One size does not fit all: Refining zircon provenance interpretations via integrated grain shape,geochronology, and Hf isotope analysis

Sediment provenance studies commonly utilize isotopic signatures to resolve detrital mineral sources and routing. However, non-unique ages and geochemical characteristics across geographically distinct crystalline source regions can lead to significant ambiguities in mineral provenance interpretations. Such ambiguity is apparent in southern Australia’s Cenozoic Eucla Basin, which hosts world-class heavy mineral sand resources. Here, new Hf isotope data are provided from four heavy mineral prospects (N = 8, n = 844 [N = samples, n = grains]). Zircon grain shape data are also presented for a suite of detrital Eucla Basin samples (N = 22, n = 35,604) and the basin’s underlying basement, the Coompana Province (N = 13, n = 824). The data are integrated with published detrital and non-detrital primary zircon data to investigate the efficacy of grain shape analysis to better resolve the basin’s mineral provenance. Zircon Hf isotope compositions indicate a primary Mesoproterozoic juvenile source for zircon melts (～1250–1000 Ma, ?2.5 < ?_Hf > ～+5) with additional contributions from a range of juvenile to evolved late Archean to Phanerozoic-aged zircon bearing magmas (?28.0 < ?_Hf > +11). U–Pb geochronology and Hf isotopes are incapable of differentiating Mesoproterozoic-aged source rocks bounding the region for the majority of heavy mineral deposits analyzed as potential sources express overlapping crystallization ages and similarities in Hf-isotope characteristics. However, distinct zircon grain shapes (i.e., perimeter, major axis and circularity) facilitate improved differentiation across these Mesoproterozoic sources. Filtering of U–Pb age, Hf isotope and shape data implicate the underlying Madura and Coompana provinces as dominant sediment sources for Eucla Basin detritus aged ～1400–1000 Ma. The lack of direct sediment pathways between the underlying basement provinces and placer sediments analyzed demonstrates the significance of zircon reworking from intermediate sedimentary basins in the formation of the economically significant Eucla Basin beach placers. Zircon grain shape represents a cheaply acquired and readily incorporated grain characteristic that can enhance provenance investigations.     

Radiometric characteristics of heavy mineral deposits along the west coast of South Africa

During the last decade, exploration and mining of modern-Tertiary heavy mineral beach and raised beach sands along the west coast of South Africa has developed into a major industry. High resolution radiometric techniques have demonstrated their use as a quantitative indicator of total heavy mineral concentration (THM) and also have the ability to discriminate between sediments derived from different provenance terrains. Results indicate that it is possible to calculate the total heavy mineral concentration from high resolution radiometric measurements, but the ability to quantify the concentration of individual mineral fractions, such as ilmenite, requires further refinement of the method. Radiometric characteristics of the light mineral fraction made it possible to distinguish between sediments from mixed marine-aeolian palaeoplacers and active present beach placers. The effects of in situ alteration of the ilmenite fraction in the palaeoplacers were also reflected by subtle contrasts in the radiometric character of the deposits. Furthermore, the uniform radiometric character of the sample population indicates a common provenance for the heavy mineral suite and supports previous results which indicate the metamorphites of the mid-Proterozoic Namaqua Province as the primary source terrain. The high resolution radiometric techniques discussed offer a broad spectrum of applications in sedimentology and have the potential to aid heavy mineral exploration. With further refinement it can possibly be used quantitatively in grade control during mining and beneficiation of heavy mineral deposits. Received: 19 July 1996 / Accepted: 7 January 1997     

Geological characteristics of 2011 Japan tsunami sediments deposited along the coast of southwestern Mexico

The tsunami of 11th March 2011 was originated at the east coast of Japan and deposited ca.1 cm thick sediment layer along the coast of southwestern Mexico up to a maximum distance of 320 m from the beach. The sedimentological, mineralogical and geochemical characteristics of the sediments deposited during the tsunami (JT) are compared with the pre-tsunami sediments (PRT). JT sediments consist of dominant coarser fractions (>54% of medium to coarse sand), whereas PRT deposits comprise abundant finer fractions (>58% of fine sand). Assemblage of mafic and heavy minerals suggests similar provenance for both. The higher abundance and variation of heavy minerals along with higher concentrations of bromine (Br) and sodium (Na) in the JT deposits reveal the influence of high energy sea waves in transportation of heavy mineral rich coarse sediments onto the coastal lowlands.     

