Continental Carbonates Research Articles

The Miocene lacustrine carbonate factory of the Ñirihuau Formation, Ñirihuau Basin, North Patagonian Andes, Argentina

AbstractFacies, microfacies and stable isotope analyses of limestone beds in the northernmost Ñirihuau Basin, North Patagonian Andes, Argentina, document and constrain the past hydrological, sedimentological and climate conditions that prevailed during the deposition of a lacustrine system between ca 15 and 13 Ma. This palaeoenvironment is recorded in the middle section of the Ñirihuau Formation, which holds significance because: (1) It was deposited during a transition from an extensional to a compressional tectonic regime; (2) it spans the Middle Miocene Climatic Optimum and the beginning of the Middle Miocene Climatic Transition; and (3) it contains limestone beds interbedded within a 600 m thick interval of mudstones and siltstones, along with intercalated sandstone and volcaniclastic bodies. Two detailed sedimentary logs were surveyed along the Arroyo Las Bayas, at the western and eastern flank of the David Syncline. Limestones from both stratigraphic sections were sampled as well as isolated limestone beds from two other sites. One facies association was defined and interpreted as a perennial lake associated with a deltaic system and dominated by detrital clastic material. It comprises Facies 1 (Marginal lacustrine) and Facies 2 (Lower delta plain); in both, the presence of grainstones and calcimudstones stands out. Through petrography and cathodoluminescence studies of these continental carbonates, nine microfacies were identified: (a) Intraclastic grainstone, (b) Homogeneous calcimudstone, (c) Silty grainstone, (d) Disrupted micrite, (e) Birds eye micrite, (f) Bioclastic mudstone, (g) Calcimudstone with sparse detrital grains, (h) Fenestral micrite, (i) Stromatolitic boundstone. These indicate mainly bio‐induced subaqueous carbonate precipitation and subordinate deposition by tractive flows with short‐distance transport on a littoral lacustrine environment. Most of these microfacies exhibit very early diagenesis (eogenesis) effects. These features, and the geochemistry results, indicate that they were deposited in a palaeolake system under temperate to warm and humid conditions.

Benthic foraminifera as depth estimators in the tropical carbonate shelf of northeastern Brazil

Benthic foraminifera are valuable proxies for oceanographic and environmental research due to their high degree of species-specific habitat use. The present study describes the distribution of benthic foraminifera across the continental carbonate shelf of northeastern Brazil, and relates their distribution patterns to environmental parameters associated with the texture of the sediment and the depth of the water. Sediment samples were collected at depths between 0.5 and 57 m. In the laboratory, the samples were analyzed following the standard protocol established for Quaternary foraminifera research. Sediments dominated by carbonate sand, which were approximately symmetric in the first depth interval (0.5–20 m), and asymmetric in the second (20–40 m) and third (40–57 m) intervals. The foraminiferal assemblages composed of 96 species, although they were dominated by the five species – Archaias angulatus, Textularia agglutinans, Amphistegina lessonii, Quinqueloculina lamarckiana, and Peneroplis carinatus. Most tests were intact and the species composition varied across the continental shelf, with Pseudotriloculina laevigata and Globobulimina? sp. Only occurring from 0.5 to 20 m depth in shelf, while Pyrgo elongata, Quinqueloculina sp. A, Quinqueloculina sp. C, Fissurina sp. A, Fissurina sp. B, Elphidium morenoi, Glabratella brasilensis, and Siphogenerina sp. Were restricted from 20 to 40 m, and Sigmoilina sp., Fursenkoina sp., and Textularia sp. A was only recorded in depth >40 m. The diversity of the foraminiferal assemblages is favored by the attenuation of hydrodynamic conditions at deeper intervals (20–57 m). Therefore, it can be concluded that the benthic foraminiferal species respond to the depth and variation in sediment characteristics through changes in their population and assemblage structures.

