Lacustrine Basin Research Articles

Element Geochemical Characteristics and Provenance Conditions of the 1st Member of Jurassic Zhongjiangou Formation in Wudun Sag, Dunhuang Basin

In order to clarify the provenance of the 1st member of the Zhongjiangou formation in Wudun sag, Dunhuang basin, the structural attributes, weathering and sedimentary characteristics of the provenance area were analyzed by means of element geochemistry, so as to determine the differences of sediment sources in different well areas. The results show that the higher the Al2O3 and K2O contents, the higher the enrichment of large ion lithophile elements and high field strength elements, while the iron and magnesium elements are relatively deficient, and there are characteristics of medium degree differentiation of light and heavy rare earth elements in Well XC1 and Well D2. The lower the Al2O3 content and the higher the SiO2 content, a loss of large ion lithophile elements and high field strength elements are observed, while the ferrophilic magnesium elements show serious loss, as shown in the characteristics of the high degree of differentiation of light and heavy rare earth elements in Well D1. In the UCC-normalized element spidergrams, the trend of Well XC1 and Well D2 is similar, which is different from that of well D1, indicating that the sediments of Well XC1 and Well D2 come from the same provenance area, while the sediment of Well D1 comes from a different provenance area. The provenance area of Well XC1 and Well D2 shows strong tectonic activity and strong weathering, while the provenance area of well D1 exhibits relatively weak tectonic activity and weathering. Combined with previous research results, Wudun sag is mainly characterized by a faulted lacustrine basin controlled by the southern boundary fault in the Jurassic layer. Therefore, the sediments of Well XC1 and Well D2 mainly come from the southern Sanweishan uplift provenance area, with strong tectonic activity; the sediments of Well D1 mainly come from the northern Beishan provenance area, with relatively weak tectonic activity.

Basin-filling processes and hydrocarbon source rock prediction of low-exploration degree areas in rift lacustrine basins: a case from the Wenchang Formation in low-exploration degree areas, northern Zhu I Depression, Pearl River Mouth Basin, E China

Hydrocarbon source rocks, as a main geologic factor of petroliferous systems in a sedimentary basin, play a key role in the accumulation of oil and gas and the formation of hydrocarbon accumulations. This study, which focuses on difficulties in prediction of hydrocarbon source rocks in basins or sags with low exploration degree and insufficient hydrocarbon source rock indicators, taking the Wenchang Formation of northern Zhu I Depression, Pearl River Mouth Basin as an example, proposed a hypothesis of “finding lakes and hydrocarbon source rocks”. Detailed steps include, first, determination of the lacustrine basin boundary according to analysis of seismic foreset facies, determination of the depositional area based on the compilation of strata residual thickness maps, determination of the lacustrine basin shape according to deciphering slope break belt system, determination of the fluctuation of paleo-water depth according to biogeochemical indicators of mature exploration areas, determination of the lacustrine basin scale based on analyses of tectonics intensity and accommodation space, which prove the existence of the lacustrine basin and identify the range of semi deep-deep lake; second, further analyses of tectonopalaeogeomorphology, paleo-provenance, palaeoclimate and paleo-water depth to reconstruct the geologic background of the original basin and semi-deep-deep lacustrine facies, to determine the distribution of semi-deep/deep lacustrine sediments in combination with studies of logging facies, core facies, seismic facies and sedimentary facies, and to rank the sags’ potential of developing hydrocarbon source rocks from controlling factors of source-to-sink system development; third, on the basis of regional sedimentary facies analysis, through identification and assessment of seismic facies types of semi-deep/deep lacustrine basins in mature areas, establishing “hydrocarbon source rock facies” in mature areas to instruct the identification and depicting of hydrocarbon source rocks in semi-deep/deep lacustrine basins with low exploration degree; fourth, through systematical summary of hydrocarbon-rich geological factors and lower limit index of hydrocarbon formation of the sags already revealed by drilling wells (e.g., sag area, tectonic subsidence amount, accommodation space, provenance characteristic, mudstone thickness, water body environment, sedimentary facies types of hydrocarbon source rocks), in correlation with corresponding indexes of sags with low exploration degree, then the evaluation and sorting of high-quality source rocks in areas with sparsely distributed or no drilling wells can be conducted with multi-factors and multiple dimensions. It is concluded that LF22 sag, HZ10 sag and HZ8 sag are II-order hydrocarbon rich sags; whereas HZS, HZ11 and HZ24 are the III-order hydrocarbon-generating sags.

