Facies associations map of the Pliocene marine deposits of the central-southern Siena Basin (Tuscany, Italy)

Abstract

The Siena Basin is a post-collisional basin of the inner Northern Apennines (Tuscany, Italy) characterized by a thick siliciclastic Neogene infill, mainly composed of marine sediments with subordinate alluvial deposits close to basin margins. The central-southern sector of the basin shows a more complex stratigraphy with the occurrence of sandy deposits also in distal areas, far from the basin margin. The aim of this paper is to provide a new 1:10,000-scale geological map of this key sector (about 45 km²) of the Siena Basin, helpful for a better reconstruction of its sedimentary evolution. The new fieldwork was based on the identification and mapping of different facies associations (expression of different sedimentary environments), whose shifts in time and space provide elements to understand the basin-fill history. The recognition of two main intra-Pliocene erosional surfaces allowed the subdivision of the succession into three alloformations. Therefore, a more complex depositional history, with respect to the previous knowledge for this key-sector of the Siena Basin, has been reconstructed, thus highlighting the importance of this kind of approach with respect to the classical lithostratigraphic criteria.

Keywords:

• Siena Basin
• Pliocene
• allostratigraphy
• facies associations

1. Introduction

The Siena Basin (Figure 1) is one of the several post-collisional basins in the western side of the Northern Apennines (Martini & Sagri, 1993). The Siena Basin has a NW-SE elongated shape (parallel to the direction of the Apenninic belt), whose boundaries are defined by pre-Neogene bedrock ridges (i.e. the Montagnola Senese and Murlo-Montalcino ridges on the western side and the Chianti Mounts – Rapolano ridge on the eastern one) and transversal subsurface bedrock highs. Such transversal highs divide the Siena Basin from the surrounding Casino and Radicofani basins (to the north and to the south, respectively) and have been interpreted as the expression of transversal tectonic elements (Pascucci, Martini, Sagri, & Sandrelli, 2007 and references therein).

Figure 1. Simplified geological map of southern Tuscany (modified from Martini et al., 2011).

The origin of the Siena Basin is still a matter of debate; traditionally, it has been interpreted as a half-graben (Costantini, Lazzarotto, & Sandrelli, 1982) formed in response to late Miocene-Quaternary extensional tectonics (Bossio et al., 1993; Carmignani et al., 2001; Pascucci, Merlini, & Martini, 1999) or as related to a low angle detachment fault (Brogi, 2011). On the contrary, a compressional regime model is proposed by other researchers (Bonini & Sani, 2002; Finetti et al., 2001).

Regardless of its structural origin, the Siena Basin hosted siliciclastic continental and marine sedimentation during the Neogene. Its sedimentary infill has been traditionally investigated through lithostratigraphic criteria (Bossio et al., 1992, 1993; Costantini et al., 1982), hence detailed sedimentological and stratigraphic studies are few (Gandin & Sandrelli, 1992; Martini, Aldinucci, Foresi, Mazzei, & Sandrelli, 2011) or limited to some fossiliferous sites (Manganelli, Martini, & Benocci, 2011; Manganelli, Spadini, & Martini, 2010). The sedimentation started in the late Miocene with the deposition of a thick fluvio-lacustrine succession (Lazzarotto & Sandrelli, 1977). Miocene continental sediments are unconformably overlain by Pliocene marine deposits generally interpreted as deposited during a single transgressive-regressive cycle, starting at the base of the Zanclean and lasting up to the late Piacenzian (Bossio et al., 1992). Pliocene marine deposits consist of nearshore sands (San Vivaldo Sand) and conglomerates (Gambassi Conglomerate) passing basinward to offshore mud (Argille Azzurre, Bossio et al., 1992) with intercalated decametres-thick sand-dominated bodies (Chiusure Sand) that have been interpreted by Gandin and Sandrelli (1992) as turbiditic sand lobes emplaced by gravity-flows. Local episodes of continental sedimentation have been reported by Aldinucci, Ghinassi, and Sandrelli (2007) and Martini et al. (2011) at the base of the Pliocene succession in the eastern margin of the basin, and by Bossio et al. (1992, 1993) within the marine deposits of the southwestern sector.