The Atlantic Jigsaw Puzzle and the geoheritage of Angola

Abstract The jigsaw-puzzle fit of South America and Africa is an icon of plate tectonics and continental drift. Fieldwork in Angola since 2002 allows the correlation of onshore outcrops and offshore geophysical and well-core data in the context of rift, sag, salt, and post-salt drift phases of the opening of the central South Atlantic. These outcrops, ranging in age from >130 Ma to <71 Ma, record Early Cretaceous outpouring of the Etendeka–Paraná Large Igneous Province (Bero Volcanic Complex) and rifting, followed by continental carbonate and siliciclastic deposition (Tumbalunda Formation) during the sagging of the nascent central South Atlantic basin. By the Aptian, evaporation of sea water resulted in thick salt deposits (Bambata Formation), terminated by seafloor spreading. The Equatorial Atlantic Gateway began opening by the early Late Cretaceous (100 Ma) and allowed flow of currents between the North and South Atlantic, creating environmental conditions that heralded the introduction of marine reptiles. These dramatic outcrops are a unique element of geoheritage because they arguably comprise the most complete terrestrially exposed geological record of the puzzle-like icon of continental drift.

Editorial for Special Issue “Geochemistry of Travertines and Calcareous Tufas”

Travertine and calcareous tufa are the lithological terms generally used to describe continental carbonates [...]

REACTIVE FLOW IN CARBONATE ROCKS FROM LACUSTRINE ENVIRONMENTS: THE EFFECTS OF PRESSURE AND FLUID SALINITY

The high production of CO2 from Brazilian pre-salt reservoirs requires research on CO2 interactions with carbonate reservoirs for enhanced oil recovery (EOR) or CO2 trapping. This interaction causes reactive phenomena that are not fully understood. Experimental tests on carbonate facies are crucial for understanding reactions and their impact on reservoir characteristics. This study investigates experimentally reactions generated by reactive fluid flow in carbonate rocks from two geological scenarios by analyzing chemical and morphological parameters before and after the tests. Salinity was the most significant parameter for reactive flow, impacting porosity for modern continental carbonate samples from Patagonia. However, the sample from the Brazilian pre-salt showed higher mineral stability and less mineral dissolution. Results provide data for simulating reactive transport in these rocks for CCS and understanding diagenetic processes' impact on reservoir quality.

Effective mechanisms in the formation of pool-rimstone dams in continental carbonate systems: The case study of Huanglong, China

The tufa/travertine landscape is an important heritage and ecological wealth given to mankind by nature, of which rimstone dam limiting pools are important features that allow their development. This study explores the mechanisms influencing the formation of rimstone dams through the study of depositional behavior in the short-term under hydrodynamic action, hydrochemical changes, and biological characterization, independent from the long-term dynamics. Numerical simulations clearly show the complex flow regimes of the paired pool and rimstone dam. Differences in flow regime and particle transport distribution in different areas of the pool lead to differences in deposition at different areas of the pool. Hydrodynamic effects generate morphological changes of the pool-rimstone dam by promoting the deposition on the outer wall of the dam and causing no-deposition and remobilization (erosion in a broad sense) in the inner wall. Significant changes in the ion concentrations of Ca2+ and HCO3− occur at short distances in the external upper and lower parts of the dam, suggesting that the thin layer of water and high velocity conditions in the outer wall of the rimstone dam facilitate precipitation. Biological processes influence the development of rimstone dam and promote its thickening. The plant remains of leaves, branches, and roots provide the basic depositional framework and depositional template for rimstone dam growth. Besides, microorganisms affect the micro-morphology of calcite through their own metabolism. This study aims to clarify the mechanisms of pool-rimstone dam formation by exploring the interactions between abiogenic and biogenic controls within a hydrodynamic model.

