Surface carbonate and land-derived clastic marine sediments from southern Chile: mineralogical and geochemical investigation

Surface carbonate and land-derived deposits in the sea off southern Chile were investigated for their mineralogical and geochemical composition. The data were related to environmental features and compared with those of similar temperate and polar carbonate deposits from Tasmania, New Zealand, Arctica, and Antarctica. The mineralogy of the siliciclastic fraction is typical of cold areas and is mainly composed of chlorite, mica, quartz, feldspars and amphibole. The CaCO₃ content varies from 30 to 90%; carbonate mineralogy is made up of low-Mg calcite, high-Mg calcite and minor amounts of aragonite. The Ca, Mg, Sr, Fe, and Mn contents of bulk carbonates and some selected skeletal hard parts are comparable to those of carbonates from Tasmania. The elemental composition is mainly related to carbonate mineralogy, skeletal components, and seawater conditions. The δ¹³C and δ¹⁸O values of carbonates are positive, and their field falls between the “seafloor diagenesis” and “upwelling water” trend lines, because the sediments are likely to be in equilibrium with waters of Antarctic origin. The mineralogical, elemental, and isotopic compositions of carbonates from southern Chile show better similarities with the “temperate” carbonates from Tasmania and New Zealand than with the “polar” carbonates from Arctica and Antarctica. Carbonate deposition is allowed by the low terrigenous input, the low SPM concentration and, probably, the upwelling of seawater from Antarctica.     

The Holocene non-tropical coastal and shelf carbonate province of southern Australia

Carbonate-dominant sediments are currently forming and accumulating over the extensive marine shelf of the passive margin of southern Australia. A dearth of continental detritus results from both a very low relief and a predominantly arid climate. The wide continental shelf is bathed by cold upwelling ocean waters that support luxuriant growths of bryozoans and coralline algae, together with sponges, molluscs, asteroids, benthic and some planktonic foraminifera. The open ocean coast is battered by a persistent southwest swell, resulting in erosion of calcrete-encrusted Pleistocene eolianites. Much sediment is reworked and overall shelf sedimentation rates are low. High-energy microtidal beach/dune systems occur between headlands and along the very long ocean beach in the Coorong region. The northern, more arid coastal areas also contain saline lakes that precipitate gypsum from infiltrated sea water, and display marginal facies of aragonite boxwork to fenestral carbonate crusts, with stromatolites and tepee structures. In contrast, the southern, seasonally humid Coorong region, has a predominantly continental groundwater regime where sulphate is rare, and the high summer evaporation precipitates dolomite, magnesite and aragonite muds. Fenestral crusts, breccias, tepees and some stromatolites are also present.

St. Vincent and Spencer gulfs both afford some protection from ocean swell, but tidal amplitude and currents increase, and a depth and inundation-related zonation of plants and animals is established. Muddy carbonate sand accumulates on the sea floor below 30 m, where filter-feeding bryozoans, bivalves and sponges dominate. In shallower regions, seagrass meadows contain a rich fauna that results in rapid accumulation of an unsorted muddy bioclastic sand. Mangrove woodlands backed by saline marsh with cyanobacterial mats are common, and accumulate mud-rich and gastropod-bearing sediment. As tidal amplitude and desiccation increase northward into both gulfs, a supratidal zone bare of vegetation (sabkha) becomes the site for deposition of gypsum-rich and fenestral calcitic mud.     

Inhibition of dissolution within shallow water carbonate sediments: impacts of terrigenous sediment input on syn-depositional carbonate diagenesis

