Petrographic Approach Research Articles

Fine-Scale Lithogeochemical Features Influence Plant Distribution Patterns in Alpine Grasslands in the Western Alps of Italy.

Bedrock geology is crucial in structuring alpine plant communities. Old studies mainly focused on the compositional differences between alpine plant communities on carbonate rocks and crystalline rocks, i.e., calcareous vs. siliceous vegetation. Increasing attention is being paid to bedrock types other than calcareous or siliceous ones, viz. those which have intermediate geochemical characteristics between pure calcareous and pure siliceous ones. Among these types of 'intermediate' bedrocks, calc-schists and serpentines are generally characterized by vegetation comprised of a mixture of basiphilous and acidophilous species. We selected several sites in alpine grasslands in the Western Italian Alps, on calc-schist and serpentine bedrocks, located at 2500 ± 100 m above sea level. X-ray fluorescence quantification of major and trace elements, combined with stereomicroscopic examination of bedrock samples with a petrographic approach, revealed a much broader range of bedrock types than recognized by inspection of geological maps. The vegetation investigated in our study was mostly composed of a set of species found more or less frequently in alpine silicicolous or calcicolous plant communities of the Alps and other European mountains. The carbonate content in the bedrock was one of the main drivers of variation in grassland vegetation, not necessarily related to soil pH. There were no distinctive species uniquely characterizing grassland vegetation on serpentines or calc-schists.

Sherds of societies: Quantitative and petrographic analysis of Neolithic ceramics in Northeast China

In China, Neolithic sites are routinely attributed to “archaeological cultures” through typological comparisons of complete pottery vessels. However, ceramic datasets have the potential to reveal more than a chrono-cultural attribution. Taking into account not only all the complete vessels but also all the sherds excavated from a site makes it possible to use quantitative analysis as well as morpho-stylistic and petrographic approaches to measure the heterogeneity of the ensemble and ask why and how it was formed. The Wutaishan site in Nong'an, Jilin province, located at the centre of Northeast China, was chosen to implement this methodology. The ceramology research on the complete pottery dataset excavated at this site sheds light on the long- and short-distance exchanges networks in Northeast China at the turn of the 4th millennium BCE, provides a new understanding of inter-site relations in the Yitong River valley, and opens perspectives on the interpretation of the functional spatial layout within the site itself.

Creating frames of reference for chert exploitation during the Late Pleistocene in Southwesternmost Iberia.

Southwestern Iberia has played a key role in characterizing Late Pleistocene human ecodynamics. Among other aspects of human behavior, chert procurement and management studies in this region have received increasing attention in the past two decades, especially focusing on the sites showing repeated human occupation, such as the case of Vale Boi (Southern Portugal). However, these studies have been very limited in their geographical scope, and mostly focused on brief macroscopic descriptions of the raw materials. To further our knowledge of the relationship between regional availability of raw materials and its impact on human adaptations and mobility, a more detailed approach to characterizing geological sources is needed. This paper characterizes chert raw materials location, diversity, and availability in a geologically well-defined region of southern Portugal - the Algarve. Through macroscopic and petrographic approaches, we provide a detailed characterization of geological chert sources to build a frame of reference for chert exploitation in the region. Our results show that there are four main chert formations in Algarve, and that despite the within-source variability, sufficient differences at macroscopic and petrographic levels are present to allow clear source attribution. These results provide a baseline for raw material studies in archaeological assemblages across southwestern Iberia, that will be essential to further characterize the dynamics of human behavior in some of the most important eco-cultural niches.

The main stages of ore formation of the Svetloe epithermal gold deposit (Khabarovsk Region) based on the study of the mineral composition of metasomatic rocks and ores

The study of the spatiotemporal relationships of metasomatites and ores of various zones of the epithermal Au-Ag deposit Svetloe (Khabarovsk Region) using a traditional mineralogical and petrographic approach made it possible to establish several stages of ore formation confined to two paleovolcanic structures of the central type. The early stage corresponds to the moderately acidic (IS) type of mineralization, the second stage of ore formation corresponds to the acidic sulfate (HS) type.Gold mineralization associated with the early stage of ore formation is represented by two main types: Au-Ag-Te and Au-Ag. The late stage of mineralization is characterized by the predominance of redeposited high-grade gold, as well as the presence of other native elements: tellurium, selenium, sulfur, and bismuth.

