Swell impact on reef sedimentary processes: A case study of the La Reunion fringing reef

Two surface‐sediment sampling campaigns were carried out in November and December 2003, before and after a strong swell event, in the back‐reef area of a microtidal fringing reef on the western coast of La Reunion, Indian Ocean. The spatial distributions of the mean grain size, sorting and skewness parameters are determined, and grain‐size trend analysis is performed to estimate the main sediment transport pathways in the reef. The results of this analysis are compared with hydrodynamic records obtained in the same reef area during fair weather conditions and during swell events. Sediment dynamics inferred from the hydrodynamic records show that significant sediment erosion and transport occur only during swell events and under strongly agitated sea states. Under normal wave conditions, there is a potential for onshore sediment transport from the reef‐flat to the back‐reef, but this transport is episodic and occurs principally during high‐tide stages. Sediment transport trends revealed by the grain‐size trend analysis method show onshore and alongshore low‐energy transport processes that are in agreement with the hydrodynamic records. The grain‐size trend analysis method also provides evidence of an offshore high‐energy transport trend that could be interpreted as a real physical process associated with return flow from the shore to the reef. The impact of swell on the reef sediment dynamics is clearly demonstrated by onshore and alongshore transport. Considering different combinations of the vector transport trends computed through the grain‐size trend analysis approach, more realistic and pertinent results can be obtained by applying an exclusive OR operation (XOR case) on the vectors. The main results presented here highlight a trend towards the accumulation of carbonate sands in the back‐reef area of the fringing reef. These sediments can only be resuspended during extreme events such as storms or tropical cyclones.     

Controls on platform‐scale patterns of surface sediments,shallow Holocene platforms,Bahamas

Isolated carbonate platforms occur throughout geological history, and commonly exhibit considerable spatial variability. To evaluate the controls on the nature of sediment accumulation across the expansive, shallow platform tops, this study systematically compares and contrasts patterns in surface sediments from several shallow (<10 m) Holocene Bahamian examples. Remote‐sensing data, field observations, petrographic characterization and quantitative grain‐size analyses reveal the spatial patterns of sediment accumulation on Crooked–Acklins Platform and the Berry Islands Bank. Integration of these data with synoptic observations of waves, tides and currents, along with regional geochemical data, provides a means to explore the factors that influence platform‐scale sedimentary patterns. These data illustrate that the platform interiors of both Crooked–Acklins Platform and Berry Islands Bank are blanketed with medium to coarse sand size sediment. Peloids are most common in the interior of Crooked–Acklins Platform, whereas the Berry Islands Bank includes more abundant composite grains. In both areas, very little mud is present, with surface sediments averaging <2% mud. Comparison of these results with published data from Little Bahama Bank, Great Bahama Bank and Caicos Platform suggest that, contrary to previous interpretations, the presence of open margins and/or brisk winds are not necessary for the occurrence of a platform top with little mud. Although the muddy sediment fraction of the interior can be suspended by elevated wave energy, wind‐generated current speeds in protected platform interiors are relatively low. Instead, in parts of the platform interiors, transport and winnowing of fines is enhanced greatly by tidal currents, which carry suspended sediments off the shallow platforms, even if shielded by islands. Beyond physical influences, however, regional geochemical compilations suggest that the Bahamian tides supply highly supersaturated waters rich in dissolved oxygen to these platform interiors. This exchange is interpreted to facilitate favourable conditions for calcium carbonate precipitation in the form of ooids, marine cements and hardened peloids across vast expanses of the platform interiors. Such fundamental controls on Holocene platform‐scale sediment dynamics are likely to have influenced carbonate systems through the geological record.     

