Search

Stromatolites in the essentially non-marine Purbeck Formation (Tithonian, Upper Jurassic) west of Aylesbury (Buckinghamshire, UK) cap an erosion surface truncating lacustrine–palustrine carbonates, previously documented from an exposure close to the site of the former Bugle Pit, Hartwell. The stromatolite occurrences in this area follow a regional northwest–southeast structural grain, evidenced by local normal faults, some demonstrably active during Late Jurassic–Early Cretaceous time. Observations from associated lithofacies and biota suggest a littoral lacustrine setting for the stromatolite 'belt'. We hypothesise that the present-day stromatolite distribution reflects a fault-controlled palaeo-shoreline, and that growth of the stromatolites was linked to lacustrine transgression driven by syn-sedimentary movement on these faults. Littoral sand associated with the stromatolites was possibly reworked from older marine sediments undergoing erosion on a nearby fault footwall, or distally, on the western margin of the Anglo-Brabant Massif. [ABSTRACT FROM AUTHOR]

Dolomite is globally present in past geological records, but rare in modern environments. The mechanisms favouring its precipitation under ambient conditions remain highly debated. This study investigates sediments, containing high concentrations of early diagenetic calcian dolomite, from alkaline Lake Van (Republic of Türkiye, formally Turkey) dating back to 252 ka bp. Powder X‐ray diffraction and scanning electron microscopy evidence suggests that dolomite formation is associated with prior dissolution of aragonite and low‐Mg calcite and a subsequent co‐precipitation with, and/or partial transformation of, high‐Mg calcite into dolomite. The infrequent presence of diatom frustules encapsulated by dolomite suggests, for Lake Van, unusually low pore‐water pH at the time of dolomite formation. Conditions facilitating the preservation of silica, as well as dissolution and subsequent reprecipitation of carbonate phases, could result from periodic reventilations of Lake Van's deep water and an advection of pore fluids with contrasting redox potential and chemical concentration gradients. This continental analogue of the coastal 'mixing‐zone' dolomitization model argues that not overcoming a single specific hydrochemical threshold, but highly dynamic and fluctuating conditions trigger dolomite formation in Lake Van. [ABSTRACT FROM AUTHOR]

In impact crater lakes, the lacustrine sedimentary records may not solely reflect climatic changes but also potential erosional effects from lithologically distinct impactite formations. The hydrochemical and biogeochemical processes during the deposition of the Nördlinger Ries impact crater lake, which fall in the range of the mid-Miocene Climate Transition, were studied by analysing microcrystalline authigenic carbonates in a drill core succession, using stable oxygen and carbon isotopes in conjunction with biomarkers. These investigations revealed an early sulfidic interval characterized by thiophenes, iso- and anteiso-C 15:0 acids derived from sulfate reducing bacteria, and dolomites with low to intermediate δ13C carb values. The subsequent distinctive interval is characterized by extremely 13C-enriched dolomite (δ13C carb up to +20.93 ± 0.05‰ V-PDB), decline of iso- and anteiso-C 15:0 acids and is rich in an Archaea-derived archaeol that is 13C-enriched (−14.7‰), indicating extensive methanogenesis in sulfate-depleted lake porewater during early diagenesis. The sulfate decline results from successive sulfate reduction when replenishment by sulfate-bearing inflow water is limited. The carbonates exhibit enriched 18O due to pronounced evaporation in a long-resided water body and enriched 13C by methanogenesis. A change in provenance of water derived from the sulfur-rich suevite (impact melt-bearing breccia) and crystalline source rocks to the sulfur-poor Bunte Breccia (continuous ejecta blanket) is required. Intermittently high Si/Al and Zr/Al at the high δ13C interval suggests sporadic short-term runoff increase, leading to fluctuating physiochemical lake conditions. A subsequent decline in both δ13C carb and archaeol indicates a decreasing lake level with intermittent subaerial exposure events, supported by bioturbation and mud cracks. The concomitant lake oxygenation is well supported by increasing Pr/Ph ratios and lipids derived from aerobic methanotrophs (13C-depleted 3-methyl-hopanoids). In the youngest unit, allochthonous lignites and biomarkers from lacustrine/soil sources appear, high total sulphur contents and thiophenes recur, and stable C and O isotope values decrease again. These observations suggest another major provenance change of the inflowing solutes, with increasing influx from weathered pyrite-bearing Jurassic claystones. These findings demonstrate that the climatic record expected from the stable carbon and oxygen isotopes of the Ries carbonates is strongly overprinted by hydrochemical and biogeochemical processes due to changing ion influx from substrate rocks, along the course of the successive ejecta erosion and catchment changes. Such an intrinsic control of lacustrine biogeochemical processes is also expected for other hydrologically closed impact crater lake basins, where catchment rocks with distinctively different lithologies are present. [ABSTRACT FROM AUTHOR]

Abstract On the Island of Samos (East Aegean region, Greece), two sedimentary basins are filled by thick continental series dated to the Late Miocene to Early Pliocene. A multidisciplinary study has been performed including (1) the definition of 21 sedimentary facies, (2) a review of the biological components and (3) carbon, oxygen and strontium stable isotope analyses. The succession is characterised by various depositional settings and hydrochemical compositions. Five main stages of basin evolution have been identified: (1) The Late Serravallian is marked by the development of alluvial fans and fan delta; (2) during the Lower Tortonian, isolated shallow lakes with variable salinity, from fresh to brackish, developed under warm and relatively humid conditions; (3) the Middle to Upper Tortonian is marked by the development of a large and deep lake with saline and alkaline waters, under colder and drier conditions; (4) the Latest Tortonian to Messinian period is represented by an ephemeral alluvial system, developed under a dry climate; (5) during the Zanclean, a palustrine and paludal wetland system, dominated by tufa carbonates, developed under moderately humid conditions. This succession is of particular interest for the reconstruction of the palaeoenvironmental evolution of the transition zone between the Mediterranean domain, and the Paratethys and circum‐Paratethys areas. The geochemical data and the presence of flora (diatoms) and fauna (gastropods) of marine affinity suggest transient ingressions of marine‐related water or groundwater inflows as early as the Lower Tortonian. The Samos succession records the complex interaction between the regional geodynamics and climate. The extensional regime of the Eastern Aegean zone generates subsidence, interrupted in the mid‐Tortonian (9 Ma) by a brief compressive event and a major exposure of the basins. Furthermore, the Late Miocene progressive aridification, followed by a change to a more humid climate (Pliocene) is also a major driver of the sedimentation.

Abundant hydrocarbons have been produced from the pre‐salt lacustrine carbonates of the Cretaceous Barra Velha Formation in the Santos Basin of offshore Brazil. The highly heterogeneous lithology and pore systems make it difficult to predict the reservoir quality via petrophysical well logs. Core description, computed tomography (CT) scanning, thin sections and wireline well logs are utilized to characterize the lithology, pore spaces are reservoir quality of the Cretaceous Barra Velha Formation pre‐salt lacustrine carbonates in the Santos Basin. The results show that the lithology mainly includes shrubstone, grainstone, laminated micrite and massive micrite. The pore systems are dominated by secondary pores, such as intergranular pores, dissolution pores and intercrystalline pores as well as modified primary pores. Geological well logs (conventional well logs, electrical and sonic image logs and nuclear magnetic resonance [NMR] logs) are integrated to build the predictable models of the carbonate reservoirs. The shrubstone, which is dominated by intergranular pores, has the highest reservoir quality, which can be derived from NMR logs. Grainstone contains both primary intergranular pores and abundant secondary dissolution pores. The laminated and massive micrite contain only micropores and therefore have the lowest reservoir quality. Lastly, conventional well logs integrated with electrical and sonic image logs are utilized to predict the distribution of lithology and reservoir quality in single wells, and the results are testified by the NMR log data. The shrubstone and grainstone are future exploration targets of the Cretaceous Barra Velha Formation in the Santos Basin. The results are hoped to give insights into the petrophysical behaviours of various lithologies, and provide theoretical and technical guidance for reservoir evaluation and prediction. [ABSTRACT FROM AUTHOR]

On the Island of Samos (East Aegean region, Greece), two sedimentary basins are filled by thick continental series dated to the Late Miocene to Early Pliocene. A multidisciplinary study has been performed including (1) the definition of 21 sedimentary facies, (2) a review of the biological components and (3) carbon, oxygen and strontium stable isotope analyses. The succession is characterised by various depositional settings and hydrochemical compositions. Five main stages of basin evolution have been identified: (1) The Late Serravallian is marked by the development of alluvial fans and fan delta; (2) during the Lower Tortonian, isolated shallow lakes with variable salinity, from fresh to brackish, developed under warm and relatively humid conditions; (3) the Middle to Upper Tortonian is marked by the development of a large and deep lake with saline and alkaline waters, under colder and drier conditions; (4) the Latest Tortonian to Messinian period is represented by an ephemeral alluvial system, developed under a dry climate; (5) during the Zanclean, a palustrine and paludal wetland system, dominated by tufa carbonates, developed under moderately humid conditions. This succession is of particular interest for the reconstruction of the palaeoenvironmental evolution of the transition zone between the Mediterranean domain, and the Paratethys and circum‐Paratethys areas. The geochemical data and the presence of flora (diatoms) and fauna (gastropods) of marine affinity suggest transient ingressions of marine‐related water or groundwater inflows as early as the Lower Tortonian. The Samos succession records the complex interaction between the regional geodynamics and climate. The extensional regime of the Eastern Aegean zone generates subsidence, interrupted in the mid‐Tortonian (9 Ma) by a brief compressive event and a major exposure of the basins. Furthermore, the Late Miocene progressive aridification, followed by a change to a more humid climate (Pliocene) is also a major driver of the sedimentation. [ABSTRACT FROM AUTHOR]

