconformably overlain by the Hervey Group. Broader considerations Figure 1 Location of the Hatchery Creek Conglomerate, the distribution of Upper Devonian “Lambie Facies” formations in the eastern part of the Lachlan Fold Belt and the rocks unconformably underlying them. C, Cambrian; O, Ordovician; S, Silurian; Smg, mid-Silurian granite; Dl, Lower Devonian; Dlg, Lower Devonian granite; Dm, Middle Devonian; Dmv, Middle Devonian volcanics. On a broader scale in the eastern Lachlan Fold Belt, contrary to what is implied by Hood and Durney (2002) in the first paragraph quoted above, there is no location where deposition has been continuous from the Early Devonian to Late Devonian and the Early Middle Devonian to Late Devonian. Only two early Middle Devonian sedimentary formations are known in the region east of the Gilmore Fault. One is the Hatchery Creek Conglomerate, discussed above, and the other is on the northwestern corner of the Mt Frome Syncline (Figure 1) where the Mt Frome Limestone extends into the first conodont zone of the Middle Devonian and is overlain conformably by the Boogledie Formation,
Multiple deformation around Taemas Bridge 829 a succession of feldspatholithic sandstone, shale and conglomerate (Meakin & Morgan 1999). This is overlain unconformably by the basal beds of the Upper Devonian Mt Frome Group. Elsewhere, east of the Gilmore Fault, the last recorded Early Devonian marine sediments are variously late Lochkovian to Emsian. Possible late Middle to early Late Devonian formations occur in two areas: (i) the Dulladerry Rift post-dating the Cowra Trough discussed above; and (ii) the Comerong Volcanics and the Boyd Volcanic Complex, which are suites that underlie the coastal strip of the Upper Devonian Merrimbula Group. The latter volcanics overlie Ordovician quartz-rich facies rocks and locally part of the Early Devonian Bega Batholith. There are no older Devonian sedimentary formations underlying these volcanics. In both cases, the volcanics are bimodal and are supposedly deposited in an extensional tectonic environment (Scheibner & Basden 1998). The Upper Devonian formations in the Lachlan Fold Belt east of the Gilmore Fault post-dating the volcanics (‘Lambie Facies’) are universally unconformable on older rocks (Figure 1). They are predominantly quartz-rich fluvial and alluvial facies deposits with a marine transgression early in the depositional history. On present biostratigraphic data, the base is late Frasnian to early Famennian. If it is diachronous, it could be a little older in the east. The Hatchery Creek Conglomerate, rather than being part of a diachronous ‘molasse’ overlap sequence, is better regarded as a short-term incursion of the fluvial facies of the Mulga Downs Group onto the western flank of the eastern Lachlan Fold Belt. An assessment of Middle Devonian deformation around the Hill End Trough was made in a series of studies by Powell and coworkers (Powell et al. 1976; Powell & Edgecombe 1978) that focused on the angular discordances between Upper and Lower Devonian rocks in thinner and less-disturbed successions on the palaeogeographical highs east and west of the trough. Even there they found that half of the 130 angular discordances located were between 10 and 20. The significant basal Upper Devonian unconformities on both highs that cut across the Lower Devonian and Silurian strata to expose Ordovician formations (Figure 1) were not taken into account. The question of the timing of deformation of the Hill End Trough was an issue I revisited (Packham 1999), reasserting my long-held view that it occurred in Middle Devonian time. The trough contains a conformable thick succession of Ludlow to upper Emsian turbidite facies rocks, the last deep-water sediments in the northeast Lachlan Fold Belt. As Upper Devonian formations are confined to the flanking highs, indirect arguments have been used to deduce the time of its deformation. The axial trough fill has been metamorphosed up to biotite grade. In samples from the Hill End goldfield, Lu et al. (1996b) obtained 40 Ar/ 39 Ar dates on biotite of 359.8 ± 0.6 and 358.2 ± 0.4 Ma, close to the Devonian– Carboniferous boundary, for the cooling of regional metamorphic biotite through the closure temperature for argon. Lu et al. (1996a, b) had found from palaeothermometry and palaeobarometric studies together with dating of the mineralisation phases that pressure had remained high and temperatures elevated well into Carboniferous time. I argued that these data indicated Middle Devonian deformation, metamorphism and limited exhumation followed by consequential slow cooling during and after the deposition of the flanking Upper Devonian formations prior to deformation later in the Early Carboniferous. The structural relationships in the Koorawatha Syncline, the Southern Cowra Trough the Currowong and Tullamore Synclines provide clear evidence of deformation in the middle part of the Devonian. This is supported by the unconformable relationship of Upper Devonian formations to older units on the Molong and Capertee High, a deduced Middle Devonian deformation of the Hill End Trough and the regional unconformity at the base of the Upper Devonian “Lambie Facies” throughout the eastern Lachlan Fold belt. This clear evidence for regional Tabberabberan events means that the four deformations identified by Hood and Durney (2002) in the Black Range Synclinorium cannot be definitively dated as Carboniferous in age. Some, and perhaps all of them, could have occurred in Middle Devonian time. REPLY D. W. DURNEY 1 and D. I. A. HOOD 2 * 1 Department of Earth & Planetary Science, Macquarie University, NSW 2109, Australia. 2 4 Cheriton Avenue, Castle Hill, NSW 2154, Australia. Packham raises some fundamental points about interpretation of the sub-Upper Devonian unconformity in the Lachlan Fold Belt. He questions the Carboniferous age of the four fold- and cleavage-forming episodes in Lower to lower Middle Devonian units of the Black Range Synclinorium (Hood & Durney 2002) that we had deduced from structural correlation with deformed Upper Devonian rocks and suggests that ‘Some, and perhaps all of them’ could be Middle Devonian. He evidently regards this method as ‘indirect’ or not ‘definitiv[e]’ and points, instead, to the widespread unconformable relations of Upper Devonian units east of the Gilmore Fault (Figure 1) and to a thermochronological interpretation (Packham 1999) of analytical data (Lu et al. 1996b, c) from the Hill End area. This general subject was discussed by Powell and coworkers and Packham (1999), but we find those assessments to be incomplete. (For example, they discussed mainly a single fold episode; we four. They assumed that the mild angular unconformable relations around the Hill End Trough were due to fold deformations; we do not.) Packham’s comments raise questions about what causes unconformities, a point that we feel must be addressed if further progress is to be made. We therefore explain what we consider to be the primary tools for assessing whether unconformities are due to folding—geometric and structural criteria—and enquire how they apply to the ca Middle Devonian discordances that he mentions. Likewise, we examine the evidence and reasoning for the *Corresponding author: david_hood@royalsun.com.au
Page 1: Australian Journal of Earth Science
Page 5 and 6: Multiple deformation around Taemas
Page 7: Multiple deformation around Taemas

View - School of Geosciences

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?