Relative Lake Level Research Articles

Architecture of sandstone bodies of Paleogene Shahejie Formation in Northern Qikou Sag, Northeast China

The conception of special geologic bodies has been applied in this study to define and illustrate economic oil and gas reservoirs in sandbodies, which is helpful for hydrocarbon forecast and exploration. Using the comprehensive analysis of drilling, logging, seismic, seismic attribute extraction, and logging constrained inversion in 3D data volume, the special geologic bodies of the Shahejie Formation (Ps 1 s) in northern Qikou sag, which have good hydrocarbon potential, are identified and described under the constraint of sequence stratigraphic framework. The dimensions, geometry, and spatial distribution features of the special geologic bodies, as well as their inner architectures and sequence-structure patterns, are also ascertained. The geologic bodies evolved from relative centralization in the middle part of the lacustrine basin in the early stage of Ps 1 s Formation, to several dispersive isolated parts in the later stage. This shows a small cycle interval (~2.5 Ma) at the end of the whole transgressive depositional process of Ps1 (~7.5 Ma) during the expansion of the lake and relative lake level rise, with the control of sediments supply and fault-related subsidence, which effect the evolution of the sedimentary system. According to the relevance and regularity of the geologic bodies’ development, different types of potential profitable reservoir traps, including the lithologic lenticular traps, lithologic updip pinchout traps, and structural-lithologic composite traps, can be preserved in the study area. In addition, the internal architecture models provide scientific basis for further hydrocarbon exploration in the frontier basin without enough data.

Division and characteristics of shale parasequences in the upper fourth member of the Shahejie Formation, Dongying Depression, Bohai Bay Basin, China

Shale parasequence analysis is an important part of sequence stratigraphy sudies. This paper proposed a systematic research method for analyzing shale parasequences including their delineation, division, characteristics and origins. The division method is established on the basis of lithofacies. Multi-method analysis and mutual verification were implemented by using auxiliary indicators (such as mineral compositions, geochemical indicators and wavelet values). A typical shale parasequence comprises a lower interval of deepening water-depth and an upper interval of shallowing water-depth (e.g., a shale parasequence including a high-total organic carbon (TOC) shale-low-TOC limy shale). Abrupt increases in pyrite content, TOC value, relative hydrocarbon generation potential ((S1+S2)/TOC), and wavelet values are indicative of parasequence boundaries. The proposed research method was applied to study the upper fourth member of the Shahejie Formation in the Dongying depression, Bohai Bay Basin. Results show that there were seven types of parasequences developed. A singular and a dual structured parasequences were identified. Three factors controlling the development of the shale parasequences were identified including relative lake level change, terrestrial input and transgression. The development of high-TOC (>2%) shale parasequences was mainly controlled by biological and chemical sedimentation. The low-TOC (<2%) shale parasequences were mainly deposited by chemical sedimentation. The diversities of shale parasequences were caused by four major controlling factors including climate, relative lake level change, terrestrial input and emergency (e.g., transgression).

Lower Cretaceous lacustrine successions, North Yellow Sea Basin, eastern China: Rift basin sequence stratigraphy and stacking patterns in response to magmatic activity

A series of Mesozoic rift basins formed in eastern China were associated with magmatic activity and subduction along the Eurasia, Izanagi and Pacific plate margins. The impact of magmatic activity on lacustrine sequence development was documented with well-log and 3-D seismic data from the Jupiter Depression in the North Yellow Sea Basin. We identified key surfaces, retrogradational and progradational parasequence sets, and defined the characteristics of systems tracts and the internal sequence components for the Lower Cretaceous (K1SQ1). A 2-D SEDPAK numerical stratigraphic forward modeling was used to further constrain sequence development in the Jupiter Depression by considering different modeling parameters and the spatial-temporal characteristics of magmatic activity. Modeling results were compared and matched with the sequence architecture observed from seismic and well interpretations. Magmatic activity impacts on the development of the K1SQ1 sequence in the North Yellow Sea Basin include topographic variation, lake level fluctuation, and sediment supply ratios. Results suggest that magmatic upwelling uplifted the northwestern area of the Jupiter Depression and formed its slope break during the late Jurassic or early Cretaceous. Along with uplifting, relative lake level dropped sharply and lake accommodation was reduced, but with increased sediment supply. Therefore, sediment accumulated along the slope break as a lowstand systems tract. Later on, as lake level continuously rose, transgressive and highstand systems tracts were developed. The proposed stacking pattern provides an analog, and a useful model, for lacustrine sequence development in response to magmatic activities in eastern China and other rift basins of similar tectonic setting.

