Jonathan Clarke

The occurrence and significance of biogenic opal in the regolith

Abstract Biogenic opal produced by vascular plants, diatoms, and siliceous sponges have been found in soils and terrestrial sediments of all continents except Antarctica since the middle of the 19th century. The opal particles range in size from fine silt to fine sand. Almost all soils contain detectable opal up to levels of 2–3%, and a significant number contain values in excess of 5%. Even higher values have been found from soils and sediments of all continents in a wide range of soil types. The most important factor is poor soil drainage and seasonal to permanent water logging. This encourages the proliferation of silica producing organisms. Such conditions have been found in the soils and aquatic sediments of the monsoonal tropics, tropical rain forests, temperate forests, tropical savanna, tropical islands, semi-arid grasslands and savanna, and temperate woodland and grassland. The presence of a volcanic substrate also appears favourable in some cases, but is not necessary. Biogenic opal preferentially collects in the A horizon of soils and, to a lesser extent, in the B horizon. This preferential distribution facilitates identification of palaeosols in stacked soil sequences. Biogenic opal is also a component of windblown dust, even in arid environments. Biogenic opal is significant to regolith processes in a number of ways. Firstly, as in the case in marine environments, it is likely to be important in silica cycling and storage because of its greater lability compared to quartz. Secondly, dissolution and reprecipitation of opal A as opal CT or micro-quartz may play a role in cementation and silicification of regolith to form silica hardpans and silcrete. Thirdly, the organisms that form biogenic opal can have considerable palaeoenvironmental significance and be valuable in reconstructing regolith evolution. Finally, some forms of biogenic silica, in particular sponge spicules, can present a health hazard. Their high abundance in some soils and sediments needs to be considered when assessing the health implications of airborne dust.

Middle to Upper Eocene stratigraphic nomenclature and deposition in the Eucla Basin

The Eucla Basin has the largest onshore extent of Cenozoic marine sediments anywhere in the world. The sediments provide a record of the evolving marine environments of the Southern Ocean and the terrestrial hinterland of the Australian continent. However, owing to its size and remoteness, the Eucla Basin is comparatively understudied. This is exacerbated by the scattered and often deeply weathered nature of the outcrops along the margins of the basin, and the inaccessibility of exposures in the basin centre, except in cliffs and caves. The extent and isolation of the Eucla Basin over two states has resulted in conflicting and overlapping stratigraphic nomenclature, especially of the marginal sediments. Therefore, we propose rationalising the nomenclature of the Eocene rocks in the region based on three guiding principles: the use of consistent terminology across the region; the recognition of the importance of allostratigraphy in defining stratigraphic architecture, in particular two 3rd‐order cycles correlated with the Tortachilla and Tuketja transgressions; and continuity with past usage wherever possible, with a minimum of new terminology. We propose eight major changes to the existing nomenclature: (i) abandoning the term Bremer Basin for the marine and marginal marine to non‐marine Eocene sediments that infill palaeovalleys and form a veneer across crystalline basement in southwest Western Australia and including these sediments in the margin of the Eucla Basin; a similar situation exists in the east, where the Eocene sediments that have been included in the Polda Basin are likewise a marginal extension of the Eucla Basin; (ii) introducing the term Maralinga Formation for all Middle Eocene non‐marine to marginal marine sediments, including those previously included in the lower part of the Pidinga Formation in South Australia, and North Royal Formation for similar sediments in Western Australia; these replace the previous informal usage of lower Pidinga and lower Werillup Formation, respectively; (iii) restricting Hampton Sandstone to its original usage for a calcareous marine sand underlying the Wilson Bluff Limestone; (iv) raising the Paling Member of the Wilson Bluff Limestone to formation status; (v) using Pidinga Formation for all Upper Eocene carbonaceous sediments on the margins of the Eucla Basin in South Australia, and Werillup Formation for all such sediments in Western Australia, including the marginal palaeovalleys; terms such as Wollubar Sandstone in the palaeovalleys of the Yilgarn Craton, and Poelpena and Wanilla Formations in the Eocene part of the former Polda Basin should be abandoned; (vi) using the term Pallinup Formation for all Upper Eocene spicule‐rich sediments along the western margin of the Eucla Basin; (vii) recognising the formation status of the Upper Eocene spicular marine sediments in the eastern Eucla Basin that were formerly termed the Khasta Member of the Hampton Sandstone and the Bring Member of the Pidinga Formation, abandoning the term Bring Member, and including those rocks, and similar sediments of the Poelpena Formation in the Polda Basin, in the new Khasta Formation; and (viii) abandoning the term Toolinna Limestone previously applied to Upper Eocene grainstone along the western margin of the Eucla Basin as it is a facies of the Wilson Bluff Limestone, whereas the grainstone at the type locality at Toolinna Cove is in fact Abrakurrie Limestone and is indistinguishable from the rest of that formation. We believe this rationalisation emphasises the unity of stratigraphy across much of southern Australia and, thus, will facilitate research on the Eucla Basin as a whole.

