Francesco Dela Pierre

Interaction of tectonic, sedimentary, and diapiric processes in the origin of chaotic sediments: An example from the Messinian of Torino Hill (Tertiary Piedmont Basin, northwestern Italy)

Geologic mapping and integrated stratigraphic and structural observations of a gypsum quarry from northwestern Italy allow evaluation of the relative contributions, the time relationships, and the causative links between tectonic, sedimentary, and diapiric processes in the genesis of chaotic sediments of Messinian age. Three chaotic units are exposed in the quarry: together, they make up a composite chaotic unit that is unconformably overlain by post-chaotic sediments. Unit 1 is composed of blocks of primary evaporites that are juxtaposed to marine marls by subvertical transpressive faults and are parallel to the fault surfaces. Unit 2 unconformably overlies Unit 1, and consists of a lenticular sedimentary body containing both angular and rounded blocks, randomly distributed in a fine-grained matrix. Unit 3 consists of a 10-m-wide body bounded by transpressive faults, and pierces both Units 1 and 2. It is composed of strongly deformed muddy deposits that envelop blocks of gypsum and carbonate rocks. Between the core and the margins, various zones have been defined based on the increasing amount of deformation toward the margins. The post-chaotic sediments unconformably overlie both Units 1 and 2, sealing the main fault systems. The composite chaotic unit is related to thrust propagation during a regional phase of deformation, and is the result of different evolutionary stages, in each of which a single genetic mechanism prevailed. Tectonic faulting prevailed during stage 1 and was responsible for the formation of a tectonically disrupted assemblage (Unit 1). During stage 2, gravity-driven sedimentary phenomena, related to slope oversteepening triggered by ongoing thrust propagation, resulted in the deposition of Unit 2. Gravity sliding was favored by the mechanical weakening of sediments caused by tectonic faulting. Over-pressure conditions resulting from the rapid deposition of Unit 2 triggered the rise of a diapir (Unit 3) that pierced Units 1 and 2. The involvement of methane-rich fluids in the formation of the diapir is suggested by the occurrence of blocks of methane-derived carbonates, found not in the quarry, but just outside it.

Palaeomagnetic and structural evidence of Neogene tectonic rotation of the Gran Sasso range (central Apennines, Italy)

Abstract Interference between discordant tectonic trends in the central Apennines (Italy) was investigated in a palaeomagnetic study of the arc-shaped E-W- to N-S-oriented overthrust units of the Lazio-Abruzzi carbonate platform (Gran Sasso), and the underlying N-S-oriented imbricates of the Marche pelagic domain (Mt. dei Fiori). Samples were taken from 26 sites in the Cretaceous-Palaeogene Scaglia formation and in the Messinian-lower Pliocene Laga flysch. Stepwise thermal demagnetization was used to remove secondary components, and a stable direction of the characteristic remanent magnetization (ChRM) was determined for 24 sites. The palaeomagnetic results point to counter-clockwise rotation of the Marche domain similar to that of the southern Umbria. Directions in both the Scaglia and Laga formations of the Lazio-Abruzzi units show counter-clockwise rotations up to 90° in the easternmost segment of the Gran Sasso belt. This setting may be framed in a structural context wherein superposition of the E-W-oriented Gran Sasso belt onto the N-S-oriented Marche imbricates derives from progressive rotation of the Lazio-Abruzzi platform, decoupled by right-lateral shearing along the N-S-oriented Rigopiano-Bussi-Rivisondoli shear zone.

Authigenic carbonates in Upper Miocene sediments of the Tertiary Piedmont Basin (NW Italy): Vestiges of an ancient gas hydrate stability zone?

A wide array of carbonate-rich rocks has been recognized in the Tertiary Piedmont Basin (NW Italy), hosted in lower Messinian slope deposits. Carbonate cements show negative δ 13 C values and positive δ 18 O values, suggesting that carbonate precipitation was induced by microbial degradation of methane produced from gas hydrate destabilization. Two groups of rocks have been distinguished: (1) Lucina -bearing mud breccias, representing the seafloor product of an ancient seepage site; and (2) Lucina -free concretions, originating below the sediment-water interface. Within this group, two subtypes have been further distinguished: stratiform concretions and cylindrical concretions. Stratiform concretions result from precipitation of dolomite in the pores of muddy sediments. Some of them display a brecciated structure; others show a network of septarian-like cracks that are empty, filled with sediments, or zoned carbonate cements. Their internal features are related to the formation of gas hydrates within the sediments and to their destabilization. Thus, these rocks mark a portion of the sedimentary column located within a (paleo) gas hydrate stability zone. Cylindrical concretions represent ancient fluid conduits related to the upward migration of CH 4 -rich fluids subsequent to gas hydrate destabilization. The carbonate-rich rocks of the Tertiary Piedmont Basin stand as one of the first examples of methane-derived rocks that record successive episodes of dissociation and re-formation of gas hydrates, and they provide precious elements to model the general evolution of a portion of the sedimentary column located within the hydrate stability zone.

