Tectono-sedimentary evolution of the External Liguride units (Northern Apennines,Italy): insights in the pre-collisional history of a fossil ocean-continent transition zone

Abstract

In the Northern Apennines, the External Liguride (EL) units are interpreted as derived from the domain that joined the Ligure–Piemontese oceanic basin to the Adriatic plate continental margin. The EL units can be divided into two different groups according to the lithostratigraphic features of the basal complexes underlying the Upper Cretaceous–Lower Tertiary carbonate flysch (e.g. Helminthoid flysch). The first group includes the western successions characterized by Santonian–Campanian sedimentary melanges where slide blocks of lherzolitic mantle, gabbros, basalts, granulites, continental granitoids are represented. The second group is represented by the eastern successions where the Cenomanian–Campanian basal complexes mainly consist of sandstones and conglomerates where the mafic and ultramafic rocks are scarce or completely lacking. Their original substrate is represented by the Middle Triassic to Lower Cretaceous, mainly platform carbonate deposits, found as slices at the base of the eastern successions.

The stratigraphic features shown by the basal complexes allow the reconstruction of their source area that is assumed to be also representative for the pre-Upper Cretaceous setting. The proposed reconstruction suggests the occurrence in the EL domain of two distinct domains. The eastern domain was characterized by a thinned and faulted continental crust belonging to the Adriatic continental margin. The western domain was instead floored by subcontinental mantle associated with lower and upper continental crust, representing the ocean–continent transition. This setting is interpreted as the result of the opening of the Ligure–Piemontese oceanic basin by passive rifting, mainly developed by simple shear, asymmetric extension of the continental crust. © 2001 Éditions scientifiques et médicales Elsevier SAS     

Petrology, Mineral and Isotope Geochemistry of the External Liguride Peridotites (Northern Apennines, Italy)

Mantle peridotites of the External Liguride (EL) units (NorthernApennines) represent slices of subcontinental lithospheric mantleemplaced at the surface during early stages of rifting of theJurassic Ligurian Piemontese basin. Petrological, ion probeand isotopic investigations have been used to unravel the natureof their mantle protolith and to constrain the timing and mechanismsof their evolution. EL peridotites are dominantly fertile spinelIherzolites partly recrystallizfd in the plagiodase Iherzplitestability field Clinopyroxenes stable in thespinel-facies assemblagehave nearly fiat REE patterns (Ce_N/Sm_N=06–08) at (10–16)C1and high Na, Sr, Ti and Zr contents. Kaersutitic-Ti-pargasiticamphiboles also occur in the spinel-facies assemblage. TheirLREE-depleted REE spectra and very low Sr, Zr and Ba contentsindicate that they crystallized from hydrous fluids with lowconcentrations of incompatible elements. Thermometric estimateson the spinelfacies parageneses yield lithospheric equilibriumtemperatures in the range 1000–1100C, in agreement withthe stability of amphibole, which implies T<1100C. Sr andNd isotopic compositions, determined on carefully handpickedclinopyroxene separates, plot within the depleted end of theMORB field (⁸⁷Sr/⁸⁶Sr=070222–070263; ¹⁴³Nd/¹⁴⁴Nd=0513047–0513205)similar to many subcontinental orogenic spinel Iherzolites fromthe western Mediterranean area (e.g. Ivrea Zpne and Lanzfl N).The interpretation of the EL Iherzolites as subcontinental lithosphericmantle is reinforced by the occurrence of one extremely depletedisotopic composition (⁸⁷Sr/⁸⁶Sr=0701736; ¹⁴³Nd/¹⁴⁴Nd=0513543).Sr and Nd model ages, calculated assuming both CHUR and DM mantlesources, range between 24 Ga and 780 Ma. In particular, the12-Ga Sr age and the 780-Ma Nd age can be regarded as minimumages of differentiation. The transition from spinel-to plagioclase-faciesassemblage, accompanied by progressive deformation (from granularto tectonite-mylonite textures), indicate that the EL Iherzolitesexperienced a later, subsolidus decompressional evolution, startingfrom subcontinental lithospheric levels. Sm/Nd isochrons onplagioclase-clinopyroxene pairs furnish ages of 165 Ma. Thisearly Jurassic subsolidus decompressional history is consistentwith uplift by means of denudation in response to passive andasymmetric lithospheric extension. This is considered to bethe most suitable geodynamic mechanism to account for the exposureof huge bodies of subcontinental lithospheric mantle duringearly stages of opening of an oceanic basin. ^*Corresponding author. Present address: Dipartimento di Stienze della Terra, Univenit di Geneva, Corso Europa 26,16132 Genova, Italy     