Late Pleistocene clastic tufa from Gafsa, southern Tunisia: Implications for paleo-landscape interpretation

The landscape reconstruction in the Jebel El Mida area during the Late Pleistocene was based on the continental carbonate record, particularly on the clastic tufa deposits. Four main lithofacies types of clastic tufa were recorgnized in the Jebel El Mida area, these are: (1) grain-supported macrodetrital phytoclast tufa, (2) grain-supported macrodetrital oncolith tufa, (3) grain-supported macrodetrital lithoclast tufa, and (4) matrix-supported microdetrital tufa. The interpretation of depositional environment reveals spring-fed streams and pools with highly CO2-enriched water helped in carbonate precipitation and incrustation on the vegetation cover. Dating of tufa deposits has been based on archeological content and radiometric (U/Th) age dating. A major lag exists between the age of the archeological content and the radiometric age for tufa (170 ky) indicating transport and re-sedimentation of older reworked tufa within the stone tool-rich sandsone and conglomeratic substrates. The major fault of Gafsa that cross-cuts the studied area is the main conduit for upwelling carbonated waters for tufa precipitation and also the re-organization of the landscape after the recurrent tectonic activity during the Late Pleistocene. The deposition of Jebel El Mida tufa indicates particular depostional settings characterized by (1) abundant rainfall on the recharge areas with high water table, (2) acidic water infiltrations through plant cover and soils, and (3) dissolution of aquiferous Cretaceous carbonates resulting in water mineralization of the groundwater during a time span that predates Late Pleistocene.

Diagenesis of continental carbonates linked to the evolution of the flexural margin of the Triassic Sorocayense-Hilario rift sub-basin, Argentina

The lithostratigraphic units that conform the Sorocayense Group and fill the Sorocayense-Hilario sub-basin represent an alluvial-fluvial-lacustrine succession with significant volcanic supply during its deposition. They are namely the Cerro Colorado del Cementerio, Agua de los Pajaritos, Monina, Hilario and El Alcázar formations and present several carbonate levels. The genesis, diagenesis, and main controlling factors on continental carbonates present a means to understand basin evolution through the study of their petrography and chemical elemental composition through cathodoluminescence techniques. We have identified six microfacies: a) homogeneous micrite, b) bioclastic micrite, c) dolomicrite, d) laminated micrite, e) oncolitic packstone, and f) sparitic carbonate. Among these microfacies, six cementation and alteration phases have been identified: a) micritization, b) mechanical compaction, c) calcitic cementation, d) sparitic cementation, e) microsparitic cementation, and f) chemical compaction. This analysis allowed establishing a chronology of the diagenetic modifications undergone by the carbonates. Results support diagenesis was controlled mainly by tectonics showing major impact in carbonate facies identified at the flexural margin of the rift. The effect would have been linked to exertion of a paleohydrological effect favoring lateral meteoric water migration through faulting. The presence of dolomite in some of the microfacies is linked to the presence of montmorillonite as the dominant Mg-rich-phyllosilicate in clay assemblages most likely acting as the potential source. In turn, Mg is more likely to be replaced by Mn leading to luminescent carbonate microfacies.

The petrophysical characteristics of Macabu formation (Aptian, Campos Basin) continental carbonates: Core and special well logs (UBI and NMR) analysis integration

To interpret carbonate heterogeneities and understand the correlation between these rock features and their petrophysical parameters, an integrated dataset formed by thin section, core sample description, image log, and nuclear magnetic resonance analysis were used. This interactive approach enables the comparison of depositional and acoustic image log facies and their permo-porous behaviors. The advantage of the proposed integration lies in the ability to expand the interpretation of depositional facies to the entire imaged well interval. The results highlight the occurrence of six depositional facies, four ultrasonic image log facies, and two intervals with different permo-porous behavior present at Macabu Formation (Aptian, Campos Basin). Based on core sample and thin section analysis, three in situ carbonates were described: F1 – shrubstones, F2 - spherulitic grainstone-wackstone and hybrid forms, F3 - stevensitic claystone with spherulites, and others like transported facies: F4 - intraclastic grainstone-rudstone, F5 - laminated Detrital packstone-grainstone, and F6 - laminated dolomudstone. According to ultrasonic signal amplitude and structural patterns, such as descriptive criteria, the ultrasonic image log facies characterization resulted in: UIF1 - bright-heterogeneous, UIF2 - dark laminated, UIF3 - bright massive, and UIF4 - dark dendritic. The NMR well log analysis helped us divide the formation into two main zones, the first with good reservoir behavior and the other with poor behavior. This approach resulted in the correlation between facies and reservoir quality, indicating those with better fluid flow potential, in which facies F1, F2, F4, and UIF4 showed the best results. Finally, the integration between the different facies analysis demonstrated that several lithofacies may have the same appearance in the acoustic image, leading them to integrate the same ultrasonic image facie, this occurs due to their similarity in roughness.

