Marnoso-arenacea Formation Research Articles

Sedimentary facies control on fracture and mechanical stratigraphy in siliciclastics: Marnoso-arenacea formation, Northern Apennines, Italy

Jointing in bedded siliciclastic sedimentary successions has been mainly analyzed in terms of fractured layers thickness-normalized joint spacing and related probability distributions, commonly neglecting the influence of primary structures on the petrophysical properties, joint height patterns, spatial arrangements and best-fitted spacing distributions. Taking into account the depositional controls on mechanical and fracture stratigraphy is crucial for improving the predictive potential of geological modelling in exploration and exploitation of underground resources, gas injection and storage. Here, we present a high-resolution, structural-stratigraphic study carried out on a 72 m-thick succession of the Marnoso-arenacea foredeep turbidites, exposed in the Northern Apennines of Italy. The systematic integration of the structural and sedimentological characterization of this basin-plain turbiditic succession, coupled with petrophysical and geomechanical measurements, represents the fracture and mechanical stratigraphy database onto which we tested the standardly used fracture density parameters and the best-fitted probability distributions, i.e. normal, log-normal, gamma and exponential, applied to fracture spacing analysis. Our results document that cumulative analysis does not identify the variability of joint height patterns and spacing distributions in siliciclastic turbidites, which instead requires considering single sedimentary facies types. Specifically, sandstones are affected by facies bounded joints and gamma spacing distributions while clay-rich and poorly sorted facies show a higher abundance of top bounded and throughgoing joints, and are best fitted by log-normal spacing distributions. We suggest that the fracture and mechanical stratigraphy should be studied at the facies scale resolution to unravel the strong control of the sedimentary facies for improving the characterization of siliciclastic reservoirs.

Spatial and spacing distribution of joints at (over-) saturation in the turbidite sandstones of the Marnoso-Arenacea Fm. (Northern Apennines, Italy)

Despite that many field studies on joint attribute statistics are available in the literature, there is still an ongoing debate on the most effective statistical treatment to properly describe the distribution of a fundamental parameter like joint spacing. This may also relate to the objective difficulty of collecting very large datasets from single linear scanlines directly in the field, because of limited exposures. To overcome such a limitation, we selected a spectacular outcrop of the Miocene Marnoso-Arenacea Formation turbidite sandstones located near the Coniale village, in the Northern Apennines (Italy), where a vertical cliff provides a 100% continuous outcrop about 90 m high and 280 m long, with well-accessible exposures at its base, along the Santerno River. This setting allows combining data acquisition on a photogrammetric DOM (digital outcrop model) covering a representative sector of the outcrop, with ground truthing by direct measurements in the field. More than 2000 joints were identified in 9 linear scanlines traced in the DOM along mechanical layers with different thickness. Comparison with field data indicates that both joint orientation and spacing data from the DOM are very reliable. The scanlines include between 549 and 37 joints, depending on layer thickness, allowing to perform robust statistical analysis. The Kolmogorov-Smirnov (K–S), Cramer-von Mises (CvM) and Anderson-Darling (A-D) nonparametric tests were used to identify the probability density function(s) that best fit spacing data among exponential, Gamma, log-normal and normal distributions. The Gamma distribution turned out as the most probable in most cases. In some specific sectors along all scanlines, joint density is significantly greater than the average for the presence of joint clusters. In some cases, joint clusters are vertically aligned with those in underlying and/or overlying scanlines, thus resulting in thoroughgoing joint corridors. Comparison of spacing statistics inside and outside joint corridors/clusters indicates a typical transition from Gamma distributions outside joint corridors/clusters, to log-normal distributions inside them. The presence of joint clusters and corridors, as well as FSR, mode/mean ratio, JSR, and FSI values, suggests that diffuse jointing in the sandstone layers of the turbiditic succession reached the saturation stage before a normal probability density function was produced, and then further strain was accommodated by localized deformation in clusters and corridors.