The early diagenetic chemical dissolution of skeletal carbonates has previously been documented as taking place within bioturbated, shallow water, tropical carbonate sediments. The diagenetic reactions operating within carbonate sediments that fall under the influence of iron‐rich (terrigenous) sediment input are less clearly understood. Such inputs should modify carbonate diagenetic reactions both by minimizing bacterial sulphate reduction in favour of bacterial iron reduction, and by the reaction of any pore‐water sulphide with iron oxides, thereby minimizing sulphide oxidation and associated acidity. To test this hypothesis sediment cores were taken from sites within Discovery Bay (north Jamaica), which exhibit varying levels of Fe‐rich bauxite sediment contamination. At non‐impacted sites sediments are dominated by CaCO₃ (up to 99% by weight). Pore waters from the upper few centimetres of cores show evidence for active sulphate reduction (reduced SO₄/Cl^? ratios) and minor CaCO₃ dissolution (increased Ca²⁺/Cl^? ratios). Petrographic observations of carbonate grains (specifically Halimeda and Amphiroa) show clear morphological evidence for dissolution throughout the sediment column. In contrast, at bauxite‐impacted sites, the sediment is composed of up to 15% non‐carbonate and contains up to 6000 μg g^?1 Fe. Pore waters show no evidence for sulphate reduction, but marked levels of Fe(II), suggesting that bacterial Fe(III) reduction is active. Carbonate grains show little evidence for dissolution, often exhibiting pristine surface morphologies. Samples from the deeper sections of these cores, which pre‐date bauxite influence, commonly exhibit morphological evidence for dissolution implying that this was a significant process prior to bauxite input. Previous studies have suggested that dissolution, driven by sulphate reduction and sulphide oxidation, can account for the loss of as much as 50% of primary carbonate production in localized platform environments. The finding that chemical dissolution is minor in a terrigenous‐impacted carbonate environment, therefore, has significant implications for carbonate budgets and cycling, and the preservation of carbonate grains in such sediment systems.     

Temperate carbonate sediments of Northern Spencer Gulf, South Australia: a high salinity 'foramol' province

Cool-water skeletal carbonate sediments are forming in Spencer Gulf, South Australia, an area of high salinity and moderate tidal range. Four environments can be distinguished: deeper marine areas (10–20 m); shallow subtidal platforms and banks (2–10 m); intertidal and supratidal zones; and coastal springs and lakes fed by saline continental groundwaters. The sediments are predominately bioclastic carbonate sands; muddy sediments occur in protected intertidal environments. The most common grain types are gastropods, bivalves, foraminifera, coralline algae and quartz. Indurated non-skeletal carbonate grains have not been seen. Composition of the sediment varies little between environments, but considerable textural variation results from variation in the stability of the substrate, hydrodynamic conditions, depth of water, period of tidal inundation, supply of terrigenous grains, temperature, and salinity. The Spencer Gulf data suggests that temperature, and particularly minimum temperature, controls the distribution of skeletal and non-skeletal grain associations in high-salinity environments. The textures of the sedimentary facies of Spencer Gulf closely parallel those of equivalent environments in warm-water carbonate provinces.     

Emplacement of the Jurassic Mirdita ophiolites (southern Albania): evidence from associated clastic and carbonate sediments

Sedimentology can shed light on the emplacement of oceanic lithosphere (i.e. ophiolites) onto continental crust and post-emplacement settings. An example chosen here is the well-exposed Jurassic Mirdita ophiolite in southern Albania. Successions studied in five different ophiolitic massifs (Voskopoja, Luniku, Shpati, Rehove and Morava) document variable depositional processes and palaeoenvironments in the light of evidence from comparable settings elsewhere (e.g. N Albania; N Greece). Ophiolitic extrusive rocks (pillow basalts and lava breccias) locally retain an intact cover of oceanic radiolarian chert (in the Shpati massif). Elsewhere, ophiolite-derived clastics typically overlie basaltic extrusives or ultramafic rocks directly. The oldest dated sediments are calpionellid- and ammonite-bearing pelagic carbonates of latest (?) Jurassic-Berrasian age. Similar calpionellid limestones elsewhere (N Albania; N Greece) post-date the regional ophiolite emplacement. At one locality in S Albania (Voskopoja), calpionellid limestones are gradationally underlain by thick ophiolite-derived breccias (containing both ultramafic and mafic clasts) that were derived by mass wasting of subaqueous fault scarps during or soon after the latest stages of ophiolite emplacement. An intercalation of serpentinite-rich debris flows at this locality is indicative of mobilisation of hydrated oceanic ultramafic rocks. Some of the ophiolite-derived conglomerates (e.g. Shpati massif) include well-rounded serpentinite and basalt clasts suggestive of a high-energy, shallow-water origin. The Berriasian pelagic limestones (at Voskopoja) experienced reworking and slumping probably related to shallowing and a switch to neritic deposition. Mixed ophiolite-derived clastic and neritic carbonate sediments accumulated later, during the Early Cretaceous (mainly Barremian-Aptian) in variable deltaic, lagoonal and shallow-marine settings. These sediments were influenced by local tectonics or eustatic sea-level change. Terrigenous sediment gradually encroached from neighbouring landmasses as the ophiolite was faulted or eroded. An Aptian transgression was followed by regression, creating a local unconformity (e.g. at Boboshtica). A Turonian marine transgression initiated widespread Upper Cretaceous shelf carbonate deposition. In the regional context, the southern Albania ophiolites appear to have been rapidly emplaced onto a continental margin in a subaqueous setting during the Late Jurassic (Late Oxfordian-Late Tithonian). This was followed by gradual emergence, probably in response to thinning of the ophiolite by erosion and/or exhumation. The sedimentary cover of the south Albanian ophiolites is consistent with rapid, relatively short-distance emplacement of a regional-scale ophiolite over a local Pelagonian-Korabi microcontinent.     

