Coalescent Alluvial Fans Research Articles

Early Pleistocene River-Fed Paleocoast in Western Umbria (Central Italy): Facies Analysis and Coastal Models

Pliocene (?)—early Pleistocene shallow marine deposits, varying from gravel to sand to clay, characterize the southernmost sector of the Valdichiana Basin, between Orte and Città della Pieve, across Tuscany, Umbria and Latium (Central Italy). Facies associations, referring to the evolution of a river-fed coast, with a sensible facies heteropy, and a sub-environment articulation, both across and alongshore, have been recently described. Although the main part of the territory responds to a wave-dominated coastal model, a clear fluvial sediment origin and the presence of localized river mouths have also been documented. Nearshore is mainly represented by interbedded sand and gravel beachface to upper shoreface deposits, in which both a mouth bar organization and a lateral distribution of gravel beaches are recognizable. Sediment origins largely depend ondebris flow processes, related to small alluvial fans/fan deltas. In constrained areas, debris flow and current continental deposits occur, referring to coalescent alluvial fans, organized as a smoothly seaward-dipping piedmont band, drained by shallow braided channels. This roughly organized fluvial system feeds a coastal area, with a fandeltabuild-up. The as-described fan delta and beach systems are characterized by a smooth seaward morphology, according to models resembling, on a coast-transverse profile, the shelf-type fan delta. Although the proposed models differ from each other’s, with respect tothe shelf-type one, this is mainly on a lateral facies distribution.

TECTONO-SEDIMENTARY EVOLUTION AND RATES OF TECTONIC UPLIFT OF THE SFAKIA COASTAL ZONE, SOUTHWESTERN CRETE

Sfakia lies within a narrow coastal zone at the southwestern foothills of the Lefka Ori Mt. Here a segment of the South Cretan margin is preserved onshore, a structure that represents a neotectonic structure with continuous activity since the Upper Miocene. This segment is characterized by a steep, E-W striking and south facing morphological escarpment that constitutes numerous E-W and ESE-WNW striking normal faults. Since the Late Miocene, marine sequences of Tortonian, Early Pliocene and Lower Pleistocene age were deposited along the coastal zone. Since the Middle Pleistocene multiple, coalescent alluvial fans covered both the alpine basement and the marine sediments. Fault-kinematic- and stratigraphie data combined with recently published palaeobathymetry reconstructions allow us to make relable estimates of both the uplift rates of fault blocks in the study area and the period that the faults that demarcate them were active. The results show that the study area is experiencing uplift already since the Middle Pliocene and that the uplift rates of the mountainous parts are higher than those of the coastal zone. The general uplift of the coastal zone seems to be controlled by offshore normal faults, south of Sfakia

The Upper Oligocene of Montgat (Catalan Coastal Ranges, Spain) : new age constrains to the western Mediterranean Basin opening.

The Oligocene deposits of Montgat are integrated in a small outcrop made up of Cenozoic and Mesozoic rocks located in the Garraf-Montnegre horst, close to the major Barcelona fault. The Oligocene of Montgat consists of detrital sediments of continental origin mainly deposited in alluvial fan environments; these deposits are folded and affected by thrusts and strike-slip faults. They can be divided in two lithostratigraphic units separated by a minor southwest-directed thrust: (i) the Turo de Montgat Unit composed of litharenites and lithorudites with high contents of quartz, feldspar, plutonic and limestone rock fragments; and (ii) the Pla de la Concordia Unit composed of calcilitharenites and calcilithorudites with high contents of dolosparite and dolomicrite rock fragments. The petrological composition of both units indicates that sediments were derived from the erosion of Triassic (Buntsandstein, Muschelkalk and Keuper facies), Jurassic and Lower Cretaceous rocks (Barremian to Aptian in age). Stratigraphic and petrological data suggest that these units correspond to two coalescent alluvial fans with a source area located northwestwards in the adjoining Collserola and Montnegre inner areas. Micromammal fossils (Archaeomys sp.) found in a mudstone layer of the Pla de la Concordia Unit assign a Chattian age (Late Oligocene) to the studied materials. Thus, the Montgat deposits are the youngest dated deposits affected by the contractional deformation that led to the development of the Catalan Intraplate Chain. Taking into account that the oldest syn-rift deposits in the Catalan Coastal Ranges are Aquitanian in age, this allows to precise that the change from a compressive to an extensional regime in this area took place during latest Oligocene-earliest Aquitanian times. This age indicates that the onset of crustal extension related to the opening of the western Mediterranean Basin started in southern France during latest Eocene-early Oligocene and propagated southwestward, affecting the Catalan Coastal Ranges and the northeastern part of the Valencia trough during the latest Chattian-earliest Aquitanian times.