Contained-Reflected Megaturbidites of the Marnoso-arenacea Formation (Contessa Key Bed) and Helminthoid Flysches (Northern Apennines, Italy) and Hecho Group (South-Western Pyrenees)

Contained-reflected beds deposited by fully-ponded or partially-reflected turbidity currents are important because their correct evaluation can give important indications on the degree of basin confinement and on the type, size and orientation of the morphological obstacle. Through a detailed facies analysis of various significant megabeds in the Marnoso-arenacea Formation, including the Contessa key bed, in the helminthoid flysches in the northern Apennines (Italy) and in the Pyrenees (megaturbidite MT5), this work proposes a depositional model that is well consistent with the recent experimental data available in the literature, discussing their strengths and limits. The Contessa and flysch megabeds fit very well with the experimental conditions because they are deposited in narrow and elongated confined basins characterized by axial flows. Indeed, in the proposed model, it is possible to recognize facies deposited by: 1) a basal underflow directed towards the bounding slope (Facies A), 2) an intermediate part of the flow characterized by lateral deflections (facies B1), 3) an upper well-developed reversing flow (facies B2) and 4) an uppermost residual reversing flow recording the final collapse of the fine-grained suspended load forming a poorly-sorted slurry facies C and a very thick mudstone unit D. Facies A, B1 and B2 are usually separated by very thin fine-grained muddy drapes rich in carbonaceous matter, which can be traced throughout the basin. These drapes - very common in contained and confined beds in these settings - can be related to internal density surfaces, along which decoupling processes, separating underflows from reversing overflows, can easily occur. Conversely, as the MT5 is characterized by a source transversal to an elongated narrow basin, the large flow volume versus basin capacity hinders the generation of reversing flows and rebound layers favoring the formation of fully-ponded pulsating overflows able to deposit alternations of laminated and massive units. This facies type can be observed in the basins that are characterized by axial flows only near the basin margins where the pulsating collapse of the reversing flow can dominate. This study shows that the integration of detailed field studies are essential to validate experimental data from an applicative point of view.

Geochemical characterization of surface sediments from the Ridracoli reservoir area and surroundings, Italy. Details on bulk composition and grain size

Sediment characterization of Ridracoli reservoir, an artificial lake for water supply, and surroundings of the Fiumi Uniti catchment in the northern Apennines, Emilia-Romagna, Italy, is carried out to evaluate the overall status of the drainage basin encompassing the reservoir. The study area combines the reservoir catchment that is part of the Foreste Casentinesi-Monte Falterona-Campigna National Park, formed by the construction of a dam operating since 1982 for drinking water storage and electricity production, and the upstream side of Fiumi Uniti catchment that flows into the Adriatic Sea, including also nearby sub-catchments of Ridracoli reservoir, named Corniolo and Pietrapazza.We collected surface sediments within the area of the reservoir and riverbed sediments of surroundings catchments, analyzed grain size distribution and the chemical composition, including carbon and nitrogen. This work investigates the chemistry and distribution of elements within the study area, evaluates the dam's effect upstream and downstream, and studies the processes that happen in rivers and the reservoir. The bulk composition is mainly represented by SiO2, Al2O3, CaO, with high Corg content reflecting the Marnoso Arenacea Formation geology of the area. Statistical data analysis on the available parameters suggests the presence of two main types of sediment, namely clayey and carbonate-dominated sediments. On the reservoir area, the relationship between elements and clay matrix and organic matter, besides iron, points to a potential grain size influence, degradation of organic matter, and mineralization processes. The grain size analysis shows that silty and clayey sediments are in the bottom of the reservoir, and sand, is apparent in sediment samples from the tributaries, due to sorting effects and likely directly explained by metals concentrations. This reasoned investigation included further samples from the literature to consider the entire surrounding area and was supported by Principal Component Analysis and Cluster Analysis.

Geology of the High Sillaro Valley (Northern Apennines of Italy)

ABSTRACT We map the High Sillaro Valley, in the Northern Apennines of Italy. Here the Ligurian wedge overthrust the foredeep deposits of the Marnoso Arenacea formation – MA. The overthrusting occurred through a shear zone with components derived from the frontal part of the Ligurian wedge and interpreted as a tectonic mélange (Sestola-Vidiciatico Tectonic unit - SVU) here subdivided into two different subunits structurally independent one from the other. In the area, the Visignano chaotic body (VIS) constitutes an intercalation within the Serravallian-Tortonian MA and derives, as the SVU, from the frontal part of the Ligurian wedge. We performed a detailed investigation of the components of the VIS that helps to better define the architecture of the frontal part of the Ligurian wedge and to constrain the timing of the deformation phases affecting this portion of the Northern Apennines during the middle-late Miocene.