The history and results of petroleum exploration in the Santos Basin, Brazil are reviewed. The regularity of hydrocarbon enrichment and the key exploration technologies are summarized and analyzed using the seismic, gravity, magnetic and drilling data. It is proposed that the Santos Basin had a structural pattern of two uplifts and three depressions and the Aram-Uirapuru uplift belt controlled the hydrocarbon accumulation. It is believed that the main hydrocarbon source kitchen in the rift period controlled the hydrocarbon-enriched zones, paleo-structures controlled the scale and quality of lacustrine carbonate reservoirs, and continuous thick salt rocks controlled the hydrocarbon formation and preservation. The process and mechanism of reservoirs being transformed by CO2 charging were revealed. Five key exploration technologies were developed, including the variable-velocity mapping for layer-controlled facies-controlled pre-salt structures, the prediction of lacustrine carbonate reservoirs, the prediction of intrusive/effusive rock distribution, the detection of hydrocarbons in lacustrine carbonates, and the logging identification of supercritical CO2 fluid. These theoretical recognitions and exploration technologies have contributed to the discovery of deep-water super-large reservoirs under CNODC projects in Brazil, and will guide the further exploration of deep-water large reservoirs in the Santos Basin and other similar regions.

The Barra Velha Formation encompasses the most important hydrocarbon reservoirs of the Pre-salt section of the Santos Basin, SE Brazil, and its lacustrine carbonates have drawn much attention in the past decade due to their sedimentological complexity and overall heterogeneity. Among the main challenges in the characterization of such rocks are the great depth of the reservoirs and the difficulty to characterize geological features in non-cored intervals. Additionally, the relationship between facies and petrophysical properties is difficult to establish with conventional well log evaluation routines. In this study, a continuous ultrasonic borehole image log (UBI) is integrated with conventional logs, nuclear magnetic resonance logs (NMR), and core samples to support the detailed geological interpretation and petrophysical evaluation of the lacustrine carbonates of the Barra Velha Formation (wholly Aptian). The systematic interpretation of the UBI log resulted in the identification of acoustic image log facies (AIFs), which were then translated into lithofacies. The entire studied interval was divided into eight zones, characterized by particular facies associations and petrophysical properties. Each zone was interpreted to have formed in depositional environments ranging from deep subaqueous to emergent conditions. A general shallowing and cleaning upward pattern was identified for the Barra Velha Formation, with a main reservoir zone associated with intermediate-to-shallow subaqueous conditions. This work aimed to explore the applications and limitations of the acoustic image log in the context of the Brazilian pre-salt and establish the relationship between image log facies and reservoir properties retrieved from NMR data. [ABSTRACT FROM AUTHOR]

The Coorong Lakes, South Australia, are one of the models for unravelling the 'Dolomite Problem'. Critically, today only a few modern environments remain where large quantities of very high magnesium calcite (VHMC; Ca0.5Mg0.5CO3; also described as protodolomite or disordered dolomite) and magnesite (MgCO3) precipitate. Previously conducted laboratory studies demonstrate that carbonate minerals can precipitate via classical and non‐classical crystallization pathways. This study uses the preserved crystal sizes, morphologies and microstructures of Ca–Mg carbonates in the Coorong Lakes (Milne Lake, Pellet Lake and North Stromatolite Lake) to evaluate which crystallization pathway most likely occurred. In the fine‐grained sediments of these lakes, very high magnesium calcite and magnesite occur as aggregate particles of nanocrystals (<100 nm). Rietveld refinements using X‐ray diffraction data give modelled Lvol–IB crystallite size values of <120 nm for all carbonates. Transmission electron microscopy shows that, within VHMC and magnesite particles, nanocrystals have an almost identical orientation of their crystal lattice fringes. This is morphologically similar to Ca–Mg carbonates formed via an amorphous carbonate precursor in non‐classical crystallization laboratory experiments. Precipitation of carbonate minerals via an amorphous‐to‐crystalline pathway requires the water to be supersaturated relative to both crystalline and amorphous phases. In the Coorong Lakes, surface water likely only becomes supersaturated relative to amorphous carbonate phases in the late summer after extensive evaporation. Observations suggest that VHMC and dolomite do not directly precipitate from bulk modern seawater, despite oversaturation relative to the crystalline phases, because seawater is undersaturated with respect to amorphous calcium magnesium carbonate, thus limiting the precipitation through a non‐classical crystallization pathway. [ABSTRACT FROM AUTHOR]

The upper Aptian lower Codó Formation is an attractive stratigraphic unit within the Parnaíba and São Luís basins, northeastern Brazil, formed during the break-up of Western Gondwana. Located in a large sag basin, the formation records a dynamic depositional history of deltaic, lacustrine, and sabkha environments. This sedimentary evolution is reflected in a diverse lithological succession characterized by intermittent terrestrial input, episodic marine incursions, and restricted lacustrine phases culminating in sulfate evaporite precipitation. While the later stages of the formation have been extensively studied, the initial sedimentary processes and environmental conditions remain poorly understood. This research aims to reconstruct the depositional paleoenvironments during the initial stages of Codó sedimentation, elucidating the onset of the alkaline hypersaline lake environment and the earlier marine incursions in the study region. To achieve this, it was conducted a high-resolution facies analysis using core descriptions, trace fossil analysis, petrography, various types of X-ray fluorescence, scanning electron microscopy, X-ray diffractometry, and isotope analyses (δ1³C, δ1⁸O, and ⁸⁷Sr/⁸⁶Sr) on a ∼63-m-thick succession from borehole 2-TV-1-MA. Thirteen facies were recognized and grouped into brackish lake, prodelta, delta front, and alkaline hypersaline lake facies successions. The lower Codó Formation exhibits cyclic sedimentation, indicating fluctuating water dynamics associated with shifts between open and closed drainage systems controlled by paleoclimate. Tectonic subsidence drove the basin's evolution from a brackish lacustrine-deltaic environment with an incipient marine influence (balanced-fill basin stage) to a restricted system with the establishment of an alkaline hypersaline lake (underfilled basin stage). • Facies analysis for the Early Cretaceous paleoenvironmental reconstruction during the western Gondwana breakup. • Stacking patterns recording contraction and expansion cycles of the water body controlled by paleoclimate. • Short-lived marine incursion in the early stages of Codó sedimentation using petrography, ichnofacies, and isotopes. • Transition from a brackish lacustrine-deltaic system to an alkaline hypersaline lake controlled by tectonic subsidence. [ABSTRACT FROM AUTHOR]

Much dissolved inorganic carbon (DIC) may exist in lacustrine carbonates in the terminal lakes of closed basins in arid areas, and inorganic carbon sinks in arid regions are an important hypothesis of global carbon cycle research in recent years. Previous studies have made significant progress in estimating the inorganic carbon content. However, the source of inorganic carbon sinks remains unclear. Here, we choose four typical arid closed basins in North China to investigate the provenance of inorganic carbon sinks. The results show that CO2 absorbed by saline‐alkaline soil is the main source of groundwater DIC in the study area, with a contribution rate of approximately 50%. Forty‐three groundwater radiocarbon ages from arid closed basins indicate that the DIC in groundwater is transported along the direction of water flow and deposited in terminal lakes, forming inorganic carbon sinks. Our study suggests that the total inorganic carbon contents of Zhuye Lake, Yanchi Lake, Huahai Lake, and Qinghai Lake in the Holocene are 0.318 Pg, 0.003 Pg, 0.160 Pg, and 3.289 Pg, respectively. The results demonstrate that inorganic carbon sinks are much larger than lake organic carbon storage, thereby serving as an important carbon cycle factor in arid regions. Plain Language Summary: As observation methods and experimental technologies have advanced, research on the global carbon cycle has penetrated all links of the Earth's surface system. The long‐neglected arid zone ecosystem exerts a significant impact on the interannual changes in global terrestrial carbon sinks. Despite this, research on the source of inorganic carbon sinks in arid areas is still insufficient. Therefore, age measurements and isotopic analyses are conducted in groundwater, river water and sediments in four arid closed basins of North China, and the main sources of inorganic carbon sinks in arid closed basins are explored. The results show that groundwater dissolved inorganic carbon is mainly derived from the CO2 absorbed by the saline‐alkaline soil through river leakage and irrigation and finally reaches the terminal lakes. Taken together, soil CO2 is one of the leading sources of inorganic carbon sinks in closed basins. Key Points: CO2 absorbed by saline‐alkaline soil is the main source of groundwater dissolved inorganic carbon (DIC) with a contribution rate of about 50%The DIC formed by the carbon absorption of saline‐alkaline soils becomes an inorganic carbon sinkThe rate of Holocene inorganic carbon sinks from lacustrine carbonates is much higher than that of organic carbon accumulation [ABSTRACT FROM AUTHOR]