The end of marine deposition in the Siena Basin is related to a general uplift affecting southern Tuscany that caused its progressive emersion. Quaternary continental deposition is represented by discontinuous alluvial deposits (Aldinucci et al., 2007) and travertine (Brogi, Capezzuoli, Aquè, Branca, & Voltaggio, 2010).

This work presents new collected data and a resulting allostratigraphic architecture of a key area of the Siena Basin (Figure 1), located in its central-southern sector, through a 1:10,000-scale facies associations map. A more detailed and complete history for this sector of the basin has been reconstructed.

2. Methods

The studied sector of the Siena Basin (about 45 km² comprised in sheets 308030, 308040, 308070 and 308080 of the topographic maps from ‘Carta Tecnica Regionale’ of Tuscany) was investigated through geological mapping at 1:10,000-scale, performed during 2009–2011. Facies analysis and allostratigraphic concepts (NACSN, 2005) were applied to the exposed Pliocene deposits. Pre-Neogene bedrock and Quaternary deposits were not differentiated since they do not constitute the topic of this work.

The studied succession was divided into alloformations bounded by unconformities: among several possible discontinuity surfaces (Bhattacharya & Walker, 1991; Walker, 1990), unconformities were chosen as boundaries since they are easily recognizable and traceable in the field. Each alloformation is composed of different facies associations (Walker, 1992) representing the expression of sedimentary environments.

Bio- and chrono-stratigraphic constraints for the recognized alloformations are taken by Martini, Arragoni, Aldinucci, and Sandrelli (2010).

3. Results and conclusions

Three alloformations (labelled A1 to A3 in upward stratigraphic order) were recognized in the investigated area, each one bounded by unconformities.

Alloformation A1 (late Zanclean) crops out at the eastern margin of the basin (Montisi – Castelmuzio area) where it unconformably overlies the deformed pre-Neogene bedrock (Figure 1). It is in turn overlain by deposits pertaining to the A3 Alloformation through an erosional surface recording subaerial erosion. A1 is mainly composed of deltaic deposits (i.e. shallow water deltas, Dunne & Hempton, 1984; Wood & Ethridge, 1988), represented by metres-thick coarsening-upward sandy lithosomes characterized by generally well-developed plane-parallel stratification that are gently inclined seaward. Sedimentary structures recording the actions of sea-waves are locally preserved (e.g.: hummocky- and swaley- cross stratification, symmetrical ripples), and fragments of marine molluscs and bioturbation are common. Deltaic sediments transitionally overlie and interfinger with alternating muddy and sandy beds (prodelta deposits), passing westward (seaward) to offshore mud, locally forming thick monotonous successions (Figure 2A).

Figure 2. A- Outcrop expression of offshore deposits; B- Alternations of sandy and muddy beds typical of offshore transition facies association; C- Outcrop view of sandy shoreface deposits (circled hammer for scale). Note the diagnostic occurrence of hummocky-cross stratified sandstone beds; D- Close-up view of a tempestite bed, note the abundant mud-clasts and the shell fragments (pencil for scale); E- Beach gravels erosionally overlying shoreface sands (circled spatula for scale); F- Clinostratified sands of A3's deltaic deposits (the building is 5 m high).

Alloformation A2 (early Piacenzian) is exposed in the westernmost part of the study area (surroundings of San Giovanni d'Asso village) where it forms a thick nearshore sand-dominated wedge, sub-parallel to the eastern margin. Alloformation A2 shows at the base offshore mud transitionally passing upward to nearshore deposits. The latter are represented by the following facies associations: (i) interbedded mud and sand (offshore transition deposits, Figure 2B), (ii) wave-influenced sandy deposits (shoreface deposits, Figures 2C–D), and (iii) gravel and sand (beach deposits, Figure 2E). Nearshore facies associations are organized in coarsening- and shallowing-upward successions. In detail, sandy shoreface sediments show well-developed plane-parallel and ripple-cross laminations. Storm-related beds and structures (e.g. hummocks, swales, tempestites; Kumar & Sanders, 1976) are locally present within shoreface deposits (Figure 2D). Beachface sediments are represented by planar cross-bedded gravels (Figure 2E), showing shape and size segregation typical of gravelly beaches (Bluck, 1967; Postma & Nemec, 1990). Clasts are mainly derived by pre-Neogene Formations (e.g. limestone, cherty limestone, radiolarite) while others are composed of recycled intra-basinal materials, like lagoonal mud clasts, sandstone concretions bored by Lithophaga, and boulders of cemented gravels.