The ancient quarrying areas of the sandstones used in the UNESCO historical centre of Urbino (Marche, Italy) as inferred from geological, petro-chemical and physical-mechanical investigations

Many buildings of the historical centre of Urbino (Marche, Central Italy), included in the UNESCO World Heritage List, are characterized by portals consisting of sedimentary rocks such as limestones or sandstones. In order to identify the possible quarries of provenance of sandstones used for architecture, we addressed our research to the Sant'Ippolito area (20 km ESE of Urbino), where historical stonemasons have been extensively documented since the 14th century. Few traces of ancient exploitation indicate some extraction sites. All the investigated rocks (both from monumental portals of Urbino and geological outcrops) consist of fine- to medium- grained sandstones with a variable content of bioclasts. Two different groups were recognized (using the petrographic Q-F-R classification diagram): (i) lithic arenites belonging to the Colombacci Formation; and (ii) feldspathic lithic arenites referable to the Tripoli Unit. In particular, petrographic data indicate that the sandstones used in the historical buildings of Urbino, come from the arenitic lithofacies of the Messinian p.p. Colombacci Formation of the Sant'Ippolito area. In addition, comparative geochemical studies of whole rock major and trace elements point out three outcrops of the above formation, as possible sites of ancient extraction. Sandstones of the Tripoli Unit and the Marnoso-Arenacea Formation, widespread in the study area and near Urbino respectively, are ruled out as source rocks, based on both petrographic and geochemical data.Anomalously high values of some heavy elements such as Pb, Zn and As, recognized in some of the portals, likely derive from a combination of various anthropogenic sources of pollution close to the investigated monuments, such as domestic heating and emissions from road traffic in the past.From a mechanical point of view, the uniaxial compressive strength tests, carried out on a representative number of selected samples of the Colombacci Formation lithic arenites, yelded low values and high variability. Other investigated physical parameters also indicate that the stones used for the portals did not represent a good choice as building material. This implies that the stonemasons of Sant’Ippolito chose local raw materials for their availability, aesthetic qualities and good workability, rather than structural properties.

The role of mechanical stratigraphy on the refraction of strike-slip faults

Abstract. Fault and fracture planes (FFPs) affecting multilayer sequences can be significantly refracted at layer–layer interfaces due to the different mechanical properties of the contiguous layers, such as shear strength, friction coefficient and grain size. Detailed studies of different but coexisting and broadly coeval failure modes (tensile, hybrid and shear) within multilayers deformed in extensional settings have led to infer relatively low confinement and differential stress as the boundary stress conditions at which FFP refraction occurs. Although indeed widely recognized and studied in extensional settings, the details of FFP nucleation, propagation and refraction through multilayers remain not completely understood, partly because of the common lack of geological structures documenting the incipient and intermediate stages of deformation. Here, we present a study on strongly refracted strike-slip FFPs within the mechanically layered turbidites of the Marnoso Arenacea Formation (MAF) of the Italian northern Apennines. The MAF is characterized by the alternation of sandstone (strong) and carbonate mudstone (weak) layers. The studied refracted FFPs formed at the front of the regional-scale NE-verging Palazzuolo anticline and post-date almost any other observed structure except for a set of late extensional faults. The studied faults document coexisting shear and hybrid (tensile–shear) failure modes and, at odds with existing models, we suggest that they initially nucleated as shear fractures (mode III) within the weak layers and, only at a later stage, propagated as dilatant fractures (modes I–II) within the strong layers. The tensile fractures within the strong layers invariably contain blocky calcite infills, which are, on the other hand, almost completely absent along the shear fracture planes deforming the weak layers. Paleostress analysis suggests that the refracted FFPs formed in a NNE–SSW compressional stress field and excludes the possibility that their present geometric attitude results from the rotation through time of faults with an initial different orientation. The relative slip and dilation potential of the observed structures was derived by slip and dilation tendency analysis. Mesoscopic analysis of preserved structures from the incipient and intermediate stages of development and evolution of the refracted FFPs allowed us to propose an evolutionary scheme wherein (a) nucleation of refracted FFPs occurs within weak layers; (b) refraction is primarily controlled by grain size and clay mineral content and variations thereof at layer–layer interfaces but also within individual layers; (c) propagation within strong layers occurs primarily by fluid-assisted development ahead of the FFP tip of a “process zone” defined by a network of hybrid and tensile fractures; (d) the process zone causes the progressive weakening and fragmentation of the affected rock volume to eventually allow the FFPs to propagate through the strong layers; (e) enhanced suitable conditions for the development of tensile and hybrid fractures can be also achieved thanks to the important role played by pressured fluids.