Changes in sedimentary profiles and stratigraphic architecture over time in lake basins are shown to be relevant markers of climate changes in terrestrial palaeoenvironments. The stratigraphic architecture of the Priabonian (late Eocene)–early Rupelian (early Oligocene) Alès–Saint‐Chaptes–Issirac saline lake system has been reconstructed from the sedimentological, biostratigraphic and magnetostratigraphic analysis of outcrops and well cores. As a function of the inflow–evaporation balance, three distinct depositional models have been proposed for the Priabonian Alès–Saint‐Chaptes–Issirac lake system which is characterized by both fault‐controlled and low‐angle ramp‐like margins: (i) freshwater to oligohaline lake; (ii) oligohaline to mesohaline closed‐lake; and (iii) hypersaline, evaporitic closed‐lake. Two major stages of lake spreading (possibly middle Priabonian and late Priabonian–early Rupelian) separated by two stages of significant lake closure represented by evaporite succession in the Alès Basin depocentre (middle to late Priabonian) have been evidenced. Regional tectonics and climate induced changes in inflow–evaporation balance of the lake and controlled the development of transgressive–regressive cycles with calcium‐sulphate sedimentation during regression in the lake depocentre (Alès Basin) and during early transgression in lake margins (Saint‐Chaptes and Issirac basins). The correlation between the palaeohydrological cycles inferred for the Alès–Saint‐Chaptes–Issirac lake system and the regional climate changes of Priabonian stage, prior to the Eocene–Oligocene transition, strongly suggests that climate significantly impacted the nature of lacustrine depositional models and the depositional architecture of Palaeogene, syn‐rift, saline lake basins from Western Europe. [ABSTRACT FROM AUTHOR]

During the Cretaceous, a high global sea level and low latitudinal temperature variations led to the growth of epeiric supergene carbonate platforms. Lacustrine carbonates are widely developed within the late Santonian stage in the Songliao Basin (Northeast China), and these lacustrine carbonate rocks have great significance for reconstructing the paleoclimate and paleoenvironment of the late Santonian stage and locating potential oil and gas resources. Based on analyses of the lithology, mineralogy, carbon and oxygen isotopes and element geochemistry of typical sections, the paleoclimate and paleoenvironment of the late Santonian stage were reconstructed, and the control of variations in the paleoclimate and paleoenvironment on lacustrine carbonate deposition was investigated. Four ostracod limestone layers (OLLs), and three dolomite concretion layers (DCLs), were identified in the typical sections. The element ratios (Fe/Mn and Mg/Ca), isotopic values (δ13Ccarb and δ18Ocarb), and clay mineral composition indicated a warm paleoclimate in the late Santonian, with alternating dry and wet climates. During the humid paleoclimate interval, a high lake level and high CO2 concentration led to an acidic–anoxic lacustrine environment, coupled with high terrigenous debris input, probably induced a mass death extinction of ostracods. However, during the semiarid paleoclimate interval, a weakly alkaline water column with low oxygen and low terrigenous detritus input promoted quasi-syngenetic dolomitization. This study combined with previous paleontological and radioisotope dating studies indicate that the lacustrine carbonate deposits in member 1 of the Nenjiang Formation were probably the continental sedimentary response to the fluctuation of the warm climate in the late Santonian. [ABSTRACT FROM AUTHOR]

Lacustrine carbonate facies distribution is controlled by multiple environmental parameters including climate, hydrology, and tectonic setting, but few published models address this complexity. In this study, seismic and borehole data, integrated with outcrop logging, correlations, and facies models, are used to create a new tectono‐sedimentary model demonstrating how extensional faults, linked by a relay ramp, control distribution of lacustrine carbonate facies in the Upper Jurassic to Lower Cretaceous Purbeck Limestone Group (Wessex Basin, UK). Accumulation occurred in half‐graben sub‐basins south of two extensional east‐west faults, with widespread subaerial emergence of footwall blocks to the north. The lacustrine limestones of the lowest unit of this Group are characterised by in‐situ microbial mounds within bedded inter‐mound packstones‐grainstones. Mounds occur in three depositional intervals separated by paleosols. The distribution of facies indicates more brackish‐water conditions shoreward to the west, and more hypersaline conditions basinward to the east. The relay ramp hosts extensive microbial carbonate buildups formed in response to carbonate‐rich waters sourced from the northern limestone footwall blocks that fed into extensive shallow‐water areas on the low‐angle relay ramp slope. [ABSTRACT FROM AUTHOR]

Lacustrine carbonate deposits mark the rift-sag sedimentation stage of the so-called pre-salt section of the Campos and Santos Basins, in the Southeast Brazilian continental shelf. These salt-sealed carbonate reservoirs form deep-water, giant and prolific oil fields that are currently responsible for approximately 70% of the Brazilian oil and gas production. In the Santos Basin, the sag stage carbonates are represented by the Aptian Barra Velha Formation, characterized by exotic textural and compositional features whose origin, development, and correlation with petrophysical properties have been broadly discussed in the last decade. A geological and petrophysical study has been performed based on core samples, plugs, and thin-sections of a well in the Barra Velha Formation from the eastern Santos Basin. The analyzed core interval was grouped into six depositional facies constituted by the association of five key textural components: (1) crystal shrubs; (2) calcite spherulites; (3) hybrid forms; (4) mud-grade carbonates and dolomite; and (5) intraclasts. The relationship between the depositional framework and the poroperm system, as evidenced by petrographic image analysis and CT-scan data, is highly complex. The depositional control over porosity is erratic, mainly due to the extensive diagenetic and possible hydrothermal overprint. Dolomite cementation, dolomitization, pervasive silicification, and dissolution processes are the main primary porosity modifiers. Petrophysical analysis indicates that intraclastic and spherulitic grainstones–rudstones, affected by matrix dissolution, may present the best reservoir properties, in certain intervals, even surpassing the shrubby framestones, which are widely regarded as the best reservoir facies. This study seeks to contribute to an emerging body of research about the Brazilian pre-salt lacustrine carbonates, broadening the geological knowledge about these deposit, while providing new data to improve hydrocarbon forecast and recovery. [ABSTRACT FROM AUTHOR]

• Six lacustrine carbonate microfacies in the Lower Cretaceous Junggar Basin are identified. • A semi-open, shallow, and hydrologically partitioned lake in the Early Cretaceous. • Aridificaiton led to transition from hydrologically partitioned to integrated lake. We report sedimentary microfacies analysis of Lower Cretaceous algal-laminated limestones interbedded with siliciclastic sediments in the Junggar Basin. We use these data to fill a knowledge gap in the Jurassic–Late Cretaceous paleoenvironmental and paleoclimate reconstructions of Central Asia. To decipher paleoenvironments and genetic mechanisms, we applied optical petrography, cathodoluminescence and fluorescence imaging, Raman spectroscopy, stable isotopic, and scanning electron microscopic analysis. Some oncoids exhibit alternating dark organic carbon-rich micrites and light sparry calcite laminae with unequal thicknesses and binding structures of microbiota. Formed by both algal binding and inorganic calcite precipitation, in-situ mixing happened during the formation of oncoidal, ooidal, and ostracodal grainstones in an agitated lakeshore and thromobolites in a relatively low-energy environment. Sparry carbonate cortices peeled off from the oncoids were transported to the shallow lake and densely packed with micrites and mud. Based on the degree of covariance between δ18O and δ13C and the wide range of δ18O, the paleo-lake was inferred to be a semi-open, shallow, and hydrologically partitioned lake in the Early Cretaceous, but evolved into a closed but well-mixed lake in the Late Cretaceous. We infer that this change is caused by the aridification in the Late Cretaceous. The Early Cretaceous lacustrine carbonates in the Junggar Basin show overall more positive δ13C than the counterparts in the Upper Cretaceous, which is attributed to the 12CO 2 absorption by microbiota. Sulfate reduction is revealed by negative sulfur isotopes of concomitant thin-layered gypsums but it exerts no measurable impact on the δ13C of the carbonates. [ABSTRACT FROM AUTHOR]

