Climatic & Tectonic Signature in the Fluvial Infill of a Late Pliocene Valley (Siena Basin, Northern Apennines, Italy) - JSR, 2007.pdf

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Journal of Sedimentary Research, 2007, v. 77, 398–414

Research Article

DOI: 10.2110/jsr.2007.039

CLIMATIC AND TECTONIC SIGNATURE IN THE FLUVIAL INFILL OF A LATE PLIOCENE VALLEY(SIENA BASIN, NORTHERN APENNINES, ITALY)

MAURO ALDINUCCI,1 MASSIMILIANO GHINASSI,2 AND FABIO SANDRELLI11 Department of Earth Sciences, University of Siena, Via Laterina, 8-53100 Siena, Italy

2 Department of Earth Sciences, University of Firenze, Via La Pira, 4-50100 Firenze, Italy

e-mail: [email protected]

ABSTRACT: Incised valleys entirely filled with fluvial deposits are rarely described in the literature, and even rarer are accountsof incised valleys whose filling is not driven by relative sea-level changes. Valleys of this type have not been widely recognized inthe stratigraphic record, because they are likely to occur in areas undergoing intense erosion or to be encased within alluvialdeposits with no strong lithologic contrast. In this paper, an upper Pliocene fluvial valley fill, encased within shallow marinemiddle Pliocene deposits and pre-Neogene bedrock (eastern Siena Basin, Northern Apennines, Italy) and accumulated inresponse to tectonics and climate changes, is described. The recognition of this fluvial body (up to 40 m thick) in an incised-valley setting is based on its overall geometry, lithofacies characteristics, gravel composition, and fossil content. Upvalley fill isrepresented by amalgamated gravels emplaced by heavily sediment-laden flows, whereas downvalley fill shows a more organizeddepositional style and consists of two fining-upward successions. Notable is the presence at the top of the lower succession of decameter-thick floodplain fines, which extend from one valley wall to another and have no fine-grained correlatives in theupvalley fill. Valley incision resulted from a drop in relative sea level, arising from late middle Pliocene regional uplift.Stratigraphic and paleontological data constrain valley filling to the late Pliocene–early Pleistocene time span. Such a fillingstemmed from an increase in sediment supply, which resulted from the interplay between uplift of the Chianti Ridge anda climatic change toward humid conditions, as recorded in the coeval strata of the adjacent Valdarno Basin. Beyond theregional significance, an important implication of the case study is that the incision and filling factors need not be geneticallyrelated. Moreover, the stratigraphic architecture of the studied fluvial fill is discussed in terms of the relations between sedimentsupply and sediment storage en route from the source area to the depositional site and their association with tectonicmovements. Specifically, the valley-wide floodplain fines are thought to record the response of the fluvial system to a tectonic

rejuvenation of catchments, which modified the valley gradient and promoted upstream gravel storage, whereas the overlyinggravels manifest the reestablishment of a new equilibrium river profile.

INTRODUCTION

Most of the incised valleys preserved in the stratigraphic record havebeen cut and filled in response to a fall and subsequent rise of relative sealevel (Dalrymple et al. 1994). Among them, most have a mixed fluvial andestuarine fill, whereas dominantly to entirely fluvial fills (Wright andMarriot 1993; Aitken and Flint 1994; Willis 1997; Blum and Price 1998;Arnott et al. 2000) are not commonly cited in the literature.

Incision and filling of a valley can be driven also by factors not relatedto relative sea-level changes, such as variations in fluvial discharge due toclimatic changes and/or stream capture and base-level changes related to

vertical tectonic movements of an inland area (Schumm et al. 1987; Blum1992; Schumm 1993; Holbrook 2001). Valleys of this type, by definition

filled with continental deposits, are common in modern settings (Blum1992; Fraser 1994), whereas they have not been widely recognized as suchin the stratigraphic record, because they are likely to be encased withinalluvial strata with no strong lithologic contrast (Zaitlin et al. 1994).

In this paper we analyze an elongated body of alluvial deposits restingon both Pliocene sediments and pre-Neogene bedrock and overlain byQuaternary alluvial deposits, at the eastern margin of the Siena Basin.

Geometry, gravel composition, facies associations, and age data allow us

to interpret such deposits as the fluvial infill of an incised valley, although

valley-scale outcrops are lacking. The purpose of this paper is to describe

the vertical and spatial development of valley-fill facies assemblages and

to combine them with regional geological constraints in order to estimate

the relative importance of possible geological factors in controlling valley

incision and filling, such as climate and tectonics. Accordingly, the

stratigraphic architecture of the valley fill is discussed in terms of

allogenic vs. authogenic processes, backfilling vs. downfilling models,

with emphasis on fluvial adjustment to tectonic rejuvenation of

catchments.

