siena basin, northern apennines, italy - GeoScienceWorld

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 1 2

AND

FABIO SANDRELLI1

Department of Earth Sciences, University of Siena, Via Laterina, 8-53100 Siena, Italy 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 accounts of incised valleys whose filling is not driven by relative sea-level changes. Valleys of this type have not been widely recognized in the stratigraphic record, because they are likely to occur in areas undergoing intense erosion or to be encased within alluvial deposits with no strong lithologic contrast. In this paper, an upper Pliocene fluvial valley fill, encased within shallow marine middle Pliocene deposits and pre-Neogene bedrock (eastern Siena Basin, Northern Apennines, Italy) and accumulated in response to tectonics and climate changes, is described. The recognition of this fluvial body (up to 40 m thick) in an incisedvalley setting is based on its overall geometry, lithofacies characteristics, gravel composition, and fossil content. Upvalley fill is represented by amalgamated gravels emplaced by heavily sediment-laden flows, whereas downvalley fill shows a more organized depositional 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 the upvalley 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 filling stemmed from an increase in sediment supply, which resulted from the interplay between uplift of the Chianti Ridge and a climatic change toward humid conditions, as recorded in the coeval strata of the adjacent Valdarno Basin. Beyond the regional significance, an important implication of the case study is that the incision and filling factors need not be genetically related. Moreover, the stratigraphic architecture of the studied fluvial fill is discussed in terms of the relations between sediment supply and sediment storage en route from the source area to the depositional site and their association with tectonic movements. 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 overlying gravels manifest the reestablishment of a new equilibrium river profile.

INTRODUCTION

Most of the incised valleys preserved in the stratigraphic record have been cut and filled in response to a fall and subsequent rise of relative sea level (Dalrymple et al. 1994). Among them, most have a mixed fluvial and estuarine fill, whereas dominantly to entirely fluvial fills (Wright and Marriot 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 related to relative sea-level changes, such as variations in fluvial discharge due to climatic changes and/or stream capture and base-level changes related to vertical tectonic movements of an inland area (Schumm et al. 1987; Blum 1992; Schumm 1993; Holbrook 2001). Valleys of this type, by definition filled with continental deposits, are common in modern settings (Blum 1992; Fraser 1994), whereas they have not been widely recognized as such in the stratigraphic record, because they are likely to be encased within alluvial strata with no strong lithologic contrast (Zaitlin et al. 1994). In this paper we analyze an elongated body of alluvial deposits resting on both Pliocene sediments and pre-Neogene bedrock and overlain by Quaternary alluvial deposits, at the eastern margin of the Siena Basin. Geometry, gravel composition, facies associations, and age data allow us

Copyright E 2007, SEPM (Society for Sedimentary Geology)

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

1527-1404/07/077-398/$03.00

JSR

FLUVIAL INFILL OF A LATE PLIOCENE VALLEY (NORTHERN APENNINES, ITALY)

399

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 et al. 1993).

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 with middle 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 halfgrabens related to post-collisional extensional tectonics thought to affect the inner side of the Northern Apennines from the late Miocene (Costantini et al. 1982; Bossio et al. 1993). However, compressive features in the basin fill led some researchers to consider the extensional tectonics interrupted by compressive pulses (Bernini et al. 1990; Boccaletti et al. 1991; Boccaletti et al. 1995). More recently it has been suggested that the compressive regime was acting until the Quaternary, and the basins have originated through reactivation of thrusts (thrust-top basins sensu Butler and Grasso 1993; Boccaletti and Sani 1998; Bonini and Sani 2002), 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 that extends from north of Lucca toward the SSE for more than 200 km up to Bolsena 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 with the 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 persisted during the Messinian up to the early Pliocene marine transgression. Two main depositional stages have been recognized in the Pliocene succession. The first one (early Pliocene) comprises basin-margin, alluvial to transitional gravels and sands, passing basinward to outer-neritic clays. The second stage (late early Pliocene–early middle Pliocene) is represented by coastal marine sands and conglomerates and correlative outer-neritic clays with turbidite sands (Gandin and Sandrelli 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 fluvial networks and local uplift of the eastern basin margin gave rise to thin and discontinuous alluvial deposits resting unconformably on Pliocene marine sediments (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 (Guo and 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).

400

JSR

M. ALDINUCCI ET AL.

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

The studied sedimentary deposits (hereafter VF) rest on both Pliocene unconsolidated 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 Quaternary sediments, is given whereas geometry, gravel composition, facies associations, and fossil content of the VF deposits are addressed in the next section. Pre-Neogene Bedrock The pre-Neogene bedrock comprises sedimentary rocks of the Santa Fiora Fm and the tectonically underlying Macigno Fm. The Macigno Fm (Oligocene) is made up of dominantly grayish turbiditic greywackes with subordinate pelites. The Santa Fiora Fm (Upper Cretaceous) consists mainly of grayish silty turbidites with interbedded yellowish to gray micrites and occasional flint-bearing grayish turbiditic calcarenites. The Pliocene Deposits The overall stratigraphy of the Pliocene successions exposed in the Siena 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 informal units: Plil and the overlying Pli2 (Fig. 3). Pli1 consists of fluvio-deltaic massive to stratified gravels, locally overlain by peaty clays with brackish-water molluscs and few sandy beds. This unit is considered to be early?–middle Pliocene in age, mainly on the basis of its stratigraphic position below the readily datable middle Pliocene deposits of the following unit. Pli2 is represented mainly by shallow marine massive to laminated sands with scattered pebbles, burrows, and shells. Massive tabular pebbly gravels representing river outflows also occur. Pebbles are frequently characterized 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 preNeogene-bedrock, Pli2 and VF deposits. It is represented by alluvialfan 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) forming terraced 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 from individual beds up to 50 cm thick along 2 m of outcrop; all clasts were broken and identified. Seven lithofacies have been defined from

JSR


401

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 on the left represent, respectively, stratigraphic diagrams (not to scale) of the northern and southern portion of the study area.

