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New findings around fluvial megafans have accrued from the world survey presented in this book, and challenge some commonly accepted generalisations. Among a list of unexpected results are that (i) megafans constitute a landform and sedimentary body of regional significance on Earth, subsidiarily on Mars, despite their relatively small number; (ii) any topographic step, high or low, can provide the anchor point for a megafan apex; (iii) most megafans are associated with tributary drainages, seldom with axial drainages; (iv) megafans form in all climates. Megafan sizes, shapes, nesting patterns, drainage configurations, tectonic settings, and sediment dispersal styles are summarised, classified, and compared to other large fluvial sediment bodies. Finer mosaics of landscape elements and landforms belonging to the rheic zone (belts of fluvial incision, narrow or wide, that cut into fan surfaces) and perirheic zone (extensive land surface beyond the reach of the fan-forming river) are reviewed from modern analogues, and their implications for identifying megafans and other distributary fluvial systems in the rock record are examined. Vocabulary defining megafans and their environments has been sharpened as a result, with some avenues for further investigation laid out in this closing chapter.
Centrally situated in France, hosting famous chateaus and vineyards yet oddly secluded, the extensive, diamond-shaped region known as Sologne is a geological elephant in the room. Nationally one of the last regions to be fully surveyed, this deceptively uniform yellow patch on small-scale geological maps is presented here and portrayed for the first time as a fluvial megafan. Its attributes are compiled and reviewed from a collection of reports, maps, handbooks and articles. Evidence shows that this ~ 100-m-thick Neogene accumulation of quartz-rich sand and clay was primarily generated by the Loire River and remained intermittently functional until early Quaternary time, whereupon dissection by the Loire River itself and by its fan-fed tributary streams prevailed. The ~ 120-km-long megafan was fed from the south by the rising volcanic swell of the Massif Central in a succession of pulses, relatively well-dated by different generations of accessory volcanic minerals. Sediment aggradation conforms to a pattern for which modern analogues may only exist in Africa, a continent hosting many dozens of swells of a similar nature to the Massif Central, with adjacent basins populated by very large megafans fed by swell-flank rivers. As such, the Loire megafan is a unique occurrence in Europe.
Discovery of the significance of fluvial megafans came about in the mid to late twentieth century. We suggest reasons why appreciation of their existence came late in the history of Earth science, even after the advent of space-based observation of planetary landscapes. The reasons are partly cultural: megafans are uncommon in the historic cradles of modern geology (Europe, North America). Reasons are also partly theoretical: rivers have been conceptualised chiefly as sediment bypass systems terminating in deltas, rather than as aggradational systems in their own right. Reasons are also perceptual: just as the megaflood origin of channeled scablands was held in disbelief, the inordinate size of megafans has stood in the way of accepting (i) the sheer magnitude of their unit-size and also (ii) their existence as active systems in modern landscapes, rather than just as stratigraphic features in the rock record. Post-1990, scientific activity around megafans accelerated and involved global mapping, classification, and regional investigations into patterns and processes. An overview of this take-off period is provided as a partial introduction to the remaining 17 chapters of this book, which are briefly outlined.
Megafans are partial cones of river sediment that reach unexpectedly large dimensions, with the largest on Earth being 700 km long. Due to recent developments in space-based observations, global mapping efforts have shown that modern megafan features cover vast landscapes on most continents. This book provides a new inventory of nearly 300 megafans across five continents. Chapters focus on regional studies of megafans from all continents barring North America and Antarctica. The major morphological attributes of megafans and multi-megafan landscapes are discussed, and the principal controls on megafan development are examined. The book also compares megafans with alluvial fans, deltas, floodplains and the recently recognised 'major avulsive fluvial system' (MAFS). The final part of the book discusses the application of megafan research to economic geology, aquifers and planetary geology including layered deposits on Mars. This is an invaluable reference for researchers in geomorphology, sedimentology and physical geography.
We present a chronology of ice recession in the eastern Pyrenees based onin situ-produced10Be data obtained from the Têt paleoglacier complex. The sampling strategy is based on the relative chronology provided by a detailed geomorphological map of glacial landforms. Results indicate that the last maximum ice advance occurred late (i.e., during Marine Isotope Stage 2) compared to the chronology currently established for the rest of the Pyrenees. Despite debatable evidence for a glacial readvance during the Oldest Dryas stade, ice-cap melt-out was rapid, residual cirque glaciers having disappeared by the Allerød interstade. This is consistent both with North Atlantic excursions established by the Greenland ice cores and paleoenvironmental data for the region. The rapid response of the east-Pyrenean ice cap to temperature variations is primarily linked to its small size compared to larger Pyrenean ice fields, to the dry Mediterranean climate, and to topography-related nonlinearities in which a small vertical rise in equilibrium line altitude generates a large change in ice mass. Possible sources of age uncertainty are discussed in the context of sampling design for single-nuclide (10Be) dating of landform sequences in formerly glaciated landscapes.
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