A. R. Milnes

Organic carbon cycling and modern phosphorite formation on the East Australian continental margin: an overview

Abstract During 1987, the Australian Bureau of Mineral Resources conducted a multidisciplinary investigation of the modern phosphorites on the continental margin of southeastern Australia between 28 and 32°S. The objectives of the work were to examine the processes controlling the cycling of organic carbon and bioactive elements, nitrogen, phosphorus, sulphur and iron in the sediments, and to investigate the roles which these processes played in the formation of the modern phosphorites. Bacterial productivities, sulphate-reduction rates, sedimentary oxygen and pore-water concentrations of nitrate, ammonia, phosphate, iron, sulphate and fluoride were measured at sea. The highest rates of microbial productivity were found in the surficial (0–20 mm) sediments of the modern phosphorite zone in 350–460 m water depth. These rates were about double those in shallower shelf (<300 m) sediments and 3–4 fold those rates in mid-slope (600–1000 m) sediments. Aerobic and anaerobic oxidation rates of organic matter, calculated from sediment oxygen profiles and sulphate-reduction rates were highest in the surface sediments in the modern phosphorite zone. The recycling of sedimentary iron, via reductive dissolution of iron oxyhydroxides and reprecipitation at the oxic/anoxic boundary results in a near-surface sedimentary trap for iron in the phosphorite zone sediments. Phosphate released from organic matter in the interfacial sediments, and fluoride from seawater, are scavenged by iron oxyhydroxides in the top few centimetres of sediment. Phosphorus, in this way, is decoupled from organic carbon in the near-surface sediments and linked to the redox cycling of iron. Phosphate and fluoride scavenged onto iron oxyhydroxides, and concentrated in the surficial sediments, are subsequently released to pore waters in the anoxic sediments when iron oxyhydroxides are buried and dissolve. The recycling process releases phosphate and fluoride for incorporation into apatite; fluoride is depleted from pore waters at depths <18 cm, phosphorite nodules form within anoxic sediments at depths <18 cm and continue to accumulate iron and phosphorus while resident in the mixed layer. Combinations of rapid sediment mixing rates, a slow sedimentation rate and a mixed layer to about 18 cm result in an average particle residence time in the phosphorite zone sediments which is about ten-fold that of the mid-slope sediments. Long residence times and rapid mixing promote the oxidation of organic carbon and release of phosphate, while the continuous recycling of iron and phosphate concentrates the phosphorus for apatite precipitation and accumulation into phosphorite nodules. Phosphorite nodules are not found in mid-slope sediments probably because of combinations of relatively rapid sedimentation rates, ineffective iron, phosphorus and fluoride recycling and trapping mechanisms, plus dilution and dissemination of any incipient apatite.

Weathering of basalt; formation of iddingsite

The formation of iddingsite by the oxidative weathering of Fo80 olivine begins by solution of Mg from planar fissures, 20 Å wide and spaced 200 Å apart, parallel to (001). Oxidation of Fe within the remaining olivine provides nuclei for the topotactic growth of goethite. Cleavage cracks < 50 Å in diameter allow Na, Al, and Ca from adjacent minerals, particularly plagioclase, to enter the altering olivine while Mg and Si diffuse away. In the early stages of weathering, strips of Fe-rich smectite (saponite), 20–50 Å wide and 1–7 layers thick, form bridges 50–100 Å long across the planar fissures. Dioctahedral smectite crystallizes on the margins of wider cleavage-controlled fissures; with further weathering halloysite is formed away from the fissure walls. In the ultimate stages of alteration, the saponite and dioctahedral smectite are lost, leaving a porous, oriented aggregate of goethite crystals each measuring about 50 × 100 × 200 Å (X, Y, Z, respectively), with sporadic veins of halloysite crossing the pseudomorph.

Pre‐ to syn‐tectonic emplacement of early Palaeozoic granites in southeastern South Australia

Abstract Stratigraphic and structural observations indicate that the Encounter Bay Granites concordantly intruded the youngest formations of the Kanmantoo Group in the Mount Lofty Ranges metamorphic belt prior to the culmination of the first phase of folding and associated schistosity development recorded during the early Palaeozoic Delamerian Orogeny. Metamorphic textures in the metasediments of the Kanmantoo Group suggest that cordierite crystallized locally near the granites prior to and during the F 1 folding, whereas andalusite crystallized on a regional scale during the F 1 folding and in the post‐F 1 and pre‐F 2 static phase. Rb‐Sr isotope data for total‐rock, feldspar, and muscovite samples of the meta‐sediment‐contaminated border facies and the uncontaminated inner facies of the Encounter Bay Granites indicate that the granites were emplaced between 515 ± 8 m.y. and 506 ± 6 m.y. ago in the Late Cambrian epoch. Rb‐Sr and K‐Ar data for biotite from the granites record variable radiogenic Sr loss un...