The significant role of sediment bio-retexturing within a contemporary carbonate platform system: Implications for carbonate microfacies development

Assessments of carbonate platform reef–lagoon sediments and benthic habitats around Rodrigues Island (south-west Indian Ocean) have been undertaken in order to examine carbonate sediment textural properties and the controls on texturally-defined sediment fabrics. Reef–lagoon sediments, sampled from across the expansive (~ 8 km wide) carbonate-dominated windward platform, principally comprise poorly sorted medium- to coarse-grained bioclastic sands, composed of a low diversity of grain constituents — predominantly non-geniculate coralline algal bioclasts. Despite a marked homogeneity in sediment compositional and grain size properties, eight distinct sediment textural groups can be identified that form a heterogeneous mosaic across the contemporary reef–lagoon system. Only along the narrow outer platform margins (reef crest, sand apron and outermost lagoon environments) do we observe consistent (predictable) transitions in sediment textural groups, where physical processes are the primary drivers of selective sediment transport and sorting. In contrast, across the main expanse of the lagoon, the sediment substrates are characterised by an irregular mosaic of texturally-defined sediment groups — formed primarily as a function of sediment bio-retexturing. The burrowing activities of alpheid and callianassid shrimps are particularly important in this respect and impart a distinctly unique textural fabric to the upper sediment horizons in the environments in which the respective organisms occur. The consequence of this is that, at the platform system scale, individual, texturally-defined sediment groups are relatively poor indicators of prevailing hydrodynamic regimes or of local sediment production, reflecting more the biological action of organisms inhabiting the substrate. This has important implications for understanding the development of carbonate sediment fabrics in the context of palaeoenvironmental reconstructions and for interpreting a key diagnostic criteria of carbonate microfacies.     

Magnetic tracing of sediment dynamics of mudbanks off southwest coast of India

In this study, we utilized environmental magnetic in combination with sedimentological and hydrodynamic data to investigate the formative processes of mudbanks along southwest coast of India. We document the linkages between enrichment of silt-sized magnetic particles and formation processes of mudbanks along Alappuzha coast. A trend of increasing magnetite concentration and coarsening in magnetic grain size is observed at mudbank stations M2 and M3, while the mud-deficient station (M1) showed an opposite trend. A strong relationship between magnetic and physical grain size for all samples implies that the magnetic particle size and clastic grain size are largely adjunct. Analysis of rock magnetic and grain size data of surficial and suspended sediments from non-mudbank (M1) and two mudbank stations (M2, M3) reflect the differential sediment partitioning and transport regimes which controlled the formation of mudbanks along Alappuzha coast. Two plausible mechanisms responsible for the formation of mudbanks are identified: grain size-selective entrainment is the dominant process during pre-monsoon; weaker hydrodynamics (waves and bottom currents) favors accumulation of silt-sized (fine and coarse) magnetic and non-magnetic fractions resulting in the formation of magnetically low-enriched sediment bed of mixed grain sizes. At the onset of monsoon, wave-induced energetic bottom currents enhance the suspension of entire sediment bedload at stations M2, M3 to form fluid mud. Concurrently, mineral-density-based selective fractionation allows the settling of coarse silt-sized magnetic particles, while the fine magnetic silt-size particles accumulate forming thick fluid mud as a suspension load resulting in the formation of mudbanks. An observed increase in magnetic susceptibility and coarsening in magnetic grain size of surficial sediments at mudbank stations (M2, M3) during monsoon period supports the interpretation. Our findings are summarized in a conceptual model which can be very well applied to investigate sediment dynamics associated with mudbank formation in coastal and shelf sedimentary systems.     

Distribution of Rare Earth Elements in Early Permian Reef-Island Ocean Sediments in Eastern Kunlun

Based on the study of stratigraphy and fossils, the Early Permian ocean in eastern Kunlun is recognized as a kind of reef-island ocean, in which there exist many different kinds of sediment, including patch carbonate platform, reef fades, transitional facies and deep basin sediments. It has been found that the total contents of REEs increase gradually from carbonate platform facies to deep basin facies. Meanwhile, sediments of different facies have different REE distribution patterns and different Ce anomalies. Most of the sediments of patch carbonate platform facies or reef facies are characterized by extremely negative Ce anomalies or moderately negative Ce anomalies (Ce/Ce*=0.33 to 0.55), and medium or thin-bedded limestones of transitional facies by moderately negative Ce anomalies (Ce/Ce*=0.49 to 0.60). However, sediments of deep basin facies show weak or no negative Ce anomalies (Ce/Ce*=0.69 to 1.47), among which the value of Ce/Ce* in the radiolarian chert is 1.47.     

