Christopher R. Fielding

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3–4 °C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1–23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

Palaeosols in Westphalian Coal-bearing and red-bed sequences, central and northern England

Westphalian A-C coal-bearing strata and red-beds in the Pennine Basin of Central and Northern England contain a wide variety of palaeosols. In coal-bearing facies associations alluvial, gley, semi-gley, organic and rare well-drained palaeosols reflect the predominance of poor drainage conditions during deposition. In red-beds more evolved ferruginous and ferallitic palaeosols testify to free drainage conditions during deposition. Catenary relationships can be inferred between most of the palaeosols in both facies associations, providing further information on the palaeogeomorphology of the depositional systems, and assisting in the understanding of the environments and conditions of coal seam formation.

U–Pb zircon geochronology of Late Devonian to Early Carboniferous extension‐related silicic volcanism in the northern New England Fold Belt*

Laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) analysis of zircons confirm a Late Devonian to Early Carboniferous age (ca 360–350 Ma) for silicic volcanic rocks of the Campwyn Volcanics and Yarrol terrane of the northern New England Fold Belt (Queensland). These rocks are coeval with silicic volcanism recorded elsewhere in the fold belt at this time (Connors Arch, Drummond Basin). The new U–Pb zircon ages, in combination with those from previous studies, show that silicic magmatism was both widespread across the northern New England Fold Belt (>250 000 km2 and ≥500 km inboard of plate margin) and protracted, occurring over a period of ∼15 million years. Zircon inheritance is commonplace in the Late Devonian — Early Carboniferous volcanics, reflecting anatectic melting and considerable reworking of continental crust. Inherited zircon components range from ca 370 to ca 2050 Ma, with Middle Devonian (385–370 Ma) zircons being common to almost all dated units. Precambrian zircon components record either Precambrian crystalline crust or sedimentary accumulations that were present above or within the zone of magma formation. This contrasts with a lack of significant zircon inheritance in younger Permo‐Carboniferous igneous rocks intruded through, and emplaced on top of, the Devonian‐Carboniferous successions. The inheritance data and location of these volcanic rocks at the eastern margins of the northern New England Fold Belt, coupled with Sr–Nd, Pb isotopic data and depleted mantle model ages for Late Palaeozoic and Mesozoic magmatism, imply that Precambrian mafic and felsic crustal materials (potentially as old as 2050 Ma), or at the very least Lower Palaeozoic rocks derived from the reworking of Precambrian rocks, comprise basement to the eastern parts of the fold belt. This crustal basement architecture may be a relict from the Late Proterozoic breakup of the Rodinian supercontinent.

Preservation of in situ, arborescent vegetation and fluvial bar construction in the Burdekin River of north Queensland, Australia

Abstract In sub-humid parts of north Queensland, NE Australia, certain types of trees are well adapted to living in river bed habitats. The bed of the tropical, variable-discharge, upper Burdekin River hosts a community dominated by the paperbark Melaleuca argentea. Trees grow preferentially in flow-parallel, linear groves, and engineer their own environment by deflecting currents, building sand and gravel bars and stabising banks. This is the first study to document in-channel bar development resulting from vegetation growth, rather than the reverse which has been inferred by previous workers. In the Burdekin River study site, individual Melaleuca range from seedlings to mature trees over 100 years old. These trees survive regular, partial to total submergence and impact damage during wet season runoff events (often reaching over 20,000 m3 s−1 at peak discharge) partly by adopting structural and growth modifications. These modifications include a reclined, downstream-trailing habit, multiple-stemmed form, modified crown with weeping foliage, development of thick, spongy bark, root regeneration and group strategies, notably development of flow-parallel, linear groves. Following death, in situ remains of trees are preserved within the mainly coarse sand to gravel channel fill, either as reclined stems/trunks stripped of branches and foliage or as more upright trunks snapped at a height of typically 1–2 m above base, both with roots. The morphological adapations and styles of preservations of in situ vegetation within the Burdekin River are considered distinctive of variable-discharge rivers. and may be useful in the identification of facies formed in such environments in the rock record, particularly when associated with bar development.

