Early Pleistocene River-Fed Paleocoast in Western Umbria (Central Italy): Facies Analysis and Coastal Models

J.M


Introduction
The coastal marine environment represents a very complex context of sedimentation, both today and in the past, due to interactions between continental and marine processes. On ancient deposits, distinctions among deltaic and non-deltaic, river-, tide-or wave-dominated coasts have been commonly used, and a large amount of literature exists, particularly for riverand wave/storm-dominated coast sedimentation models [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. In present day coasts, fluvial currents, waves and tides act together, and the role of each group of processes is often hard to discriminate; nevertheless, the aforementioned distinction permits interpretation of facies associations in terms of sedimentation models. Interaction among eustacy, tectonic subsidence and sedimentation rate determinate a high complexity in coastal depositional architecture, but also allows the analysis of deposits in terms of relative sea-level changes. Coastal and nearshore deposits are often incompletely preserved; nevertheless, they record relative sea-level changes as shift of facies associations landward or seaward, respectively [1,6,7,11,16]. Coastal systems evolution is conditioned by relative sea-level fluctuations, and often records short-and long-term cyclicity as transgressive and regressive cyclic deposits [17][18][19][20][21][22].
The study area is characterized by the occurrence of shallow water, wave-dominated deltaic deposits in the northern sector [23,24], rocky coast deposits southwards [24][25][26], and by small river-to alluvial fan-fed coasts, covering a wide part of the basin and laterally shading to reworked gravel to sandy beach systems [24,26]. The fluvial origin is often recognizable, although facies associations vary across-and alongshore [27]. Marine to continental deposits, with volcanoclastic episodes, filledup the basin from the Pliocene to late Pleistocene/Holocene [24].
The youngest sedimentary cycle, before the onset of the present-day environmental setting, was indicated as Cycle III [24] and dated to Middle-Late Pleistocene. It was characterized by the onset of lacustrine conditions (Paleo-Trasimeno) northwards [44,45], and, in the southern sector, by several volcanic and sedimentary units related to the Vulsini, Vico, and Sabatini Mts. volcanic activity [24]. Nonetheless, older Vulsini Mts. volcanic deposits were documented in both marine and continental deposits of the Valdichiana Cycle ( Figure 2, [24,46]).

Materials and Methods
A total of 27 sedimentary sections and minor outcrops, described through several research campaigns [24] have been revized here, in terms of sedimentary features. Se-lected sedimentological and stratigraphic sections have been redrawn, to put emphasis on facies associations.
Stratigraphic schemes refer to the one recently proposed for South Valdichiana [24], and the biostratigraphic scales of [42,43], for Planktonic Foraminifers and Nannofossils respectively, have been adopted. The chronostratigraphic scale refers to [57].

Textural Features
While still slightly variable from one outcrop to one another, deposits show recurrent features on lithology and petrological composition, grain size, and sedimentological parameters. The present research was focused on field facies description and interpretation. Original statistical data were partly included in a former work ( [27], with references therein).

Coarse-Grained Deposits
Clast-to matrix-supported gravel and conglomerate represent the coarser fraction for study deposits, with diameters for clasts varying between 4 mm (−2ϕ, fine pebbles) up to 0.5 m (small boulders). The mean particle size falls between 6 and 25 cm (−6ϕ/−8ϕ, vcP to cC). Morphometric features are variable; clasts vary from subangular to rounded, although the commonest shapes are "plate" or "equidimensional". Gravel/conglomerate bodies are usually poorly-to moderately-well sorted. Clast lithology varies from site to site, although limestones from both Tuscan and Umbria-Marche stratigraphic units and sandstones with prevailing Tuscan affinity prevail.
The sandy fraction varies from fS to vcS/G (5ϕ to −1ϕ). Grains are prevailing made of quartz or lithic fragments, usually angular to subangular, or plate laminae of micas, accompanied by a variable fossil component (microfossils or shell fragments), still in the range of sand. Sandy deposits are commonly moderately-well-sorted to well-sorted.

Facies Analysis and Fossil Assemblages
Seven main facies associations and their related facies have been described, associated with environments varying from proximal alluvial/coastal to distal marine conditions. Considering each association, single facies may or may not be found together in outcrop.

Facies Association A-Alluvial Fan
The facies association A groups facies referred to a slight-to-moderate organized alluvial environment.

