Alluvial sand composition as a tool to unravel the Late Quaternary sedimentation of the Modena plain, northern Italy

The Modena alluvial plain is located at the northern side of the Northern Apennines thrust-and fold-belt, where streams draining the chain flow toward the north-east into the Po River. The alluvial plain is characterized by a spectacular abundance of archaeological sites of various ages and can be considered a natural laboratory for the reconstruction of the recent sedimentary evolution of the Po Plain. Detailed modal analyses of modern sands of the Modena Plain streams indicate that the provenance signal can be distinguished on the basis of key components, such as quartz, feldspar, carbonate and lithic fragments. The compositional fields of the streams depend on the extent of the watershed, the recycling of older fluvial sediments and the sediment input from tributary streams. The modal analyses demonstrate that sand composition of the major rivers (Panaro and Secchia) has not changed during the Holocene, when sediment production, storage and dispersal were probably dominated by colluvial aggradation in an environment characterized by a dense vegetation cover. In the Late Pleistocene, fluvial sands were characterized by higher feldspar contents compared with modern and Holocene sands. This feldspar abundance could reflect a high-frequency signal in sediment supply rates linked to secular variations of weathering processes and reveals the strong denudation and sediment removal conditions of the last glacial stage (15-18 kyr). The implication of this study is that provenance of Holocene sediments now buried in the floodplain can be determined by a simple comparison with modern sand composition. Sand composition studies may represent a useful tool to reconstruct the Pleistocene-Holocene fluvial sediment supply and the evolution of human settlements as function of climate and drainage system changes.


INTRODUCTION
Sand composition studies are useful for reconstructing the sedimentary history of a basin as a function of climatic-physiographic control of sediment production, supply and dispersal (Basu, 1985;Critelli et al., 1997;Weltje et al., 1998).These studies have a particular significance in areas such as the Late Pleistocene-Holocene Modena plain, where fluvial sediments buried a spectacular number of Neolithic, Iron Age, Bronze Age, Etruscan, Roman and Longobardian archaeological sites.The evolution of these human settlements through time can be investigated in detail by reconstructing the palaeogeography of the plain, which has been done mainly by stratigraphic (Gasperi et al., 1989) and geomorphological studies (Cardarelli et al., 2004;Panizza et al., 2004).Palaeochannel traces visible on the surface, for example, may help to reconstruct the ancient local drainage patterns, but the limitation of this method is that only the very late stage evolution of the plain can be investigated.Sediment composition studies may extend our knowledge of the sedimentary supply further back in time through the stratigraphic column, provided that fluvial sediment provenance can be distinguished.Moreover, the good chronological control available for the stratigraphy of the area allow us to investigate in detail the sediment compositional variability through time as a function of late glacial-interglacial climate cycles.This paper illustrates our studies on the stream sediments in the Modena Plain, where we have compared modern and ancient sands to reconstruct their compositional evolution through time.The purpose of this study is to provide a contribution to the understanding of the fluvial sediment supply to test the possibility to investigate the evolution of human settlements as a function of climate and drainage system changes.

