Braided to meandering channel patterns in humid-region alluvial fan deposits, River Reno, Po Plain (northern Italy)

Sedimentary Geology, 31 (1982) 231--248 Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands

231

BRAIDED TO MEANDERING CHANNEL PATTERNS IN HUMIDREGION ALLUVIAL FAN DEPOSITS, RIVER RENO, PO PLAIN (NORTHERN ITALY)

GIAN GABRIELE ORI

Istituto di Geolo~a e Paleontologia, Universitd di Bologna, 40127 Bologna (Italy) (Received January 20, 1981; revised and accepted September 23, 1981) ABSTRACT Ori, G.G., 1982. Braided to meandering channel patterns in humid-region alluvial fan deposits, River Reno, Po Plain (northern Italy). Sediment. Geol., 31: 231--248. During the Quaternary, alluvial sediments filled the Po Basin under humid climate conditions. Along the southern margin of the basin, a thick wedge of coalescing fans was built: the Reno Fan is one of the best examples. The River Reno, in its mountain reach (source area), drains m~lange-like rocks (sedimentary and ophiolitic exotic bodies embedded in mud) and arenaceous,, flyseh-like units. The Reno Fan deposits are mostly composed of gravels and show, in the upper part, three facies: (1) Proximal Fan: gravel deposits (longitudinal bars) consisting of lenticular units of trough shape (2--5 m wide, 1--2 m deep) cutting each other; (2) Transitional Zone (intermediate fan): gravel bodies with epsilon (sigmoidal) cross-stratification, 1--4 m thick and 10--50 m wide, with planeparallel or slightly concave-up bases (lateral bars); and (3) Distal Fan: gravel and sand tabular bodies (2--5 m thick, 40--300 m wide), showing epsilon cross-stratification (point bars). Interchannel deposits (silt and mud) exhibit horizontal stratification, root fragments, and a terrestrial molluscan fauna. In the proximal area, the deposits show evidence of multi-channel low-sinuosity streams (braided pattern), with aggradational filling. In the transitional zone, channel sinuosity increases and lateral accretion also occurs in braided pattern deposits (lateral bars). In the distal fan, channels become single and meandering. Evidence of this downfan facies change may be found in: (a) channel geometries; (b) types of channel fills; (c) ratio of lateral extent to thickness of the bodies; and (d) spatial arrangement of the facies.

INTRODUCTION

Most available descriptions of alluvial fan deposits, both ancient and modern, concern sediments deposited under semi-arid, arid and proglacial conditions. Two prevalent sediment types occur in such fans: braided stream facies (see Rust, 1978b, 1979), and mass flow deposits (see Bull, 1972; Rust, 1979). Braided stream deposits of alluvial fans are similar to those accumulating in alluvial plains, and criteria for distinguishing them, based only on structures and textures, do not seem to exist (Rust, 1978b). Debris-flow 0037-0738/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

232

deposits represent one of the main criteria for the recognition of ancient alluvial fan deposits {fanglomerates). They have been found in ancient examples believed to have been deposited under humid climate conditions (Wasson, 1977, Heward 1978a; Crawford and Dott, 1979). Boothroyd and Nummedal (1978) have suggested that proglacial outwash may provide a "model for humid alluvial fans", but this suggestion does not seem to consider the role of vegetation in more temperate and warm climates. So the arid or proglacial alluvial fan models essentially remain the only ones available. Humid alluvial fans can be considerable physiographic features in temperate and humid regions (Gole and Chitale, 1966; Wasson, 1978; Parkash et al., 1980). They are widespread along the margin of several young mountain chains, e.g. the Apennines, Alps and Himalayas. Along the southern margin of the Po Plain, gravel sedimentation persisted during the Holocene under humid climate conditions (atlantic to sub-boreal). Among the coalescing alluvial fans of this region (Fig. lb) the River Reno Fan is the largest and best known as far as its physiography and gross stratigraphy are concerned (Francavilla et al., 1976; Ori, 1979; Francavilla et al., 1980; Francavilla and Colombetti, 1980) (Figs. lc and 2). The fan deposits

c

• quarw "~e

"f"

8.70~/~::e

".,5}

Fig. 1. Index m a p of southern Po Plain (b), and quarry locations (numbers are referred to figures) in the Reno Fan (c).

