Quaternary marine and continental unconformity-bounded stratigraphic units of the NW Sicily coastal belt

ABSTRACT In the coastal sector of NW Sicily, the regional correlation of relevant unconformities recognised within the Quaternary sedimentary successions allowed the mapping of seven unconformity-bounded stratigraphic units (UBSUs). The regional unconformities are marine or subaerial erosional surfaces, as well as non-depositional surfaces, locally marked by paleosoils. The erosional surfaces were produced from marine abrasion, surface water overland/concentrated flow, river erosion, karst solution, mass movement, or wind erosion. The main lithofacies of the Quaternary UBSUs consist of: (a) marine and coastal bioclastic calcarenites, (b) aeolian sandstones, (c) river deposits, (d) colluvial deposits, (e) talus slope deposits, (f) landslide deposits, and (g) chemical carbonates (travertines and speleothems). Quaternary environmental changes, due to tectonics, climate, and sea-level oscillations, are the causes that favoured the development of erosion/deposition processes responsible for the genesis of unconformities and deposits. As a result, through the UBSU map of the NW Sicilian coastal belt, it is possible to: (i) recognise stratigraphic units controlled by tectonic, climatic, and environmental processes (and their interplay) and (ii) detect Quaternary sedimentary evolution.


Introduction
Synthem stratigraphy (Chang, 1975) is a stratigraphic tool aimed at defining the unconformity-bounded stratigraphic units (UBSUs) as 'bodies of rocks bounded above and below by significant unconformities of regional extent' (Murphy & Salvador, 1999;Salvador, 1987). UBSUs have been established because they can fulfil a need that other kinds of stratigraphic units cannot meet, and are the expression of stratigraphic concepts for which other units are inadequate. In particular, the UBSUs are the units that best allow for a stratigraphic classification in areas affected by cyclical phenomena, such as tectonically and climatically driven sea-level changes. In these areas, affected by depositional and erosional events, UBSUs provide easy stratigraphic synthesis. They are rarely used as more attention is given to sequence stratigraphy, but synthem stratigraphy has been effectively applied to Mesozoic and Cenozoic successions around the world (Basilone, 2009;Benvenuti & Degli Innocenti, 2001;Benvenuti, Papini, & Rook, 2001;Ruban, Zerfass, & Pugatchev, 2009;Sacchi, Horvath, & Magyari, 1999;Vezina, Jones, & Ford, 1999;Zavala, 2000).
The aim of this paper, which includes the description of the UBSUs and their unconformities, is to provide a new large-scale map (1:100,000) that helps to operate intrabasinal correlations at regional scale across NW Sicily coastal belt. It is useful for understanding (i) the genetic and morpho-evolutionary meaning of the unconformities separating different units, (ii) the role of paleosoils and other indicators marking the discontinuities, and (iii) the interplay of environmental changes, climatic fluctuations, and tectonics acting during the Quaternary in NW Sicily.

Geological setting
NW Sicily is a part of the Sicilian Fold and Thrust Belt (FTB), a segment of the Alpine collisional belt, described as the result of both post-collisional convergence between Africa and Europe and roll-back of the subduction hinge of the Ionian lithosphere (Figure 1; Catalano, Valenti, et al., 2013). The rocks forming the NW Sicily FTB are made up of Meso-Cenozoic shallow-and deep-water carbonates pertaining to the deformed paleogeographic units of the Southern Tethyan margin (Basilone, Frixa, Trincianti, & Valenti, 2016;Basilone, Lena, & Gasparo Morticelli, 2014;Catalano, Basilone, et al., 2013) and tertiary clastic deposits including the deformed foredeep units (e.g. Numidian flysch) and the filling deposits of the wedge-top basins (Gasparo Morticelli et al., 2015;Gugliotta et al., 2014). The thrusted bodies were displaced along dip-slip and strike-slip NW-SE-and NE-SW-oriented faults, with up to several hundred metres downthrown ( Figure 2; Catalano et al., 2011;Catalano, Basilone, et al., 2013). This extensional and transtensional tectonics acted during the Plio-Quaternary and was related to the opening of the Tyrrhenian Sea (Billi, Presti, Orecchio, Faccenna, & Neri, 2010;Malinverno & Ryan, 1986). Slow but extended uplift and climatically driven sealevel changes have controlled the Quaternary erosion/ deposition processes along the coastal sector of N Sicily and its offshore (Di Maggio et al., 1999;Incarbona et al., 2010;Mauz, Buccheri, Zöller, & Greco, 1997;Pepe et al., 2003). Horst, graben, and half-graben bordered by wide fault scarps characterise the present-day morphotectonic setting (Agnesi et al., 2000;. The structural lows were the sites where the continental and marine Quaternary deposits accumulated ( Figure 2).