Cenozoic Eucla Basin and associated palaeovalleys,southern Australia — Climatic and tectonic influences on landscape evolution,sedimentation and heavy mineral accumulation

The Eucla Basin including the vast Nullarbor Plain lies on the margins of the Yilgarn, Musgrave and Gawler cratons in southern Australia and owes its distinctive landscape to a unique set of interactions between eustatic, climatic and tectonic processes over the last ~ 50 Ma. Understanding of the history of the basin and the palaeovalleys that drained from the surrounding cratons are important because they contain major mineral deposits, and the sediments derived from them contain remobilised gold, uranium, and heavy minerals. In particular, a remarkably preserved palaeoshoreline sequence along the north-eastern margin of the Eucla Basin is highly prospective for heavy mineral placer deposits. The record of marine, marginal marine, estuarine, fluvial and lacustrine environments, as constrained mainly by an extensive borehole dataset, reflects major depositional events during the Palaeocene–Early Eocene, Middle–Late Eocene, Oligocene–Early Miocene, Middle Miocene–Early Pliocene and Pliocene–Quaternary. These events reflect the key role of eustatic sea-level variation which, during highstands, inundated the craton margins, flooding palaeovalleys to up to 400 km inboard of the present coastline. However, a systematic eastward migration of the depocentre across the Eucla Basin during the Neogene, together with apparent flow reversals in a number of palaeovalley systems draining the Gawler Craton, suggest that the Eucla Basin has also been subject to differential vertical movements, expressed as a west-side up, east-side down tilting of ~ 100–200 m. This differential movement forms part of a broader north-down–southwest-up dynamic topographic tilting of the Australian continent associated with relatively fast (6–7 cm/yr) northward plate motion since fast spreading commenced in the Southern Ocean at ~ 43 Ma. We suggest that the evolving dynamic topography field has played a key role in facilitating development of placer deposits, largely through multistage, eastward reworking of near-shore sequences during highstand transgressive cycles on a progressively tilting platform under the influence of persistent westerly weather systems.     

Influence of nearshore sediment dynamics on the distribution of heavy mineral placer deposits in Sri Lanka

Beach sediments in Sri Lanka contain industrial-grade heavy mineral occurrences. Samples of both offshore and onshore sediments were collected to examine the provenance, mineralogy and geochemical compositions of the heavy mineral occurrences. Coastal morphodynamic changes along the coastline of Sri Lanka were analyzed using the time-series satellite images. These coastal morphodynamic changes were used to identify the prominent directions of monsoon-influenced longshore currents, coastal sediment accretion and depositional trends and their relationships to the provenance of the heavy minerals. Results show the concentrations of detrital ilmenite, zircon, garnet, monazite, and rutile vary in the onshore and offshore sediments. The heavy mineral potential of the northeastern coast is high (average contents of about 45–50% in the Verugal deposit, 70–85% in the Pulmoddai deposit, and 3.5–5.0% in offshore samples stretching from Nilaveli to Kokkilai), compared to sediments in southwest (average content about 10% in onshore sediments and 2% in offshore sediments from the mouth of the Gin River). Therefore, no economic-grade heavy mineral placers were identified in the offshore environments. The high concentrations of heavy minerals in beach sediments and low concentrations in offshore sediments suggest operation of a panning system in the surf zone to form enriched placer deposits. Major and trace element compositions of beach sediments show marked enrichments of TiO₂, Fe₂O₃, La, Ce, Zr, Cr, Nb, Th and V compared to average Upper Continental Crust (UCC) values. Analysis of prominent coastal longshore transport patterns identifies bidirectional sediment transport in the northeast coast of Sri Lanka. In the southwestern coast, two transport directions occur with anti-clockwise transport from Galle to Hambantota, and clockwise transport from Hikkaduwa to Wadduwa. The heavy minerals in the placers were mainly derived from Precambrian metamorphic rocks, and transported to the coast through the river systems of Sri Lanka.     