GEOLOGICAL SETTING

Overview

The Northern Apennines is a fold–thrust chain formed during the

Tertiary as a consequence of the interaction between various microplates

in the Africa–Eurasia collisional belt (Carmignani et al. 2001). The

Neogene–Quaternary basins of the Northern Apennines (Fig. 1) have

been intensely studied for many decades (Merla 1951; Sestini 1970;

Boccaletti et al. 1971; Barberi et al. 1973; Dewey et al. 1973; Boccaletti

Copyright E 2007, SEPM (Society for Sedimentary Geology) 1527-1404/07/077-398/$03.00



and Guazzone 1974; Locardi 1982; Wezel 1982; Patacca and Scandone

1986; Lavecchia 1988). Some basins are filled with upper Miocene– Pliocene fluvio-lacustrine and shallow-marine deposits (central basins,sensu Martini and Sagri 1993; such as the Siena Basin) or exclusively withmiddle Pliocene–Pleistocene continental deposits (peripheral basins, sensu

Martini and Sagri 1993; such as the Valdarno Basin).

These basins have been traditionally interpreted as grabens or half-grabens related to post-collisional extensional tectonics thought to affectthe inner side of the Northern Apennines from the late Miocene

(Costantini et al. 1982; Bossio et al. 1993). However, compressivefeatures in the basin fill led some researchers to consider the extensional

tectonics interrupted by compressive pulses (Bernini et al. 1990; Boccalettiet al. 1991; Boccaletti et al. 1995). More recently it has been suggestedthat the compressive regime was acting until the Quaternary, and thebasins have originated through reactivation of thrusts (thrust-top basinssensu Butler and Grasso 1993; Boccaletti and Sani 1998; Bonini and Sani2002), whereas the present-day delimiting normal faults are thought to be

of Pleistocene age.

The Siena Basin

The Siena Basin (Fig. 2) is part of the elongated tectonic depression thatextends from north of Lucca toward the SSE for more than 200 km up toBolsena Lake (Fig. 1). Syndepositional (Pascucci et al. 2007) transversal

highs, such as the Monteriggioni and Pienza ones, presently divide this

depression into delimited minor basins, such as the Siena Basin. According

to the extensional hypothesis, this basin is thought to be a half graben withthe master fault located on its eastern side (Costantini et al. 1982).

Sedimentation in the Siena Basin started in the late Miocene (Tortonian)

in a fluvio-lacustrine setting, which persistedduring theMessinian up to the

early Pliocene marine transgression. Two main depositional stages havebeen recognized in the Pliocene succession. The first one (early Pliocene)

comprises basin-margin, alluvial to transitional gravels and sands, passingbasinward to outer-neritic clays. The second stage (late earlyPliocene–early

middle Pliocene) is represented by coastal marine sands and conglomerates

and correlative outer-neritic clays with turbidite sands (Gandin andSandrelli 1992). These are sealed by shallow marine sands deposited during

a middle Pliocene marine regression which caused the basins of the inner

Northern Apennines to emerge (Bossio et al. 1993). Evolving fluvialnetworks and local uplift of the eastern basin margin gave rise to thin and

discontinuous alluvial deposits resting unconformably on Pliocene marinesediments (Magi 1992). Nevertheless, little attention has been paid to the

upper Pliocene–Quaternary deposits of the Siena Basin (Costantini et al.

1982), apart from for the well-known travertine of the Rapolano area (Guoand Riding 1992, 1994, 1998, 1999).

This study is focused on the upper Pliocene–lower Pleistocene alluvial

deposits exposed on the eastern margin of the basin (Castelnuovo

Berardenga and Rapolano area; Fig. 2).

FIG. 1.—Simplified geological maps of the inner part of the Northern Apennines where location of the study area in Tuscany is shown (modified after Bossio etal. 1993).

FLUVIAL INFILL OF A LATE PLIOCENE VALLEY (NORTHERN APENNINES, ITALY) 399J S R



STRATIGRAPHIC FRAMEWORK

The studied sedimentary deposits (hereafter VF) rest on both Plioceneunconsolidated sediments and pre-Neogene rocky substratum and are

overlain by Quaternary alluvial deposits (Fig. 3). In this section a brief description of the pre-Neogene substratum, Pliocene and Quaternarysediments, is given whereas geometry, gravel composition, faciesassociations, and fossil content of the VF deposits are addressed in thenext section.

Pre-Neogene Bedrock

The pre-Neogene bedrock comprises sedimentary rocks of the SantaFiora Fm and the tectonically underlying Macigno Fm. The Macigno Fm(Oligocene) is made up of dominantly grayish turbiditic greywackes withsubordinate pelites. The Santa Fiora Fm (Upper Cretaceous) consistsmainly of grayish silty turbidites with interbedded yellowish to graymicrites and occasional flint-bearing grayish turbiditic calcarenites.

The Pliocene Deposits

The overall stratigraphy of the Pliocene successions exposed in theSiena Basin is known (Costantini et al. 1982; Gandin and Sandrelli 1992;Bossio et al. 1993), but detailed sedimentological studies are few. In our

study area the Pliocene succession could be subdivided into two informalunits: Plil and the overlying Pli2 (Fig. 3).