Modern and Holocene carbonate sedimentology of two saline volcanic maar lakes, southern Australia

The Basin Lakes are two adjacent maar lakes located in the centre of the Western Volcanic Plains District of Victoria, Australia. Both lakes are saline and alkaline; West Basin Lake is meromictic whereas East Basin is a warm monomictic lake. The carbonate mineral suite of the modern offshore bottom sediments of these Basins consists mainly of dolomite and calcite, with smaller amounts of hydromagnesite and magnesite in West Basin and monohydrocalcite in East Basin. The dolomite, hydromagnesite, magnesite, and monohydrocalcite are endogenic in origin, being derived by primary inorganic precipitation within the water columns of the lakes or at the sediment-water interface. The calcite is biologically precipitated as ostracod valves. In addition to the carbonates in the modern offshore (deep-water) sediments, the lakes also contain a girdle of nearshore carbonate hardgrounds. Both beachrock and microbialites (algal boundstones) are present. These modern lithified carbonate units exhibit a wide range of depositional and diagenetic fabrics, morphologies and compositions. In West Basin, the hardgrounds are composed mainly of dolomite, hydromagnesite, and magnesite, whereas dolomite and monohydrocalcite dominate the East Basin sediments. Aragonite, high-Mg calcite, kutnahorite, siderite, and protohydromagnesite also occur in these lithified carbonate units. Stratigraphic variations in the carbonate mineralogy of the Holocene sediment record in the lakes were used to help decipher the palaeochemistry and palaeohydrology of the Basins. These changes, in conjunction with fluctuations in organic remains and fossil content, indicate a pattern of lake level histories similar to that deciphered from other maar lakes in western Victoria.     

Genetic sequence stratigraphy of cool water slope carbonates (Pleistocene Eucla Shelf, southern Australia)

The south Australian Eucla Shelf belongs to the world's largest cool-water carbonate sedimentary system. During the Pleistocene, it exported large amounts of sediment to the shelf edge and upper slope resulting in an expanded sedimentary wedge. Wedge-internal clinoforming seismic reflectors suggest a stacking of the deposits into genetic sequences. High-resolution stable oxygen and carbon isotope, point counting, grain size, and carbonate mineralogical XRD analyses were carried out to characterize these genetic sequences along a dip-parallel transect of three ODP Leg 182 drill holes located between the shelf edge and upper slope. Oxygen and carbon isotope fluctuations show that the genetic sequences formed as a response to sea level fluctuations. Within the genetic sequences, facies differentiation and sediment volume partitioning occur along the transect. Lowstand deposits are fine grained and contain more sponge spicules and micrite. Highstand deposits are coarse grained with tunicate spicules, brown bioclasts, as well as bryozoan and corallinacean debris. Boundaries separating highstand and lowstand deposits are triggered by sea level fall, and are expressed as abrupt grain size changes or as turning points in grain-size trends. Analyzed components vary in abundance along the transect. Genetic sequences show dip-parallel variations in thickness combined with changing relative proportions of lowstand versus highstand deposits.     

Late Quaternary shedding of shallow-marine carbonate along a tropical mixed siliciclastic–carbonate shelf: Great Barrier Reef, Australia

