Vena Del Gesso Basin Research Articles

Orbitally Forced Hydrological Balance During the Messinian Salinity Crisis: Insights From Strontium Isotopes (87Sr/86Sr) in the Vena del Gesso Basin (Northern Apennines, Italy)

AbstractThe 87Sr/86Sr ratio is a proxy of the hydrologic structure of marginal basins characterized by the mixing between marine and continental waters. This reliable paleoceanographic and paleoclimatic tool is considered a key to the reconstruction of high‐amplitude hydrologic changes that governed the evolution of the Mediterranean region during the Messinian salinity crisis (MSC), when reduced connections with the global ocean resulted in the deposition of the most studied Earth's salt giant. Paleohydrological dynamics leading to the deposition of the Primary Lower Gypsum during the first stage of the MSC were revealed by the study of the 87Sr/86Sr composition of 114 samples from the Vena del Gesso basin (Northern Apennines, Italy). The analysis of seven high‐resolution 87Sr/86Sr profiles from individual gypsum beds suggests a climatic influence of precessional‐scale orbital parameters during evaporite deposition. The variations of Sr isotope ratios over time indicate that gypsum precipitation started from a seawater body dominated by continental contributions, followed by a relative increase of marine input. The overall results show a gradual upward detachment from the global ocean 87Sr/86Sr curve, with crucial steps corresponding to the most extreme eccentricity minima, suggesting an orbital influence on the hydrological balance of the basin. Given the sedimentological and geochemical analogies shared by the Mediterranean marginal basins that hosted the Primary Lower Gypsum deposition, the 87Sr/86Sr evolution of the Vena del Gesso basin may be considered as indicative of Mediterranean‐scale hydrologic dynamics, driven by gradual reduction of exchanges with the Atlantic and variable freshwater input.

Precessional control of Sr ratios in marginal basins during the Messinian Salinity Crisis?

AbstractBased on 87Sr/86Sr data of the Primary Lower Gypsum (PLG) deposits in the Vena del Gesso basin—a marginal basin of the Mediterranean during the Messinian Salinity Crisis—a correlation between 87Sr/86Sr values and precessional forcing has recently been proposed but not yet confirmed. In this study, a box model is set up to represent the Miocene Mediterranean deep basin and a connected marginal basin. Measurements of 87Sr/86Sr in the Vena del Gesso and estimated salinity extrema are used to constrain model results. In an extensive analysis with this model, we assess whether coeval 87Sr/86Sr and salinity fluctuations could have been forced by precession‐driven changes in the fresh water budget. A comprehensive set of the controlling parameters is examined to assess the conditions under which precession‐driven 87Sr/86Sr variations occur and to determine the most likely setting for PLG formation. Model results show that precession‐driven 87Sr/86Sr and salinity fluctuations in marginal basins are produced in settings within a large range of marginal basin sizes, riverine strontium characteristics, amplitudes of precessional fresh water budget variation, and average fresh water budgets of both the marginal and deep basin. PLG deposition most likely occurred when the Atlantic‐Mediterranean connection was restricted, and the average fresh water budget in the Mediterranean was significantly less negative than at present day. Considering the large range of settings in which salinities and 87Sr/86Sr fluctuate on a precessional timescale, 87Sr/86Sr variations are expected to be a common feature in PLG deposits in marginal basins of the Mediterranean.

Age refinement of the Messinian salinity crisis onset in the Mediterranean

AbstractWe propose a revised age calibration of the Messinian salinity crisis onset in the Mediterranean at 5.971 Ma based on the recognition of an extra gypsum cycle in the transitional interval of the Perales section (Sorbas basin, Spain) and the revision of the magnetostratigraphy of the Monticino section (Vena del Gesso basin, Italy). This age re‐calibration allows to state more accurately that: (i) the interval encompassing theMSC‐onset is continuous, thus ruling out any erosional feature or stratigraphic hiatus related to a major sea‐level fall affecting the Mediterranean; (ii) the first gypsum was deposited during the summer insolation peak at 5.969 Ma associated with an eccentricity minimum and roughly coincident with glacial stageTG32; (iii) theMSC‐onset was preconditioned by the tectonically‐driven reduction of the hydrological exchanges with the Atlantic Ocean and finally triggered by glacial conditions in the northern hemisphere and by arid conditions in northern Africa.

