Stratigraphy and depositional setting of slurry and contained (reflected) beds in the Marnoso‐arenacea Formation (Langhian‐Serravallian) Northern Apennines,Italy

This work presents the stratigraphy and facies analysis of an interval of about 2500 m in the Langhian and Serravallian stratigraphic succession of the foredeep turbidites of the Marnoso‐arenacea Formation. A high‐resolution stratigraphic analysis was performed by measuring seven stratigraphic logs between the Sillaro and Marecchia lines (60 km apart) for a total thickness of about 6700 m. The data suggest that the stratigraphy and depositional setting of the studied interval was influenced by syndepositional structural deformations. The studied stratigraphic succession has been subdivided into five informal stratigraphic units on the basis of how structurally controlled topographic highs and depocentres, a consequence of thrust propagation, change over time. These physiographic changes of the foredeep basin have also been reconstructed through the progressive appearance and disappearance of thrust‐related mass‐transport complexes and of five bed types interpreted as being related to structurally controlled basin morphology. Apart from Bouma‐like Type‐4 beds, Type‐1 tripartite beds, characterized by an internal slurry unit, tend to increase especially in structurally controlled stratigraphic units where intrabasinal topographic highs and depocentres with slope changes favour both mud erosion and decelerations. Type‐2 beds, with an internal slump‐type chaotic unit, characterize the basal boundary of structurally controlled stratigraphic units and are interpreted as indicating tectonic uplift. Type‐3 beds are contained‐reflected beds that indicate different degrees of basin confinement, while Type‐5 are thin and fine‐grained beds deposited by dilute reflected turbulent flows able to rise up the topographic highs. The vertical and lateral distribution of these beds has been used to understand the synsedimentary structural control of the studied stratigraphic succession, represented in the Marnoso‐arenacea Formation by subtle topographic highs and depocentres created by thrust‐propagation folds and emplacements of large mass‐transport complexes.     

Recognition of ancient carbonate wind deposits: lessons from a modern analogue, Chrissi Island, Crete

Carbonate aeolian deposits are common along arid to semiarid, wind-exposed, present-day coastlines bordered by productive carbonate ramps. Lithified carbonate dunes (aeolianites) have been described around the world in marine terraces of Quaternary age, but these deposits have seldom been identified in the Pre-Quaternary record. Several authors have suggested that this scarcity reflects that these deposits form and are preserved only during icehouse periods characterized by high-amplitude sea-level changes. Others [e.g. McKee and Ward Carbonate Depositional Environments (1983) , AAPG Memoirs, Vol. 33, pp. 131–170] suggest that the scarcity of aeolianites in the Pre-Quaternary record could reflect the ‘great difficulty in recognising wind blown carbonate deposits and in differentiating between them [aeolianites] and other carbonate sands of nearshore environments’. It has been considered that carbonate shoreface/foreshore deposits are very difficult to discriminate petrographically from backshore deposits. This petrographic study of recent sediments from the shoreface to backshore along the northern coast of Chrissi Island, Crete, confirms that carbonate aeolian sands can be very easily misinterpreted as shoreface deposits. Textural examination of thin sections by image analysis techniques indicates, however, that grain orientation patterns differ between facies. Shoreface deposits exhibit a unimodal distribution of grain orientation (flat rose diagram), whereas backshore deposits show a tendency towards a bimodal distribution with a significant proportion of vertical grains. This observation has been confirmed in Pleistocene aeolianites from Tunisia and Western Australia. Grain verticality thus seems to be a reliable criterion for discriminating wind-lain carbonate bodies from shoreface deposits. Vertical grains in aeolian carbonate deposits could reflect gravity effects (e.g. reorientation of grains because of meteoric water percolation and air pull-up). Laboratory experiments conducted on carbonate sands under the action of percolating waters confirm this hypothesis. This reorganization process is preferentially developed in recently deposited and loosely packed sands resulting from grainfall and/or grainflow. In addition, this suggests that the presence of vertical grain orientation might be an indicator of the frequency and intensity of rainfalls during deposition.     

Experimental Investigation and Optimization of Thermodynamic Properties and Phase Diagrams in the Systems CaO-SiO2, MgO-SiO2, CaMgSi2O6-SiO2 and CaMgSi2O6-Mg2SiO4 to 1{middle dot}0 GPa