Peligros hidrometeorológicos en un valle árido: abordaje geopedológico

Se expone el análisis geopedológico del valle intermontano de Zonda, Argentina, con el objetivo de que este sea aplicado en estudios de ordenamiento territorial. Los resultados indican que los riesgos provienen de tres cuencas principales: dos locales (río San Juan, río Seco de la Ciénaga) y una regional (bajadas pedemontanas), cada una con sus propias características. Los peligros más importantes corresponden a procesos de aluviones e inundaciones provenientes de los límites del valle y desbordes de los máximos caudales estivales del río San Juan. Los resultados fueron presentados en mapas que contienen unidades de relieve y riesgos hidrometeorológicos asociados. Los asentamientos humanos crecen en el valle en contacto con el piedemonte y están amenazados principalmente por aluviones de recurrencia anual y estival.

Geomorphology of the alluvial fans in Colalao del Valle-Quilmes area (Santa María Valley, Tucumán Province, Argentina)

ABSTRACTThe geomorphological map of the alluvial fans in the Colalao del Valle-Quilmes area represents the first detailed map of the Santa María Valley, Northwest Argentina. The map was made using photointerpretation of aerial photographs and Google Earth images. The initial working scale was 1:10,000 and then reduced to 1:27,000 to fit ISO A1 size. Final drawings were produced using FreeHand 11. The area is characterized by the presence of coalescent alluvial fans developed on four levels. These fans end at the floodplain of the Santa María River. Sand mantles and dunes are present in this area, with old channels of the main river forming abandoned wetlands. A Holocene fault affects the distal area of some fans, whilst the apices of several alluvial fans have archaeological settlements. This study area presents an excellent example of regional Holocene evolution. Recent and present fluvial dynamics, aeolian activity, and the Holocene fault show high geomorphological activity. As a consequence, the information contained in the geomorphological map may be relevant to the establishment of risk areas for human settlements. Moreover, the map could be useful for planning conservation measures for the Prehispanic villages.

Multiscale map analysis in alluvial fan flood-prone areas

Several case studies of geomorphological mapping at various scales in order to identify areas prone to alluvial fan flooding are presented in this paper. The selected areas are located in southern Italy and are representative of a geomorphic unit (foothills consisting of coalescent alluvial fans) found throughout the southern Apennines. The medium-scale approach represents the best tool to identify areas susceptible to flooding, using detailed geomorphological mapping of fan systems. It can be considered a preliminary analysis, which provides important information for large areas and identifies zones which need further investigation. Large-scale map analysis may be applied to a single fan and used to distinguish fan portions prone to different degrees of hazard exposure, thus providing accurate information for decision-makers who are called upon to plan hazard management. In both cases, analysis based on detailed geological and geomorphological field surveys is often coupled with remotely sensed data.

Large Late Pleistocene landslides from the marginal slope of the Flysch Carpathians

The gently concave piedmont of the marginal slope of the Flysch Carpathians in the Czech Republic has long been considered to comprise a system of pediments or coalescent alluvial fans. However, within one of the typical sections of this piedmont, large successive landslides with long travel distances of ~2.5 km have been identified through geophysical measurements and the investigation of an extensive artificial exposure. Accelerator mass spectrometry (AMS) radiocarbon dating and pollen analysis demonstrate that the uppermost generations of landslide deposits have originated since ~56 ka BP during the warmer and more humid interpleniglacial conditions of Marine Isotope Stage 3 (MIS 3). The geomorphological evidence for landsliding during MIS 3 has almost completely disappeared from this region due to intensive periglacial processes operating during the Last Glacial Maximum and subsequent fluvial and anthropogenic processes operating during the Holocene. The considerable antiquity of the studied terrestrial landslide bodies is unique within the context of Europe. This study shows the value of re-examining landscape development using new techniques and fresh exposures.

Flood susceptibility assessment in a highly urbanized alluvial fan: the case study of Sala Consilina (southern Italy)

Abstract. This paper deals with the risk assessment to alluvial fan flooding at the piedmont zone of carbonate massifs of the southern Apennines chain (southern Italy). These areas are prime spots for urban development and are generally considered to be safer than the valley floors. As a result, villages and towns have been built on alluvial fans which, during intense storms, may be affected by flooding and/or debris flow processes. The study area is located at the foothills of the Maddalena mountains, an elongated NW-SE trending ridge which bounds to the east the wide intermontane basin of Vallo di Diano. The area comprises a wide detrital talus (bajada) made up by coalescent alluvial fans, ranging in age from the Middle Pleistocene to the Holocene. Historical analysis was carried out to ascertain the state of activity of the fans and to identify and map the zones most hit by past flooding. According to the information gathered, the Sala Consilina fans would appear prone to debris flows; in the past these processes have produced extensive damage and loss of life in the urban area. The watershed basins feeding the fans have very low response times and may produce debris flow events with high magnitudes. Taking into account the historical damage, the fan surface morphology, and the present urban development (street orientation and hydraulic network), the piedmont area was zoned and various susceptibility classes were detected. These results may represent a useful tool for studies aiming at territorial hazard mapping and civil protection interventions.