Passive‐roof thrusting in the Messinian Vena del Gesso Basin (Northern Apennines, Italy): constraints from field data and analogue models

AbstractWe present the results of a study of the Vena del Gesso Basin (Romagna Apennines, Italy) integrating field analyses and analogue modelling. This basin represents one of the best‐preserved top‐thrust basins in the Northern Apennines foreland and is one of the few examples where primary evaporites, related to the Messinian salinity crisis of the Mediterranean, widely crop out. The structural style affecting the Messinian gypsum is examined to get insights into the mechanism responsible for the overall deformation features recognizable in the area. The evaporites are completely detached at the base and widespread back‐thrusts, repeatedly doubling these deposits, strongly contrast with the regional forelandward vergence of structures in the Apennines. On the basis of the comparison between field data and experimental results, the features characterising this area can be described as the result of the deformation linked to the sequential activation of an obliquely propagating passive‐roof duplex. Analogue models evidenced the major role played (1) by syntectonic erosion that promoted the development of passive‐roof duplex style, as well as (2) the role of décollement level pinch‐out that determined an oblique progression of deformation. Finally our data lead to reconsider the palaeoenvironmental reconstruction concerning the onset of the Messinian salinity crisis in the Mediterranean. Copyright © 2007 John Wiley & Sons, Ltd.

Deep‐water clastic evaporites deposition in the Messinian Adriatic foredeep (northern Apennines, Italy): did the Mediterranean ever dry out?

AbstractA new genetic facies model for deep‐water clastic evaporites is presented, based on work carried out on the Messinian Gessoso‐solfifera Formation of the northern Apennines during the last 15 years. This model is derived from the most recent siliciclastic turbidite models and describes the downcurrent transformations of a parent flow mainly composed of gypsum clasts. The model allows clearer comprehension of processes controlling the production and deposition of clastic evaporites, representing the most common evaporite facies of the northern Apennines, and the definition of the genetic and stratigraphic relationship with primary shallow‐water evaporites formed and preserved in marginal settings. Due to the severe recrystallization processes usually affecting these deposits, petrographic and geochemical analyses are needed for a more accurate interpretation of the large spectrum of recognized gravity‐driven deposits ranging from debrisflow to low‐density turbidites. Almost all the laminar ‘balatino’ gypsum, previously considered a deep‐water primary deposit, is here reinterpreted as the fine‐grained product of high to low‐density gravity flows. Facies associations permit the framing of the distribution of clastic evaporites into the complex tectonically controlled depositional settings of the Apennine foredeep basin. The Messinian Salinity Crisis occurred during an intense phase of geodynamic reorganization of the Mediterranean area that also produced the fragmentation of the former Miocene Apennine foredeep basin. In this area, primary shallow‐water evaporites equivalent to the Mediterranean Lower Evaporites, apparently only formed in semi‐closed thrust‐top basins like the Vena del Gesso Basin. The subsequent uplift and subaerial exposure of such basins ended the evaporite precipitation and promoted a widespread phase of collapse leading to the resedimentation of the evaporites into deeper basins. Vertical facies sequences of clastic evaporites can be interpreted in terms of the complex interplay between the Messinian tectonic evolution of the Apennine thrust belt and related exhumation–erosional processes. The facies model here proposed could be helpful also for better comprehension of other different depositional and geodynamic contexts; the importance of clastic evaporites deposits has been overlooked in the study of other Mediterranean areas. Based on the Apennine basins experience, it is suggested here that evaporites diffused into the deeper portions of the Mediterranean basin may consist mainly of deep‐water resedimented deposits rather than shallow‐water to supratidal primary evaporites indicative of a complete basin desiccation.