233

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are composed mostly of clast~supported gravels, and unlike fans previously described, they exhibit evidence of meandering channels. Three facies are recognized in the Recent deposits: (1) proximal fan with braided stream deposits; (2) transition zone where lateral bar deposits (low-sinuosity meanders) coexist with braided facies; and (3) distal fan with prevalent point bar sediments (higher-sinuosity meanders). The Reno Fan provides a good opporTABLE I Q u a n t i t a t i v e c h a r a c t e r i z a t i o n s o f facies

Quarry (1)

1 2 3 4

5 6 7

Channel-bar facies

proximal proximal proximal transitional transitional distal distal distal transitional distal

Grainsize mean

Max. clast size

Sand-mud deposits

Abandoned channel

Overbank deposits

c m (2)

c m (3)

(%)

deposits

(%)

3.2 1.8 3 2.9 -1.2 1.5 0.03 1.8 --

13.6 12 12.4 11.4 11 11.5 12 sand 12.2 11.3

1 5 20 17 25 40 75 100 15 20

1 3 12 14 13 10 11 -8 5

-----17 40 100 ---

(1) N u m b e r p r o g r e s s i o n s h o w d o w n f a n direction• ( 2 ) B axis. (3) T e n largest clasts, b axis.

(%)

234

tunity to study the transition from braided to meandering channel patterns. Although this work is primarily structural and geometrical, there are corresponding textural changes (gravel-to-sand changes and improvement of sorting) (Table I). Quarries, 2 0 - 3 0 m deep, excavated in the River Reno area and several scattered elsewhere in the Po Plain have enabled me to study facies and lateral changes in the Quaternary deposits of t h e Reno Fan area and its surroundings. Deeper subsurface sediments of the Po Plain have been examined in almost 140 wells drilled by AGIP for gas exploration in older sediments. Unfortunately, no cores were obtained by AGIP in the Quaternary sediments and electric logs are in many cases absent or incomplete. SOURCE A R E A

The Po Plain is the largest alluvial plain in Italy. The Po River flows from the Westen Alps eastward into the Adriatic Sea, with its general direction parallel to the tectonic strike of the Apennines and Alps. The Apennine tributary rivers flow perpendicular to this trend (Fig. lb). In the southern part of the plain (Emilia Romagna), tributary rivers drain two main rock types in their source area (Apennines): to the east of the "Sillaro Line" (Fig. l b ) there is a turbidite formation (Marhoso Arenacea) that chiefly consists of sandstone-mudstone alternations; to the west, chaotic terranes of m~lange-type crop out (Ricci Lucchi, 1975). The latter are composed of large sedimentary formations (sandstone, cherts, limestone, marls) and ophiolitic exotic bodies embedded in muds. Typical of this source area is the availability of huge amounts of mud and gravel, with a relative deficiency of sand. A further supply of gravel lies in valley terraces eroded by the modern streams. THE PO PLAIN SETTING

The Po Basin is subsiding strongly (1 m m / y r ; Selli, 1967), and has a correspondingly high rate of sedimentation. The alluvium is thicker at, or near, the marginal escarpment and thins towards the central axis (Po River) and Adriatic Sea. Along a marginal fault near Bologna (Fig. 2), where the Reno Fan occurs, the escarpment is steeper than to the east or west. Two depositional systems are recognized: the alluvial fans and the alluvial plain; tributary rivers and main rivers (Po River) are sub-systems of the latter systems (Fig. lb). The coalescing alluvial fans border the plain in a narrow piedmont belt 6--7 km wide. The deposits show evidence of gravel-bed braided streams, and the proximal deposits of the River Reno Fan are typical (see below). Mean topographic gradient is in proximal area 5 m / k m , 3 m / k m in intermediate fan, and 1.8 m / k m in distal fan. In the sub-surface, electric logs mainly display fining upward sequences 20--60 m thick, which represent episodes of recession of the fan system. This is consistent with the surficial stratigraphy