The age of the lateritized summit surface on Kangaroo Island and adjacent areas of South Australia

Abstract Stratigraphic, geomorphological, and radiometric evidence shows that the laterite of the high plains and plateau of Kangaroo Island is older than the Middle Jurassic but younger than the Early Permian. Palaeoclimatic and palaeontological considerations suggest the Triassic as the most likely age of both the laterite and the surface on which it is developed. High‐level lateritized surfaces in the adjacent Mount Lofty Ranges and southern Eyre Peninsula are of similar age.

Sedimentation dynamics and redox iron-cycling : controlling factors for the apatite-glauconite association on the East Australian continental margin.

Abstract Detailed sedimentological and geochemical studies of phosphorites and sediments from the East Australian continental margin have shown that both apatite and glauconite are forming at a transition zone between relict, iron oxyhydroxide-rich, organic-poor (TOC<0.3%) outer shelf (200–350 m) sediments and relatively rapidly accumulating, iron oxyhydroxide-deficient, organic-rich (TOC>0.8%) deep water (460–650 m) sediments. The interaction between sediment mixing and Fe-P cycling processes (between the pore waters and the solid phase) appear critical to the formation of modern phosphorites in this area. The phosphate nodules form within the anoxic zone in the sediments at depths of approximately 10–18 cm below the sediment-seawater interface. Nodules which remain in the sediment mixed layer after they form continue to accumulate both P and Fe for up to 60 ka; during this time their apatite and iron oxyhydroxide contents more than double and the nodules become denser and more lithified. Apatite and glauconite formation are favoured by periods of high sea-level and low current velocities, as these conditions allow a relatively high organic carbon input to the sediments and thereby the maintenance of anoxia at shallow depths within the sediments. During periods of low sea-level and high current velocities, the carbon flux into the sediments decreases and the sediments become oxic. Consequently the Fe-cycling processes cease and apatite and glauconite formation stops: the glauconite is progressively transformed to goethite, and phosphorite nodules are concentrated into lag deposits and ferruginized. Alternations of high and low sea-level cycles eventually result in the formation of the massive ferruginous Neogene phosphorites that mantle much of the outer shelf. The iron enrichment processes observed in the modern to Neogene phosphorites on the East Australian continental margin provide explanations for many of the features seen in ferruginous Neogene deposits in the world’s oceans.

Interpretation of palaeoweathering features and successive silicifications in the Tertiary regolith of inland Australia

Detailed studies of morphological, micromorphological and geochemical characteristics of silcretes in the deep bleached and weathered regolith across a large area of inland Australia have provided a new interpretation of the history of the regolith and its climatic and morphological evolution during the Tertiary. Pedogenic silcretes have distinctive morphological and mineralogical features caused by a succession of phases of silica dissolution and recrystallization resulting from multiple episodes of water infiltration and percolation under alternately wet and dry climates. These are the oldest of the regolith features. Deep, bleached profiles formed over a wide area in a variety of substrates ranging from Precambrian granites to Palaeozoic sandstones, Cretaceous sediments and Tertiary deposits, and represent the second major stage in regolith development. These profiles, in which kaolinite coexists with gypsum, alunite and opal, formed by reaction of the substrates with saline groundwaters, the water-table levels of which progressively fell over the region. Extensive networks of termite burrows constructed to great depth in the bleached regolith followed the water tables down. The climate was warm and dry with a high water deficit. Groundwater silcretes formed near-horizontal lenses and pods of porcellanite and jasper in the bleached regolith. They preserve the primary fabric of the host rock. Groundwater silcretes post-date the construction of termite burrows and were formed during a rise in groundwater tables across the landscape, in places to near-surface environments in broad landscape depressions. The climate was more humid but the presence of gypsum during silicification demonstrates that the groundwaters were still saline. Red–brown hardpans are the youngest silicification features and represent periods of successive infiltration and percolation, and waterlogging, during high rainfall or flood events. They are confined to low regions in the landscape. Mineralogical and geochemical analysis of the bleached profiles, together with geochemical modelling, suggests that ferrolysis is the most likely cause of acidity in groundwater leading to the development of the bleach profiles and/or alunite. Present-day groundwater tables are both at low levels and sulphate-rich. It is possible that acidic alteration leading to bleaching is still active around the extensive playa landscapes in the region.