Basin-scale, high-resolution three-dimensional facies modeling of tectonically confined turbidites: An example from the Firenzuola system (Marnoso-arenacea Formation, northern Apennines, Italy)

Facies modeling is an important step in reservoir characterization because each facies is related to specific petrophysical properties, and the assessment of their spatial arrangement is key for hydrocarbon recovery estimations. Most data sets related to subsurface sedimentary basins in exploration areas are limited to seismic resolution, which is lower than the actual thickness range of the beds in most depositional systems. In that way, detailed studies carried out in the outcropping area of collisional margins can be used as analogs of subsurface reservoirs with similar geologic settings to better predict heterogeneities and properties distribution, with no limitation of resolution. This study provides an experimental three-dimensional high-resolution facies model based on 5500 m (∼18,000 ft) of bed-by-bed measurements divided into seven stratigraphic logs, collected over several years of field studies in the Marnoso-arenacea Formation (MAF; northern Apennines, Italy). It focuses on the upper Serravallian Firenzuola turbidite system, which was deposited in a highly confined inner foredeep during an important phase of growth and propagation of the Monte Castellaccio Thrust and Verghereto high. The synsedimentary activity of these tectonic structures deeply affected sediment dispersal patterns and facies distribution of the inner foredeep that became, in the final narrowing phase, a piggyback-type basin and induced the coeval shift of the main depositional area to an outer and relatively wider basin representing the new depocenter of the MAF foredeep. The modeling has provided quantitative analysis of turbidite facies trends and a numerical data set suitable for further geostatistical and petrophysical studies.

Palynofacies classification of submarine fan depositional environments: Outcrop examples from the Marnoso-Arenacea Formation, Italy

Basin floor fans contain some of the largest deep-water hydrocarbon accumulations discovered, however they also demonstrate extremely complex stratigraphic architecture, understanding of which is crucial for maximum recovery. Here we develop a new method, based upon palynofacies analysis, for the distinction of the different depositional environments that are commonly associated with basin floor fans. Previous studies and our sedimentological analysis allow good confidence in the discrimination of the different depositional environments of the outcropping Marnoso-Arenacea Formation fan system. One hundred and thirty-five samples were collected from mudstones in conjunction with sedimentary logging of 871 m of outcrops. Six lithofacies associations are described and interpreted to represent lobe axis, lobe fringe, fan fringe, contained interlobe, basin plain, and starved high depositional sub-environments. Palynofacies of these elements demonstrate turbidites to be rich in terrestrial organic matter, with sixteen categories of matter recognised. The abundances and proportions of particles varies between sub-environments, with lobe axis deposits containing the largest, densest particles, with a transition to ever smaller and lighter particles moving toward the basin plain. Fuzzy C-means statistical analysis was used to explore this trend. Distribution of organic matter is not random, but is dominated by hydrodynamic sorting and sequential fall-out of particles as turbidity currents passed across the basin. This allows a palynofacies classification scheme to be constructed to assist the identification of depositional environments of submarine fans, which may be combined with subsurface data to assist reservoir characterisation.