235 of the Reno Fan inferred from quarry section. The gravels of the fans pass downflow into sandy~silty ribbons, 2--3 km wide, of the Apennine tributary rivers of the alluvial plain system (Fig. lb). The ribbons represent multistorey channel bodies, oriented roughly north-south. Channels seen in quarry exposures are nearly symmetrical in cross section, 2--5 m deep and 5--10 m wide. The fill is composed of fine to coarse sandy layers, 1--20 cm thick, interbedded with mud. Other channelized bodies, with no obvious channel shape, are made up of trough cross-bedded sand. Flood-plain deposits (mottled and silty layers) lie between the ribbons or within them. Electric logs show monotonous fine-grained sediments with coarser bodies (sometimes with fining-up sequences) several meters thick. The mean topographic gradient of this sub-system varies from 0.2 to 0.8 m/km. The Po River carries a large amount of sand through the plain, forming a meander belt, with west--east trend, several kilometers wide. Wells drilled near the channel show well-defined bodies, 10--25 m thick. Three of these bodies have been traced over more than 7 km, almost transverse to flow, thus showing they belonged to a large river system. RENO FAN DEPOSITS The Reno Fan deposits are 380 m deep (Francavilla et al., 1976; data AGIP) and consist of gravel and sand sediments organized in cycles 10--30 m thick. The radial gradient along the palaeo-fan surface is approx. 2.5 m/km, and the axial length 8 km. Due to the activities of man, the modern River Reno is degradational and cuts down as deep as 5--10 m into its own older deposits.

Proximal fan The proximal fan is composed almost entirely of clast~supported gravels, poorly to moderately sorted, with no sedimentary structures. A certain degree of organization is shown by imbrication; structureless thin lenses of well-sorted fine gravel occur. Due to the lack of vertical variations of grainsize, bedding is not easily identified, but it is possible to see broad-concave erosional surfaces cutting each other, which produce lenticular units of trough shape 2--15 m wide and 1--2 m deep (Fig. 3). Medium-scale (mean set thickness 20 cm), trough cross-stratified sands sometimes fill the troughs in place of gravels; the sands are also present at channel margins as lenses 0.5 m thick. Figure 4 shows palaeocurrent pattern. These flat-bedded and lenticular massive gravel deposits are thought to have accumulated in longitudinal bars in low-sinuosity multi~channel streams (see Miall, 1977; Rust, 1978a and b). Lenticular geometry is typical, in fluviatile environments, of braided stream deposits (Allen, 1965; Williams and Rust, 1969; Eynon and Walker, 1974; Miall, 1977; Rust, 1978b). Each

236

overbank -

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braided

~

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V

Fig. 3. Braided stream deposits of the proximal fan (Birra Quarry, Location in Fig. lc).

237

n = 239

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E n=217

z

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imbrication

X-bedding (individual readings)

epsilon X-bedding ( i n d i v i d u a l readings)

vector mean

Fig. 4. Directional data from the three units o f the R e n o Fan.

trough, marked by an erosional base, represents a partial channel fill. Similar deposits also occur in the apical areas of fan systems dominated by stream flows (Boothroyd, 1972; Boothroyd and Ashley, 1975; Boothroyd and Nummedal, 1978; Rust, 1978b, 1979), and are found in ancient alluvial fans as well (McGowen and Groat, 1971; Howell and Link, 1979). The sand lenses are the fillings of abandoned channels (Howell and Link, 1979), or are deposited, during waning flow, on the sides of gravel bars (Rust, 1972; Boothroyd and Ashley, 1975). Transition zone

The gravels of this facies show sets of sigmoidal stratification 1--4 m thick, dipping at angles of 5--13 ° . Each set forms a well-defined body bound by plane-parallel or slightly curved surfaces. Sets have erosional bases and are 10--50 m wide. The shape of the bodies is tabular or gently lenticular with concave-up bases (Fig. 5). There is no evidence of any upward trend in grainsize, but in the upper part of the bodies there are at places small channels, 20--40 cm deep, filled with fine gravels or cross-bedded sands. These sig-

238

f

f

epsilon rat.

Fig. 5. Lateral bar deposits of the transition zone (intermediate fan). Bodies are gently channelled and filled with epsilon cross-stratification (Pigna Quarry, location in Fig. Ic).

moidal-bedded bodies intertongue with the braided facies found in the proximal fan area. The sigmoidal stratification may be interpreted as the record of lateral accretion of a point or side bar, i.e. as epsilon cross-stratification (Allen, 1963). In the transition zone (or intermediate area) these lateral bars occur along with braid bars, so a mixed pattern of multi-sinuous channels is inferred. The existence of " m i x e d " (both braiding and meandering) streams is observed both in sandy (Shelton and Noble, 1964; Schwartz, 1976, 1978) and pebbly (Smith, 1974; Bluck, 1974; author's observations) modern streams. Such an interpretation is suggested also by the small lateral extent of the channelled bodies (see below).