Hot Rocks: Interpreting Extremes of Earth Surface Temperatures From the Geologic Record

AbstractHow hot can we expect Greenhouse Earth to have been in the past and the future? This is a question that deep‐time paleoclimatologists have long investigated. In a new paper by Fetrow et al. (2022), https://doi.org/10.1029/2022PA004517, deconvolving the depositional environments and burial history of continental carbonates deposited during known greenhouse climates, such as the Cretaceous, highlight the way Earth scientists can embark on understanding the clumped isotope derived temperatures and properly interpreting such temperatures.

Morphological Typology and Petrography of Plio-Quaternary Continental Carbonates of Aït Chaïb, Skoura Basin, Middle Atlas, Morocco

The Plio-Quaternary Ait Chaib basin occupies the northwestern part of the Skoura Syncline, which is part of the northern Middle Atlas Folded Zone. This intracontinental basin was formed probably in the Pliocene after the Tortono-Messinian regression sea. The Plio-Quaternary deposits are bound by fluvial conglomerates and continental carbonates. They consist of barrage travertines; travertine limestones and oncholitic travertine silts and sands of the upstream and downstream deposits. These travertines laterally pass to lacustrine limestones at the bottom of the basin. The genesis of these continental carbonates, correlative to phases of karstification, has affected the Meso-Cenozoic carbonate deposits of the folded Middle Atlas, and is linked to favorable paleo-environmental conditions of geological, geomorphological, hydrogeological, hydrological and bioclimatic order. The petrographic study of these lacustrine limestones shows laminar structure allowing to assimilate into continental stromatolites. They were formed by cyanobacterial mats in a shallow lacustrine environment in which water were was non-turbid and supersaturated in limestone solution, coming from karstic springs staking out the Middle Atlasic North Accident. The important diagenesis evolution is linked to the surface water table contain, which has as a floor gypsiferous lagoonal marl of the Upper Miocene age.

Analysis of a travertine system controlled by the transpressional activity of the Alhama de Murcia fault: The Carraclaca site, eastern Betic Cordillera, Spain

Continental carbonates, such as travertines and tufas, formed from CO2-rich groundwater degassing as it emerges at the Earth’s surface, are often associated with major crustal-scale faults. The Carraclaca site, in the Lorca-Totana section of the Alhama de Murcia Fault, Spain, presents a complex geomorphological landscape controlled by active tectonics. The geology here records the interaction between Quaternary alluvial fans, travertines, and a pop-up structure developed in a transpressional section of the fault. The Alhama de Murcia Fault is an 80 km long left-lateral strike-slip fault that is one of the main seismogenic structures in the Iberian Peninsula. In this work, we examined the relation between travertine precipitation in the Carraclaca site and the tectonic activity of this fault zone through morphological and geochemical studies. The δ13C and δ18O isotopic signals indicate that the carbonate deposits are hydrothermal. In addition, the 87Sr/86Sr ratios in the samples suggest subsurface fluid interaction with the Miocene sediments and the Alpujárride basement, located below the alluvial deposits. Tectonic activity in the Alhama de Murcia Fault might generate the opening of deep water circulation in the crust every time a seismic event occurs, giving rise to hydrothermally derived carbonates precipitation. Deep waters rise and reach the surface interacting with meteoric waters, resulting in travertine formation. Therefore, the Carraclaca carbonate deposits study can inform us about the seismogenic cycle of the fault in the Lorca-Totana section.