•
Facies B 1 -Parallel-laminated to massive fine-grainedsediments (vfS to mud), with mixed brackish and marine micro-and macrofaunas, vegetal fragments, and/or sandy (vcS) bioclastic horizons and large mollusc (mainly oysters) layers.Micropaleontological assemblages with species tolerating altered salinity conditions are locally documented [60]. Interpretation: brackish coastal ponds/lagoons isolated between distributary channels or by submarine bars. • Facies B 2 -Channelled mixed gravel and sand, cross-stratified deposits, interbedded with barren to poorly fossiliferous sandy to silty layers ( Figure 3c). Channel axial directions are dispersed from SW to SE. At the base of the channels a vcP (φ = 5-6 cm) channel lag occurs, with poorly reworked marine macrofossils (mainly Ostrea lamellosa, Persististrombus coronatus, Conus sp., Thericium sp.) and shell debris. Interpretation: distributary channel deposits. • Facies B 3 -Lenticular-shaped, clast-supported gravel beds, intermingled with sand ( Figure 3e,f). Gravel beds are up to 1-1.5 m thick (in the middle part), but thickness shade laterally to a single clast. Additionally,texture tends to be progressively matrixsupported, to finally shade into sand. Through the same trend, the diameter of gravel varies from φ > 25 cm (fine boulders) to φ = 4-6 cm (vcP), with an MPS (Mean Particle Size) of~10 cm. Clasts are mainly blade-to disc-shaped, rounded to well-rounded limestones, although sandstone lithotypes also occur. Finer sediments are fS/mS, moderately well-sorted, angular, high sphericity calcarenites and micas laminae, with minor cherty grains. Interpretation: organized mouth bar deposits. • Facies B 4 -Well-sorted, biotubated fine to medium sand (fS/mS), with angular high sphericity grains. Bioclastic lags, produced by waves and partly reworked by organisms, also occur ( Figure 3e,g). Interpretation: Interdistributary bays, sheltered bay/lagoon deposits.

Facies Association C-Beachface
• Facies C 1 -Poorly organized, wedge-shaped erosional-based gravel beds, laterally (from east to west) reducing to one-clast thickness and then shading to sand (Figure 4a,b). Texture i s mainly clast-supported, but locally can vary to matrix-supported or open-work as well. Deposition close to the shoreline is testified by the occurrence of both Lithophaga boring and encrusting species (as barnacles or oysters).
Interpretation: Mixed-sand and gravel submarine dunes, longshore bar deposits.
It is not excluded that broken/unbroken mollusc layers may represent evidence of repeated storm/fairweather cycles, although they are not always strictly correlated. Interpre t ation: transition to offshore deposits.

Facies Association G-Offshore
The distal deposits are represented by massively-to thinly-laminated gray-blue silty clay, cropping out in a narrow band in the central part of the basin. Throughout the whole area, deposits show no significant sedimentological differences. Main structures are represented by horizontal laminations and fossil layers, although fossils are also sparse in the sediment. The percentage of fines φ < 63 µm (4ϕ), i.e., the upper limit for settlement [63], is commonly more than 80%, indicating a settlement-dominated environment. Nonetheless, variable percentages of coarse grains also occur. The fossil assemblages (mainly benthic and planktonic foraminifers) suggest a relatively deep offshore environment (up to 80-130 m [24,27,39]), below the storm wave-base; still close enough to the shoreline to reflect the coastal processes. At the expense of the "turbiditic" sandstones (Macigno s.l Fm) and, to a lesser extent, of the limestones (e.g., Scaglia Toscana Fm), erosion currently produces an eluvial blanket made up of heterometric clasts, immersed in an abundant silty-to-clayey matrix. The present-day hydrographical network on the slopes consists of a number of small but deep-incized streams. Now as well in the past, these features exposed the slopes both to landslides, mainly during parossistic rainfall events, and to a general remobilisation of sediments by debris flow and sheetflood processes. On the other hand, the watercourses, with a torrential regime, which can occasionally receive and sort the material coming from the slopes downstream through channelled processes, are both associated with traction currents and massflow.

Arrangement of the Mountain Reliefs and Inherited Coastal Morphology
Regardless of the relative altitudes or the recent tectonics, the mountain ridges bounding the basin should have not been very different from present-day when they were the shores of the Paleo-Tyrrhenian Sea, during the early Pleistocene.
Two aspects require a reflection: first of all, the mountain range, before the marine transgression of the lower Pliocene, must have known a moment of exposure and sub-aerial modeling, with subsequently inherited morphologies and marked lateral facies variability along and across the paleocoast.
Secondly, the processes modeling the slopes were strongly conditioned both by the climatic conditions and by the lithological and structural features of the rocky substrate.
In the case study, these characteristics determined low coasts and were strongly influenced by fluvial-alluvial processes, while the formation of high coasts, with cliffs, was much more difficult to develop.
The processes typical of the coastal marine environment, mainly the wave motion, reworked deposits, conditioning their final morphology and determining a sub-environmental zoning both along-and across-shore.