I materiali di Villa Garzoni (Collodi)

Villa Garzoni and its garden are an extraordinary example of 18th-century Tuscan taste and culture. They are located on the slope of a hill at the mouth of the Pescia di Collodi stream in the Pescia plain (northern Tuscany). The garden presents scenographic and surprise effects that are part of a single decorative ensemble of great stylistic coherence. The steeply sloping terrain influenced the planimetric organisation, which develops along a central axis of symmetry, with an arrangement of terraces. In this paper the results of a diagnostic campaign performed on the materials of the villa and garden are reported. Stones, mortars and plasters were identified and analysed according to a well-established chemical and minero petrographic approach. Local and imported stones, raw materials and technologies used for the preparation of mortars and plasters, materials used for decoration were identified. To plan the best restoration approach their state of conservation was assess.

Sourcing ritual specialists in ancient Tampa Bay (AD 650–1550): A multi-method chemical and petrographic approach

Archaeologists have long relied on material proxies of labor organization to identify different social formations. Conventional wisdom holds that specialization is particularly integral in developing hierarchical states, and that hunter-gatherers are typically “generalists” provisioning their immediate household and community. However, archaeological evidence from eastern North America challenges these assumptions in showcasing evidence of specialized production among nonhierarchical societies. Because specialization is now known to exist outside the chiefdom or state, some researchers have questioned its analytical utility. Further, recent approaches to crafting discourage the use of generalizing heuristics (e.g., specialization), and instead center the historical dimensions of community and identity. In this study, we argue that archaeological research on specialization can mature by shifting focus from determinative wholes like hierarchies, to the relationships between crafters and recipients. To demonstrate this point, we present results of a multi-method chemical and petrographic study of Late Woodland (ca. AD 650–1050) and Mississippian (ca. AD 1050–1550) pottery from the Tampa Bay region of Florida. By contextualizing these data within historical relationships between communities and crafters, our study identifies two different forms of ritual specialization among nonhierarchical hunter gatherers; one predicated on religious leadership, the other on securing access to esoteric knowledge and property.

Pottery technology and provenance in southern Tawantinsuyu. A petrographic approach to Provincial Inca style

Pottery technology and provenance in southern Tawantinsuyu. A petrographic approach to Provincial Inca style

Hydrothermal Alteration Mapping Using Landsat 8 and ASTER Data and Geochemical Characteristics of Precambrian Rocks in the Egyptian Shield: A Case Study from Abu Ghalaga, Southeastern Desert, Egypt

This study evaluates the geological attributes of rocks within the Abu Ghalaga area using spatial, geochemical, and petrographic approaches. ASTER and Landsat imagery processed using band ratio and principal component analysis were used to map hydrothermal alterations, while a regional tectonic evaluation was based on automated extraction of lineaments from a digital elevation model. Geochemical and petrographic analyses were then employed for discrete scale evaluation of alteration patterns of rocks across the study location. Based on satellite image processing, alteration patterns across the study area are widespread, while evidence from lineament analysis suggests a dominant NW–SE tectonic trend accompanied by a less dominant ENE–WSW direction. The different rock units exposed in the studied district are arranged chronologically from oldest to youngest as arc metavolcanic group (basalt and rhyolite), arc metagabbro–diorite, gneissose granite (granodiorite and tonalite), and dykes (aplite and felsite). Various types of igneous and metamorphic rocks have propylitic, phyllic, and argillic zones. Geochemical data indicate that the studied rocks are classified into granite, granodiorite, gabbroic diorite, and gabbro. Geochemically, the rocks have a sub-alkaline magma type. The granodiorite–tonalite is derived from the calc–alkaline magma nature, while gabbro and diorite samples exhibit tholeiitic to calc–alkaline affinity. The tectonic setting of the studied rocks trends toward volcanic arc granite (VAG). Based on petrographic, geochemical, and remote analyses, sericitization, chloritization, epidotization, kaolinitization, carbonatization, and silicification are the main alteration types present in the study area. As a result of lineaments analysis, the existing fractures and structural planes form valid flow paths for mineral-bearing hydrothermal solutions.

