Autogenic dynamics of alluvial fans in endorheic basins: Outcrop examples and stratigraphic significance

Abstract

Alluvial fans are relatively simple depositional systems, due to the direct coupling of sediment sources and adjacent accumulation areas. Nonetheless, general models of alluvial-fan evolution and stratigraphy remain elusive, due to the great sensitivity of such systems to allogenic controls and their strongly case-specific responses. Autogenic processes intrinsic to alluvial-fan dynamics can complicate stratigraphic architectures, with effects not easily distinguishable from those of allogenic forcing. A distinction is made here between lateral autogenic dynamics, tied to spatial sediment distribution over fan surfaces, and vertical autogenic dynamics, related to independent incision-aggradation cycles. Autogenic mechanisms have been highlighted recently by modelling studies, but remain poorly constrained in field-based studies. Examples are presented here from the margins of the Cenozoic Teruel and Ebro basins (Spain), where alluvial fans accumulated thick successions during phases of basin topographic closure and endorheic drainage which promoted forced aggradation. Fan successions consist of conformable architectures of stacked clastic sheets, laterally continuous and with no evidence of internal unconformities, inset architectures, fan segmentation or preserved incised channels. Continuous aggradation in these closed basins strongly inhibited 'vertical' autogenic dynamics in the form of fan head and through fan incision, due to the forced rise in geomorphic base level and the creation of positive accommodation. Furthermore, the lack of incised channels favoured widespread sediment transport and aggradation over broad fan sectors in relatively short time spans, in contrast to the typical occurrence of active lobes and abandoned fan surfaces caused by 'lateral' autogenic dynamics. Stratigraphic records of alluvial fans developed in endorheic basins are essentially complete and largely unaffected by autogenic processes. The latter characteristic implies that they can be more unambiguously interpreted in terms of allogenic forcing, because stratigraphic signatures are not complicated by the effects of complex fan autodynamics.

Experimental alluvial fans: Advances in understanding of fan dynamics and processes

“…The term autogenic is used to describe the process response arising from internal feedbacks with the fan system independent of alterations in the allogenic (external) controls . However, the role of autogenic processes in alluvial fans remains poorly constrained: the sporadic occurrence of effective geomorphic events in present-day alluvial fans renders the direct observation of processes unlikely, and the chaotic architecture of ancient fan successions has hampered detailed stratigraphic analysis (Ventra and Nichols, 2014). By enabling control of the external factors influencing fan systems, experimental modelling has therefore provided an avenue to explore the influence of autogenic processes on flow patterns and the resultant morphology.…”

Quantitative reconstruction of late Holocene surface evolution on an alpine debris-flow fan

“…Debris-flow fans are known to occupy different depositional lobes over time (Suwa and Okuda, 1983;Blair and McPherson, 1994;Stoffel et al, 2005;Dühnforth et al, 2007;Ventra and Nichols, 2014;d'Arcy et al, 2015), and switching between lobes has been linked to internal and external controls on fan development (Dühnforth et al, 2008;Ventra and Nichols, 2014). Schumm et al (1987) and de Haas et al (2016) documented cyclic alternations of avulsion, channelization, and backstepping of the active depocenter on experimental debris-flow fans, and de Haas et al (2016) argued that debris-flow fans are therefore likely governed by the same large-scale compensatory behavior as fluvial fans and fan deltas, albeit via different physical processes.…”

A sedimentary model for early Palaeozoic fluvial fans, Alderney Sandstone Formation (Channel Islands, UK)

“…Stages of peak discharge recur every 20 to 50 m of vertical stratigraphy, and point to a semi-cyclic behaviour of the depositional system. This could be either related to allogenic factors such as fluctuations in sediment production and routing (Gibling et al, 2011), or simply to autogenic mechanisms such as stochastic avulsions (Buehler et al, 2011;Ventra and Nichols, 2014) or fan-apex relocation (Owen et al, 2015c). A tendency towards smaller macroform accretionary packages and reactivation surfaces are indicative of a mature fluvial fan recording frequent aeolian interludes along its distal reaches (Fig.…”