Facies Architecture and Coastal Models
The described facies associations crop out in a wide area on both sides of the basin (Figure 1), and they highlight paleodepositional variability both across-and alongshore. Some sedimentological sections have been chosen, to highlight the vertical organization and the lateral (stratigraphic) relations ( Figure 6). The area responds to a wave-influenced coastal model, especially during storm events which, at least in some moments, seem to have had a short period of recurrence.
The deposits recognized within the various sub-environments have a fluvial to alluvial origin, which in many cases is still recognizable despite the action of the waves. The described facies associations allow us to propose two different sedimentary models, which can be considered as end-members in the coastal variability in the area, and which have been defined as fandelta-dominated and gravel beach-dominated coasts, respectively. In both cases, the proximal part is characterized by a smooth seaward morphology (Figure 7), and reflects acrossshore the organization of the shelf-type fan delta model [8,13,14,64]. Due to this gently seaward-dipping morphology, minimal variations on the sea level caused a large shift of facies both shore-and seawards. Such variations were documented in study outcrops ( Figure 6). Nonetheless, the proposed models differ from each other's, with respect to theshelftype one, mainly on lateral facies distribution, and some distinction between environments, associated with the two coastal models, can be proposed.
If in the proximal (nearshore) deposits the influence of both river/alluvial processes and wave action is still recognizable (although the prevailing ones are not always easily recognizable), differences within deposits become less marked across-shore. In particular, distal marine deposits, from lower shoreface to offshore, show laterally continuous homogeneous features.

Fan Delta-Dominated Coasts (Type 1 Model)
• Organized alluvial fan-The subaerial part of this coast consists of both fluid flow and sediment gravity flow deposits (facies A 1 to A 3 ; Table 1), all referable to slight organized alluvial fan (Figures 7 and 8). This environment was firstly associated with single alluvial fan [46,60], reworking locally volcanoclastic deposits, although it probably better matches with a gravel-dominated, shallow-channel braided fluvial model (e.g., "Type 1", "Scott type" models sensu [55]). In fact, this environment and its facies associations could be better considered as part of a stepped and laterally extended braided system, probably on the top of coalescent fans (Bajada [65,66]). In such an environment, local situations could also be interpreted as lahar deposits (Morgavi D., pers. comm. 2018). In any case, both massive and current processes have been documented. Interpretation: Large or coalescentalluvial fan, bajada. • Fan delta front-B 3 (organized mouth bar deposits) and secondarily B 2 (distributary channel deposits) facies prevail: gravel bodies, and subordinate sand, show a clear fluvial origin and transport, and were sedimented and partly reworkedin a shallow (10 to 20 m) coastal environment. Deposits were probably related to intermittent discharge; gravel shade to marine sand both laterally and frontally, with a prevailing progradation seaward which can be related to short frequency sea-level variations or to sedimentary supply as well. Facies B 1 was less commonly documented. Small brackish ponds could locally have formed between the channels. Mouth bars, although reworked by waves, were localized, and higher lateral facies continuity was probably reached only in correspondence of the external bars, quite far from the coast at the passage to the prodelta. Here, waves probably built submarine longshore bars (facies D 2 ) and isolated sheltered environments as lagoons or interdistributary bays (facies B 1 ). • Prodelta-It was mainly represented by facies F 1 , although a clear distinction from offshore deposits (facies G) is not easy in outcrop, and often related to macro/microfossil assemblages and laboratory analyses [27,67]. The transition to the prodelta was probably gradual, through a gently inclined coastal morphology and with a progressive increase in fine-grained fraction. Consequently, resedimented bottomset deposits (facies F 2 , Figure 5d: see below), while still possible, are less commonly documented and mainly associated with the Type 2 coastal model.  • Unorganized alluvial fan-Facies A 4 (Figure 3b,d, Table 1) characterizes the subaerial part of this coastal model. Deposits are poorly organized, and have been associated withdebris flow processes (surging debris flow sensu [10]), in which density is the main acting parameter, with subordinated traction current contribution. Gravel bodies of facies A 4 show poor lateral extension, and they were associated with heavy rainfallinduced landslides on unstable slopes, or parossistic activity of small streams along the paleoshore. Sedimentological features indicate a sudden fall in energy and a short transport. Interpretation: Small and/or isolated alluvial fans. • Mixed-gravel and sandy beachface-Bodies of gravel and sand, described as alternated and partly eteropic facies C 1 and C 2 ( Table 1). Facies C gravel can still be associated with a prograding fan delta front (mouth bar deposits), reworked as gravel beaches deposits. In fact, if the river transport origin is almost clear, originary body geometries are only barely guessable, hidden by the processes associated with wave motion. Gravel (facies C 1 ) were associated with a relative high-energy beachface, with wave motion redistributing deposits parallel to the paleocoast. Sandy dunes (facies C 2 ) are interpreted as submerged longshore bars (sensu [1,68,69]) and associated with theshoreface. In fact, a clear distinction between the foreshore and the shoreface is not possible here, and these dunes can be associated with the beachface environment as well (e.g., foreshore and upper shoreface, without a well-defined boundary). • Shoreface-Where mixed gravel and sand bodies are lacking, deposits can be generically associated with a submerged beach environment, between the average low tide surface and the fair-weather wave base (i.e., the shoreface environment). Nonetheless, in some cases the distinction between upper and lower shoreface can be proposed. The upper shorefaceis represented by alternated fair-weather deposits (facies D 3 ) and storm-induced longshore bar deposits (facies D 2 ), migrating landwards. Backset laminations, analogous to structures described here as facies D 2 , were recently interpreted in terms of supercritical backwash flows at the beachface-shoreface transition [70], still associated with storm deposits and referred to the evolution of a gravelly beach.
Although the whole suite of structures was not documented here, probably due to different wave climax, coastal morphologies or outcrop conditions, such a mechanism can be suitable for this case study as well.