Did Late Miocene (Messinian) gypsum precipitate from evaporated marine brines? Insights from the Piedmont Basin (Italy)

During the first stage of the Late Miocene Messinian salinity crisis (5.97–5.60 Ma), deposition of sulfates (the Primary Lower Gypsum) occurred in shallow silled peripheral subbasins of the Mediterranean undergoing restricted water exchange with the Atlantic Ocean. Fluid inclusions in Messinian selenite crystals from the Piedmont Basin (northwest Italy) have surprisingly low salinities (average of 1.6 wt% NaCl equivalent), suggesting that parent waters were depleted in Na + and Cl – compared to modern seawater. Modern gypsum from a Mediterranean salt work, in contrast, contains fluid inclusions with elevated salinities that match the normal evaporation trend expected for seawater. The salinity data indicate that the Messinian sulfate deposits from the Piedmont Basin formed from hybrid parent waters: seawater mixed with Ca 2+ and SO 4 2– enriched freshwaters that dissolved coeval marginal marine gypsum. Such mixed parent waters and complex recycling processes should be taken into account when explaining the genesis of other Messinian gypsum deposits across the Mediterranean Basin.

Are the large filamentous microfossils preserved in Messinian gypsum colorless sulfide-oxidizing bacteria?

The thick gypsum deposits formed in the Mediterranean Basin during the Messinian salinity crisis incorporate dense mazes of filamentous fossils, which were interpreted as algae or cyanobacteria, thus pointing to a shallow-marine subtidal or intertidal environment. The data presented here reveal that these filaments represent remains of colorless, vacuolated sulfide-oxidizing bacteria. This interpretation is supported by the presence of small crystal aggregates of iron sulfide (pyrite) and associated polysulfide within the filamentous fossils. Pyrite and polysulfide are considered to result from early diagenetic transformation of original zero-valent sulfur globules stored within the cells, which is a clade-diagnostic feature of living and degraded sulfur bacteria. In addition to filamentous fossils, the studied gypsum crystals contain remains of euryhaline and stenohaline diatoms and clay-rich aggregates interpreted as alteration products of marine snow floccules. This peculiar fossil assemblage reflects conditions of increased productivity in the water column, triggered by high fluxes of nutrients into the basin during phases of enhanced riverine runoff and freshwater discharge. This study confirms that gypsum evaporites have great potential to preserve the early stages of the taphonomic alteration of bacterial cells, shedding light on the paleoecology of ancient hypersaline environments.

The Cretaceous-Eocene succession of the Rocca Busambra (Western Sicily, Italy): a patchy record on a dissected palaeostructural high

A detailed study has been carried out, integrating biostratigraphy, sedimentology and physical stratigraphy, on Albian-Eocene pelagic sediments (Hybla and Amerillo formations) of the western sector of Rocca Busambra (Trapanese Domain, Western Sicily). They mainly consist of pink to greyish mudstones to packstones with abundant planktonic foraminifera and display the same lithofacies as the well known Scaglia formations, typical of the coeval Southern Alps and Apennine successions. These Scaglia-type sediments commonly occur as isolated sedimentary bodies with a marked lenticular geometry and highly variable thicknesses (from few decimetres to tens of metres) and ages (from late Albian to middle Eocene). They uncomformably rest, with onlap relationships, on very steep or low angle surfaces incised within the underlying formations that can be the Middle Jurassic-Lowermost Cretaceous Buccheri and Lattimusa formations or the Lower Jurassic Inici Formation. Steep surfaces, in agreement with previous authors, are considered as palaeofault planes, whereas gently sloping surfaces find a better explanation as slide scars. The occurrence of monomict and polymict breccias and megabreccias, interlayered within the succession, document debris flow and rock fall processes and support the scenario of a highly irregular sea floor topography. Albian-Eocene sediments are also found as infillings of neptunian dykes and sills present within the Buccheri or the Inici formations.Biostratigraphic data evidence the existence of two very long gaps, respectively corresponding to the Berriasian-middle Albian and the late Campanian-late Paleocene intervals, and the time correspondence of sediment preservation as neptunian dyke fills and sea floor accumulation. All these sedimentologic, geometric and biostratigraphic data show that, starting from the late Albian and up to the Eocene, the Rocca Busambra stood as a margin of a submarine plateau dissected by faults in two main distinct episodes of Albian and Paleocene-Eocene age. In such a morphostructural setting, prolonged time intervals characterized by sediment bypass alternated with tectonic events, during which the opening of fractures and fault-related depressions and the generation of slide scars provided suitable settings for trapping and preserving sediments.

The northernmost record of the Messinian salinity crisis (Piedmont basin, Italy)

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