Post-eocene biofacies,palaeoenvironments and palaeogeography of the Bengal Basin,India

The Bengal Basin is known for the extensive development of a thick Cretaceous— Tertiary sedimentary sequence in the eastern part of India. The basin has undergone several phases of transgressions, regressions, periodic uplifts and localised negative movements throughout the period of its evolution. The sedimentation up to the end of the Eocene was largely under the influence of a major transgressive phase which was later replaced by a regressive phase on account of the uplift and resulting marine retreat. in this paper, an analysis of the microfaunal distribution, biofacies and sedimentary and tectonic history is made for reconstruction of the palaeoenvironmental conditions and palaeogeography of the Bengal Basin relating to the post-Eocene regressive phase of evolution.During the Oligocene—Early Miocene, the open marine conditions existing since the Eocene were progressively modified on account of the regression into more brackish, lagoonal and estuarine types. A typical prograding delta started to spread over a large part of the marginal area of the basin. The rocks of the Barail Group, formed under the transitional environments of the delta, are found to occur in almost all the major geo-provinces of the basin. The sedimentary environments and characteristics of the rocks were ultimately controlled by the configurations of the respective geo-provinces. The negative movements in the Miocene along certain marginal areas were associated with the marine transgressive phase giving rise to the Surma Group. This transgressive phase was, however, short lived and its impact was not equally felt in all areas. The northeastern region was still uplifted which favoured an almost continuous existence of the deltaic phase.By the end of Miocene a greater part of the basin underwent a strong tectonic upheaval which was possibly the precursor of the main Himalayan orogeny. The intermontane depressions formed in the uplifted areas were the depositional sites for the Tipam Group and younger beds in Plio-Pleistocene times. The rapidly eroded debris from the highlands spread over a wide area towards the south under the influence of the river system of the Ganga and Brahmaputra which has transformed the original basinal areas into a large modern delta complex.     

Recent sedimentological conditions in the delta of the river Kyrönjoki,Western Finland

The estuary of the River Kyrönjoki was echo sounded and, on the basis of these results the delta was delimited and areas of erosion and accumulation were separated. Samples of the sediment surface (0–2 cm) from 65 sites in the estuary were analysed for organic content, C, N, P, Ca, Fe, Mn, Pb, Cu, Zn, Cd and Hg. On the basis of the chemical analyses the delta was divided into two parts according to the sedimentary conditions: (I) In the inner part, where acid fresh water of c. pH 5.5 from the river is predominant, the conditions were reducing. The Fe/Mn ratio of the sediment was high. The heavy metal content was usually low. The Hg content in the organic compounds of the sediment was, due to the acidity, higher than in the outer part. (II) In the outer part of the delta, where neutral brackish water of pH c. 7.0 dominates, the sedimentary conditions were mostly oxidizing. The Fe/Mn ratio was very low. The heavy metal and organic content was mostly high in the accumulation areas. This is because the fine-grained allochthonous material, which has a high heavy metal and organic content, flocculates and precipitates when it reaches the brackish water of the Gulf of Bothnia, and the salinity and the pH are higher.     

Growth of the Ballena fan delta on the Gulf of California (Mexico) at the close of the Pliocene Warm Period

Tectonic uplift on the shores of Bahía San Rafael in Mexico’s upper Gulf of California exposed a Pliocene delta system that covers a map area of 4 km². Subaerial dissection by arroyos entrenched during Pleistocene and post-Pleistocene time carved cross-sectional slices through the delta, showing its dominant construction resulting from massive transfers of siliciclastic sand derived from the breakdown of igneous rocks (tonolite) in a well-defined terrestrial basin. Restoration of the sedimentary structure by elimination of Pleistocene arroyos and linkage of former topographic lines reveals a triangular shape recognizable as a classic fan delta. The complex includes an alluvial fan that emerges from a small opening in the landscape connected to a semi-circular, high-walled basin with a map area of 2.6 km². Through a strictly longitudinal sequence, estimates of the excavated basin’s volume and the delta’s sedimentary volume were conducted as a mass-balance exercise that yielded a strong match. The lower central part of the delta features dense concentrations of sand dollars (Dendraster granti) that form a distinct limestone coquina not previously recognized elsewhere in Baja California. Through the regional biostratigraphy of concurrent range zones supplemented by absolute age dates from inter-bedded volcanics in other places, a later Pliocene age around 3–2 Ma is suggested for the sand dollars and the delta complex in which they are buried. Such timing corresponds to the close of the Pliocene Warm Period, during which a persistent El Niño climate brought tropical storms and excessive rainfall to the upper Gulf of California. Comparisons with other Pliocene deltas throughout the Gulf of California underscore the unique status of the complex named the Ballena fan delta.     