Provenance, tectonic setting and palaeoclimate of Proterozoic Jiran Sandstone, Southeastern Rajasthan, India: A petrographic approach

Proterozoic Jiran Sandstone rests unconformably on Binota Shale and Khori-Malan Conglomerate. The Jiran Sandstone is comprised mainly of fine to medium-grained, varicolored, thickly bedded sandstones, showing diverse primary sedimentary structures such as ripple marks, planar, and trough cross-bedding. Petrographically, Jiran Sandstone is of mainly quartzarenite which is composed of varieties of quartz with ultra-scarcity of feldspar, lithic fragments, micas, and heavy minerals. Quartz is more abundant mineral shown by X-ray Diffraction Analysis. The provenance, tectonic setting, and paleoclimatic condition of the Jiran sandstone were evaluated using integrated petrographic studies. Analysis pursuant, monocrystalline and polycrystalline quartz grains and heavy minerals are driven primarily from metamorphic and plutonic Precambrian basement source rocks of a craton interior setting with a minor quartzose recycled sedimentary source material. Intensive chemical weathering in warm and humid paleoclimate is indicated by lack of feldspar and rock fragments.

Evaluating aromatization of solid bitumen generated in the presence and absence of water: Implications for solid bitumen reflectance as a thermal proxy

Geological models for petroleum generation suggest thermal conversion of oil-prone sedimentary organic matter in the presence of water promotes increased liquid saturate yield, whereas absence of water causes formation of an aromatic, cross-linked solid bitumen residue. To test the influence of hydrogen from water, organic-rich (22 wt% total organic carbon, TOC) mudrock samples from the Eocene lacustrine Green River Formation Mahogany zone oil shale were pyrolyzed under hydrous and anhydrous conditions in closed system batch reactors at temperatures between 300 and 370 °C for 72 h. Pre- and post-pyrolysis samples were characterized using petrographic approaches including optical microscopy, reflectance, Raman spectroscopy, and scanning electron and transmission electron microscopy to quantify differences in relative appearance, abundance, and composition of solid bitumen newly generated during the pyrolysis experiments. Petrographic analyses were supplemented by geochemical screening measurements (TOC content and programmed temperature pyrolysis). Results show post-hydrous pyrolysis residues contain lower TOC, are comprised of solid bitumen with greater aromaticity, and have textures indicative of lower viscosities, relative to anhydrous residues from the same temperature pyrolysis conditions. These observations suggest solid bitumen forming from thermal conversion of oil-prone sedimentary organic matter under anhydrous conditions may be less aromatic, although more cross-linked, than solid bitumen forming under hydrous conditions at the same time-temperature combination. To explain these results, we suggest that a radical disproportionation mechanism is favored in the presence of hydrogen donated from water, and that this disproportionation promotes aromatization in the solid residue with concomitant expulsion of saturated hydrocarbons.

Calcium Isotopic Constraints on the Transition From Aragonite Seas to Calcite Seas in the Cambrian

AbstractThe primary mineralogy of marine carbonates has varied over geological time in concert with the secular evolution of global climate and seawater chemistry. Here, we employed a multi‐proxy geochemical and petrographic approach, including measuring the Ca isotope (δ44Ca) and Sr content of Ediacaran–Cambrian carbonates, to provide new insights into the timing of the transition from a “dolomite‐aragonite sea” to a “calcite sea” across the Ediacaran‐Cambrian transition. We find robust evidence for the persistence of an aragonite sea well into Cambrian Age 2 (and potentially up through Age 4). Together with an updated petrographic compilation of abiotic carbonate precipitates (i.e., ooids and cements), these new δ44Ca and Sr data provide further evidence that there was a protracted transition from aragonite seas to calcite seas in the Cambrian. We propose that this transition was mediated, in part, by changes in seawater Mg/Ca ratios potentially regulated by the global marine redox state and extents of authigenic clay precipitation.

Geochemical and petrographic approach for the origin of the limestone blocks of thewalls of the Great Temple of Hattusha, Çorum (N-Turkey)

The socle stones of the Great Temple in the Lower City of the Hittite capital city, Hattusha (Çorum Province) have been studied for the first time by petrographic and geochemical analysis (Sr isotope, stable isotope, and trace element geochemistry). Study objectives were to determine the carbonate facies of the stone samples used in the Great Temple and investigate their possible sources. Petrographic analysis of 10 limestone samples presents five clusters of different microfacies. Stable isotope (δ13C and δ18O) analysis displayed the presence of 5 groups in parallel with petrographic analysis. Trace element analysis of the same groups displayed considerable difference among the stone samples and 87Sr/86Sr isotope ratios change between 0.70697 and 0.706867. Based on the results, it can be stated that the building using stones are from probably five different sources. This is of great importance to better understand the construction process and the acquisition of the building materials.