LA-ICP-MS zircon U-Pb geochronology to constrain the age of post-Neocomian continental deposits of the Cerrillos Formation, Atacama Region , northern Chile: tectonic and metallogenic implications

New U-Pb zircon dating of volcanic intercalations in the lower conglomeratic part of the Cerrillos Formation shows that its deposition extended in time at least from 110.7±1.7 to 99.7±1.6 Ma. The significantly younger U-Pb zircon age of 69.5±1.0 Ma obtained for the upper volcanic part of the Cerrillos Formation suggests recurrence of volcanism in the Late Cretaceous instead of continual volcanic activity. A minimum late Maastrichtian age for the Cerrillos Formation and its initial deformation was determined by the U-Pb zircon age range from 66.9±1.0 to 65.2±1.0 Ma for the lower part of the unconformably overlying Hornitos Formation. The new U-Pb data for the Cerrillos and Hornitos formations poses questions about the Campanian-Maastrichtian age range currently ascribed to the latter. The lower part of the Cerrillos Formation represents a major change in the sedimentary regime from previous marine carbonate sedimentation in a back- arc basin until the late Aptian to subsequent coarse alluvial sedimentation and volcanism since the early Albian. The lower part of the Cerrillos Formation is interpreted as the development of coalescent alluvial fans thinning inland, accompanied by volcanism. These developed as the result of transpressive deformation and uplift of the area of the current Coastal Cor- dillera by late Aptian, leading to subsequent active erosion and sedimentation inland, along with the eastward shift of the magmatic foci in the region. A mineralizing period of Cu-Au porphyries overlaps in time with the deposition of the Cerrillos Formation in northern Chile; marking also a significant change in the metallogeny of the Andes of northern Chile.

Wealden of the Weald: a new model

An alternative to the deltaic model is presented. The Weald is seen as a subsiding graben-basin, with ‘Macaraibo’ sedimentary features, spasmodically open to the sea and margined by active horsts. Channel-facies are commoner in the Wealden sand and clay formations than previously suspected. The ‘normal’ environment was a variable-salinity coastal mudplain with lagoons and sandy water-courses loosely connected north-westwards with the East Anglian Sea. Channel patterns fluctuated across the low ⇋ high sinuosity transition. Periodically, increased riverflow transformed parts of the basin into sandy braidplains, culminating sometimes in coalescent alluvial fans. These interruptions were brief and generated by marginal upfaulting of surrounding blocks combined with attendant climatic changes. Large expanses of alluvial plain were bare of trees and bushes, but supported rich growths of herbaceous pteridophytes where deposition and erosion became inactive temporarily. Herds of dinosaur travelled freely across the basin and maintained themselves in it. The climate was warm, with marked wet and dry seasons and, possibly, diurnal rhythms in precipitation. Its general trend was towards ‘amelioration’, as though Britain was leaving the Purbeck semiarid zone and moving across the warm temperate belt. Following mid-Purbeck earth movements and the final Cinder Bed transgression, the London-Kent horsts dominated Hastings times, being the main suppliers of arenaceous sediment and controlling water-salinity by acting as imperfect barriers against further inroads by the northern sea. On two occasions (Ashdown and Lower Tunbridge Wells formations) the channel-networks merged to build up a single braidplain spanning the basin. Bedload from Wessex and the Isle of Wight stopped short at the Hampshire-Sussex border, though some Norman sand may have reached south-east Sussex. During Weald Clay times the London and Kent blocks ceased to be important sources of sediment, the former letting in the muddy ‘Snettisham’ Sea voluminously. Several of the brackish-marine inundations sprinkled East Anglian sand across the north-west Weald, mixing it with ‘London’ gravel gathered up in passing. But most of the sand, now sparse, was generated by jolts in distant Cornubia and Armorica, rejuvenating the rivers and causing the Wessex alluvia to probe the western Weald. After their Horsham premiere the movements weakened, the younger sands reaching less far eastwards and becoming more restricted to their channel systems. Least affected by tectonic and marine influences were the eastern parts of the basin. The regional palaeoslope in the Anglo-French area seems to have tilted northeast, away from the ruptured continental margin, preluding increased spreading rates in the mid-Cretaceous Atlantic. There is no room in the new model for large-scale classical deltaic processes or for the traditional derivation of the immature ‘western’ detritus from Hercynian granites (or any other granites). The model removes some previous difficulties, e.g. the small sizes of the London catchments, the rapidity of many sedimentological changes and the paradox that both marine and fluviatile invasions appear to come from the same general direction. In the field, the well-known ‘Rocks’ (massive sandstone members) become the merged multistorey networks of braided channel-fills formed during climactic phases of uplift and river-rejuvenation on the basin margins.