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.     

Environmental setting of San Francisco Bay

San Francisco Bay, the largest bay on the California coast, is a broad, shallow, turbid estuary comprising two geographically and hydrologically distinct subestuaries: the northern reach lying between the connection to the Pacific Ocean at the Golden Gate and the confluence of the Sacramento-San Joaquin River system, and the southern reach (herein called South Bay) between the Golden Gate and the southern terminus of the bay. The northern reach is a partially mixed estuary dominated by seasonally varying river inflow, and the South Bay is a tidally oscillating lagoon-type estuary. Freshwater inflows, highest during winter, generate strong estuarine circulation and largely determine water residence times. They also bring large volumes of dissolved and particulate materials to the estuary. Tidal currents, generated by mixed semidiurnal and diurnal tides, mix the water column and, together with river inflow and basin geometry, determine circulation patterns. Winds, which are strongest during summer and during winter storms, exert stress on the bay's water surface, thereby creating large waves that resuspend sediment from the shallow bay bottom and, together with the tidal currents, contribute markedly to the transport of water masses throughout the shallow estuary.     

Recent evaporite deposition associated with microbial mats,Al-Kharrar supratidal–intertidal sabkha,Rabigh area,Red Sea coastal plain of Saudi Arabia

The supratidal–intertidal sabkha of the Al-Kharrar area, Red Sea coast, Saudi Arabia, contains the evaporite minerals gypsum, anhydrite, and halite. Microbial mats flourish adjacent to the sabkha evaporites in tidal flats and pools of the Al Kharrar lagoon. Desiccation and decay of some microbial mats in tidal flat areas have led to precipitation of gypsum and halite there. The evaporite minerals have been precipitated through displacive, inclusive, and replacive growth within mud, sand, gravelly sand, and bioclastic sediment of the sabkha. Gypsum occurs as lenticular and tabular crystals whereas anhydrite occurs as nodular (individual, mosaic, and enterolithic) and pseudomorphs of lenticular gypsum crystals that grew displacively and replacively near the surface of the sabkha. Halite exists as a diagenetic cement within the sabkha sediment, or as primary rafts and skeletal crystals in desiccated tidal pools with salinity over 220‰. Microbial mats are growing on the surface of the upper tidal flat areas and in pools at a salinity range of 80–110‰, and they lead to biostabilization of the sediment. They have induced a range of sedimentary surface structures (MISS) including gas domes, reticulate patterns, tufts, pinnacles, wrinkles, and microbial shrinkage cracks. The occurrence, abundance, and association of evaporite minerals and MISS are controlled by local environmental factors such topography of the sabkha, emergence or submergence of tidal areas, surface area of the evaporite basin, contribution of meteoric water from floods from the adjoining Red Sea Mountains, and water salinity. These factors promote the growth of the microbial mats in the winter months, and deposition of evaporite minerals during summer months. Field and petrographic data indicate that the main recharge to the sabkha area is from tidal flow and water seepage from the Al-Kharrar lagoon. The results of this study indicate that within a small sabkha area of Al-Kharrar (3?×?17 km), a large variation in evaporite mineral types and morphologies grade into and are associated with MISS due to local environmental parameters. The interpretation of this association of evaporite minerals and MISS provides useful data for understanding the mechanisms responsible for precipitation of evaporite minerals and formation of MISS.     

The palaeolimnological potential of diatom assemblages in floodplain lakes of the Danube Delta, Romania: a pilot study

Within the framework of a palaeolimnological pilot study, the potential of sedimentary diatom assemblages for the reconstruction of the eutrophication history was studied in short sediment cores from five shallow lakes located in the Romanian Danube Delta. A total of 234 diatom taxa representing 57 genera could be identified but diatom assemblages are generally predominated by a few species only. Loss on ignition percentages, total diatom abundances and diatom downcore successions largely vary within the single sediment cores and between all five lakes. This suggests the presence of various lake environments within the Danube Delta, each characterized by lake-specific geomorphological, sedimentological, hydrological, and biological conditions. Highly variable depositional conditions are also confirmed by radiocesium measurements in two of the five cores suggesting distinct inter-lake differences in the sedimentation rate. Causes for the great intra-lake and inter-lake variability in diatom downcore successions are difficult to specify and may include changes in the physico-chemical conditions, aquatic vegetation cover, water depth, river influence, turbidity and inter-biotic interactions. However, based on this pilot study, it is likely that the spatial and temporal distribution of sedimentary diatom assemblages in Danube Delta lakes were not triggered by the trophic state. The delta lakes likely became meso- to eutrophic long before 1950, possibly during late-Holocene times.     