Palustrine limestones and calcretes as the sedimentary record of paleoenvironmental changes in a Late Permian semi-arid climate at the SE periphery of the Southern Permian Basin (the Holy Cross Mountains, Poland)

An important part of the Upper Permian (Zechstein) sequence in the Holy Cross Mountains (HCM) in Poland comprises continental carbonate facies where dryland and wetland depositional environments dominated the southeastern part of the Southern Permian Basin. Sedimentological and thin-section microfacies analysis of over 860 m of core from seven wells, supplemented with field observations from several outcrops from the HCM area, has allowed recognition of numerous siliciclastic and mixed silici‑carbonate facies which are related to alluvial fan, proximal and distal floodplains, and ephemeral lacustrine environments. Due to climatic seasonality and subsequent groundwater oscillations, the latter were frequently exposed, causing widespread desiccation cracking and pedogenesis. Continuous aggradation controlled by steady tectonic subsidence allowed the deposition and preservation of thick (> 30 m) units with numerous palustrine intervals. In such a low aggradation rate system, vertical and lateral changes in the continental facies associations reflect the climatic evolution from arid to semi-arid and then to sub-humid environments with enhanced climatic seasonality. These facies changes indicate a gradual increase in the water availability towards the Permian-Triassic boundary, which may support the hypothesis of generally increased wetting at the low-latitudes of Pangea, linked with the development of greenhouse conditions and enhancement of the intertropical convergence zone at that time. Continental carbonates are therefore recognised as high-resolution recorders of palaeoclimate and tectonics.

Controls on Mg/Ca Ratios in Recent Stromatolites: Insights from Fluvial Systems in the Iberian Range (Spain)

The utility of the Mg/Ca elemental ratio of calcite ((Mg/Ca)calcite) as a temperature indicator in continental carbonate deposits is a matter of debate due to the different results obtained by diverse authors. In this study, we aimed to test the reliability of the (Mg/Ca)calcite in fluvial carbonates. We selected the recent tufa stromatolite records of four rivers on the Iberian Peninsula for the trace element analysis based on six-monthly sampling. Previous sedimentary and hydrological studies on these fluvial basins provided the information for this work. The water temperature estimates for the stromatolite (Mg/Ca)calcite substantially differed from the measured water temperatures in most of the studied cases. We thus assessed other factors that participate in the control of the Mg partitioning between water and calcite. The correction of the detrital Mg content yielded water temperatures that matched the measured ones in one of the rivers. The (Mg/Ca)water, water discharge and calcite precipitation rates may also occasionally influence the (Mg/Ca)calcite. The six-month behaviour of some of these parameters could interfere with the relationship between the (Mg/Ca)calcite and water temperature. According to these results, and their comparison with other non-marine carbonates, the wide variety of parameters that are involved in the (Mg/Ca)calcite limit it as a geochemical thermometer in continental sedimentary environments.

Plio-Pleistocene continental carbonates and silcretes in Wadi El-Natrun, NW Egypt: Their morphology and paleoenvironmental significance

To deduce the prevailing paleoenvironmental conditions during the deposition of the Plio-Pleistocene continental carbonate sequence at Wadi El-Natrun (NW Egypt), we examined and compared the macro- and micro-morphological, mineralogical and stable isotopic characteristics of calcretes, dolocretes, palustrine carbonates, and silcretes of the lower Pliocene Meikheimin and Plio-Pleistocene Qataji formations. Calcrete occurs in both formations and exhibits a variety of forms, including powdery, nodular, fracture-infill, massive and hard laminated crusts (hardpan). The calcretes developed in mixed vadose and phreatic diagenetic environments under an arid to semi-arid climate. Palustrine carbonates occur in the upper Qataji Formation and developed as nodular, desiccated, brecciated, mottled and pisoidal limestones that are interbedded with the calcretes. They exhibit varied micro-morphological features, including circum-granular and planar cracks, nodular and clotted fabrics, peloidal textures, infilling of burrows, root molds, and fenestral pores, which indicate modifications during subaerial exposure. The dolocrete facies in the upper Qataji Formation comprises laminated carbonate crusts. These include spheroidal dolomite and dolomicrite with Mg-rich clays that formed as primary precipitates in a continental setting. The silcretes consist of spheroidal, irregular opaline accumulations that formed as replacements of calcrete and dolocrete. These features reflect silicification in near-surface environments where there was an interplay between vadose and phreatic environments during deposition and early burial diagenesis. The stable isotopic data suggest deposition under warm, seasonally arid to semi-arid climatic conditions in an area dominated by C3 vegetation. The regional water table was higher in the early Pleistocene than during the early Pliocene but was subject to large fluctuations.

