Keith A.W. Crook

Trace element characteristics of graywackes and tectonic setting discrimination of sedimentary basins

The graywackes of Paleozoic turbidite sequences of eastern Australia show a large variation in their trace element characteristics, which reflect distinct provenance types and tectonic settings for various suites. The tectonic settings recognised are oceanic island arc, continental island arc, active continental margin, and passive margins. Immobile trace elements, e.g. La, Ce, Nd, Th, Zr, Nb, Y, Sc and Co are very useful in tectonic setting discrimination. In general, there is a systematic increase in light rare earth elements (La, Ce, Nd), Th, Nb and the Ba/Sr, Rb/Sr, La/Y and Ni/Co ratios and a decrease in V, Sc and the Ba/Rb, K/Th and K/U ratios in graywackes from oceanic island arc to continental island arc to active continental margin to passive margin settings. On the basis of graywacke geochemistry, the optimum discrimination of the tectonic settings of sedimentary basins is achieved by La-Th, La-Th-Sc, Ti/Zr-La/Sc, La/Y-Sc/Cr, Th-Sc-Zr/10 and Th-Co-Zr/10 plots. The analysed oceanic island arc graywackes are characterised by extremely low abundances of La, Th, U, Zr, Nb; low Th/U and high La/Sc, La/Th, Ti/Zr, Zr/Th ratios. The studied graywackes of the continental island arc type setting are characterised by increased abundances of La, Th, U, Zr and Nb, and can be identified by the La-Th-Sc and La/Sc versus Ti/Zr plots. Active continental margin and passive margin graywackes are discriminated by the Th-Sc-Zr/10 and Th-Co-Zr/10 plots and associated parameters (e.g. Th/Zr, Th/Sc). The most important characteristic of the analysed passive margin type graywackes is the increased abundance of Zr, high Zr/Th and lower Ba, Rb, Sr and Ti/Zr ratio compared to the active continental margin graywackes.

Depositional processes along a very low-gradient, suspended-load stream: the Barwon River, New South Wales

The stratigraphy underlying four surfaces, within and adjacent to a very low-gradient (5 · 10−5) channel, is described relative to present depositional processes. The two lower surfaces within the channel (called benches) are formed by suspended-load deposition. This is indicative of the low energy gradients (<1.0 · 10−4) and Froude numbers (<0.1) which characterize within-channel flood waves. On the one hand there is the phenomenon of suspension of the medium sands found in the benches and on the other hand the phenomenon of deposition of the fine suspended load (80–95% < 2 μm). The higher bench is identified as the present flood plain. This level is flooded about once every year on the average. The two higher surfaces are relics, although some deposition occurs on them. Models are presented which see the past phases of river development and deposition as implicit in the hydraulic system. It is recognized that climatic change may play an additional role. Since similar streams and associated paleochannels characterize vast alluviated plains in Australia, these findings have wide application, particularly in relation to paleoenvironmental studies of ancient fluviatile sediments.

Fore‐arc evolution in the Tasman Geosyncline: The origin of the southeast Australian continental crust

Abstract A new general model describing the extended evolution of fore‐arc terrains is used to analyse the evolution of the southern Tasman Geosyncline and the concomitant growth and kratonisation of the continental crust of southeast Australia during the Palaeozoic. The southern Tasman Geosyncline comprises ten arc terrains (here defined), most of which are east‐facing, and several features formed by crustal extension. Each arc terrain consists of several strato‐tectonic units: a volcanic arc, subduction complex and fore‐arc sequence formed during subduction; and an overlying post‐arc sequence which post‐dates subduction and is composed of flysch, neritic sediments or subaerial volcanics. When these materials attained a thickness of c. 20 km their internal heat‐balance caused partial melting of the subduction complex and the hydrated oceanic lithosphere trapped beneath it, to yield S‐ and I‐type granitic magma. The magma rose, inducing pervasive deformation of each arc terrain and emplacement of granitoi...

Ordovician palaeogeography of the Lachlan Fold Belt: A modern analogue and tectonic constraints

Abstract Re‐examination of Late Ordovician facies patterns and sediment movement patterns suggests a palaeogeography trending northwest for part of the Lachlan Fold Belt during this time. From southwest to northeast, the palaeogeography consists of a continental shoreline and shelf in western New South Wales, Victoria and Tasmania, a marginal sea in central New South Wales and Victoria, and a line of volcanic centres running southeast from northwestern New South Wales towards the south coast of New South Wales. The present‐day Andaman‐Nicobar region of the northeastern Indian Ocean has many similarities to this Late Ordovician palaeogeography and provides an important scale perspective. Although these two systems are useful geographic analogues they are not necessarily tectonic analogues.