Composition and genesis of silcretes and silcrete skins from the Beda Valley, southern Arcoona plateau, South Australia

Abstract Detailed petrographic and chemical studies of silcretes collected from Beda Valley near the southern extremity of Lake Torrens, South Australia, have shown that there are at least two distinct types. One type, often found attached to quartzite, has very angular grains of quartz in a matrix very high in titanium. Electron probe studies clearly show that the areas between the quartz grains are mostly titania containing very little silica. These skins, besides containing 2–10% Ti, contain .05 to .25% Zr with a very high correlation between ratio of the amount of these elements in the skin to that in the quartzite and it is suggested they have been produced by the loss of silica. The other type of silcrete has subrounded grains of quartz in a matrix of chalcedonic silica. It is also low in titanium (<1%) and low in zirconium (<.04%). It is suggested this type, which in this area is massive with columnar structure, is formed by the addition of silica.

Amino acid racemization dating: Evidence of apparent reversal in aspartic acid racemization with time in shells of Ostrea

Abstract Data have been obtained for simulated laboratory aging of shell valves of Ostrea angasi Sowerby. In respect to the conversion of L to D aspartic acid in peptides of > 1000 Dallons there was consistently an initial increase in D L ratio followed by a progressive decrease in D L with time. This phenomenon provides further insight into the dynamics of the racemization process. It may also be a significant factor influencing D L ratios measured for total hydrolysable amino acids from natural fossils for the purposes of dating or correlation.

Evolution of landscapes in northern South Australia in relation to the distribution and formation of silcretes

Geomorphological features on the southwestern margins of the Lake Eyre Basin provide a basis for interpreting the evolution of old landscapes containing pedogenic and groundwater silcretes. and thick bleached and weathered profiles. Recurrent sequences of cut-and-fill and duricrust formation have been identified and related to major sea level changes and tectonic movements in the Lake Eyre Basin. An extensive high pediment had formed around a basement inlier by the late Eocene. An armour of pedogenic silcrete developed on this pediment under alternating dry and wet climates during the late Eocene and Oligocene. The characteristic kaolinite + opal + alunite + gypsum assemblage of the bleached profiles formed in acid saline groundwaters during the Miocene at a time of regional low water table and arid climate. Groundwater silcretes formed in the bleached profile in response to dissection of the high pediment. They are related to a period of high groundwater tables. humid climate. and gradual sinking of the Lake Eyre basin. In the meantime, widespread low pediments (glacis) formed in Lake Eyre catchment, possibly in the early Pliocene. There is a good correspondence between a geomorphic approach, taking in to account the relationship between weathering features and palaeolandscapes, and the information provided from stratigraphic studies in the region.

Petrology and mineralogy of ‘laterites’ in southern and eastern Australia and southern Africa

Abstract Field observations of ‘laterites’ in southern and eastern Australia and in southern Africa reveal a variety of ferruginous horizons and crusts referred to herein as ferricretes. Their geomorphic and stratigraphic relationships with bedrock, sediments and soils indicate formation throughout long intervals of geological time in landscapes which are also characterised by zones of bleached and iron-mottled materials. There does not appear to be a genetic relationship between the ferricretes and the weathered zones in the sense of the so-called ‘laterite profile’. Many of the ferricretes form part of existing soil profiles. Petrographic studies of a variety of ferricretes have identified three broad categories: (a) ferruginised bedrock; (b) Fe-impregnated and -indurated sediments, including sands, clays and organic sediments; and (c) ferricretes of complex sedimentary and pedogenic origin. Type-(a) and -(b) ferricretes characteristically have simple fabrics, often with single-generation, secondary Fe-oxides. Type-(c) ferricretes have complex fabrics, with many generations of hematite, goethite and in some variants, gibbsite, in the matrix and in ferruginous clasts and pisoliths. Maghemite is a common constituent of the pisoliths. The characteristics of ironstone gravelly duplex soils , which are common in the contemporary landscapes, provide the framework for a model involving multiple stages in the development of these ferricretes. The origins of the various secondary oxide minerals in ferricretes are assessed on the basis of knowledge about the formation of these minerals in pedogenic environments. Examples are given of the intricate patterns of distribution of the minerals in thin section from which definitive data may be obtained on environmental conditions for integration with field-based geomorphic studies.