G. H. Packham

The Melanesian Borderlands and India — Pacific plates' boundary

Abstract The Melanesian Borderlands extend from New Guinea to Tonga and, as the name implies, occupy a border position between the India and Pacific plates. They include island chains with small land surface, deep trenches, basinal seas and real and apparent fracture systems. As morphotectonic elements, most show anomalous features in terms of todays models. The complex as a whole is treated as made up of seven subdivisions, most of which are Cainozoic tectonic unities. These are: Bismarck Sea and surrounds; Solomon Block; Coral Sea and eastern extension; New Hebrides and South Fiji Basins; New Hebrides Block; Fiji Plateau and Lau Basin; Fiji Platform, Lau and Tonga Ridges. The Bismarck Sea is not the model of a marginal sea. The surrounding land areas have arc-like features and bear the impress of one or more reversals in polarity but some features, especially geochemical ones, are difficult to relate to arc development: the Solomons, New Hebrides and Fiji-Lau-Tonga land areas echo, with varying emphases, this same condition. The Solomons appear to have been shaped, for at least the last half of their history, by sinistral shearing movement between the two major plates and by collision with the Ontong Java Plateau. The New Hebrides, following apparent polarity reversal, has followed a subduction zone retreating to the southwest and is now on the point of collision with the Loyalty Ridge. This movement was coincident with creation of the Fiji Plateau and accommodated along the Hunter Fracture Zone. The Fiji Platform is peculiar in that it shows evidence of bodily rotation. It developed as an arc until this rotation became a dominant factor at least five million years ago. The Lau-Tonga ridges were part of the Fiji are until about the same time. Since then the Lau Basin has arisen as an interarc basin so that the Lau Ridge is now a remnant arc. The Coral Sea is an enigmatic basin the genesis of which is unknown although it is linked to the break-up of the northerly part of an old continental-cum-arc system bordering the Australian continent — Owen Stanley Metamorphics, Louisiade Archipelago, Rennell Ridge and New Caledonia. On the Pacific side of this Inner Melanesian system, the New Hebrides and South Fiji Basins are the remnants of a former Palaeogene marginal sea; the creation of the Fiji Plateau over the last 5–10 m.y. has effectively halved it. The northern boundary of the Fiji Plateau includes the Cape Johnson Trough and Vitiaz Trench; the latter can be extrapolated east to the northwestern termination of the Tonga Trench. This boundary has been viewed as a relic subduction zone. Much more data are needed, especially from the basinal seas, before a really convincing reconstruction can be attempted of the events which led to the present disposal of elements within the Melanesian Borderlands.

Stratigraphy and regional setting of the Cliefden Caves limestone group (Late Ordovician), central‐western New South Wales

Abstract The 460 m‐thick Cliefden Caves Limestone Group is the oldest and best exposed, extensive body of limestone in N.S.W. Its Late Ordovician faunas and floras are comparatively well preserved and it has some excellent silicified horizons, especially in the upper part of the sequence. In the nominal type area at Cliefden Caves, the limestone has been subdivided into three formations—the Fossil Hill Limestone (lower), the Belubula Limestone (middle) and the Vandon Limestone (upper). Only the Belubula Limestone remains undifferentiated. The Fossil Hill Limestone has been further subdivided into six members (Gleesons, Kalimna, Wyoming, Taplow, Dunhill Bluff and Transmission Limestone Members, respectively), and the Vandon Limestone into two members (Trilobite Hill and Mount Lewin Limestone Members). Each of these subdivisions is defined herein, with a designated type section and a description of its content and distribution. In late Gisbornian to early‐middle Eastonian time, the Cliefden Caves Limestone ...

Plate tectonics and the development of sedimentary basins of the dextral regime in western Southeast Asia