Petrogenesis of basalts and gabbros from an ancient continent–ocean transition (External Liguride ophiolites, Northern Italy)

Remnants of a fossil continent–ocean transition similar to that of the modern non-volcanic continental margins are preserved in the Jurassic External Liguride units. They consist of fertile lherzolites of subcontinental origin, MOR-type basalts and rare gabbroic intrusives, together with continental crust bodies exhumed during the rifting phases preceding the oceanization. The gabbroic rocks include troctolites, (olivine) gabbros, Fe–Ti oxide-bearing gabbros and diorites. Trace element and Nd isotope compositions indicate that these rocks were derived from N-MORB melts variably evolved through fractional crystallisation. In the gabbroic rocks, high-temperature ( 900 °C) shearing along ductile shear zones is locally overprinted by amphibolite-facies recrystallization (T  650 °C), which was most likely assisted by seawater-derived fluids. Basalts crop out as lava flows and as dykes crosscutting mantle lherzolites and gabbroic rocks. They display nearly flat REE patterns and high Y/Nb values (5–14), similar to modern N-MORB. Basalts are also characterised by weak Zr enrichment relative to neighbouring REE (Zr/Zr = 1.1–1.7) and high (Sm/Yb)_DM ratios (1.5–1.8). Their Nd isotope compositions are close to typical depleted mantle (initial _Nd = +7.6 to + 9.4). The geochemical features of parental melts of basaltic and gabbroic rocks may be attributed to melting of a MORB-type asthenospheric source. Trace element modelling shows that low-degree (≤ 6%) fractional melting of a depleted spinel peridotite cannot account for the elevated Sm/Yb ratios of basalts. Low-degree melting of a mixed source of spinel peridotite with small amounts of garnet pyroxenite has been proposed to explain the trace element signature of basalts.     

Recent and active tectonics of the external zone of the Northern Apennines (Italy)

We present a comprehensive study of the recent and active tectonics of the external part of the Northern Apennines (Italy) by using morphotectonic, geological–structural, and stratigraphic analysis, compared with the current seismicity of the region. This analysis suggests that the external part of the Northern Apennines is characterised by presence of three major systems of Quaternary compressive structures corresponding to (1) the Apenninic watershed, (2) the Apennines–Po Plain margin (pede-Apenninic thrust front), and (3) the Emilia, Ferrara, and Adriatic Fold systems buried below the Po Plain. Geological data and interpreted seismic sections indicate a roughly N–S Quaternary deformation direction, with rates <2.5 mm/year. The shortening decreased since the Pliocene, when our data indicate compression in a NNW–SSE direction and rates up to 7 mm/year. The trend and kinematics of the structures affecting the Apennines–Po Plain margin and the Po Plain subsoil fit well the pattern of the current seismicity of the area, as well as recent GPS and geodetic levelling data, pointing to a current activity of these thrust systems controlled by an overall compressive stress field. Close to the Apenninic watershed, earthquake focal mechanisms indicate that shallow extension is associated to deep compression. The extensional events may be related to a secondary extensional stress field developing on the hangingwall of the thrust system affecting the Apenninic watershed; alternatively, this thrust system may have been recently deactivated and overprinted by active normal faulting. Deeper compressive events are related to the activity of both a major basement thrust that connects at surface with the pede-Apenninic thrust front and a major Moho structure.     