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.     

Impacts of environmental changes on the hydrology and sedimentary processes at the confluence of St. Lawrence tributaries: potential effects on fluvial ecosystems

This study examines the effects of changes in St. Lawrence water levels and in hydrological regimes of two tributaries of the St. Lawrence, the Richelieu and Saint-François Rivers, on the sedimentary dynamics of their confluence in an historical context and in response to projected climate changes. Analysis of historical data indicates that alteration to St. Lawrence mean water levels and spring flood characteristics (duration, timing, and magnitude) have already impacted flow and sediment dynamics at tributary confluences. The major progradation of more than 1.5 km of the sedimentary front of the Saint-François delta into the shallow water of the fluvial Lake Saint-Pierre between 1859 and 2001 is associated with the combined effect of water level reduction in the lake, as a consequence of dredging of the navigation channel, and discharge regulation. High tributary discharges during periods of low water levels in the St. Lawrence have also contributed to accelerated delta front progression. Increases in temperature and changes in the precipitation regime projected for the current century were used into the HSAMI model to obtain hydrological simulations for the tributaries. Results indicate that the greatest hydrological changes will be observed during the winter and spring seasons, a time that is critical for many ecosystems. The mean spring discharge is projected to decrease by 17% for the Richelieu and 40% for the Saint-François by the end of the century. During this season, the frequency of discharge higher than the sediment transport threshold will decrease by 63% for the Saint-François and by 17% for Richelieu when compared to the reference period. However, during winter, the frequency of these high discharges will be four times higher for both rivers. These changes are expected to occur simultaneously with a decrease of the St. Lawrence water levels, a situation that will have significant impacts on sedimentary processes. A hydro-sedimentological modeling approach based on SEDROUT was used to investigate the morphological response of the river beds to the projected changes on the hydrology of St. Lawrence tributaries and to three base level scenarios. Results show that the combined hydrology and base level effects will lead to an increase in sediment supply compared to the current state, and to the extension of accumulation zones. While the effects will vary among the tributaries according to their particular characteristics, the projected increase in sediment supply will modify the extent of freshwater wetlands at the mouth of the St. Lawrence tributaries with feedback effects on local flow and sediment distributions.     

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.     

Rhodoliths and coralliths of Muri Lagoon,Rarotonga, Cook Islands

Free-living massive and branching spheroidal growths (about 5 cm diameter) of calcareous red algae (rhodoliths) and corals (coralliths) occur in abundance on the sea bed of shallow Muri Lagoon on Rarotonga's reef flat. The rhodoliths are composed of one or more species of Neogoniolithon, Lithophyllum, Tenarea, and Porolithon; the coralliths are Pavona varians (Verrill) and Porites lutea (Milne-Edwards and Haime). Muri Lagoon is the only area on Rarotonga's reef flat that is sheltered by reef islands from ocean waves. The tidal currents, which are predominantly unidirectional in Muri Lagoon, are concentrated by the reef islands into channels through which sand and gravel sediment is regularly transported. However, these prevailing currents do not normally roll the rhodoliths and coralliths. The results of field experiments on the pick-up velocity of the various types of spheroidal structure, combined with observations on growth histories of massive coralliths as revealed by the non-concentric nature of skeletal density banding, indicate that the rhodoliths and coralliths may remain static for periods up to several months yet maintain a complete envelope of living tissue. This downward survival may depend on the strong currents. Not only is the water flushing through the upper millimetre or so of the sediment substrate, but it is also capable of moving the sand and gravel grains which laterally support the rhodoliths and coralliths so that no one point of a spheroidal structure is in direct contact with the substrate for a fatal length of time. Massive rhodoliths have a high preservation potential as discrete spheroidal structures; in contrast, branching rhodoliths and coralliths are prone to fragmentation, and massive coralliths grow into stable microatolls. We conclude that a similar assemblage of rhodoliths, coralliths and microatolls in the fossil record may be indicative of the former existence of contemporary reef flat islands.     

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.     