Rise and fall of a small ice-dammed lake - Role of deglaciation processes and morphology

A small ice-dammed lake, which developed along the margin of Nordenskiöldbreen on the northern coast of Adolfbukta, (central Spitsbergen, Svalbard) has been studied by a combination of facies analysis, ground penetrating radar, analysis of photos and satellite imagery, and by surface mapping by Unmanned Aerial Vehicle (drone). The lake existed between the years 1990–2012 and occupied two partial depressions in the bedrock, separated by a bedrock ridge for the dominant period of its history. Whereas the eastern depression was almost completely infilled due to direct fluvial input, the western depression revealed only thin sedimentary cover and was dotted from the eastern depression by an outflow of surficial waters. Gilbert delta deposits with typical tripartite zones of topset, foreset and bottomset were recognised in the eastern depression. Topset was comprised by deposits of a braided river. Foreset is formed by deposits of sediment gravity flows (turbidity currents and debris flows). Bottomset is represented by alternating suspension deposits and deposits of hyperpycnal underflows (low-density turbidity currents). The ruling factors of the evolution of the delta were glacier retreat, bedrock morphology, both affecting the relative lake level, and the rate of sediment delivery. Glacier retreat over stepped and inclined bedrock morphology led to delta prograding and downstepping. The recognised fluvio-deltaic terraces revealed four lake level falls followed by fluvial downcutting, erosion and redeposition of the older deltaic/lake deposits, the shifting of the lake's position towards the damming glacier and the transition of the sediment input in the same direction. The termination of the lake was a result of further glacier retreat and the opening of subglacial drainage.

Multi-proxy paleoenvironmental reconstruction of saline lake carbonates: Paleoclimatic and paleogeographic implications (Priabonian-Rupelian, Issirac Basin, SE France)

A 200-m thick carbonate succession has been deposited in shallow-water, saline lake environments during the Priabonian-Rupelian in the Issirac Basin (South-East France). The palaeoenvironmental and palaeogeographic significance of such saline lake carbonates has been characterized on the basis of a multi-proxy analysis including 1) depositional and diagenetic features, 2) biological components (molluscs, ostracods, benthic foraminifers, characean) and 3) carbon, oxygen and strontium stable isotopes. Biological associations are indicative of dominantly shallow (<10m), freshwater to mesohaline (0 to 18‰) environments. The occurrence of evaporites is indicative of periods of hypersalinity. Carbonate production is dominantly associated to microbial activity (planar stromatolites) and micrite precipitation in lakes colonized by macrophyte meadows. The Priabonian-Rupelian carbonates from the Issirac Basin recorded three main cycles of lake transgression, corresponding to the three main sedimentary units (U1, U2 and U3). Relative lake-level, degree of connectivity with surrounding lakes and climate (dry versus humid) are the three key factors controlling the water composition, carbonate production and depositional environments in the Issirac lake. Although the ASCI (Alès-Issirac-Saint-Chaptes) lacustrine system likely represents an athalassic (inland) lake system evolving through times, the stable isotope composition (C, O and Sr) of carbonates strongly suggests the occurrence of transient connections of the ASCI lake water with water bodies influenced by seawater and/or fed with sulfates deriving from Triassic evaporites. The Issirac Basin may be therefore interpreted as a sill area connecting the ASCI lacustrine system with the Rhône valley (Mormoiron and Valence) saline lake systems during maximum flooding periods. Finally, changes in depositional features, biota and stable isotope composition of carbonates in unit U3 suggest a transition from relatively dry to more humid climate during the uppermost Priabonian or earliest Rupelian.

