A skeletal assemblage classification system for non-tropical carbonate deposits based on New Zealand Cenozoic limestones

Cenozoic limestones in New Zealand are mainly skeletal grainstones and packstones formed under non-tropical climatic conditions in open marine shelf or ramp environments. Following petrographic analysis of the nature and abundance of the skeletal components in nearly 500 samples of these limestones, a complete linkage cluster analysis identified seven major skeletal assemblages that may be regarded as subdivisions of the single foramol skeletal association defined by Lees and Buller (1972) for temperate-region carbonate deposits. The seven assemblages are given contracted names, as follows: (a) BARNAMOL = barnacle/bivalve-dominated; (b) BIMOL = bivalve-dominated; (c) BRYOMOL = bryozoan/bivalve-dominated; (d) ECHINOFOR = echinoderm/benthic foraminiferal-dominated; (e) NANNOFOR = nannofossil/planktonic foraminiferal-dominated; (f) RHODALGAL = calcareous red algal-dominated; and (g) RHODECHFOR = calcareous red algal/echinoderm/benthic foraminiferal-dominated. A composite triangular classification diagram has been devised for naming the skeletal assemblage of an unknown sample on the basis of its three main skeletal components. The diagram successfully characterises more than 85% of the New Zealand Cenozoic limestone samples and also appears to be applicable for the skeletal assemblage designation of many overseas examples of non-tropical carbonate deposits. Limitations relate mainly to locally common skeletal types (e.g. serpulids, brachiopods) that are presently not incorporated into the New Zealand-based scheme. The general ecological preferences of the main skeletal contributors in each of the seven skeletal assemblages form a basis for relating the assemblages to broad shelf habitats. Consequently, as well as the benefits of providing a more consistent skeletal assemblage terminology for comparative studies between different workers, the scheme can assist with the paleoenvironmental interpretation of non-tropical skeletal carbonate facies.     

Non-tropical carbonate deposits on the modern New Zealand shelf

Extensive (ca. 50,000 km²) shallow-marine platforms (< 250 m) off northern (34°S) and southern (48°S) New Zealand, and more local areas of shelf between, are blanketed by skeletal carbonate sediments > 70% CaCO₃), despite proximity to a tectonically active plate margin. In these regions the terrigenous sediment supply is presently low, and growth of epibenthos is fostered by firm substrates (rock, gravels, shells, seaweeds) and the generally energetic nature and high nutrient levels of open-shelf waters. Rapid transition into adjacent terrigenous-dominated facies is characteristic. Irrespective of water depth, the carbonates are coarse-grained and fragmental; carbonate mud is rare. Calcite dominates over aragonite. High-Mg calcite, widespread off northern New Zealand, is rare in the south. Skeletal material is dominated by bryozoans and bivalve molluscs, with significant local contributions from foraminifers, barnacles, calcareous red algae and echinoderms. The name bryomol is suggested for this distinctive temperate-region skeletal carbonate facies, which can be usefully subdivided based on dominant zoarial growth forms of the bryozoan component, known to be habitat-related. Bioerosion is an important mechanism of skeletal fragmentation and degradation. Many grains, especially aragontic bivalves, are infested by endolithic borers and have low preservation potential. Ages of skeletal material in the surficial deposits range from more than 20,000 years B.P. to modern, which is consistent with both low rates of carbonate production and sediment accumulation, and the wide range in preservation state of grains. Some data suggest that the skeletal carbonates are dispersed and mixed mainly during infrequent movement of sand ribbons, sand waves and sand sheets driven by storm-assisted tidal flows. Tracts of modern, palimpsest and relict carbonates can occur in juxtaposition.

The facies characteristics of the New Zealand shelf carbonate deposits contrast significantly with those of the classical Bahaman-type carbonate model. However, they are similar to those reported from many other mid- to high-latitude carbonate shelves, and afford good analogues for most onland occurrences of New Zealand Cenozoic limestones.     