•
In the lower shoreface deposits (facies E 1 ), massive-to-undulate slightly cross-laminated sand (fair-weather deposits of the shoaling/oscillatory wave zone) alternated to coarser deposits/fossil layers, interpreted as distal storm deposits. • Transition to offshore deposits, represented by structureless or subordinately parallelto undulate-laminated silty sand (facies E 2 ), with their litho-and bioclastic lags (distaltempestites), bioturbations, and sparse fossils, are the most commoncoastal marine deposits in the area. They were interpreted as transition to offshore deposits, with distal tempestites. Although they could also be associated with the fan-delta coast model, deposits of facies association E have been never documented together with prodelta deposits (facies association F, see below). Thus, they were presumably in lateral eteropy, and facies association E better refers to the gravel beach-dominated coastal model.

Open Marine Environment (Offshore)
Offshore clayey/silty clayey deposits (facies G) appear with similar features throughout the median part of the basin. They are invariantly massive-to thin-laminated, with rare fossil horizons and variable micro-and macrofossil content. This means a homogeneity of distal marine environments, regardless of the coastal type behind it.
In this distal marine environment, although only locally, resedimented gravel of facies F 2 occur (Figure 5d), interposed to clay: they are interpreted as deposits at the bottomset of small, isolated fans and associated with the Type 2 coastal model. At the transition to the open marine, in fact, the influence of nearshore processes was shaded and often indistinguishable.

Comparison with Deltaic and Rocky Coast Facies Distribution and Coastal Models
In the study area, two other coastal models were already proposed (Figure 8), taking into account the braided delta of Città della Pieve, northwards [23,24], and the rocky coast environments, mainly southwards [24,25]. The main differences, as expected, regard the continental and the nearshore sectors. The structured braided-river channels-alluvial plain system, as well as the brackish environment deposits, which characterized the Braideddelta model [23], were replaced by large/coalescent or small/spotted fans in the Type 1 and 2 models, respectively ( Figure 8). In its proximal part, the fan-delta front of the Type 1 model still resemble the organization of the braided delta, but it deviated distally; the outer delta large sandy macroforms [23], indeed, were not documented, and replaced by mixedsand and gravel longshore bars (Figure 8). Such fan-delta front organization did not occur in the Type 2 model, where it was replaced by beachface and shoreface deposits.
The rocky coast model [25] was clearly differentiated from the other three models (Figure 8), both in the subaerial portion, where continental/transitional environments were represented by cliffs and cliff thalus, and in the proximal marine environment, in which the sensible lateral variations of facies is replaced by a gravel beachface and a sandy/calcarenitic shoreface. Moreover, in the rocky coast model the gravel beachface showed several facies associations [25], which were not documented in the Type 2 model.
On the other hand, differences tend to progressively reduce seawards, until they disappear in correspondence with the offshore [24]. The Prodelta can be clearly associated only with Braided-delta and Type 1 models (Figure 8), whereas in the Type 2 coastal model it was replaced by a transition-to-offshore environment. In fact, a clear offshore environment cannot be distinguished from the prodelta in the Braided-delta model [23], while it could be indifferently associated with the other three models. Nonetheless, these homogeneous distal clayey to silty clayey deposits still documented minimal variations in both sedimentological features and microfossil assemblages [27,39,67,[71][72][73].