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.     

Tectono-sedimentary analysis of a complex,extensional, Neogene basin formed on thrust-faulted,Northern Apennines hinterland: Radicofani Basin,Italy

Large NW–SE oriented, Neogene–Quaternary structural depressions, up to about 200 km long and 25 km wide, have developed on the western side (hinterland) of the Northern Apennines over thrust substrate. The depressions are now, for the most part, laterally bounded by normal faults and are longitudinally separated into basins by transfer zones. A debate exists in the literature as to whether these basins have developed as half-graben under a predominantly extensional regime since late Miocene, or as thrust-top basins under a predominantly compressional regime that has continued until the Pleistocene. The Radicofani Basin is one of the best-preserved basins. It developed mainly during the late Miocene–Early Pliocene in the southern half of the Siena–Radicofani structural depression, and is now bounded on the east by normal faults that transect a thrust anticline “nose“ in the substrate, to the north by a substrate high or transfer zone, and to the south and west by Quaternary igneous/volcanic edifices. The basin experienced variable differential tectonic and associated sedimentation along linking, normal boundary faults. Along its eastern margin it shows the development of thick (∼600 m) alluvial fans that developed in relay areas between boundary faults and transverse faults and transfer zones. Well-exposed sections generally show upward transitions from conglomeratic alluvial fans, to shoreface sandstone, to offshore mudstones. Locally, the transition is marked by deltas primarily characterised by thick gravelly, sandy, stacked cross-sets The thicker, sandy-gravel to gravelly-sand cross-sets (5–8 m thick) are interpreted as Gilbert-type deltas; interstratified thinner (0.5–1 m thick), generally openwork gravelly strata are part of delta topset assemblages and probably represent prograding fluvial bars. Tectonic movements provided the accommodation space for the total, ∼2700 m thick basin fill. Sea level fluctuations that led to the repeated development of the cross-sets may also have been influenced by climatic or eustatic changes, possibly related to the effects of early Antarctic glaciations.Some features of the Radicofani Basin can be found in both extensional and compressional basins. However, the position of the basin in the mountain chain and the development of alluvial fans, fandeltas and associated deposits along the main boundary fault, combined with structural evidence from seismic lines, show that during the early Pliocene this basin best conforms to existing models of half-graben.     

Topography and palaeogeographic evolution of a middle Miocene foredeep basin plain (Northern Apennines, Italy)

The Marnoso–arenacea basin was a narrow, northwest–southeast trending, foredeep of Middle–Late Miocene age bounded to the southwest by the Apennine thrust front. The basin configuration and evolution were strongly controlled by tectonics.

Geometrical and sedimentological analysis of Serravallian turbidites deposited within the Marnoso–arenacea foredeep, combined with palaeocurrent data (turbidite flow provenance, reflection and deflection), identify topographic irregularities in a basin plain setting in the form of confined troughs (the more internal Mandrioli sub-basin and the external S. Sofia sub-basin) separated by an intrabasinal structural high. This basin configuration was generated by the propagation of a blind thrust striking northwest to southeast, parallel to the main trend of the Apennines thrust belt.

Ongoing thrust-induced sea bed deformation, marked by the emplacement of large submarine landslides, drove the evolution of the two sub-basins. In an early stage, the growth and lateral propagation of a fault-related anticline promoted the development of open foredeep sub-basins that were replaced progressively by wedge-top or piggy-back basins, partially or completely isolated from the main foredeep. Meanwhile, the depocenter shifted to a more external position and the sub-basins were incorporated within an accretionary thrust belt.     