Application of the adjoint approach to optimise the initial conditions of a turbidity current with the AdjointTurbidity 1.0 model

Abstract. Turbidity currents are one of the main drivers of sediment transport from the continental shelf to the deep ocean. The resulting sediment deposits can reach hundreds of kilometres into the ocean. Computer models that simulate turbidity currents and the resulting sediment deposit can help us to understand their general behaviour. However, in order to recreate real-world scenarios, the challenge is to find the turbidity current parameters that reproduce the observations of sediment deposits. This paper demonstrates a solution to the inverse sediment transportation problem: for a known sedimentary deposit, the developed model reconstructs details about the turbidity current that produced the deposit. The reconstruction is constrained here by a shallow water sediment-laden density current model, which is discretised by the finite-element method and an adaptive time-stepping scheme. The model is differentiated using the adjoint approach, and an efficient gradient-based optimisation method is applied to identify the turbidity parameters which minimise the misfit between the modelled and the observed field sediment deposits. The capabilities of this approach are demonstrated using measurements taken in the Miocene Marnoso-arenacea Formation (Italy). We find that whilst the model cannot match the deposit exactly due to limitations in the physical processes simulated, it provides valuable insights into the depositional processes and represents a significant advance in our toolset for interpreting turbidity current deposits.

The tectonically-confined Firenzuola turbidite system (Marnoso-arenacea Formation, northern Apennines, Italy)

The Firenzuola turbidite system formed during an important phase of thrust propagation, involving the upper Serravallian deposits of the Marnoso-arenacea Formation (MAF). During this phase the coeval growth of two major tectonic structures, the M. Castellaccio thrust and the Verghereto high, played a key role, causing a closure of the inner basin and a coeval shift of the depocenter to the outer basin. This work focuses on this phase of fragmentation of the MAF basin; it is based on a new detailed high-resolution stratigraphic framework, which was used to determine the timing of growth of the involved structures and their direct influence on sediment dispersal, as well as on lateral and vertical turbidite facies distribution. The Firenzuola turbidite system stratigraphy is characterized by the occurrence of MTCs (Mass Transport Complexes) and thick sandstone accumulation in the depocentral area, which passes to finer drape over the structural highs; the differentiation between these two zones increases over time and ends with the deposition of marly units over the structural highs and the emplacement of the Visignano MTC. According to the stratigraphic pattern and turbidite facies characteristics, the Firenzuola System (Unit V in the works by Muzzi Magalhaes and Tinterri) has been split into two sub-units, namely Firenzuola I (sub-Unit Va) and Firenzuola II (sub-Unit Vb): the former is quite similar to the underlying deposits (Unit IV), the latter shows the main fragmentation phase, testifying to the progressive isolation of the inner basin and a coeval shift of the depocenter to the outer basin.

Sedimentary facies control on mechanical and fracture stratigraphy in turbidites

Natural fracture networks exert a first-order control on the exploitation of resources such as aquifers, hydrocarbons, and geothermal reservoirs, and on environmental issues like underground gas storage and waste disposal. Fractures and the mechanical stratigraphy of layered sequences have been intensively studied to unravel the relationships between bed thickness and fracture spacing, but less attention has been paid to intrabed fracturing patterns due to the intrinsic local variability of sedimentary processes and products. Among sedimentary rocks, turbidites show great lateral and vertical variability of textural characteristics and depositional facies, which are expected to strongly influence the location and density of fractures. To better understand the contribution of stratigraphic, sedimentologic, and petrophysical properties on fracture patterns, we performed a high-resolution study on a selected stratigraphic interval of jointed foredeep turbidites in the Miocene Marnoso-Arenacea Formation (Northern Apennines, Italy). Cumulative statistical relations of field and laboratory structural, sedimentologic, and petrophysical data significantly improved when analyzed at the sedimentary facies scale. In particular, for facies recording different cross-flow (i.e., longitudinal to the paleocurrents) depositional conditions within the parent turbidity currents, we observed three-dimensional anisotropies of rock hardness (i.e., uniaxial compression) that were positively correlated with normalized fracture intensities, indicating a primary sedimentary control on fracture distribution. This type of intrabed joint distribution has crucial practical implications for the lateral prediction and evaluation of mesoscale fracture patterns in turbidite sequences.