Pli1 consists of fluvio-deltaic massive to stratified gravels, locallyoverlain by peaty clays with brackish-water molluscs and few sandy beds.This unit is considered to be early?–middle Pliocene in age, mainly on thebasis of its stratigraphic position below the readily datable middlePliocene deposits of the following unit.

Pli2 is represented mainly by shallow marine massive to laminatedsands with scattered pebbles, burrows, and shells. Massive tabular pebblygravels representing river outflows also occur. Pebbles are frequentlycharacterized by lithophaga traces and/or encrusting organisms, such as

oysters and barnacles. This unit has been referred to the middle Pliocene

on the basis of its foraminiferal and nannofossil assemblage (Costantini et

al. 1982).

Quaternary Deposits

The Quaternary deposits unconformably rest on pre-Neogene-bedrock,Pli2 and VF units, and consist of two informal stratigraphic units (Q1 and

Q2), which are made up of alluvial deposits (Fig. 3).

Q1.— This unit (up to 35 m thick) unconformably overlies pre-Neogene-bedrock, Pli2 and VF deposits. It is represented by alluvial-

fan amalgamated gravels passing downcurrent into plane-parallel-bedded

sands. These deposits, fed from the Chianti Ridge, are also exposed a few

kilometers southward, where they contain Acheulean (middle Pleistocene)

artifacts (Magi 1992).

Q2.— This unit, late Pleistocene in age, unconformably overlies both

VF deposits and the pre-Neogene bedrock. It is composed of massive to

cross-stratified fluvial gravels and pebbly sands (up to 3 m thick) formingterraced surfaces (Q2a), and massive cobble-grade gravels (up to 30 m

thick) deposited within alluvial fans (Q2b).

METHODS

Detailed mapping of lithofacies (1:10,000 scale) allowed us to

distinguish the VF deposits from the encasing deposits and, consequently,

to define its plan-view geometry, whereas its thickness has been assessed

by composite logs together with geological cross sections.

Such a body has been further differentiated from the encasing

unconsolidated Pliocene deposits through compositional analyses.Twenty samples (10 for each) were collected at different stratigraphic

levels in the VF deposits and in the laterally adjacent marine gravels.

Each sample consisted of 100 clasts with b axis . 1 cm, all collected fromindividual beds up to 50 cm thick along 2 m of outcrop; all clasts were

broken and identified. Seven lithofacies have been defined from

FIG. 2.—Schematic geological map of the eastern part of the Siena Basin (modified after Carmignani and Lazzarotto 2004).

400 M. ALDINUCCI ET AL. J S R



FIG. 3.—Simplified geological map of the study area with cross sections showing the geometry of the studied deposits ascribed to the valley fill (hereafter VF). Insets onthe left represent, respectively, stratigraphic diagrams (not to scale) of the northern and southern portion of the study area.




interpretation of sedimentological logs and photomosaics of laterallycontinuous outcrops and integrated with paleocurrent data with the aimof characterizing the depositional setting. For this purpose, several silty

samples (1 dm3 in volume) from VF deposits were sieved (63 mm) andanalyzed in order to test the presence of microfossils and/or macrofossils.

SEDIMENTARY FEATURES OF THE STUDIED DEPOSITS

Geometry

In plan view (Figs. 3, 4), the VF deposits form an elongated body withvariable width and orientation southwards (i.e., from the Chianti Ridge to

the Siena Basin). Its northern portion (from ‘‘La Selva’’ to ‘‘Casalbosco’’;Fig. 3) is 1–1.5 km in width and strikes NE–SW (Fig. 4), whereas the

southern one (from ‘‘Casalbosco’’ to Ombrone River; Fig. 3) reaches

a maximum width of 2.5 km and is oriented NW–SE (Fig. 4).Geological cross sections show that this body is characterized by a lens-

shaped geometry with a basal concave profile and an even top surface(Fig. 3). Deposits from the northern portion rest on pre-Neogene rocks

and have a maximum thickness of about 40 m (Fig. 3, geological cross

section A–A9), whereas the southern deposits are up to 35 m and overliePliocene unconsolidated sediments (Fig. 3, geological cross section B–B9).

Gravel Composition

Gravels are represented by graywacke, calcareous micrite, calcarenite,

and flint (Fig. 5A). Greywacke clasts are grayish in color and are derived

from the Macigno Fm. Calcareous micrite clasts, typically affected bya network of calcite veins, are yellowish to gray in color and are derived

from the Santa Fiora Fm, as well as grayish calcarenite. Flint clasts are

black and are derived from nodules in limestones of the Santa Fiora Fm.

Marine Pliocene gravels are richer in calcarenite (average value) and

flint clasts compared with the VF gravels, whereas graywackes andmicrite clasts are more common in the latter (Fig. 5A).