Using ecotechnology to address water quality and wetland habitat loss problems in the Mississippi basin: a hierarchical approach

Human activities are affecting the environment at continental and global scales. An example of this is the Mississippi basin where there has been a large scale loss of wetlands and water quality deterioration over the past century. Wetland and riparian ecosystems have been isolated from rivers and streams. Wetland loss is due both to drainage and reclamation, mainly for agriculture, and to isolation from the river by levees, as in the Mississippi delta. There has been a decline in water quality due to increasing use of fertilizers, enhanced drainage and the loss of wetlands for cleaning water. Water quality has deteriorated throughout the basin and high nitrogen in the Mississippi river is causing a large area of hypoxia in the Gulf of Mexico adjacent to the Mississippi delta. Since the causes of these problems are distributed over the basin, the solution also needs to be distributed over the basin. Ecotechnology and ecological engineering offer the only ecologically sound and cost-effective method of solving these problems. Wetlands to promote nitrogen removal, mainly through denitrification but also through burial and plant uptake, offer a sound ecotechnological solution. At the level of the Mississippi basin, changes in farming practices and use of wetlands for nitrogen assimilation can reduce nitrogen levels in the River. There are additional benefits of restoration of wetland and riverine ecosystems, flood control, reduction in public health threats, and enhanced wildlife and fisheries. At the local drainage basin level, the use of river diversions in the Mississippi delta can address both problems of coastal land loss and water quality deterioration. Nitrate levels in diverted river water are rapidly reduced as water flows through coastal watersheds. At the local level, wetlands are being used to treat municipal wastewater. This is a cost-effective method, which results in improved water quality, enhanced wetland productivity and increased accretion. The problems in the Mississippi basin serves as an example for other watersheds in the Gulf of Mexico. This is especially important in Mexico, where there is a strong need for economical solutions to ecological problems. The Usumacinta delta-Laguna de Terminos regional ecosystem is an example where ecotechnological approaches offer realistic solutions to environmental problems.     

Scoyenia Escape Burrows in Fluvial Pebbly Sand: Upper Triassic Sugarloaf Arkose,Deerfield Rift Basin,Massachusetts, USA

Late Triassic larvae of an insect, probably a beetle, moved diagonally upwards though fluvial pebbly sand along a thin mud layer, constructing Scoyenia burrows in the Sugarloaf Arkose, Deerfield rift basin, Massachusetts. They may have been escaping a temporary rise of the water table in the monsoonal dry season.     

Introduction

A prominent clastic wedge of latest Permian to early Triassic age (Katberg Sandstone) prograded northwestwards into the main Karoo Basin from a southerly source area as a sedimentary response to renewed tectonism associated with the Cape Fold Belt. Proximal to distal relationships within this clastic wedge and its relationships with underlying and overlying formations reveal a variety of stream types which reflect changing tectonic and climatic conditions.Towards the close of Permian times, the depositional area of the southeastern Karoo Basin was crossed by meandering river channels cutting through earlier formed floodplain deposits (Balfour Formation). Tectonic rejuvenation of the source area in early Triassic times led to steeper gradients and a sharp increase in the supply of coarser grained detritus. As a result, alluvial fans developed in areas adjacent to the source terrane and the river channels became braided, depositing only sands (Katberg Sandstone) with muds and silts being carried down into the most distal parts of the floodplain (Burgersdorp Formation). Subsequent denudation of the source area gradually reduced slopes and allowed the source-ward shift of the distal facies until it eventually overstepped the earlier formed braided stream deposits.Accompanying the changes in tectonic conditions was a change in climatic which also influenced stream type. The late Permian to early Triassic period records a general change to warmer climatic conditions following the widespread glaciation of the early to middle Permian. Thus the lowest Beaufort Group sediments were probably deposited in warm temperate to humid conditions with later deposits being laid down under an increasingly arid regime.     

