Several mountain arcs formed in the Mediterranean area during the Alpine orogeny, among them the Northern Apennines. They show diachronous development with the outer thrust front prograding eastward, and being progressively replaced to the west by relatively thin, post-orogenic extensional or transtensional basins. The outer and inner parts of the orogen are linked together through a series of transverse structural lineaments. Segments of such lineaments have through time acted as transcurrent faults, lateral ramps of thrusts, strike-and oblique-slip faults, and normal faults. The main lines of evidence about transverse lineaments of the Northern Apennines are reviewed, and their effects on some Neogene-Quaternary basins of the inner part of the orogen in Tuscany and the northern Tyrrhenian Sea shelf are assessed. New information from recently released commercial seismic profiles and from surface sedimentological studies has made it possible to confirm that the post-orogenic basins formed in tectonic depressions delimited by major, quiescent substrate thrusts. The depressions were longitudinally separated into basins by the transverse lineaments. The stratigraphy of the basins in each tectonic depression is similar; in most cases, initial narrow syn-rift sedimentation was followed by extensive post-rift successions due to thermal subsidence. However, the thickness and distribution of their sedimentary sequences vary according to different subsidence (or uplift) and extension that have occurred along each side of the same transverse lineament, or in blocks delimited by different lineaments. Furthermore, portions of the lineaments, such as those of the Livorno-Sillaro, may have temporarily acted as strike-slip faults (late Miocene-Pliocene, in this case), and equivalent substrate highs and parts of the same basin may have been shifted left-laterally for about 15-20 km. A further effect of the transverse lineaments on basin sedimentation has been the development of major alluvial fans at relay ramps developed near the intersection of lineaments and quasi-orthogonal, listric boundary faults.
The late Miocene-Pliocene sedimentary fill of the Siena Basin (Tuscany, Italy) consists dominantly of clastics and has internal architecture that reflects the interplay of tectonics, relative sea-level changes and climate variations. Pliocene sediments are extensively exposed and overlay both late Miocene deposits and pre-Neogene bedrock. Specifically, Pliocene basin margin sediments consist largely of sand with gravel and mud intercalations, deposited mainly in nearshore settings with minor fluvial depositional episodes. They grade basinward to dominantly offshore fines with intervening turbiditic sand bodies. New fieldwork revealed that basin margin deposits, notwithstanding lithologically rather homogeneous, are made of a variety of sedimentary facies and bear several unconformities. They have been traditionally described and mapped using lithostratigraphic criteria, that have proven to be unfit to represent such complex stratigraphic architectures. The aim of this paper is to describe the allostratigraphic architecture of the Pliocene deposits exposed in a marginal key-area (45 km 2 ) of the northern Siena Basin by means of a 1:10,000 scale geological map. The recognized succession of allostratigraphic units and their bounding discontinuities, along with new biostratigraphic data from calcareous plankton, provides new insights into the geological history of the Siena Basin and represents valuable constraints for long-distance correlation.
A carbonate terraced succession mainly consisting of fluvial/palustrine calcareous tufa and of lacustrine limestone was deposited during recent Quaternary in a limited segment of the Valdelsa fluvial pattern (southern Tuscany, Italy).The radiometric data obtained from three carbonate terraces indicate that the depositional/erosional history of the Valdelsa succession during Late Pleistocene–Holocene, has been constrained by the same cyclic events observed in coeval detrital lacustrine successions of Central Italy. At least three of the Valdelsa carbonate Synthems and the interposed erosional phases can be correlated with the major climatic changes recognized in the European–Mediterranean area, from the Last Glacial Interstadial through the Younger Dryas to the Atlantic ‘Optimum Climatic’, the Sub-Boreal and finally at 2.5 ka the last Sub-Atlantic oscillation.This climatic correlation and the radiometric data imply that the deposition of calcareous tufa in Valdelsa was mainly dependent on rainfall availability and, consequently, was active during the milder oscillations within the cold periods.
The detection of detached nearshore wedges formed in response to relative sea-level drops is considered one of the hottest topics in sequence stratigraphic analysis due to their importance as reservoir analogues. In fact, they usually constitute sandy and porous bodies generally encased in impermeable clay, thus presenting a good potential as traps for fluids. This paper focuses on the sequence stratigraphic analysis of the Pliocene deposits cropping out in the central-southern sector of the Siena Basin (Tuscany, Italy), a post-collisional basin of the Northern Apennines. The exposed sedimentary succession was investigated through a detailed sedimentological and stratigraphic approach, integrated by biostratigraphic analyses, aimed at a better characterization of the infilling history of this sector of the basin. Specifically, this study revealed the occurrence of repeated facies shifts that allowed the identification of two depositional sequences. In detail, a thick sand-rich body far from the basin margins, and previously considered as a turbiditic lobe, has been reinterpreted as formed in a nearshore setting during a fall in relative sea level. This body is totally encased in offshore clay, and due to the lack of physical connection with the related HST deposits, it has to be considered as a detached forced-regressive wedge. The present work led to the recognition of some sedimentological and stratigraphic features typical of falling stage systems tract deposits (e.g. presence of intrabasinal recycled materials, sedimentological evidence of a pre-existing fluvial network subsequently eroded) that can provide useful clues for the identification of detached forced-regressive nearshore wedges in core studies and poorly exposed settings
The recent recognition and description of terminal distributary channels in ancient delta-front deposits have led to important progress when it comes to understanding: delta construction processes; the plan-view shapes of both modern and ancient river-dominated deltas; and sediment dispersal within deltas. Deltaic deposits fed by terminal distributary channels can be observed in the extensive outcrops (approximately up to 300 m wide and 50 to 70 m thick) in the central sector of the Pliocene Siena Basin in Tuscany, Italy. Stratigraphic constraints indicate that the deposition occurred in relatively shallow marine settings, was not affected by tidal effects and was influenced only locally by wave action. As a consequence, these rock exposures provide an opportunity to investigate the facies assemblage of an ancient delta complex, which was built up almost exclusively as the result of river-related processes. The studied deltaic succession is composed of vertically stacked parasequence-like units, which are invariably characterized by the lack of coarse-grained distributary channel deposits at the top and an extensive network of terminal distributary channels that controlled the deltaic deposition. These deposits were interpreted previously as deep-water turbidites. This study provides new elements for the correct distinction between hyperpycnal-dominated delta fronts and basinal turbidite deposits. The terminal distributary channel facies documented in this study are more complex than those investigated in previous research. The channels are often passively filled during abandonment phases, and not just due to the aggradation and lateral/upstream migration of mouth bars, as classically proposed. Moreover, the frequent occurrence of mudstone beds just above the basal channel scours may be indicative of an abandonment phase after the channel incision, and before its infilling, with subsequent implications for the formational and infilling mechanisms of terminal distributary channels. Furthermore, the channel fills are often the result of alternating depositional and erosional phases, leading to a complex stratigraphic arrangement
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