Lithological and facies analysis of the Roseneath and Murteree shales, Cooper Basin, Australia

Unconventional shale plays have received marked attention over the last five years because of their economic potential for hydrocarbon generation, and yet they are amongst the least understood of all clastic sedimentary rock systems. The Cooper Basin is one of the largest Gondwana intracratonic basins in Australia, extending from northern South Australia into south-western Queensland and covering approximately 130,000 km2. The basin is may be prospective for shale gas, particularly within the lacustrine shales of the Permian Murteree and Roseneath formations. This study investigates lithological characteristics of these two units in relation to reservoir evaluation. Core samples representing the Dirkala-02 and Moomba-46 wells were used for petrographic analysis. A combination of wireline log analysis, thin section petrography, X-ray diffraction and pyrolysis analysis was used to define and characterize four distinct lithofacies facies within the Roseneath and Murteree shales: siliceous mudstone, organic siliceous mudstone, calcareous siliceous mudstone, and silty siliceous mudstone. The siliceous mudstone and organic siliceous mudstone are the most common. Diagenetic siderite occurs in all four lithofacies. A conceptual depositional model is developed for deposition of the Roseneath and Murteree shales. Wireline-log cross plots were interpreted and utilized in the construction of electrofacies. The study was concentrated on the northern portion of the basin between the Nappameri and Patchawarra Troughs in order to understand the nature of lithofacies and variability in reservoir architecture, which was controlled by relative lake level fluctuation. The results of this study will aid in the evaluation of shale gas potential for this portion of the basin, as well as a better understanding of shale gas opportunities in the Cooper Basin more generally.

Paleomagnetic secular variation and environmental magnetism of Holocene-age sediments from Tulare Lake, CA

The lake-level record from Tulare Lake, CA has been shown to provide valuable constraints on late Pleistocene and Holocene runoff from the Sierra Nevada mountain range into the San Joaquin Valley of California, one of the world's most prolific agricultural centers. This project uses the magnetic properties of the Tulare Lake sediments in order to date the sediments and to constrain the relative lake level at the time of deposition. Shallowing lake conditions were identified leading up to a prominent unconformity; magnetic mineralogy and grain size indicators, primarily decreasing ARM/IRM and S-Ratio values suggest coarser grain sizes and more oxidizing conditions. Approximately half of the samples possessed well-behaved paleomagnetic directions suitable for paleomagnetic secular variation dating. The results indicate that the sediments below the unconformity were deposited approximately 7600–8500 cal yr BP, and the sediments above the unconformity were deposited approximately 2500–800 cal yr BP. The ages of the corresponding sediments are consistent with the time intervals during which previous studies indicate that lake level was above the elevation of this site, before and after a mid Holocene regression.

Hydroclimatic variability in the Levant during the early last glacial (∼ 117–75 ka) derived from micro-facies analyses of deep Dead Sea sediments

Abstract. The new sediment record from the deep Dead Sea basin (ICDP core 5017-1) provides a unique archive for hydroclimatic variability in the Levant. Here, we present high-resolution sediment facies analysis and elemental composition by micro-X-ray fluorescence (µXRF) scanning of core 5017-1 to trace lake levels and responses of the regional hydroclimatology during the time interval from ca. 117 to 75 ka, i.e. the transition between the last interglacial and the onset of the last glaciation. We distinguished six major micro-facies types and interpreted these and their alterations in the core in terms of relative lake level changes. The two end-member facies for highest and lowest lake levels are (a) up to several metres thick, greenish sediments of alternating aragonite and detrital marl laminae (aad) and (b) thick halite facies, respectively. Intermediate lake levels are characterised by detrital marls with varying amounts of aragonite, gypsum or halite, reflecting lower-amplitude, shorter-term variability. Two intervals of pronounced lake level drops occurred at ∼ 110–108 ± 5 and ∼ 93–87 ± 7 ka. They likely coincide with stadial conditions in the central Mediterranean (Melisey I and II pollen zones in Monticchio) and low global sea levels during Marine Isotope Stage (MIS) 5d and 5b. However, our data do not support the current hypothesis of an almost complete desiccation of the Dead Sea during the earlier of these lake level low stands based on a recovered gravel layer. Based on new petrographic analyses, we propose that, although it was a low stand, this well-sorted gravel layer may be a vestige of a thick turbidite that has been washed out during drilling rather than an in situ beach deposit. Two intervals of higher lake stands at ∼ 108–93 ± 6 and ∼ 87–75 ± 7 ka correspond to interstadial conditions in the central Mediterranean, i.e. pollen zones St. Germain I and II in Monticchio, and Greenland interstadials (GI) 24+23 and 21 in Greenland, as well as to sapropels S4 and S3 in the Mediterranean Sea. These apparent correlations suggest a close link of the climate in the Levant to North Atlantic and Mediterranean climates during the time of the build-up of Northern Hemisphere ice shields in the early last glacial period.