We propose a new facies classification scheme of the cyclic lacustrine carbonates of the Bernburg Formation (Lower Triassic, Induan), with a major focus on the microbial biota and grains. Our data are based on a detailed bed-by-bed outcrop study in Central Germany, within a robust (litho)stratigraphic framework. We concentrate on two intervals of the Bernburg Formation: one around the Calvörde/Bernburg Formation boundary (including the HRZ – „Hauptrogensteinzone"), and a second around the ninth cycle near the upper formation boundary. The textural and mineralogical composition of the two endmember components: skeletal stromatolites and oolites, and the macro-, meso-, and microstructural characteristics (supplemented by μ-XRF-data) allow to classify the carbonates into five lithofacies types. They are interpreted with respect to the development of an ideal lacustrine depositional cycle, embedded between a lake level rise and a lake level fall. The microfacies attributes of the microbialites include various lamination types, shrubs, spherulites, and laminated/clotted/fenestral microfabrics. All sedimentologic (macro-, meso, and microscale) and geochemical data of this study indicate nearshore deposition of the carbonates at marginal shoals in a major endorheic lake (playa lake) with high alkalinity and salinity, and strongly fluctuating lake levels, under arid climates. The lacustrine carbonates are associated with maximum lake expansions, and are laterally interfingering with fan deltas, as indicated by abundant clastic grains in the intercalations of the skeletal stromatolites. The Bernburg Formation microbial buildups reflect changes in lake-level, hydrodynamics and grain supply, and therefore offer insights for the paleoenvironmental interpretation of lacustrine microbialites elsewhere. [ABSTRACT FROM AUTHOR]

The integrative analysis of a lacustrine carbonate succession from Butte Iouton hill (Vistrenque basin, SE France) brings new insights into depositional models and stacking patterns of oolitic saline lake margins and provides new details regarding the late Eocene paleogeography of southeast France. Depositional facies analysis and paleoenvironmental reconstructions allow reconstructing an oolitic lacustrine ramp model, displaying from the proximal to the distal areas: (1) shallow marginal saline lake domain with deposition of planar microbial laminites, and molluscan-ostracodal wackestone, (2) a more distal and open lacustrine environments with low to moderate energy characterized by the deposition of peloidal grainstones and (3) a domain of higher energy with accumulation of ooids mixed with peloids in the vicinity of the area of ooid production. Lake margin carbonate sedimentation dominantly occurred during stages of lake transgression while subaerial exposure surfaces developed during periods of negative inflow-evaporation balance (forced regression). Carbon and oxygen isotopes together with vertical trends in salinity inferred from molluscan associations show that lake transgression does not result from increasingly positive freshwater-evaporation balance volume but from the combination of subsidence and outflow from neighbouring saline waterbodies. The Butte Iouton carbonate margin is part of a set of interconnected saline lakes, occupying continental basins from Languedoc and Rhodanian region during the Priabonian, with a siliciclastic-dominated sedimentation in the southern margin, sourced by erosion of Pyrenean reliefs, and a carbonate-dominated northern margin with significant oolitic sedimentation in high-energy nearshore area.

Recent discoveries of giant hydrocarbon accumulations in coquinas and other lacustrine carbonate rocks of Rift and Post-rift (pre-salt) super-sequences of the Campos and Santos Basins have motivated a series of studies focused on understanding the genesis of these unique reservoirs. An integrated petrologic-sedimentologic-stratigraphic-seismic study of the Coqueiros Formation delineated the depositional model and evolution of reworked bioclastic rocks and other associated rocks deposited on the external high in the northern Campos Basin during the transition between late-rift and sag stages. Bioclastic rudstones and grainstones with moderate to high shell reworking represent 65% of the samples described. The following pieces of evidence point to a predominantly shallow lacustrine depositional environment, dominated by storm currents: (i) presence of high-angle (>14°) longshore cross-bedded rudstones; (ii) limited occurrence of muddy sediments interbedded with thick packages of amalgamated bioclastic rudstones; (iii) great lateral continuity of rudstone deposits devoid of matrix on the structural high, consistent with processes of storm action on large expanses of shallow lakes; (iv) deposits of massive rudstones, with disorganized shells, similar to the modern coastal bioclastic deposits of California and Shark Bay. Three sets of facies association are proposed: (i) high-energy facies, composed of bioclastic rudstones and grainstones devoid of matrix deposited above the fair weather wave base (FWB); (ii) low energy facies, composed of hybrid rocks with muddy or peloidal matrix and other fine grained rocks deposited below the FWB or in protected shallow environments; (iii) an alkaline set, composed of mainly magnesian clay ooids deposited in a more alkaline composition of lake water. The evolution of the facies associations over time demonstrates an overall fall of the relative lake level. High proportion of low energy facies associated with a deeper lacustrine environment occurs at the Coqueiros Fm. base, while at the top predominate high energy facies associated with a shallow lacustrine environment, in addition to magnesian clay-rich facies associated to restricted and alkaline water bodies. Paleocurrents suggest the activity of two main wind direction (northwest and southeast) over the external high that result in clean bioclastic facies with good reservoir potential on both flanks. The knowledge of wind directions during the late Barremian and early Aptian in the northern Campos Basin is extremely important for targeting the best depositional facies for hydrocarbon accumulation in the Coqueiros Formation. • A depositional model of bioclastic rocks composing giant oil reservoir is proposed on an oil field scale. • A shallow lacustrine depositional environment influenced by longshore and storm currents is identified. • The evolution of the facies associations demonstrate the general falling of lake level. • Measured paleocurrents point to two main wind directions that had an important role in the redistribution of the bioclasts. • Best depositional facies can be predicted for analogous reservoir based on the paleowind directions and facies model. [ABSTRACT FROM AUTHOR]

En este artículo se discute la evolución sedimentaria del relleno neógeno de un área situada en la parte Noroeste de la Cordillera Ibérica que presenta depósitos fluviales y lacustres, y su relación con la tectónica alpina compresiva que afectó a una falla anteriormente extensional. Los análisis estratigráficos permiten caracterizar tres unidades tectosedimentarias, con un espesor total de 120m. Las Unidades 1 y 2 están formadas dominantemente por litofacies clásticas, y están separadas por una discordancia y su correlativa conformidad. La Unidad 3 está formada por calizas tobáceas y micríticas, y representa un salto litológico brusco con la Unidad 2 en toda su extensión. Estas unidades están involucradas en dos estructuras de escala kilométrica con dirección alpina NNW-SSE: 1) la Falla de Nigüella, que pone en contacto materiales triásicos con cenozoicos en los bloques levantado y hundido, respectivamente, y 2) el Sinclinal de Nigüella, subparalelo a la falla anterior, con buzamiento de estratos entre 19º y 70º. En las unidades 1 y 2, cuatro asociaciones de facies sedimentarias representan el depósito en áreas proximales y medias de abanicos aluviales, procedentes de relieves locales, y un sistema fluvial braided con reducida llanura de inundación, procedente del Norte. Ambos ambientes registran sedimentación carbonática fluvial y lacustre. Otras tres asociaciones de facies representan el depósito dominante de carbonatos, tanto en entornos palustre-fluvial-lacustres como en lacustres. En las tres unidades, los rudstones de oncoides y fitoclastos se formaron en canales de aguas someras, con baja sinuosidad y en zonas encharcadas, con extensas zonas palustres tobáceas, con desarrollo de plantas hidrófilas. Por el contrario, las calizas micríticas con ostrácodos y las margas corresponden al depósito de calcita en zonas lacustres de aguas tranquilas y permanentes. Por tanto, el sistema sedimentario evolucionó desde un ambiente aluvial-fluvial dominante hasta un ambiente lacustre. Este hecho, junto con el desarrollo de calcretas en las unidades 1 y 2, indica condiciones más húmedas y posiblemente el paso a un lago cerrado hidrológicamente. La falla de Nigüella funcionó a principios del Jurásico como una falla normal, y durante la compresión del Cenozoico provocó la formación de una cuenca sinclinal mediante buttressing del bloque NE contra la falla. Este contexto condicionó la distribución y extensión de litofacies miocenas a lo largo del espacio y del tiempo y la localización de depocentros. La disminución de la actividad tectónica a lo largo del intervalo estudiado favoreció la incisión fluvial, la captura del acuífero mesozoico y la salida del agua subterránea rica en Ca2+ y HCO-3, dando lugar al depósito generalizado de carbonato cálcico y, finalmente, a la expansión del lago.