Late Holocene island reef development on the inner zone of the northern Great Barrier Reef: insights from Low Isles Reef

A sedimentological and stratigraphic study of Low Isles Reef off northern Queensland, Australia was carried out to improve understanding of factors that have governed Late Holocene carbonate deposition and reef development on the inner to middle shelf of the northern Great Barrier Reef. Low Isles Reef is one of 46 low wooded island-reefs unique to the northern Great Barrier Reef, which are situated in areas that lie in reach of river flood plumes and where inter-reef sediments are dominated by terrigenous mud. Radiocarbon ages from surface and subsurface sediment samples indicate that Low Isles Reef began to form at ca 3000 y BP, several thousand years after the Holocene sea-level stillstand, and reached sea-level soon after (within ～500 years). Maximum reef productivity, marked by the development of mature reef flats that contributed sediment to a central lagoon, was restricted to a narrow window of time, between 3000 and 2000 y BP. This interval corresponds to: (i) a fall in relative sea-level, from ～1 m above present at ca 5500 y BP to the current datum between 3000 and 2000 y BP; and (ii) a regional climate transition from pluvial (wetter) to the more arid conditions of today. The most recent stage of development (ca 2000–0 y BP) is characterised by extremely low rates of carbonate production and a dominance of destructive reef processes, namely storm-driven remobilisation of reef-top sediments and transport of broken coral debris from the reef front and margins to the reef top. Results of the present study enhance existing models of reef development for the Great Barrier Reef that are based on regional variations in reef-surface morphology and highlight the role of climate in controlling the timing and regional distribution of carbonate production in this classic mixed carbonate–siliciclastic environment.     

南流江河口水下三角洲表层沉积物端元分析及其沉积动力环境意义

沉积物粒度分布特征可揭示沉积动力环境。基于广西南流江河口水下三角洲的粒度分析数据,采用Folk分类法进行分类,应用端元分析对其粒度数据进行分解,探讨了南流江河口水下三角洲的物源和现代沉积动力环境。结果表明,研究区表层沉积物可划分为9类,其中含砾沉积物5类,不含砾沉积物4类,沉积物类型与河口环境动力特征相吻合。端元分析结果显示,研究区有5个不同类型的端元,分别代表着5种类型的沉积物动力环境:端元1反映了风浪和潮流对表层沉积物的动力作用;端元2指示了外海涌浪对表层沉积物的作用过程;端元3代表了南流江冲淡水引起的表层沉积物的输运;端元4代表了大陆架残留沉积物;端元5为干扰端元,可能是由于工程建设等人类活动所引起的。利用端元分析探讨复杂环境(如小型山溪性河口)下的沉积动力环境时,一方面需要综合考虑研究区域的物源与环境动力特征;另一方面可以结合参数拟合、粒径趋势分析等研究手段。     

Tepee-shaped agglutinated microbialites: an example from a Famennian carbonate platform on the Lennard Shelf, northern Canning Basin, Western Australia

Distinctive, metre‐scale antiformal structures are well developed in a Famennian carbonate platform in the Chedda Cliffs area of the Lennard Shelf reef complexes. The structures are distinguished by chevron‐shaped crests and thickened cores and contain abundant non‐skeletal allochems (ooids/pisoids, peloids and intraclasts) of silt to pebble size and variably developed laminations and fenestrae. The internal morphology and pervasive occurrence of fenestral clotted and wavy laminated fabrics suggest that these structures are microbial mounds composed of agglutinated stromatolites and thrombolites. Microbial fabrics most probably originated through sediment trapping and binding by microbial mats with early lithification involving microbial calcification and cementation of trapped sediment. The facies and stratigraphic context of the mounds support a shallow subtidal, transitional backreef to reef‐flat setting; however, alone these mounds do not provide unequivocal environmental information. Other large antiformal structures in Famennian platforms on the Lennard Shelf, previously described as tepee structures, show morphological similarities to the Chedda Cliffs mounds, which suggests that these other structures may also be microbial mounds. The presence of microbial mounds in platform successions further highlights the importance of microbial communities in the Lennard Shelf reef complexes.     