402

JSR

M. ALDINUCCI ET AL.

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

interpretation of sedimentological logs and photomosaics of laterally continuous outcrops and integrated with paleocurrent data with the aim of characterizing the depositional setting. For this purpose, several silty samples (1 dm3 in volume) from VF deposits were sieved (63 mm) and analyzed 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 with variable 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 lensshaped 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 overlie Pliocene 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 by a 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 and micrite clasts are more common in the latter (Fig. 5A). Clasts of the marine gravels, in contrast to VF clasts, are locally encrusted 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

JSR


403

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) from individual 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 minimum percentage of each lithotype, and the black area shows its most representative percentage. B) Calcareous pebble from the middle Pliocene marine deposits with lithophaga borings. C) Calcareous pebble from the VF deposits with abraded lithophaga traces pointing to its reworking from the Pliocene marine deposits.

massive fines, whereas the upper one is represented mainly by stratified pebble 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), is composed of gravel lithosomes (lithofacies HF) with minor intercalations of lens-shaped gravelly sands (lithofacies GSB). Lithofacies HF is composed of amalgamated, decimeters- to meterthick bodies of coarse cobble to boulder gravel, separated by decimetersthick 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 to subangular clasts typically occur just above the valley substratum. The matrix is commonly represented by fine pebbles and coarse-grained sands in cobble gravels, and by coarse- to medium-grained sand in pebble gravels. Textural trends are absent in most beds, except for some crudely normally graded beds with the largest clasts arranged into imbricated clusters indicating southward–southeastward paleoflows. Lithofacies GSB occurs as lens-shaped beds of coarse-grained gravelly sands, a few decimeters thick (typically , 60 cm). Basal surfaces are either erosional or transitional to the underlying gravels, whereas at the top the

404

M. ALDINUCCI ET AL.

JSR

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

JSR


405

FIG. 7.—Details of coarse lithofacies from the northern VF deposits. A) Amalgamated lens-shaped and erosionally-based gravels of lithofacies HF with minor crudely cross-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 crossstratified pebbly sand of lithofacies GSB sandwiched between gravelly beds of lithofacies HF.

gravelly sand beds are overlain erosionally by gravels of lithofacies HF. Internally, beds of lithofacies GSB are typically massive to occasionally crudely cross-stratified. The massive and clast-supported texture of the erosionally based gravel beds of lithofacies HF suggests their emplacement by sediment-bulked floodwater generating hyperconcentrated flows at the depositional stage (Nemec and Muszyn´ski 1982; Sohn et al. 1999; Benvenuti and Martini 2002). Transient turbulence originated the basal erosional scours, while dumping of the sediment load prevented the development of tractional structures. In this framework, the lensoid gravelly sand of lithofacies GSB represented local development of small-scale bedforms related to sustained flood flows in shallow channels (Miall 1996). The lithofacies assemblage of the northern portion indicates deposition in shallow, low-sinuosity bedload streams (Allen 1983; Hjellbakk 1997; Jones et al. 2001), which, at low-flow stage assumed a braided configuration 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). Such a lithosome passes abruptly to a decameter-thick (, 12 m thick) horizon of grayish clayey silts (lithofacies FO) with subordinate normally graded sandy intercalations (lithofacies SC). Basal gravels of lithofacies HF are clast-supported and poorly sorted with locally abundant matrix formed by a mixture of pebbles and coarse sand. Beds are traceable laterally for tens of meters with basal erosional surfaces showing scours up to a few decimeters deep, typically massive to rarely coarse-tail normally graded. Planar cross-stratified pebble gravels of lithofacies GB occur as lenticular beds up to 1.5 m thick and a few meters wide, typically characterized by recurring openwork sets. Basal surfaces can be either erosional 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 and up to 50 cm thick with sharp basal and top surfaces. Clayey silts of lithofacies FO are massive and characterized by abundant plant remains, organic-rich levels, root traces, occasional continental gastropods, and small carbonate nodules, whereas the normally graded medium- to fine-grained sands of lithofacies SC occur

406

M. ALDINUCCI ET AL.

JSR

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.

as tabular beds up to 10 cm thick. Topmost fines of the lower FU succession are overlain erosionally (Figs. 6, 8) by gravelly and sandy lithofacies of the upper FU succession (, 8 m thick). The latter differs from 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 with gravels (lithofacies HF) with minor massive sandy intercalations (lithofacies MS) overlain by cross-stratified pebbly sands (lithofacies SB), in turn overlain by massive (lithofacies MS) and normally graded sand (lithofacies SC).

JSR


407

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 and disorganized gravels of lithofacies HF with minor crudely cross-stratified gravels of lithofacies GB. C) Close-up view of Part B showing disorganized fabric of lithofacies HF. D) Close-up view of Part B showing poorly defined cross stratification of lithofacies GB.

