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Diverging drainage in the Marche Apennines (central Italy)
Quaternary International 101–102 (2003) 203–209
Diverging drainage in the Marche Apennines (central Italy) Olivia Nesci*, Daniele Savelli Istituto di Geologia, Universita" degli Studi, Campus Scientifico, Localita" Crocicchia, 61029 Urbino, Italy
Abstract In the Marche Apennines diverging drainage developed on the surface of mid-Pleistocene fanglomerates or, at places, on correlated erosional glacis. In mountain areas, diverging nets correspond with wide glacis shaped in severe cold climates at the end of the middle Pleistocene. Glacis were formed by deposition of coalescing alluvial fans and concurrent shaping of smooth erosional surfaces along and near their margins. In the coastal zone, wide fans and related fluvial plains were constructed in Crotonian times close to the mouth of trunk streams crossing a just emerged piedmont area. Both the control of the ‘‘cone shaped’’ topography on post-aggradational flow patterns and the entrenching of primitive distributary channels are conceivable in developing the diverging drainage of the study area. Four case studies, from areas differing both in age and in geological and structural settings, attest to the recurrence of this drainage in composing, together with other patterns, the grain of larger nets. Moreover, the diverging drainage of the coast area allows us to infer that superposition is an effective mechanism of net development on Plio-Pleistocene terrains of the Marche Apennine piedmont at the time when they became emergent. r 2002 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction Anticlinal ridges and foothills of the Marche Apennines (Central Italy) are crossed by transverse trunk valleys flowing northeastward into the Adriatic Sea. The rivers often maintain their course cutting gorges several hundred metres deep through topographic barriers, a behaviour stressed by a long-lasting debate (e.g. Mazzanti and Trevisan, 1978, and Alvarez, 1999 and references therein). The tributary net, in contrast, mostly accommodating itself to geological structure, lithology and Quaternary deposits, commonly displays a different and more complex drainage (e.g. Bartolini, 1992). Our research is focused on peculiar diverging drainage found on piedmonts of various extension, age and origin, which is of some interest in understanding substantial topics of hydrographic net development on the foothills of the Marche Apennines.
2. Morphostructural setting and drainage development The Marche Apennines (Fig. 1) consist of northeast verging thrusted anticlines involving a Meso-Cenozoic *Corresponding author. E-mail address: [email protected] (O. Nesci).
sedimentary succession. Carbonatic anticlines, generally conforming to higher relief, exert the main control on the morphostructure of the internal areas. Along the northern coast, between Gabicce and M. Conero, coastal hills (summit elevations not exceeding ca. 200 m) occur in correspondence with thrusted anticlines. To the south (e.g. near Porto S. Giorgio), similar but less pronounced coastal structures are buried beneath Plio-Quaternary roughly undeformed sediments (e.g. Cantalamessa et al., 1990). A wide hilly piedmont develops on Plio-Quaternary terrains cropping out between the chain and the Adriatic Sea (Fig. 1). North of M. Conero, as well as in the internal zones of the southern area, thrusted anticlines are responsible for a piedmont landscape characterised by anticlinal hills and homoclinal ridges of different size and orientation. In contrast, south of M. Conero promontory, an overall homoclinal arrangement of Plio-Pleistocene terrains along the coastal zone, is accountable for wide, seaward-dipping cuestas. According to the evolution model of Mayer et al. (2002), the primitive embryos of the modern hydrographic net could develop in the most internal and uplifted areas as long ago as the Messinian, in directions both transverse and/or longitudinal with respect to the chain. The final emergence, that in the piedmont areas was attained during the upper Pliocene to lower-mid
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Fig. 1. Geological sketch of the Marche Apennines and location of the study areas and localities cited in the text. CZ=coastal zone; CI=Cingoli piedmont; CS=Monti della Cesana piedmont; MT=Monte Titano area (S. Marino).
Pleistocene timespan through a complex history of transgressions and regressions (e.g. Cantalamessa et al., 1986), greatly enlarged the drained surface to the east. Thus, a significant eastward drainage extension took place, drawing transverse trunk valleys and a complex net of tributaries (e.g., strike-, dip-, antidip-tributary streams). In the mid-upper Pleistocene and Holocene, ultimately, a generalised, persistent tectonic uplift forced the drainage to entrench (cf. Bisci and Dramis, 1991; Bigi et al., 1995; Fanucci et al., 1996), often more than 30–50 m deep, allowing terrace formation, upstream extension, and captures, but preventing important lateral shifting and/or avulsions. Both before and during the net entrenchment diverging patterns of minor streams were formed in the coastal zones as well as in the internal ones. Using sample-areas of the Marche Apennines, this net arrangement, indicating similar genetic mechanisms in quite different morphostructural contexts and in separate times, is analysed.