Distinction between the Storegga tsunami and the holocene marine transgression in coastal basin deposits of western Norway

Many coastal lakes were inundated by both the Storegga tsunami (7000 ¹⁴C yr BP) and the mid-Holocene sea-level rise (the Tapes transgression) in western Norway. The tsunami eroded lake bottoms and deposited graded and/or massive beds of sand, rip-up clasts, and coarse plant material. By contrast, when the rising sea entered the lakes, it deposited only gyttja, silt and fine sand, without causing much erosion of the underlying lake sediments. Storegga tsunami deposits in some coastal lakes were interpreted previously as ordinary marine sediments from the Tapes transgression. Our reinterpretation of these deposits shows that the transgression maximum phase was reached after 6500 yr BP, more than 1000 yr later than previously inferred for the coast of Sunnmøre. The new data cannot be combined in a shoreline diagram without showing the 6000 yr BP and 7000 yr BP shorelines as slightly warped. © 1998 John Wiley & Sons, Ltd.     

Late Quaternary evolution of coastal sand Barriers, Port Stephens-Myall Lakes Area, central New South Wales, Australia

The geomorphic and stratigraphic history of six coastal embayments has been studied in the vicinity of Newcastle, New South Wales (N.S.W.), Australia, in order to determine modes of deposition, and the degree to which marine and estuarine deposits can be correlated and dated. Each embayment possesses its own distinctive suite of landforms ranging from those dominated by coastal dunes to those in which beach ridges occur. In four of the bays dual sand barriers, comprising an Inner Barrier and an Outer Barrier, provide the framework for correlation between embayments. Six stages are recognized in the deposition of late Quaternary sediments in this area: (i) Pre-Last Interglacial, involving accumulation of separate composite units consisting of estuarine clays and transgressive dune complexes; (ii) Last Interglacial stage during which Inner Barriers were formed; (iii) Last Glacial reworking of barrier and dune sands by westerly winds; (iv) Postglacial Marine Transgression during which the Outer Barriers were initiated; (v) Mid-Holocene stage following cessation of sea-level rise ca. 6000–6500 yr B.P. on this coast, and involving progradation of Outer Barriers in some embayments; and (vi) Late Holocene episodic eolian reworking of dune complexes and Outer Barriers. The relatively high wave and wind energy as well as the tectonic setting of the central N.S.W. coast results in somewhat different geomorphic histories from barrier-island coasts in North America and Europe.     

Basin-related uranium mineral systems in Australia: A review of critical features

This paper reviews critical features of basin-related uranium mineral systems in Australia. These mineral systems include Proterozoic unconformity-related uranium systems formed predominantly from diagenetic fluids expelled from sandstones overlying the unconformity, sandstone-hosted uranium systems formed from the influx of oxidised groundwaters through sandstone aquifers, and calcrete uranium systems formed from oxidised groundwaters flowing through palaeochannel aquifers (sand and calcrete). The review uses the so-called ‘source-pathway-trap’ paradigm to summarise critical features of fertile mineral systems. However, the scheme is expanded to include information on the geological setting, age and relative timing of mineralisation, and preservation of mineral systems. The critical features are also summarised in three separate tables. These features can provide the basis to conduct mineral potential and prospectivity analysis in an area. Such analysis requires identification of mappable signatures of above-mentioned critical features in geological, geophysical and geochemical datasets. The review of fertile basin-related systems shows that these systems require the presence of at least four ingredients: a source of leachable uranium (and vanadium and potassium for calcrete-uranium deposits); suitable hydrological architecture enabling connection between the source and the sink (site of accumulation); physical and chemical sinks or traps; and a post-mineralisation setting favourable for preservation. The review also discusses factors that may control the efficiency of mineral systems, assuming that world-class deposits result from more efficient mineral systems. The review presents a brief discussion of factors which may have controlled the formation of large deposits in the Lake Frome region in South Australia, the Chu-Sarysu and Syrdarya Basins in Kazakhstan and calcrete uranium deposits in the Yilgarn region, Western Australia.     

