New biostratigraphic data from the Early Pleistocene tyrrhenian palaeocoast ( western Umbria , central Italy )

Plio-quaternary marine deposits are largely documented in western Umbria (central Italy), although they still lack biostratigraphic defi nition. Contrary to published data, Early Pleistocene deposits outcrop more extensively than previously reported in the Orvieto area. A composite biostratigraphic succession, almost continuous from the top of the G. gr. crassaformis Zone to the top of the Gl. cariacoensis Zone, can be reconstructed in offshore clay sections. Nannofossil assemblages and marker events (bmG, tCm, blG, tHs, tlG) from the MNN16a to MNN19e subzones have been documented. Lower shoreface – transition to offshore sections as described, are characterized by poor planktonic assemblages; nevertheless, they are still referable to the same stratigraphic interval. Deposits can be partially inserted into the “Chiani – Tevere” depositional cycle, also documented in this area. Moreover, marine conditions persist in the area from the base of the Gelasian to the top of the Calabrian, and it can be modelled as a peripheral, survival sea-branch, cut-off from the main river supply and from continental infl uence. However, Zanclean to Piacenzian deposits occur in a small area, localized around the town of Orvieto, so the former distinction of superimposed depositional cycles can only be speculative.


INTRODUCTION
The Orvieto area (western Umbria, Central Italy) documents the evolution of the Tyrrhenian side of the Northern Apennines during the last 3.5 Ma (Fig. 1). The occurrence of coas tal marine environments is documented in the area from the Piacenzian to the Calabrian at least, and the palaeoenvironmen tal evolution has been classically reconstructed identifying two main sedimentary cycles: Piacenzian p.p. -Gelasian and MANCINI et al. (2004) proposed the use of the "Tenaglie Unit" (Zanclean -Piacenzian) and the "Chiani-Tevere Unit" (Gelasian p.p. -Santernian).
New data from the Orvieto area, (BIZZARRI et al., 2003(BIZZARRI et al., , 2004BIZZARRI, 2006), recently contributed to stratigra phic refi nement and a palaeoenvironmental review, and are only partially referable to the former stratigraphic scheme.
This paper aims to present integrated sedimentological and micropalaeontological data of some Early Pleistocene sections recently recognized in the Orvieto area, in order to place the reconstruction in the wider stratigraphic and palaeoenvironmental framework of western Umbria.

GEOLOGICAL SETTING
The study area ( Fig. 1) is located in western Umbria (central Italy), near the town of Orvieto, along the Lower Paglia -Tiber Valley, a graben basin limited by East-and West-dipping conjugated normal fault systems, formed as a consequence of Lower Pliocene tectonics (FUNICIELLO et al., 1981;AAVV, 1982). The basin accommodated coastal marine environments from the Early Pliocene to the Santernian (AM-BROSETTI et al., 1977;1978a;1987;GIROTTI & MAN-CINI, 2003;MANCINI et al., 2004) or Emilian (BIZZARRI et al., 2004;BIZZARRI, 2006;BIZZARRI & BALDANZA, 2006a, 2007. The study area is bounded to the East by the Narnese-Amerina range, to the West by the Rapolano-Cetona range, and by the Vulsini Late Pleistocene volcanic district to the South-West (Fig. 1).

Allerona quarries
In the clay quarries north of the Allerona railway station on the western side of the Paglia Valley, a composite section, about 50 m thick has been reconstructed (Figs. 1, 2), divided into lower and upper portions respectively, separated by a 10 m gap. The deposits are represented by grey-blue, massive to thin -laminated silty clay. The fossil content is mainly represented by scattered gastropods and bivalve coquinas. The microfossil assemblage is characterized by ostracods including Aurila sp., Cytheropteron alatum and C. testudo, and rich foraminifer assemblages (Fig. 3).
The basal portion of the section contains poor calcareous nannofossil assemblages with small Gephyrocapsa, Calcidiscus macintyrei and Discoaster spp., referable to the MNN16a-MNN18 Zone (Piacenzian -Gelasian pp.). Higher in the section, the calcareous nannofossil assemblages are well preserved allowing identifi cation of the Gelasian-Calabrian interval, continuous from the MNN19a to the MNN19d subzones. The bmG and blG nannofossil events are observed, marking the base of the MNN19b and MNN19b subzones, respectively.