Barnacle-dominated limestone with giant cross-beds in a non-tropical, tide-swept, Pliocene forearc seaway, Hawke's Bay, New Zealand

Pliocene, non-tropical, widespread and locally thick (up to 100 m) limestones occur in Hawke's Bay, eastern North Island, where they are intimately associated with very thick ( > 5 km), terrigenous-dominated, Neogene sequences that formed in a tectonically active convergent margin setting. The non-tropical character of the limestones is shown unequivocally by (1) the complete dominance of skeletal calcarenites and calcirudites, (2) the occurrence of oyster banks as the only in situ organic structures, (3) the dominance of barnacles, epifaunal molluscs, bryozoans, echinoderms, foraminifers, brachiopods and calcareous red algae as skeletal components, and (4) the preponderance of calcite over aragonite in the mineralogy of the skeletal grains and cements. The abundance of barnacle fragments in the limestones, and the related exclusive occurrence of only one major organic association, a barnacle-(epifaunal) bivalve-bryozoan assemblage, is striking and unusual given the extent of the limestones. Pecten and oyster valves acted as substrates for barnacle attachment, and their growth was promoted by strong tidal paleocurrents that swept the depositional setting: a long (450 km), narrow (30–50 km) forearc basin seaway, which formed between an actively deforming subduction complex to the east and an uplifting structural ridge to the west. Synsedimentary deformation promoted limestone formation on the margins of the seaway by creating current-swept, clastic-free submarine ridges that acted as the sites of carbonate production. Tidal flows dispersed the carbonate constituents and organised them into a wide spectrum of tide-influenced, cross-bedded and horizontal structures. Most spectacular are occurrences of giant tabular cross-beds, with sets 10–40 m thick and foreset dips of 7–36°, some interpreted as the deposits of major sand bars on carbonate deltas marginal to the mouths of saddles traversing the rising antiforms, and others analogous to modern linear sand ridges. The small- to large-scale planar and trough cross-beds, and the horizontal and lenticular beds that are invariably associated with the giant cross-beds and dominate most sections, represent mainly the deposits of sand waves and sand sheets at inner- to mid-shelf depths in the seaway.     

Methane-derived high-Mg calcite submarine cement in Holocene nodules from the Fraser Delta, British Columbia, Canada

Carbonate nodules and slabs in late Holocene shelly terrigenous deposits of the modern Fraser River delta (～49°N) are formed close to the seafloor by precipitation from saline pore waters of mainly fibrous to bladed crystals of high-Mg (～ 10–20 mol% MgCO₃) calcite cement as coalescing isopachous crusts on grains. Previous reports that the cement is low-Mg calcite are not supported by this study. Highly negative δ¹³C values of ? 7 to ? 59‰ for the cements indicate that the bulk of their carbonate carbon was derived from the microbiological degradation of organic matter in the deltaic deposits during shallow burial. In particular, the production of biogenic methane (CH₄) by anaerobic bacterial fermentation, its upward migration, chemical or biological oxidation to CO₂ and neutralization in the near-surface sediment, and diffusion to microenvironments relatively enriched in organic components, are a possible set of conditions influencing the process and sites of carbonate cementation. Methane-derived Mg-calcite appears also to be the major submarine cement in several other modern occurrences of lithified shallow-water terrigenous sands and muds at non-tropical latitudes.     

Seagrass development in terrigenous-influenced inner ramp settings during the middle Eocene (Urbasa–Andia Plateau,Western Pyrenees,North Spain)