GEOLOGICAL SETTING
The Modena alluvial plain area is located at the northern side of the Northern Apennines thrust-and fold-belt, where streams draining the chain flow toward the north-east into the Po River (Fig. 1).The Northern Apennines formed mainly during the Tertiary as consequence of convergence between the European and the Adria plates.The convergence consumed the interposed Tethyan oceanic crust with the formation of an accretionary prism, which, during the subsequent collisional phase, produced an orogenic wedge consisting of the following tectono-stratigraphic units (Bettelli & De Nardo, 2001): a) Tuscan-Umbria-Romagna units formed by deformation of the Adria passive margin; in the study area these units consist mainly of Upper Oligocene-Middle Miocene thick siliciclastic turbidite sequences and chaotic shaly assemblages (mélanges); b) Sub-Ligurian units consisting of Paleocene-Early Miocene siliciclastic and carbonate turbidite sandstones, conglomerates and shales; c) Ligurian units generated by the subduction of the Tethyan Ocean and represented mainly by Early Cretaceous-Early Tertiary chaotic deep water shaly rocks (mélanges and olistostromes) and calcareous and arenaceous turbidite sandstones; these units contain also Late Jurassic-Early Cretaceous ophiolites, chert and limestones, derived from the former oceanic floor sequence.
The Ligurian thrust-nappe units overlie the Sub-Ligurian thrust-nappe units and both lie on the Tuscan-Umbria-Romagna fold-and-thrust belt units.On the northern side of the chain, the Ligurian units are unconformably overlain by the Epi-Ligurian Sequence and by Miocene-Pliocene and Quaternary terrigenous deposits of the Po Plain.The Epi-Ligurian sequence consists of a thick Middle Eocene-Early Messinian succession of matrix-supported breccias (olistostromes), deep water varicolored shales and marls, muddy and sandstone turbidites, conglomerates, shallow water siliciclastic and bioclastic deposits.The Pliocene deposits consist mainly of marine deep water mudstones.The Po Plain is the syntectonic sedimentary wedge filling the Pliocene-Pleistocene Apennine foredeep.The total basin infill is up to 4 km thick and the Quaternary deposits reach a thickness of 1.5 km.

CLIMATE AND GEOMORPHOLOGY
The study area has an approximate extent of 150 km 2 and is limited by two major rivers, the Secchia River to the west and Panaro River to the east (see Tab. 1 for river data).The area is crossed by minor streams that mostly drain into the Panaro River (Fig. 2).The drainage basin climate is classified as "temperate subcontinental" (Mennella, 1972).Mean annual temperature varies from 8 to 12° C and coldest and hottest months are January and July, respectively.Precipitation varies with elevation, ranging from 800 to 2400 mm/yr and generally having two maxima during the year, at November and May, and a minimum in July.
As previously described, the streams drain areas that consist mainly of sedimentary rocks, such as arenites, siltstones and shales (Fig. 1).Magmatic rocks crop out along the main valleys as isolated ophiolite blocks including diabase, serpentinite and gabbro suites.No metamorphic rocks are present with the exception of very limited areas at the extreme tip of the Secchia river watershed, where amphibolites and quartzites crop out.River drainage basin data and estimate of the relative proportion of erodable terranes in the catchment areas are reported in Tab. 1 (Cati, 1981).Although the Secchia River has a larger drainage basin than the Panaro River (2174 versus 1784 km 2 ), the latter includes a larger proportion of erodable terranes and as a result is characterized by a higher suspended load (2030 versus 1847 t/km 2 ) and an higher inferred soil erosion (0.769 versus 0.684 mm/yr, Tab. 1).
The minor streams drain the northernmost side of the Apennine chain cutting mostly argillaceous and arenite sediments (Fig. 1).Their drainage system in the alluvial plain has been heavily modified since ancient time to limit the impact of floods on urban areas.The Cerca stream (Torrente Fossa), for example, was deviated into the Secchia River in the XV or XVI century (Fig. 2).Gasperi and Pizziolo (in press) report historical accounts that the Tiepido stream was deviated into the Panaro River; unfortunately the year when this deviation took place is unknown.