Depositional evolution and models for a deep-lacustrine gravity flow system in a half-graben rifted sag, Beibuwan Basin, South China Sea

The Paleogene Liushagang Formation is part of the Fushan Sag, a continental lacustrine basin located at theSoutheastern margin of the Beibuwan Basin, South China Sea. Further understanding of the deep-water gravityflow deposits in this formation will be conducive to lithologic reservoir exploration in the sag. In this study,three members of the Liushagang Formation, SQEls3 SQEls2 and SQEls1, from old to young, are used withcore observation, well log data, and three-dimensional seismic data to identify four deep-lacustrine gravity flowlithofacies including their vertical and lateral relationships within the depositional system. The results are thenused to establish a deep-water gravity flow depositional model. Four types of gravity flow lithofacies developed inthe sag: sandy debrite, turbidite, sandy slump, and bottom-current deposits. Sand-rich sub-lacustrine fan depositswith typical turbidite channels were developed mainly in the western depression, whereas distal isolated lobesformed by sandy debrite flow deposits occurred mainly in the eastern depression. The results obtained in this studywill be helpful in the research of gravity flows in similar continental lacustrine environments.

Tectonic uplift along the northeastern margin of the Qinghai–Tibetan Plateau: Constraints from the lithofacies sequence and deposition rate of the Qaidam Basin

The uplift processes of the Qinghai–Tibetan Plateau is of great significance for understanding the tectonic dynamics and monsoons in East Asia. In this study, the spatiotemporal characteristics of the tectonic uplift along the northeastern margin of the Qinghai–Tibetan Plateau are discussed from a basin–orogen coupling perspective on the basis of detailed analyses of stratigraphy and sedimentology of the Qaidam Basin and comparisons with those of the Xining, Lanzhou, Linxia, Guide–Xunhua, and Jiuquan basins. The asynchronous tectonosedimentary events characterized by alluvial conglomerates at the Early Jurassic–Cretaceous, ~53 Ma, ~40 Ma, ~22 Ma, ~15 Ma, and <2.5 Ma have been preserved in wells at different locations. Consistently, the deposition rates indicate three episodes of accelerated deposition at ~40 Ma, ~20 Ma, and ≤5 Ma in the Cenozoic. Therefore, we believe that the orogenic uplift along the northeastern margin of the Qinghai–Tibetan Plateau can be divided into four stages: Jurassic–Cretaceous, ~40–20 Ma, ~20–5 Ma, and <5 Ma. During the Jurassic–Cretaceous, tectonic activation and uplift occurred in the ancient Qilian orogenic belt, which formed in the Early Paleozoic. By ~40 Ma, the deformation due to the growth of the “proto-Tibetan Plateau” had propagated to the Qiman Tagh Shan–East Kunlun Shan–Ela Shan–West Qinling Mountain chain. Controlled by the tectonic thrusting of the eastern Kunlun Mountains, the Qaidam Basin entered a stage of comprehensive development. At ~20 Ma, the deformation front of the Qinghai–Tibetan Plateau reached the Qilian Shan. The unified Paleogene compressional basin in the northern West Qinling Mountains split into secondary basins, such as the Xining, Lanzhou, Linxia, Guide–Xunhua, and Gonghe basins, which evolved independently. At ~5 Ma, the entire Qinghai–Tibetan Plateau entered a rapid uplift stage, and the large lacustrine basins in and around the plateau shrank and disappeared.

Biomarkers reveal Eocene marine incursions into the Qaidam Basin, north Tibetan Plateau

The easternmost extent of the large epicontinental Proto-Paratethys Sea is recorded in Eocene sediments of the Tarim Basin in western China. However, sporadic reports of marine-derived coccoliths in its eastern neighbor Qaidam Basin, which is conventionally considered to be an isolated lacustrine basin, cast doubt on this assumption. Here we present biomarker data on a core drilled in westernmost sector of the Qaidam Basin, which reveals the presence of 24-n-propylcholestane (24-npc), a biomarker hydrocarbon derived from marine algae, whose patchy stratigraphic distribution chronicles the development of ephemeral marine conditions. Biomarker proxies reveal a dynamic system where the Qaidam Basin was in intermittent communication with the Proto-Paratethys Sea via the Tarim Basin during the middle Eocene after which it became hydrographically isolated as a shallow and stratified non-marine environment in the Late Eocene.