Determining flow directions in turbidites: An integrated sedimentological and magnetic fabric study of the Miocene Marnoso Arenacea Formation (northern Apennines, Italy)

Depositional models of turbidity flows require estimating paleocurrent directions using sedimentological indicators such as flute and ripple marks, but these are not always present in outcrop sections or drill cores. In this study, we apply the anisotropy of magnetic susceptibility (AMS) as an alternative tool to estimate paleocurrent directions in a case-study turbiditic system: the Marnoso Arenacea foredeep turbidites of Miocene age exposed in the northern Apennines of Italy. Different depositional facies have been sampled for AMS and additional rock-magnetic analyses. We observed a good agreement between paleocurrent directions from flute casts at the base of turbidite beds and mean directions of maximum magnetic susceptibility axes in organized facies such as massive and laminated sandstones, even if a relatively small but apparently consistent offset of ~15–20° seems to be present. Highly dispersed AMS fabrics were instead observed in disordered facies such as convoluted and undulated sandstones as well as debrites. This strong correlation between hydrodynamic regimes of depositional facies and AMS data represents a novel contribution and confirms the validity of the AMS method to estimate flow directions in the absence of sedimentological indicators. Finally, paleomagnetic analyses from the literature were used to reconstruct the paleogeography of the Marnoso Arenacea basin and make inferences about the origin and direction of transport of the sediments at the basin scale.

Geochemical mapping based on geological units: A case study from the Marnoso-arenacea formation (Northern Apennines, Italy)

Geochemical maps can provide us with much information on geology, earth surface processes and anthropogenic pressure and are valuable tools for ore prospecting and land management. Stream sediments represent an integral of the various possible sources of sediments upstream from the sampling point therefore there can be multiple signal sources but generally the prevailing signal source is the one related to bedrock geology. Stream sediments collected from active second-order channels including singular geological units, were selected in order to determine the geochemical characteristics of each unit. The aim of this study was to analyse their potential for using them to integrate geological interpretation and produce a geologically-oriented geochemical map. From the 770 samples collected for a regional geochemical mapping program, we selected 149 samples whose catchment basin included only one of the members recognized within the Marnoso–Arenacea formation. This Middle-Upper Miocene (Langhian–Tortonian) turbiditic unit forms the backbone of the Romagna Apennines and has been subdivided into 14 members according to age and lithostratigraphic criteria. The results indicate that there are marked differences in the composition of the members of the Marnoso Arenecea formation which indicate the provenance of the sediment and the palaeogeographic evolution of the units. By means of univariate and multivariate statistical analyses (Factor analyses) two main types of sediment compositions are identified: Tortonian members are characterized by sialic coarse grain- sediments while the Langhian–Serravallian members are richer in carbonate fraction, slightly enriched in a mafic contribution. This study elaborated the geochemical data from a geological point of view by integrating the information available in literature to spatially extend the interpretation based on limited site observation as for petrographic studies. In general, the geochemical map based on a geological unit could be a useful tool for carrying out the geological reconstruction of a complex area.

Convolute laminations and load structures in turbidites as indicators of flow reflections and decelerations against bounding slopes. Examples from the Marnoso-arenacea Formation (northern Italy) and Annot Sandstones (south eastern France)

This work discusses the significance of particular types of soft-sediment deformations very common within turbidite deposits, namely convolute laminations and load structures.Detailed facies analyses of the foredeep turbidites in the Marnoso-arenacea Formation (northern Italy) and Annot Sandstones (south eastern France) show that these deformational structures tend to increase near morphological obstacles, concomitantly with contained-reflected beds. The lateral and vertical distribution of convolute laminae and load structures, as well as their geometry, has a well-defined depositional logic related to flow decelerations and reflections against bounding slopes. This evidence suggests an interaction between fine-grained sediment and the presence of morphologic relief, and impulsive and cyclic-wave loadings, which are produced by flow impacts or reflected bores and internal waves related to impinging bipartite turbidity currents.

Geochemical mapping based on geological units: a case study from the Marnoso-arenacea formation (Northern Apennines, Italy)

