Continental Carbonates Research Articles

Lattice Boltzmann Simulations of Fluid Flow in Continental Carbonate Reservoir Rocks and in Upscaled Rock Models Generated with Multiple-Point Geostatistics

Microcomputed tomography (μCT) and Lattice Boltzmann Method (LBM) simulations were applied to continental carbonates to quantify fluid flow. Fluid flow characteristics in these complex carbonates with multiscale pore networks are unique and the applied method allows studying their heterogeneity and anisotropy. 3D pore network models were introduced to single-phase flow simulations in Palabos, a software tool for particle-based modelling of classic computational fluid dynamics. In addition, permeability simulations were also performed on rock models generated with multiple-point geostatistics (MPS). This allowed assessing the applicability of MPS in upscaling high-resolution porosity patterns into large rock models that exceed the volume limitations of the μCT. Porosity and tortuosity control fluid flow in these porous media. Micro- and mesopores influence flow properties at larger scales in continental carbonates. Upscaling with MPS is therefore necessary to overcome volume-resolution problems of CT scanning equipment. The presented LBM-MPS workflow is applicable to other lithologies, comprising different pore types, shapes, and pore networks altogether. The lack of straightforward porosity-permeability relationships in complex carbonates highlights the necessity for a 3D approach. 3D fluid flow studies provide the best understanding of flow through porous media, which is of crucial importance in reservoir modelling.

Impact of the North American monsoon on isotope paleoaltimeters: Implications for the paleoaltimetry of the American southwest

Paleoaltimetric studies have characterized in detail the relationship between carbonate oxygen isotope ratios (δ18Oc) and elevation in orogens with simple, single-moisture-source hydrological systems, and applied this relationship to ancient continental carbonates to provide constraints on their past elevation. However, mixing of different atmospheric moisture sources in low-elevation orogens should affect δ18Oc values, but this effect has not yet been confirmed unequivocally. In the American Southwest, summer monsoonal moisture, sourced in the Equatorial Pacific and the Gulf of Mexico, and winter moisture, sourced in the East Pacific, both contribute to annual rainfall. We present stable isotope results from Quaternary carbonates within the American Southwest to characterize the regional δ18Oc-elevation relationship. We then provide stable isotope results from local Eocene carbonates to reconstruct late Laramide paleoelevations. The Quaternary δ18Oc-elevation relationship in the American Southwest is not as straightforward as in more simple hydrological systems. δ18Oc changes with altitude are non-linear, scattered, and display an apparent isotopic lapse rate inversion above 1200 m of elevation. We speculate that decreasing surface temperatures at high altitudes limit the duration of carbonate growth to the summer months, biasing δ18Oc values toward higher values typical of the summer monsoon and leading to lapse rate inversion. δ18Oc-elevation relationships based on modern water isotope data or distillation models predict paleoelevations that range up to as much as 2 km higher than the modern elevations of 2000 to 2400 m for our late Eocene sites located at the southern edge of the Colorado Plateau. By contrast, our δ18Oc-elevation relationship for the American Southwest yields lower paleoelevation estimates. These alternate estimates nonetheless suggest that significant elevation (at least ∼1 km) had already been attained by the Eocene, but are also compatible with

Volcanic rock alterations of the Kwanza Basin, offshore Angola - Insights from an integrated petrological, geochemical and numerical approach

The Lower Cretaceous presalt section of the Kwanza Basin (Angola) is in the spotlight following the discoveries of petroleum systems in this basin, and more generally in the South Atlantic. These systems are mostly composed of continental carbonates in close association with volcanic rocks. This work is focused on the study of an offshore Kwanza presalt volcanic sequence characterized as Valanginian trachytic subaerial lava flows. A detailed petrological analysis of the altered trachyte in association with fluid inclusion microthermometry was conducted in order to depict the initial mineralogy (albite, sanidine, titanomagnetite) and obtain a paragenetic sequence (quartz, siderite, kaolinite, calcite). Thermodynamic equilibrium modelling of the trachytes alteration by meteoric fluids, over a range of temperatures (25 °C–200 °C) and CO2 partial pressure (pCO2: 0.01 mbar to 100 bar), were performed with PHREEQC, and compared to the observed paragenetic sequence. Some numerical simulations reflect the observed paragenesis. As a result, the pCO2 is constrained by the occurrence of siderite (from 0.1 bar at 50 °C to 30 bar at 125 °C) and kaolinite (from 0.2 bar at 50 °C to 1.2 bar at 125 °C). The simulations emphasize the need for a high pCO2 in the hydrothermal system, to achieve the observed trachyte transformation. After reaching equilibrium with the trachytes, the simulated fluids highlight a mid-alkaline to near neutral pH with high Fe, HCO3+CO3, and alkali concentrations. The palaeofluids could have evolved from Ca- and Mg-rich to Ca- and Mg-poor with increasing temperature. Inversely, Si concentrations are positively correlated with increasing temperatures. This methodology, integrating a petrological approach and numerical simulations, proves to be a powerful tool leading to better understanding of the proxies (pCO2, temperature, redox conditions) controlling paragenesis. To push further, these simulations are also a step toward improved understanding of palaeofluid evolutions in presalt systems and better prediction of reservoir quality.