SAMPLES AND METHODS
A total of 42 samples of modern sands were collected in active longitudinal bars of 9 stream channels (Fig. 2).Sampling covers the channel segments between the point where the stream enters the floodplain and the last downstream occurrence of sand in the stream beds.The sampling network was designed to obtain sand specimens at approximately the same spacing distance for the each stream and to include, where possible, upstream and downstream confluence points.A total of 9 ancient sands were collected from river cuts, archaeological sites and cores located in the channel belt of the main rivers (Fig. 2).Unfortunately no sections with archaeological remains and ancient sand sediments are available for the minor streams.
Hereafter ancient sediments younger than the Late Pleistocene are called "Holocene" sands, whereas the term "modern" is restricted to present-day settings.Sand samples were washed with dilute H 2 O 2 to remove organic matter, air dried and sieved to obtain the fine sand fraction (0.125-0.250 mm, 3-2 φ).The necessity to analyze the fine sand fraction was dictated by the lack of medium-coarse sand at some of the sampling sites.The fine sand fraction was impregnated in epoxy resin under vacuum, thin-sectioned and stained for feldspar and carbonate identification.
Point-counting under transmitted light microscopy was performed according to the Gazzi-Dickinson method (Gazzi, 1966;Dickinson, 1970;Zuffa, 1985;Ingersoll et al, 1984).At least 300 grains were point-counted for each section to achieve modal composition and 40 categories of grains were distinguished.Results of point-counting are illustrated in Tab. 2.
Components not related to the original sand composition, such as brick and pottery fragments, penecontemporaneous shell fragments and authigenic carbonate nodules, were excluded from the final calculations.
Statistically rigorous confidence regions of the studied samples were calculated according to Weltje (2002).Error bars in the diagrams were calculated according to Howarth (1998).

Modern sands
The modal analyses show that modern stream sands in the Modena plain have a similar overall compositions, but show significant variations in quartz, feldspar, carbonate and lithic fragment contents (Tab.2, Fig. 3, Fig. 4).Plotting the data in Q+F-Total carbonates-Lithic fragments ternary diagram, the compositional fields reveal that sediments from different modern streams can be clearly distinguished (Fig. 4).Streams draining the eastern sector of the study area (Panaro, Tiepido, Guerro) have a generally higher carbonate content than those draining the western part, which are enriched in quartz and feldspar (Secchia and Grizzaga).
The most distinctive compositions are those of Guerro and Tiepido, which have the highest carbonate content, mainly represented by single calcite crystals (Fig. 4; Tab. 2).The Grizzaga stream shows the greatest variation in terms of the measured parameters, and therefore has the broadest compositional field (Fig. 4).

Holocene sands (< ~7 kyr)
Ancient sands collected in the channel belt of the major rivers (ages given by Gasperi and Pizziolo, in press) provide an exceptional and unique view of the compositional variations of the sediment through time.The Panaro River deposits at Spilamberto (Fig. 2) buried a spectacular series of Neolithic, Roman and Longobardian sites, and we can observe a complete sedimentary section ranging in age from the Late Pleistocene to present day (Fig. 5).The samples younger than ~7 kyr plot very close to the compositional field of modern sands (Fig. 6).In particular, these sands are slightly enriched in carbonate and lithic fragments and impoverished in quartz, compared with the modern ones (Fig. 6; Tab. 2).We analyzed two Holocene sands from the Secchia River: one has an inferred age of between 3540 ± 50 and 4585 ± 95 B.P. (sample FS B) and the other is slightly older than 4585 ± 95 B.P. (sample FS C; Tab.2; correlation with data from Cremaschi, 2000).These two samples plot directly into the compositional field of the modern sands (Fig. 6).

Late Pleistocene sands (> 10-12 kyr)
Ancient sands from fluvial deposits that are attributed to the last glacial event (Vignola Unit, Late Pleistocene, age 15-18 kyr, Gasperi and Pizziolo, in press) have higher contents of quartz and feldspar compared to both modern and Holocene sands (Tab.2).The feldspar content in Late Pleistocene sands (age >12 kyr) from cores located a few km north of Modena range from 13.5 to 30.8% (Lugli et al., 2004), whereas the range in the modern sands is from 6.9 to 21.3% (Tab.2).Higher feldspar abundances have also been detected in the Late Pleistocene sands from the Panaro River channel belt at Spilamberto (Fig. 2).Here, as shown in Fig. 7, feldspar content is highest in the Late Pleistocene (14.9-18.4%,Tab. 2), decreases in the Holocene (10.5-13.1%)and remains constant in the modern sediments (10.3-15.3%)with the noticeable exception of sample P12 (18.1%;Tab. 2) which, as discussed later, is influenced by direct supply of eroded Pleistocene sediments.