Alternating aeolian,sabkha and shallow-lake deposits from the Middle Triassic Gipsdalen Formation,Scoresby Land,East Greenland

Gypsum-bearing deposits of the Gipsdalen Formation (Anisian ? Ladinian ?) are described and interpreted on the basis of lithology, sedimentary structures and sequential pattern.The basal Kolledalen Member consists in the northwestern part of the area of coarse-grained alluvial-fan deposits overlain by cross-bedded aeolian sandstones and finer-grained sabkha or shallow desert-lake deposits. The overlying Kap Seaforth Member displays wel;-developed cyclic sedimentation. In the southeastern part of the area the ideal cycle consists of, from bottom to top: cross-bedded aeolian sandstone, structureless or wave-rippled sandstone of sabkha flat—marginal desert-lake origin, thin wave-rippled sandstone and irregularly bedded mudstone of marginal desert-lake origin and horizontally laminated mudstone deposited in an expanded desert lake.Foreset dips of the cross-bedded aeolian sandstones suggest alternating periods of north-northeast and south-southeast palaeowinds. It is concluded that the aeolian deposits of the Gipsdalen Formation were deposited at low palaeolatitudes in the northern hemisphere. In the Middle Triassic summer seasons the Intertropical Convergence Zone probably lay north of the Jameson Land Basin, which was reached by south-southeast trade winds. During the rest of the year the zone was situated to the south and the area came under the influence of north-northeast trade winds.     

Ostracods and environmental variability in lagoons and deltas along the north-western Mediterranean coast (Gulf of Lions,France and Ebro delta,Spain)

The aim of this paper is to characterize various lagoon and delta environments through the analysis of ostracod fauna. Various aquatic environments from the Mediterranean coastline of the Gulf of Lions (Languedoc-Roussillon, France) and Ebro delta (Catalonia, Spain) were studied. The sample sites (60) are different in terms of marine and fluvial influence. Environmental parameters (salinity, water depth, sedimentary texture, plant cover) were measured and compiled from available data in order to characterize the biotopes. We interpreted the species distribution as related to the degree of isolation from the sea, the hydrological internal currents and the proximity of river mouths. Moreover, the assemblage composition seems influenced by the lagoon vegetation growth. These data can be used for the palaeoenvironmental reconstructions, particularly in Holocene deltaic context where the mobility of river mouths and the forming and evolution of sand bars and lagoons are recurring issues.     

Sequence stratigraphic analysis and sea-level history of the Upper Jurassic deposits (Mozduran Formation), south of Aghdarband,NE Iran

Abstract

The Kopet-Dagh Basin is a large sedimentary basin in northeastern Iran that host the giant Khangiran and Gonbadli gas fields. The Mozduran Formation with its various sedimentary facies is an important reservoir widely distributed in the basin. A sedimentological analysis of Upper Jurassic Mozduran Formation resulted in an accurate reconstruction of the sedimentary environments and the sequence stratigraphic framework south of Aghdarband. The strata consist of six different facies associations including 12 carbonate, one evaporate and two siliciclastic subfacies. On the basis of their various components, structural and textural characteristics, these facies were deposited on a homoclinal ramp in tidal flat to open marine environments ranging from supratidal to subtidal settings. Facies A1 and A2 represent open marine, B1-B4 Shoal, C1-C4 lagoonal and D1, D2, E, T1 and T2 tidal flat and Salina environments. In addition, based on detailed field and laboratory studies on the facies architecture, several large-scale (long-term) depositional sequences could be distinguished in the stratigraphic sections of the study area. These sequences are composed of LST, TST and HST that are separated by a SB1 and SB2 sequence boundaries. The paleogeography of the study area during the Late Jurassic time is reconstructed in five block diagrams.     

松辽盆地梨树断陷早白垩世微体古生物、沉积环境以及控油意义

根据松辽盆地梨树断陷早白垩世地层中孢粉百分含量的纵向变化特征,建立了自下而上各层组的孢粉组合。通过分析孢粉、藻类以及介形类的古环境指示特征,恢复了梨树断陷地层的气温带、古植被、干湿度以及湖水古盐度。沙河子组沉积时期为温暖潮湿的中亚热带气候,古植被为针叶林和灌草丛,湖水为淡水、微咸水;营城组沉积时期为南亚热带气候,古植被为针叶林和灌草丛,湖水同样为淡水、微咸水;到了登娄库组沉积时期,气候变得炎热干旱,为热带气候,古植被为针叶林,湖水则变为半咸水。本文的研究结果可以有效指示地层的划分与对比以及沉积环境的分析。同时,结合盆地发育讨论了古气候变化的控油意义,古气候的变化控制着烃源岩和储层的发育与分布。     