A Mediterranean Holocene restricted coastal lagoon under arid climate: Case of the sedimentary record of Sabkha Boujmel (SE Tunisia)

The Holocene sedimentary record of Sabkha Boujmel (SE Tunisia) is expressed by a shallowing-upward carbonate lagoon-tidal flat cycle (2.3 m thick) unconformably overlying continental silt-sandy sediment, Late Würmian in age. The sedimentary package of this cycle starts with transgressive marginal shallow marine (intertidal to subtidal) bioclastic sands grading upwards to black mudstone, rich in organic matter (T.O.C. up to 1.3%) deposited within a lagoon protected from the sea by Upper Pleistocene lithified sand spits.The uppermost part of the cycle is represented by oobioclastic carbonate sands covered with dead biodegraded microbial mats and/or reddish sands of aeolian origin deposited in intertidal to supratidal environments. The facies arrangement, particularly the spatial distribution of the ancient and the more recent microbial mats, records the progressive infilling of the lagoon as well as the progradation of the shoreline during the last 2000 years. The organic-rich facies which provide an age varying between 4130 and 6800 yr B.P. were deposited when the Boujmel lagoon started to be progressively separated from the Mediterranean Sea.The main factors controlling the facies and the thickness variation are the local topographic sea-floor irregularities most likely controlled by the inheritance morphology resulting from an important fluviatile digging that occurred during the last glacial maximum, the relative sea-level fluctuations, the hydro-isostatic rebound and the climate.     

Palaeoecology of the Aptian Santana Formation (Northeastern Brazil)

In the interior of northeastern Brazil there occurs the Santana Formation of Aptian age, composed lithologically of three members: Crato — an alternation of thin limestones and shales; Ipubi — gypsum; and Romualdo — almost pure limestone. The formation is very fossiliferous; pollen, plant remains, ostracodes, conchostracans, mollusks, echinoids, fishes and a few reptiles. The fishes and reptiles occur in limestone concretions. All data on sediment character and on fossils have been considered together for the interpretation of the palaeoenvironment of the formation.It was concluded that at the time of the Crato Member the deposition occurred in shallow lakes and swamps. Later (Ipubi Member), a marine invasion took place during a time of dry climate, causing anhydrite precipitation. Gradually the connection with the sea became obstructed so that the environment at the end of the depositional period became once more one of fresh water (Romualdo Member). Faunal assemblages and sediments point to a fairly great supply of river water during the whole time of basin deposition, under warm and dry climatic conditions. The connection with the sea persisted for a rather short period.     

滦河流域及周边地区花粉与植被关系的研究

滦河流域及周边地区空气中的花粉组合基本上反映了当地的植被面貌, 与植物的花期相对应, 乔木植物的花期多在春季, 草本植物的花期多在夏秋季, 该地冬季基本上无植物开花, 冬季空气中的花粉应是当年或多年春、夏、秋季散落在地上又被风吹到空气中的表土花粉和一些外来花粉 ;表土花粉随海拔高程的降低依次出现山地草甸、针阔混交林或以针叶树为主的针阔混交林、山地灌草丛、滨海平原草甸和滨海草甸或滨海盐生草甸的等 5个花粉组合带, 分别为蒿 (Artemisia)唐松草 (Thalictrum)松(Pinus)桦 (Betula) 孢粉组合带、松桦栎 (Quercus) 蒿孢粉组合带、藜科 (Chenopodiaceae)蒿禾本科 (Gramineae)中华卷柏 (Selaginellasinensis) 孢粉组合带、藜科蒿菊科 (Com positae)香蒲 (Typha) 孢粉组合带和藜科蒿莎草科 (Cyperaceae)禾本科孢粉组合带 ;由于花粉在河水中是以悬移质颗粒被搬运, 因此在不同时期沉积物花粉组合存在着一定的差异, 其中洪水期间河水对孢粉的分选作用最为明显。     

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.     

Hydrology,hydraulics, and geomorphology of the Upper Mississippi River system

The Upper Mississippi River system has been modified with locks, dams, dikes, bank revetments, channel modifications, and dredging to provide a nine-foot navigation channel. These activities have changed the river's characteristics. The historical changes in the hydrologic, hydraulic, and geomorphic characteristics were assessed and related to navigational development and maintenance activities in the Upper Mississippi River system. The hydrologic, hydraulic, and geomorphic features studied include river discharges, stages, sediment transport, river position, river surface area, island surface area, and river bed elevation. Water and sediment transport effects on dredging were also estimated. It was found that the general position of the Upper Mississippi River system has remained essentially unchanged in the last 150 years except for specific man-made developments in the river basin. The stage, velocity, sediment transport, and river and island areas were altered by development of the 2.75-m navigation system. Dredging requirements are strongly related to mean annual water discharge. Years in which water discharges were great were generally the years during which large volumes of sediment needed to be dredged from the channel. The backwater areas are experiencing some deposition. With implementation of erosion-control measures in major tributaries and upland areas, better confinement of disposed dredged materials, and better maintenance practices, the sedimentation and pertinent problems in the main channel, as well as in the backwater areas, may be reduced with time.     

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。