Abstract

The Macigno Fm. represents one of the most important foredeep clastic wedges of the Northern Apennines. It is a deep-sea turbidite unit Late Oligocene-Early Miocene in age. The Macigno Fm. is referable to the first depositional episode of the diachronously migrating foredeep system, which characterises the collisional phase of the Apennine chain (Ricci Lucchi, 1986). Although the Macigno Fm. crops out widely in Northern Tuscany, it forms only scattered outcrops in Southern Tuscany, and because of only scarce literature is available about it. In this paper, I studied four critical areas in Southern Tuscany in order to add new data for a better knowledge of the Northern Apennines foredeep sedimentation. They are located on an east-west transect, transverse to the chain-foredeep system and they are, from the innermost to the westernmost, Piombino-Baratti in the coastal area, Sassetta in the Toscana marittima, Poggio Ritrovoli in the Colline Metallifere, and Rigomagno south to the Chianti Mountains. Because of the quality of the exposures, the research methodologies provided two levels of detail. First, geological surveys, based on S/M ratio lithofacies, produced subdivisions of the turbidite bodies in depositional units, while, on a second level of detail, the study of better exposed and particularly representative sections led to facies analyses. The latter, by means of facies definitions and facies associations, allowed the recognition of turbidite environments and depositional elements characterising the architecture of the turbidite systems. Moreover, stages of deposition have been recognised. A prograding system characterises the Baratti succession. It shows lobe stages in the lower-middle portion and a channel-lobe transition stage up to a channel-slope stage in the upper portion of the system. The succession ended with a synsedimentary allochthonous thrust onto the turbidite deposits. Lobes present distal to proximal depositional aspects, whereas the main channel complex shows features of close interaction between the orogenic wedge of the slope and the turbidite sedimentation. The more external successions of the Sassetta, Poggio Ritrovoli and Rigomagno areas show a similar depositional organisation among them, but different from that of the Baratti succession. In fact, these external successions define an evolution of the sedimentation from lobe stages to a thick stage of mainly mud sedimentation. This last stage relates to the basin starvation and precedes the closure of sedimentation due to the thrusting of the allochthonous nappe. Thickening-coarsening upward trends, changing from distal to intermediate environment, sometimes to proximal lobe environment, mainly characterise lobe stages. Mudrocks and interlayered olistostromes coming from the inner slope characterise the top starved deposits. Rare limestone beds are present, particularly in the Poggio Ritrovoli succession; they testify the presence of carbonate platforms generating turbidites in the foredeep basin. The depositional system characterising the external successions started in the westernmost areas and moved toward the easternmost areas, with a progressive lateral time-space shifting. Since the depositional aspects and organisation of the successions show marked differences between the Baratti area and the other areas, two kinds of turbidite systems are identified. A low-efficiency type for the Baratti area, named "Macigno costiero", and a high-efficiency for the other, named "Macigno appenninico". Moreover, the "Macigno costiero" shows progradational evolution from basinal to slope conditions due to the advancing of the orogenic wedge, with a transversal feeding to the basin. Turbidites came from storage shallow-marine basins in turn sourced from the Corso-Sardinian continental block. In contrast, the "Macigno appenninico" turbidite system is characterised by feeding and progradation longitudinal to the basin, and it shows a lateral shifting of the sedimentation following the migration of the basin depocentres. The source area of the high-efficiency system has been placed in the continental basement cropping out in the Western-Central Alps. In conclusion, I identify a depositional-tectonic evolution for the Late Oligocene-Early Miocene foredeep system of Southern Tuscany, which fits well into the depositional model known for the more northern and external foredeep. In particular, I detected a time-space sequence. In the early stages, it provides for the development of a transverse basin low-efficiency turbidite system in the inner portion, emplaced in a partially confined open piggyback basin or in an inner portion of a complex foredeep. During the later progradational stages of this system, a more external high-efficiency turbidite system developed by means of longitudinal basin feeding. This kind of turbidite system marked the activation of the classical Northern Apennines foredeep deposition, characterised by a gradual shifting of the basinal depocentres. This phase of sedimentation generated more turbidite wedges of the same type of system, but progressively younger toward the external sectors. A typical evolution of the turbidite systems recorded for these high-efficiency systems, shows the following sequence: normal filling of the basin and development of lobes-underfeeding of the inner part of the basin and development of starved deposits and olistostromes-closure of the sedimentation by synsedimentary thrusts-shifting of the whole system. Therefore, the Northern Apennines collisional deposition went through a radical change at the transition Oligocene/Miocene

You do not currently have access to this article.