Landslides types controlled by tectonics-induced evolution of valley slopes (Northern Apennines,Italy)

This paper investigates the role played by geomorphological and tectonic processes affecting a portion of an active mountain belt in causing the occurrence of different types of landslides developed in flysch bedrock. The adopted multidisciplinary approach (geomorphology, geology and geophysics) allowed to recognize in a portion of the Northern Apennines of Italy different types of landslides that developed in response to slope dynamics, in turn dependent on broader regional-scale tectonic processes. Sedimentary bed attitude, local tectonic discontinuities and lithology only partially influenced the type of landslides, which have been deeply affected by the activity of regional-scale antiform that controlled the hillslope geomorphic evolution in different ways. The growth of this structure and the tilting of its forelimb produced gently dipping slopes that approached the threshold angle that can cause the occurrence of (mainly) translational rockslides. Conversely, high-angle normal faulting parallel to the antiform axis (related to a later stage of activity of the antiform itself) strongly controlled the stream network evolution and caused the watercourses to deeply incise portions of their valleys. This incision produced younger steep valley slopes and caused the development of complex landslides (roto-translational slides-earth/debris flow). The results of the integrated study presented in this paper allowed to distinguish two main types of landslides whose development reflects the events that led to the geomorphological and geological evolution of the area. In this perspective, within the study area, landslides can be regarded and used as indicators of broader-scale recent tectonic processes.     

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?C10?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.     

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.     

Origin and evolution of a salty gypsum/anhydrite karst spring: the case of Poiano (Northern Apennines, Italy)

Poiano is the largest karst spring of the Emilia Romagna region (northern Italy). It drains an aquifer of unique properties composed of anhydrite with halite lenses at depth and gypsum at the surface (both with high NaCl content). Hydrogeological research has been undertaken using automatically recorded hourly data on temperature, electrical conductivity, and water level. Water feeding the Poiano spring is restricted within the gypsum/anhydrite outcrop between the Lucola, Sologno and Secchia rivers. Karstification in the Upper Secchia Valley only concerns the gypsum rocks mainly present along the border and in the shallower parts of the sulfate outcrop and does not appear to occur at depth. Data strongly support the hypothesis that the salt content in the spring water derives from active halokinetic movements. For the first time, the fundamental hydrogeological importance of the anhydrite part inside the sulfate rocks is demonstrated. If gypsum prevails over anhydrite the karst drainage network can extend deep into the rocks following a network of fractures and fissures. Instead, if in the deep parts of the aquifer anhydrite prevails over gypsum, the karst evolution cannot take place at depth and the structure of the underground drainage paths only follows near-surface paths in gypsum.     

The Mesozoic continental rifting in the Mediterranean area: insights from the Verrucano tectofacies of southern Tuscany (Northern Apennines,Italy)

In the Alpine-Mediterranean region, the continental redbeds and shallow-marine siliciclastics related to the early depositional phases of the Late Permian-Mesozoic continental rifting are referred to as the most common representative of the “Verrucano tectofacies”. The Verrucano-type successions exposed in southern Tuscany are diachronous, spanning from Triassic to earliest Jurassic in age, and accumulated within the Tuscan domain, a paleogeographic region of continental crust that due to the opening of the Piedmont–Ligurian ocean formed part of the Adria passive-margin. They belong to the metamorphic Verrucano Group and the non-metamorphic Pseudoverrucano fm. Viewed overall, these Verrucano-type successions appear to manifest five episodes or pulses of an ongoing continental rifting. With the exception of the first episode that developed entirely within a terrestrial setting, each one is represented by basal Verrucano-type continental siliciclastics overlain by compositionally mixed marine deposits, which resulted from four diachronous, post-Middle Triassic transgressions. This suite of tectonic pulses produced the progressive westward widening (backstepping) of the Tuscan domain in the rifting south-Tuscany area.     