Integrated sedimentological,ichnological and sequence stratigraphical studies of the Koti Dhaman Formation (Tal Group), Nigali Dhar Syncline,Lesser Himalaya,India: paleoenvironmental,paleoecological, paleogeographic significance

Abstract

Integrated ichnology, sedimentology and sequence stratigraphy of the Lower Quartzite Member to the Arkosic Sandstone Member of the Koti Dhaman Formation (Cambrian Series 2, Stage 4), Tal Group, Nigali Dhar Syncline, Lesser Himalayan lithotectonic zone are presented. Trilobite traces of Gondwanan affinity i.e., Cruziana salomonis, Cruziana fasciculata, Rusophycus dispar and Rusophycus burjensis are recorded along with Arenicolites isp. and Skolithos isp. from the Lower Quartzite Member. A rich and diverse ichnoassemblage attributed to the Cruziana ichnofacies is described for the first time from the Arkosic Sandstone Member of the same formation. Seven ichnofossil assemblages, i.e., Cruziana-Rusophycus, Planolites-Palaeophycus, Cruziana problematica, Diplichnites, Cochlichnus anguineus, Bergaueria perata and Psammichnites gigas have been recognized in the Lower Quartzite to Arkosic Sandstone members of the Koti Dhaman Formation. Seven sedimentary facies i.e., sandstone–shale facies (FT1), cross-bedded (trough and planar) sandstone (FT2), bedded sandstone facies (FT3), shale facies (FT4), shale–sandstone facies (FT5), shale-rippled sandstone facies (FT6) and planar and trough cross-laminated sandstone (FT7) and four facies associations FA1-FA4 are identified in the Koti Dhaman Formation. The formation contains shallowing upward parasequences of a tidal flat complex. Overall, two major events are recognized: i) the break in sedimentation between the Lower Quartzite Member and the overlying Shale Member probably related to forced-regressive event and ii) the facies shift from FT6 to FT7 of the Arkosic Sandstone Member represents an erosive transgressive event; the surface is interpreted as wave ravinement surface, which also serves as a sequence boundary. Integrated ichnology, sedimentology and sequence stratigraphic studies indicate that the Lower Quartzite Member was deposited in a shallow subtidal sand sheet complex and tidal flat complex; the Shale Member was deposited in a mud flat setting of a tidal flat complex, and the Arkosic Sandstone Member in a mixed-flat (tidal flat complex) to sand sheet complex front and margin (subtidal sand sheet complex). Overall, the lower to middle part of the Koti Dhaman Formation represents a tide-dominated shallow subtidal–intertidal to mud-flat subenvironments of the tidal flat complex. A palaeogeographic reconstruction of lower Cambrian (516–514?Ma) is presented based on the distribution of trilobite traces from the Lesser Himalaya and the Bikaner–Nagaur area of Peninsular India (eastern Gondwana), Egypt, Jordan, Turkey (western Gondwana) and Canada (Avalonia).     

Channel country fluvial sands and associated facies of central-eastern Australia: Modern analogues of mesozoic desert sands of south America

The Channel Country, a region of wide fluvial plains criss-crossed by a reticulate pattern of anastomosing channels, and the adjacent sand dunes and clay pans of the Lake Eyre drainage basin occupy an area of 1.3 × 10⁶ km² of internal drainage in the arid east-central part of Australia. Beneath a surface of skin of mud, the sediment of the Channel Country is sand and some mud in the floodplain, as well as in levees and channels. The surface mud represents the overbank deposits of meandering channels that are superimposed on sheet sands of a relict braided system.With the other sediments (aeolian sand and lacustrine mud) of the Lake Eyre drainage basin, the Channel Country sediments represent the latest phase of deposition of the Cainozoic Birdsville Basin, which was initiated as an interior basin behind the dismembered rifted arch of the divergent Pacific margin. In their depositional and tectonic setting, the arid sediments of central-eastern Australia are modern analogues of the Mesozoic desert sands and other non-marine sediments that were deposited behind the pre-breakup arch and post-breakup half-arch of South America. The preceding Mesozoic Great Artesian Basin of central-eastern Australia contains volcanogenic sediment and was covered for a short time by an epeiric sea; its deposition was influenced by an uplift along the convergent Pacific margin, and it is analogous to other Gondwana basins in Antarctica, southern Africa, and South America that were yoked to the convergent Pacific margin.     