Since their first occurrence in the late Cretaceous, seagrasses have played a major role in carbonate production and sedimentation across shallow-water and nearshore environments, sustaining a prolific carbonate factory and contributing to sediment accumulation through the combination of baffling and trapping effects. Most reported Palaeogene seagrass occurrences developed in oligo?mesotrophic shallow warm-water habitats and are characterized by distinct associations of small and larger benthic foraminifers adapted to low terrigenous influence. This study describes a number of seagrass episodes interbedded in the Bartonian (middle Eocene) of San Fausto–Lazkua area (Navarra region, North Spain), within a nearshore to inner-ramp succession that, in spite of being deposited under general transgressive conditions, was highly influenced by terrigenous supply from the adjacent land. Up to twelve different seagrass bed intervals occur interbedded in a cyclical manner with high-energy nearshore siliciclastics and inner ramp bioclastic carbonates rich in mesophotic?oligophotic foraminifers and heterozoan biota (red algae, echinoderms, bryozoans). Seagrass deposits exhibit typical unsorted textures, abundant bioturbation and moderate to high terrigenous content, and comprise a characteristic skeletal association of epiphytic foraminifers, red algae and, most particularly, of abundant encrusting acervulinids, commonly with distinct hooked and tubular growth forms. This abundance of suspension-feeders relative to autotrophs and mixotrophs may be indicative of temperate waters, although the taxonomic diversity of the foraminiferal assemblages in both seagrass and non-seagrass embedding deposits supports the interpretation of shallow, warm-water conditions. The studied seagrass deposits provide evidence that high siliciclastic supply and associated nutrient input may determine the occurrence of temperate-like seagrass deposits in warm-water settings, analogous to extensive heterozoan carbonate production in modern shallow-tropical environments. Thus, the identification and correct interpretation of past seagrass-vegetated environments are crucial for reconstructing palaeoecological conditions in ancient shallow-marine environments. Therefore, in comparison with carbonate-dominated environments, the mixed terrigenous?carbonate seagrass deposits are volumetrically less important, presenting a more irregular, patchy distribution, and a skeletal assemblage dominated by heterotrophs, regardless of the water temperature.     

Cenozoic algal biostromes in the eastern Veneto (northern Italy): a possible example of non-tropical carbonate sedimentation

The rhodolith-bearing biostromes described in this paper form part of an episode of exclusively carbonate sedimentation, restricted in time (Aquitanian) but relatively extensive in space, within a molasse sedimentation basin. The biostromes correspond to an algal biocoenosis on a bar or structural high, are of cyclic character, and make up a minor sequence within the Miocene molasse megasequence.

The foramol skeletal assemblage, paleogeographic conditions of the area, and resemblance of the deposits to other documented mid-latitude limestones suggest that the biostromes are examples of non-tropical carbonate sedimentation.

Trace element contents (Mg, Sr, Mn, Fe) show two distinct diagenetic phases. The first was due to active circulation of oxygenated solutions in a phreatic marine environment, the second to poor circulation of reducing solutions in a fresh-water phreatic environment.     

碳酸盐台地的类型、特征和沉积模式--兼论华北地台寒武纪陆表海-淹没台地的沉积样式

随着对现代碳酸盐沉积环境的系统调查和解释、以及对碳酸盐沉积原理认识的深化,自20世纪60年代,一系列碳酸盐沉积相模式得以建立,其中最引人注目的是Wilson和Tucker的工作。但在镶边陆棚及缓坡模式得到了广泛认可和使用的同时,对陆表海和淹没台地型沉积未能予以足够的重视。与过去相比,现今的海平面是相对较低的,因而没有出现陆表海广泛发育的情况。但在漫长的地质历史时期,陆表海曾经覆盖了广泛的克拉通区域,是碳酸盐沉积最重要的场所之一。本文在系统总结前人成果的基础上,将用于描述大尺度碳酸盐岩沉积环境的碳酸盐台地分为镶边陆棚、缓坡、陆表海、孤立台地和淹没台地5种类型分别描述,并重点强调了陆表海和淹没台地的沉积模式。华北地台寒武系大面积分布的潮坪沉积、鲕粒滩相灰岩和竹叶状风暴砾屑灰岩,以及频繁出现的台地淹没事件,为阐明陆表海和淹没台地的沉积提供了绝佳实例。这些实例和研究表明了碳酸盐沉积环境的多样性和沉积过程的复杂性,以及将今论古的困难性,从而为碳酸盐沉积原理的认识和沉积环境的解释提供新认识和新思路。     

Secular changes in colony‐forms and bryozoan carbonate sediments through geological history