Compositional zoning of fluid inclusions in the Archaean Junction gold deposit, Western Australia: a process of fluid – wall‐rock interaction?

The Junction gold deposit, in Western Australia, is an orogenic gold deposit hosted by a differentiated, iron‐rich, tholeiitic dolerite sill. Petrographic, microthermometric and laser Raman microprobe analyses of fluid inclusions from the Junction deposit indicate that three different vein systems formed at three distinct periods of geological time, and host four fluid‐inclusion populations with a wide range of compositions in the H2O–CO2–CH4–NaCl ± CaCl2 system. Pre‐shearing, pre‐gold, molybdenite‐bearing quartz veins host fluid inclusions that are characterised by relatively consistent phase ratios comprising H2O–CO2–CH4 ± halite. Microthermometry suggests that these veins precipitated when a highly saline, >340°C fluid mixed with a less saline ≥150°C fluid. The syn‐gold mineralisation event is hosted within the Junction shear zone and is associated with extensive quartz‐calcite ± albite ± chlorite ± pyrrhotite veining. Fluid‐inclusion analyses indicate that gold deposition occurred during the unmixing of a 400°C, moderately saline, H2O–CO2 ± CH4 fluid at pressures between 70 MPa and 440 MPa. Post‐gold quartz‐calcite‐biotite‐pyrrhotite veins occupy normal fault sets that slightly offset the Junction shear zone. Fluid inclusions in these veins are predominantly vapour rich, with CO2≫CH4. Homogenisation temperatures indicate that the post‐gold quartz veins precipitated from a 310 ± 30°C fluid. Finally, late secondary fluid inclusions show that a <200°C, highly saline, H2O–CaCl2–NaCl–bearing fluid percolated along microfractures late in the deposits history, but did not form any notable vein type. Raman spectroscopy supports the microthermometric data and reveals that CH4–bearing fluid inclusions occur in syn‐gold quartz grains found almost exclusively at the vein margin, whereas CO2–bearing fluid inclusions occur in quartz grains that are found toward the centre of the veins. The zonation of CO2:CH4 ratios, with respect to the location of fluid inclusions within the syn‐gold quartz veins, suggest that the CH4 did not travel as part of the auriferous fluid. Fluid unmixing and post‐entrapment alteration of the syn‐gold fluid inclusions are known to have occurred, but cannot adequately account for the relatively ordered zonation of CO2:CH4 ratios. Instead, the late introduction of a CH4–rich fluid into the Junction shear zone appears more likely. Alternatively, the process of CO2 reduction to CH4 is a viable and plausible explanation that fits the available data. The CH4–bearing fluid inclusions occur almost exclusively at the margin of the syn‐gold quartz veins within the zone of high‐grade gold mineralisation because this is where all the criteria needed to reduce CO2 to CH4 were satisfied in the Junction deposit.