The structure and development of foredunes on a locally prograding coast: insights from ground-penetrating radar surveys, Norfolk, UK

The internal structure of coastal foredunes from three sites along the north Norfolk coast has been investigated using ground‐penetrating radar (GPR), which provides a unique insight into the internal structure of these dunes that cannot be achieved by any other non‐destructive or geophysical technique. Combining geomorphological and geophysical investigations into the structure and morphology of these coastal foredunes has enabled a more accurate determination of their development and evolution. The radar profiles show the internal structures, which include foreslope accretion, trough cut and fill, roll‐over and beach deposits. Foredune ridges contain large sets of low‐angle cross‐stratification from dune foreslope accretion with trough‐shaped structures from cut and fill on the crest and rearslope. Foreslope accretion indicates sand supply from the beach to the foreslope, while troughs on the dune crest and rearslope are attributed to reworking by offshore winds. Bounding surfaces between dunes are clearly resolved and reveal the relative chronology of dune emplacement. Radar sequence boundaries within dunes have been traced below the water‐table passing into beach erosion surfaces. These are believed to result from storm activity, which erodes the upper beach and dunes. In one example, at Brancaster, a dune scarp and erosion surface may be correlated with erosion in the 1950s, possibly the 1953 storm. Results suggest that dune ridge development is intimately linked to changes in the shoreline, with dune development associated with coastal progradation while dunes are eroded during storms and, where beaches are eroding, a stable coast provides more time for dune development, resulting in higher foredune ridges. A model for coastal dune evolution is presented, which illustrates stages of dune development in response to beach evolution and sand supply. In contrast to many other coastal dune fields where the prevailing wind is onshore, on the north Norfolk coast, the prevailing wind is directed along the coast and offshore, which reduces the landward migration of sand dunes.     

Sedimentology, stratigraphy and landscape evolution of a Holocene coastal dune system, Lodbjerg, NW Jutland, Denmark

The stratigraphy and landscape evolution of the Lodbjerg coastal dune system record the interplay of environmental and cultural changes since the Late Neolithic. The modern dunefield forms part of a 40 km long belt of dunes and aeolian sand‐plains that stretches along the west coast of Thy, NW Jutland. The dunefield, which is now stabilized, forms the upper part of a 15–30 m thick aeolian succession. The aeolian deposits drape a glacial landscape or Middle Holocene lake sediments. The aeolian deposits were studied in coastal cliff exposures and their large‐scale stratigraphy was examined by ground‐penetrating radar mapping. The contact between the aeolian and underlying sediments is a well‐developed peaty palaeosol, the top of which yields dates between 2300 BC and 600 BC . Four main aeolian units are distinguished, but there is some lateral stratigraphic variation in relation to underlying topography. The three lower aeolian units are separated by peaty palaeosols and primarily developed as 1–4 m thick sand‐plain deposits; these are interpreted as trailing edge deposits of parabolic dunes that moved inland episodically. Local occurrence of large‐scale cross‐stratification may record the head section of a migrating parabolic dune. The upper unit is dominated by large‐scale cross‐stratification of various types and records cliff‐top dune deposition. The nature of the aeolian succession indicates that the aeolian landscape was characterized by alternating phases of activity and stabilization. Most sand transported inland was apparently preserved. Combined evidence from luminescence dating of aeolian sand and radiocarbon dating of palaeosols indicates that phases of aeolian sand movement were initiated at about 2200 BC , 700 BC and AD 1100. Episodes of inland sand movement were apparently initiated during marked climate shifts towards cooler, wetter and more stormy conditions; these episodes are thought to record increased coastal erosion and strong‐wind reworking of beach and foredune sediments. The intensity, duration and areal importance of these sand‐drift events increased with time, probably reflecting the increasing anthropogenic pressure on the landscape. The formation of the cliff‐top dunes after AD 1800 records the modern retreat of the coastal cliffs.     