Abstract The western part of Southeast Asia is comprised of three tectonic plates: the Southeast Asia Plate, the Burma Plate, and the Australia-India Plate. The present regime of oblique subduction and partial coupling of the combined Australia-India Plate to the Southeast Asian part of the Eurasia Plate was initiated in the Middle Eocene. Resulting dextral shear interacting with the inertia of the largely continental plate were the driving forces of basin genesis and development, setting in train a sequence of temporally and spatially variable tectonic events, diminishing in intensity away from the active plate boundary. The effects can be identified in the east from the Malay Basin to Central Thailand. A Late Eocene to Oligocene rifting phase formed rift troughs in central Sumatra, later spreading north to the Mergui Basin and south to the Sunda Basin. The Sunda Volcanic Arc was initiated. As subduction proceeded, deformation spread out from the vicinity of the plate boundary. In the Oligocene, dextral shear, substantially along preexisting faults and consequential extensional faulting initiated the Thailand basins and the Malay Basin. Subsidence and extension continued until late Middle Miocene time, possibly with some thermal relaxation. In the Late Oligocene to Early Miocene, the back arc basins close to the plate boundary began to subside, extending out from the initial rift troughs during a quiescent interval transition between the rifting and later wrenching phases. Subsidence may have been the result of the withdrawal of heat from the asthenosphere beneath the basins by the cold subducted slab. As the Sunda arc-forearc developed by accretion and magmatism, partial coupling in the plate boundary region was enhanced and transpressional deformation commenced in the Sumatra basins in the Middle Miocene. This Sea defining the northern part of the Burma Plate. South of the Andaman Sea, deformation persisted in a broad zone along the margin of the Southeast Asia Plate. Dextral wrenching in the late Middle Miocene phase affected much of the eastern region, extending south into the Natuna Basin. Deformation diminished markedly at the end of the Middle Miocene with the inception of the Burma Plate. Transpressional tectonics continued through Late Miocene and Pliocene time in Sumatra, in a broad zone as an ill defined southern extension of the Burma Plate. The continuing movement resulted in the uplift of the Barisan Mountains and the deformation of the onshore Sumatra basins. The Sumatra forearc was transferred to the Burma Plate with the establishment of the dextral Sumatra Fault probably in the Pliocene.

Granitic and monzonitic rocks dredged from the southeast Australian continental margin

Four Middle Devonian (381 Ma) granodiorite samples have been recovered from two dredge sites approximately 65 km east of Green Cape, New South Wales. The granodiorite samples are similar in age and composition to members of the Moruya Suite and probably form an along‐strike extension of that suite. The location of granodiorite on the southeastern margin requires that a piece of continental lithosphere was located to the present east of the study area in the Devonian. This piece of lithosphere may now be located somewhere on the western Lord Howe Rise. A sample of Early Cretaceous leuco‐quartz monzodiorite was also recovered from a dredge site approximately 45 km north‐northeast of Dalmeny, New South Wales. It represents a body that was intruded at essentially the same time as, and is inferred to be of similar origin to, the syenite rocks of the nearby Mt Dromedary and Montague Island complexes.

Middle to early Late Ordovician hydrothermal veining in the Molong Volcanic Belt, northeastern Lachlan Fold Belt: Sedimentological evidence

Detrital volcanic and vein quartz, accompanied by felsic volcanic debris, occur as minor constituents in the Ordovician subduction‐related mafic volcanics of the Molong Volcanic Belt. In the western province of the Molong Volcanic Belt, detrital quartz is present in the three episodes of the mafic Volcanics. Volcanic quartz occurs in allochthonous limestone blocks in the Bendigonian Hensleigh Siltstone overlying the Mitchell Formation. The second volcanic episode (the Fairbridge Volcanics) commenced after a hiatus of approximately 20 million years and lasted around 10 million years from Darriwilian to Gisbornian time. Locally derived vein quartz, volcanic quartz and felsic detritus are concentrated at the bases of autochthonous Wahringa and Yuranigh Limestone Members of the volcanics and are extensive and abundant in basal beds of the regional Eastonian limestone body that transgressed over an eroded volcanic centre at Cargo. This early Eastonian debris, deposited early in an 8 million‐year volcanic hiatus preceding the final Ordovician Bolindian volcanism, establishes a pre‐Eastonian age for mineralisation at Cargo. It is inferred that the pauses in volcanism were preceded by magmatic fractionation, intrusion and hydrothermal activity and followed by erosion, subsidence and deposition of autochthonous limestones. Minor occurrences of vein and volcanic quartz are found in Bolindian volcanogenic sediments of the third volcanic phase. It is concluded that hydrothermal vein formation (and mineralisation by inference) was associated with pauses in volcanic activity throughout the Middle to early Late Ordovician over a wide area in the western province, culminating in the mineralisation at Cargo and Copper Hill near Molong. Volcanism in the eastern province of the Molong Volcanic Belt was continuous from at least Darriwilian to latest Ordovician time. Here, detrital hydrothermal vein quartz and volcanic quartz and felsic detritus are distributed through late Middle and early Late Ordovician turbidites of the Weemalla Formation. The possible existence of cycles in the source area like those of the Fairbridge Volcanics is masked by the distal nature of these deposits. Vein formation occurred in both provinces from late Middle Ordovician to early Late Ordovician, long before the formation of the world‐class mineral deposit at Cadia associated with the latest Ordovician Cadia Monzonite.