Ever since their first radiation in the Ordovician, bryozoans have contributed significantly to carbonate sedimentation. Most of the numerous colony‐forms developed by bryozoans have evolved repeatedly in different taxonomic groups and vary in their sediment‐producing potential. There are nine basic bryozoan colony‐forms: encrusting, dome‐shaped, palmate, foliose, fenestrate, robust branching, delicate branching, articulated and free‐living. The proportion of these morphotypes in bryozoan faunas period by period is shown to change significantly through the Phanerozoic. Notable patterns include: (i) steady increase in the number and proportion of encrusting species through time, interrupted by a transient drop in the Late Palaeozoic; (ii) post‐Triassic decrease in robust branching colonies; (iii) rise in the proportion of fenestrate colonies through the Palaeozoic, followed by their absence in the Triassic and Jurassic, rarity in the Cretaceous and reappearance in smaller proportions in the Cenozoic; and (iv) scarcity of articulated colonies and absence of free‐living colonies until the Cretaceous. Most Palaeozoic bryozoan sediments come from two architecturally distinct groups of colonies: (i) domal, delicate branching, robust branching and palmate; and (ii) fenestrate. The former generate coarse particles both as sediment and components of stromatoporoid‐coral reefs in the Early and mid Palaeozoic, whereas the delicate lacy fans of the latter create both prolific coarse sediment and form the cores of Late Palaeozoic deep‐water, sub‐photic biogenic mounds. Nearly all post‐Palaeozoic bryozoan sediments comprise cyclostomes and cheilostomes with many of the same growth forms but with the addition of free‐living colonies and significant numbers of articulated colonies. The latter produced sand and mud‐sized bryozoan sediment via disarticulation for the first time. In contrast to the Palaeozoic, post‐Palaeozoic bryozoans generated sediment varying more widely across the grain‐size spectrum, from mud to sand to gravel. This article highlights the need to consider evolutionary changes in carbonate‐producing organisms when interpreting facies changes through time.     

The Development of Miocene Biohermal Bryozoan Limestones of Kazantip Cape (Crimea): A New Insight

Doklady Earth Sciences - The complex of modern physical methods for studying authigenic carbonate crusts in the Miocene bryozoan bioherms of the Kazantip Cape (Kerch Peninsula) has made it possible...     

The transfer of organic signatures from bedrock to sediment

A case study in the Canadian Arctic demonstrates how an organic geochemical signature in the regional bedrock can be transferred by erosion and redeposition to younger geological formations and surface sediments. The hydrocarbon composition recorded in Laurentian (Lower Palaeozoic) carbonate bedrock was incorporated into overlying Miocene and Quaternary formations, and modern mass waste, alluvium, snow/ice, and proglacial deposits, and further distant in ice-rafted detritus. The retention of the original geological organic signature is reflected in consistent thermal maturities (hopane ratios) and environmental indicators (sterane distributions). In the modern sediments, the geological signature is variably mixed with a modern microbial organic signature reflected in high values of carbon preference index and diploptene. These data show that hydrocarbon occurrences in bedrock may be detected by the analysis of surface detritus, especially in carbonate terrains. However they also indicate that environmental signatures may be misleading if they are inherited from older geological units.     

Advances in Modern Phytolith Research

Phytolith has been widely used as a tool to reconstruct the paleoenvironment, and the investigation of modern phytolith is very crucial to the accurate interpretation of phytolith data in ancient sediments. Studies of modern process of phytolith primarily include the morphological analysis of phytolith in modern plants, and the relationships between the formation and growth of phytolith and environmental factors, as well as the transportation and taphonomy of phytolith in modern soils and sediments. The formation of phytolith in plants is controlled not only by genes but also by environmental factors, such as humidity, precipitation, temperature, CO₂ concentration, soil pH, and nutrient status, etc. The morphology, assemblages, ¹³C and ¹⁸O of phytolith in plants can respond sensitively to environmental variables. The phytolith assemblages can be affected by its taphonomy and transportation that may be different due to phytolith types and soils/sediments texture. It is necessary to investigate the phytolith morphology and types in modern plants, the relationship between its formation and environmental factors, and the impact of transportation and taphonomy on phytolith assemblages under different environmental conditions in order to promote the application of phytolith analysis to paeloenvironment reconstruction.     