塔中Ⅰ号坡折带奥陶系礁滩复合体发育动力学及其控储机制

塔中Ⅰ号坡折带上奥陶统良里塔格组发现了我国珊瑚-层孔虫造礁群落最早的礁体,在生态演替上具承先启后重要特征,并伴随着发育了多套礁滩相储集体。充分利用古地理、古地磁,同位素、微量元素,包裹体,岩心物性分析等数据,分析了塔里木板块奥陶纪成礁环境,特别是从构造地质学、碳酸盐岩地质学、层序地层学、古生态学、古生物学出发,剖析了塔中隆起碳酸盐岩台地演化的阶段性、海平面变化的周期性,以及奥陶纪造礁生物生态群落的演替及生物生长性,分别建立了良里塔格组各个时期礁滩复合体发育的模式,深入解析了礁滩复合体多期生长发育的动力学特征。塔中良里塔格组3期叠加发育的礁体生长与台地演化的构造阶段性密切相关,同时控制了礁滩复合体微相类型上的多样性和分异性。礁滩复合体地貌隆起和高频海平面变化所导致的暴露,以及准同生期大气淡水淋滤、溶蚀作用是控制台缘礁滩体优质储层发育的动力学基础,经后期埋藏过程中所经历的表生岩溶作用、埋藏期热水岩溶作用以及断层相关岩溶等是该礁滩复合体储层物性改良主控因素。集中揭示了塔中奥陶纪礁滩复合体发育演替几何学、动力学机制及礁滩储集体控储机制,有效指导了塔中古隆起奥陶系生物礁油气规模效益开发,对于类似油气田勘探与学科发展具有积极意义。     

Organic fades in Recent sediments of carbonate platforms: Southwestern Puerto Rico and Northern Belize

Our organic facies model of a platform environment is based on the analysis of (1) total organic carbon, (2) visual protokerogen types and hydrogen indices of the sediments, (3) sediment mineralogy and grain size, and (4) reef patterns, water circulation, and relief of the adjacent land.TOC values are lowest on reefs and outer platforms (0.2%); intermediate in sediments fringing carbonate islands (0.5–2.0%), on sea-grass meadows (0.5–0.7%), and in forereef lagoons (0.1–0.7%); highest in inner platform lagoons (0.5–5.0%) and mangrove swamps (2.7–3.5%). Paniculate organic matter comprises structured terrestrial, marine, and amorphous types. On average, terrestrial material accounts for 28% of OM in Puerto Rico and 12% in Belize. The mean hydrogen index of the sediment is 355 (mg hydrocarbons/g TOC) in Puerto Rico and 719 in Belize.This study indicates that TOC values are mainly related to platform subenvironment, while OM types are controlled by distance from the shoreline. Organic facies dominated by structural terrestrial OM occur closer to the shore, whereas toward the shelf margin marine and amorphous OM contents increase.     

塔中地区良里塔格组台缘颗粒滩沉积特征及分布规律

塔中地区良里塔格组礁滩复合体蕴含着丰富的油气资源。通过钻井、录井、岩芯、薄片及测井等大量资料的综合分析,结合前人研究结果,对塔中I号坡折带上奥陶统颗粒滩类型和特征进行了系统研究。塔中地区良里塔格组台缘颗粒滩包括内碎屑滩、生屑滩、核形石滩、鲕粒滩等类型,具有类型多、发育旋回多以及厚度大的特点。颗粒滩在纵横向上并非是孤立存在的,通常与生物礁伴生,形成纵向叠置,横向指状交叉的礁滩复合体,属油气储集的有利相带。台缘颗粒滩亚相沿塔中I号坡折带呈带状展布,自西向东分布范围逐渐变窄,表现出以（藻）砂屑滩为背景,其他类型颗粒滩呈条带状、块状、点状分布于其间的规律。     

Storm‐dominated sedimentation in a protected basin fringed by coral reefs,Nara Inlet,Whitsunday Islands,Great Barrier Reef,Australia