3. Examples of diverging drainage in the Marche Apennines 3.1. Adriatic coast south of M. Conero The coastal area between M. Conero and the Tronto River mouth (CZ, Fig. 1), consists (cf. Cantalamessa et al., 1986) of a more than 900 m thick suite of Pleistocene epi-bathyal to neritic and littoral sediments. Emilian clayey marine deposits are unconformably overlain by a less than 100 m thick Sicilian–Crotonian suite consisting, in turn, of littoral sands and gravels capped by continental gravels with subordinate sandy and silty-clay beds. The continental gravels, containing in places well preserved red paleosoils, are currently interpreted as fluvial and/or alluvial fan deposits (Cantalamessa et al., 1986) with thickness generally not exceeding 25 m ca. and tilted slightly seaward (Dramis et al., 1992). On the bases of both palaeontological data and geomorphological constraints, the
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Fig. 2. Shaded relief of the coastal zone south of Monte Conero. Fig. 3. Diverging net in the area south of Monte Conero.
uppermost marine deposits and the overlain continental facies (i.e. the emergence of this area) are reasonably attributed to the Sicilian–Crotonian (e.g. Cantalamessa et al., 1986; Coltorti et al., 1991). Close to the modern shore, the alluvial-fan gravels and the littoral deposits below are truncated by steep Holocene cliffs (Fig. 2), which also cut seaward a hilly morphology characterised by cuestas developed in both marine and continental Pleistocene deposits. Moreover, the modern position of the Pleistocene shore facies lying below the alluvial fan deposits, several metres above the modern sea level, hints at the late Pleistocene–Holocene uplift of the area. The marked entrenchment of many secondary streams recognisable near the coast (Fig. 2), in turn, may be related both to the uplift and to down cutting induced by the coast retreat. The cliffs are interrupted by several broad transverse trunk valleys perpendicular to the coast (Figs. 2 and 3), displaying more or less continuous flights of fluvial terraces, which as for the entire Marche Apennines date to the mid-late Pleistocene–Holocene (e.g. Coltorti et al., 1991; Nesci and Savelli, 1991; Fanucci et al., 1996). Most of the divides of the coastal area consist of a series of gently seaward dipping flat surfaces (Dramis et al., 1992), in many cases corresponding with the topsurface of the Sicilian–Crotonian alluvial gravels capping the Pleistocene marine suite. Despite the overall
parallel pattern of the trunk valleys, the minor streams describe a meaningful diverging net, often radiating from well-defined vertices (Figs. 2 and 3). A remarkable correspondence between radiating streams and distribution of alluvial gravels and of related flat surfaces on the divides is commonly noticed (Fig. 3), thus indicating that here diverging drainage began to develop on the aggradational surfaces of fan depositional systems.
3.2. Cingoli piedmont This sector is located between the Musone and Potenza rivers (CI, Fig. 1), on a geologically complex piedmont extending for several kms to the east of Cingoli anticlinal ridge (maximum height 790 m). Here, directly to the east of the marly calcareous Mesozoic– Paleogene units of the Cingoli structure, a narrow syncline cored by lower Pliocene sandstones and marls is found. Further to the east, is the strongly asymmetric Staffolo anticline, which is cored by Tortonian marls and separated from bordering mid-upper Pliocene mainly clayey terrains by NW–SE oriented faults (e.g. Nanni, 1996 and references therein).
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During the coldest stages dating to the end of the middle Pleistocene, as in nearby areas (cf. Savelli et al., 1994), because of climatic causes some minor streams draining the Cingoli ridge constructed wide alluvial fans on the piedmont. Nevertheless, in the northern sector of the piedmont, local morphostructural constraints prevented some of the fans from attaining a well-defined fan shape, thus developing more or less lobate forms (Fig. 4c). The remnants of the alluvial fan gravels (cf. Fig. 4b) and of related erosional surfaces are still preserved as wide and well-correlating terraces (both erosional and fill terraces), allowing a good reconstruction of a past topography characterised by a wide glacis, partly depositional and partly erosional, extending east of the Cingoli ridge. A significant diverging drainage is shown in the southernmost sector of the piedmont (Fig. 4a and b), where it is interpreted as an inheritance of midPleistocene fans. As indicated by terrace correlation, to the north the development of some minor streams has been similarly controlled by the pre-existing piedmont alluvial bodies, but as these did not attain a fan shape, patterns other than diverging were formed (Fig. 4c). Therefore, if it is true that many diverging stream patterns are derived from previous alluvial fans and/or pediment surfaces, it is equally true that because of sedimentary and topographic constraints, not all the piedmonts could develop such a flow pattern.