Enrichment of 88Sr in continental waters due to calcium carbonate precipitation

δ88/86Sr data published over the last few years suggest that continental waters are enriched with 88Sr as compared to the rocks in their drainage basins. In an attempt to understand this phenomenon, this study established the fractionation in the 88Sr/86Sr ratio during precipitation of continental carbonates (i.e., carbonates precipitated on land from surface, pedogenic, or ground waters), and evaluated the contribution of this process to the 88Sr-enrichment in rivers. For this, stable and radiogenic Sr isotopes (88Sr, 87Sr and 86Sr) were measured in calcite samples and their precipitating waters collected in various continental environments, such as soil, cave, streams and groundwater.The results indicate that continental carbonates are 88Sr-depleted relative to their precipitating waters, placing them as one of the most 88Sr-depleted reservoirs on earth. The average difference in δ88/86Sr values between waters and solid CaCO3 (tufas or speleothems) that they precipitate is Δcarb-water=−0.218±0.014‰ (1SD). An even larger fractionation (εcarb-water=−0.285±0.02‰) was measured in groundwater with particularly high carbonate-alkalinity and high carbonate precipitation rate that depleted ∼65% of the Sr in the groundwater, resulting in substantial 88Sr-enrichment in the residual dissolved Sr (δSr88/86=0.656‰). Results also suggest that pedogenic carbonate precipitation in soil profile removes 50–85% of the Sr from the recharging soil–water, thereby increasing the δ88/86Sr value of the soil-water from ∼0.18‰ to 0.3‰–0.6‰. Similar 88Sr-enrichment was observed in drip water from a karst cave.A maximum removal flux of Sr into continental carbonates of about 20 Gmol(Sr)⋅y−1 is required to yield the reported 88Sr-enrichment in global rivers (δSr88/86=0.32‰) relative to their rock sources when using the fractionation factor derived in this study, Δcarb-water=−0.218‰, and the published δ88/86Sr composition of marine carbonates of 0.16‰. This surprisingly large flux requires that ∼40% of the originally weathered Sr should co-precipitate with continental carbonates. This calculation may suggest that CaCO3 precipitation is a significant mechanism in the continental Sr cycle. It is possible however that other mechanisms such as Sr uptake by plants or incongruent weathering of silicates, could also contribute to the riverine 88Sr-enrichment. Alternatively, the average δ88/86Sr value of marine carbonate rock sections undergoing weathering is higher than suggested in current literature. It is concluded that additional δ88/86Sr data on marine carbonate rock sections should be collected to resolve the enigma of the high value of riverine δ88/86Sr.

Travertines Associated With Hyperalkaline Springs: Evaluation As A Proxy For Paleoenvironmental Conditions And Sequestration of Atmospheric CO 2