Geochemical and petrographic approach for the origin of the limestone blocks of thewalls of the Great Temple of Hattusha, Çorum (N-Turkey)

The socle stones of the Great Temple in the Lower City of the Hittite capital city, Hattusha (Çorum Province) have been studied for the first time by petrographic and geochemical analysis (Sr isotope, stable isotope, and trace element geochemistry). Study objectives were to determine the carbonate facies of the stone samples used in the Great Temple and investigate their possible sources. Petrographic analysis of 10 limestone samples presents five clusters of different microfacies. Stable isotope (δ13C and δ18O) analysis displayed the presence of 5 groups in parallel with petrographic analysis. Trace element analysis of the same groups displayed considerable difference among the stone samples and 87Sr/86Sr isotope ratios change between 0.70697 and 0.706867. Based on the results, it can be stated that the building using stones are from probably five different sources. This is of great importance to better understand the construction process and the acquisition of the building materials.

Looking for Ephesian workshops: an integrated petrographic, geochemical, and chrono-typological approach to Late Hellenistic Ephesos lamps

In this pilot study, Late Hellenistic mold-made lamps from Ephesos are investigated by combining chrono-morphological and stylistic features with thin-section petrography and neutron activation analysis. Coeval wares of Ephesian production and clay samples collected around the site as well as published data from the Bonn database are considered local reference material. The results point to a network of local/regional workshops sharing facilities and/or a labor force dedicated to distinct tasks. The whole evidence is consistent with the very beginning of a nucleated workshop industry able to optimize the productivity of mold-made wares and fill the demand of an increased regional and supra-regional market. The typological and compositional variability of the latest phases could suggest a gradual transfer of potting activities towards areas providing better-suited clay sources.

Understanding the textures of Apollo 11 high‐Ti mare basalts: A quantitative petrographic approach

AbstractThis paper represents a comprehensive crystal size distribution (CSD) study of ilmenite and plagioclase from 12 Apollo 11 basalts from four of the five compositional groups (Groups A, B1, B2, B3, and one unclassified basalt—Group “U” basalt 10062). Ilmenite was saturated in the magma at/before eruption, resulting in subsurface growth of phenocrysts (Group B1) and many small crystals upon eruption. Plagioclase always exhibits linear CSDs representing a single cooling regime in each sample, which is interpreted as crystallizing within isolated magma pockets late in the cooling of the erupted lava flow. Latent heat of crystallization and insulating effects of crystallized phases produced slower cooling and lower plagioclase nucleation densities. Exceptions are the Group B2 and B3 basalts, indicating relatively earlier crystallization of plagioclase on the lunar surface. Our study demonstrates that textures of the Apollo 11 basalts are a product of the interplay among cooling rate, bulk composition, and nucleation density during crystallization. Group A basalts have the highest cooling rates compared to the other Apollo 11 samples (except 10072,53), and were erupted through high effusion rates producing thick flows that underwent extended cooling that induced textural coarsening in both early crystallizing ilmenite and late‐stage plagioclase. Group B1 lavas had the lowest effusion rates producing the thinnest flows. The Groups B2, B3, and U basalts are intermediate between these end members. Our approach can be used to define eruption environment, crystallization sequence, and cooling rate of samples collected on the Moon from non‐bedrock sources.

Late Anisian microbe‐metazoan build‐ups in the Germanic Basin: aftermath of the Permian–Triassic crisis