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 and cross-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

408

JSR

M. ALDINUCCI ET AL.

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 erosionally overlying 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 lithofacies SB 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 trough cross-stratified sands of lithofacies SB.

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 Muszyn´ski 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 (Tunbridge 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 nontractional sand deposition (lithofacies SC) from sediment dumping due to

flow expansion in the floodplain (Jones et al. 2001; Tunbridge 1984; Lowe 1988). Fossils All of the sieved samples were barren of microfossils, except for a reworked undeterminable planktonic foraminifer. In contrast, wellpreserved continental gastropods (Fig. 11) were found in the clayey silts of lithofacies FO. Gastropods belong to the species Pomatia elegans and Retinella sp., whose age distribution spans from the earliest late Pliocene to the Recent (Esu and Girotti 1991). They point to a subaerial depositional setting, characterized by vegetated areas in humid-temperate climatic conditions. On the whole, the malacofaunal association is paleoecologically consistent with the floodplain depositional setting inferred for the fines at the top of the lower FU succession. DISCUSSION

According to geometry and facies patterns, the VF fluvial deposits represent the infill of an incised valley which emanated from the western flank of the Chianti Ridge into the Siena Basin, as suggested by

JSR


409

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.

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, indicates that VF gravels represent a mix of Chianti bedrock (S. Fiora Fm and Macigno Fm) and Pliocene gravel sources.

410

M. ALDINUCCI ET AL.

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 usually results from a relative sea-level rise, which heads the progressive backfilling of the valley, generally with marine and/or brackish sediments. The second condition is related to a significant increase in sediment production (Schumm 1977), which promotes the downfilling of the valley with alluvial deposits. Specifically, the influx increase can result from an uplift of the source area, a climate change, or both (Schumm 1977; Blum and To¨rnqvist 2000). The middle Pliocene coastal area cut by the studied valley was characterized by a steep topographic gradient, owing to the short distance (typically , 1 km) between the rocky hinterland and offshore mud and the relatively narrow zone with littoral gravel and sand. This may account for the observed downcurrent decrease in valley depth, as the erosional processes implied in valley formation were inhibited by decreasing topographic gradient. It follows that the downcurrent correlative of the studied valley fill above Pliocene offshore mud could have been represented by fluvial deposits no longer confined within an incised valley and eroded during the Pleistocene. The case study conforms to the conceptual model of Schumm (1993), who states that coastal zones with steep gradients are more prone to incision than gently inclined coastal plains, because fluvial channels are more likely confined within broad incisions, hindering lateral erosion and wandering (Wood et al. 1993a). As a consequence, the resultant incised valleys are shorter than those on broad coastal plains and have relatively steep flanks (Wood et al. 1993b). Ages of valley incision and filling are based mainly on the stratigraphic position of the valley fill, the late Pliocene–Recent age of the recovered continental gastropods, and the middle Pleistocene age (Aculean artefacts; Magi 1992) of the overlying alluvial-fan strata of Unit Q1. According to Schumm and Ethridge (1994), river incision and valley widening within unconsolidated sediments is rapid, as confirmed also by experimental studies documenting rapid incision with subsequent valley widening and aggradation (Schumm et al. 1987). As a consequence, the studied valley was likely cut between the late middle Pliocene and the earliest late Pliocene and was filled during the latest Pliocene–early Pleistocene. In this context, valley incision can reasonably be related to the late middle Pliocene relative sealevel drop recorded in all of the central basins of Tuscany, due to the development of a regional-scale dome (Bossio et al. 1993; Martini and Sagri 1993; Bossio et al. 1995; Martini et al. 2001), which is still active (Bartolini et al. 1982). Such a dome led to the occurrence of Pliocene coastlines up to 850 m above modern sea level (Disperati and Liotta 1998) and is considered to 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 tideinfluenced 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 to a relative sea-level rise. The latter case can be ruled out by taking into account 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 reentering southern 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 with sequence-stratigraphic significance, its fluvial fill cannot be treated in the Exxon sequence-stratigraphic sense, because it is not associated with any relative sea-level rise. Under these conditions, climatic and tectonic factors, such as discharge variations relative to the availability of large amounts of sediments, govern sedimentary facies patterns and valley-fill architecture (Schumm 1993). In the Mediterranean, after a cooling phase accompanied by arid conditions at the middle to late Pliocene transition, an increasingly rainy