Coal depositional models for deltaic and alluvial plain sequences

Coal depositional models attempt to predict the distribution and geometry of coal beds within given sequences or provinces. To date, models proposed for coals of alluvial and deltaic origin have emphasized depositional environment as the controlling variable. In this article, I propose that subsidence regime and, to a lesser extent, sediment supply are the dominant controls upon coal distribution rather than environment of accumulation. For this reason, most existing coal depositional models are likely to have limited predictive capability.

Variations in character and preservation potential of vegetation-induced obstacle marks in the variable discharge Burdekin River of north Queensland, Australia

Abstract Vegetation-induced obstacle marks are described from Dalrymple Bend in the Burdekin River of Queensland, Australia, and their preservation potential is discussed. The bend is divided into a sand- and gravel-covered lower bar, and a vegetated upper bar. Obstacle marks, comprising an erosional scour and a depositional sediment tail, are recognised on the lower bar and the lower margin of the upper bar. Two types of obstacle marks are dominant in the lower part of the lower bar; sediment tail-dominated obstacle marks associated with inclined trees of Melaleuca argentea (Silver-leaved Paperbark), and comparatively small obstacle marks around stranded driftwood. Obstacle marks in the upper part of the lower bar are formed around grass clumps armoured with mud and gravel. At the lower margin of the upper bar, scours and gravelly sediment tails are recognised around mature tall trees. In these cases, the scour is >4 m in width, and covered with alternating beds of fine sand and mud. Among the obstacle marks described, sediment tail-dominated obstacle marks around groves of M. argentea have the highest preservation potential. Obstacle marks in the lower margin of the upper bar are also considered to have high preservation potential. Obstacle marks around stranded driftwood have the lowest preservation potential. Obstacle marks around grass clumps survive more than 1 year, but their long-term preservation potential is nonetheless low. Implications for the stratigraphic record are also discussed.

Fossil trees in ancient fluvial channel deposits: Evidence of seasonal and longer-term climatic variability

It has been established that large numbers of certain trees can survive in the beds of rivers of northeastern Australia where a strongly seasonal distribution of precipitation causes extreme variations in flow on both a yearly and longer-term basis. In these rivers, minimal flow occurs throughout much of any year and for periods of up to several years, allowing the trees to become established and to adapt their form in order to facilitate their survival in environments that experience periodic inundation by fast-flowing, debris-laden water. Such trees (notably paperbark trees of the angiosperm genus Melaleuca) adopt a reclined to prostrate, downstream-trailing habit, have a multiple-stemmed form, modified crown with weeping foliage, development of thick, spongy bark, anchoring of roots into firm to lithified substrates beneath the channel floor, root regeneration, and develop in flow-parallel, linear groves. Individuals from within flow-parallel, linear groves are preserved in situ within the alluvial deposit of the river following burial and death. Four examples of in situ tree fossils within alluvial channel deposits in the Permian of eastern Australia demonstrate that specialised riverbed plant communities also existed at times in the geological past. These examples, from the Lower Permian Carmila Beds, Upper Permian Moranbah Coal Measures and Baralaba Coal Measures of central Queensland and the Upper Permian Newcastle Coal Measures of central New South Wales, show several of the characteristics of trees described from modern rivers in northeastern Australia, including preservation in closely-spaced groups. These properties, together with independent sedimentological evidence, suggest that the Permian trees were adapted to an environment affected by highly variable runoff, albeit in a more temperate climatic situation than the modem Australian examples. It is proposed that occurrences of fossil trees preserved in situ within alluvial channel deposits may be diagnostic of environments controlled by seasonal and longer-term variability in fluvial runoff, and hence may have value in interpreting aspects of palaeoclimate from ancient alluvial successions

Plant-material deposition in the tropical Burdekin River, Australia: Implications for ancient fluvial sediments

Abstract In the deposits of the Burdekin River (north Queensland, Australia), plant material (character and directional fabric) together with the sedimentary facies character are generally diagnostic of depositional environment. Plant material is selectively entrained, transported, and deposited in all flow conditions ranging from dry season minimal flow (

Yarrol terrane of the northern New England Fold Belt: Forearc or backarc?