Fig. 4. Shaded relief (a) and interpretation of diverging drainage (b) on the Cingoli area; drainage developed on confined alluvial fans is also shown (c). For a more detailed explanation, see the text.
3.3. Eastern flank of Monti della Cesana The area extends on the northeastern flank of the Monti della Cesana anticlinal ridge (altitude averaging 600 m) and on the adjacent Isola del Piano synclinal trough (CS, Fig. 1). The relief consists of Paleogene calcareous and marly calcareous units of the Umbria-Marche stratigraphic succession, while the foothills are shaped in softer Mio-Pliocene marls and sandstones. In this area, an overall trellis drainage is recognisable, where main streams flow along the syncline axes while minor streams, cutting the piedmont area in an approximately transverse direction, flow out from the calcareous ridge to the southwest. Several patches of terrace alluvium preserved on the divides of the piedmont zone hint at alluvial fans (Fig. 5b), dating back to the end of the middle Pleistocene (Nesci et al., 1994). Smooth, gently dipping erosional terraces are recognisable between one fan and the other, thus indicating a wide glacis partly erosional and partly depositional, which at the end of the middle Pleistocene extended along the entire foothills of the calcareous ridge. Although trellis drainage is the dominant stream pattern, some radiating streams derived from middle Pleistocene alluvial fans can be recognised (Fig. 5a and b). Some diverging streams are also recognisable in areas where no remnants of ancient fans have been detected (right side of Fig. 5a), perhaps indicating an origin
Fig. 5. Shaded relief (a) and interpretation of diverging drainage (b) on the Monti della Cesana piedmont. The diverging drainage on the right side of the shaded relief pertains to an area lacking in remnants of alluvium.
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related to pediment surfaces lacking in alluvial deposits and/or a complete removal of the alluvium. 3.4. Piedmont of M. Titano (S. Marino) The area of S. Marino (MT, Fig. 1) consists of the Epiliguride succession of M. Titano, resting above Ligurid units of the allochthonous Val Marecchia sheet (cf. Conti, 1989) embedded in lower-mid Pliocene clays. The sheet has been considered as a gravitational olisthostrome (Flores, 1955), a gravitational slide (Merla, 1951), or emplaced by a mechanism of ‘‘compression plus sliding’’ (Conti et al., 1987). The study area (Fig. 6) east of the M. Titano cliff, shaped in the bioclastic calcarenites of the Epiliguride succession, is represented by a wide pediment surface extending both on Ligurid units (chiefly clayey in this area) and on Pliocene clays. This surface dates back to the end of the middle Pleistocene and, having been deeply dissected in upper Pleistocene and Holocene times, is now represented by gently northeastwarddipping terraces 50 m-high above the modern streams (Guerra and Nesci, 1999). The primitive pediment surface, partly sculptured by areal erosion and partly formed by both slope waste and alluvial deposition, developed a diverging drainage, at least in part as a consequence of alluvial deposition. The main features of this net, although deeply entrenched because of the late Pleistocene vertical uplift of the area, are still recognisable in the whole piedmont surrounding M. Titano (Fig. 6).
Fig. 6. Diverging drainage in the Monte Titano piedmont area.
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4. Discussion and conclusions The Marche Apennines are drained by transverse trunk streams flowing in composite basins, where other than transverse sub-parallel patterns developed. Because of its genetic implications, diverging drainage has a specific interest for deciphering some mechanisms of drainage growth in the study area. Diverging drainage developed throughout the Marche Apennines independently both of geological and structural setting and of age of the terrains where it is found. Although diverging nets can originate in different ways, in the study areas (Fig. 1) this particular drainage was initiated on the depositional top-surface of fans (both alluvial fans and fan deltas, as they became emergent) or, sometimes, on related erosional glacis. Hence, part of the modern channels are inherited from the distributary channels of the aggrading fan surface, but part were formed only later because of the control of the ‘‘cone shaped’’ topography on the post-aggradational flow pattern. Both fans and erosional glacis are, in fact, ‘‘primitive’’ topographic surfaces, above which a net can develop independently of other controls (e.g. lithologic or structural constraints). The traits of a primitive diverging drainage, somewhere deeply entrenched in bedrock, are also still discernible in areas where alluvium has been completely (or almost completely) removed by erosion (e.g. Monti della Cesana). Moreover, in some areas, this imprint also remains after that successive modifications (e.g. upstream extension, captures) markedly changed the original net overshadowing the primitive distributary pattern (e.g. Monti della Cesana). Alluvial piedmonts, where drainage has been prevented from developing a fully grown diverging net because of sedimentary and/or morphostructural constraints (i.e. competition between adjacent fans, topographic barriers, lithologic and structural obstructions) are also detectable (e.g. Cingoli area). Nevertheless, even in these cases, specific features derived from superposition of distributary nets can be recognised (Fig. 4). The reported diverging nets must be regarded as a variant of superposition. Other mechanisms, as captures or antecedence, are accountable for succeeding modifications. This peculiar model of evolution, indicating more than one among the many possible mechanisms able to produce some details of a progressively growing net (e.g. Cingoli, M. Titano, Monti della Cesana), suggests how superposition must be regarded as one of the fundamental processes in drainage development on undeformed (or slightly deformed) Plio-Pleistocene sediments (e.g. the southern Marche coastal zone). Moreover, it is also a conceivable drainage-initiating process in some areas where ongoing (or succeeding) tectonic deformation or controls of other kinds forced the net to change its own primitive evolution mode.