Bauxite formation on Tertiary sediments in the coastal plain of Suriname

Lateritic bauxites in the coastal lowlands of Suriname form part of a belt along the northern margin of the Guiana Shield that has long been one of the world's major bauxite producing regions. The Surinamese deposits, many of which with an extensive mining history, originated on Tertiary siliciclastic sediments and were mostly buried under a layer of young sediments. The bauxite-bearing sequences are generally topped with an iron-rich layer largely made up of hematite and goethite. It covers a gibbsite-rich bauxite horizon that passes downward into a kaolinitic bottom section containing anatase and zircon as main accessory minerals. Weathering profiles across formerly mined deposits were analyzed for geochemical and mineralogical properties aimed at exploring compositional diversity, underlying controls of bauxite-formation and the nature of precursor sediments.Studied profiles in different parts of the coastal plain reveal overall similarities between individual deposits in showing significant depletion of Si, K, Na, Mg and Ca and strong, primarily residual, relative enrichment of Al, Ti, Zr, Nb, Hf, Ta and Th. In detail, however, there are distinct differences in major and trace-element signatures, accessory mineral assemblages, facies distribution and provenance of the terrigenous precursor sediments. Enrichments in high field-strength elements and heavy rare earth elements are largely attributable to accumulation of heavy minerals like zircon in the precursor. Petrological and trace-element evidence does not support a direct genetic relationship between bauxite and the underlying saprolitic clays. The complex petrologic characteristics and compositional heterogeneity of the coastal-plain deposits can essentially be explained by element fractionation, primarily through selective leaching, in combination with relative and absolute enrichment processes, erosion and reworking during two-stage, polycyclic bauxitization of a heterogeneous precursor.     

Mineral distribution and provenance of heavy mineral sands (zircon,ilmenite, rutile) deposits from the NW Murray Basin,far western NSW,Australia

Abstract

There is significant economic interest in the Murray Basin of southeastern Australia as it is proving to be a major heavy mineral sands (HMS) province that will be one of Australia’s major source for production of rutile, zircon and ilmenite. The distribution and provenance of HMS resources in the Murray Basin is poorly understood because of its huge size, limited exploration and the complex depositional, structural and weathering mechanisms in their development. In this paper, we focus on the Copi North and Magic deposits some 130–180?km south of Broken Hill, NSW. The heavy mineral assemblages of the Copi North and Magic deposits are very similar, with the main economic minerals being ilmenite, leucoxene, pseudorutile, rutile and zircon. Intensive fracturing and brecciation are identified in many samples and are inferred to have been initially caused by multi-stage deformational processes associated with metamorphism and then further developed through alluvial and eolian transportation. Both deposits are classified as ‘medium sands, symmetrical, mesokurtic and moderately well-sorted’. The majority of economic minerals are of low to medium sphericity and subrounded, along with abundant polished eolian quartz grains. The Copi North deposit has coarser and more poorly sorted sediments with higher HMS grades and magnetics content than the Magic deposit, reflecting a higher energy depositional environment. The main source for the HMS for the Copi North and Magic deposits is largely ascribed to the Broken Hill Block. Previous studies have shown that the Broken Hill orebody underwent substantial sub-aerial weathering over hundreds of millions of years. In addition, the complex metamorphic events experienced by the Broken Hill Block were capable of forming the broad series of minerals identified within the Copi North and Magic deposits. The HMS were believed to have been transported through paleovalleys near the Mulculca Fault in a southeast direction representing a feeder system into the NW Murray Basin. Both deposits feature a relatively linear geometry (roughly parallel to the strike of the paleoshoreline), with high HMS grades, modest tonnages, and coarser sediments when compared to WIM-style offshore deposits. Compared to other strandline HMS deposits of the Murray Basin, they are smaller in size although have similar high grades of 3.7–6.9% THM and similar proportions of the HMS assemblage of ilmenite, leucoxene, rutile and zircon. Deposits of similar size and grade are likely to occur throughout the northern part of the Murray Basin.

1. KEY POINTS

2. Both the Copi North and Magic deposits contain similar mineral assemblages with the provenance of the heavy minerals ascribed to the Broken Hill Block.

3. A relatively high energy inshore environment is inferred for the Copi North deposit while a lower energy environment associated with either a foreshore or backshore environment is inferred for the Magic deposit.

4. Deposits of similar mineralogy, grades and size are likely to occur elsewhere throughout the northern Murray Basin.

     

Identification and characterization of tsunami deposits off southeast coast of India from the 2004 Indian Ocean tsunami: Rock magnetic and geochemical approach