JSR

climate characterized the late Pliocene (Shackleton et al. 1995; Suc et al. 1995a; Suc et al. 1995b; Fauquette et al. 1999; Bertini 2001; Anadoń et al. 2002). These conditions persisted until the late early Pleistocene, when the climate cooled again and the whole Mediterranean area was arid (Suc et al. 1995a). In the inner Northern Apennines, such climatic changes coincided with late Pliocene–early Pleistocene tectonics (Boccaletti and Sani 1998; Bossio et al. 1998). These are represented by regional doming and fault-driven renewed uplift of basin margins, as attested by the presence of several unconformities in the basin fills together with the emplacement of coarsegrained alluvial fan and/or fan-delta deposits (Bossio et al. 1993; Martini and Sagri 1993). Such tectonic and climatic events have recently been well documented in the middle Pliocene–lower Pleistocene deposits (Fig. 12) of the western Upper Valdarno Basin (Ghinassi and Magi 2004; Ghinassi et al. 2004), located 10 km north of the Siena Basin and separated from it by 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 sediments and grade upward into floodbasin strata, in turn overlain by upper Pliocene–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 sediments rest unconformably on the fluvio-eolian deposits with no intervening floodbasin strata (Fig. 12C), thus testifying to the occurrence of a tectonic uplift (Albianelli et al. 1995; Ghinassi et al. 2004). The outlined stratigraphic architecture, coupled with facies assemblages and detailed palynological analyses (Albianelli et al. 1995), records the interaction between the late Pliocene tectonic uplift of Chianti Ridge and the global climatic transition from dry/cold to warm/humid conditions (Ghinassi and Magi 2004; Ghinassi et al. 2005). The geological history of the western Valdarno Basin provides valuable constraints to unravel the time of valley filling, because the incised valley was sourced by the same sector of the Chianti Ridge. Accordingly, the studied valley was likely cut during the late middle Pliocene–earliest late Pliocene following the relative sealevel drop recorded in all of the central basins of Tuscany. Although the possibility that part of the lower gravel may be related to the falling stage (Blum and To¨rnqvist 2000; Holbrook 2001) cannot be ruled out, the filling stage occurred in the latest Pliocene–early Pleistocene time span, due to the interplay between the late Pliocene uplift of the Chianti Ridge and the renewed humid climate. This promoted a remarkable increase in sediment influx to the valley (Fig. 13) resulting from enhanced erosion of Chianti Ridge and adjoining unconsolidated Pliocene sediments. Under these conditions the valley was filled by both downfilling and vertical aggradation 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 floodplain aggradation through overbank water from adjacent channel(s). However, this seems unrealistic, inasmuch as current facies models of coarse-grained braided rivers contemplate settling of thin bodies of fines within shallow pools resulting from abandoned channels (Miall 1996), whereas thick floodplain deposits flanking braided rivers are thought to be preserved only in basins with high subsidence rate (Bentham et al. 1993). As a consequence, extrinsic controls on sedimentation, such as climate change promoting less powerful aqueous flows, or source drainage evolution due to tectonics or geomorphologic causes (such as capture of the upper reach of the incised valley), may account for the deactivation of the lower braided system with deposition of such a thick horizon of fines. Furthermore, the lateral extent of this horizon within the incised-valley fill, likely extending from one flank to another, strongly supports an

JSR


411