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Such an initiating mechanism, in fact, fits with an opportunistic behaviour of drainage growth (Mayer et al., 2002) which, in turn, does not exclude, in specific places and/or times, neither the Mazzanti and Trevisan (1978) and Alvarez (1999) mechanism, nor the Oberlander (1985) one. Furthermore, if drainage starts its own development on aggradational topography, variations from diverging, fan-derived nets to subparallel ones derived from flat piedmonts (e.g. alluvial slopes, Smith, 2000) can be expected, the significance for net evolution is roughly the same. Three meaningful models of drainage evolution (a; b and c in Fig. 7) comprehensive of both trunk streams and tributaries and starting from fans, based on the examples from the coastal area south of M. Conero (Figs. 2 and 3), can be outlined as follows. In case (a), an important stream develops in the inter-fan trough because of topographic constraints. The gentle trunkvalley flanks develop rather prolonged, systematically concave upstream tributaries, joining the trunk-stream at more or less acute angles. This example fits Chienti, Ete Vivo, Menocchia and, perhaps, Potenza valleys.
(b) A simple entrenchment of the river responsible for the fan formation is required; the position of the trunk valley, therefore, may be unchanged. In this case, the steep flanks of the trunk-valley show short, straight tributaries joining the trunk stream at approximately right angles. This is an appropriate explanation for the Tesino and Aso valleys. (c) Series of complex mechanisms can drive the trunk valley in marginal positions with respect to fan. This case, displaying a mechanism similar to that already described by Mukerji (1990), accounts for trunk-stream deviation resulting from captures by minor streams developing on the same fan. The trunk stream is characterised by sharp elbows. The tributaries on the trunk-valley flanks display various patterns, in part ascribable to the previously described ones; streams flowing in direction opposite to the regional dip can develop because of elbows. This is the case for the Ete Morto and, more doubtfully, of Tenna valleys.
Acknowledgements This paper was funded by MIUR to O. Nesci (COFIN 1999). Special thanks to Larry Mayer, The University of Arizona, for critical reading of the manuscript.
References
Fig. 7. Interpretative sketches for drainage evolution in the coastal area south of Monte Conero.
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O. Nesci, D. Savelli / Quaternary International 101–102 (2003) 203–209 Conti, S., Fregni, P., Gelmini, R., 1987. L’et"a della messa in posto della Coltre della Val Marecchia. Implicazioni paleogeografiche e strutturali. Memorie della Societ"a Geologica Italiana 39, 143–164. Dramis, F., Gentili, B., Pambianchi, G., 1992. La depressione morfostrutturale di Macerata. Studi Geologici Camerti, Volume Speciale 1992/1, 123–126. Fanucci, F., Moretti, E., Nesci, O., Savelli, D., Veneri, F., 1996. Tipologia dei terrazzi vallivi ed evoluzione del rilievo nel versante adriatico dell’Appennino centro-settentrionale. Il Quaternario 9, 255–258. Flores, G., 1955. Les resultats des etudes pour la recherche petrolifere en Sicilie: discussion. In: Proceedings of the Fourth World Petroleum Congress. Casa Editrice Carlo Colombo, Rome, pp. 121–122. Guerra, C., Nesci, O., 1999. Glacis Pleistocenici nel Montefeltro (Appennino Marchigiano–Romagnolo). In: Orombelli, G. (Ed.), Studi Geografici e Geologici in onore di Severino Belloni. G. Brigati, Genova, pp. 419–428. Mayer, L., Menichetti, M., Nesci, O., Savelli, D., 2002. Morphotectonic approach to the drainage analysis in the North Marche region (Central Italy). Quaternary International, this volume. Mazzanti, R., Trevisan, L., 1978. Evoluzione della rete idrografica nell’Appennino centro-settentrionale. Geografia Fisica e Dinamica Quaternaria 1, 55–62. Merla, G., 1951. Geologia dell’Appennino settentrionale. Bollettino della Societ"a Geologica Italiana 70, 95–382.