Hydrodynamic data and samples of bed sediment were collected from Nara Inlet, a small incised embayment in the Whitsunday Islands, central Great Barrier Reef. Measured tidal currents in the inlet do not exceed 0.2 ms^–1 even at spring tides. Swell waves dominate much of the inner shelf of the Great Barrier Reef but are absent from in the inlet due to the presence of a fringing reef at the inlet mouth. On the silty sand floor of the inlet, particle size decreases towards the inlet head. Most of the bed is too coarse to be remobilised by fair‐weather wave and tides, and we predict that bedload sediment transport thresholds are only exceeded in the inlet during cyclones. The observed distribution of bed sediments is consistent with landward dispersal of sediment under storm conditions. Over 20 m of (presumably Holocene) sediments occurs in the inlet and the seismic character of the infill is consistent with the observed textural variation of the modern sediments. We infer that sediment accumulation on the floor of the inlet has been storm dominated throughout much of the Holocene.     

Sea-level related resedimentation processes on the northern slope of Little Bahama Bank (Middle Pleistocene to Holocene)

Middle Pleistocene to Holocene sediment variations observed in a 26 metre long core taken during a cruise of the RV Marion Dufresne are presented. Core MD992202 was retrieved from the northern slope of Little Bahama Bank and provides an excellent example for sedimentation processes in a mid‐slope depositional environment. The sediment composition indicates sea‐level related deposition processes for the past 375 000 years (marine isotope stages 1 to 11). The sediments consist of: (i) periplatform ooze (fine‐grained particles of shallow‐water and pelagic origin) with moderate variations in carbonate content, carbonate mineralogy and grain‐size; and (ii) coarser intervals with cemented debris consisting of massive, poorly sorted, mud‐supported or clast‐supported deposits with an increased high‐magnesium calcite content. During interglacial stages (marine isotope stages 1, 5, 7, 9 and 11) periplatform oozes (i) are characterized by higher aragonite contents, finer grain‐size and higher organic contents, whereas during glacial stages (marine isotope stages 2 to 4, 6, 8 and 10), increased low‐magnesium and high‐magnesium calcite values, coarser grain‐size and lower organic contents are recorded. These glacial to interglacial differences in mineralogy, grain‐size distribution and organic content clearly show the impact of climatically controlled sea‐level fluctuations on the sedimentation patterns of the northern slope of Little Bahama Bank. The coarser deposits (ii) occur mainly at the transitions from glacial to interglacial and interglacial to glacial stages, and are interpreted as redeposition events, indicating a direct link between sediment properties (changes in mineralogy, grain‐size distribution, variations in organic contents) and sea‐level fluctuations. Changes in hydrostatic pressure and the wave base position during sea‐level changes are proposed to have triggered these large‐scale sediment redepositions.     

Tectonic, eustatic and environmental controls on mid-Cretaceous carbonate platform deposition, south-central Pyrenees, Spain

Cenomanian–Turonian strata of the south‐central Pyrenees in northern Spain contain three prograding carbonate sequences that record interactions among tectonics, sea level, environment and sediment fabric in controlling sequence development. Sequence UK‐1 (Lower to Upper Cenomanian) contains distinct lagoonal, back‐margin, margin, slope and basin facies, and was deposited on a broad, flat shelf adjacent to a deep basin. The lack of reef‐constructing organisms resulted in a gently dipping ramp morphology for the margin and slope. Sequence UK‐2 (Upper Cenomanian) contains similar shallow‐water facies belts, but syndepositional tectonic modification of the margin resulted in a steep slope and deposition of carbonate megabreccias. Sequence UK‐3 (Lower to Middle Turonian) records a shift from benthic to pelagic deposition, as the shallow platform was drowned in response to a eustatic sea‐level rise, coupled with increased organic productivity. Sequences UK‐1 to UK‐3 are subdivided into lowstand, transgressive and highstand systems tracts based on stratal geometries and facies distribution patterns. The same lithologies (e.g. megabreccias) commonly occur in more than one systems tract, indicating that: (1) the depositional system responded to more than just sea‐level fluctuations; and (2) similar processes occurred during different times throughout sequence development. These sequences illustrate the complexity of carbonate platform dynamics that influence sequence architecture. Rift tectonics and flexural subsidence played a major role in controlling the location of the platform margin, maintaining a steep slope gradient through syndepositional faulting, enhancing slope instability and erosion, and influencing depositional processes, stratal relationships and lithofacies distribution on the slope. Sea‐level variations (eustatic and relative) strongly influenced the timing of sequence and parasequence boundary formation, controlled changes in accommodation and promoted platform drowning (in conjunction with other factors). Physico‐chemical and climatic conditions were responsible for reducing carbonate production rates and inducing platform drowning. Finally, a mud‐rich sediment fabric affected platform morphology, growth geometries (aggradation vs. progradation) and facies distribution patterns.     