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 study area. B) Scheme of the relations between depositional sequences in the southwestern Upper Valdarno Basin (rectangle inset of Part A showing the unconformity separating the fluvio-palustrine succession from the underlying fluvio-eolian deposits. This boundary marks the abrupt increase in sediment supply due to concurrent uplift of the Chianti Ridge and climatic change toward humid conditions. C) Example of the unconformity surface between fluvio-palustrine sandy gravels and fluvioeolian sands.

extrinsic control over deposition of fines. A climate change seems to be unlikely, because the occurrence of both organic-rich levels and humidtemperate gastropods in the thick horizon of fines suggests the persistence of the same climatic conditions during which the lower coarse-grained deposits 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 fines above 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 di Montalto’’ 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 and kinematics 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 gravels were 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 reestablishment of a new equilibrium profile of the river system.

412

M. ALDINUCCI ET AL.

JSR

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 the Siena 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 the incised valley following increase in sediment supply from the Chianti Ridge due to tectonic and increase in rainfall.

JSR

FLUVIAL INFILL OF A LATE PLIOCENE VALLEY (NORTHERN APENNINES, ITALY) 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 valley incision 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 coarsesediment 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 constructive criticism. The authors are also indebted to Co-Editor Colin P. North for his encouragements and comments. The manuscript benefit from helpful editorial assistance by John B. Southard. I. Peter Martini is thanked for reading an early version of this manuscript. We also thank D. Esu for fossil classification. This work was funded by PAR 2004 (F. Sandrelli). REFERENCES

AITKEN, J.F., AND FLINT, S.S., 1994, High-frequency sequences and the nature of incisedvalley fills in fluvial systems of the Breathitt Group (Pennsylvanian), Appalachian Foreland Basin, Eastern Kentucky, in Dalrymple, R.W., Boyd, R., and Zaitlin, B.A., eds., Incised-Valley Systems: Origin and Sedimentary Sequences: SEPM, Special Publication 51, p. 352–368. ALBIANELLI, A., BERTINI, A., MAGI, M., NAPOLEONE, G., AND SAGRI, M., 1995, Il bacino Plio-Pleistocenico del Valdarno superiore: eventi deposizionali, paleomagnetici e paleoclimatici: Il Quaternario, v. 8, p. 11–18. ALLEN, J.R.L., 1983, Gravel overpassing on humpback bars supplied with mixed sediment: examples from the Lower Old Red Sandstone, southern Britain: Sedimentology, v. 30, p. 285–294. ANADOŃ, P., BURJACHS, F., MARTIN, M., RODRIGUEZ-LAZARO, J., ROBLES, F., UTRILLA, R., AND VAZQUEZ, A., 2002, Paleoenvironmental evolution of the Pliocene Vollarroya Lake, northern Spain. A multidisciplinary approach: Sedimentary Geology, v. 148, p. 9–27. ARNOTT, R.W.C., ZAITLIN, B.A., AND POTOCKI, D.J., 2000, Geological controls on reservoir distribution in the Lower Cretaceous Basal Quartz, ChinCoulee–Horsefly Lake area, south-central Alberta: Bulletin of Canadian Petroleum Geology, v. 48, p. 212–229. BARBERI, F., GASPARINI, P., INNOCENTI, F., AND VILLARI, L., 1973, Volcanism of the southern Tyrrhenian Sea and its geodynamic implications: Journal of Geophysical Research, v. 78, p. 5221–5232. BARTOLINI, C., BERNINI, M., CARLONI, C.C., COSTANTINI, A., FEDERICI, P.R., GASPERI, G., LAZZAROTTO, A., MARCHETTI, G., MAZZANTI, R., PAPANI, G., PRANZINI, G., RAU, A., SANDRELLI, F., VERCESI, P.L., CASTALDINI, D., AND FRANCAVILLA, F., 1982, Carta neotettonica dell’Appennino Settentrionale. Note Illustrative: Societa` Geologica Italiana, Bollettino, v. 101, p. 523–549.