桂林洞穴滴水及现代碳酸钙(CaCO3)沉积的碳同位素记录及其环境意义

经过前期(1995~2000年)及近2 a对桂林盘龙洞13个滴水点的2 个水文年的滴水和现代碳酸钙沉积的动态监测, 发现现代洞穴碳酸钙(CaCO3)沉积有两种类型: ①常年性滴水沉积碳酸盐, 其δ13C值记录了全年气候变化特征; ②季节性滴水沉积碳酸盐, 其δ13C值记录了季节性气候变化特征。现代碳酸盐沉积监测和碳同位素分析表明, 桂林盘龙洞外部峰体主要为C3植物(几乎没有C4植物), 现代沉积碳酸盐的δ13C记录显示, 在夏半年, 夏季风强、降水丰沛、生物的活动量大, 现代碳酸盐沉积量大,δ13C值则较偏负, 平均为-13.13‰; 现代碳酸盐的δ13C全年平均值为-12.23‰, 最负值达-14.5‰; 而在冬半年, 由于降水相对较少, 新沉积碳酸盐的δ13C值, 显示稍有增加(或偏正), 其δ13C值为-10‰~-11‰。此外, 当在降大雨或暴雨后(无论是在夏半年或是在冬半年), 滴水在滞后半个月或1个月后沉积形成的碳酸盐, 其δ13C值显示突然偏负, 主要反映的是降雨效应引起的CO2 效应的影响。     

中韩第五次《东北亚地壳演化》学术研讨会在北京举行

新疆天山东部广泛发育晚石炭世碳酸盐岩隆。这些岩隆主要为生物丘,少数为生物滩及与岩隆有关的生物层,其中生物丘含大量灰泥和内碎屑。基本岩石类型为泥粒状灰岩、粒泥状灰岩、粒泥状－泥粒状灰岩,少量岩石类型包括颗粒岩、骨架岩和障积岩。成岩作用是碳酸盐岩隆发展演化的重要阶段和过程,在生物丘内,已经识别出四种重要的胶结物,即球粒泥晶、纤状方解石、放射轴状方解石和粒状方解石。尽管缺失典型的造礁生物,但通过岩石类型研究和微相分析,认为碳酸盐岩隆的形成主要取决于生物作用及古环境格局。本区碳酸盐岩隆可以同欧洲瓦尔索坦的岩隆对比     

Mound‐forming cold‐water corals and bryozoans in the Early Palaeocene of Denmark

The Faxe Quarry in south‐east Denmark offers excellent exposures of Early Palaeocene, Danian deep‐water intercalated coral and bryozoan mounds that form complexes at least 40 m thick and a few kilometres wide along and over submarine highs. The coexisting coral and bryozoan mounds represent two different biogenic carbonate factories with a highly dynamic interplay during growth. The sedimentary facies, mound geometries and the density, diversity and palaeoecology of the associated benthic invertebrates and nannofossils allow recognition of six successive growth units. Unit 1 represents an outer shelf bryozoan mound belt characterized by an oligotrophic cool‐water nannofossil assemblage. Unit 2 comprises a mixed faunal assemblage of bryozoans and octocorals with an initial sparse colonization of hexacorals. The nannofossil assemblage records a decrease in diversity and an increase in warm water forms. Unit 3 marks the onset of dense colonization of the scleractinian coral Dendrophyllia candelabrum with associated low‐diversity macrofauna and nannofossil assemblages. Unit 4 represents the main coral build‐up phase with frame‐building hexacorals of Dendrophyllia and Faxephyllia associated with a high‐diversity invertebrate fauna, and relatively low‐diversity nannofossil assemblages. Unit 5 represents the late coral mound phase showing extensive lateral distribution and finally death and erosion of the coral mounds. This event was contemporaneous with a warming trend in the pelagic environment. The succeeding Unit 6 marks the burial and overgrowth of the coral mound complex by bryozoan‐rich sediments. The coral mound complex in the Faxe Quarry initiated and terminated in global nannofossil zone NP 3 and regional nannofossil zones NNT p2G–3 suggesting a mound growth duration of ca 300 kyr and a mean vertical accretion of the coral mound of 13 cm kyr⁻¹. The mound complex probably serves as the best‐exposed analogue to modern deep and cold‐water coral mounds in the North Atlantic.     