The intrinsic effect of shape on the retrogradation motif and timing of drowning of carbonate patch reef systems (Lower Frasnian,Bugle Gap,Canning Basin,Western Australia)

The evolution and architecture of a set of retreating Lower Frasnian patch reef outcrops in the Canning Basin of Western Australia were evaluated, and their depositional and stratigraphic contacts spatially recorded using digital surveying tools. The geological data, together with high‐resolution digital elevation models, were assembled in three‐dimensional visualization and modelling software and subsequently used for building two‐dimensional surface models and three‐dimensional volumetric models. Numerical data on geometry and shape were extracted from these models and used to quantitatively assess the retrogradation motif of patch reef development. The development of the patch reefs comprises three stages. During stages 1 and 2, the patch reefs exhibited an overall retrogradational escarpment‐type configuration displayed by, on average, 60° steep reef‐margin walls that lacked the support of coeval slope deposits. The subdivision between stages 1 and 2 is based on minor backstepping reducing less than 10% of the platform‐top area. The onset of stage 3 is recognized by stromatolite development fringing reef‐margin walls. During stage 3 an aggrading accretionary reef‐margin developed, comprising allochthonous and autochthonous slope deposits. Both types of slope deposit onlap the previous stages and are distributed unevenly with allochthonous slope deposits being noticeably absent around the smaller and more elongate patch reefs. The variation in distribution of slope sediment type can be explained by the amount, linked to platform size, of platform‐top shedding. Small patch reefs were unable to fill the available accommodation adjacent to escarpments with allochthonous slope sediments and were thus encroached by autochthonous slope sediments. The variation, which cannot be explained by the size difference in the platform‐top factory, has been related to the difference in perimeter length. For patch reefs with similar platform‐top production areas, a more elongate patch reef inherits a longer perimeter and a proportionally smaller volume of allochthonous slope sediment per margin length will be transported to the flanks. Thus, the more elongate patch reef intrinsically contained more sites within which autochthonous slope sediments developed. Digital outcrop modelling and numerical evaluation of the evolution of the patch reefs revealed the major differences in retrogradation motif. The quantified variations in progressive decline of platform‐top area with height were confirmed by hypothetical decline curves for ellipse‐shaped carbonate systems for which aspect ratio (ratio between length and width) varied. This mathematical model demonstrates that the progressive decline of the production area is highly sensitive to shape and can be used to numerically assess and predict the relative timing of drowning, i.e. when the platform‐top production area becomes nil, of retrogradational isolated carbonate platforms that are controlled by high accommodation. Wider implications can be surmised for highstand systems tracts and prograding carbonate systems. For example, for equally sized platforms with hypothetically similar carbonate factories and identical external forces, the potential to prograde by platform‐top shedding is higher with a smaller aspect ratio because the shorter perimeter implies less accommodation space needing to be filled up to commence slope progradation. Clearly, there are intrinsic effects of shape on the development of carbonate platform systems.     

Three-dimensional modelling of an isolated carbonate buildup (Triassic, Dolomites, Italy)