Geochemical maps can provide us with much information on geology, earth surface processes and anthropogenic pressure and are valuable tools for ore prospecting and land management. Stream sediments represent an integral of the various possible sources of sediments upstream from the sampling point therefore there can be multiple signal sources but generally the prevailing signal source is the one related to bedrock geology. Stream sediments collected from active second-order channels including singular geological units, were selected in order to determine the geochemical characteristics of each unit. The aim of this study was to analyze their potential for using them to integrate geological interpretation and produce a geologically-oriented geochemical map. From the 770 samples collected for a regional geochemical mapping program, we selected 149 samples whose catchment basin included only one of the members recognized within the Marnoso-arenacea formation. This middle–upper Miocene (Langhian–Tortonian) turbiditic unit forms the backbone of the Romagna Apennines and has been subdivided into 14 members according to age and lithostratigraphic criteria. The results indicate that there are marked differences in the composition of the members of the Marnoso arenecea formation which indicate the provenance of the sediment and the palaeogeographic evolution of the units. By means of univariate and multivariate statistical analyses (Factor analyzes) two main types of sediment compositions are identified: Tortonian members are characterized by sialic coarse grain-sediments while the Langhian–Serravallian members are richer in carbonate fraction, slightly enriched in a mafic contribution. This study elaborated the geochemical data from a geological point of view by integrating the information available in literature to spatially extend the interpretation based on limited site observation as for petrographic studies. In general, the geochemical map based on a geological unit could be a useful tool for carrying out the geological reconstruction of a complex area.

The syntectonic evolution of foredeep turbidites related to basin segmentation: Facies response to the increase in tectonic confinement (Marnoso-arenacea Formation, Miocene, Northern Apennines, Italy)

The Marnoso-arenacea Formation (MAF) records the progressive closure of the foredeep due to the north-eastward propagation of thrust fronts. The latter produced a complex foredeep characterized by the uplift of the basin inner portions and by the north-eastward shifting of the main depocentres. This work discusses the evolution of the MAF inner basin and the first deposits of the outer basin in a transect perpendicular to the paleocurrents and the main tectonic structures that are NW–SE-oriented. Specifically, this study focuses on the upper Serravallian transitional stage, which records the closure of the inner basin and the deposition shifting to an outer basin due to the growth of an important regional structure represented by the NW–SE-oriented M. Castellaccio thrust-related fold (CTRF). The growth of this structure produces progressive segmentation of the foredeep through the progressive isolation of the inner basin that becomes, in the upper Serravallian phase of basin narrowing, a ponded piggyback basin. During this evolution, the structural control and associated morphologic confinement increased over time, controlling the lateral and vertical distribution of turbidite facies that is very similar to that of intra-slope minibasins. In such a situation, vertical and lateral facies changes, associated with evident stratigraphic pinchings and onlap relationship, have played a key role in showing this phase of strong foredeep segmentation and basin confinement. This phase is marked by a well-determined vertical facies variation: contained-reflected beds pass upward into huge accumulations of sandstone lobes featuring the appearance of Ophiomorpha trace fossils and coarse-grained massive sandstone beds often overlain by mud-draped scours and tractive structures (megaripples and traction carpets). These beds are interpreted as related to flow decelerations induced by structurally-controlled basin confinement that favour the deposition of massive sandstones through hydraulic jumps and flow decoupling with bypass of upper turbulent flows that can produce well-developed tractive structures.

Quantitative Analysis of Submarine-Flow Deposit Shape In the Marnoso-Arenacea Formation: What Is the Signature of Hindered Settling From Dense Near-Bed Layers?

Abstract Submarine sediment density flows are one of the volumetrically most important processes for sediment transport across Earth. The sediment concentration of flows that reach the deep ocean has never been measured directly, and understanding these long-runout flows remains a major challenge. The Miocene Marnoso-Arenacea Formation in the Italian Apennines is the only ancient sequence where individual submarine sediment-density-flow deposits (single beds) have been mapped out for more than 100 km down-flow. Here we document the external shape and internal architecture of thirty-two individual beds that record flow evolution and can be compared to deposit shapes in mathematical or experimental models. The large number of beds allows modes of flow behavior to be identified. Larger-volume turbidites are typically dominated by massive (TA) or planar-laminated (TB) sandstone intervals that have a broad thickness maximum. This shape is important because it suggests that massive and planar laminated sandstones record hindered settling from dense near-bed layers, which have high (> 10% by volume) sediment concentrations. Previously, some authors have inferred that planar-laminated sandstones (TB) are deposited mainly by dilute flows. The position of the broad thickness maximum moves basinward as the volume of sand in the flow increases. This is consistent with mathematical modeling that suggests the position of the thickness maximum depends on flow thickness, flow speed, and sediment settling velocity, as well as sediment concentration, variations in seawater entrainment rate, and local changes in seafloor gradient. Smaller-volume turbidite sandstone intervals are finer grained and dominated by ripple cross-lamination (TC) and have a near exponential decay in thickness that is consistent with deposition from a dilute sediment suspension. The rate of near exponential thinning is controlled by sandstone volume. In contrast, turbidite mudstone intervals show an approximately linear increase in thickness with distance. Flows that entered the basin in opposite directions produced turbidite mudstone intervals that thicken towards the same location, indicating that muddy turbidity currents can drain back over long distances to basinal lows.