A review of Tertiary brown coal deposits in Australia : Their depositional factors and eustatic correlations

The paleogeographical setting, sequence stratigraphy, and timing for six Tertiary brown coal deposits along the southern seaboard of Australia indicate a concentration of major coal-forming phases to periods of significant coastal onlap coupled with frequent sea level oscillations.

Characteristics and evolution of the Tertiary palaeovalleys in the northwest Gawler Craton, South Australia

Integrated geoscientific datasets have contributed to an understanding of the Tertiary palaeovalleys once draining the Gawler Craton. Systematic investigations of both the shape and depth of the channels are based on interpretations from field exposures, a compendium of geological and drilling data, computer modelling of ancient landscapes, topographic and evaluated digital elevation models, remote sensing imagery, magnetics, seismic, gravity, airborne and transient electromagnetics, and radiometrics. Physical property contrasts that exist between the channel sediments and the underlying bedrock, for example, can be differentiated by geophysical methods to locate the incised‐valley thalweg. Evidence from sedimentology is combined with evidence from other geological and geophysical characteristics to arrive at a general reconstruction of palaeovalley architecture and history. The palaeovalleys were originally incised into the weathered pre‐Tertiary landscape of mostly weathered basement, and Tertiary fluvial, lacustrine, estuarine and even marine sediments accumulated during the Eocene and Miocene. Marine influence extended at least 100 km up the palaeovalleys during at least three major transgressions in the Eocene and Miocene intervals. Major sedimentary phases occurred in the Paleocene to Early Eocene, Middle to Late Eocene, Oligocene to Early Miocene, and Middle Miocene to Early Pliocene times.

Concretions in exhumed and inverted channels near Hanksville Utah: implications for Mars

The landscape near Hanksville, Utah, contains a diversity of Mars analogue features. These included segmented and inverted anastomosing palaeochannels exhumed from the Late Jurassic Brushy Basin Member of the Morrison Formation that hosts abundant small carbonate concretions. The exhumed and inverted channels closely resemble many seen on the surface of Mars in satellite imagery and which may be visited by surface missions in the near future. The channels contain a wealth of palaeoenvironmental information and are potentially of astrobiological interest, but intrinsically difficult terrain would make their study challenging on Mars. We show that an un-exhumed channel feature can be detected geophysically, and this may allow their study in more easily accessed terrain. The concretions morphology and surface expression parallel the haematite ‘blue berries’ that are strewn across the surface of Meridiani Planum on Mars. They are best developed in poorly cemented medium to coarse channel sandstones and appear to have formed during deep burial.

Facies and sequence stratigraphy of Eocene palaeovalley fills in the eastern Eucla Basin, South Australia

Abstract The Eocene succession filling palaeovalleys in the northeastern Eucla Basin, South Australia, is interpreted using facies and sequence-stratigraphic models based on relative sea-level changes. The dominantly fluvial sediments were deposited in incised valleys which graded basinwards to an estuarine coastal plain under warm and humid palaeoclimatic conditions. Sedimentological examination suggests a tidal influence in this fluvial succession. Fluvial–estuarine-shoreline facies associations can be recognised in these (Eocene) sequences, each of which comprises a diverse assemblage of lithofacies that can be grouped into lowstand and/or transgressive and highstand system tracts. Since the palaeorivers had hydrological connection with the sea, deposition was dominantly controlled by sea-level changes. Results of the study indicate that two third-order Eocene eustatic cycles have largely controlled sedimentation. The resulting key surfaces (unconformity, and transgressive, tidal/wave ravinement, and maximum flooding surfaces) bound depositional sequences which extend over significant areas and may be used in basin-wide correlations of stratal packages.