The December 2004 Indian Ocean Tsunami (IOT) had a major impact on the geomorphology and sedimentology of the east coast of India. Estimation of the magnitude of the tsunami from its deposits is a challenging topic to be developed in studies on tsunami hazard assessment. Two core sediments (C1 and C2) from Nagapattinam, southeast coast of India were subjected to textural, mineral, geochemical and rock-magnetic measurements. In both cores, three zones (zone I, II and III) have been distinguished based on mineralogical, geochemical and magnetic data. Zone II is featured by peculiar rock-magnetic, textural, mineralogical and geochemical signatures in both sediment cores that we interpret to correspond to the 2004 IOT deposit. Textural, mineralogical, geochemical and rock-magnetic investigations showed that the tsunami deposit is featured by relative enrichment in sand, quartz, feldspar, carbonate, SiO ₂, TiO ₂, K ₂O and CaO and by a depletion in clay and iron oxides. These results point to a dilution of reworked ferromagnetic particles into a huge volume of paramagnetic materials, similar to what has been described in other nearshore tsunami deposits (Font et al. 2010). Correlation analysis elucidated the relationships among the textural, mineral, geochemical and magnetic parameters, and suggests that most of the quartz-rich coarse sediments have been transported offshore by the tsunami wave. These results agreed well with the previously published numerical model of tsunami induced sediment transport off southeast coast of India and can be used for future comparative studies on tsunami deposits.     

Sea-level changes and buried islands in a complex coastal palaeolandscape in the South of Western Australia: Implications for greenfield mineral exploration

Weathering intensity changes due to climatic variability across tectonically stable portions of continental crust can generate a thick and extensive weathered cover, resulting in regolith-dominated terrains (RDTs). Mineral exploration in RDTs is challenging because of the lack of bedrock outcrop, and the difficulty of linking surface regolith geochemistry to the geology at depth. Complex weathering obscures the expression of the basement geochemistry in the regolith, and therefore the footprints of mineral systems are difficult to detect. The southeast of the Yilgarn Craton and the Albany–Fraser Orogen (AFO) in the south of Western Australia is an RDT that extends along the coastline and the Eucla basin.This study proposes a landscape evolution model of the AFO, driven by transgression–regression sea-level changes that resulted in the formation of numerous islands and development of estuarine zones. This model contrasts with the river system-dominated landscape evolution present in the Yilgarn Craton. This difference has significant implications for mineral exploration and geochemical interpretation of the regolith in this region.Weathering profiles developed “on inland” and “on island” are thicker and more mature than those developed in sea-inundated areas. Even if in the Yilgarn Craton local areas display reworking of weathering profiles and other complexities from Permian, non-marine Tertiary sediments and Quaternary fluvial and aeolian sediments, at a regional scale, if vertical geochemical mobility of elements has occurred, “on inland” and “on island” are more reliable for understanding geochemical anomaly-basement relationships, whereas the “marine inundated” areas require a more detailed investigation, because of the role of marine reworking of weathering profiles and possible mixture of sediments from different provenances.Landscape changes from the topographically high, dissected Yilgarn environment with thick saprolite development and uneven basement topography, to the nearly flat regions dominated by sand dunes and thin saprolite development at the coastline. These regions are the result of the erosional and depositional effects of successive sea-level transgression–regression cycles. Within this framework, the following four different regolith settings have been identified in a progressive change from Yilgarn Craton environments to the modern coastline: (1) Albany; (2) Kalgoorlie–Norseman; (3) Esperance; and (4) Neale.Mapping the palaeocoastlines, islands and estuarine zones, as well as the region of influence of marine limestones and sediments, can significantly improve the understanding of how surface geochemistry relates to the landscape, and how it links with the geology at depth, and therefore, how it may reflect the presence of mineral systems. Understanding the difference in the landscape evolution between the AFO and Yilgarn Craton is essential to properly calibrate mineral exploration protocols in both regions.     

Depositional features in tourist beaches of Chennai Metropolis,SE coast of India: Inferences from grain size studies

The present study was undertaken to examine the depositional and transportation pattern in the tourist beaches of Chennai in SE coast of India. Twenty four samples were analyzed for textural parameters, Linear discriminate function (LDF), C-M plots and heavy/light minerals from beach, berm and dune regions. Results of textural parameters indicate that they are coarse grained in the three regions. Majority of the samples indicate that they are moderately well sorted, symmetrically skewed and leptokurtic. The LDF analysis allowed in identifying areas that are characterized by: (1) beach, (2) shallow agitated environment and (3) fluvial deposits. C-M plots indicate that the sediments in the beaches were deposited from the near shore environment. Heavy mineral analysis suggests that the heavy and light minerals were in the range of 0.5 to 16.9% and 82.08 to 98.14% and the light/dark layers are controlled by the prevailing tidal conditions in the coastal region. It is observed that sand is accumulating from the shallow agitated environment, while the transportation and deposition of sediments is mainly due to the interaction of tidal activities at the same time. C-M plots suggest that the sediments fall in the graded suspension and the saltation activities in the region also facilitate to understand the hydrodynamics of the region suggesting that they are deposited by high-energy activities.     