Clasts of the marine gravels, in contrast to VF clasts, are locallyencrusted by barnacles and oysters. Moreover, calcareous clasts from

marine deposits often show well preserved lithophaga bores (Fig. 5B),

which are rare and are always deeply abraded in the calcareous clasts of

the VF deposits (Fig. 5C).

Facies Associations and Depositional Processes

The northern and southern portions of the VF unit show differences in

mean grain size, facies assemblage, and stacking patterns. Two detailed

sedimentological sections, representative of the northern and southern

fluvial strata, respectively, are shown in Figure 6.

The northern deposits, typified by the Ambra section (log number 4 in

Fig. 4), consist of monotonous, dominantly pebble- to boulder-size

disorganized gravels.

The southern deposits, typified by the Ombrone section (log number 10

in Fig. 4), are finer grained and better organized than the northern ones.They comprise two erosionally bounded fining-upward successions. The

lower one starts with dominantly disorganized cobble gravels overlain by

FIG. 4.—Internal architecture of the valley fill based on 12 schematic logs integrated with paleocurrent data.




massive fines, whereas the upper one is represented mainly by stratifiedpebble to cobble gravels grading to massive and stratified sands.

Northern Portion.— The northern portion of the VF unit (Figs. 6, 7),cut into the pre-Neogene bedrock (Macigno Fm and Santa Fiora Fm), iscomposed of gravel lithosomes (lithofacies HF) with minor intercalationsof lens-shaped gravelly sands (lithofacies GSB).

Lithofacies HF is composed of amalgamated, decimeters- to meter-thick bodies of coarse cobble to boulder gravel, separated by decimeters-thick pebble gravel with isolated cobbles. Gravel bodies are sheet-like to

locally lenticular, with erosional basal surfaces showing scours up to a few

decimeters deep and few meters wide. Gravels are clast-supported and

poorly sorted, typically rounded to well rounded, although angular tosubangular clasts typically occur just above the valley substratum. The

matrix is commonly represented by fine pebbles and coarse-grained sandsin cobble gravels, and by coarse- to medium-grained sand in pebble

gravels. Textural trends are absent in most beds, except for some crudelynormally graded beds with the largest clasts arranged into imbricatedclusters indicating southward–southeastward paleoflows.

Lithofacies GSB occurs as lens-shaped beds of coarse-grained gravellysands, a few decimeters thick (typically , 60 cm). Basal surfaces are either

erosional or transitional to the underlying gravels, whereas at the top the

FIG. 5.— A) Density plot of gravel composition in the VF deposits and underlying coastal marine deposits. Each sample consists of 100 clasts (b axis . 1 cm) fromindividual beds up to 50 cm thick along 2 m of outcrop. The upper and lower boundaries of the gray rectangle indicate, respectively, the maximum and the minimumpercentage of each lithotype, and the black area shows its most representative percentage. B) Calcareous pebble from the middle Pliocene marine deposits with lithophagaborings. C) Calcareous pebble from the VF deposits with abraded lithophaga traces pointing to its reworking from the Pliocene marine deposits.




FIG. 6.—Sedimentological logs from thenorthern (Ambra Section) and southern(Ombrone Section) VF deposits with a brief description and interpretation of constituting lithofacies.




gravelly sand beds are overlain erosionally by gravels of lithofacies HF.Internally, beds of lithofacies GSB are typically massive to occasionallycrudely cross-stratified.

The massive and clast-supported texture of the erosionally based gravel

beds of lithofacies HF suggests their emplacement by sediment-bulkedfloodwater generating hyperconcentrated flows at the depositional stage

(Nemec and Muszynski 1982; Sohn et al. 1999; Benvenuti and Martini2002). Transient turbulence originated the basal erosional scours, whiledumping of the sediment load prevented the development of tractionalstructures. In this framework, the lensoid gravelly sand of lithofacies GSBrepresented local development of small-scale bedforms related tosustained flood flows in shallow channels (Miall 1996).

The lithofacies assemblage of the northern portion indicates depositionin shallow, low-sinuosity bedload streams (Allen 1983; Hjellbakk 1997;Jones et al. 2001), which, at low-flow stage assumed a braidedconfiguration with coarse gravels emerging as longitudinal bars (Bluck

1982; Miall 1985, 1996).

Southern Portion.— The lower FU succession of the southern portion(Figs. 6, 8, 9) starts with a bedset (, 6 m thick) of cobble to fine boulder

gravels (lithofacies HF) with lensoid crudely cross-stratified pebble

gravels (lithofacies GB) and massive pebbly sands (lithofacies MS). Sucha lithosome passes abruptly to a decameter-thick (, 12 m thick) horizon

of grayish clayey silts (lithofacies FO) with subordinate normally gradedsandy intercalations (lithofacies SC).

Basal gravels of lithofacies HF are clast-supported and poorly sortedwith locally abundant matrix formed by a mixture of pebbles and coarsesand. Beds are traceable laterally for tens of meters with basal erosional

surfaces showing scours up to a few decimeters deep, typically massive torarely coarse-tail normally graded.