Sedimentology and lithostratigraphy of Upper Eocene sponge-rich sediments, southern Western Australia

Late Eocene time in the Bremer and western Eucla Basins of southern Western Australia was a period of terrigenous clastic and abundant, unusual, biosiliceous sponge sedimentation. The Pallinup Formation (revised) consists of five units; 1 and 2 are basal sandstones, 3 and 4 are variably spiculitic mudstones, whilst the uppermost unit is spiculite and spongolite, and formalised as the Fitzgerald Member (new). The Pallinup Formation, plus coeval spiculites in palaeovalleys and carbonates in the western Eucla Basin, accumulated during one large‐scale, transgressive‐regressive relative sea‐level cycle. Drowned, low‐gradient rivers supplied mud but little sand. Instead, sand was locally sourced via transgressive shoreface erosion of deeply weathered regolith. Regression terminated shoreface erosion, eliminated the sand source, and resulted in a river‐supplied, clay‐dominated shallow‐marine depositional system. The unit 2–3 sandstone‐mudstone transition, which would normally be interpreted as transgressive drowning, is in this case the result of regressive cessation of sand supply. The peak relative sea‐level (highstand) horizon thus lies within unit 2 sandstones, a facies that would usually be considered wholly transgressive, and no highstand systems tract was deposited. The maximum flooding and downlap surfaces are the same horizon and cap the transgressive systems tract. They formed coincidentally or subsequent to peak relative sea‐level, but prior to initiation of unit 3 mudstone deposition. Upper unit 2 plus unit 3 represent a condensed section systems tract, and unit 4 plus the Fitzgerald Member comprise a regressive systems tract.

Post-glacial biota from the inner part of southwest Joseph Bonaparte Gulf

Seventeen vibrocores from the inner part of Joseph Bonaparte Gulf off northwestern Australia penetrate a range of marine and marginal‐marine sediments deposited in the post‐glacial transgression and highstand. Ranging from gravelly sand to fine silt, these sediments contain a diverse fossil biota dominated by molluscs and bryozoans, but also including ostracods and foraminifers. Minor components include solitary corals, echinoids, soft coral and sponge spicules, wood debris and bone fragments. The biota can be divided into five major marine or marginal‐marine environments (intertidal, lagoonal, estuarine, strandline and shelf) and one terrestrial (riverine) environment. The intertidal environment contains four sub‐assemblages (mangroves, salt marsh, mud flat and sand flat) and the shelf environment six sub‐assemblages (hard substrate inner shelf, sandy substrate inner shelf, muddy substrate inner shelf, epiphytic, inshore and oceanic). The most useful organisms for palaeoenvironmental reconstruction are bryozoans for differentiating various shallow‐marine substrates, and foraminifers and ostracods for defining water depths, euryhaline, freshwater and oceanic influences. Palynomorphs were the only microfossils capable of providing control on terrestrial environments. The scarcity of marine plankton and the dominance of terrestrial palynomorphs in these marine sediments provides a salutary warning of the dangers of relying on plant microfossils alone when no independent environmental data are available to test the interpretation. The mollusc and bryozoan biota in the inner part of Joseph Bonaparte Gulf superficially resembles the bryomol assemblage of cool‐water shelves. This biotic assemblage is the result of turbidity rather than water temperature. The turbidity suppresses the photosynthetic, zooxanthellate and hermatypic organisms allowing molluscs, bryozoans and other apparently cool‐water biotic elements to dominate.

A CO2–O2–light hydrocarbon–soil-gas anomaly above the Junction orogenic gold deposit: a potential, alternative exploration technique

A soil-gas survey conducted above the Junction orogenic lode-gold deposit near Kambalda in Western Australia detected strong, broadly coincident, CO2–O2–light hydrocarbon anomalies through cover sediments above known mineralization. Alternatively, only CO2–O2 aberrations (without light hydrocarbons) were detected above areas where non-gold related carbonate and sulphide mineralization exists. Oxidation of the alteration assemblage associated with mineralization and the release of the gases in the fluid inclusions they contain are proposed as the source for these CO2–O2–light hydrocarbon anomalies. We believe that soil-gas exploration for orogenic gold deposits has widespread potential as an exploration method in Western Australia where regolith cover can make detection of mineralized shear zones by traditional exploration methods for gold problematic to impossible.