239

Distal fan Point-bar geometry is more clearly seen in these deposits (Ori, 1980). Here tabular bodies, 2--5 m thick, of sand or clast~upported, moderately sorted gravels, with erosional bases, exhibit epsilon cross-stratification, dipping up to 13 ° . The exposure size does n o t usually allow one to ascertain the lateral extent which, however, varies from minima of 40 to 300 m. Graval point bar bodies include three facies arranged in sequence (Fig. 6), from the base upward: channel floor, bar core, bar top. At the base of the sequence, the channel floor is covered by imbricated gravels, 20--50 cm thick. These gravels represent riffles or longitudinal bars which m a y occur in a n u m b e r of loci in sinuous streams, such as the thalweg in the bends, or the upstream apex of point bars and crossover reaches. The bar core includes clast~upported gravels with epsilon cross-stratification (Fig. 7), and does not display a well~iefined fining-upward trend (Ori, 1980, fig. 10). The bar top is composed of sandy lenses, 0.5--1 m thick and 5--12 m long (parallel to current), with concave bases either erosional or abrupt, marked by m u d layers or gravel lags. Sands display sets, 25 cm thick, of low-angle, cross-lamination and ripple cross-lamination. Continuous or lensiform gravels 80 cm thick overlie the sands in the upper part, in places showing foresets resting tangentially on trough cross-stratified sandy bottomsets with erosional bases. The sand~ravel lenses are interpreted as chute fill and/or chute-bar deposits (Fig. 6) (McGowen and Garner, 1970; Levey, 1978) or slough fills in the downstream apices o f gravel point bars (Ori and Ricci Lucchi, 1981). In some cases, the bar top facies is absent, in others it is thicker than half the body and laterally persistent. The sandy bodies, show epsilon cross-stratification. Within the epsilon

C~ channel

Barcore ~ / ~ / Fig. 6. Reconstruction of the gravel point bars in the distal fan.

240

overbank

I

5m Fig. 7. Epsilon cross-strafitication in the gravelly point bar deposits of the distal fan. Current towards observer (Airport Quarry, location in Fig. lc). strata, medium scale (50 cm thick) cross-stratification is present, dipping contrary to the epsilon strata dip (Fig. 8). This situation is typical of some point bar deposits (Puigdefabregas, 1973; Puigdefabregas and Van Vliet, 1978; Friend et al., 1979). Overbank (interchannel) deposits, when preserved, consist of mottled grey-brown silty-muds with abundant root fragments and a terrestrial molluscan fauna. The mean fine to coarse member thickness ratio is about 0.6. Abandoned channel fills vary from clay to sandy or pebbly-sandy "plug", sometimes with coarsening upward sequences. The latter fill type is probably due to reactivation of the abandoned channel, with reworking of inactive bars. The channels were 20--40 m wide and 2--5 m deep. While the sandy or sandy-gravelly point-bar deposits are reasonably well known (see Collinson, 1978 for references) there are few descriptions of

241

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.

..

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--~

~ -

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4'

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Fig. 8, Sandy point-bar deposits showing epsilon cross-stratification. Medium-scale crossstratification climbs on the epsilon cross-strata (Airport Quarry, location in Fig. lc).

TABLE II Diagnostic criteria of Reno Fan facies

Body lateral extent (W) Body thickness (h) W/h ratio Body shape Epsilon cross-strat, Associated facies

Proximal fan (braid bars)

Transition zone (lateral bars)

Distal fan (point bars)

1--15 m

13--50 m

40--300 m

1--2

1--4 m

2--6 m

10 broadly concave present braided stream

31 tabular present overbank

m

5 channel-like absent --

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B

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~

Sandy~nt

~

0verbank

0,sTAL FAN

5m 500m

bars

Fig. 9. Facies stratigraphy of the upper part of the R e n o Fan.