When distinction matters: Carbonate shrubs from the Aptian Barra Velha Formation of Brazilian's Pre-salt

Branched, bush- or shrub-like crystals are some of the most striking components of carbonate facies from the Cretaceous South Atlantic Pre-salt carbonate reservoirs. Many different analogues have been proposed to help to understand them, including travertines, lacustrine mounds and alkaline lake deposits, amongst others. Interpretation and genesis of these special facies involve discussions about biogenic vs. abiogenic formation of these branched components, relation with Mg-clays, role of microbes and growth of spherulites, amongst others. The present work shows some examples at microscale from a drilled core from Santos Basin (Brazil).The analyses of the shrub-like crystals present in the studied section allowed the distinction between four different types: (i) Type 1: fan-shaped fibrous-radial crystals; (ii) Type 2: fan-shaped, branched fibrous-radial crystals; (iii) Type 3: dendrite crystals; and (iv) Type 4: laminated crusts. All these components show similarities with their counterparts present in travertines, suggesting similar conditions controlling crystal growth in both Pre-salt and travertines. However, microfacies relationships show clear differences between the Pre-salt and those from travertine in the literature. Such Pre-salt microfacies presented here show a clear relationship between the development of the so called shrub-like crystals and the environment's energy, allowing the establishment of a proximal–distal pattern within a lacustrine environment as well as the identification of microfacies that could be related to (subaqueous?) hydrothermal vents.Distinction between the different types of branched components described here as well as their relationships with other microfacies are critical issues for the differentiation of the processes involved and the interpretation of the environments of formation (e.g., spring-related and proximal–distal environments within a lake). The observations made on samples from a drilled core of Brazilian's Pre-salt allow the discussion on some topics and interpretations involving the Pre-salt facies, providing potential research guidelines on these challenging continental carbonates.

Hydrodynamic model for independent cold and thermo-mineral twin springs in a stratified continental karst aquifer, Camou, Arbailles Massif, Pyrénées, France

The Camou springs (Arbailles Massif, French Western Pyrenees) display an unusual close association of a typically cold karstic spring that drains the Urgonian western limb of the Arbailles, and a thermo-mineral spring (33.5°C; salinity 17.7 g/L). The latter gains its mineralization at the contact of Triassic evaporites mainly through a deep loop in the Apanicé syncline. The fast upflow of this deep water occurs at the cross of large active lines (the North-Pyrenean thrust located at depth, and the Saison transverse fault). Cave diving in the nearby Maddalen Cave allowed reaching the phreatic passage at the origin of the cold spring, which however also crosses the thermal body in the third sump (S3). Both water bodies are separated by a sharp thermocline. 6 pressure-temperature dataloggers were placed in both water bodies along the thermocline for 6 months. The dataloggers located downstream on either side of the thermocline show at the beginning of flood first a rise of the thermal body, then an invasion of the whole phreatic passage by the cold floodwater, controlled by head pressure changes in the karst aquifer. From observation of these mechanisms, we deduce a hydrodynamic model with a warm plume rising into the cold aquifer, without significant mixing. Such independence of water bodies is explained by the decrease of turbulent rate at the interface, due to the sharp density gradient. The relative absence of mixing does not actually require independent “watertight” routes, both water bodies can thus coexist even in the same conduit. This model locally implies the existence of unknown secondary passages close to the spring, which allow an independent draining of each water body toward separate outlets during low stage. Such type of stratified aquifer linked to density differences is common in coastal karst (Florida, French Calanques…), in the continental evaporite karst (Schlotten of the Harz in Germany, Kungur karst in Ural…), but remains poorly identified in continental carbonate karst areas, mainly because of the difficulty of access. Together with the Mescla spring in French Alpes-Maritimes, the Camou twin springs discharging in the same porch are an outstanding example, allowing a direct study of the stratification and the dynamic of highly contrasted water bodies.