The southwest pacific area during the last 90 million years

Abstract Maps of the southwest Pacific area at 90, 60, 53, 83, 29, 21, and 10 m.y. are accompanied by brief descriptions of the data and palaeogeographic developments in each major area within the region. There are four stages in. the regional palaeogeographic development: I (80–60 m.y.): Tasman Basin and New Caledonia Trough formed; II (60–53 m.y.): Coral Sea Basin formed; III (53–21 m.y.): A great Melanesian marginal sea formed, bounded by Cainozoic island arcs; IV (21 m.y. to present): Much of northern part of Melanesian marginal sea consumed during retrograde motion of island arcs. These developmental stages cannot be adequately explained by interaction between the Pacific and Australian crustal plates. A comprehensive explanation is given using the zonal spreading model proposed by Crook (1977).

Evaluating the impacts of huge waves on rocky shorelines: an essay review of the book ‘Tsunami – The Underrated Hazard’

Abstract ‘Tsunami – The Underrated Hazard’ draws together and summarizes a wealth of information about the nature, impacts and causes of tsunamis. Written from a natural hazards perspective, the book stresses that large tsunamis can affect even protected coasts that have not experienced large tsunamis historically. Many deposits and erosional features described and discussed as tsunami signatures have yet to be linked specifically with tsunamis. Cited references in the book do not necessarily support a tsunami origin for these features, because accepted scientific citation practices have not been followed. Although rocky shorelines in southeastern Australia, the Hawaiian Islands and elsewhere generate and preserve a variety of boulder and block deposits and erosional features, their sediment dynamics and geomorphology are poorly studied, and unequivocal criteria for distinguishing between deposits of storms and tsunamis have yet to be developed. Despite its shortcomings, this book should be on the library shelves of all marine geoscientists who study modern and ancient shorelines. However, it should be used with great circumspection, because the process:product relationships it assumes are equivocal.

The upper Devonian Boyd Volcanic complex, Eden, New South Wales

Abstract Intrusive and extrusive silicic rocks intertongue with mafic volcanics and sediments in the southern part of the Eden‐Comerong‐Yalwal Rift Zone. Late Devonian fish occur in the sediments. The present stratigraphic scheme of the ‘Eden Rhyolite’ and the ‘Lochiel Formation’ separated by an unconformity or disconformity is rejected. In its place, we rename the assemblage the Boyd Volcanic Complex, formed in a terrestrial zone of extension. The recognition that the alkaline Gabo Island Granite intrudes the Boyd Volcanic Complex, and that both are overlain by the Upper Devonian Merrimbula Group, provides a tight stratigraphic bracket on a potential point on the radiometric time scale.

Sedimentology of coarse-grained alluvial fans in the Markham Valley, Papua New Guinea

Abstract In alluvial sediment sequences recognition of a hierarchy of bounding surfaces, and the shapes and associated lithofacies of the sediment bodies they define, provide an appropriate framework for understanding associations among depositional forms, the processes responsible for them, and their controls on system development. This methodology (architectural-element analysis) integrates principles from geomorphology and sedimentology (Miall, 1985, 1988). It is used here to analyse the evolution of the modern Umi Fan and the alluvial fan part of the Pleistocene Leron Formation in the Markham Valley, Papua New Guinea. A series of terraces has developed as the Umi River has incised into its fan. Detailed stratigraphic analysis of the lowest terrace, in exposures up to 25 m high and kilometres long, reveals that the fan is dominated by sheetflood deposits, with minimal preservation of either debris flow or hyperconcentrated flood flow sediments. Channel fill elements make up a larger proportion of exposures in the proximal-fan than elsewhere, while 95% of distal-fan exposures are composed of sheetflood sequences. These depositional features likely result from massive sediment dispersal associated with the rapidly uplifting upland terrain, abundant sediment availability and flashy discharge regime. Channel fill units, along with slope-related deposits from debris and hyperconcentrated flood flows, are only likely to be preserved in the trench backfilling phase following fan entrenchment. Reworking of deposits plays a dominant role in preservation of sheetflood deposits at the expense of slope-related deposits. An hierarchical framework of lithosomes is developed which characterizes various scales and bounding surfaces of depositional units which make up the Umi Fan (cf. Miall, 1988; DeCelles et al., 1991). Smaller elements, observed in terrace exposures, are described as first- to fourth-order lithosomes. They reflect geomorphic processes which are evident in and adjacent to the modern Umi River. Deposits which infill the trench are interpreted as a fifth-order lithosome constrained by trench geometry. Proximal—distal relationships are remarkably similar in the Umi Fan and Leron Formation fan sequences. In both instances debris flow deposits are seldom observed. The largest sixth- and seventh-order lithosomes, which were deposited within a time frame of approximately one hundred thousand years, are interpreted by analysis of the Leron Formation in relation to its tectonic setting. The Umi and Pleistocene Leron Formation fans exemplify fan development in a post-collisional molasse basin, under a tropical monsoonal climate. Carbonate concretions observed in distal-fan facies may provide a possible diagnostic feature of these tropical-savanna fans.