Abstract: Travertine are found in ophiolite massifs in association with bicarbonate-depleted hyperalkaline spring waters (pH up to 11.9), in contrast with most continental carbonates (e.g., travertine, tufa, speleothems) that precipitate from calcium bicarbonate-enriched waters. Here travertines formed from bicarbonate-depleted hyperalkaline spring water were subjected to a multidisciplinary and multi-scale approach to evaluate their potential as proxies of past climatic records and sequestration of atmospheric CO 2 . Two mechanisms of calcium carbonate precipitation were apparent: 1) hydration-hydroxylation reaction due to the mixing of hyperalkaline and surface runoff waters, or 2) dissolution of atmospheric CO 2(g) into hyperalkaline waters. For two sites, the bulk chemical signature of travertines (Mg, Ca, and Sr wt%) are consistent with “prior calcite precipitation” (PCP) processes and thus likely records the environmental conditions at the time of their formation. However, for the third site, the trace-element concentrations in the various carbonate fabrics indicate some recrystallization. Constant δ 18 O values indicate that hydration and hydroxylation reactions completely buffer the oxygen isotope composition of the water (equilibrium state) from which a paleo-temperature can be estimated. In contrast, δ 13 C values reflect potential carbon sources, either from surface runoff waters or atmospheric CO 2 . Within the framework of continental carbonate, calcium carbonate formation in bicarbonate-depleted hyperalkaline environments results in a linear and positive co-variation of δ 18 O and δ 13 C values and defines a unique and distinctive stable-isotope field on a δ 18 O–δ 13 C plot, in contrast to carbonates formed in more typical bicarbonate-enriched environments. Moreover, the combined variations in δ 18 O, δ 13 C, and 87 Sr/ 86 Sr between laminae document the changes in the paleo-activity of hyperalkaline spring and surface runoff waters on the time scale of formation. The 87 Sr/ 86 Sr ratio represents a tracer for quantifying surface runoff water contribution. Furthermore, the amount of CO 2 sequestrated in travertine has been estimated following different scenarios of formation. The calculated CO 2 sequestrated for these deposits ranges from 9 kgCO 2 yr –1 to 522 kgCO 2 yr –1 .

Facies architecture, geochemistry and palaeoenvironmental reconstruction of a barrage tufa reservoir analog (Betic Cordillera, S. Spain)

Continental carbonates are currently of high interest for the oil and gas industry, as they have proved to be good reservoir rocks in Brazil and Angola offshore oil fields. In this article, a tufa system in which continental carbonates formed mainly in the Pliocene (Rambla Becerra Tufa System, Guadix Basin, Betic Cordillera, S. Spain), is described in detail, and a model proposed for its formation and evolution through time. Several stages of tufa growth have been identified in the field, intercalated between stages of progradation of alluvial fan sediments. The tufa system connects laterally with the fluvial system developed in the axial valley of the Guadix Basin during the Pliocene and the Pleistocene, indicating that the tufa formation was coetaneous with the periods of lateral expansion of the fluvial system floodplain, that in previous works have been interpreted as more humid stages and potentially warmer. The absence of extraclasts in the tufa sediments points to the low activity of the alluvial fans during the tufa formation and therefore supports the idea that the main tufa growth happened during more humid stages, and the alluvial fans developed further during the more arid periods, which is consistent with other recent findings in the Guadix Basin. The tufa growth is represented by four main stages, the second one bearing the best example to study as a potential reservoir analog, as the main outcrop is a discrete body with a specific geometry and facies distribution. This main outcrop is a mound-shaped tufa build-up formed by prograding sigmoids of clastic tufa, interpreted as the ramp-like buttresses developed after a barrage in a barrage tufa system. From a preliminary isotopic study of the carbonates forming the barrage tufa system and by comparison with other tufas and travertines, the oxygen and carbon stable isotopes point to an intermediate situation between a palustrine tufa and a travertine. This information, when added to the sedimentological data, supports the idea that water feeding the tufa system probably had different sources: a meteoric one, coinciding with the surface runoff and a groundwater one, probably in the form of springs fed by groundwater from the nearby aquifers that still exist in the Mesozoic carbonates that form the External Zones of the Betic Cordillera in the area.

Geobody architecture, genesis and petrophysical characteristics of the Budakalász travertines, Buda Hills (Hungary)

Subsurface Pre-salt Lower Cretaceous reservoir systems encountered offshore Brazil and Angola that consist of porous continental carbonates are difficult to access because of the limited core material that is under concession. Therefore, it is important to study comparable continental carbonate analogues, such as travertines and lacustrine carbonates, that potentially possess similar porosity and permeability characteristics. The Budakalasz Pleistocene travertines (Buda Hills, Hungary) have been studied from a sedimentological, geochemical and reservoir properties point of view in order to understand their formation, petrophysical properties and 3D reservoir architecture. The three-dimensional geobody architecture of the Budakalasz travertines illustrates the potential complexity of kilometre scale continental carbonate systems. Heterogeneous low angle sloping travertine lobes consisting of terrace slope, smooth slope and reed facies, are covered by a lime mudstone dominated, gastropod- and charophyte-rich marsh-pool facies. The observed facies represent a change from a subaerial travertine to a palustrine to lacustrine system, reflecting the ceasing of the system due to tectonic uplift. The dominant paleo-flow direction of the sloping lobes was towards the Danube depression, with several subordinate flow systems identified. The SrCO isotope geochemical signatures reflect travertine precipitation at ambient to superambient temperatures from meteoric derived fluids with a mixed CO2 origin of the marine carbonate substrate rocks, a potential atmospheric and a soil-derived component. Although the uniform signatures of the stable oxygen and carbon isotopes, in addition to petrographic observations suggest partial diagenetic isotopic resetting, the primary isotope trends seem to have been preserved. A reservoir-oriented facies evaluation in the three-dimensional geobody frame of the Budakalasz carbonates based on porosity and permeability core plug analyses reveals their heterogeneous nature. Terrace, smooth slope and marsh-pool facies have considerable porosities of 5–15%, while the porosity of the reed facies may reach up to 27%. Permeability of all facies is strongly anisotropic, mainly controlled by a better connectivity along layer-parallel primary pores. The reconstruction of the 3D geobody architecture of the Budakalasz lobe complex coupled to its geochemical and petrophysical properties provides a unique dataset that contributes in understanding fossil travertine systems.