Tufa domes and towers are common around the margins of Winnemucca Dry Lake, Nevada, USA, a desiccated sub‐basin of pluvial Lake Lahontan. A 2·5 m diameter concentrically‐layered tufa mound from the southern end of the playa was sampled along its growth axis to determine timing, rate and geochemical conditions of tufa growth. A radiocarbon‐based age model indicates an 8200‐year tufa depositional record that begins near the end of the Last Glacial Maximum (ca 23 400 cal yr bp) and concludes at the end of the most recent Lahontan highstand (ca 15 200 cal yr bp). Petrography, stable isotopes and major and minor elemental compositions are used to evaluate the rate and timing of tufa growth in the context of the depositional environment. The deposit built radially outward from a central nucleation point, with six decimetre‐scale layers defined by variations in texture. Two distinct tufa types are observed: the inner section is composed of two layers of thinolite pseudomorphs after ikaite, with the innermost layer comprised of very small pseudomorphs (<0·25 cm) and an outer layer composed of larger, ca 3 cm long pseudomorphs, followed by a transitional layer where thinolite pseudomorphs grade into calcite fans. The outer section consists of three distinct layers of thrombolitic micrite with a branching mesofabric. The textural change occurred as lake levels began to rise towards the most recent Lahontan highstand interval and probably was prompted by warming of lake waters caused by increased groundwater flux during highstand lake levels. The Mg/Ca and Sr/Ca variations suggest a warming trend in the tufa growth environment and may also reflect increasing growth rates of tufa associated with increased fluxes of groundwater. This systematic study of tufa deposition indicates the importance of the hydrology of the lacustrine tufa system for reconstructing palaeoenvironmental records, and particularly the interaction of ground and surface waters. [ABSTRACT FROM AUTHOR]

The lacustrine carbonate has become an important exploration target, as the carbonates in half-graben Zhanhua Sag, a typical petroliferous lacustrine deposit, are not well-documented. Here the depositional characteristics, distribution patterns and controlling factors of the lacustrine carbonate deposition in Zhanhua Sag were studied, using seismic, well logging and core data associated with optical petrography, XRD and geochemical analysis. We identified 11 microfacies and 5 facies associations. The distributions of each facies associations were clarified, with results of carbonate-developed high-gradient margins and carbonate-restricted low-gradient margins, which could be concluded into two depositional models of different structural settings in half-graben Zhanhua Sag. In addition, climate, the outcropped lithology in the catchment area and structural settings are the main factors controlling the lacustrine carbonate microfacies types and their distribution here. These results could serve as important supplements and references to other lacustrine carbonate researches. [ABSTRACT FROM AUTHOR]

The volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mg‐phyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0·2 mol l−1 and pH values of ca 9·3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon–hydrogen–nitrogen elemental analyzer, X‐ray fluorescence spectrometry and X‐ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X‐ray diffraction analyses of the <2 μm clay fraction indicate the presence of a saponite‐like mineral, a Mg‐rich smectite. The saponite‐like mineral accumulates at depth to reach up to ca 30 weight %, concurrent with a decrease of the contents of hydromagnesite and organic matter. Thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions. The formation of the saponite‐like mineral instead of Al‐free Mg‐silicates resulted from high aluminum availability, which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting. Supersaturation of the lake water relative to saponite is especially due to high pH values, themselves derived from high primary productivity. This suggests that a genetic link may exist between saponite and the development of organic‐rich carbonate rocks, which may be fuelled by the input of CO2‐rich volcanic gases. This provides novel insights into the composition and formation of saponite‐rich deposits under a specific geodynamic context such as the Cretaceous South Atlantic carbonate reservoirs. [ABSTRACT FROM AUTHOR]

Highlights • Mg-clay deposits constitute the background of the sag lacustrine sedimentation. • Calcite crusts are syngenetic precipitates formed directly on Mg-clay laminations. • The silicification process occurred relatively quickly after dolomitization. • Fault systems have concentrated the dissolution and precipitation of minerals by fluid flow. • Hydrothermal alteration has an impact on permeability, fluid flow, and dynamic behavior. [ABSTRACT FROM AUTHOR]

The composite δ13C record of Neoproterozoic carbonates is characterized by large magnitude (<18‰ Vienna Pee Dee Belemnite) swings that have been interpreted to record (a) changes in dissolved inorganic carbon in sea water driven by a carbon cycle with different boundary conditions, (b) pervasive diagenetic alteration or (c) local controls similar to those active in Cenozoic platformal successions. A sedimentological study of the Trezona Formation in the Flinders Ranges of South Australia was conducted to determine the palaeoenvironmental context of the Trezona anomaly, one of the most widely recognized examples of a Neoproterozoic mega‐excursion that occurs beneath glaciogenic sediments of the Marinoan ice age (ca 635 Ma). Stratigraphic mapping identified an unconformity at its base separating it from the shelf deposits of the underlying Enorama Shale. The lower Trezona Formation is dominantly calcareous mudstone with mudcracks, channels, and mud chip and algal flake breccias interpreted to record exposure and desiccation on mudflats, shallow pools and lagoons. A marked increase in microbial carbonate and oolites in the upper Trezona Formation records a relative decline of mud input that terminates with karst. The Trezona Formation is thus interpreted to have been deposited in shallow, localized, salt withdrawal basins on the exposed shelf creating a mosaic of poorly connected lagoons and (alkaline) lakes that were intermittently restricted or isolated from the sea. Variations in δ13C and δ18O correspond with facies changes and reflect locally evolving depositional conditions influenced by sills and channels that shifted the sea water to freshwater balance. This restricted setting and range of δ13C values is more similar to Cenozoic platformal deposits (ca 12‰) than the muted (ca 3‰) global sea water variation recorded in pelagic sediments. The anomalous δ13C values in the Trezona Formation may instead provide constraints on the changing terrestrial biosphere directly preceding the evolution and rapid expansion of metazoan life. The Neoproterozoic Trezona carbon isotope anomaly represents one of the largest interpreted variations in sea water δ13C in the geological record. Here we show that the depositional environment was a series of restricted and intermittently isolated pools, lagoons and alkaline lakes associated with a local basin formed on an exposed shelf. While rarely considered, this study shows that Neoproterozoic carbon isotope values also record restricted and non‐marine conditions that do not constrain the global carbon cycle. [ABSTRACT FROM AUTHOR]

Lakes can be classified as open, semi-closed, and closed based on the balance between accommodation space and sediment–water fill. Different lake types can coexist in adjacent basins and rapidly change throughout time from one type to another. Understanding the processes that result from shifts in a lake's hydrological properties can be challenging, as well as characterizing its sedimentary products. The Aptian Codó Formation, in the Parnaíba Basin, north–northeast Brazil, contains in its record a complex facies succession deposited in a continental lacustrine system. In order to better understand the depositional aspects of different lake types, we characterized the lower portion of the Codó Formation using a multi-proxy approach. We combined litho- and microfacies analysis, ichnology, bulk chemical data, mineralogy, isotopic geochemistry, and organic matter analysis in the recently drilled, fully recovered borehole, 2-CO-1-MA. We defined eight lithofacies: bioturbated siltstones, dark gray mudstones, laminites, stratiform and domal stromatolites, heterolites rich in ostracods and organic muddy, and anhydrite and gypsum rocks. The isotopic data and the palynofacies analysis pointed out the existence of restricted conditions (TOC up to 19.6 wt%) with intermittent connections with seawater. Integrating all techniques, we interpreted five depositional stages: expanded lake, brackish ephemeral lake, perennial shallow lake, evaporitic sabkha, and stratified lake. These stages were influenced by the relation between inflow and evaporation rate, which were the main controls on sedimentation in the lacustrine-sabkha system studied. Our findings provide a detailed description and interpretation of the paleoenvironmental conditions related to the alternation from open to closed lakes, combining sedimentological, geochemical, and palynological characteristics of an inland lake system. [ABSTRACT FROM AUTHOR]

The southern Levant is a Mediterranean climate zone of complex variability in which uncertainty remains in regional palaeoclimate reconstruction. In spite of the proven value of diatoms in circum-Mediterranean palaeoenvironmental research, their potential remains largely unexplored in the southern Levant region. In this study, we generate a new, high-resolution multi-proxy record for the last ca. 9000 cal. yr BP, supported by diatom data and key biological, mineralogical and geochemical indicators preserved in a 17.8-m-long sediment sequence recovered from Lake Kinneret (the Sea of Galilee), Israel. During the Holocene, well-correlated shifts in the diatom, minero-geochemical and palynological data indicate marked lake-level variation over time as well as changes in the trophic state of Lake Kinneret. Our results are particularly important in improving the reconstruction of Holocene lake-level variation, and thus past moisture availability. Diatom-inferred lake-level oscillations correlate well with the output from climatic models from the Levantine region and clarify previous uncertainty concerning regional variation in moisture availability. The Early Holocene (from ca. 9000 to 7400 cal. yr BP) was characterized by lake-level shifts due to fluctuating dry-wet climate conditions. During the mid-Holocene (from 7400 to 2200 cal. yr BP), a stable, deep lake-level phase persisted due to high humidity. The lake level of modern Lake Kinneret not only fluctuates seasonally with available moisture, but has also been influenced for ca. 2000 years by the impacts of water abstraction for human consumption and agriculture. Over the last 9000 cal. yr BP, the trophic state of Lake Kinneret has changed from an oligotrophic to a meso- to eutrophic environment, mainly triggered by increased human impact from around 2200 cal. yr BP onwards. The lake's ecosystem status was not strongly affected by the documented major changes in human occupation patterns during the mid- Holocene, when a relatively stable environment persisted. [ABSTRACT FROM AUTHOR]