Methodologies
Field mapping was carried out using published base maps (1:10,000 scale map of the Regional Technical Cartography -CTR -Regione Siciliana). Satellite (Google Earth) and aerial images were analysed in order to recognise the main morphostructural features. The geological map is presented at a scale of 1:100,000 in a Transverse Mercator Projection.
The geological mapping was accomplished through a detailed field survey (1:10,000 scale map) performed to collect stratigraphic and lithological data of the mapped UBSUs. Both physical-stratigraphy and facies analysis were applied to several stratigraphic sections in order to define the nature of the unconformities, their stratigraphic relationships (vertical and lateral), and the sedimentological characteristics of the deposits. Microfacies analysis was used to recognise texture and diagenetic features of the hard-rock lithofacies and microfossil content. Photo-geology was useful to define the morphotectonic setting and to classify the different orders of river and marine terraces.

The geological map
The Geological Sheet consists of the Main Map and some sketches displaying various geological outputs: . the lithological map (Figure 1/Geological Sheet) allows us to locate the mapped area within the NW Sicily FTB; . correlation between the logged sections (Figure 2/ Geological Sheet) helps us to understand the stratigraphic setting at a regional scale.
The Geological Map presents the NW Sicily coastal belt from the S. Vito lo Capo peninsula up to the Capo Plaia promontory, across the Gulfs of Castellammare, Palermo, and Termini Imerese. Shaded relief has been used as background for the map to highlight topography. The study area encompasses the northernmost sector of the outcropping Sicilian FTB where the S. Vito, Palermo, Termini, and Madonie mountain complexes are separated by the intervening Castellammare, Palermo, and Termini flat coastal areas, and by deep river valleys. The various orders of the marine or river terrace deposits (i.e. Barcarello, Buonfornello, and Imera synthems) have not been distinguished due to the small scale of the map. Instead, the map highlights the large extent of the unconformity surfaces that record the effect of environmental and sea-level changes due to climatic fluctuations and tectonic movements at regional scale.

Unconformities and related UBSUs
The main sedimentological and lithological features of the several lithofacies recognised in the mapped UBSUs are reported in Table 1 and in the legend of the map. In the following, the unconformity surfaces, deposits, and stratigraphic relationships are illustrated (Figures 3  and 4).