Geothermal Accumulation Constrained by the Tectonic Transformation in the Gonghe Basin, Northeastern Tibetan Plateau

Abstract Advances in the exploration of the geothermal resources with remarkably high temperatures in the Gonghe Basin, northeastern Tibetan Plateau, provide an enhanced understanding of the origin and emplacement of hot dry rock (HDR). Based on the integrating analysis on the boundary faults distribution and their activity histories, springs and geothermal borehole data, and magnetotelluric data, we propose that the Gonghe Basin formed in a zone of slip dissipation between two major large-scale left-lateral strike-slip faults of the Kunlun fault to the south and the Haiyuan fault to the north during the Neogene time. During the evolution of these two major strike-slip faults, the basin has experienced two-phase developments: the transrotational Gonghe-Qinghai lacustrine basin system during the Miocene and the transpressional Gonghe-Tongde basin system during the Pliocene-Quaternary. In response to the crustal transtension components of the transrotational Gonghe Basin, the partial melting zone at depths of 10–25 km in the thickened crust (~54 km) has been uplifted by ~10 km compared with adjacent regions since the Pliocene. This uplifted partial melting zone may have provided prominent potential heat energy for the HDR in the Triassic granitoid batholith at shallower depths (~3–10 km) by effective enhancement of the geothermal conduction process via deep faulting. With obliquely south-verging thrusting of the Gonghe Nan Shan thrusts in the northern, the Gonghe Basin has transformed from transrotation to transpression-domination during the 6–3 Ma, as well as accompanying with the depocentre migrating to the northwest and in turn the basement elastically uplifting in the southeast. This differential deformation of the basin floor has resulted in a northeastward upward tilting of the Triassic batholith and an isothermal surface. It finally developed the high-temperature and shallow-burial HDR with anomalously temperatures of over 100°C at a depth of 1.5 km in the Qiabuqia and Zhacang geothermal areas in the Gonghe Basin, NE margin of the Tibetan Plateau.

Middle Eocene Paleoenvironmental Reconstruction in the Gonjo Basin, Eastern Tibetan Plateau: Evidence From Palynological and Evaporite Records

The early uplift of the Tibetan Plateau (TP) had a profound influence on the paleoenvironment and paleoclimate. However, we still have little information about the link between the paleoclimatic changes and flora ecosystem caused by the uplift. The Eocene fluvial-lacustrine sequences in the Gonjo Basin, eastern TP, provide excellent archives of the paleoecological and paleoclimatic responses to the surface uplift of the TP. In this study, we investigated a section of the middle Eocene Ranmugou Formation (47.8–&amp;gt;44 Ma) and used the sporomorphs assemblages, sedimentology, and geochemistry of the evaporites to reconstruct the paleoclimate and paleovegetation. The palynological assemblages and coexistence analysis reveal that the middle Eocene ecosystem in the Gonjo Basin was dominated by warm and humid temperate deciduous broad-leaved forests. The 87Sr/86Sr ratios (0.709942–0.710062) of all of the gypsum samples are higher than those of contemporaneous seawater, while the δ34S values (10.3–11.0%) are much lower, indicating a lacustrine environment. Combined with published palynological and paleoelevation data for eastern Tibetan lacustrine basins, we infer that the paleoenvironment changed from warm and humid deciduous broad-leaved forests during 47.8–&amp;gt;44 Ma, to cool and arid temperate forest during 44–40 Ma in the Gonjo Basin, and to arid and cool steppe-desert vegetation in the late Eocene Nangqian Basin. The changes in the paleoclimate and vegetation were primarily driven by the surface uplift of the Central Watershed Mountain, the high topography of which blocked the moisture from the Asian Eocene monsoon.