413

BENTHAM, P.A., TALLING, P.J., AND BURBANK, D.W., 1993, Braided stream and floodplain deposition in a rapidly aggrading basin: the Escanilla Formation, Spanish Pyrenees, in Best, J.L., and Bristow, C.S., eds., Braided Rivers: Geological Society of London, Special Publication 75, p. 177–194. BENVENUTI, M., AND MARTINI, I.P., 2002, Analysis of terrestrial hyperconcentrated flows and their deposits, in Baker, V., Martini, I.P., and Garzoń, G., eds., Floods and Megafloods, Processes and Deposits: International Association of Sedimentologists, Special Publication 32, p. 167–193. BERNINI, M., BOCCALETTI, M., MORATTI, G., PAPANI, G., SANI, F., AND TORELLI, L., 1990, Episodi compressivi neogenico–quaternari nell’area estensionale tirrenica nordorientale. Dati di mare e di terra: Societa` Geologica Italiana, Memorie, v. 45, p. 577–589. BERTINI, A., 2001, Pliocene climatic cycles and altitudinal forest development from 2.7 Ma in the Northern Apennines (Italy): evidence from the pollen record of Stirone Section (, 5.1 To , 2.2 Ma): Geóbios, v. 34, p. 253–265. BLAIR, T.C., AND BILODEAU, W.L., 1988, Development of tectonic cyclothems in rift, pull-apart and foreland basins: sedimentary response to episodic tectonism: Geology, v. 16, p. 517–520. BLUCK, B.J., 1982, Texture of gravel bars in braided streams, in Hey, R.D., Bathurst, J.C., and Thorne, C.R., eds., Gravel-Bed Rivers: New York, John Wiley & Sons, p. 339–355. BLUM, M.D., 1992, Climatic and eustatic controls on Gulf coastal plain sedimentation: An example from the late Quaternary of the Colorado River, in Armentrout, J.M., and Perkins, R.F., eds., Sequence Stratigraphy as an Exploration Tool: Concepts and Practices in the Gulf Coast: SEPM, Gulf Coast Section, Eleventh Annual Research Conference, Houston, p. 71–84. BLUM, M.D., AND PRICE, D.M., 1998, Quaternary alluvial plain construction in response to glacio-eustatic and climatic controls, Texas Gulf coastal plain, in Shanley, K.W., and McCabe, P.J., eds., Relative Role of Eustasy, Climate, and Tectonism in Continental Rocks: SEPM, Special Publication 59, p. 31–48. BLUM, M.D., AND TO¨RNQVIST, T.E., 2000, Fluvial response to climate and sea level: A review and look forward: Sedimentology, v. 47, p. 1–48. BOCCALETTI, M., AND GUAZZONE, G., 1974, Remnant arcs and marginal basins in the Cainozoic development of the Mediterranean: Nature, v. 252, p. 18–21. BOCCALETTI, M., AND SANI, F., 1998, Cover thrust reactivations related to internal basement involvement during Neogene–Quaternary evolution of the northern Apennines: Tectonics, v. 17, p. 112–130. BOCCALETTI, M., ELTER, P., AND GUAZZONE, G., 1971, Plate tectonics model for the development of the Western Alps and Northern Apennines: Nature, v. 234, p. 108–111. BOCCALETTI, M., CERRINA FERONI, A., MARTINELLI, P., MORATTI, G., PLESI, G., AND SANI, F., 1991, L’alternanza distensione–compressione nel quadro evolutivo dei bacini neogenici dell’Appennino settentrionale: Studi Geologici Camerti, Volume Speciale 1, p. 187–192. BOCCALETTI, M., BONINI, M., MORATTI, G., AND SANI, F., 1995, Le fasi compressive neogenico–quaternarie nell’Appennino Settentrionale: relazioni con l’evoluzione dei bacini interni e con la tettonica del basamento: Studi Geologici Camerti, Volume Speciale 1, p. 51–72. BONINI, M., AND SANI, F., 2002, Extension and compression in the Northern Apennines (Italy) hinterland: Evidence from the late Miocene–Pliocene Siena–Radicofani Basin and relations with basement structures: Tectonics, v. 21, p. 1–35. BOSSIO, A., COSTANTINI, A., LAZZAROTTO, A., LIOTTA, D., MAZZANTI, R., MAZZEI, R., SALVATORINI, G., AND SANDRELLI, F., 1993, Rassegna delle conoscenze sulla stratigrafia del neoautoctono toscano: Societa` Geologica Italiana, Memorie, v. 49, p. 17–98. BOSSIO, A., COSTANTINI, A., FORESI, L.M., LAZZAROTTO, A., MAZZANTI, R., LIOTTA, D., MAZZEI, R., SALVATORINI, G., AND SANDRELLI, F., 1995, Studi preliminari sul sollevamento della Toscana meridionale dopo il Pliocene medio: Studi Geologici Camerti, Volume Speciale 1, p. 87–91. BOSSIO, A., COSTANTINI, A., FORESI, L.M., LAZZAROTTO, A., MAZZANTI, R., MAZZEI, R., PASCUCCI, V., SALVATORINI, G., SANDRELLI, F., AND TERZUOLI, A., 1998, Neogene– Quaternary sedimentary evolution in the western side of the Northern Apennines (Italy): Societa` Geologica Italiana, Memorie, v. 52, p. 513–525. BROGI, A., 2004, Faults linkage, damaged rocks and hydrothermal fluid circulation: Tectonic interpretation of the Rapolano Terme travertines (southern Tuscany, Italy) in the context of Northern Apennines Neogene–Quaternary extension: Eclogae Geologicae Helvetiae, v. 97, p. 307–320. BUTLER, R.W.H., AND GRASSO, M., 1993, Tectonic controls on base-level variations and depositional sequences within thrust-top and foredeep basins: examples from the Neogene thrust belt of central Sicily: Basin Research, v. 5, p. 137–151. CAPUZZO, N., AND WETZEL, A., 2004, Facies and basin architecture of the Late Carboniferous Salvan–Doreńaz continental basin (Western Alps, Switzerland– France): Sedimentology, v. 51, p. 675–697. CARMIGNANI, L., AND LAZZAROTTO, A., 2004, Carta Geologica della Toscana: Litografia Artistica Cartografica Srl, Firenze, CARMIGNANI, L., DECANDIA, F.A., DISPERATI, L., FANTOZZI, P.L., KLIGFIELD, R., LAZZAROTTO, A., LIOTTA, A., AND MECCHERI, M., 2001, Inner Northern Apennines, in Vai, G.B., and Martini, I.P., eds., Anatomy of an Orogen; The Apennines and Adjacent Mediterranean Basins: Dordrecht, The Netherlands, Kluwer Academic Publishers, p. 197–214. COLLINSON, J.D., 1996, Alluvial sediments, in Reading, H.G., ed., Sedimentary Environments: Processes, Facies and Stratigraphy: Oxford, U.K., Blackwell Science, p. 37–82.