Paleogeographic Distribution of the Paleocoasts
Although they reflect the depositional architecture, Type 1 and 2 coastal models are the ending points of a continuous lateral facies variation which characterized the whole intermediate part of the South Valdichiana basin during the early Pleistocene. The facies analysis suggests an interaction between continental and marine processes inherent in the domains, with the development of a wave-dominated coastal marine environment, in which the fluvial-alluvial origin of deposits was still more or less recognizable. The main distinguishing factor was represented by the occurrence of structured fan-delta systems or of spotted river mouths, both associated with seasonal streams or parossistic events rather than with a persistent fluvial environment behind. These streams mainly supplied coarse-grained deposits, reworked by waves into partly redistributed mouth bar deposits or gravel beach deposits [24] (Figure 5b). Facies associated with the Type 1 model were documented in some restricted areas (Figure 1c): in these sectors, inherited coastal morphologies probably led to the development of a narrow, gently inclined band between the reliefs and the coast line, occupied by large or coalescing fans (bajada [65,66]) and their fan-deltas. The presence of such a band, indeed, caused the slope break and the energy fall did not occur directly into the sea; conditions necessary to define a fan-delta [65,66,74].

Fossil Distribution and Stratigraphic Constraints
Both Type 1 and 2 coastal models were stratigraphically constrained to the early Pleistocene (Cycle II-Valdichiana Cycle in Figure 2, [24]), although they could still be applied to the "Pliocene" Cycle as well. In fact, former studies documented middle-to-late Villafranchian freshwater to continental fossil assemblages [34,75], and the hypothesis of an older coastal evolution for the area was already suggested [23,24], although therewas a lack of continuous biostratigraphic data.
Beyond stratigraphy, the fossil record manifests a substantial homogeneity throughout the study area, and across the two coastal models. It was discussed [26,27] how the distribution of selected benthic foraminifers, molluscs, corals (Cladocora) and lunulite bryozoans mainly depended onthe bathymetry and the lithology/grain size of the sea floor, rather than from the coastal type at the rear. Other important factors were the water temperature [26,76] and chemistry [39], the local availability of nutrients [69][70][71], and the relative sea-level rise/fall [23][24][25][26].
Lack of fossil record, common presence of vegetal remains, as well as mixed assemblages of freshwater and altered salinity-tolerant species, characterized the continental/transitional environment deposits in the area [23,24]. Following only this criterion, alluvial fan/fan delta deposits are not easily distinguishable from deposits associated with braided delta [23], particularly on small outcrops: in these cases, only a wider paleoenvironmental and depositional architecture analysis led to reliable reconstructions [24].
Fossil data support the proposed coastal models: Type 1 and Type 2 paleocoasts (as well as braided-river delta and rocky paleocoasts) laterally coexisted, gradually shading seawards into a more homogeneous distal marine environment. The main differences were led by inherited morphologies and processes active on the emerged areas (physical factors), rather than by paleoecological/paleoenvironmental conditions (biological factors) or stratigraphy (geological time) [24].

Conclusions
Through the Pliocene (?)-early Pleistocene shallow marine deposits, cropping out in the southern sector of the Valdichiana Basin (particularly in its intermediate part), facies associations allow us to reconstruct the evolution of a wave-dominated, river-fed coast [24].
Spotted to coalescent alluvial fan/fan-delta deposits, more or less reworked by waves and redistributed as gravel and sandy beaches, characterized the nearshore; theirsmooth seaward morphology, on a coast-transverse profile, resembled the shelf-type fan-delta models [8].
Two coastal models are proposed here, indicated as Type 1 (Fan-delta-dominated) and Type 2 (Gravel beach-dominated), respectively, laterally shading each one in with other, and both tending to a substantial homogeneity seawards. The main differences in the distribution of facies, indeed, both between the two models and with the braided delta and the rocky coast models (still characterizing discrete sectors of the study area [23][24][25]), concentrate in the nearshore (Figure 8).
According to biostratigraphic and paleoecological data, the four models coexisted at least during part of the early Pleistocene, presumably as a consequence of inherited geomorphologic features and the evolution of neighbouring alluvial environments.
This Miocene to recent evolution of both northern and eastern emerged areas, although partially defined [23,24,44,45,79], is still not fully assessed. Open research lines are oriented in such a direction.