Sedimentary and tectonic evolution of the Vena del Gesso basin (Northern Apennines, Italy): Implications for the onset of the Messinian salinity crisis

The integration of field and subsurface data permits a substantial revision of the sedimentary evolution of the Vena del Gesso basin, a thrust-top basin in the Northern Apennines where shallow-water primary gypsum deposits related to the Messinian salinity crisis were well developed and preserved. As inferred from lateral and vertical facies changes within the underlying deep-marine turbidites of the Marnoso-arenacea Formation, evaporite precipitation occurred in a basin bounded to the north and to the east by a thrust-related anticline actively growing since the late Tortonian. Both gypsum deposition and subsequent deformation were strongly controlled by evolving paleobathymetry driven by tectonics. Primary gypsum precipitated in a shallow, silled basin, while in the adjacent deeper and larger foredeep basin, organic-rich shales were deposited. Gypsum deposits underwent severe postdepositional deformation related to large-scale gravitational collapse, as a result of a regional uplift event, also coincident with the end of the evaporitic phase. Along the inner, shallower-dipping limb of the anticline bounding the basin, large-scale, poorly deformed gypsum slabs moved downslope along a detachment surface developed at the contact with the underlying euxinic shales, forming both extensional and compressional features and showing an overall southwestern vergence. The identification of a south-southwest–dipping paleoslope, here pointed out for the first time, suggests that the deformational features affecting the gypsum unit were probably driven by gravity and not by active thrusting, as thought up to now. The steeper frontal limb of the anticline promoted the transformation of gypsum slides into debris flows and turbidite currents that deposited their load in the adjacent deeper basin. This gravitational deformation was sealed by postevaporitic upper Messinian Lagomare deposits. The sedimentary history of the Vena del Gesso basin suggests that the Messinian salinity crisis in the Apennine foredeep, as well as in the Balearic, Tyrrhenian, Sicily, and Eastern Mediterranean Basins, was tightly linked to tectonic processes. The large-scale, postdepositional collapse of primary evaporitic deposits is a widespread feature in the Mediterranean basins, and it may have altered the original stratigraphic relationships in some places. This finding has potentially important implications for a correct paleoenvironmental reconstruction of the onset of the Messinian salinity crisis.

Release of sulfur- and oxygen-bound components from a sulfur-rich kerogen during simulated maturation by hydrous pyrolysis

An immature sulfur-rich marl from the Gessosso-solfifera Formation of the Vena del Gesso Basin (Messinian, Italy) has been subjected to hydrous pyrolysis (160 to 330°C) to simulate maturation under natural conditions. The kerogen of the unheated and heated samples was isolated and the hydrocarbons released by selective chemical degradation (Li/EtNH 2 and HI/LiAlH 4) were analysed to allow a study of the fate of sulfur- and oxygen-bound species with increasing temperature. The residues from the chemical treatments were also subjected to pyrolysis–GC to follow structural changes in the kerogens. In general, with increasing hydrous pyrolysis temperature, the amounts of sulfide- and ether-bound components in the kerogen decreased significantly. At the temperature at which the generation of expelled oil began (260°C), almost all of the bound components initially present in the unheated sample were released from the kerogen. Comparison with an earlier study of the extractable organic matter using a similar approach and the same samples provides molecular evidence that, with increasing maturation, solvent-soluble macromolecular material was initially released from the kerogen, notably as a result of thermal cleavage of weak carbon–heteroatom bonds (sulfide, ester, ether) even at temperatures as low as 220°C. This solvent-soluble macromolecular material then underwent thermal cleavage to generate hydrocarbons at higher temperatures. This early generation of bitumen may explain the presence of unusually high amounts of extractable organic matter of macromolecular nature in very immature S-rich sediments.