gravel point bars in either the ancient or modem record (Ricci Lucchi, 1975, 1978; Jackson, 1978; Gustavson, 1978; Ori, 1979; Ori and Ricci Lucchi, 1981), and their recognition is still quite questionable (see Jackson, 1978; and Rust, 1979). The recognition and interpretation of point bars in distal fans is discussed elsewhere (Ori, 1980; Ricci Lucchi et al., 1981; Ori and Ricci Lucchi, 1981), but here it is possible to define these gravel point-bar deposits as tabular bodies showing epsilon cross-stratification and erosional bases, with lateral extent at least 15 times their thickness (Table II). Other elements are the lateral and vertical association with fine,rained abandoned channel and overbank deposits (Figs. 7 and 9).

Stratigraphy The fan merges into the alluvial plain (Fig. 2) (tributary river sub-system). At the base of the alluvial fan succession fine deposits with foraminifera of lagoonal type occur; deeper, open marine sediments are of Pleistocene (PostCalabrian) age (Francavilla et al., 1980). Archaeological sites, of Bronze to Late Roman Empire age, have been found in the upper depositional sequences of the Reno Fan and correlative deposits. The stratigraphy of the upper part (about 30 m deep) of the alluvial sequence displays a general fining-upward trend in quarry sections. The proximal facies are overlain by transition facies (intermediate fan), which are in

243

turn overlain by distal fan facies (Fig. 9). This fining-upward trend is also shown by electric logs of drillings in the fan deposits (ENI 1973; AGIP unpubl. data). Other sequential features (also recognized in other deposits of the alluvial fan system) of the well logs are coarsening-upward cycles and coarsening-upward assemblages of fining-up cycles {higher rank, complex cycles). All these features are interpreted as the results of advances and retreats of the fans (due presumably to tectonic, geomorphical, eustatic or climatic causes, see Heward, 1978b for references and discussion). The main trend, however, marks a recessional evolution of the system connected with a rise in sea level or, more probably, a decrease in gradient and rate of sediment supply relative to subsidence. FACIES RELATIONSHIPS A N D THEIR G E O L O G I C SIGNIFICANCE

Within the context of an aggrading fluvial system, composed of an alluvial plain and marginal fans, a lateral gradation of facies and channel geometry can be reconstructed. In the Reno Fan in particular interesting downstream changes are found (Figs. 9 and 10). In the proximal area braided channels are

Braid bars Interfan Lateral

area

bars

10 Km

downfan not to scale

•

Braid bars

~

Lateral bars

~

Point bars

~

Overbank

//

Interfan

,Point bars

,

,,'%,"

deposits

Fig. 10. Postulated relations between the facies in a single fan body.

244

LATERAL

.

• .

BRAD BARS

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•

/

•

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oJi

2 / ~

• &~

o Vertical accretion • Lateral accretion

1 QO

4

1'0

2'0

5'0

100

21]0

Width, m Fig. 11. Plot o f t h e w i d t h versus t h e h e i g h t o f t h e c h a n n e l l e d bodies.

filled only by vertical accretion, in the transition zone the channel deposits represent a more sinuous multi-channel system, where lateral bars (lateral accretion) occur together with braided bars (vertical accretion). The extent of the lateral bar bodies is small (10--50 m), because in a braided stream, channels are not sinuous and their migration is controlled by random and sudden jumps. In contrast, individual point-bar bodies of the distal fan are wider, in response to regular shifting of a wider meander bend. Such relations are shown by a diagram (Fig. 11) where the lateral extent of each body is plotted against its thickness: the ratio of the vertically aggrading braided channel bodies (proximal fan, Fig. 3) lies below the value 7; channelled bodies with epsilon cross-stratification divide into lateral bars (Fig. 5) below the value 15, and point bars (Figs. 6, 7 and 8), above it. Further criteria for distinguishing the latter two bar types are the associated facies and the shape of the channel bodies: lateral bars are tabular, at places with gently concave-up bases, and are composed of gravel; point bars have tabular shape, and the base is cut into fine overbank sediments (Table II). The facies change, between these two bar types, is also marked by the downfan thickening of bar-top facies at the expense of b a r , o r e facies. Further downstream, the point bars are entirely sandy. So far the picture of downfan facies changes (Fig. 10) seems to support the hypothesis of a continuum of facies from gravelly braided stream to sandy meandering stream deposits, trough gravel-sandy point bar deposits. This idea, proposed by geologists of the Bureau of Economic Geology of the University of Texas (Austin) (McGowen and Garner, 1970; Brown, 1973; Gustavson, 1978) is disputed by Jackson (1978) who studied a set of modern rivers whose load ranges from mud to gravel. However, the fan envi-

