New Mesoscale Fluvial Landscapes - Seismic Geomorphology and Exploration

Summary Megafans (100-600 km radius) are very large alluvial fans that cover significant areas on most continents, the surprising finding of recent global surveys. The number of such fans and patterns of sedimentation on them provides new mesoscale architectures that can now be applied on continental fluvial depositional systems, and therefore on . Megafan-scale reconstructions underground as yet have not been attempted. Seismic surveys offer new possibilities in identifying the following prospective situations at potentially unsuspected locations: (i) sand concentrations points, (ii) sand-mud continuums at the mesoscale, (iii) paleo-valley forms in these generally unvalleyed landscapes, (iv) stratigraphic traps, and (v) structural traps. Introduction—Discovery of the widespread distribution of megafans A megafan is a low-angle, partial cone of river-laid sediments that can reach hundreds of km in length (100 km minimum length used in our global survey), with areas varying from 7000 to 200,000 km2 [1, 2]. As such, megafans are mesoscale landforms (Fig. 1) — features that have received surprisingly little attention in modern landscape studies. We know of almost none in subsurface “paleogeography” reconstructions. Inaccurately termed "inland deltas," megafans need to be distinguished specifically from coastal deltas since they require no distal water body for their development. In our global study we examined inland megafans only. Megafans form most typically at mountain fronts. The Kosi River megafan, located at the foot of the Himalaya Mts. in northern India, is one of the few well known examples. The discovery of the global distribution of megafans shows that they are at least as important as a global landform as deltas, due to their great size and far greater number (i.e., large fans vs. comparably large deltas). Based originally on astronaut images that revealed their worldwide occurrence, more than 160 modern large fans have now been documented on all continents in our mapping campaign [3] — proving that these features are not merely a freakish end-member of the alluvial fan continuum. Our global study also revealed that megafans are often clustered (the Himalayan foreland plains are a prime example), so that flat landscapes they develop can dominate extensive continental surfaces — 1.2 million km2 in South America, another classic example [1]). The sample is now large enough that controls of megafan location are well understood. Thus, the presence of megafans has been successfully predicted in modern landscapes even where diagnostic patterns were not obvious remotely or on the ground (radial stream patterns removed by erosion and overprinted by dunes and younger vegetation patterns). The world survey has consequently increased our confidence in locating these features subsurface. Present literature tends to lump together fans of all sizes [e.g., 4]. World surveys have now demonstrated con-