Late Quaternary (Weichselian) alluvial history and neotectonic control on fluvial landscape development in the southern Körös plain,Hungary

Four drill cores and a clay pit section have been examined in the southern part of the Körös plain to understand the history and controls on alluvial sedimentation for the last ~ 40 ka. Four facies groups were identified, such as channel, channel margin, floodplain and floodbasin with seven distinctive facies. Magnetic susceptibility and mineralogy have further characterized the sedimentary facies indicating shifts in humidity conditions, variations in sediment flux and pedogenesis. Detailed pollen analysis of a 7.5 m thick clayey succession indicated climatic variability within the MIS 3 period. The spatial distribution of the different facies allowed outlining alluvial architecture of the study area. Three depositional units composed of various facies were identified based on OSL and radiocarbon data. These packages correspond to three major phases of channel activity: (F-I) pre-LGM period (> 30 ka to 24 ka), (F-II) post-LGM interstadial (18–16 ka), and (F-III) Late Glacial < 15 ka to ~ 10 ka). The pre-LGM and post-LGM “interstadial” phases are characterized by meandering river patterns, while the Late Glacial fluvial activity is characterized by a braided system in the area. Higher sediment supply feeding this braided river was probably caused by neotectonic uplift of the southern margin of the basin, documented by a significant stratigraphic gap between 25 and 14 ka.     

太湖16000年来沉积环境的演变

通过对太湖钻孔岩芯岩性,结构,构造的剖析及粒度,磁化率的测试,发现冰后期东太湖形成于跑今６５００年前,在距今６５００－５８００年,为一水深约２－３ｍ的,经常受到流水作用影响的浅水湖泊,距今约５８００－５７００年,东太湖曾一度干枯或接近于干枯,距今５７００年以来湖泊变浅,平均水深只有１ｍ左右,由于湖泊变浅,湖底经常遭受波浪的扰动,形成波状层理或透镜状层理。西太湖局部洼地集水成湖的时间比东太湖早,并且     

Migratory Patterns of Wild Chinook Salmon Oncorhynchus tshawytscha Returning to a Large,Free-Flowing River Basin

Upriver movements were determined for Chinook salmon Oncorhynchus tshawytscha returning to the Yukon River, a large, virtually pristine river basin. These returns have declined dramatically since the late 1990s, and information is needed to better manage the run and facilitate conservation efforts. A total of 2,860 fish were radio tagged during 2002–2004. Most (97.5%) of the fish tracked upriver to spawning areas displayed continual upriver movements and strong fidelity to the terminal tributaries entered. Movement rates were substantially slower for fish spawning in lower river tributaries (28–40 km d^-1) compared to upper basin stocks (52–62 km d^-1). Three distinct migratory patterns were observed, including a gradual decline, pronounced decline, and substantial increase in movement rate as the fish moved upriver. Stocks destined for the same region exhibited similar migratory patterns. Individual fish within a stock showed substantial variation, but tended to reflect the regional pattern. Differences between consistently faster and slower fish explained 74% of the within-stock variation, whereas relative shifts in sequential movement rates between “hares” (faster fish becoming slower) and “tortoises” (slow but steady fish) explained 22% of the variation. Pulses of fish moving upriver were not cohesive. Fish tagged over a 4-day period took 16 days to pass a site 872 km upriver. Movement rates were substantially faster and the percentage of atypical movements considerably less than reported in more southerly drainages, but may reflect the pristine conditions within the Yukon River, wild origins of the fish, and discrete run timing of the returns. Movement data can provide numerous insights into the status and management of salmon returns, particularly in large river drainages with widely scattered fisheries where management actions in the lower river potentially impact harvests and escapement farther upstream. However, the substantial variation exhibited among individual fish within a stock can complicate these efforts.     

Palynological investigations in sediments of ancient lake Duvensee,Schleswig-Holstein (North Germany)

The Duvensee originated before the Alleroed in the Late Glacial, and had its largest areal coverage during the Preboreal. After the lake retreat, which began in early Boreal times, the marginal shore areas and nearshore islands were repeatedly inhabited by man in the early Mesolithicum. Archaeological excavations of human settlements and pollen analyses of sediment cores show evidence of lake level fluctuations in the ensuing period. The results disclose that shallow water sediments such as lake marls, algal muds and coarse detrital gyttjas predominate in the sequence. In keeping with the shallow water conditions, strong lateral facies changes were observed in the cores. The early Holocene deposits have almost the same thickness irrespective of their position in the shallow or deep parts of the lake basin. The subaerial exposure of the nearshore and island areas sometimes resulted in fern and reed peats. The last remnants of the lake, which was drained in 1850, lay in the marginal areas over shallow water sediments.     