Unconformity 1 and Marsala synthem
Unconformity 1 is a sharp marine erosional (ravinement) surface cutting the Upper Pliocene sandy marls (see Figure 5(a)) and the tectonically deformed Meso-Cenozoic carbonates and clastic rocks ( Figure 5 (e)). Above this surface, marked by rare erosional channels some metres wide, the Lower Pleistocene marine/coastal deposits (Marsala synthem) lie with onlap (landwards) and downlap (seawards) strata terminations. The unit, with tabular geometry, frequently displays clinostratification. It is well exposed along the flanks of the river valleys crossing the present flat coastal areas. Grey-bluish clays are intercalated mostly along the Palermo plain (Ficarazzi clays). Both in the Balestrate, here indicated as the type section of the unit, and in the Punta Raisi supported sections (Figure 2/Geological Sheet), it is possible to observe the internal textural and compositional features of the synthem and the characteristics of its lower unconformity ( Figure 5). Colluvial deposits and reworked soils with vertebrate rests of the Monte Pellegrino Faunal Complex are found in a karstic cave at Mount Pellegrino (Burgio & Fiore, 1997) and are believed to   Cita et al., 2006) and calcareous nannofossil (2: Rio et al., 1990) biozones, mammal faunal complex distribution (Bianca et al., 1999;Bonfiglio et al., 2008;Masini et al., 2008). Table 1. Synoptic table summarising stratigraphic and sedimentological features, thicknesses, depositional environments, fossil content, and ages of the Quaternary UBSUs (main references: Bonfiglio et al., 2003Bonfiglio et al., , 2008Burgio & Fiore, 1997;Di Maggio et al., 2009;Di Stefano & Rio, 1981;Hearty et al., 1986;Incarbona et al., 2016;Mauz et al., 1997;Ruggieri, 1973;Ruggieri, Rio, & Sprovieri, 1984 pertain to this synthem. On the basis of the fossil content (Ruggieri, 1973), the age of the synthem is referred to the Calabrian (Emilian-Sicilian 'Italian marine stages', Cohen & Gibbard, 2016 Mauz et al., 1997) shows these deposits as older than the MIS 19 ( Figure 3). The continental deposits were dated to 1.6-0.7 Ma (Bonfiglio et al., 2002;. Data show that the lower unconformity age is about 1.6-1.5 My and the unit spans between the Upper Calabrian and the early Middle Pleistocene. These deposits extensively crop out along the whole coastal belt of NW Sicily, from Marsala to Trabia flat coastal areas, and its offshore (Agate et al., 1993). In the Marsala and Castellammare del Golfo plains (see Figure 1/Geological Sheet), these deposits, 80 m thick, fill tectonic depressions opening to the sea, and inland they are bordered by wide tectonically controlled relict sea cliffs derived from former fault scarps. In the Palermo plain (Figure 1/Geological Sheet), they reach 40-50 m of thickness and are mainly represented by bluish clays at the bottom followed upwards by white to yellow calcarenites.
As suggested by the nature of the lower unconformity and the facies of the overlying deposits, tectonics and glacio-eustatic sea-level changes drove the formation of the synthems. The lowermost Pleistocene extensional tectonics produced a differential subsidence, which formed steep fault blocks where the shallow-water sediments, arranged in high-frequency (glacio-eustatic) cycles, accumulated.

Unconformity 2 and Buonfornello synthem
Unconformity 2 is an erosional marine surface (ravinement) carved into the Marsala synthem or older rocks. This surface is overlain by a few metres of thick marine, paralic and continental deposits (Buonfornello synthem). These deposits show a tabular geometry and overlay stepped surfaces of marine terraces (abandoned wave-cut platforms and sea cliffs) occurring at different altitudes (from 10 to 250 m a.s.l.) and frequently marked by red paleosoils. The suggested site to observe the main characteristics of these deposits and of their lower unconformity is around the Termini Imerese-Piano di Imera sector, where some sections have been measured (Figure 2/Geological Sheet). The marine/coastal sediments laterally pass to coeval continental deposits (colluvium and travertines) mostly outcropping in karstic depressions or abandoned sea caves. Numerical age dating carried out on littoral deposits (TL, Mauz et al., 1997) and fossil shells (amino acid epimerisation, Hearty et al., 1986) relates these deposits to the Middle Pleistocene sea-level high-stand phases, corresponding to the 'warm' MISs between 19 and 7 ( Figure 3). A similar age is confirmed by the fossil content of the heterophic continental deposits belonging to the Paleoloxodon falconeri Faunal Complex (Bonfiglio, Di Maggio, Marra, Masini, & Petruso, 2003;Masini et al., 2008). Close to Alcamo village (Figure 2), fossil remains of this faunal complex are found within travertines covering a marine terrace surface at 250 m a.s.l. (Burgio & Fiore, 1997). Radiometric age dating carried out on similar vertebrate deposits in several areas of Sicily suggests an age of 455 ±90 ka (Bada et al., 1991;Rhodes, 1996). From the above, the age of the lower unconformity is correlatable to MIS 19 and the synthem deposit age is between MISs 19 and 7 (Figure 3).
Marine terraces and related deposits are discontinuously exposed along the structural lows of the NW Sicily coastal areas, previously filled by the Marsala synthem deposits, which are the result of hundreds of metres tectonic displacement towards the sea sectors (N-wards). They are also exposed along the structural highs corresponding to the remaining uplifted hanging walls of the high-angle extensional faults, where pre-Quaternary rocks crop out. Both regional and tectonic uplift determined widespread emersion with extensive erosion; the interaction between sea-level oscillation (glacio-eustatic cycles) and tectonic uplift is the mechanism generating the unconformities and the marine terrace deposits. Moreover, alternation of arid conditions and wet-warm climate occurred during the Middle Pleistocene was responsible for genesis of colluvial deposits and travertines, respectively.