A 220-m thick carbonate-dominated succession has been deposited in shallow-water, saline lake environments during the early to middle Priabonian (MP17A-MP18 mammal zones) in the Saint-Chaptes Basin (south–east France). The palaeoenvironmental, paleoclimatic and palaeogeographic significance of such saline lake carbonates has been deciphered on the basis of a multi-proxy analyses including: 1) depositional and diagenetic features; 2) biological components (molluscs, benthic foraminifera, characean gyrogonites, spores and pollens); 3) carbon and oxygen stable isotopes; 4) trace elements; and 5) clay mineralogy. Five stages of lacustrine system evolution have been identified: 1) fresh-water closed lake under dry climate (unit U1); 2) fresh to brackish water lacustrine deltaic system with a mixed carbonate-siliciclastic sedimentation under relatively wet climatic conditions (unit U2); 3) salt-water lacustrine carbonate system under humid climatic setting (unit U3); 4) evaporitic lake (unit U4); and 5) closed lake with shallow-water carbonate sedimentation under subtropical to Mediterranean climate with dry seasons (unit U5). Upper Eocene aridification is evidenced to have started as early as the earliest Priabonian (unit U1: MP17A mammal zone). A change from humid to dryer climatic conditions is recorded between units U3 and U4. The early to middle Priabonian saline lake is interpreted as an athalassic (inland) lake that have been transiently connected with neighboring salt lakes influenced by seawater and/or fed with sulfates deriving from recycling of evaporites. Maximum of connection with neighboring saline lakes (Mormoiron Basin, Camargue and Central grabens, Hérault Basin) likely occurred during unit U3 and at the base of unit U5. The most likely sources of salts of these adjacent basins are: 1) Triassic evaporites derived from salt-diapirs (Rhône valley) or from paleo-outcrops located east of the Durance fault or offshore in the Gulf of Lion; or 2) marine incursions from the south, through Paleogene grabens in the Gulf of Lion. [ABSTRACT FROM AUTHOR]

Chemostratigraphic studies on lacustrine sedimentary sequences provide essential insights on past cyclic climatic events, on their repetition and prediction through time. Diagenetic overprint of primary features often hinders the use of such studies for palaeoenvironmental reconstruction. Here the potential of integrated geochemical and petrographic methods is evaluated to record freshwater to saline oscillations within the ancient marginal lacustrine carbonates of the Miocene Ries Crater Lake (Germany). This area is critical because it represents the transition from shoreline to proximal domains of a hydrologically closed system, affected by recurrent emergent events, representing the boundaries of successive sedimentary cycles. Chemostratigraphy targets shifts related to subaerial exposure and/or climatic fluctuations. Methods combine facies changes with δ13C- δ18O chemostratigraphy from matrix carbonates across five closely spaced, temporally equivalent stratigraphic sections. Isotope composition of ostracod shells, gastropods and cements is provided for comparison. Cathodoluminescence and back-scatter electron microscopy were performed to discriminate primary (syn-)depositional, from secondary diagenetic features. Meteoric diagenesis is expressed by substantial early dissolution and dark blue luminescent sparry cements carrying negative δ13C and δ18O. Sedimentary cycles are not correlated by isotope chemostratigraphy. Both matrix δ13C and δ18O range from ca −7·5 to +4·0‰ and show clear positive covariance ( R = 0·97) whose nature differs from that of previous basin-oriented studies on the lake: negative values are here unconnected to original freshwater lacustrine conditions but reflect extensive meteoric diagenesis, while positive values probably represent primary saline lake water chemistry. Noisy geochemical curves relate to heterogeneities in (primary) porosity, resulting in selective carbonate diagenesis. This study exemplifies that ancient lacustrine carbonates, despite extensive meteoric weathering, are able to retain key information for both palaeoenvironmental reconstruction and the understanding of diagenetic processes in relation to those primary conditions. Also, it emphasizes the limitation of chemostratigraphy in fossil carbonates, and specifically in settings that are sensitive for the preservation of primary environmental signals, such as lake margins prone to meteoric diagenesis. [ABSTRACT FROM AUTHOR]

Thick Oligocene-Miocene lacustrine limestones of the Lake Mead region in southern Nevada have few adequate modern analogues that can shed light on their origin. We contend that the Lower Pahranagat Lake (LPAH), a spring-fed, alkaline lake in east-central Nevada, provides such an analogue. Through sediment cores, sampling of the lake margin, and characterization of sediment physical and chemical properties, we show that the LPAH shares a number of lithofacies in common with the Late Oligocene to Early Miocene Rainbow Gardens (RGF) and Miocene Horse Spring Formations (HSF) in the Lake Mead area. These include: stratiform stromatolites, domal stromatolites, reed beds, and intermittently laminated and massive limestones. Oxygen and carbon isotopes from authigenic carbonates support our interpretation in that the LPAH values strongly overlap those of the Oligocene-Miocene units. As a result, we interpret the Oligocene-Miocene RGF and HSF lacustrine carbonates as having been deposited in restricted, alkaline lake basins with high salinity. These basins were not conducive to the ecological success of animals and other, large multicellular organisms and, instead, fostered the growth of significant microbial communities. Lake margins, particularly during RGF deposition, were inhabited by reedy plants and salt tolerant shore grass much as is encountered in the LPAH today. These lakes were likely sourced by springs, but were nonetheless highly evaporative. Some unique aspects of both the LPAH and the Lake Mead region Oligocene-Miocene strata may shed light on the rarity of thick lacustrine carbonates in the rock record. Both of these systems are underlain by thick, Paleozoic carbonate sequences that host the principal aquifers that feed the lakes. [ABSTRACT FROM AUTHOR]

The lithographic limestone of La Pedrera de Meià (LPM) in south-Central Pyrenees (NE Spain) is considered one of the best preserved lacustrine-coastal successions of the Early Cretaceous in Europe, hosting a taxonomically diverse record of Barremian biota. While this Konservat-Lagerstätte has been extensively surveyed for paleontological purposes, little is known about the paleoenvironmental and depositional conditions prevailing during the formation of this fossil lagerstätte deposit. LPM limestone is made up of a homogeneous, dark gray to pale brown, faintly laminated 50 m-thick succession of micrite-rich intervals. To shed light on the environmental conditions of deposition, 303 rock samples were studied using a multiproxy approach based on different techniques including petrography, SEM-EDX, XRD, ICP-MS, XRF, Rare Earth Element distributions and C and O stable isotope geochemistry. The combination of sedimentological and geochemical data reveals a vertical shift of facies from littoral to profundal environments and then returning to littoral conditions. Profundal facies are made up of filament-rich mudstones originated from in situ precipitation of micrite either through biotically mediated or physico-chemical processes in undisturbed low energy settings, under anoxic conditions, characterized by the presence of transported terrestrial and lacustrine organisms that are exquisitely well-preserved. Littoral areas are characterized by the presence of in situ charophytes, smooth ostracods, and miliolid foraminifera that accumulated in mud-rich facies indicating low/moderate-energy, shallow-water, and relatively oxygenated conditions. Stable isotopic data and elemental geochemistry suggest a hydrologically closed to semi-closed lacustrine system with a restricted water circulation encouraging dysoxic to anoxic conditions in profundal sediments. Paleoproductivity proxies suggest a good correlation between catchment-derived siliciclastic discharge to the lake and primary productivity peaks. Furthermore, a decline in terrigenous input coupled with a reduction of meteoric water entrance to the lake is interpreted based on elemental proxies derived from paleosalinity and paleoredox analysis. Collectively, the strontium/barium (Sr/Ba) ratios, the sparse presence of miliolid foraminifera, and the isotopic signatures indicate an increase in salinity toward the younger intervals of the lake infilling, although their ultimate causes are unclear (i.e., increased evaporation, or a combination of reduced meteoric input and sporadic marine inputs might explain our data). The present work shows the importance of performing detailed multiproxy geochemical-sedimentological studies to better constrain the unique paleoenvironmental and paleohydrological conditions associated with the exceptional preservation of fossil biotas in Konservat-Lagerstätte deposits in analogous depositional settings. [ABSTRACT FROM AUTHOR]

With the recent discovery of new oil fields around the Dongying Depression, Bohai Bay Basin, a better understanding of the origin of its high-quality lacustrine carbonate reservoirs has become important. The lacustrine carbonates of the upper fourth member of the Eocene Shahejie Formation (ES4s) have been examined using a variety of methods, including core and thin section observations, XRD, SEM, CL and fluid inclusion and isotope testing. The δ13C and δ18O values of the Dongying carbonates differ significantly from Eocene marine values, and the 87Sr/86Sr ratios of carbonate minerals are higher than Eocene marine water and the same as the surrounding Palaeozoic carbonate bedrock. Additionally, the distribution of the carbonates is controlled by the surrounding Palaeozoic carbonate bedrock, all of which indicate their lacustrine origin. Six major carbonate lithofacies are distinguished in the southern gentle slope zone. Bioclastic, oolitic and intraclastic grainstones dominate the littoral to sublittoral zones, while mudstones are mainly distributed in the profundal zone. The ES4s interval is composed of a third-order depositional sequence that contains several small-scale, higher frequency depositional cycles. Overall, stacking patterns of high-frequency depositional cycles are consistent with the general facies variation from basin to shore. Shallow-water grainstones contain both primary intergranular and secondary dissolution pores, whereas the dolomitized mudstones in the profundal zone contain intercrystalline pores and fractures. Diagenetic processes that improve reservoir quality include dissolution and dolomitization. Reservoir quality was diminished by burial calcite cementation and compaction. High-quality reservoirs are mostly distributed in dolomitized grainstones and wackestones, which are deposited in the shoal facies of shallow lakes. Variations in reservoir quality in the Dongying Depression are due to a combination of lithofacies type, dissolution intensity, early dolomitization and tectonic-related deformation. Due to the influx of meteoric waters and shallow burial depth, the grainstones in the littoral zone have the best reservoir quality. Integrated studies have helped to understand the reservoir heterogeneity and hydrocarbon potential of the Dongying lacustrine carbonates. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