Magnetic-Fabric Analysis As A Tool To Constrain Mechanisms of Deep-Water Mudstone Deposition In the Marnoso Arenacea Formation (Miocene, Italy)

Abstract Mudstone-dominated marine successions are common in the geological record, yet a full understanding of their depositional processes is often hampered by a lack of generally accepted diagnostic criteria to distinguish between hemipelagic settling and deposition from a flowing medium. The Marnoso Arenacea Formation, a turbidite unit of Miocene age cropping out in the northern Apennines of Italy, offers the possibility to address some of these uncertainties. A relatively small (∼ 10%) but distinctive portion of the Marnoso Arenacea Formation is composed of white marlstone beds (WM beds) that have frequently been interpreted as due to hemipelagic settling of fine-grained particles (hemipelagites). The analysis of the anisotropy of magnetic susceptibility (AMS) revealed the presence in the WM beds of maximum susceptibility axes clustered within the depositional plane along the average paleoflow direction inferred from flute casts at the bases of the nearest turbidite beds, whereas the minimum susceptibility axes are oriented perpendicular to the bedding plane. This fabric is interpreted as largely sedimentary in origin (albeit a contribution from tectonic shortening cannot be excluded) and due to the alignment within the bedding plane of paramagnetic grains (e.g., muscovite) and possibly also ferromagnetic grains (magnetite) under low-velocity currents. The trend of the maximum susceptibility axes, and hence of the paleoflow direction, is approximately oriented NNW–SSE after correction for Apennines thrust-sheet rotation since the Miocene. These results suggest that the WM beds cannot be entirely due to hemipelagic settling, as often stated in the literature. A discussion of alternative depositional mechanisms leads us to conclude that the WM beds may have been deposited under the influence of contour currents and should therefore be referred to as muddy contourites.

Space/time tectono-sedimentary evolution of the Umbria-Romagna-Marche Miocene Basin (Northern Apennines, Italy): a foredeep model

The space/time evolution of the Umbria-Romagna-Marche domains of the northern Apennine Miocene foredeep is proposed. In this period, the turbidite siliciclastic sedimentation is represented mainly by the Miocene Marnoso-Arenacea Formation, which generally ends with mainly marly deposits. From the internal Apennine sectors (Umbria-Romagna domain) to the external Adriatic Margin (Marche domain) the siliciclastic succession overlies hemipelagic marly deposits (Schlier Formation). The whole depositional area can be considered as a single wide basin with depocenter or main sedimentation areas progressively migrating eastwards. This basin is characterized by some morphological highs which did not constitute real dams for the sedimentary flows (turbidity currents). Multiple feeding (arkose, litharenites, calcarenites) from different sources is related to palaeogeographical and palaeotectonic reorganization of the most internal, previously deformed, Apennine areas. The activation of the foredeep stage is marked by the beginning of the siliciclastic sedimentation (Late Burdigalian in the most internal sector). This sedimentation ends in the most external sector in the Early Messinian, pointing to a depositional cycle of about 9–10 Ma. The diachronism of the base of the siliciclastic deposition proves to be almost 5 Ma. The syn-depositional compressional deformation, which shows a marked diachronism, affected the internal area of the foredeep in the Early-Middle Serravallian, and progressively migrated up to Late Miocene, involving more and more external sectors. The deformed siliciclastic sedimentary wedge constitutes an orogenic pile incorporated in the Apennine Chain, represented by different tectonic elements superimposed by means of NE-vergent thrusts. The main stratigraphic and tectonic events of the Toscana-Romagna-Marche Apennines are presented in a general framework, resulting also in a terminological revision.