Unconformity 3 and Imera synthem
Unconformity 3 is a river erosional surface carved on Pleistocene (Marsala and Buonfornello synthems) and pre-Quaternary rocks and covered by pebbly grains, polygenic conglomerates, fluvial channel sands, sandy silt, and clayey silt forming the fluvial deposits of the Imera synthem. These deposits, with tabular geometry, characterise the ancient deposits of most of the NW Sicily river valleys. They hang on along the valley slopes, lying on various orders of river terrace surfaces that are from several metres to one hundred of metres of height with respect to the present-day valley bottom.
Along the younger river terrace deposits, the occurrence of fossil remains of Hippopotamus pentlandi pertaining to the Paleoloxodon mnaidriensis Faunal Complex (Bonfiglio et al., 2003;Masini et al., 2008) dates these deposits up to the late Middle Pleistocene and the early Upper Pleistocene. Numerical age dating of the fluvial conglomerates, sampled along a river valley crossing the Castellammare plain, indicates 227 ± 40 ka age (Mauz et al., 1997). Rests of mammals of the Paleoloxodon falconeri Faunal Complex (Bonfiglio et al., 2003 found along the older river terrace deposits suggest an early Middle Pleistocene age. Therefore, data analysis suggests an early Middle Pleistocene age of the lower unconformity and a Middle-Upper Pleistocene age, up to MIS 5a (Bianca, Monaco, Tortorici, & Cernobori, 1999), of the synthem deposits.
The interaction between alluvial deposition and vertical to lateral river erosion, controlled by climatic changes and by the following fluctuations of the river base level, and the downward migration trend of the river base level due to tectonic uplifting, are responsible for the genesis of unconformity 3 and Imera synthem deposits.

Unconformity 4 and Polisano synthem
Unconformity 4 is partly a subaerial erosional surface and, partly, a non-depositional surface topping the younger marine terrace deposits (MIS 7) of the Buonfornello synthem, as well as older rocks. The age of this unconformity is referred to the MIS 6 ( Figure 3). Cross-bedded and cross-laminated aeolian sands and arenites lie on this unconformity forming the main deposits of the Polisano synthem (Table 1 and Figure 6), which is exposed along the flat coastal areas and at the foot of the bordering slopes. The Macari coastal plain and La Fossa-Gallo Mount sectors are useful sites to observe the main characteristics both of the unit and unconformity 4 (Figure 2/Geological Sheet and Figure 6). Coeval colluvial deposits consisting of breccias and reworked soils, with stone-line structures and mammal rests (Paleoloxodon mnaidriensis Faunal Complex) locally overlay unconformity 4. Based on their fossil content and stratigraphic position (Figure 3/Geological Sheet), the continental deposits of the Polisano synthem are dated to the Middle Pleistocene (MIS 6) corresponding to the Ionian Stage (Figure 3).
The formation of the subaerial erosional unconformity and the non-depositional surfaces occurred during both sea-level fall and the subsequent lowstand stage. Due to the arid cold climate and the following significant lacking of forest-wood vegetation, rare but intense surface water processes (overland or concentrated flows) and strong wind developed. The water processes produced erosion surfaces and colluvial deposits. The wind flows led to the transport of large amount of sands from the exposed continental platform, following sea-level fall, and their deposition along the coastal areas (forming coastal dunes) up to the foot of the adjoining slopes (forming climbing dunes).