Using experimental results at 1·0 GPa for the systemsCaO–SiO₂, MgO–SiO₂, CaMgSi₂O₆–SiO₂ and CaMgSi₂O₆–Mg₂SiO₄,and all the currently available phase equilibria and thermodynamicdata at 1 bar, we have optimized the thermodynamic propertiesof the liquid phase at 1·0 GPa. The new optimized thermodynamicparameters indicate that pressure has little effect on the topologyof the CaO–SiO₂, CaMgSi₂O₆–SiO₂, and CaMgSi₂O₆–Mg₂SiO₄systems but a pronounced one on the MgO–SiO₂ binary. Themost striking change concerns passage of the MgSiO₃ phase fromperitectic melting at 1 bar to eutectic melting at 1·0GPa. This transition is estimated to occur at 0·41 GPa.For the CaMgSi₂O₆–SiO₂ and CaMgSi₂O₆–Mg₂SiO₄ pseudo-binaries,the size of the field clinopyroxene + liquid increases withincreasing pressure. This change is related to the shift ofthe piercing points clinopyroxene + silica + liquid (from 0·375mol fraction SiO₂ at 1 bar to 0·414 at 1·0 GPa)and clinopyroxene + olivine + liquid (from 0·191 molfraction SiO₂ at 1 bar to 0·331 at 1·0 GPa) thatbound the clinopyroxene + liquid field in the CaMgSi₂O₆·SiO₂and CaMgSi₂O₆·Mg₂SiO₄ pseudo-binaries, respectively. KEY WORDS: CaO–SiO₂; CaMgSi₂O₆–Mg₂SiO₄; CaMgSi₂O₆–SiO₂; experiments; MgO–SiO₂     

Origin of polyhalite deposits in the Zechstein (Upper Permian) Zdrada platform (northern Poland)

Polyhalite deposits in the Zechstein (Upper Permian) of northern Poland occur in the Lower Werra Anhydrite. In the Zdrada Sulphate Platform, the polyhalite appears to be a very early replacement of anhydrite. The replacement was caused by the halite-precipitating brines which contained potassium and magnesium ions. The formation of polyhalite was preceded by the syndepositional anhydritization of the original gypsum deposit which has often preserved its primary textures. This anhydritization on the platform and its slopes was a reaction of the precipitated gypsum in a hydrologically open evaporite basin, with brines of salt basins adjacent to the sulphate platform. These brines, when nearly saturated with respect to halite, and potassium and magnesium rich, reacted with anhydrite to precipitate polyhalite along the slopes of the Zdrada Platform. The oxygen and sulphur isotopic compositions of sulphate evaporites indicate that marine solutions were the only source of sulphate ions supplied to the Zechstein basin, and that anhydrite was transformed to polyhalite by reaction with marine brines more concentrated than those that precipitated precursor calcium sulphate minerals.     

LIDAR DETERMINATION OF THE ENTRAINMENT ZONE THICKNESS AT THE TOP OF THE UNSTABLE MARINE ATMOSPHERIC BOUNDARY LAYER

The thickness of the entrainment zone at the top of the marine atmosphericboundary layer (MABL) has been documented by an airborne lidar on twoconsecutive days during a cold-air outbreak episode over the Mediterranean.In addition to the lidar observations, in situ turbulent flux measurementsat three levels in the MABL were made by a second aircraft. The flights' tracksare broken down in segments 25–30 km long and the data are filtered for theparametrization of turbulent entrainment in the MABL at scales smaller thana few kilometres. The structural parameters of the entrainment zone aredetermined by lidar from the distributions of the instantaneous MABL topheight. The average values Ph0 and Ph2 of the cumulativeprobability distributions are used to define the bottom and top heights of the entrainment zone h0 and h2, respectively. The parameters h0 andh2 are calculated by reference to a linear vertical buoyancy flux profilein the framework of a first-order jump model. The model is constrained by bothlidar and in situ data to determine Ph0 and Ph2 and so h0and h2. In unstable conditions theaverage fraction Ph0 is estimated to be 6.0 ± 1%. It is shown to beslightly sensitive to the presence of cloud at small cloud fractions.The mean value of the ratio of the inversion level buoyancy flux to the surfacebuoyancy flux ARv is found to range from 0.15 to 0.30 depending on the shearin the MABL. The average value is 0.22 ± 0.05. Our resultsare in good agreement with previous analysis at comparable spatial scales.In purely convective conditions, the value of ARv given by theparametrizations fitted to our results is about 0.10–0.12, a value smallerthan the commonly accepted value of 0.2. When compared to previousparametrization results, our proportionality constant for the mechanicalproduction of turbulent kinetic energy is also found to be scaled down, ingood agreement with large-eddy simulation results. It is suggestedthat mesoscale organized motions in the MABL is the source of thisdifference.     

Authigenic natroalunite in middle Miocene evaporites from the Gulf of Suez (Gemsa, Egypt)

This paper deals with a new occurrence of sedimentary natroalunite (Na, K) Al₃, (SO₄)₂ (OH)_6, in the caprock of a diapir of middle Miocene evaporites from the Gemsa peninsula, located on the southwestern coast of the Gulf of Suez. Field observations, petrographic examinations and stable isotope (¹⁸O, ³⁴S) measurements on the associated authigenic phases of anhydrite and native sulphur argue for the genesis of natroalunite at a rather high temperature (i.e. higher than 75°C) by the reaction of clay minerals with sulphuric acid. The sulphuric acid was produced by the concatenation of the following reactions which are thought to increase the diagenetic temperature: bacterial reduction of sulphate evaporites, and oxidation of hydrogen sulphide to native sulphur and sulphate where aerated conditions prevailed in the pore fluids. These changes through time from reducing to oxidizing diagenetic conditions were controlled by the progressive uplift of the diapiric system toward the land surface so that the draining ground waters became progressively oxygenated.