The Upper Devonian to Lower Carboniferous volcanosedimentary rocks of the Yarrol terrane of the northern New England Fold Belt have previously been ascribed to a forearc basin setting. New data presented here, however, suggest that the Yarrol terrane developed as a backarc basin during the Middle to early Late Devonian. Based on field studies, we recognise four regionally applicable stratigraphic units: (i) a basal, ?Middle to Upper Devonian submarine mafic volcanic suite (Monal volcanic facies association); (ii) the lower Frasnian Lochenbar beds that locally unconformably overlie the Monal volcanic facies association; (iii) the Three Moon Conglomerate (Upper Devonian ‐ Lower Carboniferous); and (iv) the Lower Carboniferous Rockhampton Group characterised by the presence of oolitic limestone. Stratigraphic and compositional differences suggest the Monal volcanic facies association post‐dates Middle Devonian silicic‐dominated magmatism that was coeval with gold‐copper mineralisation at Mt Morgan. The Lochenbar beds, Three Moon Conglomerate and Rockhampton Group represent a near‐continuous sedimentary record of volcanism that changed in composition and style from mafic effusive (Late Devonian) to silicic explosive volcanism (Early Carboniferous). Palaeocurrent data from the Three Moon Conglomerate and Rockhampton Group indicate dispersal of sediment to the west and northwest, and are inconsistent with derivation from a volcanic‐arc source situated to the west (Connors‐Auburn Arch). Geochemical data show that the Monal volcanic facies association ranges from tholeiitic subalkaline basalts to calc‐alkaline basaltic andesite. Trace and rare‐earth element abundances are distinctly MORB‐like (e.g. light rare earth element depletion), with only moderate enrichment of the large‐ion lithophile elements in some units, and negative Nb anomalies, suggesting a subduction‐related signature. Basalts of the Monal volcanic facies association are best described as transitional between calc‐alkali basalts and N‐MORB. The elevated high field strength element contents (e.g. Zr, Y, Ti) are higher than modern island‐arc basalts, but comparable to basalts that floor modern backarc basins. This geochemical study, coupled with stratigraphic relationships, suggest that the eruption of backarc basin basalts followed widespread Middle Devonian, extension‐related silicic magmatism (e.g. Retreat Batholith, Mt Morgan), and floored the Yarrol terrane. The Monal volcanic facies association thus shows similarities in its tectonic environment to the Lower Permian successions (e.g. Rookwood Volcanics) of the northern New England Fold Belt. These mafic volcanic sequences are interpreted to record two backarc basin‐forming periods (Middle ‐ Late Devonian and Late Carboniferous ‐ Early Permian) during the Late Palaeozoic history of the New England Orogen. Silicic‐dominated explosive volcanism, occurring extensively across the northern New England Fold Belt in the Early Carboniferous (Yarrol terrane, Campwyn Volcanics, Drummond and Burdekin Basins), reflects another period of crustal melting and extension, most likely related to the opening of the Drummond Basin.

Permian Evolution of Sandstone Composition in a Complex Back-Arc Extensional to Foreland Basin: The Bowen Basin, Eastern Australia

ABSTRACT The Bowen Basin is a Permo-Triassic, back-arc extensional to foreland basin that developed landward of an intermittently active continental volcanic arc associated with the eastern Australian convergent plate margin. The basin has a complex, polyphase tectonic history that began with limited back-arc crustal extension daring the Early Permian. This created a series of north-trending grabens and half grabens which, in the west, accommodated quartz-rich sediment derived locally from surrounding uplifted continental basement. In the east, coeval calc-alkaline, volcanolithic-rich, and volcaniclastic sediment was derived from the active volcanic arc. This early extensional episode was followed by a phase of passive thermal subsidence accompanied by episodic compression during the late Early Permian to early Late Permian, with little contemporaneous volcanism. In the west, quartzose sediment was shed from stable, polymictic, continental basement immediately to the west and south of the basin, whereas volcanolithic-rich sediment that entered the eastern side of the basin during this time was presumably derived from the inactive, and possibly partly submerged, volcanic arc. During the late Late Permian, flexural loading and increased compression occurred along the eastern margin of the Bowen Basin, and renewed volcanism took place in the arc system to the east. Reactivation of this arc led to westward and southward spread of volcanolithic-rich sediment over the entire basin. Accordingly, areas in the west that were earlier receiving quartzose, craton-derived sediment from the west and south were overwhelmed by volcanolithic-rich, arc-derived sediment from the east and north. This transition from quartz-rich, craton-derived sediments to volcanolithic-rich, arc-derived sediments is consistent with the interpreted back-arc extensional to foreland basin origin for the Bowen Basin.