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 (< 10 m), 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. [ABSTRACT FROM AUTHOR]

An integrated petrologic-sedimentologic-stratigraphic-seismic study of the Lagoa Feia Group, rift section of the Campos Basin, has shown that rift sedimentation was dominantly intrabasinal (carbonate and stevensitic deposits), with siliciclastic deposits restricted to the proximity of graben border faults. The bivalve bioclastic rudstones (“coquinas”) that constitute the rift reservoirs show commonly limited abrasion of the bioclasts, and were deposited both on basement highs and lows throughout the rift section. Although in situ deposits of stevensite ooids and peloids occur dominantly at the base of the succession, these particles are ubiquitous to the entire rift section, mixed in variable proportion with siliciclastic and carbonate sediments. The environmental conditions required for the formation of stevensite and the growth of bivalves are mutually exclusive, as stevensite forms only at pH greater than 10, while bivalves cannot tolerate pH greater than 9. The common mixture of well-rounded basaltic rock fragments with angular, granitic-gneissic rock fragments and feldspars in the sandstones and conglomerates indicates recycling of epiclastic deposits from the early rift section, combined with first-cycle contribution from the plutonic basement. The studied cores show no evidence of subaerial exposure, and there is a lack of bioturbation, suggesting harsh environmental conditions. The rift deposits are dominantly massive or faintly-laminated, with diffuse facies boundaries. Structures indicative of unidirectional or oscillatory flow are subordinate. Integration of seismic, sedimentologic and petrographic evidence indicates that the Campos Basin rift section is formed mostly by re-sedimented gravitational deposits. The onset of the rift sedimentation occurred in synformal depressions, where bivalve banks or stevensite ooids were formed in shallow lacustrine environments under variable alkalinity conditions. With the development of half-grabens and concentration of the tectonic activity along the border faults, recurrent tectonic events promoted the mixing and gravitational re-deposition of stevensitic, clastic and bioclastic sediments in deeper, fault-bounded troughs. Large-scale units, hundreds of meters thick, were generated by major tectonic events, whereas compositional variations in the scale of meters were possibly a product of lake-level climatic fluctuations. Thus, due to syn-rift and mainly to post-rift erosion (the Neo-Aptian unconformity), the preserved rift section of the Lagoa Feia Group comprises mostly sediments deposited in the central troughs of the half-graben structures. Marginal sediments were extensively eroded and re-sedimented as gravity-driven mixed deposits. This new model, constructed form the integration of seismic, stratigraphic, sedimentologic and petrologic data, diverges substantially from the presently accepted model for the sedimentation of the rift section, opening new possibilities for the exploration of Campos Basin, as well as of similar settings, as in the adjacent Santos Basin. [ABSTRACT FROM AUTHOR]

ABSTRACT The Last Glacial to Interglacial Transition (LGIT) is a period of climatic instability. δ18O records are ideal for investigating the LGIT as this proxy responds rapidly to even minor climatic oscillations. Lacustrine carbonates offer the opportunity to investigate spatial diversity in patterns of climatic change during the LGIT but this requires the generation of δ18O records from a range of latitudinal and longitudinal settings. This study presents a coupled pollen and stable isotopic study of lacustrine carbonates spanning the Windermere Interstadial (the British equivalent of Greenland Interstadial 1, the Lateglacial Interstadial) from the site of Tirinie in the Scottish Highlands, a region where δ18O records are absent. The Interstadial is characterized by three δ18O peaks, warm intervals, and two δ18O declines, cold episodes, the timing of which is constrained by the presence of crypto-tephra. The landscape at Tirinie was highly responsive to these climatic oscillations as the sedimentary and pollen record respond to each isotopic shift. The paper concludes by highlighting that, across the British Isles, lacustrine δ18O records of the Interstadial have a consistent stratigraphy/structure, although the magnitude of the isotopic shifts is regionally variable. Potential causes of this variability are discussed. [ABSTRACT FROM AUTHOR]

Aptian carbonate rocks of Santos Basin in the Brazilian southeastern continental margin show a highly heterogeneous reservoir quality conditioned by many controlling factors. This study presents a quantitative approach based on an integrated petrographic, mineralogical, and geochemical dataset of the Barra Velha Formation (BVF) to understand the spatial and temporal distribution of depositional and diagenetic aspects and the parameters controlling the nature and distribution of the diagenetic phases in Santos Basin pre-salt carbonate reservoirs. In this formation 3 units can be differentiated. The facies with higher Mg-clay content predominate in lower structural positions and regions with low relative relief. Higher proportions of spherulites occur in the transitional zones, and fascicular calcite occurs preferentially in the transitional and higher structural positions. Grainstones are observed in all depositional environments, but with variations in their composition (intraclasts, siliciclastic/volcanoclastic grain, and clay content). Stratigraphically, facies with higher Mg-clay content predominate in Units 3 (base) and 2 (intermediate), whereas Unit 1 (top) is marked by a greater proportion of grainstones and fascicular calcite. In Unit 1, fascicular calcite crusts tend to expand from the structural high, toward lower areas. The distribution of the main diagenetic products and their association with different facies highlight the role of depositional setting and primary constituents on the diagenetic processes. The tectonic and climatic context of the BVF favored the preferential precipitation and accumulations of Mg-clays in lower structural portions of the basin. The preservation of Mg-clays is also strongly linked to the structural setting, with more intense diagenetic alterations in higher structural areas. Dolomite is a major diagenetic phase in the studied samples, followed by silica. These diagenetic phases are largely associated with Mg-clay alteration and predominate in Unit 1. Mg-clays dissolution, as well as major alteration of carbonate phases, are concentrated in higher structural positions. Given the higher occurrence of faults in these areas and the presence of saddle dolomite, barite, celestine, and fluorite, which are typical products of hydrothermal alteration, part of the dissolution processes may be related to hydrothermal alteration. The depositional and diagenetic aspects indicate a close connection between the hydrochemical evolution of the lake waters with the origin and diagenesis of these deposits. • A quantitative petrographic approach associated with geochemical data aims to understand the spatial and stratigraphic distribution of facies and diagenetic phases of pre-salt carbonate reservoirs. • Field-scale distribution of diagenetic phases highlights the role of depositional setting and primary constituents on the diagenetic processes. • The occurrence and distribution of the different Mg-clay minerals and their alterations suggest control by the hydrochemical evolution of lake water. • Besides spherulites and fascicular calcites, dolomite is one of the most relevant diagenetic phases, primarily related to Mg-clay replacement. [ABSTRACT FROM AUTHOR]

Strontium isotopes from lacustrine carbonates record detailed weathering histories of exposed bedrocks, and thus are potentially useful for understanding interactions between tectonics and climate that may be driven by local or regional factors in basin-scale hydrologic systems. We combine extensive 87 Sr/ 86 Sr and δ 18 O datasets from the Green River Formation in order to identify environmental factors driving the evolution of Sr concentration and isotopic composition in lacustrine systems through the use of Sr mass balance modeling. Two models are developed. The first tests the effect of drainage capture that drove a constant-volume lake from balance-filled to overfilled conditions, and is applied to the Laney Member of the Green River Formation. The second model focuses on lacustrine evolution in response to changing the concentration, composition, and mass of water influx in a variable-size Sr reservoir (lake). This model is applied to the Wilkins Peak Member of the Green River Formation. 87 Sr/ 86 Sr ratios are correlated with δ 18 O values only during the Laney Member, when both are readily explained by a common forcing mechanism (drainage capture). Modeling of Sr isotope compositions and concentrations indicates a short residence time for Sr (~ 10 3 –10 4 years or less) in both balance-overfilled and underfilled phases of the Green River Formation lacustrine system. This in turn suggests that paleo-lacustrine sediments in most lakes can preserve Sr isotope records with high-resolution (~ 10 2 years) timescales. Rates of Sr sequestration in carbonate are shown to have a strong influence on lacustrine Sr concentration, and high Sr concentrations of lacustrine carbonate consistent with high salinity are observed in the underfilled Wilkins Peak Member and the balance-filled Tipton and lower Laney Members. In the Laney Member, 87 Sr/ 86 Sr mass-balance modeling results provide additional support for previous interpretations that the introduction of a large drainage system produced an isotopic shift across the lower LaClede/upper LaClede boundary. Major drainage reorganization is not required to drive high variability in 87 Sr/ 86 Sr ratios, however. Modeling shows that variability in 87 Sr/ 86 Sr ratios of ~ 0.004 observed in the Wilkins Peak Member can be explained by change in the characteristics of intrabasinal water sources during highstand vs. lowstand conditions. [ABSTRACT FROM AUTHOR]