Abstract The north-east Australian margin is the largest modern example of a tropical mixed siliciclastic/carbonate depositional system, with an outer shelf hosting the Great Barrier Reef (GBR) and an inner shelf dominated by fluvially sourced siliciclastic sediment wedges. The long-term interplay between these sediment components and sea level is recorded in the Queensland Trough, a 1–2 km deep N–S elongate basin situated between the GBR platform and the Queensland Plateau. In this paper, 154 samples from 45 surface grabs and six well-dated piston cores were analysed for total carbonate content, carbonate mineralogy and Sr concentration to establish spatial and temporal patterns of carbonate accumulation in the Queensland Trough over the last 300 kyr. Surface carbonate contents are lowest on the inner-shelf (<5%) and in the trough axis (<60%) because of siliciclastic dilution. Carbonate on the shelf is mostly Sr-rich aragonite and high-Mg calcite (HMC), whereas that in the basin is mostly low-Mg calcite. Once normalized to remove the effects of siliciclastic dilution, surface Sr-rich aragonite and HMC abundances decrease linearly to background levels ≈ 100 km seaward of the shelf edge. Core samples show that, over time, normalized aragonite and Sr abundances are greatest during periods of shelf flooding and lowest when sea level drops below the shelf edge. This is consistent with changes in the production of coral and calcareous algae, and the shedding of their debris from the shelf. Interestingly, normalized HMC concentrations on the slope peak during periods of major transgression, perhaps because of maximum off-shelf transport from inter-reef areas or intermediate water dissolution. After accounting for siliciclastic dilution, there are strong similarities in both spatial and temporal patterns of carbonate minerals between slopes and basins of the north-east Australian margin and those of pure carbonate margins such as the Bahamas. A limited set of basic processes, including the formation and breakdown of carbonate on the shelf, the transport of carbonate off the shelf and eustatic sea level, probably controls carbonate accumulation in slope and basin settings of tropical environments, irrespective of proximal siliciclastic sediment sources.     

Origin of a cool-water, Oligo-Miocene deep shelf limestone, Eucla Platform, southern Australia

The Abrakurrie Limestone is an areally extensive, bryozoan-rich unit within the Eucla Platform, a Tertiary carbonate shelf which caps the central part of the southern Australian continental margin. The onshore portion, the topic of this study, has been exposed since middle Miocene time and lies beneath the Nullarbor Plain. The rocks are fine-sand- to granule-sized calcarenites, composed of bryozoans, bivalves, benthic foraminifera and echinoids with lesser numbers of brachiopods, solitary corals and serpulids. They conspicuously lack significant numbers of planktonic foraminifera and coralline algae. Most bryozoan remains are from delicate branching cyclostomes although delicate branching, robust branching, foliose, fenestrate, multilaminar encrusting and free-living cheilostomes are variably abundant in specific units. The poorly lithified sequence is punctuated by well-cemented layers with erosional tops, which are interpreted as hardgrounds. The limestone is interpreted as a cool-water, deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the Eucla Platform. A model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation. The shelf is envisaged as partitioned by the depth of the zone of wave abrasion. Sediments were produced throughout, but accumulated only below this depth. When the seafloor was above this depth it was an environment of cementation and erosion. The vertical sequence correlates in a general way with the global sea-level model for the mid-Cenozoic. While accumulation rates for southern Australian carbonates are similar to rates of cool-water carbonate deposition elsewhere (c. 2.5 cm kyr^-1), the rate of Abrakurrie accumulation is much less, suggesting that significant time periods are represented by the hardgrounds.     

Modern and Holocene foraminifera and sediments on the continental shelf off Troms, North Norway

Based on studies of gravity cores from two transverse troughs on the shelf and earlier investigations, the surface sediments are divided into three main facies: bouldery and pebbly sand on the banks and the shelf break; sand on the flanks and outer parts of the troughs and sandy mud in the inner parts of the troughs. Besides a depth control, the distribution must have been influenced by relatively rapidly moving bottom currents in the outer parts of the troughs. The distribution and composition of the modern benthic foraminiferal fauna (e.g. C. lobatulus/T. angulosa in the outer reaches and C. obtusalBolivina spp. in the inner reaches) is mainly controlled by the bottom current regime and sediments. The planktic fauna dominated by N. pachyderma (R) correlates well with the winter surface temperatures. The stratigraphi-cal analysis shows that the 10,000–9,600 years B.P. period experienced high rates of deposition probably due to meltwater runoff from the continental ice sheet. At ca. 9,700 B.P. a minimum in the production of N. pachyderma (R) indicates a temporary cooling of the surface water. During the 9,600–7,800 B.P. period the rate of deposition was reduced. At the end of this period the foraminiferal fauna changed towards one like the modern fauna, reflecting improving ecological conditions. At ca. 7,800 B.P. the sediments became coarser due to reduced input of detrital sediments and an increased production of sand-sized biogenic material. Since then the shelf environment has been fairly stable up to the present time.     

