Subaerial terminoglacial fans III: overview of sedimentary characteristics and depositional model

Abstract

A general model is presented for the small type of fans (not to be confused with sandurs) that develop subaerially in the zone immediately before an ice front. These fans have in common with other fans that a proximal, a middle and a distal subenvironment -with distinctly different sedimentary fades -can be distinguished. The characteristics of these fans differ in several respects, however, from those formed under other conditions, particularly by the high proportion of mass-flow deposits in the proximal part, by the relative scarcity of fine particles in the middle fan, and by the relatively uniform character of the sediments in the distal fan.The special character of this type of fan is ascribed to the interaction of a continuously changing distance between the ice front and the fan (as a result of alternating phases of ice advance and ice retreat), its position that may be surrounded by ice for a large part, and the irregular supply of debris-laden meltwater that comes sometimes even in the form of more or less catastrophic floods.Due to the fact that terminoglacial fans have a good preservation potential only during phases of ice retreat, these fas tend to show a slight fining-upward tendency. The slope of terminoglacial fans tends to be more gentle (rarely over 2-5°) than that of fans formed under different conditions.

Sedimentation model of gravel-dominated alluvial piedmont fan, Ganga Plain, India

“…Formed at erosional channel bases as diffused bedforms by turbulent flows (Teisseyre 1976;Zielinski and Loon 2000).…”

“…Vertical accretion under upper flow regime in aggrading channels (Teisseyre 1976;Zielinski and Loon 2000).…”

“…Conversely, in case of the Gaula Fan, sustained deeper flows in channels favoured development of high relief bedforms producing large-scale cross-beds and effective sorting of material to generate lateral facies differentiation (cf. Teisseyre 1976;Blair and McPherson 1994b;Zielinski and Loon 2000). Integrated outcrop and borehole information indicates that the Gaula Fan is the product of multiple fan expansion cycles (Figs.…”

“…Despite their tectonic and palaeoclimatic implications for the Pleistocene and the Holocene times in the Indo-Gangetic Basin, the smaller piedmont fans have not yet been properly studied, although some brief comments have been made by many workers (Ghosh and Singh 1988;Shukla and Bora 2003). Extensive work on the physiography, geomorphology and sedimentology has been undertaken on alluvial fans developed under different tectono-climatic settings (Hooke and Le 1967;Bull 1977;Harvey 1987;Nemec and Steel 1988;Beaty 1990;Wescott and Etrhridge 1990;Nemec and Postma 1993;Zielinski and Loon 2000;Krapf et al 2005). Nevertheless, probably because of limited exposures and outcrops, increased human activities and dense vegetation cover systematic sedimentological investigations on fans in Shukla et al 2001).…”

Sedimentation model for the quaternary intramontane Bhimtal–Naukuchiatal Lake deposits, Nainital, India

“…Horizontally bedded gravel 2-5 m thick units with ample sand matrix, 2-10 cm sized clasts forming horizontal bands having gentle inclination show imbrication. Sheet flows under upper flow regime conditions (Zielinski and van Loon, 2000). Ripple-cross-laminated sand (Figs.…”