Pliocene sedimentation in a shallow, cool-water, estuarine gulf, Murray Basin, South Australia

The Pliocene Norwest Bend Formation is a well‐preserved succession of terrestrial and shallow‐marine deposits in the Murray Basin, South Australia. Sediments in this unit consist of two discrete terrigenous clastic‐rich, decametre‐scale sequences, or informal members, which record episodes of marine incursion during the Early and Late Pliocene respectively. The base of each sequence is a transgressive lag and/or strandline deposit that is transitional upwards into a highstand, subtidal, terrigenous clastic and cool‐water carbonate sediment accumulation. The top of each sequence is incised by fluvial channels that are filled by river deposits which formed as relative sea‐level fell and terrestrial environments prograded basinward. Sedimentological data suggest that gross stratigraphic architecture was primarily determined by glacioeustasy. Differences in sedimentary style between these two sequences, however, reflect a major climatic change that took place in southern Australia during the mid‐Pliocene. The lower quartzose sand member is formed of siliciclastic sediment derived from prolonged, deep, subaerial weathering and contains a bivalve‐dominated, cool‐temperate, open‐marine mollusc assemblage. These sediments accumulated under an equitable, relatively warm, humid climate. The Murray Basin during this time, because of high fluvial discharge, was a salt‐wedge estuary with typical estuarine circulation. In contrast, the upper, oyster‐rich member is typified by large monospecific oyster buildups that grew in restricted coastal environments. Strandline deposits contain a warm‐temperate skeletal assemblage. Contemporaneous aeolian sediments accumulated under warm, semi‐arid climatic conditions. Well‐developed ferricrete, silcrete and calcrete horizons reflect cyclic conditions of rainwater infiltration and evaporation in the seasonally dry climate that typifies southern Australia today. Highly seasonal rainfall produced an estuary that fluctuated annually from being well to partially mixed. These Pliocene sediments support the notion that mollusc‐rich facies are the signature of cool‐water carbonate accumulations in inboard neritic environments. Unlike bryozoans that dominate the outer parts of Cenozoic cool‐water carbonate shelves, molluscs evolved to exploit an array of coastal ecosystems with wide salinity variations and variable sedimentation rates.     

山东峡山湖东岸现代滩坝沉积模式及控制因素*

滩坝作为发育于滨岸带非常重要的沉积储集层类型之一,已成为隐蔽油气藏勘探的目标。为进一步明确峡山湖东岸滨浅湖滩坝沉积模式,以现代沉积考察为基础,结合探地雷达技术和粒度分析资料,对东岸研究区滩坝砂体的沉积特征以及控制因素进行详细分析。结果表明: 峡山湖东岸主要发育风成沙丘及滩坝2种沉积体系。早期风成沉积物以发育连续性好的大型板状交错层理细砂为主,分选较好,粒度概率曲线以两段式为特征;滩坝沉积物以发育连续性差的板状交错层理、平行层理中砂为主,主要为反粒序或者复合粒序,具冲刷构造,岩性较混杂并以砂泥薄互层为特征,发育大量植物根系,粒度概率曲线以三段式为主,跳跃组分分为2段,分选较好。探地雷达资料表明研究区下部主要以厚层层状反射结构砂体为主,与风成沙丘沉积反射特征较为一致,环境敏感粒度组分对滩坝沉积较强的水动力作用有较好的指示。总体上,风力作用控制了风成砂体的沉积发育并且改变了古地貌条件,风浪作用冲刷改造了早期砂体,为滩坝的发育提供了物源,控制了滩坝发育的规模和地理位置,可概括为“风运—湖改”滩坝沉积模式,可为滩坝体系的识别和有利储集层预测提供指导。     