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Mukerji, A.B., 1990. The Chandigarh Dun alluvial fans: an analysis of the process-form relationship. In: Rachocki, A.H., Church, M. (Eds.), Alluvial Fans—A field Approach. Wiley, Chichester, pp. 131–149. Nanni, T., 1996. Geological characteristics of the basin of River Musone. In: Nanni, T. (Ed.), The Musone River Basin: Geology, Geomorphology and Hydrogeology. Grafiche Scarponi, Osimo (AN), 1997, pp. 193–215. Nesci, O., Savelli, D., 1991. Successioni alluvionali terrazzate nell’Appennino nord-marchigiano. Geografia Fisica e Dinamica Quaternaria 14, 149–162. Nesci, O., Savelli, D., Tramontana, M., Fattori, D., 1994. Evoluzione tardo-pleistocenica delle dorsali calcaree nordmarchigiane: un esempio dai Monti della Cesana. Il Quaternario 7, 139–144. Oberlander, T.M., 1985. Origin of drainage transverse to structures in orogens. In: Hack, J.T., Morisawa, M. (Eds.), Tectonic Geomorphology. Allen and Unwin, Boston, pp. 155–182. Savelli, D., Nesci, O., Mengoni, S., 1994. From alluvial fans to intravallive floodplain: a case study from the piedmont terrace alluvium of the upper Musone River basin (Marche Apennines). Giornale di Geologia Serie 3a, 27–42. Smith, G.A., 2000. Recognition and significance of streamflowdominated piedmont facies in extensional basins. Basin Research 12, 399–411.
Diverging drainage in the Marche Apennines (central Italy) Olivia Nesci*, Daniele Savelli Istituto di Geologia, Universita" degli Studi, Campus Scientifico, Localita" Crocicchia, 61029 Urbino, Italy
Abstract In the Marche Apennines diverging drainage developed on the surface of mid-Pleistocene fanglomerates or, at places, on correlated erosional glacis. In mountain areas, diverging nets correspond with wide glacis shaped in severe cold climates at the end of the middle Pleistocene. Glacis were formed by deposition of coalescing alluvial fans and concurrent shaping of smooth erosional surfaces along and near their margins. In the coastal zone, wide fans and related fluvial plains were constructed in Crotonian times close to the mouth of trunk streams crossing a just emerged piedmont area. Both the control of the ‘‘cone shaped’’ topography on post-aggradational flow patterns and the entrenching of primitive distributary channels are conceivable in developing the diverging drainage of the study area. Four case studies, from areas differing both in age and in geological and structural settings, attest to the recurrence of this drainage in composing, together with other patterns, the grain of larger nets. Moreover, the diverging drainage of the coast area allows us to infer that superposition is an effective mechanism of net development on Plio-Pleistocene terrains of the Marche Apennine piedmont at the time when they became emergent. r 2002 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction Anticlinal ridges and foothills of the Marche Apennines (Central Italy) are crossed by transverse trunk valleys flowing northeastward into the Adriatic Sea. The rivers often maintain their course cutting gorges several hundred metres deep through topographic barriers, a behaviour stressed by a long-lasting debate (e.g. Mazzanti and Trevisan, 1978, and Alvarez, 1999 and references therein). The tributary net, in contrast, mostly accommodating itself to geological structure, lithology and Quaternary deposits, commonly displays a different and more complex drainage (e.g. Bartolini, 1992). Our research is focused on peculiar diverging drainage found on piedmonts of various extension, age and origin, which is of some interest in understanding substantial topics of hydrographic net development on the foothills of the Marche Apennines.
2. Morphostructural setting and drainage development The Marche Apennines (Fig. 1) consist of northeast verging thrusted anticlines involving a Meso-Cenozoic *Corresponding author. E-mail address: [email protected] (O. Nesci).