SEDIMENT DIAGENESIS
Correct interpretation of compositional data requires an understanding of the diagenetic processes that may have affected ancient sands.In the Modena plain, post-depositional modifications that may have altered the provenance signal of the sands are: chemical weathering of feldspar grains to form clay minerals and Al-hydroxides, dissolution of carbonate clasts and growth of carbonate concretions.
Survival of feldspar in the rock record is strongly influenced by climatic conditions during sediment production, storage and dispersal (James et al., 1981;McBride et al., 1996;Critelli et al., 2003).Feldspar preservation during burial diagenesis may be roughly evaluated by the relative proportions of altered versus unaltered grains.Point-counting of feldspar populations in the modern sands indicates that content of altered and unaltered feldspar grains is approximately the same (49-66%).The proportion does not vary downstream in modern sands and remains approximately the same in the Holocene buried sediments (Panaro River).
On the other hand, the most ancient sands that we analyzed (Late Pleistocene, age 15-18 kyr) have more abundant feldspar than the modern and Holocene sands (age <~7 kyr; Lugli et al., 2004) suggesting that post depositional chemical weathering during burial has not caused any significant selective destruction of feldspar grains (Fig. 3e).
Carbonate sand grains can be dissolved by groundwater, but our observations suggest that dissolution phenomena have not significantly changed the composition of the analyzed sediments.Observations under the microscope show that most calcite clasts (single calcite crystals) are not corroded and show distinctive rhombohedral angular shapes (Fig. 3b and c), a feature that is incompatible with post-depositional dissolution.Removal of carbonate components also appears to have been negligible because buried ancient sands have generally higher carbonate content than the modern ones (Panaro River, Fig. 6).Limited partial dissolution preferentially affected microcrystalline carbonate grains, but total dissolution leading to grain removal was probably mainly concentrated in the very fine grained fractions (<0.125 mm), and thus did not obliterate the original composition revealed by point-counting of the 0.125-0.250mm size fraction.Caliche formation is the main diagenetic process in the ancient sediments and affects also relatively young sediments that buried Roman Age artefacts (I-II AD).These microcrystalline carbonate nodules range in size from fractions of mm to a few cm and are usually concentrated in thin layers.The caliches have grown displacively into the fine-grained sediments (clay and silt) and are commonly associated with roots (Fig. 3f).Very small nodules, up to a few fractions of mm in size, may be present also in the sand layers.Their formation is early and linked to groundwater table oscillations and to the development of soils (Maffei, 2001).As discussed earlier, their presence in the ancient sand does not compromise the definition of the original composition because they can be easily recognized under the microscope as secondary particles and excluded from the calculations.In particular cases, such as recycling of older sediments, some of the eroded caliche fragments may be difficult to distinguish from microcrystalline carbonate grains.As a consequence, pointcounting could overestimate the microcrystalline carbonate content of some modern sand, but, as discussed earlier, the carbonate content is slightly higher in Holocene than in modern sands (Panaro River, Tab.2; Fig. 6).