Structural evolution of the Tuscan Nappe in the southeastern sector of the Apuan Alps metamorphic dome (Northern Apennines,Italy)

Structural analysis carried out in the Tuscan Nappe (TN) in the southeastern sector of the Apuan Alps highlights a structural evolution much more complex than that proposed so far. The TN has been deformed by structures developed during four deformation phases. The three early phases resulted from a compressive tectonic regime linked to the construction of the Apenninic fold‐and‐thrust‐belt. The fourth phase, instead, is connected with the extensional tectonics, probably related to the collapse of the belt and/or to the opening of the Tyrrhenian Sea. Our structural and field data suggest the following. (1) The first phase is linked to the main crustal shortening and deformation of the Tuscan Nappe in the internal sectors of the belt. (2) The second deformation phase is responsible for the prominent NW–SE‐trending folds recognized in the study area (Mt. Pescaglino and Pescaglia antiforms and Mt. Piglione and Mt. Prana synforms). (3) The direction of shortening related to the third phase is parallel to the main structural trend of the belt. (4) The interference between the third folding phase and the earlier two tectonic phases could be related to the development of the metamorphic domes. The two directions of horizontal shortening induced buckling and vertical growth of the metamorphic domes, enhancing the process of exhumation of the metamorphic rocks. (5) The exhumation of the Tuscan Nappe occurred mostly in a compressive tectonic setting. A new model for the exhumation of the metamorphic dome of the Apuan Alps is proposed. Its tectonic evolution does not fit with the previously suggested core complex model, but is due to compressive tectonics. Copyright © 2004 John Wiley & Sons, Ltd.     

Influence of syn-sedimentary faults on orogenic structures in a collisional belt: Insights from the inner zone of the Northern Apennines (Italy)

This paper discusses the possible influence of syn-sedimentary structures on the development of orogenic structures during positive tectonic inversion in the inner Northern Apennines (Italy). Examples from key areas located in southern Tuscany provided original cartographic, structural and kinematics data for Late Oligocene-Early Miocene thrusts, organized in duplex systems, verging in the opposite direction of the foreland propagation (back-thrusts), which affected the Late Triassic-Oligocene sedimentary succession of the Tuscan Domain, previously affected by pre-orogenic structures. These latter consist of mesoscopic-to cartographic-scale Jurassic syn-sedimentary normal faults and extensional structures, which gave rise to effective stratigraphic lateral variation and mechanical heterogeneities. Structural analysis of both syn-sedimentary faults and back-thrusts were therefore compared in order to discuss the possible role of the pre-existing anisotropies in influencing the evolution of the back-thrusts. As a result, it can be reasonably proposed that back-thrusts trajectories and stacking pattern were controlled by relevant syn-sedimentary normal faults; these latter were reactivated, in some cases, if properly oriented. Such an issue adds new inputs for discussing the potential role of structural inheritance during tectonic inversions, and helps to better understand the processes suitable for the development of back-thrusts in the inner zones of orogenic belts, as it is the case of the inner Northern Apennines.     

Kinematic evolution of thrust-related structures in the Umbro-Romagnan parautochthon (northern Apennines, Italy)

In the internal part of the Umbro-Marchean-Romagnan Apennines, the foredeep clastic wedge constituting the Neogene part of the sedimentary cover is completely detached from the underlying Mesozoic–Palaeogene succession. The resulting (Umbro-Romagnan) parautochthon consists of tectonostratigraphic units with a general geometry of broad synclinal blocks separated by narrow faulted anticlines.
Thrust-related structures observed in the field require thrust ramp propagation to have occurred within already folded rocks; therefore, they cannot be restored using simple fault-bend fold or fault-propagation folding models. Evidence for a passive fold origin in the studied rocks suggests that an early detachment folding episode preceded ramp propagation. The latter was facilitated by the enhanced thickness of incompetent material in the cores of detachment anticlines, which became the preferential sites where thrust ramps cut up-section. Depending on the trajectory of such thrust ramps, different types of fault-related structures could develop. Hanging-wall anticlines which give way to monoclinal structures higher up in the section are associated with listric thrust ramps, whereas hanging wall monoclines approximately parallel to the underlying fault surface are associated with straight-trajectory ramps.
This kinematic evolution, which occurred partly during syn-depositional compression, also accounts for the observed lithofacies distribution. The latter reflects an early differentiation of the foredeep trough into sub-basins that are progressively younger towards the foreland. The detachment anticlines that originally bounded such sub-basins were the site of later thrust propagation, leading to a tectonic juxtaposition of different tectonostratigraphic units consisting of broad NW-SE elongate synclinal blocks.     