Volcano-sedimentary processes at Las Derrumbadas rhyolitic twin domes, Serdán-Oriental Basin, Eastern Trans-Mexican Volcanic Belt

Abstract The eruption of the c. 10 km 3 rhyolitic Las Derrumbadas twin domes about 2000 years ago generated a wide range of volcano-sedimentary deposits in the Serdán-Oriental lacustrine basin, Trans-Mexican Volcanic Belt. Some of these deposits have been quarried, creating excellent exposures. In this paper we describe the domes and related products and interpret their mode of formation, reconstructing the main phases of the eruption as well as syn- and post-eruptive erosional processes. After an initial phreatomagmatic phase that built a tuff ring, the domes grew as an upheaved plug, lifting a thick sedimentary pile from the basin floor. During uplift, the domes collapsed repeatedly to form a first generation of hetero-lithologic hummocky debris avalanche deposits. Subsequent dome growth produced a thick talus and pyroclastic density currents. Later, the hydrothermally altered over-steepened dome peaks fell to generate second-generation, mono-lithologic avalanches. Subsequently, small domes grew in the collapse scars. From the end of the main eruptive episode onwards, heavy rains remobilized parts of the dome carapaces and talus, depositing lahar aprons. The Las Derrumbadas domes are still an important source of sediments in the basin, and ongoing mass-wasting processes are associated with hazards that should be assessed, given their potential impact on nearby populations.

Effects of gypsum-salt rock on mineral transformations in a saline lacustrine basin: Significance to reservoir development

Gypsum-salt rock is typically developed in carbonate reservoirs, and this rock has both constructive and destructive effects on the reservoir. The ways in which gypsum rock controls reservoir development are closely related to the diagenetic conditions. In this study, a typical saline lacustrine basin, the Qaidam Basin in China, was selected to examine the influence of gypsum-salt rock on the development of the carbonate reservoir under different diagenetic conditions. Four main geological factors were assessed: formation condition (temperature), typical salt mineral (anhydrite), and fluid properties (Ca2+ and Mg2+), along with multi-group fluid–rock chemical reaction models devised using multiphase-flow solute-transport simulation technology. Mineral dissolution, precipitation, and transformation in the reservoir under various temperature, pressure, fluid, and mineral conditions were analyzed, and the change of reservoir porosity was calculated. The results showed that the concentration of Ca2+ in fluid controls the dissolution and precipitation of carbonate minerals in the reservoir; moreover, continuous and sufficient Mg2+ is a necessary condition for dolomitization. Precipitation of anhydrite decreases with increase of temperature, verifying that anhydrite precipitates more easily at low temperature. Dissolution of calcium-containing minerals in overlying gypsum-salt rock can lead to mineral precipitation in the subjacent reservoir and reduce its quality.

The Process of Tectonic Inversion and Disintegration in the Dasanjiang Unified Basin, Northeast China: Constraint From Low-Temperature Thermochronology

The Dasanjiang basin group in Northeast China contains more than ten Mesozoic–Cenozoic sedimentary basins. Much evidence shows that they were a unified large-scale depression lacustrine basin in the Early Cretaceous; however, destruction processes and mechanisms after the formation of the unified lacustrine basin are some of the key issues restricting basic research and oil and gas exploration in the Dasanjiang area. In this study, we carried out low-temperature thermochronology and thermal history inversion on samples from the main basins in the Dasanjiang area to finely restore the destruction process of the unified basin. The results show that since the Early Cretaceous, the Dasanjiang area has experienced three major positive tectonic inversions: 100 Ma~90 Ma, 73 Ma~40 Ma, and 23 Ma~5 Ma. The unified basin was destroyed by compression and uplift and gradually disintegrated. The basin gradually changed from initial unified evolution to differential evolution and finally formed the isolated appearance of each basin. The aforementioned three-stage positive tectonic inversion time limits basically corresponded to the changing periods in the movement direction, subduction angle, and movement speed of the paleo–Pacific Ocean plate. It is believed that the movement and reorganization events of the plates on the Pacific side dominated the formation, destruction, and disintegration of the Dasanjiang prototype basin, which was the main dynamic mechanism of the tectonic evolution of the Mesozoic and Cenozoic basins in the study area and Northeast China.