DOWNSTREAM COMPOSITIONAL VARIATIONS: RECYCLING AND MIXING
The broad field composition of modern sand is due to both mixing of different sands at confluence points and recycling of older sediments along stream.Ancient sand deposits are cut and eroded by the main rivers (between sampling sites P 12 and P A for the Panaro River and between FS A and FS 9 for the Secchia River, Fig. 2).Because the compositions of the Holocene sands are similar to those of the modern sands, mixing of similar end members would not cause significant deviation of sediment composition downstream.
The only difference could be an increase of shale grains due to erosion of fine-grained sediments associated with the Holocene sands (Fig. 5).By contrast, Late Pleistocene sands are not usually associated with fine-grained sediments and differ considerably in composition.For this reason, recycling of Late Pleistocene sands produces a large compositional variation in modern sands.
The complex downstream compositional variations for the Secchia and Panaro Rivers are illustrated in Figs 8 and 9, respectively.The Secchia River compositional diagram can be divided in two parts: upstream and downstream of sampling site FS A (Fig. 8).In the upstream part the compositional pattern is characterized by a decrease in feldspar and lithic fragments and an increase in quartz content.This represents the expected trend by downstream labile grain destruction during transport.An exception is represented by the content of single calcite crystals, which increase downstream.This suggests that the main grain degradation mechanism is probably not abrasion, but mechanical breakage along cleavage planes.This process produces new smaller calcite rhombohedra, multiplying their content in the fine sand fraction.This hypothesis is supported by the observation that most single calcite grains show distinct rhombohedral shapes (Fig. 3a and d) in all sand fractions and that microcrystalline carbonate rock fragments decrease downstream, rather than increase.Mixing of sand input from the tributaries does not appreciably influence the composition of the Secchia River sediments, with the exception of the Fossa stream.The Fossa sand has a higher content of micritic carbonate grains, causing a reversal in the general decreasing trend shown by this grain type just after the confluence (between sampling sites FS 11 and FS 2, Fig. 8).
The downstream part of the Secchia River diagram shows a marked feldspar increase.This behaviour is related to new detrital input resulting from erosion of Late Pleistocene sediments, which have higher feldspar contents than modern and Holocene sands.This recycled sediment input, as we have verified in the field, is restricted to between sampling sites FS A and FS 7; at sample site FS 9 further downstream, the expected pattern of labile grain decrease (Fig. 8) returns.
In the Panaro River, downstream compositional variations are more complex.Most labile grains such as shale, siltstone and micritic carbonate fragments show such dramatic fluctuations that they cannot be simply related to mixing of different sands derived from the tributaries (Fig. 9).In this case, erosion of Holocene fine-grained river sediments results in multiple inputs of labile fragments that produce concurrent dilution effects exceeding by far the variations that related to the tributary streams.The marked increase in feldspar content at sampling site P 12 is the result of recycling of feldspar-rich Late Pleistocene sediments that are directly eroded in the riverbed.The feldspar signal is then attenuated after a few km possibly by grain abrasion/breakage, but most probably by downstream dilution, as testified by increase of other labile grains such as shale and siltstone fragments derived through erosion of Holocene river sediments (Fig. 9).Ophiolite fragments, which are by far the less abundant of the lithic fragments, appear to behave conservatively in both the Panaro and Secchia Rivers (Figs 8 and 9).Their content does not significantly change downstream in all studied streams and appears to be transport invariant, as discussed by Weltje and Von Eynatten, (2004).
The complex downstream compositional variations of the sands are shown schematically in Fig. 10.Although the interpretations proposed in this study should be confirmed by further analyses, it is important to note that the number of samples examined from this relatively small system is high compared with most similar studies reported in the literature

Modern sands composition: extent of watershed and recycling
On the basis of the available data, the compositional fields of the modern stream sands appear to be linked to: a) the extent of the watershed, b) recycling of ancient fluvial sediments, and c) sediment input from tributary streams.
The extent of the watershed appears to be the dominant factor controlling sand composition.This effect is directly related to the setting of bedrock units that are mostly oriented parallel to the Apennine margin and normal to the drainage systems (Fig. 1).The most distinctive sand compositions are those of the streams characterized by smaller watershed where sediment supply is deriving from only one or a few geological formations.This is the case of the Guerro and Tiepido streams, which display the most extreme sand compositions, exemplified by their high carbonate content (Fig. 4).
Recycling of ancient sediments, in particular the Late Pleistocene sands, which are particularly rich in feldspar, is the main factor that causes widening of the modern sediment composition fields.This effect is particularly evident for the Panaro River, which also receives a significant input of labile grains by erosion of ancient sediments (Fig. 9).Sediment input from tributaries has a subordinate effect on sediment composition because minor streams generally provide sand that is not dramatically different in composition.However, the Nizzola stream that is a tributary of the Panaro River (Tab.2; Fig. 2) represent an exception to this rule: sands differ significantly, being the Nizzola sand higher in quartz and lower in carbonates, but sediment input at confluence is very scarce and does not influence the Panaro River sand composition.