The so-called Permian–Triassic mass extinction was followed by a prolonged period of ecological recovery that lasted until the Middle Triassic. Triassic stromatolites from the Germanic Basin seem to be an important part of the puzzle but have barely been investigated so far. Here, we analysed late Anisian (upper Middle Muschelkalk) stromatolites from across the Germanic Basin by combining petrographic approaches (optical microscopy, micro X-ray fluorescence, Raman imaging) and geochemical analyses (sedimentary hydrocarbons, stable carbon and oxygen isotopes). Palaeontological and sedimentological evidence, such as Placunopsis bivalves, intraclasts and disrupted laminated fabrics, indicate that the stromatolites formed in subtidal, shallow marine settings. This interpretation is consistent with δ13Ccarb of about −2.1‰ to −0.4‰. Occurrences of calcite pseudomorphs after gypsum possibly suggest occasionally elevated salinities, which is well in line with the relative rarity of fossils in the host strata. Remarkably, the stromatolites are composed of microbes (perhaps cyanobacteria and sulphate-reducing bacteria) and metazoans such as non-spicular demosponges, Placunopsis bivalves and/or microconchids. Therefore, these ‘stromatolites’ should more correctly be referred to as microbe-metazoan build-ups. They are characterized by diverse lamination types, including planar, wavy, domal and conical ones. Microbial mats likely played an important role in forming the planar and wavy laminations. Domal and conical laminations commonly show clotted to peloidal features and mesh-like fabrics, attributed to fossilized non-spicular demosponges. Our observations not only point up that non-spicular demosponges are easily overlooked and might be mistakenly interpreted as stromatolites, but also demonstrate that microbe-metazoan build-ups were widespread in the Germanic Basin during Early to Middle Triassic times. In the light of our findings, it appears plausible that the involved organisms benefited from elevated salinities. Another (not necessarily contradictory) possibility is that the mutualistic relationship between microbes and non-spicular demosponges enabled these organisms to fill ecological niches cleared by the Permian–Triassic crisis. If that is to be the case, it means that such microbe-metazoan associations maintained their advantage until the Middle Triassic.

An integrated geochemical, spectroscopic, and petrographic approach to examining the producibility of hydrocarbons from liquids-rich unconventional formations

The geochemical and petrophysical complexity of source-rock reservoirs in liquids-rich unconventional (LRU) plays necessitates the implementation of a more expansive analytical protocol for initial play assessment. In this study, original samples from selected source-rock reservoirs in the USA and the UK were analyzed by 22 MHz nuclear magnetic resonance (HF-NMR) T1-T2 mapping, followed by hydrous pyrolysis, and a modified Rock-Eval pyrolysis method (multi-heating step method-MHS). The above methods were complemented by organic petrography under reflected white and UV light excitation of the original and pyrolyzed samples. The analytical protocol presented attempts to better qualify and quantify different petroleum fractions (mobile, heavy hydrocarbons, viscous, solid bitumen), thus provide valuable and refined information about producibility of target intervals during appraisal. Results show how the hydrocarbon fractions interpreted from peak locations and intensities on NMR T1-T2 maps are in good agreement with those from MHS pyrolysis in terms of hydrocarbon mobility/producibility. Results from HP (Hydrous Pyrolysis) experiments show that an exception to this general agreement between NMR and MHS estimates occurs for the Kimmeridge Blackstone Clay samples, where MHS shows an increase of >90% in producible hydrocarbon yields vs. minimal to no presence of mobile hydrocarbons in NMR T1-T2 maps. This study clarifies the role of pore structure and networks in these discrepancies of producible oil estimates when comparing programmed pyrolysis to NMR-based techniques. This novel, multi-step and multidisciplinary approach provides a more advanced screening protocol for identifying zones of higher oil-in-place (OIP) and predicting fluid mobility prior to drilling or completions.

Depositional environment and source rock quality of the Woodbine and Eagle Ford Groups, southern East Texas (Brazos) Basin: An integrated geochemical, sequence stratigraphic, and petrographic approach

X-ray fluorescence chemostratigraphy of the Cenomanian–Turonian Woodbine and Eagle Ford Groups in the southern parts of the East Texas Basin highlights significant mudstone chemical heterogeneities that commonly are difficult to observe or quantify at the macroscale. Several key elements, namely, Ca, Si, Mo, Mn, and Ni, were correlated to depositional conditions and used in a hierarchical cluster analysis to characterize five chemical facies (i.e., “chemofacies”) across 10 cored intervals of the Woodbine and Eagle Ford Groups: (1) argillaceous, organic-matter poor; (2) transitional, organic-matter poor; (3) transitional, organic-matter moderate; (4) calcareous, organic-matter rich; and (5) calcareous, organic-matter moderate. Characterizations of organic matter richness, mineralogy, and environmental conditions of deposition were established by correlating between key element abundances, total organic carbon measurements, x-ray diffraction measurements, and petrographic observations of lithologic composition, bioturbation, and sedimentary textures. Combined analysis of elemental geochemistry, stratigraphy, and petrographically observed sedimentary textures indicates that all chemofacies were deposited in an intrashelf basin above storm wave base. The most organic-rich chemofacies was deposited on a dominantly dysoxic distal shelf. Mudstone organic matter enrichment is driven dominantly by the minimization of siliciclastic dilution and secondarily enhanced by oxygen restriction. Regional correlations of chemofacies within a sequence stratigraphic framework developed from previous outcrop and subsurface work indicate a clear relationship between interpreted stratigraphy and chemofacies deposition. Generally, the highstand sequence sets of the Woodbine Group and upper Eagle Ford formation are dominated by mineralogically clay-rich, organic matter–lean, siliciclastic sedimentation and contain poor-quality source rock. Conversely, the transgressive sequence set of the lower Eagle Ford formation is dominated by organic matter–rich pelagic carbonate accumulation and contains excellent-quality source rock.