Fault reactivation in travertine and its impact on hydraulic transmissibility: laboratory experiments and mesoscale structures

Direct shear experiments were undertaken to investigate the effect of faulting and reactivation on the hydromechanical characteristics of faults in continental carbonate samples. The tested rock is a travertine of continental, microbial origin with a calcite content of 99 wt%, with a strongly laminated texture. Analyses of the intact and sheared samples performed using medical X-ray computed tomography (CT) revealed that the porosity is mainly composed of subplanar pores and vugs. Permeability is high along the laminations, controlled by interconnected pores and fractures. The travertine is a lithological analogue for Aptian pre-salt oil reservoir rocks found in South Atlantic offshore basins. Three samples, with dimensions of 240 × 110 × 150 mm, were sheared to a maximum displacement of 120 mm under different levels of effective vertical stress (6–45 MPa), resulting in the formation of cataclastic fault gouge surrounded by a dense fracture network. A new experimental method was used to reactivate the artificially formed fault by performing cyclic vertical loading at different shear displacements on a previously sheared sample, while keeping a constant pore-pressure differential throughout the test. Pore-fluid responses across the fault zone were monitored continuously during both deformation (dynamic transmissibility) and hold periods (static transmissibility). Results show that the transmissibility reduces in all the samples for all values of the applied effective vertical stress and during shear reversal. The static transmissibility also decreases over time, which may be a result of creep deformation or the blocking of pore channels with gouge material. Our results indicate that once the gouge material is developed in the core of a carbonate fault zone, the dynamic transmissibility across that fault is permanently decreased, with little dependence on subsequent kinematics of reactivation, or changes in stress, so long as the gouge zone is not breached by a new structure.

Sedimentologic study of palustrine continental carbonate deposits of the Tajerouine area, NW Tunisia

Les calcaires palustres d'âge quaternaire (Pleistocene moyen et superieur), objet de cette etude, se trouvent au niveau du massif El Gara Tajerouine (Nord-Ouest de la Tunisie). L'etude de ces depots a differentes echelles, macroscopique (description lithologique) et microscopique (petrographie et mineralogie), montre qu'ils ont une structure tabulaire tout en reposant en discordance angulaire sur la serie marno-calcaire de l'Aptien-Albien. Ces depots montrent egalement des variations laterales et verticales de facies : conglomerats, calcaires et stromatolithes. Enfin, l'epaisseur des couches tend aussi a augmenter progressivement d'est en ouest et vers le sud de la zone d'etude. En raison de fluctuations de la nappe phreatique, ces carbonates ont subi des periodes d'inondation et d'emersion periodiques provoquant de nombreuses modifications par une pedogenese d'emersion plus ou moins profonde (nodulisation, traces de racines, pseudo-microkarst), celles-ci influant sur les cimentations au cours de la diagenese (alternances de ciment phreatique et vadose) qui justifient l'appellation des calcaires palustres.

Continental carbonate facies of a Neoproterozoic panglaciation, north‐east Svalbard