Camorena
The Camorena section outcrops in badlands 3 km south-east of Orvieto (Fig. 1), where an Early Pleistocene marine succession is exposed. The section is about 50 m thick (Fig. 2) and is exclusively composed of laminated, grey-blue clay/ silty clay. Analysis concentrated on continuous measurement and sampling of a 36 m thick clay succession in the upper part of the section. Samples from the basal part show micropalaeontological assemblages characteristic of the base of the Calabrian, with the occurrence of Globorotalia infl ata and Hyalinea balthica. Two pyroclastic, pyroxene-bearing interbeds occur (BIZZARRI et al., 2004) between 2.20 m and 3.50 m and at 15.50 m respectively. These are in the MNN19c and MNN19d subzones, are biostratigraphically constrained at 1.619 Ma and ~1.50 Ma respectively and correlate to volcanoclastic deposits documented in the Il Caio section (BIZZARRI et al., 2003).
The Camorena section covers the interval from the MNN19b to MNN19e subzones. A basal fossil assemblage is characterized by small Gephyrocapsa, medium Gephyrocapsa and Calcidiscus macintyrei, marking an Early Pleistocene age referable to the MNN19b Subzone. The Last Oc currence (LO) of C. macintyrei, followed by the First Occurrence (FO) of large Gephyrocapsa, allows identifi cation of the MNN19c subzone. Finally the LO of large Gephyrocapsa marks the base of the MNN19e subzone. The LO of Helicosphaera sellii and a scattered increase of Braarudosphaera bigelowii occur into the MNN19d subzone.
Among foraminifers, Globigerina cariacoensis occurs at the base of the section, in an assemblage with Globorotalia infl ata. Hyalinea balthica is also present.

Sugano well
On the western side of the Paglia Valley, 4 km south of Orvieto, near Sugano, a water well was recently drilled through  landslide deposits (4 m) and Vulsini Mts. volcanic deposits (9 m), reaching the underlying grey-blue marine clay. Samples recovered, contain both a rich and well preserved microfauna and pyroclastic minerals (pyroxenes) and pumice. The foraminifer assemblages (Fig. 3) are associated with the ostracods Cytheropteron alatum and Aurila sp.
The section also produced a very rich calcareous nannofossil assemblage which is characterized, in the lowermost sample, by small Gephyrocapsa and medium Gephyrocapsa referable to the MNN19c subzone. The fi rst occurrence of large Gephyrocapsa specimens, in the uppermost part of the section, marks the base of the MNN19d subzone.

Osarella II
The section (BIZZARRI, 2001(BIZZARRI, , 2006 is located along the SS 79 bis road at about 275 m a.s.l. It is represented by 18 m of sandy deposits, and comprises three intervals: Interval 1 -The lowermost 7 m are represented by well sorted fi ne sand to silty sand. Massive, bioturbated levels and thin laminated horizons almost regularly alternate. Planar to undulate lamination is organized in 10-40 cm thick horizons which regularly alternate with massive beds. Lamina sets (<5cm thick) repeat at a 1-1.5 cm interval, and each lamina is 3-4 mm in thickness. Lamination is marked by rhythmic colour and minimal grain size variations. Water escape structures (convolute lamination) also occur locally.
Interval 2 is 5 m thick and composed of moderately sort ed medium to coarse grained sand beds. In spite of diffuse bioturbation, still referable to Thalissinoides isp., each bed is characterized by an erosional base, a litho-and bioclastic lag, normal gradation and, in the uppermost part, faint cross-lamination. The fossil content is poor, but still represented predominantly by molluscs.
Interval 3 in the uppermost part of the section, alternates well sorted fi ne sand beds with horizons composed of large oysters, pectinids and other bivalves. Thalassinoides traces still occur, often emphasizing the original morphology at the base of shell beds. Furthermore, a 25 cm thick, oligotypic large Glicymeris sp. horizon characterizes the base of the interval, whereas a 50 cm thick bed composed of echinoid fragments occurs in the uppermost part.
This section contains nannofossil assemblages referable to the MNN19b and c subzones. The foraminiferal assemblages are poor and dominated by shallow water benthic species (Fig. 3). It is interesting to note the presence, although rare, of Globigerina cariacoensis in the assemblage with Globorotalia infl ata.