sedimentary succession. Carbonatic anticlines, generally conforming to higher relief, exert the main control on the morphostructure of the internal areas. Along the northern coast, between Gabicce and M. Conero, coastal hills (summit elevations not exceeding ca. 200 m) occur in correspondence with thrusted anticlines. To the south (e.g. near Porto S. Giorgio), similar but less pronounced coastal structures are buried beneath Plio-Quaternary roughly undeformed sediments (e.g. Cantalamessa et al., 1990). A wide hilly piedmont develops on Plio-Quaternary terrains cropping out between the chain and the Adriatic Sea (Fig. 1). North of M. Conero, as well as in the internal zones of the southern area, thrusted anticlines are responsible for a piedmont landscape characterised by anticlinal hills and homoclinal ridges of different size and orientation. In contrast, south of M. Conero promontory, an overall homoclinal arrangement of Plio-Pleistocene terrains along the coastal zone, is accountable for wide, seaward-dipping cuestas. According to the evolution model of Mayer et al. (2002), the primitive embryos of the modern hydrographic net could develop in the most internal and uplifted areas as long ago as the Messinian, in directions both transverse and/or longitudinal with respect to the chain. The final emergence, that in the piedmont areas was attained during the upper Pliocene to lower-mid
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Fig. 1. Geological sketch of the Marche Apennines and location of the study areas and localities cited in the text. CZ=coastal zone; CI=Cingoli piedmont; CS=Monti della Cesana piedmont; MT=Monte Titano area (S. Marino).
Pleistocene timespan through a complex history of transgressions and regressions (e.g. Cantalamessa et al., 1986), greatly enlarged the drained surface to the east. Thus, a significant eastward drainage extension took place, drawing transverse trunk valleys and a complex net of tributaries (e.g., strike-, dip-, antidip-tributary streams). In the mid-upper Pleistocene and Holocene, ultimately, a generalised, persistent tectonic uplift forced the drainage to entrench (cf. Bisci and Dramis, 1991; Bigi et al., 1995; Fanucci et al., 1996), often more than 30–50 m deep, allowing terrace formation, upstream extension, and captures, but preventing important lateral shifting and/or avulsions. Both before and during the net entrenchment diverging patterns of minor streams were formed in the coastal zones as well as in the internal ones. Using sample-areas of the Marche Apennines, this net arrangement, indicating similar genetic mechanisms in quite different morphostructural contexts and in separate times, is analysed.
3. Examples of diverging drainage in the Marche Apennines 3.1. Adriatic coast south of M. Conero The coastal area between M. Conero and the Tronto River mouth (CZ, Fig. 1), consists (cf. Cantalamessa et al., 1986) of a more than 900 m thick suite of Pleistocene epi-bathyal to neritic and littoral sediments. Emilian clayey marine deposits are unconformably overlain by a less than 100 m thick Sicilian–Crotonian suite consisting, in turn, of littoral sands and gravels capped by continental gravels with subordinate sandy and silty-clay beds. The continental gravels, containing in places well preserved red paleosoils, are currently interpreted as fluvial and/or alluvial fan deposits (Cantalamessa et al., 1986) with thickness generally not exceeding 25 m ca. and tilted slightly seaward (Dramis et al., 1992). On the bases of both palaeontological data and geomorphological constraints, the
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Fig. 2. Shaded relief of the coastal zone south of Monte Conero. Fig. 3. Diverging net in the area south of Monte Conero.
uppermost marine deposits and the overlain continental facies (i.e. the emergence of this area) are reasonably attributed to the Sicilian–Crotonian (e.g. Cantalamessa et al., 1986; Coltorti et al., 1991). Close to the modern shore, the alluvial-fan gravels and the littoral deposits below are truncated by steep Holocene cliffs (Fig. 2), which also cut seaward a hilly morphology characterised by cuestas developed in both marine and continental Pleistocene deposits. Moreover, the modern position of the Pleistocene shore facies lying below the alluvial fan deposits, several metres above the modern sea level, hints at the late Pleistocene–Holocene uplift of the area. The marked entrenchment of many secondary streams recognisable near the coast (Fig. 2), in turn, may be related both to the uplift and to down cutting induced by the coast retreat. The cliffs are interrupted by several broad transverse trunk valleys perpendicular to the coast (Figs. 2 and 3), displaying more or less continuous flights of fluvial terraces, which as for the entire Marche Apennines date to the mid-late Pleistocene–Holocene (e.g. Coltorti et al., 1991; Nesci and Savelli, 1991; Fanucci et al., 1996). Most of the divides of the coastal area consist of a series of gently seaward dipping flat surfaces (Dramis et al., 1992), in many cases corresponding with the topsurface of the Sicilian–Crotonian alluvial gravels capping the Pleistocene marine suite. Despite the overall
parallel pattern of the trunk valleys, the minor streams describe a meaningful diverging net, often radiating from well-defined vertices (Figs. 2 and 3). A remarkable correspondence between radiating streams and distribution of alluvial gravels and of related flat surfaces on the divides is commonly noticed (Fig. 3), thus indicating that here diverging drainage began to develop on the aggradational surfaces of fan depositional systems.