Carbonate mud sedimentation on a temperate shelf: Bass Basin, southeastern Australia

Bass Basin (66,000 km²) is unusual as a carbonate-dominated basin because calcitic carbonate muds are accumulating in relatively shallow (70–85 m) water depths in the central part of the basin (20,000 km²). The carbonate muds are produced by the primary accumulation and disintegration of nannoplankton, as well as through the biodegradation of skeletal carbonate grains accumulating on the sea floor. The muds are transported to the south of the basin to the end of tidal current transport paths, where they accumulate in the lowest-energy environment available in enclosed Bass Basin, although they are still subject to periodic reworking by storm energy. The basin margins consist of coarser, partially palimpsest, carbonate sands, whereas the central muds overlie a Late Pleistocene disequilibrium surface. In cores these muds are up to 1 m thick and thin away from the centre towards the margins. Bioturbation is important in modifying the textural character of the muds by increasing the grain size as faecal pellets, and therefore creating a sea floor not in equilibrium with modern hydrodynamic conditions. Radiocarbon dates for an early post-glacial marine transgression embayment facies in the basin gave ages of 10,290 ± 250 to 11,660 ± 300 years B.P. An age of 8700 ± 710 years B.P. was obtained for the base of the modern strait facies. These ages and facies thicknesses were used to establish Holocene sedimentation rates of < 12 cm/1000 y for the basin centre, falling to < 6 cm/1000 y towards the margins.     

Oceanographic controls on shallow‐water temperate carbonate sedimentation: Spencer Gulf,South Australia

Spencer Gulf is a large (ca 22 000 km²), shallow (<60 m water depth) embayment with active heterozoan carbonate sedimentation. Gulf waters are metahaline (salinities 39 to 47‰) and warm‐temperate (ca 12 to ?28°C) with inverse estuarine circulation. The integrated approach of facies analysis paired with high‐resolution, monthly oceanographic data sets is used to pinpoint controls on sedimentation patterns with more confidence than heretofore possible for temperate systems. Biofragments – mainly bivalves, benthic foraminifera, bryozoans, coralline algae and echinoids – accumulate in five benthic environments: luxuriant seagrass meadows, patchy seagrass sand flats, rhodolith pavements, open gravel/sand plains and muddy seafloors. The biotic diversity of Spencer Gulf is remarkably high, considering the elevated seawater salinities. Echinoids and coralline algae (traditionally considered stenohaline organisms) are ubiquitous. Euphotic zone depth is interpreted as the primary control on environmental distribution, whereas seawater salinity, temperature, hydrodynamics and nutrient availability are viewed as secondary controls. Luxuriant seagrass meadows with carbonate muddy sands dominate brightly lit seafloors where waters have relatively low nutrient concentrations (ca 0 to 1 mg Chl‐a m^?3). Low‐diversity bivalve‐dominated deposits occur in meadows with highest seawater salinities and temperatures (43 to 47‰, up to 28°C). Patchy seagrass sand flats cover less‐illuminated seafloors. Open gravel/sand plains contain coarse bivalve–bryozoan sediments, interpreted as subphotic deposits, in waters with near normal marine salinities and moderate trophic resources (0·5 to 1·6 mg Chl‐a m^?3) to support diverse suspension feeders. Rhodolith pavements (coralline algal gravels) form where seagrass growth is arrested, either because of decreased water clarity due to elevated nutrients and associated phytoplankton growth (0·6 to 2 mg Chl‐a m^?3), or bottom waters that are too energetic for seagrasses (currents up to 2 m sec^?1). Muddy seafloors occur in low‐energy areas below the euphotic zone. The relationships between oceanographic influences and depositional patterns outlined in Spencer Gulf are valuable for environmental interpretations of other recent and ancient (particularly Neogene) high‐salinity and temperate carbonate systems worldwide.     

Quantitative mineralogical analysis of carbonate sediments by X-ray diffraction: a new, automatic method for sediments with low carbonate content

A new X-ray diffraction method has been developed whereby the weight percentages of aragonite and low and high-magnesium calcite are determined from the integrated peak areas of samples. Peak areas are measured by a step scanning method. The weight percentages of MgCO₃ in calcite are determined from the angular position of the calcite peak. This technique uses a direct calculation method which simplifies the preparation of the samples and the calibration processes and increases the quality of the results. The fully automatic method uses a desk-top computer to guide the diffractometer and to carry out the necessary calculations. Tests on precision and accuracy of the method indicate that results with less than ± 4% error (mineral %) and ± 0.6% error (MgCO₃%) are obtainable for all samples even those with a low (10%) carbonate content.     