Sulphur and oxygen sequestration of n-C 37 and n-C 38 unsaturated ketones in an immature kerogen and the release of their carbon skeletons during early stages of thermal maturation

Sedimentary rock from the Gessoso-solfifera Formation (Messinian) in the Vena del Gesso Basin (northern Italy) containing immature ( R o = 0.25%) S-rich organic matter was artificially matured by hydrous pyrolysis at temperatures from 160 to 330°C for 72 h to study the diagenetic fate of n-C 37 and n-C 38 di- and tri-unsaturated methyl and ethyl ketones (alkenones) biosynthesised by several prymnesiophyte algae. During early diagenesis, the alkenones are incorporated into the kerogen by both sulphur and oxygen cross-linking as indicated by chemical degradation experiments with the kerogen of the unheated sample. Heating at temperatures between 160 and 260°C, which still represents early stages of thermal maturation, produces large amounts (up to 1 mg/g TOC) of S-bound, O-bound, and both S- and O-bound n-C 37 and n-C 38 skeletons, saturated n-C 37 and n-C38 methyl, ethyl, and mid-chain ketones, C 37 and C 38 mid-chain 2,5-di- n-alkylthiophenes, C 37 and C 38 1,2-di- n-alkylbenzenes, and C 37 and C 38 n-alkanes. With increasing thermal maturation, three forms of the n-C 37 and n-C 38 skeletons are relatively stable (saturated hydrocarbons, 1,2-di- n-alkylbenzenes and saturated ketones), whereas the S- and O-bound skeletons are relatively labile. These results suggest that in natural situations saturated ketones with an n-C 37 and n-C 38 skeleton can be expected as well as the corresponding hydrocarbons.

Impact of dia- and catagenesis on sulphur and oxygen sequestration of biomarkers as revealed by artificial maturation of an immature sedimentary rock

Hydrous pyrolysis of an immature ( R o ≈ 0.25%) sulphur-rich marl from the Gessoso-solfifera Formation (Messinian) in the Vena del Gesso Basin was carried out at 160°C ≤ T ≤330°C for 72 h, to study the effect of progressive diagenesis and early catagenesis on the abundance and distribution of sulphur-containing and sulphur- and oxygen-linked carbon skeletons in low-molecular-weight and high-molecular-weight fractions (e.g. kerogen). To this end, compounds in the saturated hydrocarbon fraction, monoaromatic hydrocarbon fraction, polyaromatic hydrocarbon fraction, alkylsulphide fraction and ketone fraction were quantified, as well as compounds released after desulphurisation of the polar fraction and HI LiAlH 4 treatment of the desulphurised polar fraction. Sulphur-bound phytane and (20 R)-5α,14α,17α(H) and (20 R)-5β,14α,17α(H) C 27–C 29 steranes in the polar fraction become less abundant with increasing maturation temperature, whereas the amount of their corresponding hydrocarbons increases in the saturated hydrocarbon fraction. Carbon skeletons that are bound in the kerogen by multiple bonds (e.g. C 38 n-alkane and isorenieratane) are first released into the polar fraction, and then as free hydrocarbons. These changes occur at relatively low levels of thermal maturity ( R o <0.6%), as evidenced by the “immature” values of biomarker maturity parameters such as the ββ/(ββ + αβ + βα) C 35 hopane ratio and the 22 S/(22 S + 22 R)−17α,21β(H) C 35 hopane ratio. Sulphur- and oxygen-bound moieties, present in the polar fraction, are not stable with increasing thermal maturation. However, alkylthiophenes, ketones, 1,2-di- n-alkylbenzenes and free n-alkanes seem to be stable thermal degradation products of these sulphur- and oxygen-bound moieties. Thus, apart from free n-alkanes, which are abundantly present in more mature sedimentary rocks and crude oils, alkylthiophenes, 1,2-di- n-alkylbenzenes and ketones can also be expected to occur. The positions of the thiophene moiety and the carbonyl group coincide with the original positions of the functional groups of their precursors. Thus, important information about palaeobiochemicals is retained throughout the sequestration/degradation process.