The discovery of the Barmer Basin, Rajasthan, India, and its petroleum geology

The discovery of oil and gas in the Barmer Basin in northwest India was one of the more significant global discoveries in the decade 2001–2010. The basin’s presence was suspected from gravity and magnetic data in the late 1980s but not confirmed until 1999 from seismic and drilling. The basin is a lacustrine failed rift. Biostratigraphic data, however, indicate it was intermittently connected to marine waters via either the Cambay Basin, the Kutch Basin, or across the Devikot high, temporarily forming a large, shallow estuary. At least six major tectono-stratigraphic events have caused relative lake level falls and translation of clastic reservoirs basinward. Upward of 6 km (∼20,000 ft) of Cenozoic and Mesozoic sedimentary rocks have been preserved. Prolific source rocks occur from the Mesozoic through Eocene strata. Tectonically, the basin is divided into a northern and a southern province. The north province continues to undergo inversion and erosion, and has not been buried as deeply as the south. Kinetics of the major source facies in the north are substantially different from those in the south, as well as the present-day and paleo-heat flow. These differences have made the northern part of the basin predominantly an oil province and the southern part a mixed oil and gas province. The prolific Paleocene Fatehgarh Formation contains the bulk of the 7.3 billion barrels of stock tank oil in place (STOIP) identified to date, but other reservoirs from the Mesozoic to the late Cenozoic are common and may yield significant future resource additions.

The Alpar canyon system in the Pannonian Basin, Hungary – its morphology, infill and development

Giant incised canyons were recently recognized in Late-Miocene post-rift sediments in the central part of the Pannonian Basin. Though not connected to the world seas, Lake Pannon shows significant signs of relative lake level variations controlled by tectonics and climate changes. The incision surface of the Alpar canyon system is connected to SB Pa-4 (6.8Ma sensu Vakarcs, 1997), earlier reported to represent a significant relative base-level fall in the basin, however, debated recently.Incised several hundred meters in the preexisting substrate, the individual canyon valleys of the Alpar canyon system are enormous in size and display a multi-story nature. They loose topographic expression headwards and basinward. Widths of individual valleys range from 5 to 10km, with smaller tributaries. In the study area several adjacent canyon valleys can be seen on seismic profiles. The valley depth is greatest near their confluence, where a major trunk valley (600–700m deep) was formed by eroding most of the Upper Miocene succession. The canyons are filled with clay marls. They are incised into an extremely thick aggrading deltaic complex and are overlain by fluvial sediments, suggesting a major transgression in between.The Late Miocene Alpár canyon system developed on the southern margin of the Mid-Hungarian Mobile Belt, the latter is characterized by NE-SW oriented fold axis and NE-SW oriented left lateral strike-slip faults. The canyon system coincides with a large releasing bend and/or extensional duplex of the Paks–Szolnok strike-slip system being active as sinistral during the Late Miocene.Presumably, the formation of the deep canyons was generated by the close interaction of several factors and events in space and time, among them tectonic uplift forced relative base-level fall, the reactivation and bending/duplexing of a strike-slip fault system located near the coeval zone of the lake shoreline and shelf edge, and the possible change of sediment supply carried by overfed rivers.

Water-level changes and their effect on deepwater sand accumulation in a lacustrine system: a case study from the Late Miocene of western Pannonian Basin, Hungary

Major deepwater sand accumulations are known to be connected to sea-level lowstands in marine basins, due to the relative lack of accommodation space on the shelf. This model was tested for the Late Miocene Lake Pannon, a large endorheic lake depositing a sedimentary succession up to 3–4 km thick. The progradation of the shelf between 9.7 and 6 Ma was tracked along a 2D seismic network. The shelf-edge trajectory indicates continuous, moderate lake-level rise between 9.7 and 9 Ma. This is followed by alternating intervals of quasi-steady and rapidly rising relative lake level with a period of ca. 250 ky. However, due to the lack of significant lake-level drops, no third-order sequence boundary was identified in the studied time span (9.7–6 Ma). The sand ratio of the deepwater deposits does not show strong correlation with these lake-level changes; moreover, the units deposited under rising lake-level conditions generally contain slightly larger proportion of sand. These features can be explained by the climatic origin of water-level rises: increasing precipitation enhances not only water inflow, but also sediment influx, which can overwhelm the well-known effect of accommodation space on the sand transport toward the deep water. The results suggest that shelf-margin trajectory might not be an efficient tool for predicting deepwater accumulations of sand in endorheic lacustrine basins.