AbstractThe Marinoan panglaciation (ca 650 to 635 Ma) is represented in north‐east Svalbard by the 130 to 175 m thick Wilsonbreen Formation which contains syn‐glacial carbonates in its upper 100 m. These sediments are now known to have been deposited under a CO2‐rich atmosphere, late in the glaciation, and global climate models facilitate testing of proposed analogues. Precipitated carbonates occur in four of the seven facies associations identified: Fluvial Channel (including stromatolitic and intraclastic limestones in ephemeral stream deposits); Dolomitic Floodplain (dolomite‐cemented sand and siltstones, and microbial dolomites); Calcareous Lake Margin (intraclastic dolomite and wave‐rippled or aeolian siliciclastic facies); and Calcareous Lake (slump‐folded and locally re‐sedimented rhythmic/stromatolitic limestones and dolomites associated with ice‐rafted sediment). There is no strong cyclicity, and modern analogues suggest that sudden changes in lake level may exert a strong control on facies geometry. Both calcite and dolomite in stromatolites and rhythmites display either primary or early diagenetic replacive growth. Oxygen isotope values (−12 to +15‰VPDB) broadly covary with δ13C. High δ13C values of +3·5 to +4·5‰ correspond to equilibration with an atmosphere dominated by volcanically degassed CO2 with δ13C of −6 to −7‰. Limestones have consistently negative δ18O values, while rhythmic and playa dolomites preserve intermediate compositions, and dolocretes possess slightly negative to strongly positive δ18O signatures, reflecting significant evaporation under hyperarid conditions. Inferred meltwater compositions (−8 to −15·5‰) could reflect smaller Rayleigh fractionation related to more limited cooling than in modern polar regions. A common pseudomorph morphology is interpreted as a replacement of ikaite (CaCO3·H2O), which may also have been the precursor for widespread replacive calcite mosaics. Local dolomitization of lacustrine facies is interpreted to reflect microenvironments with fluctuating redox conditions. Although differing in (palaeo)latitude and carbonate abundance, the Wilsonbreen carbonates provide strong parallels with the McMurdo Dry Valleys of Antarctica.

Comparison of lacustrine successions and their palaeohydrological implications in two sub‐basins of the Triassic Cuyana rift, Argentina

AbstractContinental carbonates are rich in palaeoclimatic, palaeoenvironmental and palaeontological information. While carbonate accumulation mechanisms have been described for many types of continental environments, especially in extensional basins, there are still uncertainties that existing facies models fail to address. The Triassic Cerro de las Cabras and Cerro Puntudo formations are alluvial–fluvial–lacustrine sequences that represent a part of the sedimentary infill of two sub‐basins of the Cuyana Basin during the early stages of the Triassic rift in west‐central Argentina. Previous work has provided absolute dates, confirming that these deposits are coeval (Anisian) allowing a comparative study of carbonate sedimentation in an extensional tectonic context. The description and origin of freshwater carbonate deposits and their surrounding siliciclastic sediments in specific areas of the Cuyana rift, gives insight into the major factors that control carbonate precipitation in all rift basins, including the characterization of the palaeohydrology and the importance of provenance. The Cerro de las Cabras Formation represents an ephemeral, playa‐lake depositional system with subaerial exposure and pedogenesis. Its aggradational succession corresponds to the evaporative facies association lake type, diagnostic of underfilled lake basins where persistently closed surface hydrology can lead to thick evaporites. However, this formation lacks thick evaporites and has microbialitic limestones, pointing to an open groundwater supply. The Cerro Puntudo Formation represents an alkaline playa‐lake system fed by groundwater and ephemeral surface‐water input. The unit is an aggradational–minor progradational succession, pointing to a fluctuating profundal facies association, suggesting a balanced‐filled lake type. These two synchronous, lacustrine depositional systems were influenced by tectonics and climate. Provenance and hydrology are key controls in carbonate accumulation in continental rift basins that must be included in future facies models for continental carbonates. Comparison with other rift basins suggests that application of lake‐type characterizations coupled with palaeohydrology and provenance patterns will aid in developing new sedimentation models for freshwater limestones in extensional settings.

Continental carbonates as a hydrocarbon reservoir, an analog case study from the travertine of Saturnia, Italy

The Pleistocene Saturnia travertine (central Italy) represents a possible analog of the pre-salt continental carbonate reservoirs discovered in the Santos and other basins in the South Atlantic margin of Brazil. Two subhorizontal travertine tabular bodies, several tens of meters thick and extending over an area of (), have been studied in two quarries. Facies variations and associated petrophysical properties were reconstructed applying a multidisciplinary approach. The Saturnia travertine, formed from a warm water spring, is composed of various stacked carbonate banks, separated by subaerial erosive phases and paleosols. The lacustrine tabular bodies, terraces, and sills are made of crystalline crust, shrub, pisoid, paper-thin raft, coated bubble, reed, and lithoclast-breccia facies. The (from +4‰ to +8‰) supports an interpreted volcanic mantle source, whereas, the (from −9‰ to −5‰) is in agreement with warm meteoric waters. The ratio isotopic signature indicates a carbonate from dissolution of deep-seated carbonates. The facies reservoir properties were studied via porosity and permeability analysis of plugs, three-dimensional x-ray computer tomography, as well as image analysis on microscale under thin section and macroscale on large rock slabs to define various porosity indices. A strong heterogeneity of the petrophysical properties and variable connectivity were observed (porosity from 4% to 30% and permeability up to hundreds of md), but no compartmentalization of the carbonate bodies is present.