Mineralogy and geochemistry of the Middle Permian Pingdiquan Formation black shales on the eastern margin of the Junggar Basin, north‐west China: Implications for palaeoenvironmental and organic matter accumulation analyses

The Middle Permian black shales from the eastern Junggar Basin of northern Xinjiang, north‐west China are one of the richest and thickest lacustrine hydrocarbon source rocks in China. The palaeoenvironmental settings and controlling factors of organic matter accumulation along the margins of the Middle Permian lacustrine basin remain unclear, although the same works have been undertaken in the centre of the Middle Permian lacustrine basins. Here, we performed a combination of mineralogical and geochemical analyses on the black shales of the Middle Permian Pingdiquan Formation from the eastern margin areas of the Junggar Basin. The results show that the Pingdiquan Formation black shales in the study areas are organic‐rich, with total organic carbon (TOC) contents of 0.74–17.61 wt% (avg. 5.81 wt%). Salinity proxies including Sr/Ba and Rb/K ratios, combined with traces of gypsum and halite in a few samples, suggest brackish to saline water conditions. Palaeoclimate proxies indicate a warm‐humid environment during the deposition of the Pingdiquan Formation black shales. The redox proxies V/(V + Ni), V/Cr and U/Th ratios, and Eu and Ce anomalies together indicate dysoxic‐anoxic conditions of lake water. Strong positive correlations between palaeoproductivity proxies (P/Al, Ni/Al, Cu/Al, and Zn/Al) and TOC contents imply that palaeoproductivity may have played an essential role in organic matter accumulation during the deposition of the black shales. In contrast, negative correlations between terrestrial detrital input proxies (Al and Ti) and TOC contents indicate that terrestrial detrital input may have acted as a diluent to organic matter accumulation. Additionally, although the correlation between redox proxies [V/(V + Ni) and U/Th] and TOC contents is not straightforward, dysoxic‐anoxic conditions of lake water are an essential prerequisite for organic matter accumulation in the Pingdiquan Formation black shales.

Significantly enhanced mid Holocene fluvial activity in a globally important, arid‐zone wetland: The Okavango Delta, Botswana

AbstractInformation on past dryland environments is commonly derived from geomorphological landforms and sediments (‘geoproxies’). The Okavango Delta in the middle Kalahari, Botswana, has been subject to a long history of arid–humid transitions but its potentially rich archive of fluvial geoproxies is largely untapped. Previous palaeoenvironmental studies in the Delta region have focused mainly on aeolian dunes, lacustrine beach ridges, and rare pollen sequences in surrounding locations, and the Delta's channel dynamics have remained poorly constrained, both chronologically and in quantitative palaeohydrological terms. Focusing on the Delta's Xugana region, we present the first optically stimulated luminescence ages for palaeochannel fills and scroll bars, revealing significantly enhanced fluvial activity in the ~7–4 ka interval. Along 120–150 m‐wide, sand‐bed, sinuous palaeochannels, lateral migration and bend cutoffs were driven by palaeodischarges up to ~350–450 m3 s−1, at least nine times those of the present‐day channels. These palaeochannels represent the last major phase of meander activity in the middle and lower Delta. The palaeodischarges imply significantly higher rainfall over the catchment in the mid Holocene, possibly resulting from a southwards shift of the African tropical rainbelt. Over the last few thousand years, diminished fluvial activity in the Xugana region has continued; &lt;20 m‐wide channels with discharges typically &lt;40 m3 s−1 have been more laterally stable, albeit subject to local avulsions. These findings provide the ‘missing link’ between evidence for past intervals of enhanced rainfall in the Delta's tributary catchments in the northwest and west and the filling of large lacustrine basins to the southeast and east. The findings contribute to improved understanding of changing Kalahari hydroclimates and support growing evidence indicating that the mid to late Holocene was a time of significant fluvial transformation across many of the world's drylands.