La Casella
The section has been described along the SS 71 road at the 41 km mark (BIZZARRI, 2001(BIZZARRI, , 2006, and extends for about 25 m, between 150 and 175 m a.s.l. (Fig. 1). Sedimentological features allow differentiation of two intervals: Interval 1 is represented in the lowermost 13 m by well sorted, grey to blue very fi ne sand and silty sand. Beds appear massive, due to bioturbation (mainly Thalassinoides isp., but also "v" shaped indeterminate forms). They are occasionally interbedded with up to 10 cm thick, medium to coarse grained sandy lithoclastic lags. The bioturbation increases in the last few metres, where wavy to gently cross laminations also occur. Gastropods and bivalves (Thericium sp., Hynia sp., Natica sp., Glans sp. and Ostrea spp.) also increase upwards, although shell fragments are documented throughout the section.
Interval 2 is characterised by 10 m of well sorted fi ne to very fi ne grained, yellowish sand. The sediments are massive, with dispersed large scattered bivalves and gastropods and common Thalassinoides isp. traces. They are interrupted by irregular intervals of bioclastic lags, often bioturbated, and by horizons of large oysters and pectinids, mainly made of Ostrea lamellosa, Chlamys spp. and Flabellipecten fl abelliformis. Barnacles also locally occur.
The microfossil content (Fig. 3) is dominated by shallow water benthonic taxa. The section comprises calcareous nannofossil assemblages characterized by small Gephyrocapsa, medium Gephyrocapsa and C. macintyrei in the basal samples. The disappearance of C. macintyrei, in the middle part of the section, allows identifying both the MNN19b and 19c subzones.
The section allows reconstruction of a succession of calcareous nannofossil events which document the MNN19 a and b subzones.

Castellunchio
The 10 m reconstructed section, located in the vicinity of the Baschi railway section (Fig. 1), is represented by massive, gray-blue very fi ne sand and silty sand. G. cariacoensis and G. infl ata are recognized from the base of the section, whereas Hyalinea balthica occurs in the uppermost part. The outcrop is attributable to the MNN19b and c Nannoplancton subzones.

Previous reference sections for the study area
Rocca Ripesena -A section in clay deposits, approximately 60 m thick, was reconstructed by CARBONI (1975), a few kilometres west from Orvieto, not far from the Sugano well site, (Fig. 1). Samples were collected from between 200 -250 m a.s.l. The author, attributed the section to the Piacenzian, mainly on the basis of G. gr. crassaformis distribution. The landscape has been strongly modifi ed during the last 30 years, and the exposure today appears very different with respect to the original. Yet, the Rocca Ripesena area has been newly sampled, between 250 -260 m a.s.l., which is about at the top of the old section of CARBONI (1975). Samples document a very rich and preserved nannofossil assemblage, referable to the MNN 19 c subzone. H. balthica and G. cariacoensis also occur. As a Pliocene age cannot be totally rejected for the base of the section, the new data allow reference of the top to the Calabrian.
Pod. Palombaro -CARBONI & DI BELLA (1996) analyzed the Podere Palombaro section (near Orvieto) and documented a Pleistocene age for this clayey sequence outcropping on the left bank of the Paglia River, which was attributed to the Piacenzian (CARBONI, 1975;BARBERI et al., 1994) or Pliocene/Pleistocene age (AMBROSETTI et al., 1987). The studied sediments were correlated to the Chiani-Tevere sandy clay formation (Argille e Sabbie del Chiani-Tevere, AMBROSETTI et al., 1987), on the base of benthic foraminiferal assemblages. The P. Palombaro represented the "key section" that allowed the authors to identify the Early Pleistocene (G. cariacoensis Zone), where the age of exposed Pleistocene sediments "has always been debated" because of the lack of planktonic markers (CARBONI & DI BELLA, 1996). Several samples (not in situ) were recently collected nearby, which confi rm the previous data, and furthermore, Hyalinea balthica is also present. The nannofossil assemblages are attributable to the MNN19c subzone, thus confi rming the presence of Early Pleistocene deposits in the Pod. Palombaro area.
Il Caio -Detailed analysis of the Il Caio section (Figs.1, 2) is reported in BIZZARRI et al. (2003) and BIZZARRI (2006). Deposits are represented by shoreface sand, with distal alluvial gravel, in the lowermost part, by prodelta silt and clay in the intermediate part and by alluvial fan gravel and sand in the upper part (Fig. 2). A fan delta environment was reconstructed (BIZZARRI et al., 2003;BIZZARRI, 2006). New sampling of this section, complemented by mineralogical analysis of volcanoclastic episodes, allows better understanding of its stratigraphy. The base of the section is devoid of calcareous nannofossils because of the predominantly medium sand grain size. The middle-upper portion is instead characterized by a thin interval of silt to clay sediments; the samples here contain good calcareous nannofossil assemblages which are characterized by abundant small Gephyrocapsa, common medium Gephyrocapsa and very few specimens of C. macintyrei. The microfossil assemblages are re presented by Globorotalia infl ata and Globigerina cariacoensis. This interval has been attributed to the Early Pleistocene (MNN19b Subzone) and the presence of G. cariacoensis documents a Santernian age. The occurrence of py ro xene crystals, micas and pumice dispersed in marine fi ne grained sediments, documents the presence of a volcanoclastic event that can be attributed to the Santernian and precedes those found in the Camorena section.