Marine influence and incursion in the Gondwana basins of Orissa, India: A review

The Permian–Triassic succession of the Indian Gondwana Sequence was previously considered to have been deposited in a fluviatile-lacustrine environment. Similarly, earlier Lower Gondwanas of Orissa State (a major part of the Mahanadi Master basin) were considered entirely fresh water deposits. Faunal evidence is still scanty in this master basin. Ichnology and palynology along with a few sedimentary records are reviewed and analysed for inferring marine signature. The marine nature of the Talchir, Karharbari, Barakar, Barren Measures and Kamthi sediments of three major basins (Talcher, Ib River and Athgarh) in Orissa State was predicted on the basis of typical marine ichnofossils. Most of these sediments also contain acritarchs reflecting marine marginal environment throughout the Permian. Moreover, evidence of wave activity, salinity raise and discovery of phosphorite in Permian sediments also strengthen this view.Hence, the previous model of continental facies for the Lower Gondwanas is found to be incorrect. The ichnofossils (Skolithos and Cruziana ichno-facies), acritarchs (Foveofusa, Leiosphaeridia, Greinervillites, etc.) and other palynofossils of marine origin can be utilized as a tool for palaeoenvironmental reconstruction. In the Gondwana basins of Orissa (Mahanadi Master basin), consistent occurrence of marine acritarchs and trace fossils with some typical sedimentary structures such as wave ripples has been studied and reviewed from the Talchir (Early Permian) to Upper Kamthi (Triassic) formations at various time intervals. Here marine incursion could have occurred due to the well known global transgressions during Permian and Triassic.     

Spatial distribution of phytoplankton in Lake Baikal, Spring 1991

• 1 Studies carried out in Lake Baikal in late spring (late May-early June) 1991 showed marked differences in the species composition and abundance of phytoplankton in different regions of the lake. The south and north basins were characterized by small forms of algae. The middle basin, Maloe More and the shallow waters of the Selenga had species with large cells including Aulacoseira islandica subsp. helvetica, Dinobryon cylindricum and D. divergens.
• 2 Areas of high biomass were correlated with shallow waters and river inputs. This was especially apparent in the region of the Selenga delta. The vertical distribution of phytoplankton indicated the non-synchronous start of the spring homothermy throughout the lake. Phytoplankton were concentrated in the upper 100 m layer with subsurface maxima resulting from the sinking of large algae.
• 3 The concentration of phytoplankton biomass in general at this time characterized the lake as moderately productive.

     

Microbial-Activated Sediment Traps Associated with Oncolite Formation Along a Peritidal Beach,Northern Arabian (Persian) Gulf,Kuwait

Identification of microbial communities within shoreline sediments and sediment precipitates from the Tigris-Euphrates delta (northern Kuwait) were determined by microscopic/nanoscopic studies, and by molecular analysis. Oncolites are syn-diagenetic carbonate precipitates that are surviving in a shallow subtidal to intertidal siliciclastic environment with periodically excessive hydraulic energy, extreme salinity (up to 47 per mil), and high concentrations of organic matter. X-ray diffraction techniques reveal that oncolite cortices are predominantly composed of calcite, quartz, halite and dolomite, associated with minor fractions of clay minerals. Quantitative analysis of the Corey Shape Factor reveals distinct morphological populations but with local overlap. A plot of the Equivalent Diameter vs. Corey Shape Factor provided the best indicator of the morphological relationships within the total oncolite population, indicating a hydrodynamically controlled morphological distribution defining intertidal and subtidal oncolite classes. Direct microscopic examination of the samples indicates that diatoms are the most abundant eukaryotic algae in subtidal sediments and within actively precipitating carbonate cements, especially the genus Navicula. In contrast, filamentous cyanobacteria from the genus Anabaena are most abundant in the intertidal zone sediments. The PCR-DGGE of the 16SrRNA gene of the cyanobacteria shows a higher diversity for this genus of bacteria in all sediment samples and that the cyanobacterial population in the diagenetically precipitating oncolites are closely related to the population found in the subtidal sediments. Dunaliella viridis dominates the culturable algae obtained from the four tidal zones. Our results indicate that a range of microbial populations are actively contributing to the formation of microbially-induced sedimentary structures in the extreme conditions of the southern Tigris-Euphrates delta.