The Middle Triassic carbonate buildups of the Dolomites show facies similarities with mud mounds but display apparent architectural elements of flat‐topped carbonate platforms. In order to test whether the facies similarities to mounds are also reflected in the internal buildup architecture, a three‐dimensional modelling study of the Middle Triassic Monte Cernera buildup has been carried out. The Cernera buildup exhibits apparent geometries suggesting a mounded platform in the lower and uppermost part of the buildup, separated by an interval with apparent platform geometry and a retrogradational platform interior, which is difficult to explain with a flat top platform model. For this purpose, a number of three‐dimensional models were constructed using the three‐dimensional modelling programme petrel ^TM. Key geological horizons were constructed based on outcrop measurements, intermediate horizons were calculated in the modelling program, and the intersections of the modelled layers with a digital topography surface were displayed and compared with outcrop photographs. The models were refined stepwise until a best fit with the actual bedding architecture was achieved. The best fit model shows that the mounded geometries in the lower and uppermost part of the buildup are real architectural elements. The intermediate platform stage, about 1·5 km across, had probably retained a mounded top with a relief of up to 50 m, which is difficult to distinguish from an absolutely flat top, but necessary to explain the retrogradational platform interior. The present study shows that Monte Cernera was dominated by mounded geometries at all stages of platform development. The mounded geometry plus facies data suggest that the platform is a deep‐water accumulation, below the zone of intense wave energy, but within the photic zone. The Cernera represents a tropical buildup type, which did not have the capacity to grow into continuously wave‐swept environments because of the small size and the absence of a wave‐resistant energy barrier. Such buildup types are probably common after major crises in earth history, when reef organisms were virtually absent.     

Controls of heavy minerals and grain size in a holocene regressive barrier (Ilha Comprida,southeastern Brazil)

Ilha Comprida is a regressive barrier island located in southeastern Brazil that was formed essentially by Quaternary sandy sediments. Ilha Comprida sediments were analyzed to assess heavy mineral indices and grain size variables. The spatial variation of heavy minerals and grain size was interpreted in terms of the present barrier dynamics and the barrier’s evolution since the Middle Holocene. These analyses allowed for the identification of the main factors and processes that control the variation of heavy minerals and grain size on the barrier. Rutile and zircon (RZi) and tourmaline and hornblende (THi) are significantly sensitive to provenance and exhibit the contributions of the Ribeira de Iguape River sediments, which reach the coast next to the northeastern end of Ilha Comprida. In addition to the influence of provenance, TZi responds mainly to hydraulic sorting processes. This agrees with a sediment transport pattern characterized by a divergence of two resultant net alongshore drifts southwest of the barrier. The sediments from the Ribeira de Iguape River reach the barrier directly through the river mouth and indirectly after temporary storage in the inner shelf. The combination of grain size and heavy mineral analyses is a reliable method for determining sediment transport patterns and provenance.     

Spatial variations in grain size distribution and selected metal contents in the Xiamen Bay,China

The present work attempts to describe the spatial distribution of heavy metals in the surficial sediments of the Xiamen Bay influencing by the characters of the sediments. In this paper, the distribution of trace metals, as well as the main properties (i.e. grain size, TOC, AVS) based on surface sediments collected at 40 stations across the Xiamen Bay was done. The relation of the sediment properties to heavy metal concentrations was evaluated with Pearson’s correlation analysis. According to the grain size distribution, cluster analysis is used to classify the sampling stations into groups regarding the depositional environment. Basing on the data, the hydrodynamic feature and the contamination of surface sediment by selected metals were discussed. The results show that the distribution of various types of sediments and the variations of their concentrations of heavy metals are controlled largely by the hydrodynamic conditions. As for the surface sediments, Xiamen West Harbour shows a serious situation with Arsenic, Pb and Zn.     

潮滩表层沉积物磁性特征的季节性变化及其对沉积动力作用的响应:以杭州湾北岸为例

根据2004年9月—2005年8月杭州湾北岸中潮滩固定观测点重复采集的表层沉积物的磁学参数和粒度分析,探讨了研究区域潮滩沉积物磁学性质的季节性变化及其对沉积动力的响应。 结果表明, 该观测点潮滩沉积物磁学参数存在显著的月际变化,2004年12月底—2005年5月上旬的半年中,SIRM、χARM、χfd%、χARM/χ、χARM/SIRM等参数出现高值,指示了沉积物中含有较多的细晶粒亚铁磁性矿物,对应于潮滩淤积时段和较细的沉积物粒级组成,其余各月上述磁参数值较低,对应于潮滩沉积物较粗的侵蚀期。上述结果表明,沉积物的磁性特征可以很好地反映潮滩冲淤过程中沉积动力的变化。