Tectonics and quaternary evolution of the Northern Apennines watershed area (upper course of Arno and Tiber rivers,Italy)

This work examines the connection between Quaternary tectonics and erosion/incision processes in the primary Tuscan‐Romagna watershed of the Northern Apennines, which essentially coincides with the topographic culmination of the Nero Unit structural ridge. Tectonic and geomorphic information were collected in the area where this ridge is crossed by the upper Tiber River course forming a deep gorge. Structural analysis and field mapping have revealed that the region experienced polyphase tectonics with superposed thrust folding events identifiable both at the map and mesoscopic scales. Hinterland‐SSW‐verging thrusts and thrust‐related folds deformed the whole thrust pile during the latest deformation phase. Backthrusts/backfolds controlled the development of intermountain basins nearby the main watershed during the Early Pleistocene and seemingly deformed, in the Tiber gorge, a low‐relief landscape developed in the Early Pleistocene (ca. 1.1 Ma). Successively, the upper Tiber River course area and Apennines axial zone underwent a generalized uplift, which is manifested by the deep incision of palaeo‐morphologies. This proposed sequence of events correlates well with the major geodynamic change of the Apennines revealed by an acceleration of uplift rates in the Middle–Late Pleistocene. This latter event may also correlate with increased rates of river incision recorded in Europe as a consequence of uplift and/or climate change. Copyright © 2008 John Wiley & Sons, Ltd.     

Lithologic transition and bed thickness periodicities in turbidite successions of the Antola Formation, Northern Apennines, Italy

The Antola Formation of Upper Cretaceous age crops out extensively in the Northern Apennines and consists of graded units of calcareous sandstones, sandstones, marlstones, and shales. It can be subdivided into the Cerreto, Antola Marlstone, Bruggi, and S. Donato Members on the basis of bed thicknesses and percentage of shales. Although the whole formation is interpreted as a deep-sea basin plain deposit, the members constitute lateral facies subdivisions which range from proximal, thick-bedded turbidities that show a prevalence of thinning upward cycles in bed thicknesses to distal turbidites that show predominantly thickening upward cycles and have a high percentage of shale. Repetitive patterns in the lithological sequence of the turbidite association are generally distinctive and are satisfactorily described as first order Markov chains. Only the Antola Marlstone Member has an additional second order Markov property. Imaginary eigenvalues of the transition probability matrices of all but the Bruggi Member demonstrate a strong cyclic character in the lithologic ordering within the formation. The behaviour of the Antola Marlstone and of the Bruggi may reflect the influence of a secondary ophiolitic intra-basinal source of clastics that contributed sandy turbidites and olistostromes. Systematic long-term variations in the sequence of bed thickness development in some sections of the Antola Formation are often subtle and equivocal, and pose special problems in interpretation. Fourier analysis was applied to the task of partitioning fundamental wavelengths from “background noise” introduced by essentially random depositional processes. In all members there is (1) strong short-term wavelength of two to three beds indicative of alternating thin and thick beds and judged to be typical of turbidite sequences; (2) an intermediate wavelength ranging from about five beds (proximal facies), eight beds (distal) to nine beds (very distal), which have both thinning and thickening upward trends, interpreted respectively as valley fill due to shifting talwegs of low density turbidity currents, and to progradational, flat turbidite lobes; (3) a poorly defined long-term wave-length of from thirty to greater than sixty beds that may be related to an unspecified trend in the evolution of the sedimentary basin. Phase angles associated with the coniputed power spectra give indications as to the asymmetry (thickening or thinning upward) or symmetry of the representative units.