Novel applications of fluid inclusions and isotope geochemistry in unravelling the genesis of fossil travertine systems

AbstractThe Denizli Basin is a fault‐bounded Neogene–Quaternary depression located in the Western Anatolian Extensional Province, Western Turkey. The basin is a unique geological site with abundant active and fossil (Quaternary) travertine and tufa deposits. Fluid inclusion microthermometry and isotopic analysis were applied to study the genesis of the Ballık fossil travertine deposits, located in the south‐eastern part of the basin. Microthermometry on fluid inclusions indicates that the main travertine precipitating and cementing fluids are characterized by low salinity (<0·7 wt% NaCl equivalent) and variable temperatures that cluster at <50°C and ca 100°C. Fluids of meteoric origin have been heated by migration to the deeper subsurface, possibly in a local high geothermal gradient setting. A later uncommon cementation phase is related to a fluid with a significantly higher salinity (25·5 to 26·0 wt% bulk). The fluid obtained its salinity by interaction with Late Triassic evaporite layers. Strontium isotopes indicate that the parent carbonate source rock of the different travertine precipitates is very likely to be the Triassic limestone of the Lycian Nappes. Carbon isotopes suggest that the parent CO2 gas originated from thermal decarbonation of the Lycian limestones with minor contributions of magmatic degassing and organic soil CO2. Oxygen isotopes confirm the meteoric origin of the fluids and indicate disequilibrium precipitation because of evaporation and degassing. Results were integrated within the available geological data of the Denizli Basin in a generalized travertine precipitation model, which enhanced the understanding of fossil travertine systems. The study highlights the novel application of fluid inclusion research in unravelling the genesis of continental carbonates and provides several recommendations for hydrocarbon exploration in travertine‐bearing sedimentary basins. The findings suggest that travertine bodies and their parent carbonate source rocks have the potential to constitute interesting subsurface hydrocarbon reservoirs.

Acoustic properties in travertines and their relation to porosity and pore types

Sonic velocities of Pleistocene travertines were measured under variable confining pressures. Combined with petrographical characteristics and petrophysical data, i.e. porosity, permeability and density, it was determined that travertine porosity, pore types and cementation control compressional-wave (Vp) and shear-wave velocity (Vs). At 40 MPa confining pressures, Vp ranges between 3695 and 6097 m/s and Vs between 2037 and 3140 m/s. Velocity variations in travertines are, as with all carbonates, primarily linked to sample heterogeneity, i.e. differences in fabric, texture and porosity. They thus not necessarily emanate from changes in mineralogy or composition. Body wave velocities have a positive correlation with sample density and an inverse correlation with porosity. The travertines, sampled in extensional settings with normal faulting activity, define a specific compressional-wave velocity (y-axis) versus porosity (x-axis) equation, i.e. (log(y) = −0.0048x + 3.7844) that differs from the Vp-porosity paths defined by marine carbonates. Acoustic wave velocities are higher for travertines than for marine carbonates. Travertine precipitates form rigid rock frames, often called framestone, with large primary pores. Marine carbonates on the other hand often consist of (cemented) transported sediments, resulting in a rock frame that permits slower wave propagation when compared to the continental limestones.Acoustic velocity variations are linked to variations in pore types. Mouldic pores (macropores) show faster wave propagation than expected from their total porosities. Microporosity, interlaminar and interpeloidal porosity result in slower acoustic velocities. Framework pores and micro-moulds are associated with lowered acoustic velocities, while vug porosity is found above, on and below the general velocity-porosity trend. Not only the pore type, but also pore shapes exert control on body wave velocities. Cuboid-and rod-like pore shapes increase the velocity, while plate-and blade-like pore shapes have a negative effect on the velocity. The study demonstrates how seismic sections in travertine systems can contain seismic reflections that are not caused by non-carbonate intercalations, but relate to geobody boundaries, in which the seismic expression is function of porosity, pore types and shapes. This study provides and relates petrophysical data, i.e. porosity, permeability and acoustic velocities of travertines and is of importance for the interpretation of seismic reflection data in subsurface continental carbonate reservoirs.