Planar cross-stratified pebble gravels of lithofacies GB occur aslenticular beds up to 1.5 m thick and a few meters wide, typicallycharacterized by recurring openwork sets. Basal surfaces can be eithererosional or transitional to underlying coarse gravels of lithofacies HF.Imbrication of clasts and dip directions in cross-stratified beds indicate

southeastward paleoflows.

Massive pebbly sands of lithofacies MS are typically coarse grained andup to 50 cm thick with sharp basal and top surfaces.

Clayey silts of lithofacies FO are massive and characterized byabundant plant remains, organic-rich levels, root traces, occasionalcontinental gastropods, and small carbonate nodules, whereas the

normally graded medium- to fine-grained sands of lithofacies SC occur

FIG. 7.—Details of coarse lithofacies from the northern VF deposits. A) Amalgamated lens-shaped and erosionally-based gravels of lithofacies HF with minor crudelycross-stratified gravelly sands of lithofacies GSB. B) Close-up view of Part A showing the disorganized fabric of lithofacies HF. C) Close-up view of Part A showing cross-stratified pebbly sand of lithofacies GSB sandwiched between gravelly beds of lithofacies HF.




as tabular beds up to 10 cm thick. Topmost fines of the lower FUsuccession are overlain erosionally (Figs. 6, 8) by gravelly and sandylithofacies of the upper FU succession (, 8 m thick). The latter differsfrom the lower succession in the finer grain size of gravels, their

moderately developed fabric, and the higher content of sand.

The upper FU succession (Figs. 6, 8, 10) starts dominantly withgravels (lithofacies HF) with minor massive sandy intercalations(lithofacies MS) overlain by cross-stratified pebbly sands (lithofaciesSB), in turn overlain by massive (lithofacies MS) and normally graded

sand (lithofacies SC).

FIG. 8.—Panoramic view of the southern VF deposits (Ombrone Section) lying between lower?–middle Pliocene gravels and middle Pleistocene deposits of unit Q1.Note the abrupt grain-size change, from pebbles and cobbles to sandy silt, at the top of the lower FU succession.




Gravels of lithofacies HF are cobble to pebble grade, clast-supported,

and poorly sorted with sparse mud clasts. They form tabular to lenticular,

erosionally based massive beds, locally characterized at the top by

discontinuous and open-framework pebble gravels. Lithofacies GB are

pebble-grade gravels occurring as crudely planar cross-stratified, tabular

to lenticular beds, locally marked by open-framework cross-sets.

Lithofacies SB consists of erosionally based, lensoid beds of trough

cross-stratified coarse-grained gravelly sands with a common basal gravel

veneer showing imbricated clasts and mud clasts. Imbricated clasts andcross-stratification point to southwestward paleoflows. Lithofacies MS

and SC show the same characters of the homonymous lithofacies of the

lower FU succession.

Erosionally based FU successions of clast-supported, massive to well

stratified, poorly sorted gravels and sands are typically referred to

FIG. 9.—Facies details from the lower FU succession of VF deposits (Ombrone Section). A) Abrupt transition between the basal coarse gravels and the overlying fines

representing the top of the lower FU succession. B) Amalgamated coarse deposits at the base of the FU succession represented mainly by erosionally based anddisorganized gravels of lithofacies HF with minor crudely cross-stratified gravels of lithofacies GB. C) Close-up view of Part B showing disorganized fabric of lithofaciesHF. D) Close-up view of Part B showing poorly defined cross stratification of lithofacies GB.




a braided-river setting (Miall 1996). Accordingly, sedimentation occurred

mostly during flood events, with coarse gravels of lithofacies HF referable

to hyperconcentrated flows (Nemec and Muszynski 1982; Smith 1986;

Hjellbakk 1997; Sohn et al. 1999; Benvenuti and Martini 2002)

developing at the depositional stage of major floods (similarly to the

upvalley fill), and massive pebbly sands of lithofacies MS dumped from

lower-energy turbulent flows (Miall 1996; Hjellbakk 1997), as flow

velocity dropped rapidly, possibly resulting from flow expansion (Tun-

bridge 1984; Lowe 1988). The cross-stratified lithofacies GB and SB are

evidence of relatively diluted and sustained flows. Specifically, lithofacies

GB is referable to transverse bars (Miall 1996), where variations in

hydraulic conditions (Steel and Thompson 1983) or fluvial discharge

(Shih and Komar 1990) originated foresets with different fabrics, whereas

lithofacies SB documents bedform progradation within minor channel

scours (Allen 1983; Collinson 1996).

In this picture, the thick horizon of fines (see discussion for its

stratigraphic significance) at the top of the lower succession represents an

aggrading poorly drained floodplain (Miall 1996; Jones et al. 2001), with

suspension fallout of clayey silt (lithofacies FO) and subordinate non-

tractional sand deposition (lithofacies SC) from sediment dumping due to

flow expansion in the floodplain (Jones et al. 2001; Tunbridge 1984; Lowe1988).