Sedimentological evidences and palaeoenvironmental reconstruction
The study sections belong to a distal low energy marine environment, in front of a beach, alternately dominated by deposition and wave action. The correlatable proximal deposition, (not considered here except for the Il Caio section), is represented by beachface gravel and cross-stratifi ed upper shoreface sand. All proximal and distal deposits belong to a river-fed coastal system, supplied by a number of local alluvial fans draining both sides of the valley (BIZZARRI, 2006(BIZZARRI, , 2007. The monotonous clay-silty clay sequences of the Allerona quarries, Camorena and Sugano well outcrops were deposited below wave base, and provide evidence of an offshore environment, with a maximum depth ranging from 80 to 120 m. Sieve analyses reveal a high percentage (98.5 to 99.3%) of grains fi ner than 0.63 mm (4 φ of the Hudden-Wentworth scale). According to DUNBAR & BARRETT (2005), grain size 4 φ represents the real borderline for settling processes, at least in marine environments, where more than 80% of grains fi ner than 4 φ is indicative of a settling-dominated offshore environment, with a minimum 60 m water depth. The foraminiferal assemblages, indeed, are dominated by planktonic specimens and by low-oxygen taxa rather than by deep water benthonic specimens (Fig. 3).
Alternatively, the other four sections document a faint wave action, and are referable to an environment across the transition to offshore and the lower shoreface. Normally grad ed beds, as well as bioclastic lags and shell beds, are interpreted as tempestites. The Baschi railway and Castellunchio sections document no environmental modifi cation. Conversely, the La Casella and Osarella II sections show a slight shallowing upward trend, from transition to offshore to lower shoreface. Concerning the foraminifers (Fig. 3), these four sections are dominated by shallow water proximal specimens, whereas planktonic taxa rarely occur.

Stratigraphic data
The seven sections presented above allow recognition of an almost continuous stratigraphic succession of marker events, from the Piacenzian to the uppermost Calabrian.
The nannofossil assemblages are of medium -good preservation, and are characterized by the common presence of Gephyrocapsidae, Reticulofenestridae, Calcidiscidae and Coccolithus pelagicus. Furthermore, the low abundance of specimens only allows a semi-quantitative approach, to evaluate the distribution of the most signifi cant taxa. Moreover, a variable number of reworked specimens (from Cretaceous to Miocene) are present. Calcareous nannofossil events documented in the seven successions are totally comparable to those found in the western Mediterranean (DI STEFANO, 1998;DE KAENEL et al., 1999;RAFFI, 2002).
The datum agrees with the occurrence of G. infl ata in all of the sections (except for the La Casella section), and with the occurrence of Gl. cariacoensis in four of the sec-tions (Figs. 2, 3), documenting the G. infl ata and Gl. cariacoensis Zones, according to the schemes of COLALONGO & SARTONI (1979) and IACCARINO & PREMOLI SILVA (2007). The lowermost Allerona quarries section, on the basis of the nannofossil and foraminiferal assemblages, belongs to the G. gr. crassaformis Zone and ranges from the MNN16a to MNN18 Nannofossil Zones.
Biostratigraphic data and sedimentological analyses, document uninterrupted Late Gelasian-Calabrian marine sedimentation in the Orvieto area for the fi rst time. Moreover, the offshore succession seems to be continuous and concordant, at least since the Late Pliocene.
Finally, a brief remark on the benthonic specimen Hyalinea balthica is opportune. H. balthica is commonly documented at 6 of the sections, in sediments varying from clay to silt, to fi ne sand, proving that its presence is independent of grain size. The lowermost fi rst local datum for the taxon occurs inside the MNN19b subzone, at the base of the Calabrian, still supporting the Early Pleistocene age for the Orvieto area deposits. The fi rst appearance datum of H. balthica, well-known as a "cold guest" marked the Santer nian -Emilian boundary (AZZAROLI et al., 1997), particularly in the Adriatic successions. In the Orvieto area, the fi rst occurrence of H. balthica occurs in the Santernian, and is thus signifi cantly earlier.
In our sections, H. balthica is widely and continuously documented in offshore deposits (Fig. 2), and often in an assemblage with both deep water benthic and deeper planktonic microorganisms (Fig. 3). Furthermore, the occurrence of H. balthica suggests isolation and cooling at the sea-fl oor (ROSS, 1984;BERGAMIN & DI BELLA, 1997), which is a possible consequence of sea-water column stratifi cation. These palaeoecological inferences are preliminary, and a broa der approach is needed. At the moment, our data lower the taxon appearance, which thus can be considered a seafl oor palaeotemperature proxy, whereas its stratigraphic importance needs to be reconsidered.
New analyses involving the Il Caio section, (still in progress (PANDOLFI, 2006;PECCERILLO et al., 2010aPECCERILLO et al., , 2010b), attest to an affi nity with the Roman Comagmatic Province. The new calcareous nannofossil stratigraphic data indicate that the lowermost volcanoclastic event occurred during the MNN 19b subzone, whereas a Late Villafranchian freshwater mollusc assemblage in the upper part of the section (BIZZARRI et al., 2003), indicates an age older than 1.4 Ma for its uppermost part (Fig. 2). As a consequence, the age of the uppermost volcanic products can be approximated in a range between 1.4 Ma and 1.62 Ma. Similar age con-straints characterize marine deposits in the other four outcrops, where distal fallout deposits cover a time span of about 300 ky (MNN 19 b-d subzones). Biostratigraphic constraints clearly mark the occurrence of three successive steps in volcanic activity, from the MNN 19b to the MNN 19d subzones (Fig. 2).
The source of volcanics was probably a small, still unknown eruptive centre, and its discontinuous activity covered a time span of at least 300 ky (PECCERILLO et al., 2010b). The correlation of these events looks reliable, and its age constraint is different from that of the Middle Pleistocene "Paleobolsena" volcanic event (SANTI, 1990;GILLOT et al., 1991;NAPPI et al., 1994).