245 ronment may be affected by a complex geomorphic history, e.g. entrenchments, terracing, abrupt changes in tectonic setting, coalescence of other fans, and it may also be influenced by the lithology of the drainage area. The features of the Reno Fan in part resemble those of other fans, in part show specific differences. In particular, the point-bar deposits, so far considered typical of the alluvial plain environment, are extensive in this fan. From the Reno Fan and previously described examples, it is clear that isolated observations on facies and textures may be inadequate to distinguish alluvial fan from alluvial plain deposits. The geologic-geographic setting must be taken into account (Bull, 1972; Heward, 1978b; Wasson, 1978; Rust, 1979). For example, the presence of an escarpment is the first condition for the existence of a fan (Wasson, 1979). In addition to setting, the processes must also be considered; in this respect, Wasson (1979) affirms that the process pattern may be dependent on the fan development. In the light of this evidence, the main features aiding in fan recognition, can be summarized: (a) marginal position in the basin (along a fault scarp); (b) rapid downfan change of facies and downfan pinchout of deposits (Figs. 2 and 9); (c) abrupt distal boundary; and (d) depositional direction normal to main basin trend (alluvial plain). The ancient and modern examples of fan deposits may be divided into two types: "disordered fans" (scarcely differentiated and dominated by mass flows), and fans dominated by stream flows (respectively, dry and wet fans of Schumm, 1977) the Reno Fan belongs to the latter type. Both fan types form alluvial and deltaic (fan delta) environments; in the latter case, only the distal area interfinger with marine deposits and the other fan facies remain largely the same as in continental setting. Fan deposits quite similar to those of the Reno Fan (i.e., with the same facies arrangement (Fig. 10) and/or point bar deposits), both in alluvial and deltaic settings, were frequently formed from the Late Messian to present day at the southern edge (Apennines margin) of the Po Basin (Ricci Lucchi, 1975; Ricci Lucchi et al.,

1981). CONCLUSIONS (1) The Po Plain is composed of marginal fans and an alluvial plain, and the main river (Po River) direction is normal to that of the alluvial fans (Fig. Ib). (2) The Reno Fan lies at the southern rim (Apenninic margin) of the plain. The source area outcrops are of m61ange-like rocks, producing a large amount of gravel and mud. The fan exhibits deposits of braided streams (longitudinal bars) in the proximal area, braided stream deposits associated with small bodies of sigmoidal cross-bedding (longitudinal bars plus lateral bars) in the middle, and laterally persistent tabular bodies with epsilon crossstratification (point bars) in distal areas (Fig. 10). (3) The clast-supported, gravel point-bar deposits are typically character-

246

istic of the fan, displaying a sequence of gross fining-up type, with deposits, at the top, which may be interpreted as chute bar, chute channel fill or slough fill. The core of the sequence is represented by gravel epsilon crossstratification (Fig. 6). (4) Lateral bar deposits and point bar deposits can be distinguished using the following criteria (Table II): (a) channel geometries; (b) types of channel fills; (c) ratio of lateral extent to thickness of the bodies; and (d) spatial arrangement of the facies. (5) The Reno Fan, with its textures, facies change, and point-bar deposits, is not an isolated example; in fact other similar deposits are known (Ricci Lucchi, 1975; Ricci Lucchi et al., 1981; Ori and Ricci Lucchi, 1981) in the Apenninic margin of the Po Basin. ACKNOWLEDGEMENTS

I wish to thank F. Ricci Lucchi for helpful advice and guidance, as well as L. Venturi for valuable assistance. AGIP Company provided stratigaphies and electric logs of the wells drilled in the Po Plain, and C. Venturini helped me in the elaboration of material. Further aid was given by F. Ricci Lucchi, P.F. Friend, K.A.W. Crook and A. Ramos, who critically read the manuscript. P. Ferrieri prepared the photographic materials.

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