Minor Elements Utility in Paleo-lake Evolvement and Signification of Lower Tertiary Fning Formation，Gaoyou Sag, Subei Basin

According to the study vertical variations of trace elements: Boron(B) and Strontium(Sr) contents testing in clay rock of tertiary Funing Formation(from bottom to up :E1f1、 E1f2、 E1f3、 E1f4), Gaoyou sag, Subei Basin of China., the water of palaeolake in Gaoyou sag of Subei Basin changed from freshwater(E1f1、E1f3) gradually to salt water(E1f2、E1f4). The calcareous cement is formed in early diagenesis. It also means that Gaoyou palaeolake was twice opened to the sea from the first section of Fning Formation to the fourth (E1f1-E1f4). During the second and fourth of Funing Formation (E1f2、E1f4）, the Gaoyou palaeolake was the deepest and the area was largest. Comprehensive analysis of trace elements indicates that palaeo-salinity and depth fluctuated during different sedimentary periods. The sedimentary periods of Funing Formation can be divided into 9 intermediate cycles with relative lake level change. The paleolake level was relative rising quickly in the second and fourth of Funing Formation (E1f4) and relative descending quickly in the first Funing Formation (E1f1). Key words: palaeolake； trace elements； lower tertiary Fning Formation； Gaoyou sag

RESERVOIR POTENTIAL OF A LACUSTRINE MIXED CARBONATE / SILICICLASTIC GAS RESERVOIR: THE LOWER TRIASSIC ROGENSTEIN IN THE NETHERLANDS

The Lower Triassic Rogenstein Member of the Buntsandstein Formation produces gas at the De Wijk and Wanneperveen fields, NE Netherlands and consists mainly of claystones with intercalated oolitic limestone beds. The excellent reservoir properties of the oolites (φ= 20‐30%; k = 5‐4000 mD) are predominantly controlled by depositional facies. Oolitic limestones are interpreted as the storm and wave deposits of a shallow, desert lake located in the Central European Buntsandstein Basin.The vertical sequence of lithofacies in the Rogenstein Member indicates cyclic changes of relative lake level. The reservoir rock is vertically arranged in a three‐fold hierarchy of cycles, recognised both in cores and wireline logs. These cycles are a key to understanding the distribution of reservoir facies, and are used as the basis for a high‐resolution sequence stratigraphic correlation of the reservoir units.Slight regional‐scale thickness variations of the Rogenstein Member (in the order of tens of metres) are interpreted as the effects of differential subsidence associated with the inherited Palaeozoic structural framework. The depositional basin can be subdivided into subtle palaeo‐highs and ‐lows which controlled facies distribution during Rogenstein deposition. Oolitic limestones show their greatest lateral extent and thickest development in the Middle Rogenstein during large‐scale maximum flooding.Potential reservoir rocks (decimetre to metres thick) are present in the NE Netherlands, in particular in the Lauwerszee Trough and the Lower Saxony Basin, where abundant gas shows of 200 ‐ 4000 ppm CH4 have been recorded. Preserved primary porosity is interpreted to be a result of rapid burial in subtle depositional palaeo‐lows in this area. The thickest, amalgamated oolite intervals (tens of metres thick) occur in the eastern part of the Central Netherlands Basin. Because of poor reservoir properties, other areas appear to be less promising in terms of Rogenstein exploration potential.

Periglacial sediment variations controlled by late Quaternary climate and lake level change at Elgygytgyn Crater, Arctic Siberia

Late Quaternary sediments in a permafrost environment recovered from the Elgygytgyn Impact Crater were studied to determine regional palaeoenvironmental variability and infer past water‐level changes of the crater lake. Stratigraphic analysis of a 5 m long permafrost core is based on various lithological (grain size, total organic carbon, magnetic susceptibility) and hydrochemical (oxygen isotope composition, major cation content) properties and pore ice content. The results show that alluvial sediments accumulated on top of cryogenically weathered volcanic rock. Changes in the hydrochemical properties reflect different stages of cryogenic weathering. The lithological characteristics mark the transition from an erosive site to a site with accumulation. This environmental change is linked to a relative lake level highstand at &gt;13 000 yr BP, when a shoreline bar was formed leading to slope sedimentation. Lake level dropped by 4 m during the Holocene.