Fringing reef growth on a terrigenous mud foundation, Fantome Island, central Great Barrier reef, Australia

The leeward fringing reef at Fantome Island (central Great Barrier Reef province) is a carbonate body which has developed under the influence of terrigenous sedimentation. The reef flat is up to 1000 m wide and is surfaced by mobile sand and gravel, with almost all live corals restricted to the seaward rim. The reef slope has coral columns and heads on the upper part, but below 5 m water depth it is a muddy substrate with scattered mounds of branching corals. Three high recovery cores show the reef is up to 10 m thick and developed over a gently sloping terrace of weathered Pleistocene alluvium. Three post-glacial stratigraphic units are recognised: (1) carbonate reef top unit of coral rudstone and framestone including Sinularia spiculite; (2) lower slope unit of coral floatstone in a terrigenous muddy matrix; and (3) transgressive basal unit of skeletal arkosic sand. The acid insoluble content of matrix and of individual corals increases downwards. Coral growth rates decrease downwards, reflecting slower growth in muddier environments. Radiocarbon dating shows that the reef prograded seaward at almost stable sea level. An average vertical accumulation rate of 6.7 mm yr^-1 is indicated. Two age reversals are interpreted as material transported by storms or by erosion in response to a late Holocene sea-level fall. The carbonate reef top unit has developed adjacent to, and is environmentally compatible with a muddy terrigenous, lower slope unit. Terrigenous influx has not changed during the Holocene, and terrigenous content of sediments is controlled by deposition on the reef slope of fine sediment winnowed from the reef flat and concentration of coarse sediment in the transgressive basal sheet.     

Holocene carbonate sedimentation on the west Eucla Shelf, Great Australian Bight: a shaved shelf

The southern continental margin of Australia is a cool-water carbonate sedimentary province located in a high-energy, swell-dominated oceanographic setting. A vibrocore transect of ¹⁴C-dated sediments across the centre of the Eucla Shelf is the first record of Holocene shelf deposition in the Great Australian Bight. Much of the seafloor shallower than 70 m water depth, the base of wave abrasion, is bare Cenozoic limestone, in some places encrusted by (?) Late Pleistocene, coral-rich, limestone that is cemented by high-magnesium calcite (12 mole% MgCO₃). The areally extensive, 100 km-wide, hard, bored substrate supports an epibiota of coralline algae, minor bryozoans and soft algae or is covered by patches of Holocene sediment up to 1.5 m thick; generally a basal bivalve lag (< 3 ka) overlain by quartzose-bioclastic palimpsest sand. This pattern of active carbonate production but little accretion on the wave-swept mid- to inner-shelf is similar to that on other parts of the southern Australian continental margin. The term shaved shelf is proposed for this style of carbonate platform, formed by alternating periods of sediment accretion, cementation and erosion.

The palimpsest sand is typically rich in bivalves, coralline algae and locally, detrital dolomite. Outer shelf Holocene sediment, below the base of wave abrasion but inboard of the shelf edge, is a metre-thick unit of fine, microbioclastic muddy sand with minor delicate bryozoans overlying a 9–13 ka rhodolith gravel. Some of this outer shelf sediment appears to have been resedimented. The shelf edge is a sandy and rocky seafloor with active bryozoan growth and sediment production.

The Holocene sediments are enriched in coralline algal particles and conspicuous large foraminifers (cf. Marginopora) and depleted in bryozoans, as compared to coeval deposits on the Lacepede and Otway shelves off southeastern Australia. These differences are interpreted to reflect warmer waters of the Leeuwin Current and prevalent downwelling in this area as opposed to the general upwelling and colder waters in the east.     

Salinity reflux and dolomitization of southern Australian slope sediments: the importance of low carbonate saturation levels