Unconformity 5 and Barcarello synthem
Along the coastal areas, unconformity 5 is a marine abrasion (ravinement) surface, above which coastal/ marine deposits with Persististrombus latus onlap the underlying aeolian sands of the Polisano synthem or older rocks; these deposits (Strombus limestone) occur in two orders of marine terraces exposed between 0 and 25 m a.s.l. Landwards, the unconformity is a subaerial erosional surface produced by surficial water overland/concentrated flow and is covered by colluvial deposits (reworked soils and breccias) containing fossil mammals (Paleoloxodon mnaidriensis Faunal Complex). The coastal and continental deposits of the Barcarello synthem (Figure 7) discontinuously crop out along the present-day coastal belt. The Sferracavallo area, where the Barcarello type section was measured, is an excellent site to observe the lithological features of the marine deposits of this synthem and the characteristics of unconformity 5 (Figure 2/Geological Sheet and Figure 7). The warm-temperate 'Senegalensis fauna' (Table 1) and numerical age dating restrict the age of the Barcarello synthem with the last interglacial s.s. (MIS 5, Tyrrhenian, Figure 3).
Glacio-eustatic oscillations and tectonic uplift (Antonioli et al., 1999;Di Stefano et al., 2012;Sulli, Lo Presti, Gasparo Morticelli, & Antonioli, 2013) controlled the genesis and evolution of coastal unconformity 5 and deposits. Wet-warm climate, when chemical weathering and pedogenetic processes prevailed, favoured soil development. Minor climate oscillations towards arid conditions permitted the development of overland/concentrated flows. The latter processes eroded and transported the unconsolidated soils previously formed and deposited them as colluvial sediments.
The stratified slope deposits (Table 1 and Figure 8) crop out along the flanks and at the foot of the highdipping carbonate rock slopes. The Raffo Rosso type section shows the main textural and compositional features of the deposits forming the synthem and the characteristics of unconformity 6 (Figure 2/Geological Sheet and Figure 8). Seawards, crossed stratified quartz and calcareous aeolian sandstones (coastal or climbing dunes), 1-2 m thick, crop out along the flat coastal areas; landwards, 1-7 m thick colluvial deposits with fossil mammals (Pianetti Faunal Complex and Castello Faunal Complex, Bonfiglio et al., 2003) dated to the late Upper Pleistocene (Burgio & Fiore, 1997) crop out both in karstic caves and at the foot of the coastal relief.
The genesis of unconformity 6 is related to the last glacial period (MISs 4-2), during which subaerial erosion or non-deposition processes affected the relief. During this event, a semi-arid and cold climate promoted strong physical weathering of the rocks (Agnesi et al., 2000), producing slope deposits that were rapidly cemented due to abundant vadose water circulation. Aeolian processes and overland/concentrated flows, leading to the formation of dune deposits and colluvial deposits, respectively, were also favoured by a significant lack of forest-wood vegetation.
4.1.7. Unconformity 7 and Capo Plaia synthem Unconformity 7 is a paraconformity surface or a subaerial erosional surface where the mostly unconsolidated deposits of the Capo Plaia synthem lie above the stratified and cemented screes of the RFR or older deposits. The Capo Plaia sector is the suggested site to observe the main characteristics of this synthem (see Main Map). The unconformity is related to the sea-level lowstand developed during the last glacial maximum (MIS 2; Figure 3). The synthem encompasses all the uppermost Pleistocene-Holocene deposits, widely cropping out along the study area, that were formed during the present interglacial climatic event (MISs 2-1). They display various lithologies from coastal to continental facies (Table 1) and assume an important multidisciplinary role for geological mapping work, archaeological investigations, and applied sciences (engineers, architecture, geotechnical).

Discussion and conclusion
Detailed stratigraphic studies and facies analysis of the marine, paralic, and continental Quaternary deposits allowed for the identification of unconformities and their correlation at a regional scale. This approach was useful for defining and mapping seven UBSUs in the NW Sicily coastal belt.

Software
Using GIS software, we create a geo-database including geo-referenced (datum epsg 32633) vector data related to attitudes, tectonic features, and lithostratigraphy (points, lines, and polygons, respectively). Adobe Illustrator (AI) was used to draft the geological map. Sketches and photos on the Geological Sheet and in the manuscript were adapted using both Adobe Photoshop and AI. Three-dimensional analysis of the topography was performed using Global Mapper.