Fossils

All of the sieved samples were barren of microfossils, except fora reworked undeterminable planktonic foraminifer. In contrast, well-preserved continental gastropods (Fig. 11) were found in the clayey siltsof lithofacies FO. Gastropods belong to the species Pomatia elegans and

Retinella sp., whose age distribution spans from the earliest late Plioceneto the Recent (Esu and Girotti 1991). They point to a subaerial

depositional setting, characterized by vegetated areas in humid-temperateclimatic conditions. On the whole, the malacofaunal association ispaleoecologically consistent with the floodplain depositional settinginferred for the fines at the top of the lower FU succession.

DISCUSSION

According to geometry and facies patterns, the VF fluvial depositsrepresent the infill of an incised valley which emanated from the western

flank of the Chianti Ridge into the Siena Basin, as suggested by

FIG. 10.—Facies details from the upper FU succession of southern VF deposits (Ombrone Section). A) Basal gravels and sands of the upper FU succession erosionallyoverlying massive clayey silts at the top of the lower FU succession. B) Close-up view of Part A showing erosionally based and trough cross-stratified sands of lithofaciesSB with minor massive sand intercalations of lithofacies MS. C) Close-up view of Part A showing disorganized gravels of lithofacies HF sandwiching a bedset of troughcross-stratified sands of lithofacies SB.




paleocurrents. Such an inference, although the flanks of a valley form are

not clearly visible, is strongly supported by fossil content (late Pliocene– Recent continental gastropods) and gravel composition. In particular,

quantitative analysis clearly differentiates gravels from VF and Pli2 units.

Moreover, the fact that the VF unit is cut into the Chianti Ridge bedrock,

along with the presence of clasts with abraded lithophaga traces, indicatesthat VF gravels represent a mix of Chianti bedrock (S. Fiora Fm and

Macigno Fm) and Pliocene gravel sources.

FIG. 11.—Gastropods from the clayey silt of lithofacies FO representing the topmost part of the lower FU succession: A, B) Retinella sp.; C, D) Pomatia elegans.




According to Zaitlin et al. (1994), a valley becomes a depositional zone:1) when the influx of sediments is constant but the ebb decreases, 2) when

the ebb is constant but the influx increases. The first condition usuallyresults from a relative sea-level rise, which heads the progressivebackfilling of the valley, generally with marine and/or brackish sediments.

The second condition is related to a significant increase in sedimentproduction (Schumm 1977), which promotes the downfilling of the valleywith alluvial deposits. Specifically, the influx increase can result from an

uplift of the source area, a climate change, or both (Schumm 1977; Blumand Tornqvist 2000).

The middle Pliocene coastal area cut by the studied valley wascharacterized by a steep topographic gradient, owing to the short distance(typically , 1 km) between the rocky hinterland and offshore mud andthe relatively narrow zone with littoral gravel and sand. This may accountfor the observed downcurrent decrease in valley depth, as the erosional

processes implied in valley formation were inhibited by decreasingtopographic gradient. It follows that the downcurrent correlative of thestudied valley fill above Pliocene offshore mud could have beenrepresented by fluvial deposits no longer confined within an incised

valley and eroded during the Pleistocene. The case study conforms to theconceptual model of Schumm (1993), who states that coastal zones withsteep gradients are more prone to incision than gently inclined coastal

plains, because fluvial channels are more likely confined within broadincisions, hindering lateral erosion and wandering (Wood et al. 1993a).As a consequence, the resultant incised valleys are shorter than those onbroad coastal plains and have relatively steep flanks (Wood et al. 1993b).

Ages of valley incision and filling are based mainly on the stratigraphicposition of the valley fill, the late Pliocene–Recent age of the recoveredcontinental gastropods, and the middle Pleistocene age (Aculean artefacts;

Magi 1992) of the overlying alluvial-fan strata of Unit Q1. According toSchumm and Ethridge (1994), river incision and valley widening withinunconsolidated sediments is rapid, as confirmed also by experimental

studies documenting rapid incision with subsequent valley widening andaggradation (Schumm et al. 1987). As a consequence, the studied valley waslikely cut between the late middle Pliocene and the earliest late Pliocene andwas filled during the latest Pliocene–early Pleistocene. In this context, valleyincision can reasonably be related to the late middle Pliocene relative sea-

level drop recorded in all of the central basins of Tuscany, due to thedevelopment of a regional-scale dome (Bossio et al. 1993; Martini and Sagri1993; Bossio et al. 1995; Martini et al. 2001), which is still active (Bartolini etal. 1982). Such a dome led to the occurrence of Pliocene coastlines up to

850 m above modern sea level (Disperati and Liotta1998) and is consideredto be the greatest surface uplift recorded in the inner Northern Apennines(Serri et al. 2001).