The geographic position of A2 nearshore deposits with respect to the older A1 nearshore sediments indicates that a significant basinward facies shift has occurred. Considering the areal distribution of the coastal deposits, it is estimated that the facies shift was of at least 3.5 km towards the sea (distance between the Castelmuzio-Montisi area to the San Giovanni d'Asso area). According to Martini et al. (2010), the movement of the coastal system was related to an important sea-level fall that occurred at the Zanclean-Piacenzian boundary, and this event determined the emersion and erosion of the A1 deltaic deposits and the deposition of A2 sediments on top of A1's offshore sediments. Consequently, the A2 basal boundary is represented by an erosional surface that corresponds to a regressive surface of marine erosion (sensu Plint, 1988). The upper boundary of alloformation A2 is a high-relief unconformity (Figures 3A-B-C), whose origin is related to a further sea-level fall, that caused the emersion and subsequent erosion of the A2 nearshore wedge.

Figure 3. A- Panoramic view of the boundary between A2 and A3 in the surroundings of San Giovanni d'Asso village; B- Sketch of Figure 3A, highlighting the stratigraphic relation between the two alloformations. The solid red line indicates the unconformity which separates A2 and A3; C- Close-up view of the unconformity (highlighted by the solid red line) between A2 and A3 (road cut along the road connecting San Giovanni d'Asso and Montisi villages); D- Angular unconformity (dashed red line) between A3 and pre-Neogene bedrock (surroundings of Montisi village).

The presence of intra-basinal recycled materials and coarse-grained bedrock clasts within A2's beach deposits suggests the existence of a drainage system capable of transporting coarse materials from the bedrock ridge and the exposed older Pliocene sediments. Nonetheless, no traces of fluvial systems are preserved in the study area, probably due to the strong erosion related to the formation of the unconformity on top of the A2 deposits.

Finally, alloformation A3 (late Piacenzian) rests unconformably on A1 and A2 deposits and pre-Neogene bedrock (Figure 3D). It is mainly represented by offshore marine deposits cropping out in the central part of the study area and passing landward (eastward) to deltaic and coastal deposits (Montisi-Castelmuzio area, Figure 2F). Close to the basin margin, the sandy deltaic deposits pertaining to A3 show strong lithological and sedimentological similarities with the underlying A1 deposits.

A3 deposits testify an important sea-level rise which re-established marine conditions in a previously emerged area. This caused an abrupt facies superimposition, especially in the central part of the basin where marine A3's offshore mud directly overlies A2's beach and shoreface deposits.

However, typical transgressive-related deposits like shell beds (Kidwell, 1991), lags and other condensed beds (Amorosi, 1995; Chiarella, Longhitano, Sabato, & Tropeano, 2012), are lacking in the investigated area.

In conclusion, this new fieldwork, based on the mapping of both the areal distribution of facies associations and major erosional surfaces, permitted the subdivision of the studied succession into three alloformations (Figure 4). The resulting stratigraphic framework provides a more complex depositional history than previously known. The origin of such surfaces is related to base-level variations that produced mappable seaward and landward facies-shifts. Furthermore, the detailed geological map of the distribution of facies associations provides helpful elements for quantifying the magnitude of such events, hence highlighting greater effects on the sedimentary succession than the ones observable at outcrop-scale.

Figure 4. Simplified geological scheme of the investigated area (not in scale). Solid red lines indicate unconformities (boundaries of each alloformation). The white area is indicative of the lowermost part of the Neogene succession, not exposed in the distal sector of the investigated area.

Software

The geological map and associated geological section were compiled by scanning hand drafts as black and white TIF files, and then digitizing the linework using the Adobe Illustrator CS5 graphics package.

Acknowledgements

This work derives from the MSc thesis of the first author and forms part of the PhD thesis of Ivan Martini. Antonella Mancini is thankfully acknowledged for the assistance in drawing the map. Thanks to Regione Toscana for permission to publish the topographic map of the studied area. Dr. Domenico Chiarella (Weatherford Petroleum Consultants, Bergen, Norway) and two other anonymous reviewers are thanked for their constructive comments on the manuscript and helpful suggestions that improved the final paper.