Variations in origin and composition of kerogen constituents as revealed by analytical pyrolysis of immature kerogens before and after desulphurization

Kerogens isolated from nine samples from a single marl bed of the Gessoso-solfifera formation in the Vena del Gesso basin (Upper Miocene, Italy) were treated with CrCl 2 and Li/EtNH 2 to remove inorganic and organic sulphur, respectively. The untreated and “desulphurized” (CrCl 2 and Li/EtNH 2) kerogens were qualitatively and quantitatively analyzed by flash pyrolysis-gas chromatography-mass spectrometry. Typically, significant variations in the relative contributions and abundances of n-alk-1-enes/ n-alkanes, alkylated pyrroles, thiophenes and supposedly S-bound moieties released as phytenes and sterenes were observed. The first series of products ( n-hydrocarbons) contributed relatively more to the pyrolyzates of the desulphurized kerogens, suggesting an enrichment of microalgal-derived aliphatic macromolecules. These marine micro-organisms are thought to belong in part to the class of Eustigmatophyceae. Alkylated pyrroles were barely detectable in the untreated kerogen pyrolyzates, whereas they contributed significantly to those from the treated kerogens. Similar treatment of a porphyrin, octaethylporphyrin, revealed that alkyl porphyrins are thermally too stable to be cleaved upon pyrolysis. However, Li/EtNH 2 treatment reduced the porphyrin standard, such that it could generate mainly monopyrroles upon pyrolysis. It is concluded that Li/EtNH 2 treatment reduced tetrapyrrole moieties bound to the desulphurized kerogen network.

Molecular indicators for palaeoenvironmental change in a Messinian evaporitic sequence (Vena del Gesso, Italy): III. Stratigraphic changes in the molecular structure of kerogen in a single marl bed as revealed by flash pyrolysis

Kerogens of nine samples from a single marl bed of the Gessoso-solfifera Formation in the Vena del Gesso basin (Messinian, Italy) were qualitatively and quantitatively studied by analytical pyrolysis. Relationships between the nature of the pyrolysis products and the source organisms were determined. The high abundance of (i) algal-derived components ( n-alkanes and n-alk-1-enes) and (ii) sulphur-containing and sulphur-bound products signifying a high degree of early diagenetic sulphurization were observed. Presence of photic zone anoxia during the deposition of these sediments was discussed via the presence of 1,2,3,4-tetramethylbenzene in the pyrolysates. Unusual distributions were found for three series of pyrolysis products (i.e. C 13 and C 14 n-alkylated thiophenes and thiolanes and C 12 and C 13 n-1,3-alkadienes were by far the major products of these series) in the pyrolysates of two samples corresponding to the middle of the cycle (samples 3 and 5) which also have the highest δ 13C TOC values and the highest pyrolysis yields. Therefore, the contribution of organisms biosynthesizing resistant algal biopolymer was considered as much more important during the deposition of the sediments associated with samples 3 and 5. We finally proposed partial structural elements from which the specific low molecular weight compounds of samples 3 and 5 can be derived upon pyrolysis.

Thermal stability of thiophene biomarkers as studied by hydrous pyrolysis

An immature ( R o = 0.25%) sulphur-rich calcareous shale from the Gessoso-solfifera Formation (Messinian) in the Vena del Gesso Basin (northern Italy) was artificially matured by hydrous pyrolysis at constant temperatures ranging from 160 to 330°C for 72 h to study the applicability of alkylthiophenes as molecular indicators during progressive diagenesis and early catagenesis. Artificial maturation at temperatures up to 330°C generated large amounts of alkylthiophenes from the thermal degradation of kerogen. The importance of the kerogen in the production of alkylthiophenes is supported by the observation that the isomer distributions of alkylthiophenes at high temperatures coincide with the isomer distributions of alkylthiophenes in the Curie-point pyrolysate of the isolated kerogen of the original sample. The generation profiles of the thiophenes with a phytane carbon skeleton and the 3,4-di- n-alkylthiophenes suggest that continuous degradation of alkylthiophenes is also taking place, so that the overall effect is alkylthiophene concentrations with intermediate values. The structural isomer distribution of linear alkylthiophenes does not change significantly when these compounds are generated at different temperatures. This implies that important information regarding the original positions of functionalities in the precursor molecules is retained and, thus, that alkylthiophenes are useful biomarkers at relatively high levels of thermal maturity (up to R o ≈ 0.95–1.1%).