Depositional Models of Deep-Water Gravity-Flow in Lacustrine Basin and Its Petroleum Geological Significance—A Case Study of Chang 6 Oil Group in Heshui Area, Ordos Basin, China

Gravity-flow can carry a large number of sediments and organic matters from shallow water to deep lakes with its strong transporting energy, directly or indirectly facilitating the formation of deep-water tight reservoirs and shale reservoirs. Therefore, studying the genetic types, dynamic mechanisms, and depositional models of gravity-flow deposits is essential in the exploration of unconventional petroleum in large lacustrine basins. This research studied the genetic types, dynamic mechanisms, and sedimentary models of the gravity-flow deposits of the Chang 6 oil group in the Heshui Area, Ordos Basin, China, aiming to reveal its petroleum geological significance. Core observation, microscopic thin section identification, particle size analysis, and determination of rare earth elements were carried out. As a result, three types of gravity-flow deposits are detected, namely, slide-slump, sandy debris flow, and turbidity current. A certain slope gradient in bed form is the necessary geomorphic condition for gravity flow formation, and determines its development level, distribution range, and flow transformation efficiency. Sufficient provenance lays the material foundation and determines its depositional composition and development type. Other factors include earthquakes, volcanoes, and floods, which serve as triggering forces. In addition, fragmentation, liquefaction, and fluid mixing are the main dynamic mechanisms driving flow transformation. Based on the flow type of gravity flow, particle size characteristics, gravity-flow transformation relations, development mechanism, and spatial distribution pattern, we distinguished two depositional gravity-flow models, i.e., slump turbidite body and sublacustrine fan. Re-portrait the spatial distribution of deep-water gravity flow in the study area. From the perspective of sedimentology, explain the genesis of sand bodies in the northeast and southwest. The sandy debris flow in the middle fan braided channel microfacies of the sublacustrine fan sways the development of thick massive sand bodies in the study area. Hybrid event beds formed by the fluid transformation in a slump turbidite are the potential dessert area for deep-water tight oil and gas.

Molecular and isotopic compositions of hydrocarbon gases generated from anomalously 13C-enriched sapropelic sedimentary organic matter in lacustrine basins

For lacustrine basins, the enrichment of 13C in sedimentary organic matter is closely related to the growth of plankton biological productivity. Consequently, there are many cases where the sapropelic organic matter is more 13C-enriched than the humic organic matter. However, the geochemical characteristics of the pyrolysis products produced by 13C-enriched lacustrine sapropelic organic matter are less discussed. Herein, two lacustrine sapropelic kerogen samples with remarkably different carbon isotopic ratios (–23.2‰ and –28.4‰) and a coal sample (–25.6‰) were performed by a closed and dry pyrolysis system at temperatures ranging from 300 °C to 600 °C. The results indicated that the carbon isotopic compositions of hydrocarbon gas products were significantly affected by the initial carbon isotopic composition of their sources. At similar simulated temperatures, the carbon isotopic compositions of methane generated from the 13C-enriched sapropelic sample and the coal sample were more consistent. However, owing to the difference in their gas-generation mechanism, the natural gas generated from the coal sample had a more 13C-enriched isotopic composition of ethane (δ13C2) and a higher dryness, particularly at higher maturity levels. The hydrogen isotopic composition of methane (δ2HC1) was markedly enriched in deuterium as the thermal maturity increased and may have been independent of the genetic type and salinity. Therefore, we used the “Dryness vs. δ2HC1” and “δ13C2 vs. δ2HC1” diagrams to confirm the genetic type of natural gas in lacustrine basins.

Controlling factors of matrix acidizing potential of low permeability clastic reservoir in faulted lacustrine basin

Based on constant rate mercury injection experiment, casting thin section identification, scanning electron microscope observation, clay mineral X-ray and rock specific surface area analysis, the controlling factors of matrix acidizing potential of low-permeability sandstone reservoir in fault depression lacustrine basin were determined from three aspects: pore filling, throat filling and pore-throat combination characteristics. It is concluded that provenance controls the plane partition of reservoir pore fillings. Burial depth controls the longitudinal zoning of key filling material in reservoir throat. The difference of rock structure in sedimentary facies - microfacies controlling zone leads to the change of pore-throat assemblage pattern. The matrix acidizing scheme of low permeability sandstone reservoir in fault depression basin can be formulated according to the law of “provenance zoning, buried depth zoning and being controlled by microfacies in zone”, and the implementation scheme of pre-acid, main acid and post-acid can be put forward respectively. This method can effectively promote the integration process of exploration and development of low permeability clastic reservoir in mature exploration area.