埃迪卡拉系微生物碳酸盐岩沉积特征及其地质意义:以川中磨溪8井区灯影组四段为例*

四川盆地高石梯—磨溪地区埃迪卡拉系灯影组广泛发育一套微生物碳酸盐岩。以磨溪8井区灯影组四段(灯四段)为例,通过精细解析高频向上变浅序列的岩石学特征,发现灯四段微生物云岩及序列建造具有如下特征: (1)向上变浅序列主要由凝块石云岩、叠层石云岩和(含微生物)泥晶云岩3类构成,伴以少量微生物粘结颗粒云岩;(2)完整的典型向上变浅序列自下而上由(含微生物)泥晶云岩(A段)、平直状叠层石云岩(B段)、小型丘状叠层石云岩(C段)、分散状凝块石云岩(D段)、格架状凝块石云岩(E段)、微生物粘结颗粒云岩(F段)等6个岩性单元构成,但大量的序列由A-C-E、A-C-D、B-D、B-C-D-E等不完整的岩性单元构成,且序列顶底皆以凝块石云岩/(含微生物)泥晶云岩、叠层石云岩/(含微生物)泥晶云岩等不平整的岩性突变面或暴露面区分;(3)埃迪卡拉系微生物碳酸盐岩发育于浪基面之上至平均海平面附近的相对高能区域,且凝块石云岩较叠层石云岩形成的环境能量高。该研究结果不仅揭示出较高能的浅水环境控制了规模性微生物岩分布这一规律,而且对区域储集层预测具有指导意义,同时也因建立了有别于现代微生物的环境分布模式而具有重要的沉积学意义。     

Granularity and textural analysis as a proxy for extreme wave events in southeast coast of India

Extreme wave events of 1000 and 1500 years (radiocarbon ages) have been recently reported in Mahabalipuram region, southeast coast of India. Subsequently, we carried out extensive sedimenttological analysis in regions covering a total lateral coverage of 12 km with a new archeological site as the central portion of the study area. Twelve trenches in shore normal profiles exhibit landward thinning sequences as well as upward fining sequences confirming with the global signatures of extreme wave events. The sediment size ranges from fine-to-medium and moderately well sorted-to-well sorted, and exhibit positive skewness with platykurtic-to-leptokurtic nature. We now propose the abrupt winnowing or back and forth motion including unidirectional transport of these deposited sediments, which results in positive skewness. Textural analyses derived from scanning electron microscope studies (SEM) demonstrate the alteration produced, in the ilmenite mineral with vivid presence of pits and crescents with deformation observed on the surface due to extreme wave activities. This is further confirmed with the predominance of high-density mineral such as magnetite (5.2) and other heavy minerals in these deposits inferred the high-intensity of the reworking process of the beach shelf sediments.     

Late Quaternary marine deposition in New South Wales and southern Queensland — An evolutionary model

From new data on coastal and continental shelf morphology, sediments, stratigraphy and chronology, it is possible to formulate a general model of late Quaternary marine sedimentation, for New South Wales and southern Queensland. This model integrates various factors influencing deposition in coastal and shelf environments, in relation to glacio‐eustatic sea level oscillations.

The model involves several components, including (i) very slow to negligible continental margin subsidence during the Quaternary, (ii) an inherited geomorphic framework; (iii) oscillations of sea level of c 100 m amplitude every 100 000 years, with interglacial high sea levels being close to present and only the Last Interglacial being significantly higher; and (iv) a wave climate that induces a potential south to north littoral sand transport at all sea level positions.

Terrigenous sediment that is moved from the hinterland through embayments to the shelf is either stored as barrier, estuarine or inner shelf deposits, or lost to depositional sinks on the continental slope or into coastal dune fields. Over many glacial‐interglacial cycles, sand has been progressively moved northward and has accumulated in vast aeolian sand deposits in southern Queensland. Littoral sand transport was especially effective during sea levels lower than present. The relatively shallow and lower gradient shelf north of Newcastle (33°S) has encouraged preservation at the coast of a wide range of depositional morphologies, including Pleistocene barriers, whereas the steeper southern shelf has induced net sediment loss seawards and shoreline erosion, excpt in the Holocene. To account for Holocene barrier development in the southern region, the model invokes reworking of sand deposits stranded high on the inner shelf at the end of the Pleistocene Epoch. These were in disequilibrium with Postglacial marine processes that operated at a lower level of the sea than did those during the Last Interglacial maximum.