Elucidating aeolian dune history from lacustrine sand records in the Lake Michigan Coastal Zone, USA

Aeolian sand in two small lakes within a Lake Michigan coastal-dune complex southwest of Holland, Michigan, provides a more detailed higher-resolution history of dune activity, during the later half of the Holocene, than do combined palaeosol and OSL ages from the dunes themselves. The sand signal from four cores within these lakes consists of visible sand laminae and invisible sand peaks within mud and sapropel units. Early in the history of dune growth the sand may have come from backdunes immediately adjacent to the lakes, but in the upper portions of the core aeolian sand is interpreted as having been transported from 30—40 m high parabolic dunes ~200 m from the lakes. All but one episode of dune mobility suggested by radiocarbon-dated dune palaeosols or OSL-dated dune sand are represented by a peak in the radiocarbon-dated lacustrine sand signal. However, sand peaks occur in the lacustrine record that do not correspond to palaeosol or OSL ages, suggesting that the sand record from small lakes gives a more complete record of aeolian activity. Heterogeneity in sand deposition related to bathymetry suggests that the study of multiple cores from the same lake will yield the best results . No strong correlation between rising lake levels on the Lake Michigan relative lake level curve and aeolian activity was found, although such a correlation has been reported elsewhere.

Lower Triassic sequence stratigraphy of the western part of the Germanic Basin (west of Black Forest): Fluvial system evolution through time and space

The aim of this paper is to analyse the fluvial evolution of the Lower Triassic in the western part of the Germanic Basin through time and space, as well as the impact of the geodynamic and climatic setting on the preservation of fluvial deposits. The Lower Triassic crops out only in the Vosges Massif and the Black Forest, so well-log studies are required to realise sequence stratigraphy correlations and establish comparisons with others parts of the Germanic Basin. In a first step, we use well-log data analyses to characterise the electrofacies associations in the Triassic and then define the well-log signatures of each formation. In a second step, the characterisation and recognition of genetic sequences and their stacking pattern allow us to define seven minor cycles integrated into two major cycles. Finally, the quantification of the lithologies at different stages of basin evolution leads to the reconstruction of paleoenvironmental maps to illustrate facies evolution through space and time. A comparison with cycles defined in the Germanic Basin allows us to propose correlations of the Lower Triassic on either side of the Rhine Graben and leads to a discussion of the evolution of fluvial systems through time and space. During the Scythian, the fluvial style is characterised by braided fluvial systems evolving laterally into lake deposits towards the central part of the Germanic Basin. During this stage, the basin was a huge depression with very few marine connections in its extreme eastern part. The stratigraphic cycles represent rhythmic fluctuations in relative lake level that could be attributed to sediment supply and/or lake level variations in an arid setting. Four minor stratigraphic cycles are observed that are integrated within a single major stratigraphic cycle. During the period of the stratigraphic base-level rise of the major cycle, a maximum of 233 m of sediment would represent a duration of sedimentation in the Paris Basin of at least 1.8 m.y. During the period of the stratigraphic base-level fall of the major cycle, a maximum of 65 m of sediment would have accumulated over 2.5 m.y. On the western edge of the Germanic Basin, the top of the Scythian is marked by a major sedimentary break characterised by a planation surface, with preservation of the first paleosols, followed by the Hardegsen unconformity. This unconformity is tectonically deformed, leading to the development of a new sedimentation area in the west of the basin. The fluvial sedimentation above this discontinuity shows a trend towards enhanced development of floodplain or lacustrine-type environments at its western margin, with the formation of paleosols. The fluvial systems are linked with sabkhas, and then with a shallow sea connected to the Tethys Ocean. In this context, the stratigraphic cycles are caused by variations in relative sea level and/or sediment supply. The fluvial deposits are preserved in an exoreic basin.

Solar and temporal effects on Escherichia coli concentration at a Lake Michigan swimming beach.