Although extensive studies have been conducted on unconventional mudstone (shales) reservoirs in recent years, little work has been performed on unconventional tight organic matter-rich, fine-grained carbonate reservoirs. The Shulu Sag is located in the southwestern corner of the Jizhong Depression in the Bohai Bay Basin and filled with 400–1000 m of Eocene lacustrine organic matter-rich carbonates. The study of the organic matter-rich calcilutite in the Shulu Sag will provide a good opportunity to improve our knowledge of unconventional tight oil in North China. The dominant minerals of calcilutite rocks in the Shulu Sag are carbonates (including calcite and dolomite), with an average of 61.5 wt.%. The carbonate particles are predominantly in the clay to silt size range. Three lithofacies were identified: laminated calcilutite, massive calcilutite, and calcisiltite–calcilutite. The calcilutite rocks (including all the three lithofacies) in the third unit of the Shahejie Formation in the Eocene (Es3) have total organic carbon (TOC) values ranging from 0.12 to 7.97 wt.%, with an average of 1.66 wt.%. Most of the analyzed samples have good, very good or excellent hydrocarbon potential. The organic matter in the Shulu samples is predominantly of Type I to Type II kerogen, with minor amounts of Type III kerogen. The temperature of maximum yield of pyrolysate (T max) values range from 424 to 452 °C (with an average of 444 °C) indicating most of samples are thermally mature with respect to oil generation. The calcilutite samples have the free hydrocarbons (S1) values from 0.03 to 2.32 mg HC/g rock, with an average of 0.5 mg HC/g rock, the hydrocarbons cracked from kerogen (S2) yield values in the range of 0.08–57.08 mg HC/g rock, with an average of 9.06 mg HC/g rock, and hydrogen index (HI) values in the range of 55–749 mg HC/g TOC, with an average of 464 mg HC/g TOC. The organic-rich calcilutite of the Shulu Sag has very good source rock generative potential and have obtained thermal maturity levels equivalent to the oil window. The pores in the Shulu calcilutite are of various types and sizes and were divided into three types: (1) pores within organic matter, (2) interparticle pores between detrital or authigenic particles, and (3) intraparticle pores within detrital grains or crystals. Fractures in the Shulu calcilutite are parallel to bedding, high angle, and vertical, having a significant effect on hydrocarbon migration and production. The organic matter and dolomite contents are the main factors that control calcilutite reservoir quality in the Shulu Sag. [ABSTRACT FROM AUTHOR]

Isotope geochemistry of lacustrine carbonate represents a powerful tool to reconstruct paleoclimatic and paleoenvironmental conditions. Here, we present a comprehensive set of long-term oxygen (Isotope geochemistry of lacustrine carbonate represents a powerful tool to reconstruct paleoclimatic and paleoenvironmental conditions. Here, we present a comprehensive set of long-term oxygen (δ18O) and carbon (δ13C) stable isotope records from 5 Chattian to Burdigalian lacustrine sequences distributed over the Central Anatolian Plateau. Field relationships combined with stable isotope geochemistry indicate a relatively humid subtropic Late Oligocene climate with an environment characterized by large, temporally open freshwater lakes. Approximately during the middle Aquitanian, a 4%o-5%o increase in lake δ18O values indicates changes in regional climate including more arid conditions and an increasing dominance of closed saline lake conditions in the central plateau region. This time period was also characterized by frequent climatic fluctuations such as short-lived humid periods, possibly recording the influence of seasonality, topography, and the waxing and waning of aridity. In general, relatively high Oligo-Miocene δ18O lake water values within the modern plateau interior, even for the least evaporative sequences, suggest the absence of significant orographic barriers at both the northern and southern plateau margins prior to 20-16 Ma. 18O) and carbon (δ13C) stable isotope records from 5 Chattian to Burdigalian lacustrine sequences distributed over the Central Anatolian Plateau. Field relationships combined with stable isotope geochemistry indicate a relatively humid subtropic Late Oligocene climate with an environment characterized by large, temporally open freshwater lakes. Approximately during the middle Aquitanian, a 4%o-5%o increase in lake δ18O values indicates changes in regional climate including more arid conditions and an increasing dominance of closed saline lake conditions in the central plateau region. This time period was also characterized by frequent climatic fluctuations such as short-lived humid periods, possibly recording the influence of seasonality, topography, and the waxing and waning of aridity. In general, relatively high Oligo-Miocene δ18O lake water values within the modern plateau interior, even for the least evaporative sequences, suggest the absence of significant orographic barriers at both the northern and southern plateau margins prior to 20-16 Ma. [ABSTRACT FROM AUTHOR]

Abstract: During mid Eocene times, the northwestern margin of the Gosiute Lake (Wyoming) has been characterised by the extensive development of a microbialite dominated carbonate sequence corresponding to the Laney Member of the Green River Formation, thus providing a potential analogue for petroleum carbonate reservoirs developed in similar marginal lacustrine settings. A detailed sedimentological study carried out on the Little Mesa plateau near LaBarge (SW Wyoming) allows the reconstruction of: 1) lateral and vertical facies variations of the carbonate sequence across the margin, 2) development patterns of complex microbial bioconstructions, and 3) the evolution of the margin through time. The carbonate sequence, up to 18m-thick, starts with the deposition of sandy limestones which fill channels in the underlying siliciclastic unit of the Cathedral Bluff Formation. The increasing microbial contribution in the upper part of the sequence results in the development of large insect-microbial build-ups which represent up to 80% of the total volume of the carbonate deposits and which aggregated to form reef-like geometries up to several tens of metres in diameter, including bioherms, spur and groove alignments and reef-flat structures. The initiation and development of the lacustrine carbonate platform was controlled both by the general lake expansion associated to the Eocene Climatic Optimum and by local drainage diversions responsible for a new water inflow in the basin from the northern volcanic provinces. Short-term lake level changes driven by high-frequency climate fluctuations which are superimposed to this general transgressive trend may explain the discontinuous and polygenic growth of the individual insect-microbial build-ups. The basin infilling by prograding alluvial volcaniclastic deposits originated from the northwestern volcanic province caused the rapid turn off of the marginal lacustrine carbonate factory during mid Eocene time. [Copyright &y& Elsevier]

Abstract: Pseudospherulitic fibrous calcite (PFC) has been found as a major constituent (85–90%) within thin massive limestone beds of the Quaternary mudflat-shallow lacustrine facies association (1.5–2m thick) that forms part of combined facies associations of the Quaternary clastic-carbonate unit (25–30m thick) at Bir-Karawein area in the Farafra Oasis, Western Desert, Egypt. The thin massive limestone beds (2–5cm thick) are devoid of pedogenic features and marine fossils. They form a rhythmic cyclic succession with thin massive mudrocks (5–10cm thick). The mudflat-shallow lacustrine facies association herein occurs within a depositional sequence of distal alluvial-floodplain (6–12m thick) and palustrine (1.5–4.5m thick) facies associations. The PFC is a composed of loosely packed rounded to sub-rounded single low-Mg-calcite crystals (150–250μm-sized) with intracrystalline fibrous microfabric marked by fibers (150–250μm long and 10–20μm wide) radiating from the center of the individual crystals and displaying irregular internal growth with lobate pattern. The PFC crystals show non-planar to highly irregular intercrystalline boundaries. Under SEM, the individual crystal fibers group of PFC form ellipsoid to sub-globular bodies. Each PFC crystal exhibits successive zones of thick non-luminescence and thin brightly orange to dull luminescence. The matrix (10–15%) between the PFC crystals is mainly a honeycomb-like smectite. The PFC is postulated to be a primary precipitate. This concept is reached because the PFC: (i) does not display the criteria of typical Microcodium structures, root-calcification, speleothem structures, calcite spherulites of laminar calcretes, and calcitization of precursor dolomite or aragonite, (ii) possesses homogenous compositional and textural characteristics, and (iii) occurs within limestone beds that lie in between impermeable massive mudrock beds that dampen diagenesis. A role for possible bacterial contribution in crystallization of the PFC is assumed in the view of its internal microfabric characteristics (pseudospherulitic and lobate internal growth patterns), and morphological features (ellipsoidal to subglobular appearance), in addition to its slightly negative values of δ13C (−0.51‰ to −2.19‰). The low concentration of Na (0.11–0.20%), Sr (70–110ppm) and Mn (0.04–0.31%), in addition to the negative values of δ18O (−4.65‰ to −5.96‰) in the PFC reflect its deposition from oxygenated freshwater. In addition, the absence of covariance between δ13C and δ18O values (r =−0.202) of the PFC indicates precipitation in a hydrologically-open, short-lived lake setting. In summary, the PFC is of low-Mg type and formed in a hydrologically-open, short-lived, freshwater lake as a primary precipitate with possible bacterial contribution. [Copyright &y& Elsevier]