Holocene sands: a constant composition
Our data demonstrate that sand composition of the main rivers has not varied significantly in the last 7 kyr.Although constant sand composition for this time span can be demonstrated only for the major rivers (Panaro and Secchia), it seems reasonable that sediment composition of minor streams would also not have varied.On the other hand, palaeochannel traces and historical records indicate that the minor streams changed flow patterns many times, both by natural avulsions and artificial diversions (Gasperi and Pizziolo, in press).The result is that minor stream sediment supply patterns changed markedly through time, as a consequence of different confluence networks and different contributions of recycled older sand in their pathway.Thus, ancient sand compositions of minor streams are probably different to the modern streams.We are currently investigating this hypothesis by sampling palaeochannel sediments.

Late Pleistocene sand composition: the glacial signal
The significant shift in sand composition for sediments older than ~12 kyr, with an overall higher content of quartz and feldspar components could be explained by various factors: a) tectonics, b) diagenesis and c) climatic effects.
A change in the bedrock lithology induced by neotectonics could modify the sediment supply to the basin.Tectonics and climate are considered to be the major controlling factors for the Quaternary alluvial architecture at the southern margin of the Po basin in the Bologna area (Fig. 1; Amorosi et al., 1996).On the other hand, the latest documented tectonic uplift of the chain ended in the Reggio Emilia sector in the Late Pleistocene and the continental Late Quaternary deposits onlapping the Apennine margin appear undeformed (Barbacini et al., 2002).Moreover, as discussed earlier, post-depositional diagenetic changes, such as feldspar alteration and carbonate dissolution, do not seem to have varied the sand composition.
Climate changes related to glacial-interglacial phases appear to be the most important factor that modified sediment composition at ~12 kyr by changing bedrock weathering rate and removal of weathering products.This shift in composition represents a clear supply signal in the basin fill record.High-frequency variations in the rate of sediment supply (order of 10 kyr) are recorded as secular variations of the extent of weathering (Weltje et al., 1998).The change in composition suggests that sediments are today subjected to more prolonged chemical weathering during storage in the source area, but that alteration was negligible during storage in the alluvial sequence, at the contrary of what is reported for tropical climate settings (Johnsson et al., 1991).Thus, the sediment supply shift could reflect changes from a stage of marked denudation, such as during the last glacial maximum (15-18 kyr, Orombelli and Ravazzi, 1996) to a stage of colluvial aggradation and soil formation with dense vegetation cover (since ~7 kyr).

CONCLUSIONS
In the Late Pleistocene-Holocene sedimentary record of the Modena floodplain, the composition of fluvial sands has provided an unique opportunity to recognize and isolate the signature of factors controlling sediment supply and the nature of their complex interactions.Modal analyses of modern sands from the Modena Plain streams indicate that their provenance signal can be clearly distinguished.The compositional field of modern sands from each stream depends, in order of importance, on the extent of the watershed, the recycling of older fluvial sediments and, subordinately, on the sediment input from tributary streams.

Fig. 1 -
Fig. 1 -Geological sketch of the drainage basins of the Secchia and Panaro River in the northern Apennines.Simplified from Bettelli and De Nardo (2001).

Fig. 2 -Fig. 3 -
Fig. 2 -Simplified sketch of the fluvial drainage network in the Modena Plain with sampling sites of modern and ancient sands.

Fig. 8 -Fig. 9 -
Fig. 8 -Plot of downstream compositional variability of Secchia River sands as a function of distance from source and confluence points of tributary streams (arrows).a) recalculated parameters, b) all grain types.Errors bars are not reported for simplicity; error bar amplitudes

Fig. 10 -
Fig. 10 -Sketch summarizing the downstream compositional variations of the modern stream sands as a function of transport and erosion of ancient sediments.