High potential of calcareous tufas for integrative multidisciplinary studies and prospects for archaeology in Europe

Calcareous tufas are continental carbonates from open-air conditions, specific to wet and warm periods. They contain abundant remains of fauna and flora fossilised in situ and may accumulate regularly over thousands of years offering high stratigraphic resolution for palaeoenvironmental reconstructions. As they are mainly comprised of calcite, tufas allow direct and precise dating as well as geochemical reconstructions of past climates. Additionally, recent investigations have highlighted their strong potential for archaeology as several studied sequences provided high quality record of in situ prehistoric levels. Tufas are thus a unique archive in continental areas for development of synergic multidisciplinary investigations of past human societies and associated environmental and climate evolution. We emphasise that calcareous tufa are key-deposits to investigate human–environment–climate interactions during interglacial periods, from Lower Palaeolithic to Antiquity, in Europe.

Interdigitated fluvial clastic deposits and calcareous tufa testifying an uplift of the catchment area: An example from the Pianizzoli area (southern Tuscany, Italy)

Calcareous tufa, as well as other types of continental carbonates, are largely sensitive to climate variations. As a consequence, the relationship between climate variations and tufa deposition is relatively well known and calcareous tufa deposits are often used as a reliable climatic proxy in many geological settings. Conversely, the response of tufa to tectonics and uplifting processes is relatively poorly known, due to their genetic independence from geothermally heated groundwater springs and associated tectonic-controlled upwelling conduits in tectonically active settings.The aim of this paper is to describe a peculiar stratigraphic succession where calcareous tufa deposits are closely associated and interbedded with coarse-grained clastic deposits. The study area is located in southern Tuscany (Massa Marittima), where extensional tectonic processes, responsible for the emplacement of magmatic bodies at shallow crustal levels, have been active from Pliocene to Quaternary age, and caused a diffuse uplift as well as local volcanic eruptions and hydrothermal fluid circulation.The succession described in the paper recorded phases of calcareous tufa-dominated deposition (i.e. overall, medium/low hydrodynamic energetic settings) repeatedly interbedded with coarse-grained clastic fluvial sediments which testify an abrupt change in both the energy of the depositional environment and the sediment supply. Nevertheless, during the phases dominated by clastic sedimentation, tufa deposition was not inhibited as it would be expected for climate-influenced settings, suggesting that the observed stratigraphic settings were a consequence of a local uplift of the catchment area.These findings indicate that calcareous tufa deposits could be a sensitive proxy to tectonics when they are associated with coarse-grained clastic deposits, in a geological setting where the origin of the clastic sediments cannot be explained by autocyclic or climate-induced factors.

Pleistocene calcareous tufa from the Ellera basin (Umbria, central Italy) as a key for an integrated paleoenvironmental and tectonic reconstruction

Continental carbonates (calcareous tufa and associated deposits) represent a natural archive of paleoenvironmental and tectonic information that can be fully disclosed only by the application of a multidisciplinary approach. The aim of this paper is to describe the geological features of the Ellera basin (Umbria, central Italy) by integrating physical (stratigraphy, sedimentology, geochemistry, tectonics) and biological (paleontology, palynology) data. The study of the Ellera basin represents a starting point for the improvement of a methodology that could be applied to similar sedimentary systems, regardless of their actual location, extension and age.Two stratigraphic units were recognized in the Ellera basin, both characterized by multiple events of calcareous tufa deposition during the Pleistocene. In the lower San Biagio Unit, the concurrent finding of Hippopotamus (?H. antiquus) and Leptobos remains, associated with Late Villafranchian molluscs, suggests an attribution to the Early Pleistocene (Tasso or Farneta Faunal Unit, about 1.8–1.5 Ma). Both sedimentological and paleontological data indicate a periodically flooded alluvial plain depositional environment, and isotopic values of δ13C and δ18O register the establishment of a wet paleoclimate. The overlying Santa Sabina Unit lies with an angular unconformity on the San Biagio Unit, and was dated to 115,000 ± 8000 years by the application of the U–Th method. The age of the Santa Sabina Unit falls within a period of warm and humid climate (MIS 5d–e), and paleoclimatic inferences are supported by the isotopic record (δ13C and δ18O). The unit deposited in a well-vegetated fluvio-lacustrine and fluvio-palustrine environment locally dominated by herbaceous taxa, as revealed by palynological analysis.Despite the 1.5 Ma time lag between the two units, their depositional evolution shared the presence of calcareous tufa, whose deposition was controlled by similar paleoenvironmental and paleoclimatic conditions (hot and humid interglacial conditions with high rainfalls characterized both periods), as well as by the local tectonic setting and activity. In both units the maximum thickness/abundance of the calcareous tufa occurs at the intersection between two regional tectonic lineaments, controlling both the basin subsidence and the ascent of CO2-rich water.