Anomalously saline waters in Ocean Drilling Program Holes 1127, 1129, 1130, 1131 and 1132, which penetrate southern Australian slope sediments, and isotopic analyses of large benthic foraminifera from southern Australian continental shelf sediments, indicate that Pleistocene–Holocene meso‐haline salinity reflux is occurring along the southern Australian margin. Ongoing dolomite formation is observed in slope sediments associated with marine waters commonly exceeding 50‰ salinity. A well‐flushed zone at the top of all holes contains pore waters with normal marine trace element contents, alkalinities and pH values. Dolomite precipitation occurs directly below the well‐flushed zone in two phases. Phase 1 is a nucleation stage associated with waters of relatively low pH (ca 7) caused by oxidation of H₂S diffusing upward from below. This dolomite precipitates in sediments < 80 m below the sea floor and has δ¹³C values consistent with having formed from normal sea water (? 1‰ to + 1‰ Vienna Pee Dee Belemnite). The Sr content of Phase 1 dolomite indicates that precipitation can occur prior to substantial metastable carbonate dissolution (< 300 ppm in Holes 1129 and 1127). Dolomite nucleation is interpreted to occur because the system is undersaturated with respect to the less stable minerals aragonite and Mg‐calcite, which form more readily in normal ocean water. Phase 2 is a growth stage associated with the dissolution of metastable carbonate in the acidified sea water. Analysis of large dolomite rhombs demonstrates that at depths > 80 m below the sea floor, Phase 2 dolomite grows on dolomite cores precipitated during Phase 1. Phase 2 dolomite has δ¹³C values similar to those of the surrounding bulk carbonate and high Sr values relative to Phase 1 dolomite, consistent with having formed in waters affected by aragonite and calcite dissolution. The nucleation stage in this model (Phase 1) challenges the more commonly accepted paradigm that inhibition of dolomitization by sea water is overcome by effectively increasing the saturation state of dolomite in sea water.     

Calcium carbonate dissolution in deep sea sediments: reconciling microelectrode, pore water and benthic flux chamber results

We report the benthic fluxes of O₂, titration alkalinity (TA), Ca²⁺, NO₃⁻, PO₄³⁻, and Si(OH)₄ from in situ benthic flux chamber incubations on the Ceara Rise and Cape Verde Plateau and compare them to previously published results. We find within analytical uncertainty that the TA flux is twice the calcium flux, suggesting that dissolution/precipitation of CaCO₃ is the principal mechanism controlling benthic TA and Ca²⁺ fluxes. At sites where the sediments contain significant (>35%) CaCO₃ and the overlying waters are supersaturated with respect to CaCO₃, the ratios of the total dissolution rate to the remineralization rate are significantly less than at all other study sites. We propose that these observations can be explained by precipitation of fresh CaCO₃ at the supersaturated sediment surface followed by redissolution deeper in the sediments because of metabolically-produced CO₂. A numerical simulation is presented to demonstrate the feasibility of this explanation. In addition, surface exchange reactions in high-CaCO₃ sediments coupled with high rates of particle mixing may also impact rates of metabolic dissolution and depress chamber-derived estimates of carbonate alkalinity and calcium benthic fluxes. These results suggest that at supersaturated, high CaCO₃ locations, previous models of sediment diagenesis may have overestimated the impact of metabolic dissolution on the preservation of CaCO₃ deposited on the sea floor.     

Hyperspectral logging of middle Cambrian marine sediments with hydrocarbon prospectivity: a case study from the southern Georgina Basin,northern Australia

The Georgina Basin is a Neoproterozoic–Paleozoic basin that spans parts of the Northern Territory and Queensland in northern Australia. The basin is prospective for petroleum, phosphate and base metals (copper, lead and zinc). The Dulcie and Toko synclines in the southern part of the basin are prospective for petroleum, where a thick Cambro-Ordovician succession of marine carbonates hosts several source rocks and associated oil and gas shows. The key source rock units occur within the middle Cambrian Narpa Group, including both the Thorntonia Limestone (Series 2 and 3) and the Arthur Creek Formation (Series 3). The base of the Arthur Creek Formation is characterised by organic-rich ‘hot’ shales (associated with a prominent gamma spike in well logs) that have been targeted by petroleum explorers for both conventional and unconventional oil and gas. For this study, hyperspectral logging data collected by HyLogger? instruments were evaluated from 13 wells in the southern Georgina Basin, including petroleum, mineral and stratigraphic wells. Formation boundaries are commonly (but not always) characterised by distinctive changes in mineralogy, as determined by spectral and X-ray diffraction data. Key source rock units in the southern Georgina Basin were characterised and mapped in terms of their mineralogy, and other spectral properties (e.g. Short-Wave Infrared (SWIR) reflectance and spectral contrast). Interpretation of the hyperspectral data alongside wireline log data supports the differentiation of two successions within the Arthur Creek Formation that are each characterised by basal organic-rich shales, previously distinguished on the basis of biostratigraphic and well-log data. The older succession in the Dulcie Syncline is spectrally characterised as being quartz and carbonate dominated. The younger succession, distributed across the eastern part of the Dulcie Syncline and fully across the Toko Syncline, is spectrally characterised as quartz and carbonate dominated, with variable white-mica contributions. Key associations are observed between the HyLogger mineralogy and geophysical-log data. Peaks in the gamma log intensity in the middle Cambrian sediments commonly correspond to elevated measured total organic carbon contents, decreased carbonate contribution, SWIR reflectance and spectral contrast, and relatively increased proportions of white micas and quartz. This study demonstrates that HyLogging data can provide an improved understanding of the sedimentological, mineralogical and diagenetic characteristics, as well as associated spatial heterogeneity, of prospective hydrocarbon formations in sedimentary basins.     