3.2. Cingoli piedmont This sector is located between the Musone and Potenza rivers (CI, Fig. 1), on a geologically complex piedmont extending for several kms to the east of Cingoli anticlinal ridge (maximum height 790 m). Here, directly to the east of the marly calcareous Mesozoic– Paleogene units of the Cingoli structure, a narrow syncline cored by lower Pliocene sandstones and marls is found. Further to the east, is the strongly asymmetric Staffolo anticline, which is cored by Tortonian marls and separated from bordering mid-upper Pliocene mainly clayey terrains by NW–SE oriented faults (e.g. Nanni, 1996 and references therein).
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During the coldest stages dating to the end of the middle Pleistocene, as in nearby areas (cf. Savelli et al., 1994), because of climatic causes some minor streams draining the Cingoli ridge constructed wide alluvial fans on the piedmont. Nevertheless, in the northern sector of the piedmont, local morphostructural constraints prevented some of the fans from attaining a well-defined fan shape, thus developing more or less lobate forms (Fig. 4c). The remnants of the alluvial fan gravels (cf. Fig. 4b) and of related erosional surfaces are still preserved as wide and well-correlating terraces (both erosional and fill terraces), allowing a good reconstruction of a past topography characterised by a wide glacis, partly depositional and partly erosional, extending east of the Cingoli ridge. A significant diverging drainage is shown in the southernmost sector of the piedmont (Fig. 4a and b), where it is interpreted as an inheritance of midPleistocene fans. As indicated by terrace correlation, to the north the development of some minor streams has been similarly controlled by the pre-existing piedmont alluvial bodies, but as these did not attain a fan shape, patterns other than diverging were formed (Fig. 4c). Therefore, if it is true that many diverging stream patterns are derived from previous alluvial fans and/or pediment surfaces, it is equally true that because of sedimentary and topographic constraints, not all the piedmonts could develop such a flow pattern.
Fig. 4. Shaded relief (a) and interpretation of diverging drainage (b) on the Cingoli area; drainage developed on confined alluvial fans is also shown (c). For a more detailed explanation, see the text.
3.3. Eastern flank of Monti della Cesana The area extends on the northeastern flank of the Monti della Cesana anticlinal ridge (altitude averaging 600 m) and on the adjacent Isola del Piano synclinal trough (CS, Fig. 1). The relief consists of Paleogene calcareous and marly calcareous units of the Umbria-Marche stratigraphic succession, while the foothills are shaped in softer Mio-Pliocene marls and sandstones. In this area, an overall trellis drainage is recognisable, where main streams flow along the syncline axes while minor streams, cutting the piedmont area in an approximately transverse direction, flow out from the calcareous ridge to the southwest. Several patches of terrace alluvium preserved on the divides of the piedmont zone hint at alluvial fans (Fig. 5b), dating back to the end of the middle Pleistocene (Nesci et al., 1994). Smooth, gently dipping erosional terraces are recognisable between one fan and the other, thus indicating a wide glacis partly erosional and partly depositional, which at the end of the middle Pleistocene extended along the entire foothills of the calcareous ridge. Although trellis drainage is the dominant stream pattern, some radiating streams derived from middle Pleistocene alluvial fans can be recognised (Fig. 5a and b). Some diverging streams are also recognisable in areas where no remnants of ancient fans have been detected (right side of Fig. 5a), perhaps indicating an origin
Fig. 5. Shaded relief (a) and interpretation of diverging drainage (b) on the Monti della Cesana piedmont. The diverging drainage on the right side of the shaded relief pertains to an area lacking in remnants of alluvium.
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related to pediment surfaces lacking in alluvial deposits and/or a complete removal of the alluvium. 3.4. Piedmont of M. Titano (S. Marino) The area of S. Marino (MT, Fig. 1) consists of the Epiliguride succession of M. Titano, resting above Ligurid units of the allochthonous Val Marecchia sheet (cf. Conti, 1989) embedded in lower-mid Pliocene clays. The sheet has been considered as a gravitational olisthostrome (Flores, 1955), a gravitational slide (Merla, 1951), or emplaced by a mechanism of ‘‘compression plus sliding’’ (Conti et al., 1987). The study area (Fig. 6) east of the M. Titano cliff, shaped in the bioclastic calcarenites of the Epiliguride succession, is represented by a wide pediment surface extending both on Ligurid units (chiefly clayey in this area) and on Pliocene clays. This surface dates back to the end of the middle Pleistocene and, having been deeply dissected in upper Pleistocene and Holocene times, is now represented by gently northeastwarddipping terraces 50 m-high above the modern streams (Guerra and Nesci, 1999). The primitive pediment surface, partly sculptured by areal erosion and partly formed by both slope waste and alluvial deposition, developed a diverging drainage, at least in part as a consequence of alluvial deposition. The main features of this net, although deeply entrenched because of the late Pleistocene vertical uplift of the area, are still recognisable in the whole piedmont surrounding M. Titano (Fig. 6).