High-latitude pelmatozoan–bryozoan mud-mounds from the late Ordovician northern Gondwana platform

Mud-mound complexes identified within the early to middle Ashgill Cystoid Limestone Formation of northeastern Spain are the first fossil build-ups to be described in the high latitude north-facing margin of Gondwana. Mud-mound complexes comprise individual lenticular mounds (composed of floatstones, cephalopod-rich mudstones and cementstones), flanks and intermound deposits (including pelmatozoan packstones and floatstones). The small mounds are mainly composed of bryozoans, cystoids and crinoids, and were developed on outer ramp environments. Mound initiation depended upon the stabilization and colonization of densely packed lenses of pelmatozoan-rich sediments. In a mid-ramp setting, pelmatozoan–bryozoan meadows were episodically degraded by common wave- and storm-induced processes, the development of semi-consolidated substrates, and the periodic influx of terrigenous material. Finally, during the Hirnantian regression, the Iberian mixed (carbonate–siliciclastic) platform was exposed to subaerial conditions sufficiently for erosion and karstification to occur. From a palaeogeographical point of view, the pattern of the Ashgill Iberian platform deposition is characterized by episodic exclusion of carbonates from most nearshore environments by a shoreline source of siliciclastic sediments. A similar interpretation to that made on the Iberian Cystoid Limestone Formation, in terms of gradual proximality–distality changes, is proposed for comparable facies types in Ashgill limestones described in southwestern Europe. © 1998 John Wiley & Sons, Ltd.     

Heterozoan carbonate deposition on a steep basement escarpment (Late Miocene,Almería,south‐east Spain)

Heterozoan temperate‐water carbonates mixed with varying amounts of terrigenous grains and muddy matrix (Azagador limestone) accumulated on and at the toe of an inherited escarpment during the late Tortonian–early Messinian (late Miocene) at the western margin of the Almería–Níjar Basin in south‐east Spain. The escarpment was the eastern end of an uplifting antiform created by compressive folding of Triassic rocks of the Betic basement. Channelized coralline‐algal/bryozoan rudstone to coarse‐grained packstone, together with matrix‐supported conglomerate, are the dominant lithofacies in the higher outcrops, comprising the deposits on the slope. These sediments mainly fill small canyon‐shaped, half‐graben depressions formed by normal faults active before, during and after carbonate sedimentation. Roughly bedded and roughly laminated coralline‐algal/bryozoan rudstone to coarse‐grained packstone are the main lithofacies forming an apron of four small (kilometre‐scale) lobes at the toe of the south‐eastern side of the escarpment (Almería area). Channelized and roughly bedded coralline‐algal/bryozoan rudstone to coarse‐grained packstone, conglomerates, packstone and sandy silt accumulated in a small channel‐lobe system at the toe of the north‐eastern side of the escarpment (Las Balsas area). Carbonate particles and terrigenous grains were sourced from shallow‐water settings and displaced downslope by sediment density flows that preferentially followed the canyon‐shaped depressions. Roughly laminated rudstone to packstone formed by grain flows on the initially very steep slope, whereas the rest of the carbonate lithofacies were deposited by high‐density turbidite currents. The steep escarpment and related break‐in‐slope at the toe favoured hydraulic jumps and the subsequent deposition of coarse‐grained, low‐transport efficiency skeletal‐dominated sediment in the apron lobes. Accelerated uplift of the basement caused a relative sea‐level fall resulting in the formation of outer‐ramp carbonates on the apron lobes, which were in turn overlain by lower Messinian coral reefs. The Almería example is the first known ‘base of slope’ apron within temperate‐water carbonate systems.     