CONCLUSION
The stratigraphic reconstruction contributes to our understanding of evolution of the Tuscan-Umbria-Latium area. Start ing from the mid 1970's, Pliocene and Pleistocene deposits in the area have been referred to two 3 rd order sedimentary cycles, separated by the "Acquatraversa" erosion/ tectonic phase (BLANCH, 1955;AMBROSETTI et al., 1977;1987;GIROTTI & MANCINI, 2003;MANCINI et al., 2004). None of the authors recorded marine deposits after the Santernian, so the ultimate marine regression must have already occurred in the Early Pleistocene.
All of the studied sections belong to the same environmental domain, and to the same depositional cycle ("Chiani -Tevere" cycle: AMBROSETTI et al., 1987;GIROTTI & MANCINI, 2003;MANCINI et al., 2004), even though the temporal extension of that cycle (Gelasian pp.-Santernian) seems to be limited.
The micropalaeontological content allows detailed stratigraphic reconstruction, with identifi cation of the calcareous nannoplankton Zone MNN19 (RIO et al., 1990) and relative MNN19a to MNN19e subzones. Moreover, lithological and sedimentological data confi rm both lateral and vertical environmental continuity, and the lack of major unconformities do not allow defi nition of a Lithostratigraphic Unit limit. Con sidering the new data presented herein, stratigraphic revision seems to be necessary, as well as re-interpretation of the palaeogeographic patterns. The data confi rm the persistence of marine conditions in the Orvieto area from the Gelasian to the top of the Calabrian at least, either un-or only partially infl uenced by continental progradation, unlike the southern areas, where a continental environment is already documented, from the Santernian (GIROTTI & MANCINI, 2003;MANCINI et al., 2004). The Orvieto area can be illustrated as a restricted satellite basin in which marine conditions remain, protected from the infl uence of the main rivers during the Early Pleistocene and accommodating only negligible supply from tributary rivers (BIZZARRI, 2006).
Contrary to former palaeogeographic interpretations, the occurrence of Early Pleistocene marine deposits is largely documented in the Orvieto area. The offshore clay deposits allow reconstruction of a composite biostratigraphic succession (Fig. 2), almost continuous from the top of the G. gr. crassaformis Zone to the top of the Gl. cariacoensis Zone.
As expected, the datum becomes less clear in more proximal marine deposits. Nevertheless, nannofossil assemblages and bioevents still allow reference of these deposits to the MNN19a, MNN19b and MNN19c subzones. Deposits can be partially attributed to the "Chiani -Tevere" depositional cycle, which thus also appears largely documented in the study area.