Sedimentary evolution of tectonically-controlled Cenozoic shallow marine glaucony, fluviatile and lacustrine-palustrine deposits at Bahariya region, Egypt

The Cenozoic basin of the tectonically-active Bahariya region (Western Desert, Egypt) contains a unique paleoenvironmental, paleotectonic and paleoclimatic record displaying a large variety of sedimentary facies, stratigraphic discontinuities, and geochemical changes. This study deals with the sedimentary evolution and tectono-stratigraphic modelling of shallow marine Upper Eocene glaucony facies (Upper Hamra Formation) that changes upward into a sequence of Oligocene continental fluviatile sandstone-calcrete (Radwan Formation) and then into Miocene lacustrine-palustrine deposits (Continental Carbonate Unit). The tectonic instability of the Bahariya basin can be deduced from occurrence of sedimentary discontinuities, development of pedogenic features, and unconformable relations among rock units. Both sedimentation pattern and facies distribution seem to have been controlled by fault activity through time. Two types of glaucony facies (pelloidal and massive smectitic glaucony) were deposited in a narrow sub-basin along main faults reaching the basin center. These faults also conditioned the deposition of the Oligocene fluviatile sandstone and Miocene lacustrine-palustrine carbonates. A transition from greenhouse (global warming event of the Middle Eocene Climatic Optimum) to icehouse climatic conditions (Eocene-Oligocene climatic cooling) is probably represented by the deposition of the shallow marine glaucony facies during the Priabonian. Semi-arid to sub-humid conditions were dominant during the formation of the Oligocene calcretes and the Miocene lacustrine-palustrine carbonates. The latter carbonate deposits consist of basal thin-bedded chert deposits displaying hydro-plastic sedimentary structures that are possibly coeval with the Miocene basaltic extrusions along faulted and low-lying areas. We conclude that both paleoclimate and intraplate tectonic activity related to the Red Sea rifting influenced the pattern of sea level, continental drainage, sedimentation rate and clastic supply since the Priabonian up to the middle Miocene in the Bahariya basin.

Controlling factors of acoustic properties in continental carbonates: Implications for high-resolution seismic imaging

Continental carbonates are characterized by multi-scale heterogeneous porous networks, making the geological interpretation of seismic imaging difficult. We investigate two sedimentary sections exhibiting a similar facies succession, combining geological characterizations and multi-scale acoustic measurements. Based on outcrop investigation and petrographic description, we define nine sedimentary facies displaying contrasted early diagenetic evolutions. According to the vertical facies variations, we develop a depositional model corresponding to a low gradient valley fed by freshwaters, subdivided into three main domains (alluvial plain, palustrine and paludal). To understand the acoustic properties of the studied sedimentary rocks while remaining representative of their multi-scale heterogeneity, we acquire acoustic measurements at two different scales: i) at log-scale, directly on the outcrop surfaces using a frequency of 250 kHz; and ii) at plug-scale as usually done in laboratory using a frequency of 500 kHz. Based on these multi-scale geophysical acquisitions, we link in-situ P-wave velocities with the different sedimentary facies while characterizing centimeter-scale Representative Elementary Volumes (REVs). Conversely, based on laboratory measurements and thin-section petrography, we define relationships between P-wave velocity, porosity, facies, and diagenesis, corresponding to millimeter-scale REVs. Using both in-situ P-wave velocity measurements and plug densities, we construct 1-D synthetic seismograms showing meter-scale seismic reflectors equivalent to crosswell seismic frequency ranges. This approach shows the following: i) high-amplitude seismic reflectors fit with facies changes associated to diagenetic contrasts (e.g. cemented versus uncemented carbonates); ii) reflection free-zones match with a succession of facies changes affected by diagenetic homogenization (e.g. intensely to pervasively recrystallized and cemented carbonates). Our work highlights the importance of relating an extensive geological description of carbonates (facies, depositional model, diagenesis) together with multi-scale acoustic measurements and synthetic seismic modelling to predict the high-resolution heterogeneities of subsurface reservoirs.

Continental freshwater carbonate coated grains: oncoids in Quaternary deposits of the Serra da Bodoquena region, Central-West Brazil

Continental freshwater carbonate coated grains: oncoids in Quaternary deposits of the Serra da Bodoquena region, Central-West Brazil