Autogenic cyclicity of foreset sorting in experimental Gilbert-type deltas

Abstract

Gilbert-type deltas are commonly characterised by steep foreset bedding. Changes in the foreset characteristics such as grain size, grading, alternating sand and gravel are often interpreted as records of changes of base level or sediment input due to diurnal, seasonal or climatic forcing. To aid such interpretations, an important question is to what extent cyclicity in the foreset characteristics can be explained by autogenic processes rather than exogenic forcing. Experimental deltas were generated with gravelly sand under constant boundary conditions and a varying width-depth ratio of the feeder system. The foresets of the narrow feeder system are regular and show gradual upward fining. The foresets of the wide feeder system, in contrast, show quasi-cyclic patterns of coarsening and fining during the delta progradation. The cyclicity is caused by the emergence of a channelised point feeder system with migrating gravel side bars, which distributes the sand and gravel laterally in a non-uniform manner. This spectacular change of foreset architecture is fully explained by the autogenic response of the feeder system to the width. These experiments are contrasted with experiments and data from literature in the framework of the delta classification of Postma [Postma, G., 1990. Depositional architecture and facies of river and fan deltas: a synthesis.

Reciprocal Changes In Foreset To Bottomset Facies In A Gilbert-Type Delta: Response To Short-Term Changes In Base Level

“…However, the changes in base level are obviously not a sole factor controlling the hydraulic regime and morphodynamics of the delta front. The deltaic depositional system inevitably acquires ''noise'' due to its own autogenic variability with lateral shifting of distributaries (Kleinhans 2005;Longhitano 2008), and due to allogenic factors such as climate seasonality and possibly regional climatic changes (see the Pleistocene climatic fluctuations in the East Mediterranean reviewed by Bottema and van Zeist 1981, Roberts and Wright 1993, Landman et al 1996. In short, the base-level behavior alone imposes little more than particular recognizable tendencies in the pattern of subaqueous sediment dispersal, as the facies record of base-level changes is by no means free of ''noise.''…”

Gilbert‐type deltas recording short‐term base‐level changes: Delta‐brink morphodynamics and related foreset facies

“…These deltas have attracted considerable research interest during the last three decades or so, including modern cases (Prior et al ., ; Kostaschuk & McCann, ; Prior & Bornhold, , , ; Corner et al ., ; Bell, ; Saito, ) and many ancient examples (Postma, ; Postma & Roep, ; Colella, ,b; Postma & Cruickshank, ; Colella & Prior, ; Ori et al ., ; Dart et al ., ; Dorsey et al ., ; Massari, ; Chough & Hwang, ; Sohn et al ., ; Nemec et al ., ; Dorsey & Umhoefer, ; Lønne et al ., ; Lønne & Nemec, ; Ilgar & Nemec, ; Mortimer et al ., ; García‐García et al ., ; Breda et al ., ; Ford et al ., ; Longhitano, ; Backert et al ., ; Eilertsen et al ., ; Ilgar et al ., ; Gobo et al ., ), as well as computer modelling (Muto & Steel, ; Syvitski & Daughney, ; Hardy et al ., ; Uličný et al ., ) and laboratory experiments (Kleinhans, ; Rohais et al ., ; Bijkerk et al ., ; Ferrer‐Boix et al ., ). A comprehensive review of the delta‐slope processes and facies was given by Nemec (), including the origin of slope chutes, gullies and cross‐strata backsets (Fig.…”

“…The delta brink (Fig. 1A) is a crucial morphodynamic zone for sediment transfer from the upper delta front, dominated by river effluent regime, to the lower delta front dominated by basin hydraulic processes and the delta-slope realm dominated by gravitational sediment transport (Prior et al, 1981;Massari & Parea, 1990;Prior & Bornhold, 1990;Lønne & Nemec, 2004;Kleinhans, 2005;Longhitano, 2008).…”

米粒子を用いた実験マイクロデルタ堆積物中にみられる堆積過程を反映した二種類の3次元粒子配列

“…However, the avalanches on the lee slopes of deltas and dunes commonly develop as tongue-shaped flows (e.g., Allen, 1963Allen, , 1982Carling and Glaister, 1987). Sedimentary structures observed through the sidewall of a flume can be influenced by the width of the flume (Kleinhans, 2005b). A study conducted by Carling and Glaister (1987) showed that the orientation of pebbles at the toe of the foreset was more variable than that at the upper foreset.…”