Tight Oil Accumulation Dynamics of Source-Reservoir Integration in a Slope Belt in a Lacustrine Basin: A Case Study of the Third Member of the Palaeogene Shahejie Formation, Qibei Slope, Qikou Sag

Tight Oil Accumulation Dynamics of Source-Reservoir Integration in a Slope Belt in a Lacustrine Basin: A Case Study of the Third Member of the Palaeogene Shahejie Formation, Qibei Slope, Qikou Sag

Late Triassic paleowinds from lacustrine wave ripple marks in the Fleming Fjord Group, central East Greenland

Geological evidence of paleoclimate conditions provide important constraints on paleoclimate models. Paleowinds are one of the central paleoclimate parameters in global circulation models used to reconstruct past climatic conditions. However, only a few climate proxies such as eolian dunes and shallow-water wave ripples provide information on paleowind directions. In this study, we present new paleoclimatic data based on lacustrine sediments with wave ripple marks in the Upper Triassic Fleming Fjord Group, central East Greenland deposited at very shallow water depth in the northern part of Pangaea in a mid-latitude, warm-temperate climate. Wave-ripple orientations are measured on 380 individual fossilized wave-rippled beds at four localities in the ~8500 km 2 lacustrine basin. These ripple data are used to establish a geologic wind proxy and are compared to previously published general circulation models. Two wave ripple orientations are observed: The dominant ripple orientation and wave ripple characteristics indicate paleowinds from the Late Triassic SSE and NNW (154.5° and 334.5°), while a subordinate orientation indicate paleowinds from W or E (270° or 90°). This paleowind regime remained stable for at least ~7.5 million years during the mid-late Norian (218.5-211 Ma). These results correspond very well with some general circulation model results that suggest summer winds from the NNW and winter winds from the SSE. Based on the great compatibility between empirical data and general circulation model results, the wind pattern seems to have been greatly influenced by regional, Late Triassic mountain topography in the present-day Greenland and Norway channeling the paleowind in the northernmost part of the pre-Atlantic rift basin (the mid-Norwegian-east Greenland rift system).

GIS MULTITEMPORAL ANALYSIS OF REMOTE SENSING IMAGES TO ASSESS THE SILTING OF ANTHROPOGENIC DAM LAKES IN THE OUTER SUBCARPATHIAN AREA. CASE STUDY: PUCIOASA ACCUMULATION LAKE, ROMANIA

On Romania's territory, after the major floods that took place during the 70s, the hydrotechnical setup of the large rivers that drain the peri-Carpathian areas was made. The purpose of building these anthropogenic dam lakes is complex, given that extreme hydrological phenomena occurred in late spring and early summer, caused by a significant rainwater supply (about 70%), the difference being made up by the underground supply. Such a hydrotechnical setup was made in the Subcarpathian areas, on large rivers, to mitigate flood waves, but also to create water resources to supply local communities. The main problem facing these lakes is the massive silting due on the one hand to the geological and geomorphological characteristics of the Subcarpathians and, on the other hand, to the high degree of anthropization. The Subcarpathians are, from a geological point of view, made up of friable deposits (marls, clays, marl-clays, gypsums, conglomerates), which trigger a series of geomorphological processes of slope erosion (areal streaming, ravine creation, runoffs, torrential rainfall, and landslides), all of which are involved in the hydrological drainage regime. The Subcarpathians also make up a geographical area with a high density of population and, implicitly, of anthropogenic activities, which under certain conditions favour the erosion and degradation of land. These factors ensure that the solid flow recorded on the Subcarpathian Rivers is between 5-25 million tons/ha per annum. All this leads to the intense silting of anthropogenic lakes, which causes the volume of water stored in the tank to sometimes decrease to 1/3 of its initial volume. Under these conditions, in the extremity of basins, lacustrine plains were formed, and within the dam area, the pelitic sedimentary deposits exceeded 7-10 m in thickness. The aim of this study is to perform a multitemporal analysis of lake silting in the Subcarpathian area by using GIS and integrating the suite of bathymetric elevations and the series of satellite images existing from the time of the lake's construction to present-day.

Genesis and Distribution of Pyrite in the Lacustrine Shale: Evidence from the Es3x Shale of the Eocene Shahejie Formation, Zhanhua Sag, East China.