Mineralogical characterization of low-grade nickel laterites from the North Oman Mountains: Using mineral liberation analyses – scanning electron microscopy-based automated quantitative mineralogy

Afterwards to the Late Cretaceous obduction of the Tethyan Semail ophiolite suite intense chemical weathering involved the Oman Mountains region. This weathering produced complex mineral and chemical assemblages that led to the formation of a Ni lateritic paleosol. In the present study, five laterite sections were sampled and studied by using a quantitative mineralogical and petrographic approach. Mineral liberation analyses (MLA) were carried out for the first time for several selected samples.The MLA results, supported by the Electron probe micro-analyzer (EPMA) and Energy-dispersive X-ray spectroscopy (EDX) analyses, revealed that the main ore minerals of the analyzed laterite are chlorite and goethite as the main Ni-bearing hosts. Magnetite and chromite are also present as by-product minerals.The Ni-bearing chlorite in the Saqah-3 and Al Russayl profiles have coarser grains compared to those in the other Saqah localities, while the goethite grains are coarse in most of the analyzed samples. The presence of approximately 60 to 80% of both the Ni-bearing minerals (chlorite and goethite) as liberated phases (>95% free) is a potential indicator for their future exploration.Chromite and magnetite grains are present in a wide range of grain sizes. Seventy percent of the chromite grains in Saqah-4 and Al Russayl profiles and 70% of the magnetite in Saqah-2 occur as liberated phases (>95%) compared to their presence in other profiles, where only half of them occur as liberated phases.The grain-size-liberation class relationships showed that Ni-bearing mineral phases (chlorite and goethite) are present as liberated phases (high-grade materials) followed by chromite and magnetite, which are interlocked with other phases to lower grade materials.A better understanding of the mineralogical and geochemical quantitative characterization, such as grain size, modal analyses, mineral association, and percentage of interlock between minerals in the deposit, can avoid the metallurgical problems, thus possibly reducing the operational costs and improving the recovery of metals.The above-obtained MLA parameters can build a good feasibility study during the exploration stage of these low-grade Ni deposits in the Northern Oman Mountains, as well as later during their beneficiation and processing methods, if other economic and mining factors are deemed favorable.

Massive dolomites in the Messinian evaporitic sequence (Sicily, Italy): multi-analytical characterization and implications for the dolomitization processes

A combined geochemical, mineralogical, petrographic and geological approach has allowed to constrain the dolomitization process responsible for the formation of massive dolomitic body within the Messinian evaporitic series. We focused our investigations on an example of massive dolomite body, outcropping over the marginal area of the evaporitic basin in the Peloritani Mountains (NE area of Sicily region, Italy). Field observations allowed recognizing several sedimentary structures, such as erosion surfaces, lenses of conglomerates and ripples highlighting a clastic origin. The petrographic study revealed fine-grained and massive structure, scarce porosity, light to pink color and absence of micro-fossils. The chondrite-normalized trace element pattern shows a general enrichment in LREE and depletion in HREE. The isotopic signatures of the studied dolomites (δ18O = – 4.38–1.24‰, V-PDB; δ13C = – 1.48–1.94‰, V-PDB) are comparable with those of type-3 “Calcare di Base” which is the common reference to record the Messinian salinity crisis. The positive co-variant relationship between the δ18O and δ13C values suggests a dolomitization process in the seawater/freshwater mixing zone. The occurrence of the studied dolomites in the evaporitic sequence coupled with the results we gained point towards an origin from “early diagenetic” processes. As a consequence, a model explaining the genetic mechanism for the studied dolomitic rocks can be essentially summarized as: (1) the massive dolostones were originally deposited as evaporitic limestones under rather high-salinity conditions; (2) the dolomitization process occurred in the seawater/freshwater mixing zone that provided Mg-rich fluids over uplifted submarine masses.