The valley fill consists of alluvial strata with no evidence of tide-influenced or brackish-water deposition and could represent either an

incised valley filled entirely with fluvial deposits or the fluvial portion of a major valley (Segment 3 of Zaitlin et al. 1994), whose filling was due toa relative sea-level rise. The latter case can be ruled out by taking intoaccount that the overall uplift of the inner Northern Apennines,combined with the presence of Pliocene inherited morphostructural sills

and possibly enhanced by this uplift, prevented the sea from reenteringsouthern Tuscany after the middle Pliocene regression (Bossio et al. 1993;Martini and Sagri 1993; Bossio et al. 1998). Thus, whereas the basal

surface of the incised valley is a regional unconformity surface withsequence-stratigraphic significance, its fluvial fill cannot be treated in theExxon sequence-stratigraphic sense, because it is not associated with anyrelative sea-level rise. Under these conditions, climatic and tectonicfactors, such as discharge variations relative to the availability of large

amounts of sediments, govern sedimentary facies patterns and valley-fillarchitecture (Schumm 1993).

In the Mediterranean, after a cooling phase accompanied by aridconditions at the middle to late Pliocene transition, an increasingly rainy

climate characterized the late Pliocene (Shackleton et al. 1995; Suc et al.1995a; Suc et al. 1995b; Fauquette et al. 1999; Bertini 2001; Anadon et al.

2002). These conditions persisted until the late early Pleistocene, when theclimate cooled again and the whole Mediterranean area was arid (Suc etal. 1995a).

In the inner Northern Apennines, such climatic changes coincided with

late Pliocene–early Pleistocene tectonics (Boccaletti and Sani 1998; Bossioet al. 1998). These are represented by regional doming and fault-driven

renewed uplift of basin margins, as attested by the presence of severalunconformities in the basin fills together with the emplacement of coarse-grained alluvial fan and/or fan-delta deposits (Bossio et al. 1993; Martiniand Sagri 1993). Such tectonic and climatic events have recently been welldocumented in the middle Pliocene–lower Pleistocene deposits (Fig. 12)

of the western Upper Valdarno Basin (Ghinassi and Magi 2004; Ghinassiet al. 2004), located 10 km north of the Siena Basin and separated from itby the Chianti Ridge (Fig. 12A).

There (Fig. 12B), fluvio-eolian deposits, referable to the Gauss– Matuyama magnetochron transition (Albianelli et al. 1995; Ghinassi et

al. 2004), rest unconformably on middle Pliocene fluvio-deltaic sedimentsand grade upward into floodbasin strata, in turn overlain by upperPliocene–lower Pleistocene fan-delta and fluvio-palustrine sediments,

bearing at the base a 2.21 6 0.09 Ma ash layer (Ghinassi et al. 2004).

Close to the Chianti Ridge (Fig. 12B), such fluvio-palustrine sedimentsrest unconformably on the fluvio-eolian deposits with no interveningfloodbasin strata (Fig. 12C), thus testifying to the occurrence of a tectonic

uplift (Albianelli et al. 1995; Ghinassi et al. 2004). The outlinedstratigraphic architecture, coupled with facies assemblages and detailedpalynological analyses (Albianelli et al. 1995), records the interaction

between the late Pliocene tectonic uplift of Chianti Ridge and the globalclimatic transition from dry/cold to warm/humid conditions (Ghinassiand Magi 2004; Ghinassi et al. 2005). The geological history of thewestern Valdarno Basin provides valuable constraints to unravel the time

of valley filling, because the incised valley was sourced by the same sectorof the Chianti Ridge. Accordingly, the studied valley was likely cut duringthe late middle Pliocene–earliest late Pliocene following the relative sea-

level drop recorded in all of the central basins of Tuscany. Although thepossibility that part of the lower gravel may be related to the falling stage

(Blum and Tornqvist 2000; Holbrook 2001) cannot be ruled out, thefilling stage occurred in the latest Pliocene–early Pleistocene time span,

due to the interplay between the late Pliocene uplift of the Chianti Ridgeand the renewed humid climate. This promoted a remarkable increase insediment influx to the valley (Fig. 13) resulting from enhanced erosion of

Chianti Ridge and adjoining unconsolidated Pliocene sediments. Underthese conditions the valley was filled by both downfilling and verticalaggradation of fluvial strata.

In this context, the floodplain fines at the top of the lower FU succession

of downvalley fill point to an abrupt change in depositional processes,whose significance requires a discussion in terms of autocyclic vs.allocyclic processes. Intrinsic control on sedimentation implies avulsion

of the lower-succession braided system followed by long-lived floodplainaggradation through overbank water from adjacent channel(s). However,this seems unrealistic, inasmuch as current facies models of coarse-grainedbraided rivers contemplate settling of thin bodies of fines within shallowpools resulting from abandoned channels (Miall 1996), whereas thick

floodplain deposits flanking braided rivers are thought to be preservedonly in basins with high subsidence rate (Bentham et al. 1993). Asa consequence, extrinsic controls on sedimentation, such as climate changepromoting less powerful aqueous flows, or source drainage evolution due

to tectonics or geomorphologic causes (such as capture of the upper reachof the incised valley), may account for the deactivation of the lowerbraided system with deposition of such a thick horizon of fines.