Fig. 6. Diverging drainage in the Monte Titano piedmont area.
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4. Discussion and conclusions The Marche Apennines are drained by transverse trunk streams flowing in composite basins, where other than transverse sub-parallel patterns developed. Because of its genetic implications, diverging drainage has a specific interest for deciphering some mechanisms of drainage growth in the study area. Diverging drainage developed throughout the Marche Apennines independently both of geological and structural setting and of age of the terrains where it is found. Although diverging nets can originate in different ways, in the study areas (Fig. 1) this particular drainage was initiated on the depositional top-surface of fans (both alluvial fans and fan deltas, as they became emergent) or, sometimes, on related erosional glacis. Hence, part of the modern channels are inherited from the distributary channels of the aggrading fan surface, but part were formed only later because of the control of the ‘‘cone shaped’’ topography on the post-aggradational flow pattern. Both fans and erosional glacis are, in fact, ‘‘primitive’’ topographic surfaces, above which a net can develop independently of other controls (e.g. lithologic or structural constraints). The traits of a primitive diverging drainage, somewhere deeply entrenched in bedrock, are also still discernible in areas where alluvium has been completely (or almost completely) removed by erosion (e.g. Monti della Cesana). Moreover, in some areas, this imprint also remains after that successive modifications (e.g. upstream extension, captures) markedly changed the original net overshadowing the primitive distributary pattern (e.g. Monti della Cesana). Alluvial piedmonts, where drainage has been prevented from developing a fully grown diverging net because of sedimentary and/or morphostructural constraints (i.e. competition between adjacent fans, topographic barriers, lithologic and structural obstructions) are also detectable (e.g. Cingoli area). Nevertheless, even in these cases, specific features derived from superposition of distributary nets can be recognised (Fig. 4). The reported diverging nets must be regarded as a variant of superposition. Other mechanisms, as captures or antecedence, are accountable for succeeding modifications. This peculiar model of evolution, indicating more than one among the many possible mechanisms able to produce some details of a progressively growing net (e.g. Cingoli, M. Titano, Monti della Cesana), suggests how superposition must be regarded as one of the fundamental processes in drainage development on undeformed (or slightly deformed) Plio-Pleistocene sediments (e.g. the southern Marche coastal zone). Moreover, it is also a conceivable drainage-initiating process in some areas where ongoing (or succeeding) tectonic deformation or controls of other kinds forced the net to change its own primitive evolution mode.
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Such an initiating mechanism, in fact, fits with an opportunistic behaviour of drainage growth (Mayer et al., 2002) which, in turn, does not exclude, in specific places and/or times, neither the Mazzanti and Trevisan (1978) and Alvarez (1999) mechanism, nor the Oberlander (1985) one. Furthermore, if drainage starts its own development on aggradational topography, variations from diverging, fan-derived nets to subparallel ones derived from flat piedmonts (e.g. alluvial slopes, Smith, 2000) can be expected, the significance for net evolution is roughly the same. Three meaningful models of drainage evolution (a; b and c in Fig. 7) comprehensive of both trunk streams and tributaries and starting from fans, based on the examples from the coastal area south of M. Conero (Figs. 2 and 3), can be outlined as follows. In case (a), an important stream develops in the inter-fan trough because of topographic constraints. The gentle trunkvalley flanks develop rather prolonged, systematically concave upstream tributaries, joining the trunk-stream at more or less acute angles. This example fits Chienti, Ete Vivo, Menocchia and, perhaps, Potenza valleys.
(b) A simple entrenchment of the river responsible for the fan formation is required; the position of the trunk valley, therefore, may be unchanged. In this case, the steep flanks of the trunk-valley show short, straight tributaries joining the trunk stream at approximately right angles. This is an appropriate explanation for the Tesino and Aso valleys. (c) Series of complex mechanisms can drive the trunk valley in marginal positions with respect to fan. This case, displaying a mechanism similar to that already described by Mukerji (1990), accounts for trunk-stream deviation resulting from captures by minor streams developing on the same fan. The trunk stream is characterised by sharp elbows. The tributaries on the trunk-valley flanks display various patterns, in part ascribable to the previously described ones; streams flowing in direction opposite to the regional dip can develop because of elbows. This is the case for the Ete Morto and, more doubtfully, of Tenna valleys.
Acknowledgements This paper was funded by MIUR to O. Nesci (COFIN 1999). Special thanks to Larry Mayer, The University of Arizona, for critical reading of the manuscript.
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Fig. 7. Interpretative sketches for drainage evolution in the coastal area south of Monte Conero.
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