414

M. ALDINUCCI ET AL.

COSTANTINI, A., LAZZAROTTO, A., AND SANDRELLI, F., 1982, Il graben di Siena, CNR, Progetto finalizzato Energetica, Sottoprogetto Energia Geotermica, Final Report, Pisa, 170 p. CREWS, S.G., AND ETHRIDGE, F.G., 1993, Laramide tectonics and humid alluvial fan sedimentation, NE Uinta uplift, Utah and Wyoming: Journal of Sedimentary Petrology, v. 63, p. 420–436. DALRYMPLE, R.W., BOYD, R., AND ZAITLIN, B.A., 1994, History of research, types and internal organization of incised-valley systems: introduction to the volume, in Dalrymple, R.W., Boyd, R., and Zaitlin, B.A., eds., Incised-Valley Systems: Origin and Sedimentary Sequences: SEPM, Special Publication 51, p. 3–10. DE BROEKERT, P., AND SANDIFORD, M., 2005, Buried inset-valley in the eastern Yilgarn Craton, Western Australia: Geomorphology, age, and allogenic control: Journal of Geology, v. 113, p. 471–493. DEWEY, J.F., PITMAN, W.C, III, RYAN, W.B.F., AND BONNIN, J., 1973, Plate tectonics and the evolution of the Alpine system: Geological Society of America, Bulletin, v. 81, p. 3137–3180. DISPERATI, L., AND LIOTTA, D., 1998, Estimating uplift of clay-filled extensional basin through the porosity–depth curve: the case of the Radicofani Basin (Italy): Annales Tectonicae, v. 12, p. 162–176. ESU, D., AND GIROTTI, O., 1991, Late Pliocene and Pleistocene assemblages of continental molluscs in Italy. A survey: Il Quaternario, v. 4, p. 137–150. FAUQUETTE, S., SUC, J.-P., GUIOT, J., DINIZ, F., FEDDI, N., ZHENG, Z., BESSAIS, E., AND DRIVALIARI, A., 1999, Climate and biomes in the West Mediterranean area during the Pliocene: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 152, p. 15–36. FRASER, G.S., 1994, Sequences and sequence boundaries in glacial sluiceways beyond glacial margins, in Dalrymple, R.W., Boyd, R., and Zaitlin, B.A., eds., Incised-Valley Systems: Origin and Sedimentary Sequences: SEPM, Special Publication 51, p. 337–351. GANDIN, A., AND SANDRELLI, F., 1992, Caratteristiche sedimentologiche dei corpi sabbiosi intercalati nelle argille plioceniche del Bacino di Siena: Giornale di Geologia, v. 54, p. 55–65. GHINASSI, M., AND MAGI, M., 2004, Variazioni climatiche, tettonica e sedimentazione al passaggio Pliocene Medio-Pliocene Superiore nel bacino del Valdarno Superiore (Appennino Settentrionale): Societa` Geologica Italiana, Bollettino, v. 123, p. 301–310. GHINASSI, M., MAGI, M., SAGRI, M., AND SINGER, B.S., 2004, Arid climate 2.5 Ma in the Plio-Pleistocene Valdarno Basin (Northern Apennines, Italy): Palaeogeography, Palaeoclimatology, Palaeoecology, v. 207, p. 37–57. GHINASSI, M., ABBAZZI, L., ESU, D., GAUDANT, J., AND GIROTTI, O., 2005, Facies analyses, stratigraphy and palaeontology (molluscs and vertebrates) in the Upper Pliocene sandy flood-basin deposits of the Upper Valdarno Basin (Northern Apennines): Rivista Italiana di Stratigrafia e Paleontologia, v. 111, p. 467–487. GUO, L., AND RIDING, R., 1992, Micritic aragonite laminae in hot water travertine crust, Rapolano Terme: Sedimentology, v. 39, p. 1067–1079. GUO, L., AND RIDING, R., 1994, Origin and diagenesis of Quaternary travertine shrub facies, Rapolano Terme, Central Italy: Sedimentology, v. 41, p. 499–520. GUO, L., AND RIDING, R., 1998, Hot spring travertine facies and sequences, late Pleistocene, Rapolano Terme, Italy: Sedimentology, v. 45, p. 163–180. GUO, L., AND RIDING, R., 1999, Rapid facies changes in Holocene fissure ridge hot spring travertines, Rapolano Terme, Italy: Sedimentology, v. 46, p. 1145–1148. HJELLBAKK, A., 1997, Facies and fluvial architecture of a high-energy braided river: the Upper Proterozoic Seglodden Member, Varanger Peninsula, northern Norway: Sedimentary Geology, v. 114, p. 131–161. HOLBROOK, J.M., 2001, Origin, genetic interrelationships, and stratigraphy over the continuum of fluvial channel form bounding surfaces: An illustration from middle Cretaceous strata, southeastern Colorado: Sedimentary Geology, v. 144, p. 179–222. JONES, S.J., FROSTICK, L.E., AND ASTIN, T.R., 2001, Braided stream and flood plain architecture: the Rio Vero Formation, Spanish Pyrenees: Sedimentary Geology, v. 139, p. 229–260. LAVECCHIA, G., 1988, The Tyrrhenian–Apennines system: Structural setting and seismotectogenesis: Tectonophysics, v. 147, p. 263–296. LOCARDI, E., 1982, Individuazione di strutture sismogenetiche dall’esame dell’evoluzione vulcano-tettonica dell’Appennino e del Tirreno: Societa` Geologica Italiana, Memorie, v. 24, p. 569–596. LOWE, D.R., 1988, Suspended load fallout rate as an independent variable in the analysis of current structures: Sedimentology, v. 35, p. 765–776. MAGI, M., 1992, Depositi fluviali e di conoidi alluvionali del Pleistocene nell’alta Valle dell’Ombrone (Siena): Societa` Geologica Italiana, 76u Riunione Estiva, Riassunti, 183 p. MARTINI, I.P., AND SAGRI, M., 1993, Tectono-sedimentary characteristics of the late Miocene–Quaternary extensional basins of the Northern Apennines, Italy: EarthScience Reviews, v. 34, p. 197–233. MARTINI, P., SAGRI, M., AND COLELLA, A., 2001, Neogene–Quaternary Basins of the Inner Apennines and Calabrian Arc, Italy, in Vai, G.B., and Martini, I.P., eds., Anatomy of an Orogen; The Apennines and Adjacent Mediterranean Basins: Dordrecht, The Netherlands, Kluwer Academic Publishers, p. 375–400. MERLA, G., 1951, Geologia dell’Appennino Settentrionale: Societa` Geologica Italiana, Bollettino, v. 70, p. 95–382. MIALL, A.D., 1985, Architectural-element analysis: a new method of facies analysis applied to fluvial deposits: Earth-Science Reviews, v. 22, p. 261–308. MIALL, A.D., 1996, The Geology of Fluvial Deposits: Heidelberg, Springer-Verlag, 582 p.