Furthermore, the lateral extent of this horizon within the incised-valleyfill, likely extending from one flank to another, strongly supports an




extrinsic control over deposition of fines. A climate change seems to be

unlikely, because the occurrence of both organic-rich levels and humid-

temperate gastropods in the thick horizon of fines suggests the persistence

of the same climatic conditions during which the lower coarse-graineddeposits were emplaced. Likewise, river capture is also improbable, given

that no coeval fluvial valley has been documented in proximity of the

studied valley. We interpret the abrupt sedimentation of floodplain finesabove the braided-river gravels as the response of the fluvial system to

a tectonic rejuvenation of catchment area. Late Pliocene–Quaternary fault

activity within an overall extensional regime has recently been documented

a few kilometers south of the study area (Brogi 2004), along the eastern

margin of the Siena Basin. Moreover, the area between the ‘‘Castello diMontalto’’ and ‘‘La Selva’’ localities (Fig. 3) is characterized by natural

CO2 exhalations along fault-related-damaged rocks and travertine

deposition, although detailed structural studies on fault orientation andkinematics are lacking. In our view, such a rejuvenation led to

a modification of valley gradient, promoting coarse-sediment starvation

in the downcurrent valley reach, which turned into a floodplain, as gravelswere trapped upstream (the fine-free upvalley fill). Specifically, the deeply

scoured erosional surface at the base of the downvalley-fill upper

succession marks the abrupt progradation of gravels with the reestablish-

ment of a new equilibrium profile of the river system.

FIG. 12.—Geological setting of the middle–upper Pliocene deposits of the Upper Valdarno Basin. A) Location of the Upper Valdarno Basin with respect to the studyarea. B) Scheme of the relations between depositional sequences in the southwestern Upper Valdarno Basin (rectangle inset of Part A showing the unconformityseparating the fluvio-palustrine succession from the underlying fluvio-eolian deposits. This boundary marks the abrupt increase in sediment supply due to concurrentuplift of the Chianti Ridge and climatic change toward humid conditions. C) Example of the unconformity surface between fluvio-palustrine sandy gravels and fluvio-eolian sands.




FIG. 13.—Model of the sedimentary evolution of the incised valley. A) Sedimentation of middle Pliocene marine coastal gravels and sands at the eastern margin of theSiena Basin. B) Incision of the valley due to relative sea-level fall. Entrenchment was greatest where the local gradient was steep enough for trenching. C) Filling of theincised valley following increase in sediment supply from the Chianti Ridge due to tectonic and increase in rainfall.




CONCLUSIONS

This paper provides new insights into the poorly documented late

Pliocene–Quaternary geological history of the eastern Siena Basin.

Nevertheless, beyond the regional significance, this study describes a kind

of incised valley not commonly identified in the geological record and

discusses the vertical development of facies assemblages in terms of

allogenic vs. authogenic factors.

The valley basal surface is a sequence boundary, inasmuch as valleyincision was promoted by a relative sea-level drop, whereas its fluvial fill

was not driven by a relative sea-level rise but resulted from an increase in

sediment supply due to the interplay between climate and tectonics.

Accordingly, because most of the incised valleys documented in the

literature were cut and filled in response to variations of the same

controlling factors (e.g., relative sea-level changes), the case study attests

that valley incision and filling need not be genetically related, as recently

documented by de Broekert and Sandiford (2005).

Furthermore, of particular interest are changes in sediment supply and

sediment storage en route from the source area to the depositional site

and their relationships with tectonic movements. Indeed, a tectonic pulse

that occurred during the valley-filling stage altered the geomorphic profile

of the valley and promoted a rapid transition from gravelly braided

deposits into floodplain fines in the downcurrent valley reach, whereas

coarser sediments were trapped close to the basin margin (the fine-free

upvalley fill). The case study conforms to conceptual models (Smith and

Smith 1980), field data (Blair and Bilodeau 1988; Crews and Ethridge

1993; Capuzzo and Wetzel 2004), and results of analogous simulations

(Paola et al. 1992; Strong et al. 2005), all inferring that differential

subsidence caused by tectonic processes commonly results in coarse-

sediment starvation and downcurrent sedimentation of fines.

ACKNOWLEDGMENTS

We are grateful to Associate Editor Sue Marriot, reviewer John Holbrook,and an anonymous reviewer for their critical comments and constructivecriticism. The authors are also indebted to Co-Editor Colin P. North for hisencouragements and comments. The manuscript benefit from helpful editorialassistance by John B. Southard. I. Peter Martini is thanked for reading an

early version of this manuscript. We also thank D. Esu for fossilclassification. This work was funded by PAR 2004 (F. Sandrelli).

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Received 4 August 2005; accepted 29 October 2006.


Climatic & Tectonic Signature in the Fluvial Infill of a Late Pliocene Valley (Siena Basin, Northern Apennines, Italy) - JSR, 2007.pdf

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