The Narooma Terrane offshore: a new model for the southeastern Lachlan Orogen using data from rocks dredged from the New South Wales continental slope

ABSTRACT Eight dredges from the southern New South Wales continental slope sampled the offshore extension of the Lachlan Orogen. Two rock suites were recovered: (1) lower greenshist facies limestones, felsic volcanics, sandstones, mudstones and Moruya Suite granodiorite correlate with the onshore Silurian to mid-Devonian orogenic phase; and (2) a strongly deformed greenschist to lower amphibolite facies mafic volcanics, cherts, marbles, pelites and serpentinites correlate in part with the Cambro-Ordovician Wagonga Group of the Narooma Terrane. The mafic volcanic rocks have ocean island, tholeiitic and boninitic basalt affinities. The offshore distribution of ocean island basalt that correlates with medial Cambrian basalt breccias at Batemans Bay suggests a large seamount or seamount complex. The boninites, tholeiites and ultramafics could be part of a forearc-generated ophiolite. The Narooma Terrane basement is interpreted as the part of the bonititic arc postulated to have collided with Vandieland in late early Cambrian time. Mid-Cambrian rifting of the oceanward part of this arc remnant, generated the Albury–Bega Terrane oceanic basement exposed in the Howqua Valley in the west and Melville Point in the east. Overlying are upper–mid-Cambrian to lowermost Ordovician black shale and chert, Lower Ordovician to Gisbornian Adaminaby Group quartz turbidites and Gisbornian to lower Bolindian Bendoc Group black shales. Batemans Bay exposures are reinterpreted as a dismembered basin margin succession onlapping the west-facing attenuated flank of the Narooma Terrane. The Narooma Cambro-Ordovician cherts and mudstones were initially deposited outboard on the more elevated seamount flank elevated above the clastic-filled basin to the west. Benambran deformation commenced in latest Ordovician time uplifting the outer Narooma Terrane, shedding debris from the seamount and its flanks, culminating in allochthonous displacement of chert masses to the basins eastern margin to Narooma, and emplacing them as a succession of thrust sheets. Contemporaneously, silt and mud of the Bogolo Formation, deposited from the west, were mixed with olistostomal basalt and chert debris from the east. Early Silurian westward tectonic transport of the Narooma Terrane ruptured the Albury-Bega basin floor at Batemans Bay, thrusting it and its sedimentary cover over its eastern margin as a series of thrusts each floored by melange (mapped Bogolo Formation), derived from the slope debris and its overpressured sedimentary cover. Offshore, the metamorphosed Benambran phase rocks are unconformably overlain by Tabberabberan cycle sediments and volcanics intruded by granodiorite. Our interpretation of the boundary between the Albury-Bega and Narooma terranes as a thrusted passive margin accumulation is incompatible with models of a Narooma Accretionary Complex formed by the subduction of the Paleopacific Plate.

Late Silurian and Early Devonian biostratigraphy in the Hill End Trough and the Limekilns area, New South Wales

The first recorded graptolites from the Chesleigh Formation of the Limekilns district are Monograptus prognatus, ?M. transgrediens, Pristiograptus sp. cf. P. shearsbyi and Linograptus posthumus, which unequivocally indicate a Prídolí (Late Silurian) age for the formation. The Silurian-Devonian boundary appears best placed at approximately the boundary between the Chesleigh Formation and the overlying Cookman Formation, despite the presence of the early Lochkovian graptolite Monograptus uniformis uniformis much higher stratigraphically in the Limekilns Formation. In the Hill End Trough succession, the Turondale Formation has yielded conodonts including Amydrotaxis praejohnsoni, which indicates the delta to pesavis conodont Zones of late Lochkovian (Early Devonian) age. Conodonts from the upper part of the Cunningham Formation, at a locality in the Trough west of Mudgee, include Polygnathus nothoperbonus / P. inversus, indicating the perbonus zone of middle Emsian age. The base of the Cunningham Formation is diachronous, being late Lochkovian (delta Zone) — the same age as the Turondale Formation — on the west flank of the Trough as previously reported, and late Pragian in the east. The youngest faunas found in the unit are late Emsian.