Studies on solar inactivation of Escherichia coli in freshwater and in situ have been limited. At 63rd St. Beach, Chicago, Ill., factors influencing the daily periodicity of culturable E. coli, particularly insolation, were examined. Water samples for E. coli analysis were collected twice daily between April and September 2000 three times a week along five transects in two depths of water. Hydrometeorological conditions were continuously logged: UV radiation, total insolation, wind speed and direction, wave height, and relative lake level. On 10 days, transects were sampled hourly from 0700 to 1500 h. The effect of sunlight on E. coli inactivation was evaluated with dark and transparent in situ mesocosms and ambient lake water. For the study, the number of E. coli samples collected (n) was 2,676. During sunny days, E. coli counts decreased exponentially with day length and exposure to insolation, but on cloudy days, E. coli inactivation was diminished; the E. coli decay rate was strongly influenced by initial concentration. In situ experiments confirmed that insolation primarily inactivated E. coli; UV radiation only marginally affected E. coli concentration. The relationship between insolation and E. coli density is complicated by relative lake level, wave height, and turbidity, all of which are often products of wind vector. Continuous importation and nighttime replenishment of E. coli were evident. These findings (i) suggest that solar inactivation is an important mechanism for natural reduction of indicator bacteria in large freshwater bodies and (ii) have implications for management strategies of nontidal waters and the use of E. coli as an indicator organism.

Sequence stratigraphy of the Westphalian in the northern part of the Southern North Sea

Abstract A series of stacked fluvio-lacustrine sequences in the Westphalian of the northern part of the Southern North Sea can be correlated over areas of greater than 2500 km 2 . Each sequence is similar in terms of thickness (100–300 m) and the internal arrangement of lithofacies but they differ in colour and in the amount of sandstone and coal that they contain. A typical sequence consists of a sand-rich lower unit, a mud-dominated middle unit which fines upwards towards its top and a mud-dominated upper unit which generally coarsens upwards. The main reservoir-prone intervals are confined to the lower-middle parts of sand-rich units and consist of coarse-grained braided or low-sinuousity channel deposits. These fluvial sandstones are laterally extensive and are neither typical of incised valley fills (in terms of stratal geometries) nor of progradational systems tracts (in terms of facies distribution). Instead they appear to have been deposited during periods of widespread fluvial aggradation caused by transgression as a result of relative lake-level rise. Each lower sand-rich unit is overlain by a dark mudstone with a high gamma ray expression which is interpreted as an initial (lake) flooding surface marking the onset of predominantly lacustrine and mire sedimentation associated with the development of the fining-upwards mud-dominated unit. The top of each fining-upwards unit is marked by a maximum gamma ray peak interpreted as a maximum flooding surface which defines the end of lake transgression. Above the maximum flooding surface, each coarsening-upwards mud-dominated unit shows increasing deltaic influence upwards associated with a slowing in the rate of relative lake level rise. The tops of some of these units are eroded suggesting that in some case deltaic progradation was followed by a relative fall in lake level; in other cases the boundary with the overlying transgressive sand-rich unit in the next sequence is wholly conformable. Coal mires developed in coastal areas, particularly during transgression when coarse-grained sediment was trapped up-dip on the coeval alluvial plain. Relative changes in lake level had a periodicity of 1.5 Ma or less indicating that they may be related to third order eustatic cycles.

The contribution of TOPEX/POSEIDON to the global monitoring of climatically sensitive lakes

Remote sensing and long‐term monitoring of closed and climatically sensitive open lakes can provide useful information for the study of climatic change. Satellite radar altimetry offers the advantages of day/night and all‐weather capability in the production of relative lake level changes on a global scale. A simple technique which derives relative lake level changes is described with specific relevance to the TOPEX/POSEIDON geophysical data record data set. An assessment of the coverage and global tracking performance of both the NASA radar altimeter and the solid state altimeter over these lakes is discussed, and results are presented for the first 1.75 years of the mission. Lake level time series were acquired for 12 closed lakes, six open lakes, and three reservoirs, providing information in many cases where ground gauge data are unobtainable or the lake is inaccessible. The results, accurate to ∼4 cm rms, mark the beginning of a very accurate lake level data set, showing that TOPEX/POSEIDON can successfully contribute to the long‐term global program.

Radar altimetry: A new concept in monitoring lake level changes

Remote sensing of closed and climatically sensitive open lakes can provide useful information for hydrological applications and, through long‐term monitoring, for climatic change. One new method that offers a unique way of providing relative lake level data in areas where ground gauge measurements are unavailable uses satellite radar altimetry.Global measurement of climate‐related variables will help to detect and monitor climate changes as they occur and validate climate models and related process studies. Because lake volumes respond to changes in precipitation integrated over their catchment basins [Street‐Perrott et al., 1986], they can act as important indirect indicators of climatic change. This response is particularly marked for closed lakes which have no significant surface or subsurface outflow.