Distributions of pigments in the organic matter of marls from the Vena del Gesso evaporitic sequence

The porphyrins and carotenoids of marl horizons within the Messinian evaporitic cycles of the Vena del Gesso basin show variations in abundance and composition throughout a sequence of ten cycles. Significant variations are also apparent within individual marl horizons. The differences in pigment distribution reflect significant changes in the environmental conditions during sediment deposition. The presence of significant amounts of isorenieratene and co-occurrence of an alkyl porphyrin with a C 34 skeleton provide evidence for anoxic conditions extending into the photic zone of the palaeo-water column. Variations in the free pigment contents of the marls reflect a regressive trend both during deposition of an individual marl, and over the entire sequence.

A novel sterane, 27-nor-24-methyl-5α-cholestane, in sediments

A novel sterane, 27-nor-24-methyl-5α-cholestane, has been identified in sedimentary hydrocarbon mixtures by coinjection with an authentic standard. The C27 sterane occurs in silica-rich sediments from the Monterey Formation, USA, the Onnagawa Formation, Japan, and the Menilite Shale, Poland, and in bituminous marls from the Vena del Gesso basin, Italy. The compound seems to be biosynthetically related to 24-norcholestanes which co-occurs in sometimes relatively high amounts. The 13C-content of the sterane in the Monterey Formation indicates an algal source similar to other steranes present in this sediment. A diatom or a dinoflagellate source is suggested for this compound.

Di- or polysulphide-bound biomarkers in sulphur-rich geomacromolecules as revealed by selective chemolysis

Three types of sulphur-rich high-molecular-weight material in the alkylsulphide, the polar, and the asphaltene fractions isolated from the bitumen of an immature bituminous shale from the Vena del Gesso basin (Italy) were desulphurised using Raney Ni and were treated with MeLi/MeI, a chemical degradation method which cleaves selectively and quantitatively di- or polysulphide linkages. The products formed were characterised by gas chromatography-mass spectrometry. Raney Ni desulphurisation revealed that these S-rich macromolecules are in substantial part composed of sulphur-linked biomarkers with linear, branched, isoprenoid, steroid, hopanoid, and carotenoid carbon skeletons. MeLi/Mel treatment provided evidence that a major part of the total amount of macromolecularly bound biomarkers are linked via di- or polysulphide moieties to the macromolecular network. Since the di- or polysulphide linkages are attached at specific positions of the bound biomarkers it is proposed that they are formed by intermolecular incorporation reactions of HS x − into low-molecular-weight functionalised biological lipids during early diagenesis. The different properties (solubility and molecular weight) of the sulphur-rich macromolecules in the alkylsulphide, the resin, and the asphaltene fractions can be explained simply by differences in degree of sulphur cross-linking.

Algal crusts, autochthonous and clastic gypsum in a cannibalistic evaporite basin: a case history from the Messinian of Northern Apennines

ABSTRACTThe Messinian Vena del Gesso Basin in the Northern Apennines is filled by very thick (up to 35 m) beds of coarse crystalline gypsum (selenite) associated with thinner carbonate and shaly (euxinic) intercalations. The conventional Usiglio model of salt fractionation does not apply to this evaporitic sequence for the following reasons: carbonate which underlies gypsum is not evaporitic but algal in origin; most gypsum did not precipitate from surface brines but at and below a sediment‐water interface occupied by algal mats; a significant portion (10–80%) of gypsum beds is composed of redeposited selenite which was removed from the margins and transported toward the centre of the basin by slope‐controlled currents and gravity flows (debris flows).We call this process cannibalistic because of its intraformational character (connected with evaporative fall of water level) and volumetric importance.A recurrent vertical pattern of six main facies (euxinic to gypsum fanglo‐merates) is interpreted as a bathymetric, regressive cycle controlled by both sedi‐mentological and tectonic‐eustatic factors. The inferred environmental setting is a residual turbidite trough (Marnoso‐arenacea) evolving abruptly toward lagoonal conditions and filled up to sea level by evaporitic and mechanical (mostly fluvial) processes. Repeated inundations of restricted‐marine water started the depositional cycle thirteen or fourteen times.