JSR

NEMEC, W., AND MUSZYN´SKI, A., 1982, Volcaniclastic alluvial aprons in the Tertiary of Sofia district (Bulgaria): Annales Societatis Geologorum Poloniae, v. 52, p. 239–303. PAOLA, C., HELLER, P.L., AND ANGEVINE, C.L., 1992, The large-scale dynamics of grainsize variation in alluvial basin, 1: theory: Basin Research, v. 4, p. 73–90. PASCUCCI, V., MARTINI, I.P., SAGRI, M., AND SANDRELLI, F., 2007, Effects of the transverse structural lineaments on the Neogene–Quaternary basins of Tuscany (inner Northern Apennines, Italy), in Nichols, G., Paola, C., and Williams, E.A., eds., Sedimentary Processes, Environments and Basins—A Tribute to Peter Friend: International Association of Sedimentologists, Special Publication 38, in press. PATACCA, E., AND SCANDONE, P., 1986, Struttura geologica dell’Appennino emilianoromagnolo: ipotesi sismotettoniche: Atti Convegni Lincei, v. 80, p. 157–176. SCHUMM, S.A., 1977, The Fluvial System: New York, John Wiley, 338 p. SCHUMM, S.A., 1993, River response to base-level changes: Implications for sequence stratigraphy: Journal of Geology, v. 101, p. 279–294. SCHUMM, S.A., AND ETHRIDGE, G., 1994, Origin, evolution and morphology of fluvial valley, in Dalrymple, R.W., Boyd, R., and Zaitlin, B.A., eds., Incised-Valley Systems: Origin and Sedimentary Sequences: SEPM, Special Publication 51, p. 11–27. SCHUMM, S.A., MOSLEY, M.P., AND WEAWER, W.E., 1987, Experimental Fluvial Geomorphology: New York, John Wiley, 413 p. SERRI, G., INNOCENTI, F., AND MANETTI, P., 2001, Magmatism from Mesozoic to present: Petrogenesis, time–space distribution and geodynamic implications, in Vai, G.B., and Martini, I.P., eds., Anatomy of an Orogen; The Apennines and Adjacent Mediterranean Basins: Dordrecht, The Netherlands, Kluwer Academic Publishers, p. 77–102. SESTINI, G., 1970, Postgeosynclinal deposition: Sedimentary Geology, v. 4, p. 481–520. SHACKLETON, N.J., HALL, M.A., AND PATE, D., 1995, Pliocene stable isotope stratigraphy of Site 846: Ocean Drilling Program, Proceedings, Scientific Results, v. 138, p. 337–355. SHIH, S.M., AND KOMAR, P.D., 1990, Hydraulic controls of grain-size distributions of bedload gravels in Oak Creek, Oregon, USA: Sedimentology, v. 37, p. 367–376. SMITH, D.G., AND SMITH, N.D., 1980, Sedimentation in anastomosed river systems: examples from alluvial valleys near Banff, Alberta: Journal of Sedimentary Petrology, v. 50, p. 157–164. SMITH, G.A., 1986, Coarse-grained nonmarine volcaniclastic sediments: terminology and depositional process: Geological Society of America, Bullettin, v. 97, p. 1–10. SOHN, Y.K., RHEE, C.W., AND KIM, B.C., 1999, Debris flow and hyperconcentated flood-flow deposits in an alluvial fan, northwestern part of the Cretaceous Yongdong Basin, central Korea: Journal of Geology, v. 107, p. 111–132. STEEL, R.J., AND THOMPSON, D.B., 1983, Structures and textures in Triassic braided stream conglomerates (‘‘Bunter’’ Pebble Beds) in the Sherwood Sandstone Group, North Staffordshire, England: Sedimentology, v. 30, p. 341–367. STRONG, N., SHEETS, B., HICKSON, T., AND PAOLA, C., 2005, A mass-balance framework for quantifying downstream changes in fluvial architecture, in Blum, M.D., Marriot, S.B., and Leclair, S.F., eds., Fluvial Sedimentology VII: International Association of Sedimentologists, Special Publication 35, p. 243–253. SUC, J.P., BERTINI, A., COMBOURIEU-NEBOUT, N., DINIZ, F., LEROY, S., RUSSO-ERMOLLI, E., ZHENG, Z., BESSAIS, E., AND FERRIER, J., 1995a, Structure of West Mediterranean vegetation and climate since 5.3 ma: Acta Zoologica Cracoviensia, v. 38, p. 3–16. SUC, J.P., DINIZ, F., LEROY, S., POUMOT, C., BERTINI, A., DUPONT, L., CLET, M., BESSAIS, E., ZHENG, Z., FAUQUETTE, S., AND FERRIER, J., 1995b, Zanclean (, Brunssumian) to early Piacenzian (, early–middle Reuverian) climate from 4u to 54u north latitude (West Africa, West Europe and West Mediterranean areas): Rijks Geologische Dienst, Mededelingen, v. 52, p. 43–56. TUNBRIDGE, I.P., 1984, Facies model for a sandy ephemeral stream and clay playa complex: the Middle Devonian Trentishoe Formation of North Devon, U.K.: Sedimentology, v. 31, p. 697–715. WEZEL, F.C., 1982, The Tyrrhenian Sea: a rifted krikogenic-swell basin: Societa` Geologica Italiana, Memorie, v. 24, p. 531–568. WILLIS, B.J., 1997, Architecture of fluvial-dominated valley-fill deposits in the Cretaceous Fall River Formation: Sedimentology, v. 44, p. 735–757. WOOD, L.J., ETHRIDGE, F.G., AND SCHUMM, S.A., 1993a, An experimental study of the influence of subaqueous shelf angles on coastal plain and shelf deposits: American Association of Petroleum Geologists, Memoir 58, p. 381–391. WOOD, L.J., ETHRIDGE, F.G., AND SCHUMM, S.A., 1993b, The effect of base-level fluctuations on coastal-plain shelf and slope depositional systems: an experimental approach, in Posamentier, H.W., Summerhayes, C.P., Haq, B.U., and Allen, G.P., eds., Sequence Stratigraphy and Facies Associations: International Association of Sedimentologists, Special Publication 18, p. 43–53. WRIGHT, V.P., AND MARRIOT, S.B., 1993, The sequence stratigraphy of fluvial depositional systems: the role of floodplain sediment storage: Sedimentary Geology, v. 86, p. 203–210. ZAITLIN, B.A., DALRYMPLE, R.W., AND BOYD, R., 1994, The stratigraphic organization of incised-valley systems associated with relative sea-level change, in Dalrymple, R.W., Boyd, R., and Zaitlin, B.A., eds., Incised-Valley Systems: Origin and Sedimentary Sequences: SEPM, Special Publication 51, p. 45–60. Received 4 August 2005; accepted 29 October 2006.