Late Silurian or Early Devonian corals from the continental slope off southern New South Wales

Latest Silurian or Early Devonian tabulate corals have been recovered in a dredge haul from submarine bedrock at a water depth of 2620 – 2155 m on the continental slope off southern New South Wales 45 km east of Tuross Heads from the submerged and unexplored part of the eastern Lachlan Orogen. The corals have been identified as latest Silurian to Early Devonian taxa, Favosites gothlandicus and Squameofavosites squamuliferus forma nitidus. The presence of shallow water deposits containing these forms indicates that a crustal thickening event or events took place near the end of Silurian or early in Devonian time in the eastern Lachlan Orogen east of the present coastline. This occurred well after the end of subduction in Early Silurian time which is marked by the welding of the Cambrian — Late Ordovician Wagonga Group accretionary wedge or collided terrain on to the eastern margin of the orogen and significantly earlier than the emplacement of the Moruya Suite granites in Middle Devonian time.

U–Pb SHRIMP ages of volcanic zircons from the Merrions and Turondale Formations, New South Wales, and the Early Devonian time‐scale: a biostratigraphic and sedimentological assessment

U–Pb ages for volcanic zircons from the Lower Devonian Turondale and Merrions Formations initially dated by Jagodzinski and Black (1999) have been recalculated by Compston (2000, 2001). The Turondale, Waterbeach and Merrions Formations are, in ascending stratigraphic order, three of the Lower Devonian deep‐water formations of the Hill End Trough. The recent discovery of a Lochkovian conodont fauna of the delta zone in derived limestone blocks from the uppermost beds of the Turondale Formation has established a maximum age for that horizon. The principal lithological assemblages of the formations are volcanic (megaturbidite and lavas), epiclastic turbidites and essentially hemipelagic lithologies. The mean zircon ages suggest a long duration for the Merrions Formation, 615 m thick and 91% volcanic, and a very much shorter interval for the upper two‐thirds of the Turondale Formation, 360 m thick and 34% volcanic, plus the intervening non‐volcanic Waterbeach Formation, 512 m thick. A sedimentological model based on scaled depositional rates for the facies has been used to estimate the relative accumulation rates and durations of the succession from the base of the Turondale Formation to the top of the Merrions Formation. The model indicates that the Merrions Formation accumulated in a much shorter time than the interval from the top of the lower Turondale Formation to the base of the Merrions Formation. The analytical uncertainties of the Jagodzinski and Black (1999) zircon ages are large enough to be compatible with the sedimentological model, but the smaller analytical uncertainties calculated by Compston (2000, 2001) fall far from the scaled sedimentological model. Grouping together of the lower Merrions Formation and lower Turondale Formation zircon dates in Compston (2000) for a single mid‐Lochkovian age is in conflict with the biostratigraphic and sedimentological evidence available and the derived time‐scale for the Early Devonian is highly questionable. Given the incompatibility of the recalculated zircon ages with the depositional history and the uncertainty of biostratigraphic dating of the formations, neither of the quite different sets of ages quoted in Compston (2000, 2001) are useful for the definition of a numerical time‐scale.

Late Paleocene foraminiferal faunas with Chapmanina and Reticulophragmium from offshore southeastern Australia: a Tethyan influence

Samples from off southern New South Wales in approximately 1750 m water depth have yielded late Paleocene faunas of benthic and planktonic foraminifera and other invertebrates. Two faunal associations with a strong Tethyan influence, dominated respectively by Chapmanina conjuncta sp. nov. and Reticulophragmium naroomaensis sp. nov., are described; this is the first formal record of these genera from Australasia. The age of the host sediment is based on both benthic species and few planktonic foraminifera. The environment at the time was warm temperate to subtropical at the southwestern extremity of a counter-clockwise Pacific Ocean gyre. The new occurrence extends the record of Palaeogene marine sediments and palaeoenvironments along the eastern margin of Australia and confirms earlier observations that conditions in the marine environment at that time were considerably warmer along the east coast of Australia than those along the southern margin; the biogeographic association was Tethyan. The circulation model may go some way to explaining coeval warmth along the east Antarctic coast.