Pyrite is a common mineral in sedimentary rocks and is widely distributed in a variety of different morphologies and sizes. Pyrite is also widely distributed in the Es3x shale of the Eocene Shahejie Formation in the Zhanhua Sag, Bohai Bay Basin. A combination of geochemical and petrographic studies has been applied to address the formation and distribution of pyrite in the Es3x shale. The methods include thin section analysis to identify the representative samples of the shale-containing pyrite, total organic carbon (TOC) content analysis, X-ray fluorescence, X-ray diffraction, electron probe micro-analysis, and field emission scanning electron microscopy (FE-SEM) coupled with the energy dispersive spectrometer, to observe the characteristics, morphology, and distribution of pyrite in the lacustrine shale. The content of pyrite in the Es3x shale ranges from 1.4 to 11.2% with an average content of 3.42%. The average contents of TOC and total organic sulfur (TS) are 3.48 and 2.53 wt %, respectively. Various types of pyrites are observed during the detailed FE-SEM investigations including pyrite framboids, euhedral pyrite, welded pyrite, pyrite microcrystals, and framework pyrite. Pyrite framboids are densely packed sphere-shaped masses of submicron-scale pyrite crystals with subordinate large-sized euhedral crystals of pyrite. Welded pyrite forms due to the overgrowth and alteration of pyrite crystals within the larger pyrite framboids. Pyrite microcrystals are the euhedral-shaped microcrystals of pyrite. The framework pyrite is also observed and is formed due to the pyritization of plant/algal tissues. Based on the growth mechanism, the pyrites can be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. The presence of pyrite, especially the abundance of pyrite framboids, suggests that the environment during the Es3x shale deposition in the lacustrine basin was anoxic. Their dominant smaller size suggests the presence of an euxinic water column, which is consistent with the lack of in-place biota and high TOC contents. This research work not only helps to understand the pyrite mineralization, role of organic matter, and reactive iron in pyrite formation in the shale but also helps to interpret the paleoredox conditions during the deposition of shale. This research work can also be helpful to other researchers who can apply these methods and conclusions to studying shale in other similar basins worldwide.

Conglomerate to mudstone lacustrine cycles revealed in Junggar Basin, Northwest China: Middle Permian Lucaogou and Jingjingzigou formations

The Junggar Basin is one of the very large, long-lived, Late Paleozoic lacustrine basins of China, developed by the closure of the early Paleozoic Junggar Ocean. The novelty of the reported research is outcrop measurements through the mid-Permian Jingzingzigou and Lucaogou formations, allowing reconstruction of interacting very coarse and very fine-grained alluvial-to-lacustrine depositional cycles within a km-thick succession of organic-rich mudstones. The deposits are interpreted as coarse grained fan deltas that extend subaqueously into Junggar lacustrine basin, depositing fine-grained muddy, sediment gravity flow deposits. The coarseness and frequency of both alluvial and subaqueous sediment gravity flows that thin and pinch out over short distances suggests that the margin of the basin was likely steep and fringed by relatively deep lake waters, but was not the main margin of sediment supply. The proposed depositional model highlights the process response to climate-driven lake-level changes around the steep lake margin. However, the coarse-grained alluvial to fan-delta cycles and the muddy lacustrine cycles occur at different thickness scales. The former (5–13 m thick) contain internal erosion surfaces and are dominated by river sheet flood and debris flow deposits and are likely incomplete due to frequent shifting, erosion and reworking of the sediments. The lacustrine cycles (few meters thick) are dominated by current-driven, very fine sandstone, siltstone and organic-rich mudstone beds with ripple lamination and graded beds, as well as dolomites during times or places where subaqueous currents were lacking. The age dating control is not detailed but we propose that depositional cycles were likely a response to Milankovitch orbital cyclicity, also suggested by previous Permian climate modeling, that cause alternating wet and dry periods and trigger tens of meters lake level changes with abrupt grainsize changes in the shallow proximal areas and more subtle changes in sediments of distal/deep lacustrine regions. The climate changes therefore cause both sediment supply changes subaerially and lake accommodation changes subaqueously, though these would also be modulated to a lesser extent in the resultant stratigraphy by autogenic responses and tectonic forcing of sediment yield.