Tectonic control on travertine and calcareous tufa deposition in a low-temperature geothermal system (Sarteano, Central Italy)

This paper presents an integrated study directed toward an understanding of the tectonic control on the hydrothermal circulation and resulting deposition of Middle–Late Pleistocene travertine in a low-temperature geothermal field of southern Tuscany, located in the Sarteano area (Central Italy). The study area is characterized by thermal springs ( c . 25 °C) and related continental carbonate deposits, structurally controlled by SW–NE-trending oblique to strike-slip faults dissecting previous normal faults. Sedimentological–stratigraphical data and 230 Th/ 234 U radiometric age determination for the travertine indicate that continental carbonates were deposited in a tectonically controlled, perched-spring system from 268 ± 22 ka (at least) to the present. The hydrothermal system is (and was) characterized by meteoric waters with a minor contribution from fluids of deeper origin. Meteoric waters infiltrated to depth through tectonically damaged zones of Mesozoic and Cenozoic carbonate rocks and overheated as a result of localized, anomalous geothermal gradient. Then, fluids enriched in bicarbonate–sulphate moved up along highly fractured rock masses at the intersection between strike-slip faults and normal faults. This paper highlights the use of continental carbonates in the reconstruction of palaeohydrothermal systems, in terms of the location of the main conduits for hydrothermal fluid flow, age of faulting and physical properties of hydrothermal fluids.

Porosity and hydric behavior of typical calcite microfabrics in stalagmites

Petrophysical techniques commonly used for material characterization are applied for the first time to speleothem samples to investigate the porosity and hydric behavior of calcite stalagmites used in paleoclimatology. These techniques allow the determination of the stalagmites' potential to undergo diagenetic transformations when substantial changes in drip waters occur in the cave environment. The petrophysical techniques include water absorption under vacuum and by capillarity, nuclear magnetic resonance, environmental scanning electron microscopy, and mercury intrusion porosimetry. The studied samples comprise five common calcite microfabrics, which have markedly different porosities and hydric behaviors and, as a consequence, different sensibilities to diagenetic processes related to the influx of water. The experiments show that stalagmites can behave as complex, small-scale hydrological systems and that the circulation of water through them by complex nets of interconnected pores might be common. As the circulation of water favors diagenetic transformations that involve geochemical and isotopic changes, the characterization of flow patterns is key for outlining areas that are susceptible to such modifications, which is critical to paleoclimatic studies that are based on speleothems because geochemical and stable isotopic data are used as paleoenvironmental proxies and absolute ages are obtained by using radioactive isotope ratios. These potential modifications also have obvious implications for studies based on fluid inclusions in speleothems.The integrated methodology, which uses primarily non-destructive techniques, shows a high potential for characterization of any type of speleothem and other continental carbonates such as tufas or sinters.

Il significato paleoclimatico dei carbonati continentali olocenici centro-sahariani: memoria delle forzanti orbitali e di eventi climatici rapidi

Palaeoclimatic significance of Holocene continental carbonates from the central Sahara: memory of orbital forcing and high frequency events.We identified in the hyperarid central Sahara several outcrops of freshwater continental carbonates, formed in correspondence of former springs and in lake sedimentary environments. They are not in equilibrium with the present-day environmental and climatic conditions and therefore they indicates a wetter period dating to the Holocene. Sampled deposits were submitted to radiometric dating (U-series and 14C) and stable isotopes were measured to get palaeoclimatic information. Calcareous tufa from fossil springs in the Tadrart Acacus massif confirm higher precipitation for the early Holocene and a their progressive decline later on; springs dried out at c. 8000 years BP. At the mean time, the outcrop of the water table in the edeyen of Murzuq allowed the formation of constant and freshwater piezometric lakes. As a consequence of decreased rainfalls, the level of the lakes fell at c. 8000 years BP; after few centuries of aridity, the level of the water raised again. Geochemical, sedimentological, and paleontological analyses demonstrate that lakes suffered seasonal oscillations of the level. Our data confirm that the African Humid Period began soon after the Pleistocene/Holocene transition and the intensity of precipitation strongly decreased during the middle Holocene, in accordance with the modifications of the orbital forcing. Moreover, the central Saharan carbonates recorded also a short arid phase that is in relation with the 8.2 BP event.