Ordovician and Silurian history of the southeastern part of the Lachlan Geosyncline

Abstract During the Ordovician and Silurian Periods that part of the Lachlan Geosyncline lying between Yass, N.S.W., and the Victorian border changed from a deep-sea region with basic volcanics, volcanic graywackes, and cherts, to a shallow marine to subaerial platform dominated by acid volcanics, volcaniclastics, mudstones, and limestones. In the course of this development a quartz-graywacke distal flysch body spread throughout the region during the Late Ordovician. This was deformed during the Benambran Orogeny at the end of the Ordovician by gravity tectonics consequent upon the anatexis, metamorphism, folding, and uplift of the Wagga Metamorphic Belt to the west. The region remained deeply submerged after the Benambran Orogeny, and a series of submarine fans comprising a quartz graywacke proximal flysch was developed along its western margin during the late Llandoverian. Distal equivalents may be present, unrecognized, within the Ordovician flysch to the east. Early in the Wenlockian the Quidongan Oro...

Fore-arc evolution and continental growth: a general model

Abstract The extended evolution of fore-arc regions which leads to their eventual incorporation into stable kratonic continental crust is elucidated by a general model based upon observations from the modern circum-Pacific and the Palaeozoic Tasman Geosyncline. Fore-arc regions widen during subduction in the manner described by Karig & Sharman (1975). Their history, after subduction has ceased, depends upon the thickness of the accretionary prism formed during subduction. Where the prism is thick ( ca. 20 km) kratonization is a single-step process. The fore-arc region remains above sea-level; post-arc silicic volcanics accumulate due to granitoid plutonism, the magmas being derived by melting of the subduction complex and from the oceanic lithosphere trapped beneath it. The volcanic arc subsides, becoming the site of a fore-deep. Intermediate-thickness accretionary prisms ( ca. 16 km) are kratonized in a two-step process. They remain at shelf depths, while their associated volcanic arcs sink to comparable depths. Both acquire a post-arc shallow marine sequence of typical platform-cover facies. They are then deformed and intruded by granitoids when the crust attains critical thickness ( ca. 20 km). Thin accretionary prisms (≤ 12 km) require a three-step process for kratonization. They and their associated arcs sink to bathyal depths. They are overwhelmed by prograding post-arc flysch deposits of continental origin. Deformation of the post-arc flysch and plutonism occur when critical crustal thickness ( ca. 20 km) is attained. A transitional tectonic regime ensues, with molasse-like transitional basins preferentially sited over the extinct volcanic arcs and the thinner parts of buried accretionary prisms. The model satisfactorily explains the Late Proterozoic-Palaeozoic evolution of southeast Australia, where a 1000 km wide tract of continental crust was accreted to the Australian Kraton in 250–300 Ma, beginning as a S.W. Pacific-type oceanic terrain. It has been found useful for interpreting geosynclinal terrains in other continents. According to the model, the dynamic processes that contribute to kratonization are systematically causally connected. Kratonization is a unified, internally deterministic and self-sustaining phenomenon. The model has implications for the origin, ‘stratigraphy’ and composition of upper and lower continental crust; the origins and tectonic settings of ophiolites, granitoids, paired metamorphic belts and transitional basins; and for the nature and causes of orogenesis.