Mineral Types of Gold Deposits and Regularities of Their Localization in Southeastern East Sayan

The gold-bearing deposits of southeastern East Sayan have been categorized according to mineral composition. The most important classification criterion is the composition of productive ore mineral assemblages specific to each of the distinguished types, whereas the use of other criteria results in the inevitable overlap of different structural, compositional, or genetic features of deposits. Eight mineral types of deposits have been determined, which characterize the main gold-bearing mineral ore parageneses: gold-polysulfide, gold-quartz, gold-telluride, gold-tetradymite, gold-antimony, gold-bismuth-sulfosalt, gold-pyrrhotite, and gold-fahlore. The regional metallogenic units are structural-metallogenic zones somewhat differing by the nature of mineralization. Thus, in the Bokson-Gargan metallogenic zone, the gold-quartz, gold-polysulfide, and gold-pyrrhotite types of deposits prevail, while in the western part of the zone, the gold-telluride and gold-bismuth-sulfosalt types are widespread. In the Ilchir zone, gold-fahlore deposits are developed, while in the Khamsarinskaya zone, gold-tetradymite and gold-stibnite. It has been established that the mineral types of deposits depend on the composition of the host rock complexes: gold-quartz, gold-polysulfide, and gold-pyrrhotite types form in association with ophiolites and Archean basement rocks. At granitoid-massif-related deposits, base-metal minerals assume the leading role: Bi sulfosalts, stibnite, tetradymite, and tellurides. The gold-fahlore type forms in carbonate sequences. The proposed classification makes it possible to group all of the known gold-bearing deposits of southeastern East Sayan; it can also be applied to adjacent regions.

     

The genesis of calcite in carbonate rocks of the sedimentary basins: Evidence from the stable carbon and oxygen isotopes composition

The distributions of stable carbon and oxygen isotopes in modern and ancient limestones of various types were studied. Carbonate samples from modern sediments were collected in the Black and Barents Seas. Ancient carbonates were represented by Upper Jurassic (Kimmeridgian-Tithonian) limestones from the central part of the West Siberian basin. Carbonate samples include remains of modern and Upper Jurassic fauna, carbonate crust from sediments of the Black Sea, carbonate tube from sediments of the Barents Sea, and Upper Jurassic limestone from the carbonate layer found at top of Abalak, bottom of Bazhenov deposits in the central part of the West Siberian basin. According to the results of the isotope analysis and comparison with modern carbonates, Upper Jurassic limestones of the West Siberian basin belong to the group of methane-derived carbonates and precipitated as a result of anaerobic oxidation of methane (AOM). Fractures in limestones are filled with secondary calcite.     

Growth history of shallow-water carbonates: control of accommodation on ecological and depositional processes.

We explore the role played on the growth of shallow-water carbonates by changes in accommodation using case studies from Late Quaternary to modern carbonate environments, i.e. coral reefs and carbonate platforms and ramps. Accommodation appears not to influence either primary productivity or community structure (K- versus r-strategists) as these are directly governed by nutrient supply; but it does control submarine depositional profiles and their stratigraphic expression (i.e. volumetric sedimentary partitioning). The growth of communities from incipient to senescent stages is dictated by relative rates of sea-level variations, although ambient temperatures and trophic conditions may modify the community response. A decrease or stabilization of accommodation favours lateral migration, thus generating ecological and sedimentary innovations. Organic carbon production tends to be positively correlated to that of carbonate, both declining as accommodation decreases. Shallow water carbonates develop as a result of the interplay of factors, including the types and rates of biological and physical processes. Applying depositional models of Recent shallow-water carbonates to the interpretation of ancient carbonate platforms first needs to acknowledge the respective role of the two contrasting phases that operated during the post-glacial sea-level rise (an earlier, fast-rising phase versus a later, slowly rising to stabilizing phase); the period of relative sea-level stabilization (i.e. the past 6,000 years) appears a relevant reference for still-stand-related carbonate systems. Furthermore, the oligotrophic nutrient model, a dominant feature of the modern shallow-water tropics, can be used as an analogue in reconstructing carbonate growth histories during greenhouse episodes of Earth history.     

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.