Three‐dimensional seismic analysis of sediment waves and related geomorphological features on a carbonate shelf exposed to large amplitude internal waves,Browse Basin region,Australia

This study analyses the three‐dimensional geometry of sedimentary features recorded on the modern sea floor and in the shallow subsurface of a shelf to upper slope region offshore Australia that is characterized by a pronounced internal wave regime. The data interpreted comprise an extensive, >12 500 km² industrial three‐dimensional seismic‐reflection survey that images the northern part of the Browse Basin, Australian North West Shelf. The most prominent seismic–morphological features on the modern sea floor are submarine terrace escarpments, fault‐scarps and incised channels, as well as restricted areas of seismic distortion interpreted as mass wasting deposits. Besides these kilometre‐scale sea floor irregularities, smaller bedforms were discovered also, including a multitude of sediment waves with a lateral extent of several kilometres and heights up to 10 m. These sedimentological features generally occur in extensive fields in water depths below 250 m mostly at the foot of submerged terraces, along the scarps of modern faults and along the shelf break between the outer shelf and the upper continental rise. Additional bedforms that characterize the more planar regions of the outer shelf are elongate, north‐west/south‐east oriented furrows and ridges. The formation of both sediment waves and furrow‐ridge systems requires flow velocities between 0·3 m sec^?1 and 1·5 m sec^?1, which could be generated by oceanic currents, gravity currents or internal waves. In the studied setting, these velocities can be best explained as being generated by bottom currents induced by internal waves, an interpretation that is discussed against oceanographic background data and modelling results. In addition to the documentation of three‐dimensional seismic–geomorphological features of the modern sea floor, it was also possible to map kilometre‐scale buried sediment wave fields in the seismic volume down to ca 500 ms two‐way‐time below the present sea floor, indicating the general potential for the preservation of such bedforms in the sedimentary record.     

Geochemistry of clastic sediments from Sargur supracrustals and Bababudan group,Karnataka: Implications on Archaean Proterozoic Boundary

The sediments from three stratigraphic levels in the Bababudan schist belt of Dharwar craton exhibit great diversity in major, trace and rare earth element (REE) geochemistry and thus interpreted to represent significant compositional variation in the source rocks. Detailed geological and geochemical studies have been carried out on clastic rocks constituting the Archaean Sargur supracrustals and the Bababudan belt of Dharwar craton (DC), southern India for understanding the geochemical characteristics and to define the Archaean-Proterozoic Boundary (APB/QPC) in southern India. There is significant contrast in the geochemical signatures for the sediments from these stratigraphic levles. The Sargur enclave population is characterised by slight LREE enrichment with (La/Sm)_N ranging from 1.45 to 3.58, almost flat HREE with (Gd/Yb)_N ranging from 0.65 to 1.29 with Eu/Eu* ranging from 0.49 to 0.91 suggesting mafic-ultramafic source rocks in the provenance. On the other hand, the Post QPC (PQPC) rocks are characterised by LREE enrichment with (La/Sm)_N ranging from 2.66 to 7.07, nearly flat HREE with (Gd/Yb)_N ranging from 0.58 to 0.95 and significant depletion of Eu with Eu/Eu* ranging from 0.34 to 0.85, indicating felsic province in the source area. The conglomerates and quartzites representing the QPC are showing mixed nature of these, reflecting the transitional character in depositional environment. Increase in abundance of REE, K₂O/Na₂O, Th/Sc, La/Sc, Th/U, Hf/Ta and Zr/Y ratios are characteristic of the QPC. The PQPC sediments are enriched in Th, U and HFSE like Hf, Nb, Zr and Y, and depleted in Co and Eu than their older counterparts. These geochemical signatures signify the dominance of mafic-ultramafic rocks in the source area for Sargur rocks and the existence of granite-granodiorite for PQPC clastics. Thus, the unconformity related oligomictic quartz pebble conglomerates (QPC) and quartzites at the base of Bababudan Group resembling the QPC of Witswaterand, South Africa signifies that a stable continental crust had already developed in southern India prior to ∼3.0Ga.